WO1998025361A1

WO1998025361A1 - An optical amplifier and a method of preventing emission therefrom of optical power exceeding a prescribed safety limit

Info

Publication number: WO1998025361A1
Application number: PCT/DK1997/000518
Authority: WO
Inventors: Claus Friis Pedersen; Søren Morten NIELSEN
Original assignee: Dsc Communications A/S
Priority date: 1996-11-15
Filing date: 1997-11-14
Publication date: 1998-06-11
Also published as: DE69713902D1; US6243195B1; EP0956666A1; DE69713902T2; EP0956666B1; AU4940897A; DK129396A; DK0956666T3

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.

Description

An optical amplifier and a method of preventing emission therefrom of optical power exceeding a prescribed safety limit

The invention relates to an optical amplifier comprising an active fibre, a pump unit spaced from the active fibre and adapted to give a nominal, continuous pump power in an operational state, and a pump fibre adapted to trans- fer pump power from the pump unit to the active fibre.

The invention moreover relates to a method of preventing emission of optical power exceeding a prescribed safety limit upon interruption of an optical fibre which trans- fers pump power from a pump unit to an active fibre.

Optical fibre amplifiers for amplifying optical 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 signal to be amplified, and a communications signal is launched into the amplifier, a signal coherent with the signal on the input will occur on the output of the active 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 km) from the pump laser, in which case the amplifiers are referred to as remote-pumped amplifiers. With e.g. remote-pumped preamplifiers, also called RILP (Remote In-Line Preamplifier) , the active fibre is thus spaced from the actual receiver of 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 transmitted in such fibres, in the form of com- munications signals or pump power, is typically harmful to the human eye. Therefore, because of situations with access to fibre ends or non-connected connectors, it is prescribed by various standards how much optical power may be transmitted from an open fibre end in these situa- tions. These situations may e.g. occur in case of repair, maintenance and testing of systems, or when a fibre has broken, or a connector is disassembled. It is the temporal mean power of the light that is harmful to the eye.

To achieve the desired function 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 pump power in the event that the fibre transmitting the pump power is interrupted between the pump laser and the active fibre.

Further various communications equipment standards pre- scribe that the equipment must be capable of automatically resuming normal operation when the transmission path has been re-established after a break and transmission signals are transmitted again. For remote-pumped amplifiers, 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 communications signals arriving at the receiver after the re-establishment of the transmission path, will frequently be below the sensitivity limit of the receiver because of the reduced pump power. This problem has previously been solved e.g. by using an additional fibre from the receiver to the active fibre. This fibre, in combination with the transmission fibre, is used for passing a control signal from the receiver to the active fibre and back to the receiver. When the con^¬ trol signal is present, there is no break on the fibre and consequently no access to the strong optical pump power, and the pump laser can therefore pump with full power. When, on the other hand, the control signal is ab- sent, this indicates a break on the fibre, 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 reduced if an alarm signal is received from the receiver in the other end of a transmission fibre, said alarm 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 and, therefore, this system also has the above-mentioned drawback. Further, the system is not suitable for reducing pump power, unless a special detector unit as above is provided at the active fibre for generation of an alarm signal.

A similar system is known from US 5 428 471 in which two parallel fibres are used for transmission in respective directions. When a fibre amplifier in one direction detects an absent input signal a message is sent via the opposite fibre back to the previous fibre amplifier to reduce 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 capable of returning to full pump power when the fibre connection has been re-established. This may take place by using the existing fibre or fibres, which means that no additional fibre is required exclusively for this purpose, and that an additional detector or coupler at the active fibre is obviated.

This is achieved according to the invention in that, in a safety state, the pump unit is moreover adapted to give a pulsed pump power whose mean power is lower than a prescribed safety limit. Pulsing of the pump power ensures that its mean power can be kept so low in the safety state that the emitted light is unharmful 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 and to inform 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 absorbed by the active fibre which, in response to the pulse, simultaneously generates a spontaneous noise called ASE (Amplified Spontaneous Emission) , and this ASE signal may then be returned to the pump unit.

The pump unit, which generates the required pump power, may be constructed in different ways. In an expedient embodiment defined in claim 2, a pump laser is used.

When, as stated in claim 3, the pump unit is adapted to detect whether an optical signal is returned from the active fibre in response to the pulsed pump power, it is ensured that the pump unit can switch between the opera- tional state and the safety state in dependence on the returned signal.

When, 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. Therefore, 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 power, 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 operational state if it is detected that such a signal is returned. As stated in claim 8, switching from the safety state to the operational state may 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 may be given a sufficient pump power for it to operate practically normally and therefore to amplify any communications sig^¬ nals, while enabling it to be controlled by means of the pulses whether the connection is still intact until com- munications signals proper are received. Expediently, as stated in claim 9, the superimposed pulsed signal in the intermediate state may have the same shape as the pulsed pump power in the safety state. As a result, the same detector circuit may be used in the two states.

As stated in claim 10, it will therefore be expedient that optical information signals may moreover be transferred from 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 pump fibre, and moreover capable, in the safety state, of transferring the pulsed pump power and the possible response 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, as stated in claim 12. When the pump unit is in the safety state, it may be adapted to remain in this state if no returned optical signal in response to the pulsed pump power is detected, and to switch to the intermediate state if such a signal is detected, as stated in claim 13.

When the pump unit is in the intermediate state, it may be adapted to switch to the operational state if information signals are received, to switch to the dwell state if no returned optical signal in response to the pulsed pump power is detected, and to remain in the intermediate state if a returned optical signal in response to the pulsed pump power is detected and no information signals are received, as stated in claim 14.

Finally, as stated in claim 15, the pump unit may be adapted to inhibit the detection of whether an optical signal in response to the pulsed pump power 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 reflections that will be returned from the pump fibre, irrespective of whether this is intact or broken, and instead exclusively detects the ASE noise which can only originate from the active fibre, and which will last considerably longer than the reflections from the pump fibre .

As mentioned, the invention also relates to a method of preventing emission of optical power exceeding a prescribed safety limit on interruption of an optical fibre which transfers pump power from a pump unit to an active fibre. This method is stated in claim 16. When the mean power of the pump power is changed in response to a sig- nal received from the active fibre such 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, it is ensured that the mean power may automatically be reduced to a safe level when a break occurs on the optical fibre.

As stated in claim 17, this may expediently take place in that the mean power below said safety limit is generated by pulsing the pump power with a given frequency, and that, as stated in claim 18, the signal received from the active fibre is detected by detecting whether a sig^¬ nal with the given pulsation frequency is received.

The invention will now be described more fully below with reference to the drawing, in which

fig. 1 shows an example of a remote-pumped optical amplifier 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,

fig. 3 shows curve shapes of signals in the receiver and pump unit when this is in a safety state, and

fig. 4 shows the curve shape of a pump signal in the receiver and pump unit when this is in an intermediate state .

Fig. 1 shows an example of an optical remote-pumped amplifier of 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 pump fibre 3, which may typi- cally have a length of 10-50 km. In the receiver and pump unit 2, the light arriving from the fibre 3 passes via a wavelength multiplexer 4 to a receiver or amplifier circuit 5, in which the transmis^¬ sion or information signals contained in the light may be received and optionally be passed on for further proces^¬ sing. A pump laser 6 generates optical pump power which is transmitted via the wavelength multiplexer 4 out on the fibre 3 in a direction toward the erbium-doped fibre 1. Typically, the light with the information signals may have a wavelength of 1550 nm, while oppositely directed pump light may have a wavelength of 1480 nm, thereby enabling the wavelength multiplexer 4 to transmit the in^¬ formation signals from the fibre 3 to the receiver circuit 5 and the pump power from the pump laser 6 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-doped fibre 1 would therefore involve the risk that a harmful quantity of light might hit an eye if no safety measures were taken.

The receiver and pump unit 2 is therefore adapted to be able to assume three states.

In an operational state assumed when the receiver 5 detects a communications signal, the pump laser gives full nominal pump power, as the received communications 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 from the receiver and pump unit 2 to the erbium-doped fibre 1, the pump signal is pulsed so that its mean power is below 10 mW, which means i.a. that the equipment may be categorized as safety class 1 according to the IEC 825 recommendations .

An intermediate state is assumed when connection to an erbium-doped fibre is detected, while the receiver 5 has not yet detected a communications signal. In this state, the pump power is detected so that the mean power consti^¬ tutes about 2/3 of the nominal pump power.

If the fibre 3 is intact when the system is in the safety state or the intermediate state, the pump pulses will reach the erbium-doped fibre 1, and the optical power contained in the pulses will be absorbed by the erbium- doped fibre, while a spontaneous noise pulse of so-called ASE noise (Amplified Spontaneous Emission) is generated in response to each pulse in the erbium-doped fibre 1. These ASE noise pulses will then be returned via the fibre 3 to the unit 2, where, as will be described more fully below, they can be detected to indicate that there is no break on the fibre 3.

If, on the other hand, there is a break on the fibre 3, the 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 es- tablished. When this has been found to be the case, the unit 2 switches to the intermediate state until the receiver 5 detects a transmission signal. In the intermedi^¬ ate state where the pump power is about 2/3 of the nominal value, the transmission quality is just slight infe- rior relative to normal function, and the rest of the system is therefore capable of performing a normal start- up procedure. When the receiver 5 detects a transmission signal, 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, the 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 pump 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 frequency in the states 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 received from the fibre 3 may optionally be amplified in an optical amplifier 7, following which it is split into two branches in the optical coupler 8. 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 continuously 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 further processing and also informs the control unit 16 that communications signals are received at the moment. 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 to appear, it informs the control unit 16 which immediately sets the pump laser 6 in the safety state via the connection 9, where pump power is transmitted in pulses determined by the clock generator 17. The pulsed pump power may e.g. look as shown on curve A in fig. 3. The repetition frequency of the pulses is here selected to be 75 Hz, and the duty cycle is selected such that the resulting mean power is below 10 mW. Typically, the nomi- nal power will be 110 mW, and the duty cycle will then be 1/11 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 from the fibre 3. This signal may e.g. look as shown on curve B in fig. 3. It is noted that 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 passed via the fibre 3 back to the receiver and pump unit 2. The ASE noise will be generated as long the pulse lasts, and will then decrease according to an exponential curve whose time constant is long with respect to the transmission time on the fibre 3. The ASE noise received on the receiver and pump unit 2 may look as shown on curve C in fig. 3. These ASE noise pulses are used in the receiver and pump unit 2 as an indication that the fibre 3 is intact. Since, however, the signal received from the fibre 3 is the sum of curves B 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 the transmitted pump pulse, which will be about 0.5 msec, with a fibre length of 50 km. As mentioned below, a reflected signal will return also if the fibre is broken, but also this signal will at most be of the same duration. The inhibition circuit 12 is also controlled by the clock generator 17. Thus, only the exponential "tail" of the ASE noise pulse will be present on the output 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 bandpass filter having a centre frequency of the 75 Hz and a bandwidth of e.g. 15 Hz to filter out partly components from a possible communications signal partly signals originating from a constant spontaneous emission in the erbium-doped fibre 1.

The bandpass-filtered signal is then fed to a sample-and- hold circuit 14 which samples with the same frequency as the pulsed pump signal so as to provide a sampling value for each pulse. The sampled values are lowpass-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 that the ASE noise pulses are above the threshold value, it instructs the pump laser 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 the erbium-doped fibre 1, as the pump pulses do not reach it. But 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 both curves B and C in fig. 3; but this reflection will be over at the latest simultaneously with curve B and will therefore be blocked by the inhibition circuit 12, so that the control unit 16 does not detect any signal. This is an indication of a break on the fibre, and the control unit therefore informs the pump laser 6 to remain in the safety state.

When the control unit 16 has established that ASE pulses return, the unit switches to the intermediate state, 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 nominal power.

If the fibre 3 is still intact, a signal corresponding completely to the one described above and shown in fig. 3 will be returned, 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 communications signals. When these are detected 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 maintained when communications signals are received. Therefore, in this circuit, it will not be expedient to switch directly from the safety state to the operational state. If the ASE pulses disappear in the intermediate state, this indicates that the fibre has been interrupted again, and the control unit 16 will therefore return to the safety state.

The repetition frequency of the emitted pulses is here selected at 75 Hz; but may also assume other values of course. It must be sufficiently low so that the next ASE pulse is not emitted before the ASE pulse caused by the pulse has died away, and the lower limit of the frequency is determined by the maximum time it may take the system to switch from the intermediate state to the safety state .

Claims

P a t e n t C l a i m s

1. An optical amplifier comprising • 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), c h a r a c t e r i z e d 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 a prescribed safety limit.

2. An optical amplifier according to claim 1, c h a r a c t e r i z e d in that the pump unit (2) com^¬ prises a pump laser (6) for generating the optical power.

3. An optical amplifier according to claim 1 or 2, c h a r a c t e r i z e d in that the pump unit (2) is moreover adapted to detect whether an optical signal is returned from the active fibre (1) in response to the pulsed pump power.

4. An optical amplifier according to claims 1-3, c h a r a c t e r i z e d in that the pump unit (2) is adapted to generate the pulsed pump power as pulses which are repeated with a given frequency.

5. An optical amplifier according to claim 4, c h a r a c t e r i z e d in that the pump unit (2) is adapted to perform the detection of whether an optical signal is returned from the active fibre (1) in response to the pulsed pump power, by detecting whether an optical signal with the given pulsation frequency is received.

6. An optical amplifier according to claim 3, c h a r a c t e r i z e d in that the pump unit (2) is adapted to remain in the safety state if it is detected that no optical signal is returned from the active fibre (1) in response to the pulsed pump power.

7. An optical amplifier according to claim 3, c h a r a c t e r i z e d in that the pump unit (2) is adapted to switch to the operational state if it is detected that an optical signal is returned from the active fibre in response to the pulsed pump power.

8. An optical amplifier according to claim 7, c h a r a c t e r i z e d in that the pump unit (2) is adapted to switch from the safety state to the operational state via an intermediate state and, in this in- termediate state, to give a continuous pump power superimposed by a pulsed signal.

9. An optical amplifier according to claim 8, c h a r a c t e r i z e d in that the superimposed pulsed signal in the intermediate state has the same shape as the pulsed pump power in the safety state.

10. An optical amplifier according to claim 8 or 9, c h a r a c t e r i z e d in that it is moreover poss- ible to transfer optical information signals from the active fibre (1) to the pump unit (2), and that the pump unit (2) comprises means (11) for detecting whether such information signals are received.

11. An optical amplifier according to claim 10, c h a r a c t e r i z e d in that said optical informa^¬ tion signals are transferred from the active fibre (1) to the pump unit (2) via the pump fibre (3) .

12. An optical amplifier according to claim 10 or 11, c h a r a c t e r i z e d in that the pump unit (2) , when in the operational state, is adapted to remain in said state as long as information signals are received, and to switch to the safety state if no information sig- nals are received.

13. An optical amplifier according to claims 8-12, c h a r a c t e r i z e d in that the pump unit (2) , when in the safety state, is adapted to remain in said state if no returned optical signal is detected in re^¬ sponse to the pulsed pump power, and to switch to the in^¬ termediate state if such a signal is detected.

14. An optical amplifier according to claims 10-13, c h a r a c t e r i z e d in that the pump unit (2) , when the intermediate state, is adapted to switch to the operational state if information signals are received, to switch to the dwell state if no returned optical signal is detected in response to the pulsed pump power, and to 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, c h a r a c t e r i z e d in that the pump unit (2) is adapted to inhibit the detection of whether an optical signal is returned from the active fibre (1) in response to the pulsed pump power, until a selected period of time has elapsed after the emission of each pulse from the pump unit (2) .

16. 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), c h a r a c t e r i z e d by 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.

17. A method according to claim 16, c h a r a c t e r ^¬ i z e d by generating the mean power below said safety limit by pulsing the pump power with a given frequency.

18. A method according to claim 17, c h a r a c t e r i z e d by detecting the signal received from the active fibre (1) by detecting whether a signal with the given pulsation frequency is received.