Title of Invention

"AN OPTICAL AMPLIFIER AND A METHOD OF PREVENTING EMMISION THEREFROM OF OPTICAL POWER EXCEEDING A PRESCRIBED SAFETY"

Abstract

An optical amplifier comprises an active fibre (1), a pump unit (2) spaced from the active fibre and adapted to give a nominal, continuous pump power in an operational state, and a pump fibre (3) adapted to transfer optical pump power from the pump unit (2) to the active fibre (1). Moreover, in a safety state the pump unit (2) is adapted to give a pulsed pump power whose mean power is lower than a prescribed safety limit. A method of preventing emission of optical power exceeding a prescribed safety limit on interruption of an optical fibre (3) which transfers pump power from a pump unit (2) to an active fibre (1), comprises changing the mean power of the pump power in response to a signal received from the active fibre (1) so that the mean power assumes a value below said safety limit if said signal is not received, and assumes a nominal value if said signal is received.

Full Text

The inven tion rela.es to an optical. amplifier comprising an active fibre, a punp unit spaced from the active Tibre and adapted to give a nominal/ continuous pump power in 331 operational s~at€, and a punp fibre adapted to trans¬fer pump power from. the pump unit to the active fibre. The in-ention moreover relates to a methcc. of preventing emission of optical power exceeding a prescribed safety limit upon interruption of an optical fibre which trans-fars pump cower frorr a pump unit to an active fibre. Optical fibre amplifiers for amplifying cptical signals typically consist of a length of active fibre, which may e.g. be an erbium-doped fibre, and a unit for generating pump power, e.g. a pump laser. When the active fibre is pumped with a strong optical signal (the pump signal) having a wavelength range different from that of the sig¬nal to be amplified, and a. concnuni cat ions signal is launched into the amplifier, a signal coherent with the signal on the input will occur on the output cf the ac¬tive fibre. The gain is determined i.a. by the power of the pump signal. The active fibre may be arranged at a considerable dis¬tance (e.g. 10-50 lea) from the pump laser, jn which case "the amplifiers' are referred to as remote-pumped amplifi¬ers. with e.g. remote-pumped preamplifiers, also called FiLP (Remote In-Line Preamplifier), the active fibre is thus spaced from the actual receiver cf the optical sig¬nals, and it is pumped from the receiver. This takes place via an optical fibre, typically, but not necessar- ily, the same fibre as transmits the communications sig¬nals from the active fibre to the receiver. The light cransmitted in such fibres, in the form of com-munications signals or pump power, is typically harmful to the cuman aye. Tnerefcr.e, because cf situations with access tc- fibre ends or non-connected connectors., it is prescribed by various standarc.s how much optical power may he transmitted froir. an open fibre end in thesa situa¬tions. These situations may e.g. occur in case of repair, maintenance anc. testing of systems/ or whan a fibre has broken, or a connector is disassembled. It is tha tempo¬ral mean power c-' Che light that is harmful to ths eye. To achieve the desired functicn of a remote-pumped ampli¬fier, it is necessary to emit levels of pump power in the fibre from the pump laser which significantly exceed the mentioned safety limits. To comply with the safety stan¬dards, it is therefore necessary to reduce the punp power in the event that the fibre transmitting the pump power is interrupted between the punp laser and the active fibre. Further various communications equipment standards pre¬scribe that the equipment must be capable of automati¬cally resuming normal opera-ion when the -ransmission path has been rs-eszablished after a breax and transmis¬sion signals are transmitted again. For remo'ue-pumpjsd am¬plifiers, such as e.g. RILP, this requirement, however, is not easy to satisfy, as the reduced pump power results in a considerable reduction In the gain of the active fibre. Therefore, the comiriuni cat ions signals arriving at the receiver after the re-esteblishment of the transmis¬sion path, will frequently be below the sensitiv_ty limit of the receiver because of the reduced puxrp power. This problem has previously been solved e.g. by using ar, additional fibre from. the receiver to the active fibre. This fibre, in ccnbination with the transmission fibre, is used for passing a control signal from the receiver to tha active fibre and back ~o the receiver. When the 'con¬trol sigr.al is presant, there is no break en the tibre and -consequently no access to the strong optical pump power, and the pump laser can wherefore pump with full power. When, on the other hand, the conr.rol signal is ab-ssnt, this -indica-93 a break on the fibr^, involving the risk that the optical power hits an eye, and the pump .power is therefore reduced to a safe level until the con¬trol signal is-present again. Although this solution is technically adequate, it is vi¬tiated by the serious drawback that it requires an addi¬tional fibre typically of a length of 10-50 km. Moreover, a detector or a coupler capable of returning the control signal to the receiver must necessarily be provided at the active fibre. Systems which are able to reduce the optical output power from a fibre amplifier in case of a broken fibre are also known. These systems do not involve remote-pumped amplifiers and, therefore, they only reduce the power of the communications signals because the pump power never leaves the fibre amplifier itself. Such a system is described in DE 42 22 270 in. which the pump power to the active fibre is reducsd if an alarm signal is received frorr. the receiver in the other end of a transmission fibre, said alarxr. signal .Indicating that the communications signals are not received,, e.g. because of a broken fibre. However, this can only be done if there is an extra fibre or another transmission channel •for transfer of the alarm signal ana, therefore, this system also has the above-sientiened drawoack. Further, the sysitem is r.oz suitable for reducing pump power, unless a special detector unt as above is provided at the active fibre for generaticn of an alarm sigr.al. A similar syster. is known fron US 5 4123 471 in which two parallel fibres are used for transmission in respective directions, when a fihre amplifier in one direction detects an absent input signal a message is sent via the opposite fibre back tc the previous fibre ampiafire to reducc or shut down its optical power level. Therefore, also this system has the above-mentioned drawbacks. The invention provides an optical amplifier of the stated type which, in case of a pump fibre break, is capable of complying with the standards of how much light may be transmitted on the fibre, and which is simultaneously r.a-pable of returning to full pump power when the. fibre con¬nection has been re-established. This may cake piece by using the existing fibre or fibres, which rtieans that no additional fibre is required exclusively for this pur¬pose, and that an additional detector or coupler at the active fibre is obviated. This .i s achieved according to the invention in that, in a safety state, the pump unit is ir.oreover adapted to give a pulsed pump power whose mean power is lover than a pre¬scribed safety limit. Rising of the-pump power ensures that its mean power can te kept so low in. the safety state that the emitted light is unhartiful to the human eye, while the instantaneous power of the pulses, is sufficiently high for the active fibre to respond on reception of these pulses ar.d to in¬form the pump unit - via the.pump fibre or optionally an¬other existing fibre - that the pump fibre is now intact again. When - and if - a pump pulse arrives at the active fibre, the optical power -contained in the pulse will be abscrbec by the active fibre which, in response tc the pulse, simultaneously generates a spontaneous noise called.ASE (Amplified Spontaneous Emission), and this ASE signal may then be returned to tha pump unit. The puir.p ur.it, which generates the required pump power, may be constructed in different ways. In an expedient en-cediment defined in claim 2, a pump laser is used. When/ as stated ir. claim. 5, the pump .unit is adapted to detect-whether an optical signal is it-turned frcir. the ac¬tive fibre in-.response to the pulsed pump power, it is ensured that the puir.p unit can switch between the = opera-lional state and the safety state in dependence on the returned signal. Wnen, as stated in claim 4, the pump unit is adapted to generate the pulsed pump power as pulses repeated with a given frequency, it is ensured that also the returned ASE noise, in the situation where the pulses arrive' at • the active fibre, will have this frequency, a corresponding ASE pulse being returned for each emitted pulse. There¬fore, as stated in claim 5, the pump unit may expediently be adapted to perform the detection .of whether an optical signal is returned from the active fibre in response to the pulsed pump powor, by detecting whether an optical signal with the given pulsation frequency is.received. Then, as stated in claims 6 and 7, the pump unit may be adapted to remain in the .safety state if it is detected that no optical signal is returned from the active fibre in response to the pulsed pump power, and to switch to the operatiorial state if it is detected that such a sig¬nal is returned. As siated in claim a, switching from the safety state to the operational state nay take place via an intermediate state in which the pump unit can give a continuous pump power superimposed by a pulsed signal. This ensures that in. this intermediate state the active fibre nay be given a sufficient punp power for it to operate practically normally and therefore to amplify any communications sig-awlo, whilc enabling it to toe controlled by means nf the pulses whether the connection is still intact unf:il COM.-MANicatloas signals proper are received. Expediently/ as seated in claiir 9, the superimposed pulsed .signal ir. the intermediate state may hava the same shape as the pulsed pump power in the safety state. As a result, the same de-tector circuit may te used in the two states. As stated in claim 10, it will therefore be expedient that optical information signals may moreover be trans¬ferred fron the active fibre to the pump unit, and that the pump unit comprises means for detecting whether such Information signals are received. A particularly expedient embodiment, which is defined in claim 11, is obtained when the said optical information signals are transferred from the active fibre to the pump unit via the pump fibre, as the system then. just needs one fibre capable of serving as a transmission fibre and purip fibre, and moreover capable, in the safety state, of transferring the pulsed pump power and the possible re¬sponse to this. When the pump unit is in the operational .state, it may be adapted TO remain in this state as long as information signals are received, and to switch to the safety state if. no information signals are received, ss stated in claim 12.. When the pump unit is in the safety state, it may be adapted to remain in this slate if r.c returned optical signal in response to the pulsed pun-p power is detected, and to switch to the intermediate state if such a signal i:3 detected, as stated in claim 13. When the pump unit '. s in the intermediale state, it may be adapted to switch to the operational .stats if informa¬tion signals ere received/ to switch to the dwell state i. no returned optical signal in response t:o the puisec pump power is detected, and to remain in the intermediate siate if a. returned optical signal in response to the pulsed pump power it detected and no information signals ate received, • as stated in claim. 14. Finally, as stated in claim. 15, the pump unit may be adapted to inhibit the detection cf whether an optical siqnal ir. response to the pulsed pump pcwer is returned from the active fibre, until, a selected period of time has elapsed after the transmission of each pulse from the pump unit. This ensures -that the detector circuit- ignores the rcflsctiorns that will be returned from. the pump fibre, irrespective of whether this is intact or broken, and instead exclusively detects the A5E noise which can only originate from the active fibre, and which will last considerably .longer than the reflections from -he pump fibre. As mentioned, the invention- also relates to a method of preventing emission of optical power exceeding a pre¬scribed safety Unit on interruption of an optical fibrs which transfers pump power from a pump unit to an active fibre. This method is stated in claim 16. Wher. the mean power of the pump power is changed in response to a sig¬nal received fron the .active fibre such that the mean power assumes a value below said safety limit if said signal is net received, and aosumaes a nominal value if said signal is received, .it is ensured "hat the mean p^wer nay automatically be reduced to a sale level when a break occurs en the optical fibre. As stared ir. claim 17, this may expediently take place in tiat the mean power below said safety .limit is generated by pulsing the pump power with a given frequency, ar:d that, as stated in claim. 19, the signal received from the accive fibre is detected by detecting whether a sig¬nal with the given pulsation frequency is 'received. The invention will row be described more fully below with reference to the drawing, in which fig. 1 shows an example of a reracte-puir.ped optical ampli¬fier consisting of an -erbium-doped, fibre, a receiver and pump unit and a transmission and pump fibre, fig. 2 shows the receiver and pump unit of fig. 1 in greater detail, lig. 3 shews curve shapes of signals in the receiver and pump unit when -his is in a safety state, and fig. 4 shows the curve shape of a pump signal in the re¬ceiver . and punp unit when this is ir. an intermediate. sr.ate. . . • ' Fig. 1' shows an example of an optical remcte-puinped am¬plifier c;f the invention. The example involves an optical preamplifier consisting of an erbium-doped fibre 1 and a receiver and pump unit 2 connected to the erbium-doped fibre by a transmission and punp fibre '3, which may typi¬cally have a length of 10-50 km I- the receiver and pump unit 2, the light arriving from L.:e fibre 3 passes via a wavelength multiplexer 4 to a receiver or amplifier circuic 5, in which the transmis¬sion cr information signals contained in the light may be received and cpionally be passed or. tor further proces¬sing. A pump laser 6 generates optical pump power which 1 - transmitted via the wavelength multiplexer 4 out cr. the fibre 3 in ?. direction toward the erbiun-deped fibre 1. Typically, the light with the information signals may have z wavelength of 1550 nm.. while oppositely directed pluap iight may have a wavelength of 1430 nm, thereby en- aoling -he wavelength mult-pleset 4 tc transmit the in- forir.ation signals trom the fibre 3 to the receiver cir¬ cuit 5 and the pump power from the pump aser 5 to the fibre 3. The mean power of the pump signal will usually be consid¬erably higher than the mean power of , the transmission signals, and interruption of the fibre 3 between the unit 2 and the erbium-dcped fibre 1 would therefore involve the risk that a harmful quantity of light might hit an eyeif ne safety measures were taken. The receiver and pump unil 2 is therefor* adapted to be able to assume three states. In an operational state assumed when the receiver 5 de¬tects a comnunications signal, the pump laser gives fuii nominal pump power, as the received communicatior.s signal is a guarantee that the fibre 3 is intact all the way to the erbium-doped fibre 1. In a safety state assumed when there is no connection fron the receiver and pump unit 2 to the erbium-doped fibre 1, the pump signal is pulsed so. that Lts mean pcwer is below 10 mK, which means i.a. that the equipment may be categorised s.s safety class 1 according to the IEC 925 An intermediate state is assumed when correction to an eirbiuir.- doped fibre is detected, while the receiver 5 has not yec delected i coiatiunicaticns signal. In this state, the punp power is detected sc tnal that aear. power consti¬tutes ate at 2/3 of -ho nominal pump power. IF the fiore 3 is in-act when the system is in the safety STaf.p cr the interr.ediate state, the pump pulses will roach the erbium-doped fibre 1, and the optice.1 power contained in the pulses will be absorbed by the ernium-dopsd fibre, while a spontaneous noise pulse of so-cai_sd ASE noise (Amplified Spontaneous Emission} is gsnerated in resporse to each pulse in the erbiam-dcped fibre 1. These ASK noise pulses will than be returned via the . fibre 3 to the unit 2, where, as will be dascribed more fully below, they can be detected to indicate that there is no break on the fibre 2. If, on the other hand, there is a break on the fibre 3, tie pulses emitted from the pump laser 6 will not reach the erbium doped fibre 1, and thus no ASE noise pulses will be generated. When the system is started, the receiver and pump unit 2 will first assume, the safety state, while it is checked whether connection to an erbium-doped fibre has been cs-tabiishec.. When this has been fcund to be the case, the unit 2 switches to the intermediate state until the re-ceiver 5 detects a, transmission signal. In the intermedi-ate stale where the pump power is about 2/3 of the nomi-nal valus, the transmission quality, is jus'c slight infe-rior relative to normal function, and the rest: of the system it, therefore capable of performing a norms.1 start- up procedure. When the receiver 5 .detects a transmission 3gnal, the unit 2 switches to the normal operational state.If it is detected at any time while the system is in the operational state that the receiver 5 no longer detects a communications signal, che unit 2 immediately switches to the safety state, as the missing communications signal may e.g. be caused by a fibre break between the unit 2 and the erbium-doped fibre 1.Fig. 2 shows in. greater detail how the receiver and pimp unit 2 may be constructed. As will be seen, the pump la¬ser 6 is controllable partly from. a control unit 16 and partly-from a clock generator 17. The control unit 16 de¬cides which of the three above-mentioned states the unit is to assume, while the clock generator 17 determines the pulse Trequency in the stales where the pump laser is pulsed. The pulse frequency may e.g. be selected at 75 Hz. Having passed the wavelength multiplexer 4, the light re-ceived from the fibre 3 nay optionally be ' amplified in an optical amplifier -7, following which it is split into two branches in the optical coupler 6. The branch having the units 12-15, which will be described more fully below, detects whether the received light includes ASE noise pulses with the .pulse frequency, while the detector 11 detects whether the light contains communications sig¬nals. In the operational state, the pump laser 6 pumps continu¬ously with the nominal pump power, and the communications signals' received from the fibre 3 • reach the detector 11 via the wavelength multiplexer 4, the amplifier 7 and the coupler 8. The .detector 11 passes the signals on for farther process ing and also informs Che control unit 16 Inat communications signals are received at; the moner.t. The control unit 16 therefore ensures that the laser 6 continues to give full pump power. if the detector 11 detects that the communications sig¬nals fail tc appear, it inforns the ccntrol unit 16 which immediatety sets the pump laser 5 in the safety state via tae connection 9, ahere pump power is transmitted in pulses determined by the clock generator 17. The pulsed punp power may e.g. look as shown or. curve A in fig. 3. The repetition frequency of the pnlsss is here selected to be 75 Hz, and the duty cycle is selected such that the resulting mean cower'is below 1C mW. Typically, the nomi¬nal power will be 110 mW, and the duty cycle will then be l/ll or less. When the fibre 3 is intact, the pulses will move along it until they reach the erbium-doped fibre 1, and part of the pulse will be reflected on the way because of Rayleigh scattering, and, therefore, a reflected signal will return to. the receiver and pump unit 2 fron the fibre 3. This signal may e.g. look ao chown on curvo B in fig. 3. It is noted thai the amplitude of the reflected signal is considerably smaller than the emitted pulses. When the pump pulse reaches the erbium-doped fibre 1, this will be active and start generating ASE noise, which is likewise pass.ed via the;fibre 3 back to the receiver and pump unit 2. The ASE noise will be generated as long the pulse lasts, ard will then decrease according to an exponential curve whose time constant is long with re¬spect to the transmission time on the fibre 3. The ASS roise received on the receiver and pimp unit 2 may look as shown on curve c in fig. 3.. These ASE no:.se pulses are used in the receiver and pump unit 7 as an indication that the fibre 3 is intact. since, hewever, the signal received froir. the fibre 3 is the sum of curves 3 and C, the Inhibition circuit 12 pro-vides for blocking of the received signal as long as the signal reflected from the fibre 3 lasts (curve B; . Be¬cause of the pulse transit time in the fibre, this will be a period after the end of t.ne transmitted pump pulse, wnich will be abou- 0.5 msec, with a fibre ier.gt'n of 50 kr.; As mentioned below, 3 reflected signal will return also if the fibre is broker,, but also this signal will at rr.Dst be of the same duration. The inhibition circuit 12 is also controlled by the clocz. generater 17. Thus, only the exponential "tail" of the A5E noise pulse will be present on the ouiput of the inhibition circuit; 12, as shown on curve D in fig. 3. This signal, like the emitted pulses, has a repetition frequency of 75 Hz, and it is now passed through a band¬pass filter having a centre frequency of the 75 Hz and a bandwidth of e.g. 15 HZ to filter out partly components froir. a possible communications signal partly signals originating from a constant spontaneous emission in the erbiurc.-dcped f ibre 1.. The bandpass-filtered signal is then fed to a sample-aad-hcld circuit 14 which samples with the saitie frequency as the pulsed pump signal so as to provide a sampling value for each pulse. The sampled values are Icwpass-filtered in the lowpass filter 15 and are then compared in the control unit 16 with a threshold value to decide whether a sufficiently great value of the ASE noise is received. If the control unit 16 detects chat the ASE noise pulses are above the threshold value, it instructs the pump la¬ser ' to switch to the intermediate state, which will be described below, as the fibre 3 must be intact. if. on the other hand, the fibre 3 is broken, no ASE pulses can come from -he eibium.-doped fibre 1, as the' panp pulses dc not reach at aut then there will be a strong reflection of the emitted pulse from the break. Depending on the distance from the break/ this reflection will usually have a considerably greater amplitude than bath curves B and C in fie. 3; but this reflection will 02 over at the latest simultaneously with curve a and will therefore oe blocked by the inhibition circuit 12, so that the 'control unit 16 dees not detect any signal. This is an indication of a break- on the - fibre, and the control unit therefore informs the pump letsei. 6 to remain in. the safety state. when the control unit 16 has established that ASE pulses return the unit switches to the intermediate szats, as mentioned, where the pump laser emits a signal, as shown in fig. 4. The power level between the pulses is selected at about 2/3 of the nominal pump power, and the peak level of the pulses corresponds to the ncninal power. If the fibre 3 is still intact, a signal corresponding completely to the one described above and shown in fig. 3 will be- reLurzisd, the amplitude, of the signals being merely smaller. The difference is just that the erbium-doped fibre 1 will now receive sufficient pump power to make it capable of passing on corwnuni cat ions signals. When these are detested by the detector 11, the control unit switches to the normal operational state. The inter¬mediate state is necessary, because the operational state can only be naintained when communicationn aighalc ore received. Therefore, in this circuit, it will not' be ex¬pedient to switch directly from the safety state to the operational stare, I! -he ASE pulses disappear in the intermediate state, this indicates that the fibre has been interrupter again, and the control unit 16 will therefore return tc the safety state. The repetition frequency of the emitted pulses is here selected at .75 Hz; but may also assume ether values of course. It must be sufficiently low so thai the aext- ASE pulse is not • emitted before the ASE culse caused by the pilse has died away, and the lower Unit of the trequency is determined by the maximum time it may take the system to switch 'fron the intermediate state to the safety state. patent claims 1. An optical amplifier comprising • an active fibre (1) • a pump unit (2) spaced from the active fibrs and adapted to give a normal, continuous pump power in an operational state, and • a pump fibre (3) adapted to transfer optical pump power from the pump ur.it (2) to the active fibre (1), characterized in that, in a safety state, the pump unit (2; is moreover adapted to give a pulsed pump power whose mean power is lower than g prescribed safety limit. 2. An optical amplifier according to,claim 1/ characterized in that the pump unit (2) con- prises a pump laser (€}- for generating the cptical power. 3. An optical amplifier according to claim 1 or 2, characterized in that the puir.p unit {2} is moreover' auapted to detect whether an optical signal is returned from the active fibxe (1) in response to the pulsed pump power. 4. An optical amplifier according to claims 1-3, characterized in that the pump unit (2) is adapted to generate the pulsed puir.p power aw pulses which are repeated v/ith a given frequency-. 5. An optical amplifier according to claim 4, characterized in that the puir.p unit (2) is adapted to perform the detection cf whether ai- cptical signal is returned from the active fibre {1} in response to the pitsed punp power, by detecting whether an optical signal with the givea pulsation frequency is received. 6. Ar: optical amplifier according to clair. 5, characterized in that the pump unie (2; is adapted to remain in che safety state if it is detected t-iat no cpticil signal is re turned from the acrive fibre (i) in response to the pulsed rump power. 7. Ar -optical amplifier according to clam 3, c r. a r a c t - r i z e d in that the pump ur.ic (2) is adapted to switch to the operational &tat5 if it is de¬tected that an optical signal is returned from the active fibre in response to the pulsed puinp power. 8. An optical amplifier according to claim 7• characterized in that the pump unit (2) is adapted to switch from the safety state to the opera¬ tional state via an intermediate state and, in this . in¬ termediate slaty, to give a continuous punp power super¬ imposed by a pulsed signal. 9. An optical amplifier according to claim 9, characterized in that the superimposed pulsed signal in the intermediate state has the sane shape as the pulsed pump powe in the safety stale. 1C. An optical amplifier according to claim 8 or 9, characterized in that it is moreover poss¬ible to transfer optical information signals, frcin the ac¬tive fib::e (1) to the pump unit (2)~, and that the puinp unit (2) comprise? means (115 for detecting whether such information signals are received. 11. An optical amplitier according to claim 10, characterized in that said optical infcrraa-tion signais are transferred from the active fibre (11 to t'-e pvimp unit (2) via the camp fibre (3) . 12. An optical amplifier according to claim 10 or 11, characterized ir. that the pump unit (2), wien in the operational slate, is adapted to roaiair. in said state as Icng as information signals are received, a.-.d to switch to the safety state it no information sig¬ nals are received. 13. Ar. optical amplifier according to claims 8-12, characterized in that the pump ur.it (2}, . wnen in the safety state, is adapted to remain in said slate if no returned optical signal is detected in re¬ sponse to the pulsed pump power, and tc switch to the in¬ termediate state if such 5 signal is detected. 14. .An optical amplifier according to claiir.s 10-13, c h a r a c t e r i 2 e d in that the pjir.p unit (2), wr.en the internediate state, ls.aclapi.ad to switch to the operational ste~.e if information signals are received, to switch to the dwell state if no returned optical signal is detected in response to the pulsed pionp power, and tc remain.'"in the intermediate state if a returned optical signal is detected in response; to the pulsed pump power and no information signals are received. 15. An optical amplifier according to claims 3-14, characteri z. e d in that the pur.ap unit (2) is adapted to inhibit the detection of whether an optical signal is -ietuined. fiom the active fibre (1) ir. response to the pulsed pump power, until a selected period of time r.as elapsed after the emission of each pulse from the pump ur.it: (2) . 13. A method of preventing enission of opt ical power ex ceedinc a prescribcc safety limit on interruption of ar optical fibre -3) which transfers pump power fron a pump Vtti7 (2) to.ft^civa fibre f- (-) c h a r a c t e r ' _ i z e d by changinc the mean power of the pump ;power ir. Xesccuse tc a signal receive from the active fibre (1, so that the mean power assumes a value below said safety limit il said signal is not -received, and essuiucs a n a t value if said signal is teceived 17. A method according to claim 16, character- 12 e d by generating th* nuan power below said safety limit by pulsing the pump powcir with a giver, frequency. 18. A method according to claim 17, character- i zed by detecting the signo1. received from the active fibre (1) fay delecting whether a signal with the given pulsation frequency is received. 19. An optical amplifier substantially as herein reference to the accompanying drawings. described with

Documents:

3272-del-1997-abstract.pdf

3272-del-1997-claims.pdf

3272-del-1997-correspondence-others.pdf

3272-del-1997-correspondence-po.pdf

3272-del-1997-description (complete).pdf

3272-del-1997-drawings.pdf

3272-del-1997-form-1.pdf

3272-del-1997-form-13.pdf

3272-del-1997-form-19.pdf

3272-del-1997-form-2.pdf

3272-del-1997-form-3.pdf

3272-del-1997-form-4.pdf

3272-del-1997-form-6.pdf

3272-del-1997-gpa.pdf

3272-del-1997-pct-210.pdf

3272-del-1997-pct-409.pdf

3272-del-1997-petition-138.pdf