WO2001054237A1

WO2001054237A1 - Hybrid gain control on optical amplifiers

Info

Publication number: WO2001054237A1
Application number: PCT/US2000/032595
Authority: WO
Inventors: Gregory J. Cowle; Thomas W. Mcnamara; Chia C. Wang
Original assignee: Corning Incorporated
Priority date: 2000-01-21
Filing date: 2000-11-30
Publication date: 2001-07-26
Also published as: AU2001236350A1

Abstract

An apparatus is provided for controlling an optical amplifier. The apparatus includes an optical gain control laser cavity and an electronic control feedback loop. The optical gain control laser cavity controls the inversion of the optical amplifier. The electronic control feedback loop measures the power level in the optical gain control laser cavity and adjusts the amount of pump power provided to the amplifier according to changes in the power level. Since the inversion of the optical amplifier is controlled to within pre-selected limits by the optical gain control laser cavity and the power level in the optical gain control cavity is fixed by the electrical feedback control loop, the spectral-hole depth at the optical gain control laser wavelength is kept constant. As such, the power level from the amplifier is kept constant.

Description

HYBRID GAIN CONTROL ON OPTICAL AMPLIFIERS

BACKGROUND OF THE INVENTION 1. Technical Field

The present invention generally relates to optical amplifiers and, more particularly, to a gain control apparatus for an optical amplifier. 2. Discussion

Wavelength add/drop multiplexing systems require gain control to ensure that a near constant power level is provided by the system. In certain cases, the input channels delivered to the system change. This may occur, for example, when one of two channel sources stops functioning. In this case, the number of channels delivered to the system changes yet the power level provided by the system needs to be maintained. As such, gain control over the surviving channels is required.

One system in which the above scenario is particularly relevant is erbium- doped fiber amplifiers. Since erbium-doped fibers are largely homogeneous, optical systems have been successfully used for gain control. Optical gain control is advantageous because it is passive and self-contained. That is, the optical gain control is independent of gain ripple, input conditions, and pump powers.

Optical gain control typically involves the formation of a laser cavity about the amplifier. Due to the optical feedback of the optical gain control laser, the optical gain through the amplifier is equal to the optical loss in the cavity. As a result, once the cavity loss is fixed, the optical gain is controlled.

Unfortunately, erbium-doped fibers are not purely homogeneous. As such, optical gain control systems on erbium-doped fibers exhibit certain degrees of spectral hole burning. For example, when the power of the optical gain control laser is increased, such as by dropping channels, the spectral hole gets deeper.

This results in a reduction in the effective gain at the optical gain control laser wavelength. Consequently, the optical feedback of the optical gain control laser cavity increases the inversion of the erbium-doped fiber amplifier so that, at the bottom of the spectral hole, the optical gain equals the total optical loss experienced by the optical gain control laser. In other words, when the power of the optical gain control laser changes, the spectral hole depth changes. This causes gain errors in the surviving channels.

In view of the foregoing, it would be desirable to provide a gain control apparatus to reduce the gain error caused by spectral hole burning in an optically gain controlled erbium-doped fiber amplifier.

SUMMARY OF THE INVENTION

The above and other objects are provided by an apparatus for controlling an optical amplifier. The apparatus includes an optical gain control laser cavity and an electronic control feedback loop. The optical gain control laser cavity controls the inversion of the optical amplifier. The electronic control feedback loop measures the power level in the optical gain control laser cavity and adjusts the amount of pump power provided to the amplifier according to changes in the power level in the gain control cavity. Since the inversion of the optical amplifier is controlled to within pre-selected limits by the optical gain control laser cavity and the power level in the optical gain control cavity is fixed by the electrical feedback control loop, the spectral-hole depth at the optical gain control laser wavelength is kept constant. As such, the power level of the gain control cavity is kept constant.

In a preferred embodiment of the present invention, the optical gain control laser cavity includes a variable optical attenuator coupled to the optical amplifier by wavelength selective couplers. The electronic control feedback loop taps the optical gain control laser power which is monitored by a photodetector. A trans- impedance amplifier converts the optical signal from the photodetector into an electrical signal. An error circuit generates a control signal in response to the changing laser power. A pump providing power to the optical amplifier changes its output in response to the control signal.

BRIEF DESCRIPTION OF THE DRAWINGS In order to appreciate the manner in which the advantages and objects of the invention are obtained, a more particular description of the invention will be rendered by reference to specific embodiments thereof which are illustrated in the appended drawings. Understanding that these drawings only depict embodiments of the present invention and are not therefore to be considered limiting in scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which:

FIG. 1 is a schematic diagram of the hybrid gain control apparatus of the present invention;

FIG. 2 is a graph illustrating the measured output spectra of the amplifier in FIG. 1 with the optical gain control laser cavity open and closed;

FIG. 3 is a graph illustrating the gain error of the surviving channels measured with the feedback electronics of FIG. 1 enabled and disabled; FIG. 4 is a graph illustrating the output spectrum of the amplifier of FIG. 1 when fully loaded;

FIG 5 is a graph illustrating the output spectrum of the amplifier of FIG. 1 when there is only one surviving channel;

FIG. 6 is a graph illustrating the output spectrum of the amplifier of FIG. 1 with 25 surviving channels; and

FIG. 7 is a graph illustrating the output spectrum of the amplifier of FIG. 1 when it is loaded with three channels.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention is directed towards a hybrid gain control apparatus for an optical amplifier. In accordance with the teachings of the present invention, an optical gain control laser cavity is employed to fix the inversion of the amplifier. An electronic control loop is disposed between the optical gain control laser cavity and the amplifier for monitoring power changes in the cavity and changing a level of power supplied to the amplifier in accordance therewith. In this way, steady state gain error caused by spectral hole burning is reduced.

Turning now to the drawing figures, FIG. 1 illustrates a hybrid gain control apparatus 10 according to the present invention. The apparatus 10 includes an amplifier 12 disposed along a fiber 14 extending between a multiwavelength laser source 16 (such as WDM transmitter, for example) and a laser destination 18. The amplifier 12 is preferably an erbium-doped fiber amplifier although other homogeneous or in-homogeneous amplifiers may substitute therefore. An optical gain control laser cavity 20 is formed about the amplifier 12.

That is, the gain control cavity 12 surrounds the amplifier. An electronic control feedback loop 22 is coupled between the optical gain control laser cavity 20 and the amplifier 12. The optical gain control laser cavity 20 includes a first wavelength selective coupler 24 coupled to the fiber 14 downstream of the amplifier 12. The first wavelength selective coupler 24 diverts a pre-selected wavelength from the amplifier 12 through the optical gain control laser cavity 20. This is the so called optical gain control laser wavelength.

The optical gain control laser cavity 20 includes a variable optical attenuator 26 disposed downstream of the first wavelength selective coupler 24. The variable optical attenuator 26 fixes the total optical loss within the optical gain control laser cavity 20. As such, the inversion of the erbium-doped fiber amplifier 12 is controlled as long as the laser source 16 is above the threshold of the variable optical attenuator 26. A second wavelength selective coupler 28 is interposed between the variable optical attenuator 26 and the amplifier 12 along the fiber 14. The second wavelength selective coupler 28 feeds the optical gain control laser wavelength back to the amplifier 12. To control the direction of the radiation through the optical gain control laser cavity 20, an isolator 30 is coupled to the fiber 14 between the amplifier 12 and the first wavelength selective coupler 24. In the illustrated embodiment, the radiation circulates clockwise.

The electronic control feedback loop 22 is coupled to the optical gain control laser cavity 20 by a tap 32. The tap 32 is also coupled to control electronics 34. The control electronics 34 measure the power level in the optical gain control laser cavity 20 at the optical gain control laser wavelength. The control electronics 34 also generate a control signal corresponding to the measured power level. The control signal is used to keep the laser power constant. To accomplish the foregoing, the control electronics 34 preferably include a photodetector for detecting the laser power, a trans-impedance amplifier for converting the optical signal from the photodetector to an electrical signal, and an error circuit for generating a control signal according to the electrical signal.

The control electronics 34 are electrically connected to a pump 36. The pump 36 is coupled to the amplifier 12. The pump 36 provides different levels of power to the amplifier 12 according to the control signal from the control electronics 34. As stated above, the control signal corresponds to the power level in the optical gain control laser cavity 20. Preferably, a wavelength division multiplexer 38 is interposed between the pump 36 and the amplifier 12 along the fiber 14. In operation, radiation flows clockwise through the optical gain control laser cavity 20 due to the presence of the isolator 30. A pre-selected wavelength is divided out by the first wavelength selective coupler 24 and diverted through the cavity 20. The cavity 20 is tapped by the tap 32 and the power level therein is monitored by the control electronics 34.

When the power level through the cavity 20 increases or decreases, for example, due to signal channels in the amplifier 12 being dropped or added, the control electronics 34 detect the change and generate a control signal. The control signal is delivered from the control electronics 34 to the pump 36. The pump then adjusts the power delivered to the amplifier 12 to keep the laser power level in the optical gain control cavity 20 constant.

Simultaneously with the above, the value of the variable optical attenuator 26 is set in order to fix the total optical loss in the laser cavity 20. As a result, the inversion of the erbium-doped fiber amplifier 12 is kept within known limits. Because the inversion of the amplifier 12 is essentially fixed by the optical gain control laser cavity 20 and the laser power output by the amplifier 12 is fixed by the electronic control feedback loop 22, the spectral hole depth at the optical gain control wavelength is kept constant. As a result, the inversion change of the erbium-doped fiber amplifier 12 caused by the spectral hole burning is reduced. If the control electronics 34 are fast enough, the relaxation oscillations of the laser source 16 can also be reduced.

Referring now to FIG. 2, to demonstrate the performance of the hybrid gain control apparatus of the present invention, an erbium-doped fiber amplifier was built according to the configuration shown in FIG. 1. The laser wavelength was about 1527 nm. When the feedback control electronics were enabled, the laser power in the gain control cavity 20 was kept constant. The gain spectra of the amplifier were measured with 32 channels and a power of about -18.5 dBm per channel. A 980-nm laser diode was used for the pump. As illustrated in FIG. 2, this amplifier has an average gain of about 18 dB when it is fully loaded with 32 channels. With the optical gain control laser cavity open and the feedback electronics disabled, the amplifier required a pump power of 102 mW to achieve the gain spectrum shown in FIG. 2. The pump power requirement was increased to 106 mW when the optical gain control laser cavity was closed and the laser was running. This pump power difference shows that the pump power penalty for having the optical gain control laser cavity on the amplifier to be only 0.2 dB.

Turning now to FIG. 3, to compare the performance of the hybrid gain control apparatus of the present invention to optical gain control only, the gain error of the surviving channels in channel-drop events was measured with the feedback electronics enabled and disabled. The gain error of each surviving channel when all other 31 channels were dropped was measured under these two conditions and is shown in FIG. 3. The surviving channel gain error is defined to be the deviation from the gain spectrum shown by the solid triangles in FIG. 2. The maximum gain error occurred when all other channels were dropped. When the feedback electronics were disabled and the amplifier had only optical gain control through the laser cavity, the 1532.7 nm channel had a 2.2 dB gain error with all other 31 channels dropped and a pump power of 106 mW as shown in FIG. 3. With the feedback electronics enabled and the laser power kept constant through the method of the present invention, the maximum gain error was reduced to about 0.7 dB, as shown in FIG. 3.

Referring now to FIG. 4, the gain error of the surviving channels in the apparatus of the present invention is limited by the unavoidable signal spectral hole burning rather than by the apparatus itself. FIG 4. shows the output of the amplifier when it is fully loaded with 32 channels and both the feedback electronics and optical gain control of the present invention enabled. The 1532.7 nm channel has an output power of 1.97 dBm and the laser has an output power of about -6 dBm. Turning now to FIG. 5, when 31 out of 32 channels are dropped, the output power of the only surviving channel (the 1532.7 nm channel) increased to 2.62 dBm. This results in a 0.65 dB gain error which is within the measurement accuracy shown in FIG. 3. Further, as shown in FIGS. 6 and 7, the spectral hole burning of adjacent channels plays an important role in the gain error of the surviving channel. As compared to the loading condition in FIG. 7, the loading condition in FIG. 6 is much closer to the fully loaded case illustrated in FIG. 4. However, in FIG. 6, the 1532.7 nm channel has an output power of 2.56 dBm or a gain error of 0.59 dB which is close to the results shown in FIG. 5. On the other hand, in FIG. 7, there are only three surviving channels yet the 1532.7 nm channel has an output power of 1.95 dBm, which is very close to the 1.97 dBm output power from the fully loaded condition shown in FIG. 4.

The counter intuitive result between FIG. 6 and FIG. 7 is due to the signal spectral hole burning. That is, each adjacent channel of the 1532.7 nm channel bums a spectral hole around the channel wavelength. The overlapping of these spectral holes results in a lower effective gain (or a spectral hole) at 1532.7 nm.

As a result, when the adjacent channels of the 1532.7 nm appear, the optical gain of the 1532.7 nm channel reduces.

Referring again to FIG. 6, although most of the channels are present, all of the adjacent channels of the 1532.7 nm channel are dropped. The 1532.7 nm channel is then unaffected by the spectral hole burning of its adjacent channels.

It therefore has a higher effective optical gain to cause significant gain error. On the other hand, as shown in FIG. 7, most adjacent channels of the 1532.7 nm channel are present to bum overlapping spectral holes. As a result, the effective gain of the 1532.7 nm channel is reduced and the output power is close to that measured with the fully loaded condition illustrated in FIG. 4. It is worth noting that the leakage power of the laser bypassing the output wavelength selective coupler is kept to about -6 dBm. This indicates that the feedback electronics are functioning properly. Thus, the present invention utilizes a hybrid gain control apparatus including an optical gain control laser cavity and feedback electronics to keep laser power in the gain control cavity 20 constant. Since amplifier inversion is essentially fixed by the optical gain control laser cavity and laser power is fixed by feedback electronics, the spectral hole burned by the laser remains constant. As such, the gain error caused by the laser spectral hole burning is greatly reduced.

Those skilled in the art can now appreciate from the foregoing description that the broad teachings of the present invention can be implemented in a variety of forms. For example, while it is preferred to have the control electronics adjust the pump power of the optical amplifier to keep the laser power constant, the control electronics could adjust the loss of the laser cavity to keep the laser power constant. Therefore, while this invention has been described in connection with particular examples thereof, the true scope of the invention should not be so limited since other modifications will become apparent to the skilled practitioner upon a study of the drawings, specification, and following claims.

Claims

CLAIMSWhat is claimed is:

1. An apparatus for controlling an amplifier comprising: a gain control cavity for controlling a level of inversion of the amplifier; and an electronic control feedback loop coupled between the gain control cavity and the amplifier for monitoring power changes in the gain control cavity and adjusting power provided to the amplifier according to the power changes in the gain control cavity such that a power level of the gain control cavity is kept constant.

2. The apparatus of Claim 1 further comprising a variable attenuator disposed along the gain control cavity for fixing an amount of loss within the gain control cavity.

3. The apparatus of Claim 1 further comprising a wavelength selective coupler interposed between the amplifier and the gain control cavity for diverting a pre-selected wavelength from the amplifier through the gain control cavity.

4. The apparatus of Claim 3 further comprising an isolator interposed between the amplifier and the wavelength selective coupler for dictating a direction of circulation through the gain control cavity.

5. The apparatus of Claim 1 , 2, or 4 further comprising a tap interconnecting the electronic control feedback loop and the gain control cavity for enabling monitoring of a power level in the gain control cavity by the electronic control feedback loop.

6. The apparatus of Claim 1 , 2, 3, 4, or 5 further comprising a photodetector disposed along the electronic control feedback loop and coupled to the gain control cavity for detecting a power level in the gain control cavity.

7. The apparatus of Claim 6 further comprising a trans-impedance amplifier disposed along the electronic control feedback loop and coupled to the photodetector for converting an optical signal from the photodetector to an electrical signal, and an error circuit coupled to the trans-impedance amplifier for generating a control signal according to the electrical signal for keeping a power level of the amplifier constant.

8. The apparatus of Claim 7 further comprising a pump disposed along the electronic control feedback loop and coupled to the amplifier for providing different levels of power to the amplifier according to the control signal from the error circuit.

9. The apparatus of Claim 8 further comprising a wavelength division multiplexer interposed between the pump and the amplifier along the gain control cavity for reintroducing radiation from the gain control cavity to the amplifier.

10. A method of reducing inversion change in an amplifier comprising: controlling an inversion of the amplifier to within pre-selected limits with a gain control cavity; and fixing a power level in the gain control cavity with an electronic feedback control loop coupled to the gain control cavity and the amplifier.

11. The method of Claim 10 wherein the controlling step further comprises applying a variable attenuator to an output of the amplifier diverted through the gain control cavity and returned to the amplifier.

12. The method of Claim 10 or 11 wherein the fixing step further comprises: monitoring the power level in the gain control cavity; and adjusting a power input to the amplifier according to changes in the power level in the gain control cavity.

13. The method of Claim 12 wherein the monitoring step further comprises detecting the power level in the gain control cavity with a photodetector.

14. The method of Claim 12 wherein the adjusting step further comprises: generating a control signal according to the changes in the power level in the gain control cavity; sending the control signal to a pump coupled to the amplifier; and varying a power output of the pump according to the control signal.

15. A hybrid gain control apparatus for reducing inversion change in an erbium-doped fiber amplifier due to spectral hole burning comprising: an optical gain control laser cavity coupled to the amplifier, the cavity including: a first wavelength selective coupler disposed downstream of the amplifier; a variable optical attenuator disposed downstream of the first wavelength selective coupler; and a second wavelength selective coupler disposed downstream of the variable optical attenuator and upstream of the amplifier; and an electronic control feedback loop coupled to the amplifier and the cavity, the feedback loop including: a tap coupled to the cavity downstream of the first wavelength selective coupler and upstream of the variable optical attenuator; control electronics disposed downstream of the tap for monitoring a power level in the cavity and generating a control signal according to the power level; and a pump electrically coupled to the control electronics and disposed upstream of the amplifier, the pump supplying power to the amplifier according to the control signal.

16. The apparatus of Claim 15 wherein the control electronics further comprise: a photodetector coupled to the tap; a trans-impedance amplifier coupled to the photodetector; and an error circuit coupled to the trans-impedance amplifier and communicating with the pump.

17. The apparatus of Claim 15 or 16 further comprising an isolator disposed downstream of the amplifier and upstream of the first wavelength selective coupler.

18. The apparatus of Claim 15,16 or 17 further comprising a wavelength division multiplexer interposed between the pump and the amplifier downstream of the second wavelength selective coupler.

19. An apparatus for controlling an amplifier comprising: a gain control cavity for controlling a level of inversion of the amplifier; and an electronic control feedback loop coupled between the gain control cavity and the amplifier for monitoring power changes in the gain control cavity and adjusting power provided to the amplifier according to the power changes in the gain control cavity such that gain spectrum of the amplifier is kept constant.