WO2002080318A1

WO2002080318A1 - Noise-compensating gain controller for an optical amplifier

Info

Publication number: WO2002080318A1
Application number: PCT/US2002/004639
Authority: WO
Inventors: Muhidin Lelic
Original assignee: Corning Incorporated
Priority date: 2001-03-31
Filing date: 2002-02-15
Publication date: 2002-10-10

Abstract

A noise-compensating gain controller (1) is provided for an optical amplifier (3), such as a multi-channel erbium-doped optical amplifier. The controller (1) includes a gain detecting circuit (22), a set-point circuit (32) for providing a signal indicative of a selected gain level of the amplifier (3), and a gain compensating circuit (34) for providing a signal indicative of the relative strengths of the signal component and noise component of the output caused by amplified spontaneous emission. A digital signal processor (20) receives signals generated by the gain detecting circuit (22), the set-point circuit (32), and the gain compensating circuit (34) and adjusts the output of the optical amplifier (3) by discounting the strength of the noise component so that the gain of the signal component is equal to the selected gain level.

Description

NOISE-COMPENSATING GAIN CONTROLLER FOR AN OPTICAL AMPLIFIER

FIELD OF THE INVENTION

[0001] This invention generally relates to control systems for optical amplifiers and is specifically concerned with a method of operating a noise-compensating gain controller for an erbium-doped fiber amplifier that avoids overshoot of a selected gain level during amplification transients.

BACKGROUND OF THE INVENTION

[0002] Erbium-doped fiber amplifiers (EDFAs) are used in optical transmission networks to extend transmission distances and to compensate for losses from various network elements. Such amplifiers typically comprise a pump laser whose output is optically coupled to the input of two, serially connected coils of erbium-doped optical fiber. In operation, the output of the pump laser excites the atoms of erbium dopant within the serially connected coils of doped fibers. These excited atoms release their excess energy in proportion to the strength of the incoming optical signal, which results in an amplified output. When such EDFAs are used simply as amplification relay stations along a single, long-distance optical circuit, there is little need for a device to specifically control the amount of gain that the amplifier imparts on the incoming optical signal. However, as optical systems have become more complex, the need for such gain control systems has increased. Such a need may arise, for example, when an optical network is installed around an urban area. Under such circumstances, the distances between the optical amplifiers may be very different. If the EDFAs in the system all have the same amplification capacity, this capacity must be adjusted by way of a gain control device so that the signal strength remains uniform throughout all branches of the network.

[0003] In the past, such gain control has typically been achieved by the combination of a digital signal processor in combination with a power regulation circuit that modulates the amount of electrical power applied to the pump laser. The digital signal processor generates a control signal that instructs the power regulation circuit to deliver electrical power to the pump laser at a level consistent with a selected gain set-point. The specific control signal associated with a particular set-point is determined by an empirically derived control algorithm which is programmed into the memory of the signal processor. Hence, when the set-point of the gain controller is selected to be, for example, at 25 decibels (db), the digital processor generates a control signal that causes the pump laser to amplify the incoming optical signal until the strength of the output corresponds to the amount selected at set-point, i.e., 25 db.

[0004] While such EDFA gain controllers can work well for their intended purpose, the applicant has observed that a significant problem arises when the incoming optical signal is significantly contaminated with a noise component known as amplified spontaneous emission (ASE) in the art. Because such prior art gain controllers amplify the total output to a desired gain level, and because the optical output is nearly always the combination of an amplified signal plus a variable amount of amplified ASE, such controllers under-amplify the signal in direct proportion to the power of the ASE component mixed therewith. Such under-amplification is much worse for low input signals, when the ASE power content may be larger than the signal power. In all cases, the resulting under-amplification of the optical input signal can lead to undesirable non- uniformities in the strength of the signals transmitted through the optical network. Moreover, due to the variability of the ASE power component and the optical inputs, the under-amplification problem cannot be solved by a single, empirically derived algorithm programmed into the signal processor at the time of its manufacture. [0005] To solve these problems, the applicant has developed a noise-compensating gain controller capable of amplifying the signal component of the optical output to the desired gain level selected by the operator. This controller is described in detail herein, and generally comprises a gain detecting circuit, a set-point circuit for providing a signal indicative of a selected gain level of the amplifier, a gain compensating circuit for providing a signal indicative of relative strengths of the signal component and the noise component of the amplifier output, and a digital signal processor for adjusting the gain level so that the gain of the signal component of the output is equal to a select gain level. In operation, upon selection of a specific gain level via the set-point circuit, the digital signal processor compares the selected gain level with the actual gain level indicated by the gain detecting, and computes an amplification difference necessary to equalize the actual gain with the selected gain. At the same time, in response to signals received by the gain compensating circuit, the processor computes the amount that the gain will have to be adjusted to bring the signal component of the amplifier output to the selected gain level. The processor than proceeds to change the amplification by the computed difference while adjusting this difference to bring the gain of the signal component of the output to the selected gain level.

[0006] While such a noise compensating gain controller represents a substantial advance in the prior art, the applicants have observed that an amplification overshoot problem may occur during amplification transients, which could happen as a result of rapid fluctuations in the power of the optical input, or a rapid change in the gain set- point by the system operator. The applicants have further observed that, because the power of the ASE component of the optical input varies with input power levels, overshoot of the desired amplification level can occur if the digital signal processor is not capable of computing the amount of noise-compensating adjustment required in the amplification as fast as it computes the amount of amplification difference necessary to bring the actual gain to the same level as the selected gain. Such differences in computation rate occur as a result of the greater complexity of the amplification compensation calculations, and the operating speed limitations of most commercially available processors. Under such circumstances, the processor proceeds to change the amplification level by the difference it rapidly computes from the signal provided by the gain detecting circuit, but then attempts to adjust this difference on the basis of a pre-transient amplification level due to the computational lag time. This causes a localized spike or overshoot of the steady state value of the final amount of amplification will occur, as is indicated in Figure 6. In the graph of Figure 6, the overshoot is approximately 2dB, and may occur when the total input power of the optical signal changes between -26 and -1 ldB. Such change could easily occur under normal operating conditions of the amplifier as the result of the adding of channels, which typically could occur in time periods of less than 100 micro seconds. The resulting 2dB overshoot or spike in amplification is highly undesirable in an optical network of interconnected amplifiers, as each amplifier in the network would amplify the overshoot to an even greater height relative to the signal received. The overall effect would be a deterioration in the bit error rate (BER) in the transfer of data. [0007] Clearly, there is a need for a method of operating in noise-compensating gain controller during amplification transients which avoids such spikes of amplification overshoots. Ideally, such a method would not require the addition of any new components or alteration of the connections of the noise-compensating optical gain controller, and could be easily and simply implemented merely by the programming of relatively simple control algorithms into the digital signal processor.

SUMMARY OF THE INVENTION

[0008] The invention is a method for operating a noise-compensating gain controller that avoids undesirable overshooting of the gain level during amplification transients. The method is particularly adapted for use of a gain controller having a gain detecting circuit that continuously monitors the gain of the optical input, a set point circuit that provides a signal indicative of a selected gain level of the amplifier, a gain compensating circuit that provides a signal indicative of the relative strengths of the signal component and the noise or ASE component of the amplifier or output, and a digital processor circuit that receives signals from the previously mentioned circuit components, and then proceeds to adjust the gain level so that the gain of the signal component of the optical output is equal to the selected gain level. [0009] In the first step of the method, the digital signal processor first determines a difference in the amount of amplification necessary to bring the combination of the signal and noise component forming the optical output to the selected gain level.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Figure 1 is a schematic diagram of a first embodiment of the noise- compensating gain controller connected to an EDFA;

[0011] Figure 2 is a second embodiment of the noise-compensating controller which utilizes an optical power monitor so that the ASE at two or more frequencies in the amplified output can be monitored;

[0012] Figure 3 is a third embodiment of the noise-compensating gain controller in combination with a variable optical attenuator which functions to remove undesirable tilt across the channels of the optical output of the amplifier, and [0013] Figure 4 is a fourth embodiment wherein a dynamic gain flattening filter has been substituted for the variable optical attenuator of the embodiment illustrated in Figure 3.

DETAILED DESCRIPTION OF THE INVENTION

[0014] With reference to Figure 1, wherein the noise-compensating gain controller 1 of the invention is particularly adapted for use with an EDFA-type optical amplifier 3 well known in the optical transmission arts. Such an amplifier 3 includes an optical input waveguide 5 for receiving an optical input, and a wavelength division multiplexer 7 for coupling the output of a pump laser 9 into the amplifier 3. The output of the wavelength division multiplexer 7 is connected to an upstream amplification coil 11 formed from a coiled length of erbium-doped optical fiber. The output of the upstream amplification coil 11 is in turn connected to a gain flattening filter 13 which is pre- calibrated at a particular gain level to reduce tilt in the output of the amplifier 3 by attenuating the strength so the most amplified channels so that they are roughly equal to the strength of the least amplified channels. A second coil 15 of erbium-doped optical fiber is connected to the output of the gain flattening filter 13, and an additional wavelength division multiplexer 16 is provided to couple the output of a second pump laser 17 into the amplifier 3. The output of the second wavelength division multiplexer 16 is connected to an optical output waveguide 19 as shown. Finally, the outputs of the pump lasers 9, 17 are controlled by a digital signal processor 20 which transmits electrical control signals to power circuits 21a, b which are respectively connected to the pump laser 9, 17. Each of the pump lasers 9, 17 may be, for example, a 980 nm pump manufactured by Lasertron located in Bedford, Massachusetts. Alternatively, a 1480 nm pump may be used The digital signal processor 20 may be a Model DSP56311 manufactured by Motorola located in Austin, Texas.

[0015] The gain controller 1 of the invention includes a gain detecting circuit 22 for continuously making an on-line, real time determination of the output gain of the amplifier 3, a manually operable set-point circuit 32 for providing a set-point signal indicative of a selected gain level, and a gain compensating circuit 34 for determining what percentage of the power of the amplifier output is directly attributable to amplified spontaneous emission (ASE).

[0016] The gain detecting circuit 22 includes upstream and downstream optical taps 24a, b. The small amount of light diverted from these taps (typically only about 2% of the through-put) is directed onto photodiodes 26a, b. The electrical signals generated by the photodiodes 26a, b are conducted to transimpedance amplifiers 28a, b via connectors as shown. The output of the amplified electrical signals generated by the transimpedance amplifiers 28a, b is relayed to a ratio circuit 30 which may form part of the digital signal processor 20. The ratio circuit 30 transmits a signal to the digital signal processor 20 as to the relative strengths of the total optical input and the total optical output and hence indicates the overall gain P_out of the amplifier 3, i.e.,. the gain of the combination of the signal component and the noise or ASE component that forms the optical input.

[0017] While the set-point circuit 32 has been shown independently of the digital signal processor 20, it may also be integrated into the circuitry of the processor 20. The set- point circuit 32 includes a knob, dial, or some other control which allows a system operator to select a particular gain level that he or she wishes the amplifier 3 to operate at. Such set-point circuits 32 are well known in the prior art and do not form any part of the invention per se. [0018] The gain compensating circuit 34 includes an optical output tap 36 for diverting a small percentage of the optical output to a filter 38. Filter 38 is preferably a narrowband optical filter such as a Bragg grating or Fabri-Perot filter capable of filtering out the ASE component of the total optical output of the amplifier 3. This component of the output is directed onto a photodiode 39. The resulting electrical signal is amplified by a transimpedance amplifier 40 equivalent to the previously discussed transimpedance amplifiers 28a, b. The resulting signal P_ASE is indicative of the power of the ASE component included within power of the optical output P_out [0019] In operation, the digital signal processor 20 receives a signal from the gain detecting circuit 22 indicative of the overall gain of the amplifier 3, or P_out/P_ιn- The processor 20 also receives a signal from the set-point circuit 32 indicative of a desired gain of only the signal component of the optical input, i.e., (P_out - P_ASE) P_IΠ- The processor 20 then proceeds to subtract the power of the ASE component (P_ASE) detected by the gain compensating circuit from the total output power (P_out) detected by the gain detecting circuit 22. The processor 20 then increases the gain of the amplifier 3 until the gain of the signal component (P_OUI-P_ASE) of the optical output P_out is equal to the selected gain indicated by the gain point circuit 32. Hence, if the power of the ASE component P_ASE were 25% of the total optical output power P_ou, of the amplifier 3, then the digital signal processor 20 would increase the overall gain P_out/P_m by one-third, or 33 1/3 % so that the gain of the signal component would be equal to the gain selected by the set-point circuit 32.

[0020] Figure 2 illustrates an alternative embodiment 42 of the noise-compensating gain controller. In this embodiment, all of the circuits are identical with the circuits previously discussed in the Figure 1 embodiment with the exception of the gain compensating circuit 44. Specifically, in lieu of the narrow-band optical filter 38, which is capable of isolating the ASE component for only a single frequency between the multiple channels typically contained with the optical input P,_n, circuit 44 utilizes an optical power monitor 48. Such optical power monitors 48 are known in the prior art and contain a plurality of narrow-band electrical filters and photodiodes capable of isolating the ASE component of the optical power for at least two frequencies in the optical output 19. Hence the optical power monitor 48 generates at least two electrical signals indicative of the ASE components at two different points within the output spectrum, which are in turn respectively amplified by transimpedance amplifiers 50a, b. The outputs of each of the amplifiers 50a, b are conducted to a summation circuit 52, which generates a signal indicative of an average ASE power component contained within the two or more frequencies monitored by the optical power monitor 48. This signal, along with the signals generated by the gain detecting circuit 22 and the set- point circuit 32, are processed in exactly the same way by the digital signal 20 as described with respect to the embodiment illustrated in Figure 1. The advantage of this embodiment 42 over the previously described embodiment 1 is that a more accurate indication of the average P_ASE across the output spectrum is provided by the optical power monitor 48 and acted on by gain-compensating circuit 44. [0021] Figure 3 illustrates still another embodiment 54 of the gain controller in combination with a variable optical attenuator 56 which advantageously eliminates undesirable tilt across the output spectrum of channels. The use of a variable optical attenuator 56 (or dynamic gain flattening filter 62, shown in Figure 4) for the purpose of eliminating or at least reducing undesirable tilt in the output spectrum is the subject of a separate patent application entitled, "Dynamic Controller for a Multi-Channel Optical Amplifier," and assigned to the same assignee of the present application, Corning Incorporated, located in Corning, New York. Applicant hereby incorporates the entire specification of the application entitled "Dynamic Controller for a Multi- Channel Optical Amplifier" within the present application by express reference thereto. When the optical amplifier 3 is modified by the addition of the variable optical attenuator, the gain detecting circuit may be split into two such circuits 22a, b which measure the total gain of the optical signal transmitted from each of the coils 11 and 15, respectively. Such a duplication of the gain detecting circuit 22a, b results in a more accurate determination of the total gain of the flattened optical output signal created by the variable optical attenuator 56. The gain-compensating circuit 34 used in embodiment 54 is the same as that used in conjunction with the embodiment illustrated in Figure 1.

[0022] Figure 4 illustrates another embodiment 60 of the invention which is similar to that illustrated in Figure 3, with the exceptions that (1) the variable optical attenuator 56 is replaced with a dynamic gain flattening filter 62 and (2) the gain detecting circuit 22 is not split into two duplicate circuits, but remains a single circuit 22 in the same configuration as that discussed with respect to Figure 1. In addition to providing a relatively tilt-free or tilt reduced output spectrum, the embodiment 60 of Figure 4 has the additional advantage over the embodiment 54 illustrated in Figure 3 that the dynamic gain flattening filter can operate to flatten the optical spectrum with only 5 db loss versus losses of up to 19 db associated with presently known, state-of-the-art variable optical attenuators. The Figure 3 embodiment 54 and Figure 4 embodiment 60 operate in the same manner as the embodiment 1 described with respect to Figure 1 with the exception that the digital signal processor 20 also modulates the tuning of the variable optical attenuator 56 or the dynamic gain flattening filter 62 along with the power levels of the pump lasers 9, 17 to achieve a substantially tilt-free optical output, as described in detail in the previously referred co-pending application. [0023] While this invention has been described with respect to several preferred embodiments, various modifications and additions to the invention will become evident to persons of skill in the art. For example, the gain compensating circuit 44, optical filter 38, and transimpedance amplifier 40 may be positional in front of the first amplification coil 11 without interfering with the operation of the invention. All such variations, modifications, and additions are intended to be encompassed within the scope of this patent, which is limited only by the claims appended hereto.

PARTS LIST

I . Noise-compensating gain controller (first embodiment) 3. EDFA-type optical amplifier

5. Optical input waveguide

7. Wavelength division multiplexer

9. Pump laser

I I . Upstream amplification coil 13. Gain flattening filter

15. Downstream amplification coil

16. Wavelength division multiplexer

17. Pump laser

19. Optical output waveguide

20. Digital processor

22. Gain detecting circuit

24. Upstream and downstream taps a, b

26. Photodiodes a, b

28. Transimpedance amplifiers a, b

30. Ratio circuit

32. Set-point circuit

34. Gain-compensating circuit

36. Output tap

38. Optical filter

39. Photodiode

40. Transimpedance amplifier 42. Second embodiment

44. Gain compensating circuit

46. Output tap

48. Optical power monitor

50. Transimpedance amplifiers a, b

52. Summation circuit

54. Third embodiment 56. Variable optical attenuator

60. Fourth embodiment

62. Dynamic gain flattening filter

Claims

WHAT IS CLAIMED IS:

1. A noise-compensating gain controller for an optical amplifier, comprising: a gain detecting circuit for providing a signal indicative of a gain level of an output of said amplifier, wherein said output includes a signal component and a noise component; a set point circuit for providing a signal indicative of a selected gain level of said amplifier; a gain compensating circuit for providing a signal indicative of relative strengths of said signal component and said noise component of said output, respectively, and a digital processor circuit for receiving said gain level signal, selected gain level signal, and said relative strength signal for adjusting said gain level so that the gain of said signal component of said output is equal to said selected gain level.

2. The noise-compensating gain controller defined in claim 1, wherein said gain detecting circuit includes first and second optical taps coupled to an input and said output of said amplifier.

3. The noise-compensating gain controller defined in claim 2, wherein said gain detecting circuit includes first and second photosensitive components for producing first and second electrical signals from light diverted from said first and second taps that are indicative of relative amplitudes of said amplifier input and output, respectively.

4. The noise-compensating gain controller defined in claim 3, wherein said gain detecting circuit also includes first and second amplifiers for amplifying said first and second electrical signals.

5. The noise-compensating gain controller defined in claim 1, wherein said gain compensating circuit includes an optical tap coupled to the amplifier output, and a filter for separating the noise component from the signal component in output light diverted by the tap.

6. The noise-compensating gain controller defined in claim 5, wherein said gain compensating circuit further includes a photosensitive component for converting light separated by said filter into an electrical signal.

7. The noise-compensating gain controller defined in claim 6, wherein said separated light is light corresponding to said noise component of said amplifier output.

8. The noise-compensating gain controller defined in claim 6, wherein said gain compensating circuit further includes an amplifier for amplifying said electrical signal.

9. The noise-compensating gain controller defined in claim 7, wherein said amplifier is a multi-channel optical amplifier, and said gain compensating circuit produces an electrical signal indicative of the noise component present in at least two different points of the output spectrum of said amplifier.

10. The noise-compensating gain controller defined in claim 9, wherein said gain compensating circuit includes an optical power monitor for providing at least two separate electrical signals indicative of the strength of the noise component at two different points in the amplifier output spectrum, and a summation circuit for providing a signal proportional to the amplitude of the sum of said two separate electrical signals.

11. An ASE compensating gain controller for an erbium doped fiber amplifier, comprising: a gain detecting circuit for providing a signal indicative of a gain level of an output of said amplifier, wherein said output includes a signal component and a ASE component; a set point circuit for providing a signal indicative of a selected gain level of said amplifier; a gain compensating circuit for providing a signal indicative of relative strengths of said signal component and said ASE component of said output, respectively, and a digital processor circuit for receiving said gain level signal, selected gain level signal, and said relative strength signal for adjusting said gain level so that the gain of said signal component of said output is equal to said selected gain level.

12. The ASE compensating gain controller according to claim 11, wherein said gain detecting circuit includes first and second optical taps coupled to an input and said output of said amplifier.

13. The ASE compensating gain controller according to claim 12, wherein said gain detecting circuit includes first and second photodiodes for producing first and second electrical signals from light diverted from said first and second taps that are indicative of relative amplitudes of said amplifier input and output, respectively.

14. The ASE compensating gain controller according to claim 13, wherein said gain detecting circuit also includes first and second amplifiers for amplifying said first and second electrical signals.

15. The ASE compensating gain controller according to claim 11, wherein said gain compensating circuit includes an optical tap coupled to the amplifier output, a filter for separating the ASE component from the signal component in output light diverted by the pat, a photodiode for converting diverted light into an electrical signal, and an amplifier for amplifying said signal.

16. The ASE compensating gain controller according to claim 11, wherein said amplifier is a multi-channel optical amplifier, and said gain compensating circuit produces an electrical signal indicative of the ASE component present in at least two different points of the output spectrum of said amplifier.

17. The ASE compensating gain controller according to claim 16, wherein said gain compensating circuit includes an optical power monitor for providing at least two separate electrical signals indicative of the strength of the noise component at two different points in the amplifier output spectrum, and a summation circuit for providing a signal proportional to the amplitude of the sum of said two separate electrical signals.

18. The ASE compensating gain controller according to claim 11, further comprising a variable optical attenuator for removing tilt from said output of said amplifier.

19. The ASE compensating gain controller according to claim 11, further comprising a dynamic gain flattening filter for removing tilt from said output of said amplifier.

20. The ASE compensating gain controller according to claim 11 , wherein said amplifier includes first and second serially connected coils of erbium doped optical fiber, and wherein said gain detecting circuit provides a signal indicative of a gain level associated with said first coil of fiber, and further comprising a second gain detecting circuit for providing a signal indicative of a gain level associated with said second coil of fiber.