Title of Invention

"OPTICAL WAVELENGTH FILTER"

Abstract

A low-loss, high-efficiency optical demultiplexer is provided. The optical demultiplexer has a plurality of first optical devices and a plurality of second optical devices serially connected among the first optical devices. Each first optical device has first through third ports, receives an input optical signal via the first port, directs the optical signal to the second port, and directs an optical signal returning via the second port only to the third port. Each second optical device has fourth and fifth ports, is connected between the first port of a corresponding first optical device and the second port of another corresponding first optical device, reflects only an optical signal having a corresponding wavelength component in the optical signal received via the fourth port, and passes an optical signal having the other wavelength components to the fifth port. In the second optical devices, different wavelength components of an optical signal received via the fourth port are reflected. In addition, optical signals having respective wavelength components reflected from the second optical devices are received in the first optical device via the second port thereof, and output from the first optical device via the third port thereof. In the present invention, power loss of a divided optical signal is far less than in a conventional Ix n coupler method.

Full Text

BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to an optical wavelength filter for a receiver of a wavelength division multiplexing (WDM) transmission system, and more particularly, to an optical wavelength filter and an optical demultiplexer for a low-loss, high-efficiency WDM transmission system. Description of the Related Art A WDM transmission system multiplexes the wavelength area of an optical fiber into several channels by simultaneously transmitting signals of several wavelength bands, relying on wavelength characteristics of an optical signal. In the WDM transmission system, an input optical signal, having been multiplexed to have several wavelength components, is demultiplexed at the receiver and recognized in the respective channels. The optical demultiplexer for the receiver of the conventional WDM transmission system includes a Ixn coupler and first through nth band pass filters. Here, n represents the number of channels of a transmitted optical signal. A coupler is a passive device for branching or coupling optical signals, that is, for branching an input channel into several output channels or coupling several input channels into an output channel. The Ix n coupler branches an input optical signal, produced by multiplexing optical signals having many wavelength components, for example, λ1 ,λ2,..., λn, into n branch optical signals Pout(λ1), λ2,..., λn), and outputs them via n respective ports. Here, the power of each branch optical signal Pout(λ1, λ2 ,...,λn) is (l/n)th that of the input optical signal of the Ix n coupler. The first through nth band pass filters receive the branch optical signals Pout(λ1, λ2,..., λn) from the n ports, pass only their corresponding wavelength components, and output optical signals Pout( λ1), Pout( λ2), ..., Pout (λn) of n channels having wavelength components λ1- λn, respectively. Hence, the power of each of the n optical signals Pout( λ1), Pout( λ2), ..., Pout (λn) is (l/n)th that of the input optical signal Here, λ1- λn, and P0 represent the wavelength components and the power value of the input optical signal Pin(λ1, λ2,..., λn), respectively. The branch optical signal Pout(λ1, λ2,..., λn) has (1/n)th the power of the input optical signal Pin(λ1, λ2,..., λn), while keeping the wavelength components of the input optical signal Pin(λ1, λ2,..., λn). Here, the vertical axis on the graphs indicates the powers P of the optical signals, and the horizontal axis indicates the wavelengths λ, of the optical signals. P0 denotes the power value of the input optical signal Pin(λ1, λ2,..., λn), and λ1- λn denote the wavelength components multiplexed in the input optical signal Pin(λ1, λ2,..., λn). The power of the branch optical signal Pout(λ1, λ2,..., λn) output from the Ix n coupler is (l/n)th that of the input optical signal Pin(λ1, λ2,..., λn), that is, P0/n. Thus, each of the optical signals Pout(λ1), Pout(λ2),..., Pout(λn) having their respective wavelength components, which are output from the first through nth band pass filters, also have (l/n)th the power of the input optical signal Pin(λ1, λ2,..., λn), that is, P0/n. In the conventional WDM transmission system, use of the Ix n coupler for demultiplexing a multiplexed optical signal at a receiver offers only (l/n)th the power of an input optical signal of the Ix n coupler. In order to make up for the power loss caused by this Ix n coupler, the optical demultiplexer for the receiver in the conventional WDM transmission system further includes an optical amplifier for amplifying an optical signal to increase the power by n times before it is input to the Ix n coupler. The optical demultiplexer has an optical amplifier, a Ix n coupler, and first through nth band pass filters. Here, n denotes the number of channels of a transmitted optical signal. An optical signal P1(λ1, λ2,..., λn) received in the optical amplifier is produced by multiplexing optical signals of many wavelength components, for example, λ1, λ2,..., λn. The optical amplifier amplifies the input optical signal PI(λ1, λ2,..., λn) by two or more times the number of wavelength components included in the input optical signal P1(λ1, , λ2,..., λn), and outputs an amplified input optical signal P2(λ1, λ2,..., λn). The Ix n coupler receives the amplified input optical signal P2(λ1, λ2,..., λn), branches the amplified signal, and outputs n branch input optical signals P3(λ1, λ2,..., λn). Here, the n branch input optical signals each have (l/n)th the power of the amplified input optical signal P2(λ1, λ2,..., λn), that is, a power value as great as or greater than the output of the input optical signal P1(λ1, λ2,..., λn), while keeping the wavelength components included in the input optical signal P1(λ1, λ2,..., λn). The first through nth band pass filters separate optical signals P4(λ1), P4(λ2),..., P4( λn) of their corresponding wavelength components from the branch input optical signals P3(λ1, λ2,..., λn). Here, the powers of the optical signals P4(λ1), P4(λ2), P4(λn) each are larger than that of the input optical signal P1(λ1, λ2,..., λn). SUMMARY OF THE INVENTION It is an object of the present invention to provide an optical wavelength filter for reflecting an optical signal having a predetermined wavelength component without power loss. It is another object of the present invention to provide an optical demultiplexer for a receiver in an optical WDM transmission system, which allows little power loss. Accordingly, to achieve the above first object, there is provided an optical wavelength filter. The optical wavelength filter has a first optical device and a second optical device. The first optical device includes first, second, and third ports, receives an input optical signal having a plurality of wavelength components, directs the input optical signal to the second port, and directs an optical signal returning from the second port only to the third port. The second optical device includes fourth and fifth ports. The fourth port is connected to the second port of the first optical device. The second optical device reflects only an optical signal having a predetermined wavelength component in the optical signal received via the fourth port and passes an optical signal having the other wavelength components to the fifth port. To achieve the second object, there is provided an optical demultiplexer. The optical demultiplexer has a plurality of first optical devices and a plurality of second optical devices. The plurality of first optical devices each have first through third ports, receive an input optical signal via the first port, directs the input optical signal to the second port, and directs an optical signal returning from the second port only to the third port. The plurality of second optical devices each have fourth and fifth ports which are connected between the second port of a corresponding first optical device and the first port of another corresponding first optical device, reflect only an optical signal having a predetermined wavelength component in an optical signal received via the fourth port backward from a light traveling direction, output the reflected optical signal to the corresponding first optical device via the second port thereof, and pass an optical signal having the other wavelength components to the other first optical device connected to the fifth port via the first port thereof. STATEMENT OF THE INVENTION According to the present invention there is provided an optical wavelength filter comprising: a first optical device (750) having a first port (751), a second.port (752) and a third port (753), for directing a first input optical signal having a plurality of wavelengths received through the first port (751) to the second port (752) and directing a second input optical signal received' through the second port (752) to the third port (753); and a second optical device (850) having a fourth port (854) connected to the second port (752) and a fifth port (855), for receiving the plurality of wavelengths through the fourth port (854), reflecting an optical signal having a predetermined wavelength among the plurality of wavelengths to the fourth port (854) and passing other wavelengths to the fifth port (855)/ wherein the predetermined wavelength among the plurality of wavelengths input to the optical wavelength filter is output from the optical wavelength filter. BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS The above objects and advantages of the present invention will become more apparent by describing in detail a preferred embodiment thereof with reference to the attached drawing(s) in which: Fig. 1 is a block diagram of an optical demultiplexer for a receiver of a conventional wavelength division multiplexing (WDM) transmission system; FIG. 2 is a waveform diagram illustrating the power of an optical signal received in the Ixn coupler shown in FIG. 1; FIG. 3 is a waveform diagram illustrating the power of an optical signal received in each of the first through nth band pass filters from the Ix n coupler shown in FIG. 1; FIG. 4A is a waveform diagram illustrating the power of an optical signal output from the first band pass filter shown in FIG. 1; FIG. 4B is a waveform diagram illustrating the power of an optical signal output from the second band pass filter shown in FIG. 1; FIG. 4C is a waveform diagram illustrating the power of an optical signal output from the nth band pass filter shown in FIG. 1; FIG. 5 is a block diagram of an optical demultiplexer for compensating for power loss caused by a 1 x n coupler in a receiver of the conventional WDM transmission system; FIG. 6 is a block diagram of an optical wavelength filter according to an embodiment of the present invention; FIG. 7 is a block diagram of an optical demultiplexer according to another embodiment of the present invention; and FIG. 8 is a block diagram of the first optical wavelength filter shown in FIG. 7. DESCRIPTION OF THE PREFERRED EMBODIMENTS In figure 1, the optical demultiplexer for the receiver of the conventional WDM transmission system includes a Ixn coupler 100 and first through nth band pass filters 200-300. Here, n represents the number of channels of a transmitted optical signal. A coupler is a passive device for branching or coupling optical signals, that is, for branching an input channel into several output channels or coupling several input channels into an output channel. The 1x n coupler 100 branches an input optical signal, produced by multiplexing optical signals having many wavelength components, for example, λ1, λ2,..., λn, into n branch optical signals Pout(λ1, λ2,..., λn), and outputs them via n respective ports. Here, the power of each branch optical signal Pout(λ1, λ2,..., λn) is 0/n)th that of the input optical signal of the Ix n coupler 100. The first through nth band pass filters 200-300 receive the branch optical signals Pout(λ1, λ2,..., λn) from the n ports, pass only their corresponding wavelength components, and output optical signals Pout (λ1), Pout( λ2),..., Pout( λn) of n channels having wavelength components λ1- λn, respectively. Hence, the power of each of the n optical signals Pout( λ1), P0ut( λ2),..,Pout( λn) is (l/n)th that of the input optical signal Pin(λ1, λ2,..., λn). FIG. 2 is a waveform diagram illustrating the power of the input optical signal Pin(λ1, λ2,..., λn) of the Ix n coupler shown in FIG. 1. Here, λ1- λn and P0 represent the wavelength components and the power value of the input optical signal Pin((λ1, λ2,..., λn), respectively. FIG. 3 is a waveform diagram illustrating the power of the branch optical signal Pout(λ1, λ2,..., λn) output from the Ix n coupler shown in FIG. 1 to each band pass filter. In figure 3, the branch optical signal POUT(λ1, λ2,..., λn) has (l/n)th the power of the input optical signal Pin(λ1, λ2,..., λn), while keeping the wavelength components of the input optical signal Pin((λ1, λ2,..., λn). FIGs. 4A-4C are waveform diagrams illustrating the powers of optical signals P0ut(λ1), Pou(λ2) and Pout(λn) output from the first, second, and nth band pass filters shown in FIG. 1. Here, the vertical axis on the graphs indicates the powers P of the optical signals, and the horizontal axis indicates the wavelengths λ, of the optical signals. P0 denotes the power value of the input optical signal Pin(λ1, λ2,..., λn), and λ1-λn denote the wavelength components multiplexed in the input optical signal Pin(λ1, λ2,..., λn). As shown in figure 3, the power of the branch optical signal Pout(λ1, λ2,..., λn) output from the Ix n coupler is (l/n)th that of the input optical signal Pin(λ1, λ2,..., λn), that is, P0/n. Thus, each of the optical signals Pout( λ1), P0ut( λ2),... P0ut(λn) having their respective wavelength components, which are output from the first through nth band pass filters 200-300, also have (l/n)th the power of the input optical signal Pin(1, λ2,..., λn), that is, P0/n. In the conventional WDM transmission system, use of the Ix n coupler for demultiplexing a multiplexed optical signal at a receiver offers only (l/n)th the power of an input optical signal of the Ix n coupler. In order to make up for the power loss caused by this Ix n coupler, the optical demultiplexer for the receiver in the conventional WDM transmission system further includes an optical amplifier for amplifying an optical signal to increase the power by n times before it is input to the Ix n coupler. FIG. 5 is a block diagram of the optical demultiplexer further including an optical amplifier, for the receiver in the conventional WDM transmission system. Referring to figure 5, the optical demultiplexer has an optical amplifier 400, a Ix n coupler 100, and first through nth band pass filters 200-300. Here, n denotes the number of channels of a transmitted optical signal. An optical signal P1(λ1, λ2,..., λn) received in the optical amplifier 400 is produced by multiplexing optical signals of many wavelength components, for example, λb λ2,..., λn. The optical amplifier 400 amplifies the input optical signal P1, (λ2,..., λn) by two or more times the number of wavelength components included in the input optical signal P1(λ1, λ2,..., λn), and outputs an amplified input optical signal P2(λ1, λ2,..., λn). The 1x n coupler receives the amplified input optical signal P2(λl5 λ2,..., λn), branches the amplified signal, and outputs n branch input optical signals P3(λ1, λ2,..., λn). Here, the n branch input optical signals each have (l/n)th the power of the amplified input optical signal P2(λ1, λ2,..., λn), that is, a power value as great as or greater than the output of the input optical signal P1(λ1, λ2,..., λn), while keeping the wavelength components included in the input optical signal P1(λ1, λ2,..., λn). The first through nth band pass filters 200-300 separate optical signals P4(λ1), P4(λ2),..., P4(λn) of their corresponding wavelength components from the branch input optical signals P3(λ1, λ2,..., λn). Here, the powers of the optical signals P4(λ1), P4(λ2),..., P4(λn) each are larger than that of the input optical signal P1(λ1, λ2,..., λn) Referring to FIG. 6, the optical wavelength filter according to the embodiment of the present invention has a circulator 750 and a fiber grating reflection filter 850. The circulator 750 has first through third ports 751-753. The first port 751 receives an optical signal having a plurality of wavelength components, for example, (λl, λ2,...,λn) and directs the received optical signal to the second port 752. The circulator 750 directs an optical signal having only a wavelength component, for example, λ1 which is reflected back from the fiber grating reflection filter 850 to the second port 752, only to the third port 753. The fiber grating reflection filter 850 can reflect only an optical signal having a predetermined wavelength component backward from a signal traveling direction by periodically varying the refractive index of a fiber sensitive to ultraviolet rays, that is, relying on Bragg conditions by varying the refractive index of the fiber through irradiation of ultraviolet rays onto the fiber sensitive to the ultraviolet rays. The fiber grating reflection filter 850 includes fourth and fifth ports 854 and 855. The fourth port 854 is connected to the second port 752. The fiber grating reflection filter 850 reflects only the optical signal having the predetermined wavelength component lj among the wavelength components, for example, λ1, λ2, ..., λn, of the optical signal received from the fourth port 854 backward from the light traveling direction to the second port 752 of the circulator 750, and outputs an optical signal having the other wavelength components λ2,..., λn to the fifth port 855. The optical wavelength filter according to the embodiment of the present invention is provided with the circulator 750 and the fiber grating reflection filter 850, thus separating only an optical signal having a predetermined wavelength component from an optical signal having a plurality of multiplexed wavelength components. Therefore, application of the optical wavelength filter to a WDM transmission system obviates the need for an optical amplifier used to compensate for division-incurring power loss of an optical signal in a receiver. FIG. 7 is a block diagram of an optical demultiplexer according to an embodiment of the present invention to achieve another object of the present invention. Referring to FIG. 7, the optical multiplexer has first through nth optical wavelength filters 700-900 which are serially connected. The first through nth optical wavelength filters 700, 800 and 900 have input ports 701, 801 and 901, first output ports 702, 802 and 902, and second output ports 703, 803 and 903. The first optical wavelength filter 700 receives an input optical signal having a plurality of wavelength components, for example, λb λ2,..., λn via the input port 701, and outputs only an optical signal having a wavelength component, for example, λ1, among the wavelength components, for example, λl5 λ2,..., λn of the input optical signal, via the first output port 702. The first optical wavelength filter 700 outputs an optical signal having the other non-λ1 wavelength components, that is, λ2,..., λn received from the first output port 702, via the second port 703. The second optical wavelength filter 800 receives the optical signal having the other non-λ1 wavelength components λ2,..., λn via the input port 801 from the second output port 703. Similarly, the second optical wavelength filter 800 outputs an optical signal having a wavelength component, for example, λ2 among the wavelength components λ2,..., λn via the first output port 802, and an optical signal having the other wavelength components 13,..., λn via the second output port 803. Through this procedure, the (n-l)th optical wavelength filter 900 receives an optical signal having wavelength components λn-1 and λn via the input port 901, and outputs an optical signal having a wavelength component, for example, λn-1 via the first output port 902 and an optical signal having the other wavelength component λn via the second output port 903. FIG. 8 is a block diagram of the first optical wavelength filter 700 shown in FIG. 7. The optical wavelength filter 700 includes a circulator 770 and a fiber grating reflection filter 870. The circulator 770 has first, second, and third ports 771, 772, and 773. The first port 771 is connected to the input port 701 of the optical wavelength filter 700. The circulator 770 receives an optical signal having a plurality of wavelength components, for example, λb λ2,..., λn via the first port 771 and outputs the optical signal to the fiber grating reflection filter 870 via the second port 772. The circulator 770 directs an optical signal having only one wavelength component, for example, lb reflected back from the fiber grating reflection filter 870 via the second port 772, only to the third port 773. The fiber grating reflection filter 870 can reflect only an optical signal having a predetermined wavelength component backward from a signal traveling direction by periodically varying the refractive index of a fiber sensitive to ultraviolet rays, that is, relying on Bragg conditions by varying the refractive index of the fiber through irradiation of ultraviolet rays onto the fiber sensitive to the ultraviolet rays. The fiber grating reflection filter 870 has fourth and fifth ports 874 and 875, and the second port 772 of the circulator 770 is connected to the fourth port 874. The fiber grating reflection filter 870 receives the optical signal having the wavelength components λ1, λ2,..., λn via the fourth port 874, reflects only the optical signal having the wavelength component γ1 backward from a light traveling direction to the circulator 770 via the second input port 772, and outputs the optical signal having the other wavelength components γ,2,..., γn via the fifth port 875. The optical signal having the other wavelength components is output from the fifth port 875 to the first optical wavelength filter 700 via the second output port 703. As described above, by constituting an optical demultiplexer of serially connected optical wavelength filters each including a circulator and a fiber grating reflection filter, power loss of an optical demultiplexer, which is caused by an 1x n coupler of an optical demultiplexer in a receiver of the conventional WDM transmission system, can almost be eliminated. Power loss generated when an optical signal of a plurality of wavelength components is divided into optical signals each having a wavelength component in the conventional optical demultiplexer and the optical demultiplexer in the present invention will be described. For example, it is assumed that the number of channels to be transmitted is 10 and the input power of each channel is lOmW. In the conventional optical demultiplexer, 10 optical signals, branched from an Ix n coupler and having their corresponding respective wavelength components by band pass filters, each have ImW, that is, a l/10th of lOmW. However, in the optical demultiplexer, a total loss of 2dB is produced due to insertion loss of the circulator itself while an input lOmW optical signal is received in the circulator, reflected by a fiber grating reflection filter to be an optical signal having a predetermined wavelength component, and output from the circulator. That is, the output power of the optical signal having the predetermined wavelength component divided from the lOmW input optical signal is 6.3mW. Therefore, there is no need for an optical amplifier for compensating for power loss of an optical signal caused by 1 'n coupling in an optical demultiplexer of the conventional WDM transmission system. In the present invention, by constituting an optical demultiplexer of a plurality of serially connected optical wavelength filters each having a circulator and a fiber grating reflection filter, power loss of an optical signal caused by 1 'n coupling in an optical demultiplexer of a receiver in the conventional WDM transmission system cannot be produced. Further, the optical demultiplexer of the present invention is useful to a high-density WDM transmission system for increasing transmission capacity since there is no limit to the number of divided wavelengths. While the present invention has been illustrated and described with reference to specific embodiments, further modifications and alterations will occur to those skilled in the art within the spirit and scope of this invention. We Claim: 1. An optical wavelength filter comprising: a first optical device having first, second, and third ports, for receiving an input optical sig'nal having a plurality of wavelength components, directing the input optical signal to the second port, and directing an optical signal returning from the second port to the third port; and a second optical device having a fourth port connected to the second port of the first optical device and a fifth port, for reflecting only an optical signal having a predetermined wavelength component in the optical signal received via the fourth port and passing an optical signal having the other wavelength components. 2. The optical wavelength filter as claimed in claim 1, wherein the second optical device is a fiber Bragg grating reflection filter for making a refractive index difference with a grating period having regular intervals using light interference, and reflecting only a predetermined wavelength backward from a light traveling direction. 3. The optical wavelength filter as claimed in claim 2, wherein the predetermined wavelength can be set according to user demands by controlling the grating period according to Bragg conditions for an intended wavelength. 4. The optical wavelength filter as claimed in claim 1, wherein the first optical device comprises a circulator having an input port, a first output port, a second output port, for directing an input optical signal from the input port to the first output port, and directing an optical signal received via the first output port only to the second output port. 5. The optical wavelength filter as claimed in claim in 4, wherein the total power loss produced while the optical signal is received in the circulator via the input port, directed to the output port, reflected as an optical signal having a predetermined wavelength component back via the first output port, and output from the circulator via the second output port, is 2dB due to insertion loss in the circulator. 6. . An optical demultiplexer comprising: a plurality of first optical devices each having first through third ports, for receiving an input optical signal via the first port, directing the input optical signal to the second port, and directing an optical signal returning from the second port to the third port; and a plurality of second optical devices each having fourth and fifth ports which are connected between the second port of a corresponding first optical device and the first port of another corresponding first optical device, for reflecting only an optical signal having a predetermined wavelength component in an optical signal received via the fourth port backward from a light traveling direction, outputting the reflected optical signal to the corresponding first optical device via the second port thereof, and passing an optical signal having the other wavelength components to the other first optical device connected to the fifth port via the first port thereof. 7. The optical demultiplexer as claimed in claim 6, wherein the second optical devices comprise fiber Bragg grating reflection filters for making a refractive index difference with grating periods having regular intervals using light interference, and reflecting only a predetermined wavelength backward from a light traveling direction. 8. The optical demultiplexer as claimed in claim 7, wherein the predetermined wavelength can be set according to user demands by controlling the grating period according to Bragg conditions for an intended wavelength. 9. The optical demultiplexer as claimed in claim 7, wherein the plurality of second optical devices have different grating periods, reflect only optical signals having different wavelength components, and pass optical signals having the other wavelength components, so that an optical signal having a corresponding wavelength component is separated whenever an optical signal having a plurality of wavelength components pass through the plurality of second optical devices. 10. The optical demultiplexer as claimed in claim 6, wherein the plurality of first optical devices each comprise a circulator having an input port, and first and second output ports, for receiving an input optical signal via the input port, directing the input optical signal to the first output port, and directing an optical signal received via the first output port only to the second output port. 11. The optical demultiplexer as claimed in claim 10, wherein the total power loss produced while the optical signal is received in the circulator via the input port, directed to the output port, reflected as an optical signal having a predetermined wavelength component back from a corresponding second optical port, and output from the circulator via the second output port, is 2dB due to insertion loss in the circulator. 12. An optical wavelength substantially as hereinbefore described with reference to the accompanying drawings. 13. An optical demultiplexer substantially as hereinbefore described with reference to the accompanying drawings.

Documents:

3423-del-1997-abstract.pdf

3423-del-1997-claims.pdf

3423-del-1997-correspondence-others.pdf

3423-del-1997-correspondence-po.pdf

3423-del-1997-description (complete).pdf

3423-del-1997-drawings.pdf

3423-del-1997-form-1.pdf

3423-del-1997-form-19.pdf

3423-del-1997-form-2.pdf

3423-del-1997-form-3.pdf

3423-del-1997-form-4.pdf

3423-del-1997-form-6.pdf

3423-del-1997-gpa.pdf

3423-del-1997-petition-137.pdf

3423-del-1997-petition-138.pdf