Title of Invention	OPTICAL FIBER AND OPTICAL FIBER PREFORM
Abstract	An optical fiber is constituted by: a three-layer structured core which includes, a first core (having a relative refractive index difference of Δ1 in a region of a radius of Rl), a second core (having a relative refractive index difference of Δ2 in a region from the radius Rl to a radius of R2), and a third core (having a relative refractive index difference of Δ3 in a region from the radius of R2 to a radius of R3), wherein the relative refractive index differences have relationships of Δ1> Δ2, Δ3> Δ2, and Δ3> Δ1, when Δ1-Δ2=X and Δ3-Δ2=Y, (X+Y)>0.4% is satisfied, and X and Y satisfy 0.25%<X<0.6%, 0.1%<Y<0.6%, and a relationship of (2*X-0.7)%<Y<(X/2+0.4)%, thereby satisfying the G652 standard and having an SBS threshold equal to or higher than that of an optical fiber having the same MFD by +3 dB or higher.

Title of Invention

OPTICAL FIBER AND OPTICAL FIBER PREFORM

Abstract

An optical fiber is constituted by: a three-layer structured core which includes, a first core (having a relative refractive index difference of Δ1 in a region of a radius of Rl), a second core (having a relative refractive index difference of Δ2 in a region from the radius Rl to a radius of R2), and a third core (having a relative refractive index difference of Δ3 in a region from the radius of R2 to a radius of R3), wherein the relative refractive index differences have relationships of Δ1> Δ2, Δ3> Δ2, and Δ3> Δ1, when Δ1-Δ2=X and Δ3-Δ2=Y, (X+Y)>0.4% is satisfied, and X and Y satisfy 0.25%<X<0.6%, 0.1%<Y<0.6%, and a relationship of (2*X-0.7)%<Y<(X/2+0.4)%, thereby satisfying the G652 standard and having an SBS threshold equal to or higher than that of an optical fiber having the same MFD by +3 dB or higher.

Full Text	DESCRIPTION OPTICAL FIBER AND OPTICAL FIBER PREFORM Technical Field [0001] The present invention relates to an optical fiber and an optical fiber preform. There was a problem in that when an optical analog signal or an optical baseband signal is transmitted for a long distance by using an optical fiber, due to an effect of stimulated Brillouin scattering (hereinafter, referred to as SBS), only a predetermined intensity of light (SBS threshold power) can be transmitted through the optical fiber although light with higher power was intended to be transmitted, and the remaining light becomes backscattered light and returns to the incoming side, so that transmissible power of signal light is limited. The invention relates to an optical fiber capable of suppressing occurrences of SBS and transmitting higher power of signal light. Priority is claimed on Japanese Patent Application No. 2006-249360, filed on September 14, 2006, the content of which is incorporated herein by reference. Background Art [0002] Recently, fiber to the home (FTTH) services in which homes are connected via optical fibers to exchange information using the optical fibers have been expanded. In FTTH services for transmitting various types of information, there is a system for simultaneously transmitting a broadcasting signal and other communications signals in their respective modes using a single optical fiber. In general, in this system, in many cases, the broadcasting signal is an analog signal, a baseband signal, or an optical SCM signal. The characteristics of this system from the point of optical fiber as a transmission medium are as follows: (i) FTTH is based on a typical double-star PON (passive optical network), and a distribution loss increases. (ii) In order to transmit an analog signal, a baseband signal, or an optical SCM signal, a receiver needs a high CNR (carrier-to-noise ratio), so that the minimum signal light power of a light receiving unit needs to be greater than that in digital transmission used for communications. [0003] As described above, when analog transmission using intensity modulation is performed for video transmission, high power transmission is needed to compensate for a distribution loss and guarantee a high CNR. However, there was a problem in that only a predetermined intensity of light (SBS threshold power) can be transmitted through the optical fiber although higher power is intended to be transmitted, and the remaining light becomes backscattered light and returns to the incident side, so that transmissible signal light power was limited. [0004] As means for suppressing SBS, there is a technique for changing a dopant concentration and a residual stress in a longitudinal direction (for example, see Patent Document 1). In this technique, by changing the dopant concentration or the residual stress in the longitudinal direction, it is possible to enlarge the Brillouin spectrum and suppress occurrences of SBS. In addition, techniques for giving such a refractive index profile for an optical fiber that SBS can be suppressed, are proposed (for example, see Patent Documents 2 to 5, and 7). Patent Document 1: Japanese Patent No. 2584151 Patent Document 2: WO 2004/100406 Patent Document 3: U.S. Patent No. 7082243 Patent Document 4: Japanese Unexamined Patent Application Publication No. 2006-154707 Patent Document 5: Japanese Unexamined Patent Application Publication No. 2006-184534 Patent Document 6: Japanese Unexamined Patent Application Publication No. 2006-133314 Patent Document 7: Japanese Unexamined Patent Application Publication No. 2006-154713 Non-Patent Document 1: "Design concept for optical fibers with enhanced SBS threshold" Optics Express, Vol. 13, Issue 14, p. 5338 (July 2005), Andrey Non-Patent Document 2: "Nonlinear Optical Fibers with Increased SBS Thresholds" OFC/NFOEC 2006, OtyA3, Scott Bickham, Andrey Kobyakov, Shenping Li Disclosure of the Invention Problems that the Invention is to Solve [0005] As a technique for suppressing SBS, as described above, the technique for changing a dopant concentration or a residual stress in a longitudinal direction is reported (Patent Document 1). However, in this technique, optical characteristics are changed in the longitudinal, so that this technique is not preferable for practical use. [0006] In addition, techniques for suppressing SBS by giving a refractive index profile for an optical fiber (Patent Documents 2 to 5 and 7) are reported. In theses techniques, a change in optical characteristics in a longitudinal direction does not occur, However, a structure having a refractive index profile suitable for desired characteristics is needed. [0007] In Patent Documents 2, 3, and 5, for an optical fiber described which has a three-layer structured refractive index profile, the refractive index profile is set to a suitable condition, thereby suppressing SBS and obtaining the same optical characteristics as ITU-T Recommendation G.652 (hereinafter, referred to as G652). However, not all structures described in Patent Documents 2, 3, and 5 satisfy the same optical characteristics as G652, and when manufacturing is practically performed on the basis of these conditions, a suitable design value for each condition is needed. [0008] In Patent Document 4, a uniform bending loss becomes worse, and in consideration of handling of the optical fiber, this phenomenon is not preferable. In Patent Document 6, fluorine needs to be added at an intentional position, so that manufacturing a preform using a VAD method is difficult. In Patent Document 7, only the shape of a refractive index profile is described, and detailed parameters and the like are not mentioned. Means for Solving the Problems [0009] The invention is designed to solve the above-mentioned problems. An object of the invention is to provide an optical fiber and an optical fiber preform which have stable characteristics in a longitudinal direction, compatibility with G652, excellent manufacturability, and suppressed SBS, by giving a structure design value for a suitable refractive index profile. [0010] According to a first aspect of the invention, there is provided an optical fiber having a refractive index profile constituted by: a three-layer structured core which includes, in the central portion of the core, a first core having a substantially uniform positive relative refractive index difference Δl in a region from the center of the core to a radius of Rl urn, a second core which surrounds and comes in contact with the first core and has a substantially uniform positive relative refractive index difference Δ2 in a region from the radius of Rl µm to a radius of R2 µm, and a third core which surrounds and comes in contact with the second core and has a substantially uniform positive relative refractive index difference Δ3 in a region from the radius of R2 µm to a radius of R3 µm; and a cladding which surrounds and comes in contact with the three-layer structured core and has a substantially uniform refractive index, wherein Δ2 is equal to or less than 0.4%, Δl, Δ2, and Δ3 have relationships of Δ1>Δ2, Δ3>Δ2, and Δ3>Δ1, when Δl, Δ2, and Δ3 have relationships of Δ1-Δ2=X and Δ3-Δ2=Y, (X+Y)>0.4% is satisfied, and X and Y satisfy 0.25% (2X-0.7)% (Δ2+Δ3)+1.0 less than 1260 nm, a mode field diameter is in the range of 7.9 to 10.2 µm at a wavelength of 1.31 µm, a zero-dispersion wavelength is in the range of 1300 to 1324 nm, a zero-dispersion slope is equal to or less than 0.093 ps/(nm2-km), a uniform bending loss is equal to or less than 2 dB/m at a diameter of 20 mm and a wavelength of 1.31 µm, and an SBS threshold at a wavelength of 1.55 µm is higher than that of a single-mode optical fiber having a typical step-index profile and the same mode field diameter by +3 dB or higher. According to a second aspect of the invention, there is provided an optical fiber having a refractive index profile constituted by: a three-layer structured core which includes, in the central portion of the core, a first core having a maximum relative refractive index difference Δ1 in a region from the center of the core to a radius of Rl µm, a second core which surrounds and comes in contact with the first core and has a minimum relative refractive index difference Δ2 in a region from the radius of Rl urn to a radius of R2 µm, and a third core which surrounds and comes in contact with the second core and has a maximum relative refractive index difference Δ3 in a region from the radius of R2 µm to a radius of R3 µm; and a cladding which surrounds and comes in contact with the three-layer structured core and has a substantially uniform refractive index, wherein Δ2 is equal to or less than 0.4%, Δ1, Δ2, and Δ3 have relationships of Δ1> Δ2, Δ3> Δ2, and Δ3> Δ1, when Δ1, Δ2, and Δ3 have relationships of Δ1-Δ2=X and Δ3-Δ2=Y, (X+Y)>0.4% is satisfied, X and Y satisfy 0.25% a relationship of (2X-0.7)% relationships of (Δ2+Δ3)+1.0 wavelength is less than 1260 nm, a mode field diameter is in the range of 7.9 to 10.2 µm at a wavelength of 1.31 µm, a zero-dispersion wavelength is in the range of 1300 to 1324 nm, a zero-dispersion slope is equal to or less than 0.093 ps/(nm2.km), a uniform bending loss is equal to or less than 2 dB/m at a diameter of 20 mm and a wavelength of 1.31 urn, and an SBS threshold at a wavelength of 1.55 urn is higher than that of a single-mode optical fiber having a typical step-index profile and the same mode field diameter by +3 dB or higher. According to a third aspect of the invention, there is provided an optical fiber preform having a refractive index profile constituted by: a three-layer structured core which includes, in the central portion of the core, a first core having a substantially uniform positive relative refractive index difference Δ1 in a region from the center of the core to a radius of Rl µm, a second core which surrounds and comes in contact with the first core and has a substantially uniform positive relative refractive index difference Δ2 in a region from the radius of Rl µm to a radius of R2 µm, and a third core which surrounds and comes in contact with the second core and has a substantially uniform positive relative refractive index difference Δ3 in a region from the radius of R2 µm to a radius of R3 µm; and a cladding which surrounds and comes in contact with the three-layer structured core and has a substantially uniform refractive index, wherein Δ2 is equal to or less than 0.4%, the Δ1, Δ2, and Δ3 have relationships of Δ1> Δ2, Δ3> Δ2, and Δ3> Δ1, when Δ1, Δ2, and Δ3 have relationships of Δ1-Δ2=X and Δ3-Δ2=Y, (X+Y)>0.4% is satisfied, and X and Y satisfy 0.25% (2X-0.7)% (Δ2+Δ3)+1.0 preform is drawn into an optical fiber, a cable cutoff wavelength is less than 1260 nm, a mode field diameter is in the range of 7.9 to 10.2 µm at a wavelength of 1.31 µm, a zero-dispersion wavelength is in the range of 1300 to 1324 nm, a zero-dispersion slope is equal to or less than 0.093 ps/(nm2-km), a uniform bending loss is equal to or less than 2 dB/m at a diameter of 20 mm and a wavelength of 1.31 µm, and an SBS threshold at a wavelength of 1.55 µm is higher than that of a single-mode optical fiber having a typical step-index profile and the same mode field diameter by +3 dB or higher. According to a fourth aspect of the invention, there is provided an optical fiber preform having a refractive index profile constituted by: a three-layer structured core which includes, in the central portion of the core, a first core having a maximum relative refractive index difference Δ1 in a region from the center of the core to a radius of Rl µm, a second core which surrounds and conies in contact with the first core and has a minimum relative refractive index difference Δ2 in a region from the radius of Rl µm to a radius of R2 µm, and a third core which surrounds and comes in contact with the second core and has a maximum relative refractive index difference Δ3 in a region from the radius of R2 µm to a radius of R3 µm; and a cladding which surrounds and comes in contact with the three-layer structured core and has a substantially uniform refractive index, wherein Δ2 is equal to or less than 0.4%, Δ1, Δ2, and Δ3 have relationships of Δ1> Δ2, Δ3> Δ2, and Δ3> Δ1, when Δ1, Δ2, and Δ3 have relationships of Δ1-Δ2=X and Δ3-Δ2=Y, (X+Y)>0.4% is satisfied, and X and Y satisfy 0.25% and a relationship of (2X-0.7)% relationships of (Δ2+Δ3)+l .0 optical fiber preform is drawn into an optical fiber, a cable cutoff wavelength is less than 1260 nm, a mode field diameter is in the range of 7.9 to 10.2 µm at a wavelength of 1.31 µm, a zero-dispersion wavelength is in the range of 1300 to 1324 nm, a zero-dispersion slope is equal to or less than 0.093 ps/(nm -km), a uniform bending loss is equal to or less than 2 dB/m at a diameter of 20 mm and a wavelength of 1.31 (am, and an SBS threshold at a wavelength of 1.55 µm is higher than that of a single-mode optical fiber having a typical step-index profile and the same mode field diameter by +3 dB or higher. Brief Description of the Drawings [0011] FIG. 1 is a view showing a relationship between incoming light power and transmitted, backscattered light power to show occurrences of SBS. FIG. 2 is a view illustrating a configuration of an SB S threshold measurement system. FIG. 3 is a view showing MFD dependence of an SBS threshold of an SMF having a refractive index profile of step-index model illustrated in FIG. 4. FIG. 4 is a view showing the refractive index profile of step-index model. FIG. 5 is a view showing a first example of a refractive index profile of an optical fiber related to the invention. FIG. 6 is a view showing a second example of a refractive index profile of an optical fiber related to the invention. FIG. 7 is a view showing a relationship between X+Y and an SBS suppression effect (SBSeff) on an SMF having the same MFD when Δ1-Δ2=X and Δ3-Δ2=Y. FIG. 8 is a view showing a relationship of (X,Y) satisfying a zero-dispersion wavelength of 1300 to 1324 nm and SBSeff>+3 dB. FIG 9 is a view showing a relationship between Δ2+Δ3 and R2/R1 satisfying MFD=7.9tol0.2um. FIG. 10 is a view showing a relationship between A3-A1 and SBSeff. FIG. 11 is a view showing a relationship between Δ1-Δ2 and SBSeff. FIG. 12 is a view showing a relationship between Δ1-Δ2 and SBSeff. FIG. 13 is a view showing relationships between SBS thresholds and MFDs of Examples 1 and la to lg. FIG. 14 is a view showing relationships between SBS thresholds and MFDs of Examples 1 and la to lv. FIG 15 is a view showing relationships between SBS thresholds and MFDs of Examples 2a to 2f. FIG. 16 is a view showing relationships between SBS thresholds and MFDs of Examples 2g to 2m. FIG. 17 is a view showing a refractive index profile of an optical fiber of Example 3. FIG. 18 is a view showing a refractive index profile of an optical fiber preform of Example 5. FIG. 19 is a view showing relationships between SBS thresholds and MFDs of Example 5. FIG. 20 is a view showing a refractive index profile of an optical fiber preform of Example 6. FIG. 21 is a view showing relationships between SBS thresholds and MFDs of Example 6. FIG. 22 is a view showing a refractive index profile of an optical fiber preform of Example 7. Description of Symbols [0012] 1: LIGHT SOURCE AT WAVELENGTH OF 1.32 µm 2: LIGHT SOURCE AT WAVELENGTH OF 1.55 µm 3: EDFA 4: POWER METER FOR MEASURING BACKSCATTERED LIGHT POWER 5: 9:1 COUPLER 6: POWER METER FOR MEASURING INCOMING LIGHT POWER 7: POWER METER FOR MEASURING TRANSMITTED LIGHT POWER 8: OPTICAL FIBER TO BE MEASURED Best Mode for Carrying Out the Invention [0013] An optical fiber of the invention has a refractive index profile constituted by: a three-layer structured core which includes, in the central portion of the core, a first core having a substantially uniform positive relative refractive index difference Δ1 in a region from the center of the core to a radius of Rl µm, a second core which surrounds and comes in contact with the first core and has a substantially uniform positive relative refractive index difference Δ2 in a region from the radius of Rl urn to a radius of R2 µm, and a third core which surrounds and comes in contact with the second core and has a substantially uniform positive relative refractive index difference Δ3 in a region from the radius of R2 urn to a radius of R3 µm; and a cladding which surrounds and comes in contact with the three-layer structured core and has a substantially uniform refractive index, or has a refractive index profile constituted by: a three-layer structured core which includes, in the central portion of the core, a first core having a maximum relative refractive index difference Δ1 in a region from the center of the core to a radius of Rl µm, a second core which surrounds and comes in contact with the first core and has a minimum relative refractive index difference Δ2 in a region from the radius of Rl µm to a radius of R2 µm, and a third core which surrounds and comes in contact with the second core and has a maximum relative refractive index difference Δ3 in a region from the radius of R2 µm to a radius of R3 µm; and a cladding which surrounds and comes in contact with the three-layer structured core and has a substantially uniform refractive index. In the optical fiber, Δ2 is equal to or less than 0.4%, Δ1, Δ2, and Δ3 have relationships of Δ1> Δ2, Δ3> Δ2, and Δ3> Δ1, when Δ1, Δ2, and Δ3 have relationships of Δ1-Δ2=X and Δ3-Δ2=Y, (X+Y)>0.4% is satisfied, and X and Y satisfy 0.25% 0.1% satisfy relationships of (Δ2+Δ3)+1.0 cutoff wavelength is less than 1260 nm, a mode field diameter is in the range of 7.9 to 10.2 urn at a wavelength of 1.31 µm, a zero-dispersion wavelength is in the range of 1300 to 1324 nm, a zero-dispersion slope is equal to or less than 0.093 ps/(nm2-km), a uniform bending loss is equal to or less than 2 dB/m at a diampter of 20 mm. and a wavelength of 1.31 µm, and an SBS threshold at a wavelength of 1.55 µm is higher than that of a single-mode optical fiber having a typical step-index profile and the same mode field diameter by +3 dB or higher. [0014] In addition, the optical fiber of the invention may be manufactured by drawing a preform manufactured by a typical VAD method. However, the manufacturing method of the preform is not limited to the VAD method, and well-known OVD and CVD methods can be used. [0015] FIG 1 is a graph showing occurrences of SBS in an optical fiber. As shown in FIG. 1, when the power of light entering the optical fiber is gradually increased, at a specific value, the power of backscattered light sharply increases, and most of the power of the incoming light becomes backscattered light and returns to the incoming side. Therefore, as shown in FIG. 1, incoming light power at the maximum change rate (second-order differential equation of backscattered light) of a gradient of the backscattered light with respect to a change in the incoming light power is defined as a threshold (SBS threshold) of an occurrence of SBS. [0016] FIG. 2 is a view illustrating a configuration of an SBS threshold measurement system. In FIG. 2, reference numeral 1 denotes a light source at a wavelength of 1.32 µm, reference numeral 2 denotes a light source at a wavelength of 1.55 µm, reference numeral 3 denotes an EDFA, reference numeral 4 denotes a power meter for measuring backscattered light power, reference numeral 5 denotes a 9:1 coupler, reference numeral 6 denotes a power meter for measuring incoming light power, reference numeral 7 denotes a power meter for measuring transmitted light power, and a reference numeral 8 denotes an optical fiber to be measured. In this measurement system, the three power meters 4, 6, and 7 are connected through the 9:1 coupler 5 to measure the power of incoming light, backscattered light, and transmitted light of the optical fiber 8 to be measured. In addition, incoming light power at the maximum second-order differential equation of the backscattered light with respect to the incoming light becomes an SBS threshold. [0017] In Patent Documents 2 and 3, an SBS threshold is evaluated by using the same measurement system and definition. Here, the definition of the SBS threshold is considered as definition 4 in the following document. Shimizu, "A Study on Definition of the SBS Threshold in a Single-Mode Optical Fiber", Institute of Electronics, Information and Communication Engineers, General Conference in 2005, B-10-66. [0018] The SBS threshold is dependent on a mode field diameter (hereinafter, referred to as MFD). A plot result obtained by calculating the MFD dependence of the SBS threshold of a single-mode optical fiber (hereinafter, referred to as SMF) which has a typical refractive index profile of step-index model as shown in FIG. 4 and satisfies the G652 standard is shown in FIG. 3. As shown in FIG. 3, in the range of 7.9 to 10.2 µm of an MFD at a wavelength of 1.31 µm that is in the G652 standard, the SBS threshold of the SMF changes in the range of 7.4 to 9.7 dBm. Accordingly, when the SBS threshold is compared, an optical fiber having the same MFD needs to be compared. [0019] The optical fiber of the invention has optical characteristics described in G652 in » that a cable cutoff wavelength is less than 1260 nm, an MFD at a wavelength of 1.31 µm is in the range of 7.9 to 10.2 µm, a zero-dispersion wavelength is in the range of 1300 to 1324 nm, a zero-dispersion slope is equal to or less than 0.093 ps/(nm2km), a uniform bending loss is equal to or less than 2 dB/m at a bending diameter of 20 mm and a wavelength of 1.31 µm, and an SBS threshold is equal to or higher than two times (+3 dB) that of an SMF having the same MFD. [0020] FIG. 5 is a view illustrating a first example of a refractive index profile of the optical fiber of the invention. The refractive index profile is constituted by: a three-layer structured core which includes, in the central portion of the core, a first core having a substantially uniform positive relative refractive index difference Δ1 in a region from the center of the core to a radius of Rl µm, a second core which surrounds and comes in contact with the first core and has a substantially uniform positive relative refractive index difference Δ2 in a region from the radius of Rl µm to a radius of R2 µm, and a third core which surrounds and comes in contact with the second core and has a substantially uniform positive relative refractive index difference Δ3 in a region from the radius of R2 µm to a radius of R3 urn; and a cladding which surrounds and comes in contact with the three-layer structured core and has a substantially uniform refractive index, wherein Δ1> Δ2 and Δ3> Δ2. [0021] The refractive index profile of the optical fiber of the invention, as illustrated in FIG. 6, may allow relative refractive index differences not to have uniform values. FIG. 6 is a view illustrating a second example of a refractive index profile of the optical fiber of the invention. The refractive index profile is constituted by: a three-layer structured core which includes, in the central portion of the core, a first core having a maximum relative refractive index difference Δ1 in a region from the center of the core to a radius of Rl µm, a second core which surrounds and comes in contact with the first core and has a minimum relative refractive index difference Δ2 in a region from the radius of Rl µm to a radius of R2 (am, and a third core which surrounds and comes in contact with the second core and has a maximum relative refractive index difference Δ3 in a region from the radius of R2 urn to a radius of R3 µm; and a cladding which surrounds and comes in contact with the three-layer structured core and has a substantially uniform refractive index, wherein Δ1> Δ2 and Δ3> Δ2. [0022] For the optical fiber having the refractive index profile constituted by the three-layer structured core and the one-layer structured cladding for surrounding the core, in order to obtain an optical fiber which has the above-mentioned optical characteristics, that is, characteristics compatible with G652 and an SBS threshold equal to or higher than two times that of (by +3 db) the SMF having the same MFD as the optical fiber of the invention, detailed examinations were repeated. As a result, it was found that relationships between Δ1, Δ2, A3, Rl, R2, and R3 were limited. [0023] FIG. 7 is a view showing a relationship of a SBS suppression effect SBSeff on an SMF when Δ1 -A2=X and Δ3-Δ2=Y. Here, SBSeff is defined by the following expression. SBSeff=SBS threshold of the optical fiber of the invention-SBS threshold of the SMF having the same MFD as the optical fiber of the invention [0024] As can be seen from FIG. 7, by allowing X+Y to be greater than 0.4%, it is possible to improve SBSeff to be equal to or higher than +3 dB and increase the SBS threshold. However, by only using the condition, in some cases, an optical fiber having the optical characteristics compatible with the G652 cannot be obtained. Specifically, in order to obtain an optical fiber having a zero-dispersion wavelength in the range of 1300 to 1324 nm, it is preferable that X and Y have relationships of X relationships of X and Y satisfying this condition are shown in FIG. 8. [0025] In addition, in order to allow the MFD at the wavelength of 1.31 µm to be 7.9 µm, that is, the lower limit of the G652 standard, or greater, while the condition is satisfied, it is preferable that Δ2 be equal to or less than 0.4%. In addition, in order to allow the MFD at the wavelength of 1.31 µm to be in the range of 7.9 to 10.2 µm, the position of the third core in the radial direction represented as R2/R1 needs to be suitably determined depending on the sum of Δ2 and A3, that is, Δ2+Δ3. [0026] In order to obtain an optical fiber for enabling the MFD at the wavelength of 1.31 µm to be in the range of 7.9 to 10.2 µm, it is preferable that (Δ2+Δ3) and R2/R1 satisfy relationships of (Δ2+Δ3)+1.0 relationships between (Δ2+Δ3) and R2/R1 satisfying the condition are shown in FIG. 9. [0027] In addition, from the point of manufacturability of an optical fiber, it is preferable that Δ3> Δ1. FIG. 10 is a view showing a relationship between A3-A1 and SBSeff. As shown in FIG. 10, when A3-A1 is negative, SBSeff significantly changes with a slight change in relative refractive index difference. On the other hand, when A3-A1 is positive, the change rate of SBSeff with respect to the change in relative refractive index difference is small. In addition, when SBSeff is approximated to a quadratic equation of A3-A1, the approximation curve is a parabola which opens down, and the inflection point is A3- Δ1>0. It can be seen that the effect of SBSeff caused by the change in relative refractive index difference is small when A3-A1 is positive. When an optical fiber preform is manufactured, in some cases, due to fluctuation in dopant concentration, the relative refractive index difference is changed from a target by about ±0.05%. In this case, there is a possibility that SBSeff is decreased to be lower than a target. When A3-A1 is positive, the change rate of SBSeff caused by the change in relative refractive index difference is small. Therefore, SBSeff does not significantly change with respect to the change in relative refractive index difference caused by the fluctuation in dopant concentration, and SBS characteristics that are always stable can be obtained. In addition, in the refractive index profile of FIG. 10, Δ1 is 0.5%, Δ2 is 0.22%, A3 is in the range of 0.40 to 0.65% at an interval of 0.025%, and R2/R1 is 2.2. However, as shown in Table 1, in other combinations of Δ1, Δ2, A3, Rl, R2, and R3, the inflection point is A3- Δ1>0, and it can be seen that SBS characteristics which are always stable can be obtained when A3-A1 is positive. It is preferable that Δ1 -A2 be equal to or greater than 0.25%. FIG. 11 is a view showing a relationship between Al-A2 and SBSeff. As shown in FIG. 11, when Δ1 is greater than Δ2 by 0.25% or higher, SBSeff can be significantly increased as compared with the case where the difference between Δ1 and Δ2 is equal to or smaller than 0.25%, thereby obtaining a higher SBS suppression effect. In addition, when Δ1 -A2 is equal to or greater than 0.25%, the effect of the change in relative refractive index difference caused by fluctuation in dopant concentration during manufacturing of the optical fiber preform is reduced. Therefore, even when Δ1-Δ2 is changed, SBSeff does not change significantly, and degradation in yield can be prevented. In the refractive index profile of FIG 11, Δ1 is in the range of 0.44% to 0.56% at an interval of 0.03%, Δ2 is 0.24%, Δ3 is 0.55%, and R2/R1 is 2.2. However, as shown in FIG. 12, in a refractive index profile using other combinations of Δ1, Δ2, A3, Rl, R2, and R3, that is, when Δ1 is in the range of 0.44% to 0.56%, Δ2 is in the range of 0.18% to 0.26%, Δ3 is in the range of 0.45% to 0.65%, and R2/R1 is in the range of 1.8 to 2.6, the same tendency can be obtained. Particularly, in a refractive index profile in which Δ1-Δ2 is equal to or greater than 0.25%, a higher SBS suppression effect can be obtained, and dependence of SBSeff on a change in Δ1-Δ2 is reduced. Table 1 In addition, by disposing the third core as described above, it is possible to obtain the SBS threshold higher than that of a conventional optical fiber by +3 dB or higher and obtain characteristics compatible with the G652. Examples [0029] Example 1 and Comparative Example 1 In Table 2, structural parameters and optical characteristics of an optical fiber of Example 1 which has the refractive index profile of FIG. 5 are shown. In addition, structural parameters and optical characteristics of an optical fiber of Comparative Example 1 are shown. The optical fiber of Comparative Example 1 is an SMF having a step-index profile as illustrated in FIG. 4. [0030] Table 2 As shown in Table 2, the optical fiber having the structural parameters of Example 1 related to the invention had an SBS threshold of 12.2 dBm for a length of 20 km, and could obtain a higher suppression effect than the optical fiber of Comparative Example 1 which had the same MFD by +3.5 dB. In addition, the optical fiber of Example 1 had the same optical characteristics as the SMF of Comparative Example 1 and satisfied the G652 standard. [0032] Examples 1a to 1g Table 3 shows results in the case where structural parameters of Example 1 are represented by using X, Y, and R2/R1 described above. [0033] [0034] The optical fibers having the structural parameters of Examples la to lg shown in Table 3 had SBS thresholds of 12.4 to 13.3 dBm for a length of 20 km as shown in FIG. 13 and could obtain suppression effects higher than the SMF having the same MFD by +3.7 to +4,6 dB. In addition, the optical characteristics of the optical fibers of Examples la to lg all satisfied the G652 standard. Examples lh to lv Tables 4 and 5 show results in the case where structural parameters of Example 1 are represented by using X, Y, and R2/R1 described above. The optical fibers having the structural parameters of Examples la to lg shown in Table 3 and Examples lh to lv shown in Tables 4 and 5 had SBS thresholds of 10.9 to 13.8 dBm for a length of 20 km as shown in FIG. 14 and could obtain suppression effects higher than the SMF having the same MFD by +3.1 to +4.5 dB. In addition, the optical characteristics of the optical fibers of Examples lh to lv all satisfied the G652 standard. [0035] Examples 2a to 2f Table 6 shows optical characteristics in the case where structural parameters of a refractive index profile of the optical fiber having the refractive index profile of FIG. 6 are represented by using X, Y, and R2/R1 described above. [0036] [0037] The optical fibers having the structural parameters of Examples 2a to 2f shown in Table 6 had SBS thresholds of 12.0 to 13.7 dBm for a length of 20 km as shown in FIG. 15 and could obtain suppression effects higher than the SMF having the same MFD by +3.3 to +5.0 dB. In addition, the optical characteristics of the optical fibers of Examples 2a to 2f all satisfied the G652 standard. In addition, by changing the refractive index of the third core as illustrated in FIG. 6, the amount of dopant GeO2 in the core can be reduced, so that it is possible to reduce a loss in the optical fiber. Examples 2g to 2m Table 7 shows optical characteristics in the case where structural parameters of a refractive index profile of the optical fiber having the refractive index profile of FIG. 6 are represented by using X, Y, and R2/R1 described above. The optical fibers having the structural parameters of Examples 2a to 2f shown in Table 6 and Examples 2g to 2m shown in Table 7 had SBS thresholds of 10.8 to 14.3 dBm for a length of 20 km as shown in FIG 16 and could obtain suppression effects higher than the SMF having the same MFD by +3.2 to +4.7 dB. In addition, the optical characteristics of the optical fibers of Examples 2g to 2m all satisfied the G652 standard. [0038] Examples 3 and 4 FIG. 17 shows a refractive index profile of an optical fiber of Example 3 related to the invention. The refractive index profile in Example 3, as shown in FIG. 17, is constituted by: a three-layer structured core which includes, in the central portion of the core, a first core in a region from the center of the core to a radius of Rl µm, a second core which surrounds and comes in contact with the first core in a region from the radius of Rl µm to a radius of R2 µm, and a third core which surrounds and comes in contact with the second core in a region from the radius of R2 µm to a radius of R3 µm; and a cladding which surrounds and comes in contact with the three-layer structured core. However, unlike in Examples 1 and 2, the refractive index profile of the core smoothly changes, and the boundary thereof is obscure. Therefore, by using the change rate (dA/dr) of the relative refractive index difference in the radial direction, the diameter of each layer was determined. The relative refractive index difference Δ1 of the first core, as represented by the following equation (1), is defined as A which becomes equivalently uniform in the region from the center of the core to the radius Rl, the relative refractive index difference Δ2 of the second core is defined as a relative refractive index difference that becomes a minimum value in the region between the radii Rl and R2 µm, and the relative refractive index difference Δ3 of the third core is defined as a relative refractive index difference that becomes a maximum value in the region between the radii R2 and R3 µm. [0039] Equation 1 [0040] The structural parameters of the optical fiber of Example 3 defined as described above and the optical characteristics thereof are shown in Table 8. In addition, in Table 8, the structural parameters of the optical fiber of Example 4 having the same refractive index profile as Example 3 and the optical characteristics thereof are shown. [0041] [0042] As shown in Table 8, the optical fibers of Examples 3 and 4 had SBS thresholds of 12.2 to 12.7 dBm for a length of 20 km, and could obtain higher suppression effects than the SMF having the same MFD by +3.5 to +4.0 dB. In addition, the optical fibers of Examples 3 and 4 all satisfied the G652 standard. [0043] Example 5 FIG. 18 is a refractive index profile of an optical fiber preform of Example 5. The optical fiber preform in this example, as shown in FIG. 18, is constituted by: a three-layer structured core which includes, in the central portion of the core, a first core having a substantially uniform positive relative refractive index difference Δ1 in a region from the center of the core to a radius of Rl µm, a second core which surrounds and comes in contact with the first core and has a substantially uniform positive relative refractive index difference Δ2 in a region from the radius of Rl µm to a radius of R2 µm, and a third core which surrounds and comes in contact with the second core and has a substantially uniform positive relative refractive index difference Δ3 in a region from the radius of R2 µm to a radius of R3µm. Like Examples 1 and 2, the three-layer structured core is included. Structural parameters of the optical fiber preform of Example 5 are represented in Tables 9 and 10 by using X, Y, and R2/R1 described above, and optical characteristics that were exhibited when the preform was drawn into an optical fiber are shown. The optical fiber drawn from the optical fiber preform of Example 5 had an SBS threshold of 10.9 to 13.8 dBm for a length of 20 km as shown in FIG. 19, and obtained a suppression effect higher than the SMF having the same MFD by +3.1 to +4.5 dB, so that the G652 standard was further satisfied. [0044] Example 6 FIG. 20 shows a refractive index profile of an optical fiber preform of Example 6. The optical fiber preform in this example, as shown in FIG. 20, is constituted by: a three-layer structured core which includes, in the central portion of the core, a first core having a maximum relative refractive index difference Δ1 in a region from the center of the core to a radius of Rl µm, a second core which surrounds and comes in contact with the first core and has a minimum relative refractive index difference Δ2 in a region from the radius of Rl µm to a radius of R2 µm, and a third core which surrounds and comes in contact with the second core and has a maximum relative refractive index difference Δ3 in a region from the radius of R2 urn to a radius of R3 jam. Like Examples 1 and 2, the three-layer structured core is included. Structural parameters of the optical fiber preform of Example 6 are represented in Tables 11 and 12 by using X, Y, and R2/R1 described above, and optical characteristics that were exhibited when the preform was drawn into an optical fiber are shown. The optical fiber drawn from the optical fiber preform of Example 6 had an SBS threshold of 10.8 to 14.3 dBm for a length of 20 km as shown in FIG. 21, and obtained a suppression effect higher than the SMF having the same MFD by +3.2 to +4.7 dB, so that the G652 standard was further satisfied. [0045] Example 7 FIG. 22 shows a refractive index profile of an optical fiber preform of Example 7. The optical fiber preform in this example, as shown in FIG. 22, is constituted by: a three-layer structured core which includes, a first core disposed in a region from the center of the core to a radius of Rl, a second core which surrounds and comes in contact with the first core and is disposed in a region from the radius of Rl µm to a radius of R2 µm, and a third core which surrounds and comes in contact with the second core and is disposed in a region from the radius of R2 µm to a radius of R3 um. Like Examples 1, 2, 5, and 6, the three-layer structured core is included. However, unlike in Examples 1, 2, 5, and 6, the refractive index profile smoothly changes, and the definition of the boundary thereof is the same as in Examples 3 and 4. Structural parameters of the optical fiber preform of Example 7 are represented in Table 13, and optical characteristics that were exhibit when the preform was drawn into an optical fiber are shown. The optical fiber drawn from the optical fiber preform of Example 7 had an SBS threshold of 12.6 dBm for a length of 20 km and obtained a suppression effect higher than the SMF having the same MFD by +3.8 dB, and the G652 standard was satisfied. Industrial Applicability [0046] According to the invention, for the optical fiber having a segment-core type refractive index profile, the relationships between the relative refractive index differences Δ1, Δ2, and Δ3 of the layers are suitably designed, and the position of the third core is suitably determined. Therefore, it is possible to increase an SBS threshold by +3 dB or higher as compared with the SMF having the same MFD while maintaining the optical characteristics described in G652. In addition, by allowing the relative refractive index difference of the third core to be greater than that of the first core, it is possible to improve manufacturability of the optical fiber preform. CLAIMS 1. An optical fiber having a refractive index profile constituted by: a three-layer structured core which includes, in the central portion of the core, a first core having a substantially uniform positive relative refractive index difference Δ1 in a region from the center of the core to a radius of Rl µm, a second core which surrounds and comes in contact with the first core and has a substantially uniform positive relative refractive index difference Δ2 in a region from the radius of Rl urn to a radius of R2 µm, and a third core which surrounds and comes in contact with the second core and has a substantially uniform positive relative refractive index difference Δ3 in a region from the radius of R2 µm to a radius of R3 µm; and a cladding which surrounds and comes in contact with the three-layer structured core and has a substantially uniform refractive index, wherein A2 is equal to or less than 0.4%, Δ1, Δ2, and Δ3 have relationships of Δ1> Δ2, Δ3> Δ2, and Δ3> Δ1, when Δ1, Δ2, and Δ3 have relationships of Δ1-Δ2=X and Δ3-Δ2=Y, (X+Y)>0.4% is satisfied, and X and Y satisfy 0.25% relationship of (2X-0.7)% Δ2, A3, Rl, and R2 satisfy relationships of (Δ2+Δ3)+1.0 a cable cutoff wavelength is less than 1260 nm, a mode field diameter is in the range of 7.9 to 10.2 µm at a wavelength of 1.31 µm, a zero-dispersion wavelength is in the range of 1300 to 1324 nm, a zero-dispersion slope is equal to or less than 0.093 ps/(nm2.km), a uniform bending loss is equal to or less than 2 dB/m at a diameter of 20 mm and a wavelength of 1.31 µm, and an SBS threshold at a wavelength of 1.55 µm is higher than that of a single-mode optical fiber having a typical step-index profile and the same mode field diameter by +3 dB or higher. 2. An optical fiber having a refractive index profile constituted by: a three-layer structured core which includes, in the central portion of the core, a first core having a maximum relative refractive index difference Δ1 in a region from the center of the core to a radius of Rl µm, a second core which surrounds and comes in contact with the first core and has a minimum relative refractive index difference Δ2 in a region from the radius of Rl µm to a radius of R2 µm, and a third core which surrounds and comes in contact with the second core and has a maximum relative refractive index difference Δ3 in a region from the radius of R2 µm to a radius of R3 urn; and a cladding which surrounds and comes in contact with the three-layer structured core and has a substantially uniform refractive index, wherein A2 is equal to or less than 0.4%, Δ1, Δ2, and Δ3 have relationships of Δ1> Δ2, Δ3> Δ2, and Δ3> Δ1, when Δ1, Δ2, and Δ3 have relationships of Δ1-Δ2=X and Δ3-Δ2==Y, (X+Y)>0.4% is satisfied, X and Y satisfy 0.25% (2X-0.7)% Δ2, A3, Rl, and R2 satisfy relationships of (Δ2+Δ3)+1.0 a cable cutoff wavelength is less than 1260 nm, a mode field diameter is in the range of 7.9 to 10.2 µm at a wavelength of 1.31 µm, a zero-dispersion wavelength is in the range of 1300 to 1324 nm, a zero-dispersion slope is equal to or less than 0.093 ps/(nm -km), a uniform bending loss is equal to or less than 2 dB/m at a diameter of 20 mm and a wavelength of 1.31 µm, and an SBS threshold at a wavelength of 1.55 µm is higher than that of a single-mode optical fiber having a typical step-index profile and the same mode field diameter by +3 dB or higher. 3. An optical fiber preform having a refractive index profile constituted by: a three-layer structured core which includes, in the central portion of the core, a first core having a substantially uniform positive relative refractive index difference Δ1 in a region from the center of the core to a radius of Rl µm, a second core which surrounds and comes in contact with the first core and has a substantially uniform positive relative refractive index difference Δ2 in a region from the radius of Rl µm to a radius of R2 µm, and a third core which surrounds and comes in contact with the second core and has a substantially uniform positive relative refractive index difference Δ3 in a region from the radius of R2 µm to a radius of R3 µm; and a cladding which surrounds and comes in contact with the three-layer structured core and has a substantially uniform refractive index, wherein Δ2 is equal to or less than 0.4%, Δ1, Δ2, and Δ3 have relationships of Δ1> Δ2, Δ3> Δ2, and Δ3> Δ1, when Δ1, Δ2, and Δ3 have relationships of Δ1-Δ2=X and Δ3-Δ2=Y, (X+Y)>0.4% is satisfied, and X and Y satisfy 0.25% relationship of (2X-0.7)% Δ2, A3, Rl, and R2 satisfy relationships of (Δ2+Δ3)+1.0 when the optical fiber preform is drawn, into an optical fiber, a cable cutoff wavelength is less than 1260 nm, a mode field diameter is in the range of 7.9 to 10.2 µm at a wavelength of 1.31 (am, a zero-dispersion wavelength is in the range of 1300 to 1324 nm, a zero-dispersion slope is equal to or less than 0.093 ps/(nm2.km), a uniform bending loss is equal to or less than 2 dB/m at a diameter of 20 mm and a wavelength of 1.31 µm, and an SBS threshold at a wavelength of 1.55 µm is higher than that of a single-mode optical fiber having a typical step-index profile and the same mode field diameter by +3 dB or higher. 4. An optical fiber preform having a refractive index profile constituted by: a three-layer structured core which includes, in the central portion of the core, a first core having a maximum relative refractive index difference Δ1 in a region from the center of the core to a radius of Rl µm, a second core which surrounds and comes in contact with the first core and has a minimum relative refractive index difference Δ2 in a region from the radius of Rl µm to a radius of R2 µm, and a third core which surrounds and comes in contact with the second core and has a maximum relative refractive index difference Δ3 in a region from the radius of R2 µm to a radius of R3 µm; and a cladding which surrounds and comes in contact with the three-layer structured core and has a substantially uniform refractive index, wherein Δ2 is equal to or less than 0.4%, Δ1, Δ2, and Δ3 have relationships of Δ1> Δ2, Δ3> Δ2, and Δ3> Δ1, when Δ1, Δ2, and Δ3 have relationships of Δ1-Δ2=X and Δ3-Δ2=Y, (X+Y)>0.4% is satisfied, and X and Y satisfy 0.25% relationship of (2X-0.7)% Δ2, A3, Rl, and R2 satisfy relationships of (Δ2+Δ3)+l.0 when the optical fiber preform is drawn into an optical fiber, a cable cutoff wavelength is less than 1260 nm, a mode field diameter is in the range of 7.9 to 10.2 µm at a wavelength of 1.31 µm, a zero-dispersion wavelength is in the range of 1300 to 1324 nm, a zero-dispersion slope is equal to or less than 0.093 ps/(nm2-km), a uniform bending loss is equal to or less than 2 dB/m at a diameter of 20 mm and a wavelength of 1.31 µm, and an SBS threshold at a wavelength of 1.55 µm is higher than that of a single-mode optical fiber having a typical step-index profile and the same mode field diameter by +3 dB or higher. An optical fiber is constituted by: a three-layer structured core which includes, a first core (having a relative refractive index difference of Δ1 in a region of a radius of Rl), a second core (having a relative refractive index difference of Δ2 in a region from the radius Rl to a radius of R2), and a third core (having a relative refractive index difference of Δ3 in a region from the radius of R2 to a radius of R3), wherein the relative refractive index differences have relationships of Δ1> Δ2, Δ3> Δ2, and Δ3> Δ1, when Δ1-Δ2=X and Δ3-Δ2=Y, (X+Y)>0.4% is satisfied, and X and Y satisfy 0.25% 0.1% G652 standard and having an SBS threshold equal to or higher than that of an optical fiber having the same MFD by +3 dB or higher.

Full Text

DESCRIPTION
OPTICAL FIBER AND OPTICAL FIBER PREFORM
Technical Field
[0001]
The present invention relates to an optical fiber and an optical fiber preform.
There was a problem in that when an optical analog signal or an optical baseband signal
is transmitted for a long distance by using an optical fiber, due to an effect of stimulated
Brillouin scattering (hereinafter, referred to as SBS), only a predetermined intensity of
light (SBS threshold power) can be transmitted through the optical fiber although light
with higher power was intended to be transmitted, and the remaining light becomes
backscattered light and returns to the incoming side, so that transmissible power of signal
light is limited. The invention relates to an optical fiber capable of suppressing
occurrences of SBS and transmitting higher power of signal light.
Priority is claimed on Japanese Patent Application No. 2006-249360, filed on
September 14, 2006, the content of which is incorporated herein by reference.
Background Art
[0002]
Recently, fiber to the home (FTTH) services in which homes are connected via
optical fibers to exchange information using the optical fibers have been expanded. In
FTTH services for transmitting various types of information, there is a system for
simultaneously transmitting a broadcasting signal and other communications signals in
their respective modes using a single optical fiber. In general, in this system, in many
cases, the broadcasting signal is an analog signal, a baseband signal, or an optical SCM

signal. The characteristics of this system from the point of optical fiber as a
transmission medium are as follows:
(i) FTTH is based on a typical double-star PON (passive optical network), and a
distribution loss increases.
(ii) In order to transmit an analog signal, a baseband signal, or an optical SCM
signal, a receiver needs a high CNR (carrier-to-noise ratio), so that the minimum signal
light power of a light receiving unit needs to be greater than that in digital transmission
used for communications.
[0003]
As described above, when analog transmission using intensity modulation is
performed for video transmission, high power transmission is needed to compensate for a
distribution loss and guarantee a high CNR. However, there was a problem in that only
a predetermined intensity of light (SBS threshold power) can be transmitted through the
optical fiber although higher power is intended to be transmitted, and the remaining light
becomes backscattered light and returns to the incident side, so that transmissible signal
light power was limited.
[0004]
As means for suppressing SBS, there is a technique for changing a dopant
concentration and a residual stress in a longitudinal direction (for example, see Patent
Document 1). In this technique, by changing the dopant concentration or the residual
stress in the longitudinal direction, it is possible to enlarge the Brillouin spectrum and
suppress occurrences of SBS. In addition, techniques for giving such a refractive index
profile for an optical fiber that SBS can be suppressed, are proposed (for example, see
Patent Documents 2 to 5, and 7).
Patent Document 1: Japanese Patent No. 2584151

Patent Document 2: WO 2004/100406
Patent Document 3: U.S. Patent No. 7082243
Patent Document 4: Japanese Unexamined Patent Application Publication No.
2006-154707
Patent Document 5: Japanese Unexamined Patent Application Publication No.
2006-184534
Patent Document 6: Japanese Unexamined Patent Application Publication No.
2006-133314
Patent Document 7: Japanese Unexamined Patent Application Publication No.
2006-154713
Non-Patent Document 1: "Design concept for optical fibers with enhanced SBS
threshold" Optics Express, Vol. 13, Issue 14, p. 5338 (July 2005), Andrey
Non-Patent Document 2: "Nonlinear Optical Fibers with Increased SBS
Thresholds" OFC/NFOEC 2006, OtyA3, Scott Bickham, Andrey Kobyakov, Shenping Li
Disclosure of the Invention
Problems that the Invention is to Solve
[0005]
As a technique for suppressing SBS, as described above, the technique for
changing a dopant concentration or a residual stress in a longitudinal direction is reported
(Patent Document 1). However, in this technique, optical characteristics are changed in
the longitudinal, so that this technique is not preferable for practical use.
[0006]
In addition, techniques for suppressing SBS by giving a refractive index profile
for an optical fiber (Patent Documents 2 to 5 and 7) are reported. In theses techniques,

a change in optical characteristics in a longitudinal direction does not occur, However,
a structure having a refractive index profile suitable for desired characteristics is needed.
[0007]
In Patent Documents 2, 3, and 5, for an optical fiber described which has a
three-layer structured refractive index profile, the refractive index profile is set to a
suitable condition, thereby suppressing SBS and obtaining the same optical
characteristics as ITU-T Recommendation G.652 (hereinafter, referred to as G652).
However, not all structures described in Patent Documents 2, 3, and 5 satisfy the same
optical characteristics as G652, and when manufacturing is practically performed on the
basis of these conditions, a suitable design value for each condition is needed.
[0008]
In Patent Document 4, a uniform bending loss becomes worse, and in
consideration of handling of the optical fiber, this phenomenon is not preferable.
In Patent Document 6, fluorine needs to be added at an intentional position, so
that manufacturing a preform using a VAD method is difficult.
In Patent Document 7, only the shape of a refractive index profile is described,
and detailed parameters and the like are not mentioned.
Means for Solving the Problems
[0009]
The invention is designed to solve the above-mentioned problems. An object
of the invention is to provide an optical fiber and an optical fiber preform which have
stable characteristics in a longitudinal direction, compatibility with G652, excellent
manufacturability, and suppressed SBS, by giving a structure design value for a suitable
refractive index profile.

[0010]
According to a first aspect of the invention, there is provided an optical fiber
having a refractive index profile constituted by: a three-layer structured core which
includes, in the central portion of the core, a first core having a substantially uniform
positive relative refractive index difference Δl in a region from the center of the core to a
radius of Rl urn, a second core which surrounds and comes in contact with the first core
and has a substantially uniform positive relative refractive index difference Δ2 in a region
from the radius of Rl µm to a radius of R2 µm, and a third core which surrounds and
comes in contact with the second core and has a substantially uniform positive relative
refractive index difference Δ3 in a region from the radius of R2 µm to a radius of R3 µm;
and a cladding which surrounds and comes in contact with the three-layer structured core
and has a substantially uniform refractive index, wherein Δ2 is equal to or less than 0.4%,
Δl, Δ2, and Δ3 have relationships of Δ1>Δ2, Δ3>Δ2, and Δ3>Δ1, when Δl, Δ2, and Δ3
have relationships of Δ1-Δ2=X and Δ3-Δ2=Y, (X+Y)>0.4% is satisfied, and X and Y
satisfy 0.25% (2*X-0.7)% (Δ2+Δ3)+1.0 less than 1260 nm, a mode field diameter is in the range of 7.9 to 10.2 µm at a
wavelength of 1.31 µm, a zero-dispersion wavelength is in the range of 1300 to 1324 nm,
a zero-dispersion slope is equal to or less than 0.093 ps/(nm2-km), a uniform bending loss
is equal to or less than 2 dB/m at a diameter of 20 mm and a wavelength of 1.31 µm, and
an SBS threshold at a wavelength of 1.55 µm is higher than that of a single-mode optical
fiber having a typical step-index profile and the same mode field diameter by +3 dB or
higher.

According to a second aspect of the invention, there is provided an optical fiber
having a refractive index profile constituted by: a three-layer structured core which
includes, in the central portion of the core, a first core having a maximum relative
refractive index difference Δ1 in a region from the center of the core to a radius of Rl µm,
a second core which surrounds and comes in contact with the first core and has a
minimum relative refractive index difference Δ2 in a region from the radius of Rl urn to
a radius of R2 µm, and a third core which surrounds and comes in contact with the
second core and has a maximum relative refractive index difference Δ3 in a region from
the radius of R2 µm to a radius of R3 µm; and a cladding which surrounds and comes in
contact with the three-layer structured core and has a substantially uniform refractive
index, wherein Δ2 is equal to or less than 0.4%, Δ1, Δ2, and Δ3 have relationships of
Δ1> Δ2, Δ3> Δ2, and Δ3> Δ1, when Δ1, Δ2, and Δ3 have relationships of Δ1-Δ2=X and
Δ3-Δ2=Y, (X+Y)>0.4% is satisfied, X and Y satisfy 0.25% a relationship of (2*X-0.7)% relationships of (Δ2+Δ3)+1.0 wavelength is less than 1260 nm, a mode field diameter is in the range of 7.9 to 10.2 µm
at a wavelength of 1.31 µm, a zero-dispersion wavelength is in the range of 1300 to 1324
nm, a zero-dispersion slope is equal to or less than 0.093 ps/(nm2.km), a uniform bending
loss is equal to or less than 2 dB/m at a diameter of 20 mm and a wavelength of 1.31 urn,
and an SBS threshold at a wavelength of 1.55 urn is higher than that of a single-mode
optical fiber having a typical step-index profile and the same mode field diameter by +3
dB or higher.
According to a third aspect of the invention, there is provided an optical fiber
preform having a refractive index profile constituted by: a three-layer structured core

which includes, in the central portion of the core, a first core having a substantially
uniform positive relative refractive index difference Δ1 in a region from the center of the
core to a radius of Rl µm, a second core which surrounds and comes in contact with the
first core and has a substantially uniform positive relative refractive index difference Δ2
in a region from the radius of Rl µm to a radius of R2 µm, and a third core which
surrounds and comes in contact with the second core and has a substantially uniform
positive relative refractive index difference Δ3 in a region from the radius of R2 µm to a
radius of R3 µm; and a cladding which surrounds and comes in contact with the
three-layer structured core and has a substantially uniform refractive index, wherein Δ2 is
equal to or less than 0.4%, the Δ1, Δ2, and Δ3 have relationships of Δ1> Δ2, Δ3> Δ2, and
Δ3> Δ1, when Δ1, Δ2, and Δ3 have relationships of Δ1-Δ2=X and Δ3-Δ2=Y, (X+Y)>0.4%
is satisfied, and X and Y satisfy 0.25% (2*X-0.7)% (Δ2+Δ3)+1.0 preform is drawn into an optical fiber, a cable cutoff wavelength is less than 1260 nm, a
mode field diameter is in the range of 7.9 to 10.2 µm at a wavelength of 1.31 µm, a
zero-dispersion wavelength is in the range of 1300 to 1324 nm, a zero-dispersion slope is
equal to or less than 0.093 ps/(nm2-km), a uniform bending loss is equal to or less than 2
dB/m at a diameter of 20 mm and a wavelength of 1.31 µm, and an SBS threshold at a
wavelength of 1.55 µm is higher than that of a single-mode optical fiber having a typical
step-index profile and the same mode field diameter by +3 dB or higher.
According to a fourth aspect of the invention, there is provided an optical fiber
preform having a refractive index profile constituted by: a three-layer structured core
which includes, in the central portion of the core, a first core having a maximum relative

refractive index difference Δ1 in a region from the center of the core to a radius of Rl µm,
a second core which surrounds and conies in contact with the first core and has a
minimum relative refractive index difference Δ2 in a region from the radius of Rl µm to
a radius of R2 µm, and a third core which surrounds and comes in contact with the
second core and has a maximum relative refractive index difference Δ3 in a region from
the radius of R2 µm to a radius of R3 µm; and a cladding which surrounds and comes in
contact with the three-layer structured core and has a substantially uniform refractive
index, wherein Δ2 is equal to or less than 0.4%, Δ1, Δ2, and Δ3 have relationships of
Δ1> Δ2, Δ3> Δ2, and Δ3> Δ1, when Δ1, Δ2, and Δ3 have relationships of Δ1-Δ2=X and
Δ3-Δ2=Y, (X+Y)>0.4% is satisfied, and X and Y satisfy 0.25% and a relationship of (2*X-0.7)% relationships of (Δ2+Δ3)+l .0 optical fiber preform is drawn into an optical fiber, a cable cutoff wavelength is less than
1260 nm, a mode field diameter is in the range of 7.9 to 10.2 µm at a wavelength of 1.31 µm, a zero-dispersion wavelength is in the range of 1300 to 1324 nm, a zero-dispersion
slope is equal to or less than 0.093 ps/(nm -km), a uniform bending loss is equal to or less
than 2 dB/m at a diameter of 20 mm and a wavelength of 1.31 (am, and an SBS threshold
at a wavelength of 1.55 µm is higher than that of a single-mode optical fiber having a
typical step-index profile and the same mode field diameter by +3 dB or higher.
Brief Description of the Drawings
[0011]
FIG. 1 is a view showing a relationship between incoming light power and
transmitted, backscattered light power to show occurrences of SBS.

FIG. 2 is a view illustrating a configuration of an SB S threshold measurement
system.
FIG. 3 is a view showing MFD dependence of an SBS threshold of an SMF
having a refractive index profile of step-index model illustrated in FIG. 4.
FIG. 4 is a view showing the refractive index profile of step-index model.
FIG. 5 is a view showing a first example of a refractive index profile of an
optical fiber related to the invention.
FIG. 6 is a view showing a second example of a refractive index profile of an
optical fiber related to the invention.
FIG. 7 is a view showing a relationship between X+Y and an SBS suppression
effect (SBSeff) on an SMF having the same MFD when Δ1-Δ2=X and Δ3-Δ2=Y.
FIG. 8 is a view showing a relationship of (X,Y) satisfying a zero-dispersion
wavelength of 1300 to 1324 nm and SBSeff>+3 dB.
FIG 9 is a view showing a relationship between Δ2+Δ3 and R2/R1 satisfying
MFD=7.9tol0.2um.
FIG. 10 is a view showing a relationship between A3-A1 and SBSeff.
FIG. 11 is a view showing a relationship between Δ1-Δ2 and SBSeff.
FIG. 12 is a view showing a relationship between Δ1-Δ2 and SBSeff.
FIG. 13 is a view showing relationships between SBS thresholds and MFDs of
Examples 1 and la to lg.
FIG. 14 is a view showing relationships between SBS thresholds and MFDs of
Examples 1 and la to lv.
FIG 15 is a view showing relationships between SBS thresholds and MFDs of
Examples 2a to 2f.

FIG. 16 is a view showing relationships between SBS thresholds and MFDs of
Examples 2g to 2m.
FIG. 17 is a view showing a refractive index profile of an optical fiber of
Example 3.
FIG. 18 is a view showing a refractive index profile of an optical fiber preform
of Example 5.
FIG. 19 is a view showing relationships between SBS thresholds and MFDs of
Example 5.
FIG. 20 is a view showing a refractive index profile of an optical fiber preform
of Example 6.
FIG. 21 is a view showing relationships between SBS thresholds and MFDs of
Example 6.
FIG. 22 is a view showing a refractive index profile of an optical fiber preform
of Example 7.
Description of Symbols
[0012]
1: LIGHT SOURCE AT WAVELENGTH OF 1.32 µm
2: LIGHT SOURCE AT WAVELENGTH OF 1.55 µm
3: EDFA
4: POWER METER FOR MEASURING BACKSCATTERED LIGHT POWER
5: 9:1 COUPLER
6: POWER METER FOR MEASURING INCOMING LIGHT POWER
7: POWER METER FOR MEASURING TRANSMITTED LIGHT POWER
8: OPTICAL FIBER TO BE MEASURED

Best Mode for Carrying Out the Invention
[0013]
An optical fiber of the invention has a refractive index profile constituted by: a
three-layer structured core which includes, in the central portion of the core, a first core
having a substantially uniform positive relative refractive index difference Δ1 in a region
from the center of the core to a radius of Rl µm, a second core which surrounds and
comes in contact with the first core and has a substantially uniform positive relative
refractive index difference Δ2 in a region from the radius of Rl urn to a radius of R2 µm,
and a third core which surrounds and comes in contact with the second core and has a
substantially uniform positive relative refractive index difference Δ3 in a region from the
radius of R2 urn to a radius of R3 µm; and a cladding which surrounds and comes in
contact with the three-layer structured core and has a substantially uniform refractive
index, or has a refractive index profile constituted by: a three-layer structured core which
includes, in the central portion of the core, a first core having a maximum relative
refractive index difference Δ1 in a region from the center of the core to a radius of Rl µm,
a second core which surrounds and comes in contact with the first core and has a
minimum relative refractive index difference Δ2 in a region from the radius of Rl µm to
a radius of R2 µm, and a third core which surrounds and comes in contact with the
second core and has a maximum relative refractive index difference Δ3 in a region from
the radius of R2 µm to a radius of R3 µm; and a cladding which surrounds and comes in
contact with the three-layer structured core and has a substantially uniform refractive
index.
In the optical fiber, Δ2 is equal to or less than 0.4%, Δ1, Δ2, and Δ3 have

relationships of Δ1> Δ2, Δ3> Δ2, and Δ3> Δ1, when Δ1, Δ2, and Δ3 have relationships of
Δ1-Δ2=X and Δ3-Δ2=Y, (X+Y)>0.4% is satisfied, and X and Y satisfy 0.25% 0.1% satisfy relationships of (Δ2+Δ3)+1.0 cutoff wavelength is less than 1260 nm, a mode field diameter is in the range of 7.9 to
10.2 urn at a wavelength of 1.31 µm, a zero-dispersion wavelength is in the range of
1300 to 1324 nm, a zero-dispersion slope is equal to or less than 0.093 ps/(nm2-km), a
uniform bending loss is equal to or less than 2 dB/m at a diampter of 20 mm. and a
wavelength of 1.31 µm, and an SBS threshold at a wavelength of 1.55 µm is higher than
that of a single-mode optical fiber having a typical step-index profile and the same mode
field diameter by +3 dB or higher.
[0014]
In addition, the optical fiber of the invention may be manufactured by drawing a
preform manufactured by a typical VAD method. However, the manufacturing method
of the preform is not limited to the VAD method, and well-known OVD and CVD
methods can be used.
[0015]
FIG 1 is a graph showing occurrences of SBS in an optical fiber. As shown in
FIG. 1, when the power of light entering the optical fiber is gradually increased, at a
specific value, the power of backscattered light sharply increases, and most of the power
of the incoming light becomes backscattered light and returns to the incoming side.
Therefore, as shown in FIG. 1, incoming light power at the maximum change rate
(second-order differential equation of backscattered light) of a gradient of the
backscattered light with respect to a change in the incoming light power is defined as a

threshold (SBS threshold) of an occurrence of SBS.
[0016]
FIG. 2 is a view illustrating a configuration of an SBS threshold measurement
system. In FIG. 2, reference numeral 1 denotes a light source at a wavelength of 1.32 µm, reference numeral 2 denotes a light source at a wavelength of 1.55 µm, reference
numeral 3 denotes an EDFA, reference numeral 4 denotes a power meter for measuring
backscattered light power, reference numeral 5 denotes a 9:1 coupler, reference numeral
6 denotes a power meter for measuring incoming light power, reference numeral 7
denotes a power meter for measuring transmitted light power, and a reference numeral 8
denotes an optical fiber to be measured. In this measurement system, the three power
meters 4, 6, and 7 are connected through the 9:1 coupler 5 to measure the power of
incoming light, backscattered light, and transmitted light of the optical fiber 8 to be
measured. In addition, incoming light power at the maximum second-order differential
equation of the backscattered light with respect to the incoming light becomes an SBS
threshold.
[0017]
In Patent Documents 2 and 3, an SBS threshold is evaluated by using the same
measurement system and definition. Here, the definition of the SBS threshold is
considered as definition 4 in the following document.
Shimizu, "A Study on Definition of the SBS Threshold in a Single-Mode Optical
Fiber", Institute of Electronics, Information and Communication Engineers, General
Conference in 2005, B-10-66.
[0018]
The SBS threshold is dependent on a mode field diameter (hereinafter, referred
to as MFD). A plot result obtained by calculating the MFD dependence of the SBS

threshold of a single-mode optical fiber (hereinafter, referred to as SMF) which has a
typical refractive index profile of step-index model as shown in FIG. 4 and satisfies the
G652 standard is shown in FIG. 3. As shown in FIG. 3, in the range of 7.9 to 10.2 µm of an MFD at a wavelength of 1.31 µm that is in the G652 standard, the SBS threshold of
the SMF changes in the range of 7.4 to 9.7 dBm. Accordingly, when the SBS threshold
is compared, an optical fiber having the same MFD needs to be compared.
[0019]
The optical fiber of the invention has optical characteristics described in G652 in
»
that a cable cutoff wavelength is less than 1260 nm, an MFD at a wavelength of 1.31 µm
is in the range of 7.9 to 10.2 µm, a zero-dispersion wavelength is in the range of 1300 to
1324 nm, a zero-dispersion slope is equal to or less than 0.093 ps/(nm2km), a uniform
bending loss is equal to or less than 2 dB/m at a bending diameter of 20 mm and a
wavelength of 1.31 µm, and an SBS threshold is equal to or higher than two times (+3
dB) that of an SMF having the same MFD.
[0020]
FIG. 5 is a view illustrating a first example of a refractive index profile of the
optical fiber of the invention. The refractive index profile is constituted by: a
three-layer structured core which includes, in the central portion of the core, a first core
having a substantially uniform positive relative refractive index difference Δ1 in a region
from the center of the core to a radius of Rl µm, a second core which surrounds and
comes in contact with the first core and has a substantially uniform positive relative
refractive index difference Δ2 in a region from the radius of Rl µm to a radius of R2 µm,
and a third core which surrounds and comes in contact with the second core and has a
substantially uniform positive relative refractive index difference Δ3 in a region from the

radius of R2 µm to a radius of R3 urn; and a cladding which surrounds and comes in
contact with the three-layer structured core and has a substantially uniform refractive
index, wherein Δ1> Δ2 and Δ3> Δ2.
[0021]
The refractive index profile of the optical fiber of the invention, as illustrated in
FIG. 6, may allow relative refractive index differences not to have uniform values. FIG.
6 is a view illustrating a second example of a refractive index profile of the optical fiber
of the invention. The refractive index profile is constituted by: a three-layer structured
core which includes, in the central portion of the core, a first core having a maximum
relative refractive index difference Δ1 in a region from the center of the core to a radius
of Rl µm, a second core which surrounds and comes in contact with the first core and
has a minimum relative refractive index difference Δ2 in a region from the radius of Rl µm to a radius of R2 (am, and a third core which surrounds and comes in contact with the
second core and has a maximum relative refractive index difference Δ3 in a region from
the radius of R2 urn to a radius of R3 µm; and a cladding which surrounds and comes in
contact with the three-layer structured core and has a substantially uniform refractive
index, wherein Δ1> Δ2 and Δ3> Δ2.
[0022]
For the optical fiber having the refractive index profile constituted by the
three-layer structured core and the one-layer structured cladding for surrounding the core,
in order to obtain an optical fiber which has the above-mentioned optical characteristics,
that is, characteristics compatible with G652 and an SBS threshold equal to or higher
than two times that of (by +3 db) the SMF having the same MFD as the optical fiber of
the invention, detailed examinations were repeated. As a result, it was found that

relationships between Δ1, Δ2, A3, Rl, R2, and R3 were limited.
[0023]
FIG. 7 is a view showing a relationship of a SBS suppression effect SBSeff on
an SMF when Δ1 -A2=X and Δ3-Δ2=Y. Here, SBSeff is defined by the following
expression.
SBSeff=SBS threshold of the optical fiber of the invention-SBS threshold of the
SMF having the same MFD as the optical fiber of the invention
[0024]
As can be seen from FIG. 7, by allowing X+Y to be greater than 0.4%, it is
possible to improve SBSeff to be equal to or higher than +3 dB and increase the SBS
threshold. However, by only using the condition, in some cases, an optical fiber having
the optical characteristics compatible with the G652 cannot be obtained.
Specifically, in order to obtain an optical fiber having a zero-dispersion
wavelength in the range of 1300 to 1324 nm, it is preferable that X and Y have
relationships of X relationships of X and Y satisfying this condition are shown in FIG. 8.
[0025]
In addition, in order to allow the MFD at the wavelength of 1.31 µm to be 7.9
µm, that is, the lower limit of the G652 standard, or greater, while the condition is
satisfied, it is preferable that Δ2 be equal to or less than 0.4%. In addition, in order to
allow the MFD at the wavelength of 1.31 µm to be in the range of 7.9 to 10.2 µm, the
position of the third core in the radial direction represented as R2/R1 needs to be suitably
determined depending on the sum of Δ2 and A3, that is, Δ2+Δ3.
[0026]

In order to obtain an optical fiber for enabling the MFD at the wavelength of
1.31 µm to be in the range of 7.9 to 10.2 µm, it is preferable that (Δ2+Δ3) and R2/R1
satisfy relationships of (Δ2+Δ3)+1.0 relationships between (Δ2+Δ3) and R2/R1 satisfying the condition are shown in FIG. 9.
[0027]
In addition, from the point of manufacturability of an optical fiber, it is
preferable that Δ3> Δ1.
FIG. 10 is a view showing a relationship between A3-A1 and SBSeff. As
shown in FIG. 10, when A3-A1 is negative, SBSeff significantly changes with a slight
change in relative refractive index difference. On the other hand, when A3-A1 is
positive, the change rate of SBSeff with respect to the change in relative refractive index
difference is small. In addition, when SBSeff is approximated to a quadratic equation of
A3-A1, the approximation curve is a parabola which opens down, and the inflection point
is A3- Δ1>0. It can be seen that the effect of SBSeff caused by the change in relative
refractive index difference is small when A3-A1 is positive.
When an optical fiber preform is manufactured, in some cases, due to fluctuation
in dopant concentration, the relative refractive index difference is changed from a target
by about ±0.05%. In this case, there is a possibility that SBSeff is decreased to be lower
than a target. When A3-A1 is positive, the change rate of SBSeff caused by the change
in relative refractive index difference is small. Therefore, SBSeff does not significantly
change with respect to the change in relative refractive index difference caused by the
fluctuation in dopant concentration, and SBS characteristics that are always stable can be
obtained.
In addition, in the refractive index profile of FIG. 10, Δ1 is 0.5%, Δ2 is 0.22%,

A3 is in the range of 0.40 to 0.65% at an interval of 0.025%, and R2/R1 is 2.2.
However, as shown in Table 1, in other combinations of Δ1, Δ2, A3, Rl, R2, and R3, the
inflection point is A3- Δ1>0, and it can be seen that SBS characteristics which are always
stable can be obtained when A3-A1 is positive.
It is preferable that Δ1 -A2 be equal to or greater than 0.25%. FIG. 11 is a view
showing a relationship between Al-A2 and SBSeff. As shown in FIG. 11, when Δ1 is
greater than Δ2 by 0.25% or higher, SBSeff can be significantly increased as compared
with the case where the difference between Δ1 and Δ2 is equal to or smaller than 0.25%,
thereby obtaining a higher SBS suppression effect. In addition, when Δ1 -A2 is equal to
or greater than 0.25%, the effect of the change in relative refractive index difference
caused by fluctuation in dopant concentration during manufacturing of the optical fiber
preform is reduced. Therefore, even when Δ1-Δ2 is changed, SBSeff does not change
significantly, and degradation in yield can be prevented.
In the refractive index profile of FIG 11, Δ1 is in the range of 0.44% to 0.56% at
an interval of 0.03%, Δ2 is 0.24%, Δ3 is 0.55%, and R2/R1 is 2.2. However, as shown
in FIG. 12, in a refractive index profile using other combinations of Δ1, Δ2, A3, Rl, R2,
and R3, that is, when Δ1 is in the range of 0.44% to 0.56%, Δ2 is in the range of 0.18%
to 0.26%, Δ3 is in the range of 0.45% to 0.65%, and R2/R1 is in the range of 1.8 to 2.6,
the same tendency can be obtained. Particularly, in a refractive index profile in which
Δ1-Δ2 is equal to or greater than 0.25%, a higher SBS suppression effect can be obtained,
and dependence of SBSeff on a change in Δ1-Δ2 is reduced.
Table 1

In addition, by disposing the third core as described above, it is possible to
obtain the SBS threshold higher than that of a conventional optical fiber by +3 dB or
higher and obtain characteristics compatible with the G652.
Examples
[0029]
Example 1 and Comparative Example 1
In Table 2, structural parameters and optical characteristics of an optical fiber of
Example 1 which has the refractive index profile of FIG. 5 are shown. In addition,
structural parameters and optical characteristics of an optical fiber of Comparative
Example 1 are shown. The optical fiber of Comparative Example 1 is an SMF having a
step-index profile as illustrated in FIG. 4.
[0030]
Table 2

As shown in Table 2, the optical fiber having the structural parameters of
Example 1 related to the invention had an SBS threshold of 12.2 dBm for a length of 20
km, and could obtain a higher suppression effect than the optical fiber of Comparative
Example 1 which had the same MFD by +3.5 dB. In addition, the optical fiber of
Example 1 had the same optical characteristics as the SMF of Comparative Example 1
and satisfied the G652 standard.
[0032]
Examples 1a to 1g
Table 3 shows results in the case where structural parameters of Example 1 are
represented by using X, Y, and R2/R1 described above.
[0033]

[0034]
The optical fibers having the structural parameters of Examples la to lg shown
in Table 3 had SBS thresholds of 12.4 to 13.3 dBm for a length of 20 km as shown in FIG.
13 and could obtain suppression effects higher than the SMF having the same MFD by
+3.7 to +4,6 dB. In addition, the optical characteristics of the optical fibers of
Examples la to lg all satisfied the G652 standard.
Examples lh to lv
Tables 4 and 5 show results in the case where structural parameters of Example
1 are represented by using X, Y, and R2/R1 described above. The optical fibers having
the structural parameters of Examples la to lg shown in Table 3 and Examples lh to lv
shown in Tables 4 and 5 had SBS thresholds of 10.9 to 13.8 dBm for a length of 20 km as
shown in FIG. 14 and could obtain suppression effects higher than the SMF having the
same MFD by +3.1 to +4.5 dB. In addition, the optical characteristics of the optical
fibers of Examples lh to lv all satisfied the G652 standard.

[0035]
Examples 2a to 2f
Table 6 shows optical characteristics in the case where structural parameters of a
refractive index profile of the optical fiber having the refractive index profile of FIG. 6
are represented by using X, Y, and R2/R1 described above.
[0036]

[0037]
The optical fibers having the structural parameters of Examples 2a to 2f shown
in Table 6 had SBS thresholds of 12.0 to 13.7 dBm for a length of 20 km as shown in FIG.
15 and could obtain suppression effects higher than the SMF having the same MFD by
+3.3 to +5.0 dB. In addition, the optical characteristics of the optical fibers of
Examples 2a to 2f all satisfied the G652 standard.
In addition, by changing the refractive index of the third core as illustrated in
FIG. 6, the amount of dopant GeO2 in the core can be reduced, so that it is possible to
reduce a loss in the optical fiber.
Examples 2g to 2m
Table 7 shows optical characteristics in the case where structural parameters of a
refractive index profile of the optical fiber having the refractive index profile of FIG. 6
are represented by using X, Y, and R2/R1 described above. The optical fibers having
the structural parameters of Examples 2a to 2f shown in Table 6 and Examples 2g to 2m
shown in Table 7 had SBS thresholds of 10.8 to 14.3 dBm for a length of 20 km as shown
in FIG 16 and could obtain suppression effects higher than the SMF having the same
MFD by +3.2 to +4.7 dB. In addition, the optical characteristics of the optical fibers of
Examples 2g to 2m all satisfied the G652 standard.

[0038]
Examples 3 and 4
FIG. 17 shows a refractive index profile of an optical fiber of Example 3 related
to the invention. The refractive index profile in Example 3, as shown in FIG. 17, is
constituted by: a three-layer structured core which includes, in the central portion of the
core, a first core in a region from the center of the core to a radius of Rl µm, a second
core which surrounds and comes in contact with the first core in a region from the radius
of Rl µm to a radius of R2 µm, and a third core which surrounds and comes in contact
with the second core in a region from the radius of R2 µm to a radius of R3 µm; and a
cladding which surrounds and comes in contact with the three-layer structured core.
However, unlike in Examples 1 and 2, the refractive index profile of the core smoothly
changes, and the boundary thereof is obscure. Therefore, by using the change rate
(dA/dr) of the relative refractive index difference in the radial direction, the diameter of
each layer was determined. The relative refractive index difference Δ1 of the first core,
as represented by the following equation (1), is defined as A which becomes equivalently
uniform in the region from the center of the core to the radius Rl, the relative refractive
index difference Δ2 of the second core is defined as a relative refractive index difference
that becomes a minimum value in the region between the radii Rl and R2 µm, and the
relative refractive index difference Δ3 of the third core is defined as a relative refractive
index difference that becomes a maximum value in the region between the radii R2 and
R3 µm.
[0039]
Equation 1

[0040]
The structural parameters of the optical fiber of Example 3 defined as described
above and the optical characteristics thereof are shown in Table 8. In addition, in Table
8, the structural parameters of the optical fiber of Example 4 having the same refractive
index profile as Example 3 and the optical characteristics thereof are shown.
[0041]

[0042]
As shown in Table 8, the optical fibers of Examples 3 and 4 had SBS thresholds
of 12.2 to 12.7 dBm for a length of 20 km, and could obtain higher suppression effects
than the SMF having the same MFD by +3.5 to +4.0 dB. In addition, the optical fibers
of Examples 3 and 4 all satisfied the G652 standard.
[0043]
Example 5
FIG. 18 is a refractive index profile of an optical fiber preform of Example 5.
The optical fiber preform in this example, as shown in FIG. 18, is constituted by: a
three-layer structured core which includes, in the central portion of the core, a first core
having a substantially uniform positive relative refractive index difference Δ1 in a region
from the center of the core to a radius of Rl µm, a second core which surrounds and
comes in contact with the first core and has a substantially uniform positive relative
refractive index difference Δ2 in a region from the radius of Rl µm to a radius of R2 µm,
and a third core which surrounds and comes in contact with the second core and has a
substantially uniform positive relative refractive index difference Δ3 in a region from the
radius of R2 µm to a radius of R3µm. Like Examples 1 and 2, the three-layer
structured core is included.
Structural parameters of the optical fiber preform of Example 5 are represented
in Tables 9 and 10 by using X, Y, and R2/R1 described above, and optical characteristics
that were exhibited when the preform was drawn into an optical fiber are shown. The
optical fiber drawn from the optical fiber preform of Example 5 had an SBS threshold of
10.9 to 13.8 dBm for a length of 20 km as shown in FIG. 19, and obtained a suppression
effect higher than the SMF having the same MFD by +3.1 to +4.5 dB, so that the G652

standard was further satisfied.

[0044]
Example 6
FIG. 20 shows a refractive index profile of an optical fiber preform of Example 6.
The optical fiber preform in this example, as shown in FIG. 20, is constituted by: a
three-layer structured core which includes, in the central portion of the core, a first core
having a maximum relative refractive index difference Δ1 in a region from the center of
the core to a radius of Rl µm, a second core which surrounds and comes in contact with
the first core and has a minimum relative refractive index difference Δ2 in a region from
the radius of Rl µm to a radius of R2 µm, and a third core which surrounds and comes in
contact with the second core and has a maximum relative refractive index difference Δ3 in a region from the radius of R2 urn to a radius of R3 jam. Like Examples 1 and 2, the
three-layer structured core is included.
Structural parameters of the optical fiber preform of Example 6 are represented
in Tables 11 and 12 by using X, Y, and R2/R1 described above, and optical characteristics
that were exhibited when the preform was drawn into an optical fiber are shown. The
optical fiber drawn from the optical fiber preform of Example 6 had an SBS threshold of
10.8 to 14.3 dBm for a length of 20 km as shown in FIG. 21, and obtained a suppression
effect higher than the SMF having the same MFD by +3.2 to +4.7 dB, so that the G652
standard was further satisfied.

[0045]
Example 7
FIG. 22 shows a refractive index profile of an optical fiber preform of Example 7.
The optical fiber preform in this example, as shown in FIG. 22, is constituted by: a
three-layer structured core which includes, a first core disposed in a region from the
center of the core to a radius of Rl, a second core which surrounds and comes in contact
with the first core and is disposed in a region from the radius of Rl µm to a radius of R2 µm, and a third core which surrounds and comes in contact with the second core and is
disposed in a region from the radius of R2 µm to a radius of R3 um. Like Examples 1,
2, 5, and 6, the three-layer structured core is included. However, unlike in Examples 1,
2, 5, and 6, the refractive index profile smoothly changes, and the definition of the
boundary thereof is the same as in Examples 3 and 4.
Structural parameters of the optical fiber preform of Example 7 are represented
in Table 13, and optical characteristics that were exhibit when the preform was drawn
into an optical fiber are shown. The optical fiber drawn from the optical fiber preform
of Example 7 had an SBS threshold of 12.6 dBm for a length of 20 km and obtained a
suppression effect higher than the SMF having the same MFD by +3.8 dB, and the G652
standard was satisfied.

Industrial Applicability
[0046]
According to the invention, for the optical fiber having a segment-core type
refractive index profile, the relationships between the relative refractive index differences
Δ1, Δ2, and Δ3 of the layers are suitably designed, and the position of the third core is
suitably determined. Therefore, it is possible to increase an SBS threshold by +3 dB or
higher as compared with the SMF having the same MFD while maintaining the optical
characteristics described in G652.
In addition, by allowing the relative refractive index difference of the third core
to be greater than that of the first core, it is possible to improve manufacturability of the
optical fiber preform.

CLAIMS
1. An optical fiber having a refractive index profile constituted by:
a three-layer structured core which includes, in the central portion of the core, a
first core having a substantially uniform positive relative refractive index difference Δ1
in a region from the center of the core to a radius of Rl µm, a second core which
surrounds and comes in contact with the first core and has a substantially uniform
positive relative refractive index difference Δ2 in a region from the radius of Rl urn to a
radius of R2 µm, and a third core which surrounds and comes in contact with the second
core and has a substantially uniform positive relative refractive index difference Δ3 in a
region from the radius of R2 µm to a radius of R3 µm; and
a cladding which surrounds and comes in contact with the three-layer structured
core and has a substantially uniform refractive index,
wherein
A2 is equal to or less than 0.4%,
Δ1, Δ2, and Δ3 have relationships of Δ1> Δ2, Δ3> Δ2, and Δ3> Δ1,
when Δ1, Δ2, and Δ3 have relationships of Δ1-Δ2=X and Δ3-Δ2=Y,
(X+Y)>0.4% is satisfied, and X and Y satisfy 0.25% relationship of (2*X-0.7)% Δ2, A3, Rl, and R2 satisfy relationships of
(Δ2+Δ3)+1.0 a cable cutoff wavelength is less than 1260 nm,
a mode field diameter is in the range of 7.9 to 10.2 µm at a wavelength of 1.31 µm,
a zero-dispersion wavelength is in the range of 1300 to 1324 nm,

a zero-dispersion slope is equal to or less than 0.093 ps/(nm2.km),
a uniform bending loss is equal to or less than 2 dB/m at a diameter of 20 mm
and a wavelength of 1.31 µm, and
an SBS threshold at a wavelength of 1.55 µm is higher than that of a
single-mode optical fiber having a typical step-index profile and the same mode field
diameter by +3 dB or higher.
2. An optical fiber having a refractive index profile constituted by:
a three-layer structured core which includes, in the central portion of the core, a
first core having a maximum relative refractive index difference Δ1 in a region from the
center of the core to a radius of Rl µm, a second core which surrounds and comes in
contact with the first core and has a minimum relative refractive index difference Δ2 in a
region from the radius of Rl µm to a radius of R2 µm, and a third core which surrounds
and comes in contact with the second core and has a maximum relative refractive index
difference Δ3 in a region from the radius of R2 µm to a radius of R3 urn; and
a cladding which surrounds and comes in contact with the three-layer structured
core and has a substantially uniform refractive index,
wherein
A2 is equal to or less than 0.4%,
Δ1, Δ2, and Δ3 have relationships of Δ1> Δ2, Δ3> Δ2, and Δ3> Δ1,
when Δ1, Δ2, and Δ3 have relationships of Δ1-Δ2=X and Δ3-Δ2==Y,
(X+Y)>0.4% is satisfied,
X and Y satisfy 0.25% (2*X-0.7)%
Δ2, A3, Rl, and R2 satisfy relationships of
(Δ2+Δ3)+1.0 a cable cutoff wavelength is less than 1260 nm,
a mode field diameter is in the range of 7.9 to 10.2 µm at a wavelength of 1.31
µm,
a zero-dispersion wavelength is in the range of 1300 to 1324 nm,
a zero-dispersion slope is equal to or less than 0.093 ps/(nm -km),
a uniform bending loss is equal to or less than 2 dB/m at a diameter of 20 mm
and a wavelength of 1.31 µm, and
an SBS threshold at a wavelength of 1.55 µm is higher than that of a
single-mode optical fiber having a typical step-index profile and the same mode field
diameter by +3 dB or higher.
3. An optical fiber preform having a refractive index profile constituted by:
a three-layer structured core which includes, in the central portion of the core, a
first core having a substantially uniform positive relative refractive index difference Δ1 in a region from the center of the core to a radius of Rl µm, a second core which
surrounds and comes in contact with the first core and has a substantially uniform
positive relative refractive index difference Δ2 in a region from the radius of Rl µm to a
radius of R2 µm, and a third core which surrounds and comes in contact with the second
core and has a substantially uniform positive relative refractive index difference Δ3 in a
region from the radius of R2 µm to a radius of R3 µm; and
a cladding which surrounds and comes in contact with the three-layer structured
core and has a substantially uniform refractive index,

wherein Δ2 is equal to or less than 0.4%,
Δ1, Δ2, and Δ3 have relationships of Δ1> Δ2, Δ3> Δ2, and Δ3> Δ1,
when Δ1, Δ2, and Δ3 have relationships of Δ1-Δ2=X and Δ3-Δ2=Y,
(X+Y)>0.4% is satisfied, and X and Y satisfy 0.25% relationship of (2*X-0.7)% Δ2, A3, Rl, and R2 satisfy relationships of
(Δ2+Δ3)+1.0 when the optical fiber preform is drawn, into an optical fiber, a cable cutoff
wavelength is less than 1260 nm, a mode field diameter is in the range of 7.9 to 10.2 µm at a wavelength of 1.31 (am, a zero-dispersion wavelength is in the range of 1300 to 1324
nm, a zero-dispersion slope is equal to or less than 0.093 ps/(nm2.km), a uniform bending
loss is equal to or less than 2 dB/m at a diameter of 20 mm and a wavelength of 1.31 µm,
and an SBS threshold at a wavelength of 1.55 µm is higher than that of a single-mode
optical fiber having a typical step-index profile and the same mode field diameter by +3
dB or higher.
4. An optical fiber preform having a refractive index profile constituted by:
a three-layer structured core which includes, in the central portion of the core, a
first core having a maximum relative refractive index difference Δ1 in a region from the
center of the core to a radius of Rl µm, a second core which surrounds and comes in
contact with the first core and has a minimum relative refractive index difference Δ2 in a
region from the radius of Rl µm to a radius of R2 µm, and a third core which surrounds
and comes in contact with the second core and has a maximum relative refractive index
difference Δ3 in a region from the radius of R2 µm to a radius of R3 µm; and

a cladding which surrounds and comes in contact with the three-layer structured
core and has a substantially uniform refractive index,
wherein Δ2 is equal to or less than 0.4%,
Δ1, Δ2, and Δ3 have relationships of Δ1> Δ2, Δ3> Δ2, and Δ3> Δ1,
when Δ1, Δ2, and Δ3 have relationships of Δ1-Δ2=X and Δ3-Δ2=Y,
(X+Y)>0.4% is satisfied, and X and Y satisfy 0.25% relationship of (2*X-0.7)% Δ2, A3, Rl, and R2 satisfy relationships of
(Δ2+Δ3)+l.0 when the optical fiber preform is drawn into an optical fiber, a cable cutoff
wavelength is less than 1260 nm, a mode field diameter is in the range of 7.9 to 10.2 µm at a wavelength of 1.31 µm, a zero-dispersion wavelength is in the range of 1300 to 1324
nm, a zero-dispersion slope is equal to or less than 0.093 ps/(nm2-km), a uniform bending
loss is equal to or less than 2 dB/m at a diameter of 20 mm and a wavelength of 1.31 µm,
and an SBS threshold at a wavelength of 1.55 µm is higher than that of a single-mode
optical fiber having a typical step-index profile and the same mode field diameter by +3
dB or higher.

An optical fiber is constituted by: a three-layer structured core which includes, a
first core (having a relative refractive index difference of Δ1 in a region of a radius of
Rl), a second core (having a relative refractive index difference of Δ2 in a region from
the radius Rl to a radius of R2), and a third core (having a relative refractive index
difference of Δ3 in a region from the radius of R2 to a radius of R3), wherein the relative
refractive index differences have relationships of Δ1> Δ2, Δ3> Δ2, and Δ3> Δ1, when
Δ1-Δ2=X and Δ3-Δ2=Y, (X+Y)>0.4% is satisfied, and X and Y satisfy 0.25% 0.1% G652 standard and having an SBS threshold equal to or higher than that of an optical
fiber having the same MFD by +3 dB or higher.

Documents:

http://ipindiaonline.gov.in/patentsearch/GrantedSearch/viewdoc.aspx?id=vKdCCXMgF0fhzE5RpB/4hQ==&loc=wDBSZCsAt7zoiVrqcFJsRw==

« Previous Patent

Next Patent »

Patent Number

278322

Indian Patent Application Number

1056/KOLNP/2009

PG Journal Number

53/2016

Publication Date

23-Dec-2016

Grant Date

20-Dec-2016

Date of Filing

19-Mar-2009

Name of Patentee

FUJIKURA LTD.

Applicant Address

5-1, KIBA 1-CHOME, KOHTOH-KU, TOKYO 1338512

Inventors:

#	Inventor's Name	Inventor's Address
1	YOSHIDA, TAKESHI	C/O FUJIKURA LTD., SAKURA WORKS, 1440, MUTSUZAKI, SAKURA-SHI, CHIBA 2858550
2	NUNOME, TOMOHIRO	C/O FUJIKURA LTD., SAKURA WORKS, 1440, MUTSUZAKI, SAKURA-SHI, CHIBA 2858550

PCT International Classification Number

G02B 6/036

PCT International Application Number

PCT/JP2007/067830

PCT International Filing date

2007-09-13

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1	2006-249360	2006-09-14	Japan