Method and apparatus for amplified spontaneous emission corrected automatic signal power control of an optical amplifier

An optical amplifier system and controller and a method for automatically controlling the usable signal power of an optical amplifier are provided. The method differentiates between the total optical power that includes the amplified spontaneous emission (ASE), and the useful amplified optical signal power at the output of the amplifier. The optical amplifier system comprises an optical amplifier, a first and a second photodetector operable to measure the power of the input and output signals of the optical amplifier and an amplification controller with a control input. The amplification controller is operable to compensate for the ASE power when operating in automatic signal power control mode.

FIELD OF THE INVENTION

The invention pertains to the field of optical amplifiers, and more particularly to the field of automatic signal power control of optical amplifiers.

BACKGROUND OF THE INVENTION

For many applications, for example in optical pre-amplifiers, it is desirable to operate an optical amplifier in a mode of operation such that either the output optical power of the amplifier or the gain of the optical amplifier is maintained at a constant level. These modes of operation are referred to as constant output power mode and constant gain mode respectively.

The total optical output power of an amplifier consists of both amplified signal power and amplified spontaneous emission (ASE) power. Existing amplifiers rely on automatic power control (APC) to maintain the total optical power of the amplifier at a constant level. Therefore, existing APC implementations are unable to determine the usable optical signal power and optical network engineers are forced to allow additional margins in their optical power-budget design or operate the amplifier in automatic gain control (AGC) where an ASE correction is easier to implement.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, there is provided a method comprising: measuring power of an input optical signal; determining a target usable signal power at the output of the amplifier; amplifying the input optical signal to produce an amplified optical signal comprising an amplified input optical signal and amplified spontaneous emission (ASE); determining an estimate of the ASE power in the amplified optical signal; and controlling the amplifying such that the amplified input optical signal has a power substantially equal to the target usable signal power.

According to another aspect of the present invention, there is provided an optical amplifier system with an optical input and an optical output and a control input comprising: an optical amplifier in a signal path between the optical input and the optical output, operable to amplify an input optical signal to produce an amplified optical signal comprising an amplified input optical signal and amplified spontaneous emission (ASE); a first photodetector operable to measure power of the input optical signal at the optical input; a second photodetector operable to measure power of the amplified optical signal at the optical output; an amplification controller functionally connected to the optical amplifier, the control input and the first and second photodetectors, operable to: determine a target usable signal power; determine an estimate of ASE power in the amplified optical signal; and control the optical amplifier such that the amplified input optical signal at the output of the amplifier has a power substantially equal to the target usable signal power.

According to still another aspect of the present invention, there is provided a method for controlling an optical amplifier comprising: determining target usable signal power; estimating ASE power in an amplified optical signal comprising an amplified input optical signal and ASE as a function of an input optical signal power and the target usable signal power; and controlling at least one control signal such that the amplified input optical signal at the output of the amplifier has a power substantially equal to the target usable signal power.

According to yet another aspect of the present invention, there is provided a computer readable medium having recorded thereon statements and instructions for execution by a computer to carry out the method for controlling an optical amplifier comprising: determining target usable signal power; estimating ASE power in an amplified optical signal comprising an amplified input optical signal and ASE as a function of an input optical signal power and the target usable signal power; and controlling at least one control signal such that the amplified input optical signal has a power substantially equal to the target usable signal power.

According to a further aspect of the present invention, there is provided a controller comprising: a first input operable to receive a measurement of power of an input optical signal of at least one optical amplifier operable to amplify the input optical signal to produce an amplified optical signal comprising an amplified input optical signal and amplified spontaneous emission (ASE); a second input operable to receive a measurement of power of the amplified optical signal of the at least one optical amplifier; control logic operable to: determine a target usable signal power; determine an estimate of ASE power in the amplified optical signal; and provide at least one control signal operable to adjust a gain of the at least one optical amplifier such that the amplified input optical signal has a power substantially equal to the target usable signal power; and at least one control signal output operable to output the at least one control signal.

Other aspects and features of the present invention will become apparent, to those ordinarily skilled in the art, upon review of the following description of the specific embodiments of the invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS OF THE INVENTION

A system designer who wishes to operate an optical amplifier with APC is generally interested in knowing the useful signal power, not the total power. Unfortunately, existing APC implementations fail to compensate for the ASE power. Because of this, existing APC solutions are of limited value to system designers.

Various methods and devices to perform amplified spontaneous emissions (ASE) corrected automatic signal power control (ASPC) in an optical amplifier are provided. The methods and devices for doing this make use of measured input and output power levels, and calculate the ASE power for a target output signal power level. A gain of the optical amplifier can then be automatically adjusted in order to keep the output signal power at the target output signal power level regardless of the presence of ASE. The type of optical amplifier that might be used is an implementation specific detail and might include doped fiber amplifiers, such as erbium doped fiber amplifiers, as well as semiconductor optical amplifiers.

The systems and devices described below have the advantage that they compensate for ASE when performing ASPC.

FIG. 1is a block diagram of an example of an optical amplifier system40in accordance with an embodiment of the invention in which optical amplification is controlled by an amplification controller38that compensates for amplified spontaneous emissions (ASE) while performing automatic signal power control (ASPC). The optical amplification system40has an input11and an output15. The input11is connected to an input of a first optical tap12. The first optical tap12has a first output and a second output; the first output is connected to a first input of an optical amplifier (OA)50, while the second output is connected to a first photodetector (PD)26. The first PD26receives an optical signal from the second output of tap12, which is proportional to the optical power at the input11of the optical amplifier40. The optical amplifier50has a second input that is functionally connected to an amplification controller38via control signals52. The control signals52may comprise one or more signals. The optical amplifier has an output that is connected to the input of a second optical tap24. The second optical tap24has a first output and a second output; the first output of the second optical tap24is connected to the output15of the optical amplifier system40, while the second output of the second optical tap24is connected to the input of a second photodetector32. The second PD32receives an optical signal from the second output of tap24, which is proportional to the optical power at the output15of the optical amplifier40. The outputs of the first and second photodetectors26and32are functionally connected to first and second inputs of the amplification controller38, which also has a control input42.

In some implementations, the amplification controller38might be implemented as an application specific integrated circuit (ASIC) or in a logic device such as a field programmable gate array (FPGA) or a programmable logic device (PLD). In general, an amplification controller might be implemented as hardware, software, firmware or combinations thereof, which are capable of implementing control logic.

While the first optical tap12and the first photodetector16are shown as separate components, they may be provided as a single component, for example a tap-type photodetector. The same is true of the second optical tap24and the second optical detector32.

The optical amplifier50is any type of optical amplifier that generates ASE, for example a doped fiber amplifier, such as an erbium doped fiber amplifier, a Raman amplifier or a semiconductor optical amplifier.

In operation, an optical signal that is to be amplified is applied to the input11of optical amplifier system40. The first optical tap12splits the input optical signal into a first signal and a second signal and passes the second signal to the first photodetector26and the first signal to the optical amplifier50. The tap ratio of the first optical tap12, which is the ratio between the first signal and the second signal, is an implementation specific detail. The first optical tap12might, for example, split the optical input signal such that substantially all of the optical input signal is passed to the first input of the optical amplifier50, while very little of the optical input signal is passed to the first photodetector26. The first photodetector26measures the second signal that is proportional to the input signal to the amplifier, and transmits the measurement information to the amplification controller38. The optical amplifier50amplifies the first signal from the first optical tap12and also generates ASE power. The output from the optical amplifier50, which comprises the amplified first signal and the ASE power, is then split by the second optical tap24into a third signal and a fourth signal. The second optical tap24passes the fourth signal to the second photodetector32and the third signal to the output15of the amplifier system40. The second photodetector32measures the fourth signal that is proportional to the output optical power of the amplifier, and transmits the measurement information to the amplification controller38. Like the tap ratio of the first optical tap12, the tap ratio of the second optical tap24is an implementation specific detail. The second optical tap24might for example split the output of the optical amplifier50such that substantially all of the output of the optical amplifier50is passed to the output15of the optical amplifier system40, while very little of the optical input signal is passed to the second photodetector32. For example, in some embodiments, the tap ratios of the first optical tap12and the second optical tap24might both be 90:10.

The operation of the amplification controller38is setup through the control input42. The control logic of the amplification controller38utilizes the measurement information from the first photodetector26and the second photodetector32to control the optical amplifier50via control signals52. In some embodiments, the optical amplifier may be a dynamic gain-flattened amplifier with one or more variable optical attenuators, and the control signals52may comprise signals for controlling the attenuation of the one or more variable optical attenuators. Other examples of control signals include inputs for controlling laser diodes operating as EDFA pumps. In general, the control signals52may comprise any signals that adjust the output signal of the optical amplifier50. As stated above, the output of the optical amplifier50, and thus the output15of the optical amplifier system40, comprises a desired output signal, comprising the amplified first signal, and an ASE component. Using operating characteristics of the optical amplifier50, which can be determined during a calibration, the power of the ASE component of the output15is calculated and the gain of optical amplifier50is controlled such that the output15of the optical amplifier system40is maintained at a power level equal to the sum of a target output signal power and the calculated power of the ASE component; thereby carrying out ASE compensated ASPC.

In some implementations, the power of the ASE component of the output15is calculated as a function of the gain of the optical amplifier50, wavelength and input signal power. The calibration process is an implementation specific detail and examples of calibration processes for specific embodiments will be given later.

In some implementations, the control input42is operable to interface with a user interface (UI) device, such as a computer with a graphical user interface (GUI) that allows a user to configure, monitor and control the optical amplifier system40with the amplification controller38. A mouse pointer or a touch screen may be used to interact with the information that is displayed on the UI device, for example, turning the ASE compensated ASPC mode of the amplifier system40on or off and displaying the operating characteristics of the amplifier system40, such as the signal wavelength λs, the effective gain, the true gain, the target usable signal power St, the measured input signal power Sin, the output power Pout, the ASE calibration constants, which are described below with reference to the methods shown inFIGS. 3,4and5, and the calculated ASE power. In other embodiments the UI device may be a wireless handheld computer. More generally, the optical amplifier system40can be configured, monitored and controlled by any UI device with the capability to communicate with the amplification controller38via the control input42and the capability to allow a user to input data and to interact with the data that is displayed.

An example of a method for controlling an optical amplifier, which might for example be recorded as statements and instructions for execution by a computer on a computer readable medium and/or implemented as the control logic in the amplification controller38shown inFIG. 1, will now be described with reference toFIG. 8. The method begins at step8-1, in which a target usable signal power is determined. In step8-2an estimate of ASE power in an amplified optical signal comprising an amplified input optical signal and ASE is determined as a function of the power of the input optical signal and the target usable signal power. In step8-3a control signal is controlled such that the amplified input optical signal has a power substantially equal to the target usable signal power.

An example of a method for ASE compensated ASPC for an optical amplifier will now be described with reference toFIG. 2. The method might for example be implemented in the optical amplifier system40shown inFIG. 1. The method begins at step2-1, in which the input optical signal power of the amplifier system is measured. In step2-2a target usable signal power is set. In step2-3the input optical signal is amplified by the optical amplifier to produce an amplified input optical signal and ASE. In step2-4an estimate of the ASE power in the amplified optical signal is determined based on the target usable signal power. In step2-5the amplification of the optical amplifier is controlled such that the amplified input optical signal has a power substantially equal to the target usable signal power.

In general, the steps of the method shown inFIG. 2may be re-ordered or new method steps added such that amplification of an optical amplifier system is automatically controlled to compensate for ASE as a function of the measured input and output optical signal power. For example, to operate an optical amplifier in a constant output signal power mode; the target usable signal power may be specified as a first step of the method. The input signal power may be measured as a second step and the gain of the optical amplifier determined in a third step based on the input signal power and the target usable signal power. The method might then continue with steps2-3to2-5.

In some implementations, the ASE power is calculated as a function of the measured input and output signal power and signal wavelength.

In some implementations the method shown inFIG. 2might further comprise a calibration step to determine ASE calibration constants.

Specific examples of the method shown inFIG. 2operable to perform ASE compensated ASPC of specific types of optical amplifiers will now be given.

Static Flat-Gain Optical Amplifier

A static flat-gain optical amplifier has flat gain as a function of wavelength (λ) only at a designed-flat-gain level. For any other gain level, the Gain-versus-λ curve will tilt. In general the flat gain at the designed-flat-gain level is achieved through the use of a GFF. A relationship between ASE power and gain in a static flat-gain EDFA is given by:
PASE(λs)=(G/Gflat)0.025(λs−λc)[A1G2+A2G+A3St]  (1)
where A1, A2and A3are ASE calibration constants, λcis the wavelength at which the EDFA is calibrated; for example, λcmight be 1546 nm for a C-band EDFA, λsis the signal wavelength, Stis the target usable signal power, Gflatis the flat gain level and G is the gain of the EDFA as a function of λs. The ASE calibration constants for the amplifier must first be determined by carrying out a calibration. In some embodiments, the calibration process comprises setting the gain of the amplifier that is to be calibrated to the flat gain level Gflatand measuring the output optical power of the amplifier at a first wavelength λs, such that λsis equal to λc. Second and third measurements at the first wavelength can be taken for second and third levels of gain and the first, second and third measurements can be used to solve for the three ASE calibration constants A1, A2and A3in equation (1) above.
Static Flat-Gain Optical Amplifier: Single-Channel Operation at Constant Output Signal Power Mode

An example of a method for ASE compensated ASPC for a flat-gain EDFA in constant output signal power mode will now be described with reference toFIG. 3. The method might for example be implemented in the amplification controller38that is part of the optical amplifier system40. The method begins at step3-1, in which operating parameters comprising a target usable signal power St and a signal wavelength λsare provided. In step3-2the input signal power Sinis dynamically measured. In step3-3additional operating parameters comprising the flat gain level Gflat, the calibration wavelength λcand the ASE calibration constants A1, A2and A3are retrieved. In step3-4the desired gain G is calculated from the input signal power Sin and the target usable signal power St, such that
G=St/Sin(2)
and then the ASE power PASEis calculated using equation (1). In step3-5the target output power Pt, which is the sum of the target usable signal power Stand the calculated ASE power, is calculated. In step3-6the gain of the amplifier is adjusted such that the total output power Poutof the amplifier is equal to the target output power Ptand the method returns to step3-2, which sets up a feedback control loop that maintains the output power Poutat the target output power Pt; thereby performing ASE compensated ASPC in constant output signal power mode.
Static Flat-Gain Optical Amplifier: Multi-Channel Operation at Constant Output Signal Power Mode

In some embodiments, the input signal is a multi-channel signal, with a different wavelength for each channel. In these embodiments, an average of the different wavelengths may be used as the λsterm when calculating the ASE power in step3-4of the above-described method. For a multi-channel EDFA, with different wavelength λ for each channel, the method shown inFIG. 3will maintain the composite output signal power of a static-gain EDFA at a constant level. Constant channel signal output power can be achieved by using a dynamic gain EDFA and a method that will be described later.

Static Flat-Gain Optical Amplifier: Single or Multiple Channel Operation at Constant Gain Mode

The method shown inFIG. 3and described above can easily be modified to perform ASE compensated ASPC for a static-gain EDFA in constant gain mode. The only changes that have to be made to the method are rather than providing the target usable signal power Stin step3-1, the desired gain G is provided and rather than calculating G in step3-4, the target usable signal power Stis calculated from the input signal power Sinand the desired gain G, such that
St=GSin(3)
The remaining steps of the method remain unchanged.
Single-Channel Optical Amplifier

A single-channel optical amplifier has no gain-flattening filter, so that gain and ASE power are a function of wavelength in any operation case. A relationship between ASE power and gain in a single channel EDFA is
PASE(λs)=(GR(λs))0.025(λs−λc)[A1(GR(λs))2+A2GR(λs)+A3St]  (4)
GR(λs)=G(λs)/Gc(λs)  (5)
where A1, A2and A3are the ASE calibration constants, λcis the wavelength at which the EDFA is calibrated, for example, λcmight be 1546 nm for a C-band EDFA, λsis the signal wavelength, Stis the target output signal power, Gc(λs) is a gain curve as a function of wavelength measured with the average gain value close to which the EDFA will generally be operated and G(λs) is the gain of the EDFA as a function of wavelength. The calibration process for the single-channel EDFA is the same as the calibration process described above for the static flat-gain EDFA with the exception of first determining Gc(λs) by setting the gain level at the calibration wavelength λcequal to the expected average general operating level and measuring the gain as a function of wavelength.
Single-Channel Optical Amplifier—Constant Output Signal Power Mode

An example of a method for ASE compensated ASPC for a single-channel EDFA will now be described with reference toFIG. 4. The method might for example be implemented in the amplification controller38that is part of the optical amplifier system40. The method begins at step4-1, in which operating parameters comprising a target usable signal power Stand a signal wavelength λsare provided. In step4-2, the input signal power Sinis dynamically measured. In step4-3additional operating parameters comprising the calibration gain curve Gc(λs), the calibration wavelength λcand the ASE calibration constants A1, A2and A3are retrieved. In step4-4the desired gain G(λs) is calculated from the input signal power Sinand the target usable signal power St, such that
G(λs)=St/Sin(λs)  (6)
and then the ASE power PASEis calculated using equations (4) and (5). In step4-5the target output power Pt, which is the sum of the target usable signal power Stand the calculated ASE power, is calculated. In step4-6the gain of the amplifier is adjusted such that the total output power Poutof the amplifier is equal to the target output power Ptand the method returns to step4-2, which sets up a feedback control loop that maintains the output power Poutat the target output power Pt; thereby performing ASE compensated ASPC in constant output signal power mode.
Single-Channel Optical Amplifier—Constant Gain Mode

The method shown inFIG. 4and described above can easily be modified to perform ASE compensated ASPC for a single-channel EDFA in constant gain mode. The only changes that have to be made to the method are rather than providing the target usable signal power St in step4-1, the desired gain G(λs) is provided and rather than calculating G(λs) in step4-4, the target usable signal power St is calculated from the input signal power Sin and the desired gain G(λs), such that
St=G(λs)Sin(7)
The remaining steps of the method remain unchanged.
Dynamic Flat-gain Optical Amplifier

In a dynamic flat-gain optical amplifier, the true gain of the amplifying element, such as the true gain of a length of erbium doped fiber, is maintained at a constant level and the effective gain is adjusted with a variable optical attenuator. Because the true gain of the amplifying element is kept constant, gain-tilt does not occur. In this case, the ASE power is independent of signal wavelength and equation (1) simplifies to:
PASE=[A1G2+A2G+A3St]  (8)

In the case of a dynamic flat-gain optical amplifier, the ASE calibration constants can be determined by making at least three measurements of the ASE power at different effective gain levels at the same input signal power in order to solve for the three calibration constants A1, A2and A3.

Dynamic Flat-gain Optical Amplifier—Constant Output Signal Power Mode

An example of a method for ASE compensated ASPC for a dynamic-gain EDFA will now be described with reference toFIG. 5. The method might for example be implemented in the amplification controller38that is part of the optical amplifier system40if a VOA that was controlled by the amplification controller38was included in the signal path between the input and the output of the optical amplifier system40and the true gain of the first and second length of optical fiber24and26was held constant. The method begins at step5-1, in which operating parameters comprising a target usable signal power Stand a signal wavelength λsare provided. At step5-2, the input signal power Sinis dynamically measured. In step5-3additional operating parameters comprising the ASE calibration constants A1, A2and A3are retrieved. In step5-4the desired gain G is calculated from the input signal power Sin and the target usable signal power St, such that
G=St/Sin(9)
and then the ASE power PASEis calculated using equation (8). In step5-5the target output power Pt, which is the sum of the target usable signal power Stand the calculated ASE power, is calculated. In step5-6the effective gain of the amplifier is adjusted by controlling the attenuation of one or more VOAs such that the total output power Poutof the amplifier is equal to the target output power Ptand the method returns to step5-2, which sets up a feedback control loop that maintains the output power Poutat the target output power Pt; thereby performing ASE compensated ASPC in constant signal output power mode.

Since the effective gain in a dynamic flat-gain EDFA is flat with respect to wavelength, if the input signal to the amplifier comprises a multi-channel input signal, the ASE compensated ASPC of a dynamic flat-gain EDFA in constant signal output power mode described above will maintain each individual channel at a constant output signal power while maintaining the composite signal power constant.

In some implementations, the signal wavelength λs is not provided in step5-1, as the ASE power is flat for a dynamic-gain amplifier.

Dynamic Flat-gain Optical Amplifier—Constant Gain Mode

The method shown inFIG. 5and described above can easily be modified to perform ASE compensated ASPC for a dynamic-gain EDFA in constant gain mode. The only changes that have to be made to the method are rather than providing the target usable signal power Stin step5-1, the desired gain G is provided and rather than calculating G in step5-4, the target usable signal power Stis calculated from the input signal power Sinand the desired gain G, such that
St=GSin(10)

The remaining steps of the method remain unchanged.

In some implementations, the methods shown inFIGS. 3,4and5might be used with optical amplifiers other than an EDFA, for example a semiconductor optical amplifier or other doped fiber amplifiers.

FIG. 6shows two curves that represent the measured ASE power of an EDFA at two different output signal powers but with the same gain level. The two curves illustrate that the ASE power is dependent on input signal power. In some embodiments, the dependence of ASE power on signal power can be ignored, which means that the A3Stterm in equation (1) can be replaced with a calibration constant A3′, such that
PASE(λs)=(G/Gflat)0.025(λs−λc)[A1G2+A2G+A3′]  (11)

In some embodiments, the signal power dependent A3Stterm in equations (4) and (8) might also be replaced with the signal power independent term A3′.

While the above examples are directed to ASE compensated automatic signal power control of an optical amplifier, the calculation of ASE power based on a measured input signal power and a target signal power might also be employed in monitoring the noise figure of an optical amplifier. An example of an equation for estimating the noise figure of an optical amplifier as a function of ASE power and gain is as follows:
NF=PASE/(GNg)  (12)
where
G=St/Sin(13)
and NF is the noise figure of an optical amplifier, PASEis the calculated ASE power, G is the gain of the optical amplifier, Stis the target usable signal power, Sinis the input signal power and Ngis the quantum noise measured in a 0.1 nm resolution bandwidth, which is equal to 16×10−10watt. Therefore, determining the ASE power and gain of an optical amplifier according to the methods of the present invention also provides for determining the noise figure of the optical amplifier.

Two specific examples of implementations of the optical amplifier system shown inFIG. 1will now be given with reference toFIGS. 7A and 7B, in which the optical amplifier50is a double-pump EDFA. A double-pump EDFA is a two-stage amplifier in which each stage includes a length of erbium doped fiber that is pumped with pump laser light to provide optical amplification.

FIG. 7Ais a block diagram of an example of an amplifier system40in accordance with an embodiment of the invention in which optical amplification of a double-pump erbium doped fiber amplifier is controlled by an amplification controller38that compensates for amplified spontaneous emissions (ASE) while performing automatic signal power control (ASPC). An input11of the optical amplifier system40is connected to an input of a first optical tap12. The first optical tap12has a first and a second output; the first output is connected to a first optical isolator14, while the second output is connected to a first photo detector (PD)26. The first optical isolator14has a single output that is connected to a first input of a first optical coupler16. A second input of the first optical coupler16is connected to the output of a first laser diode (LD)28. The first optical coupler16has a single output that is connected to the input of a first length of erbium doped fiber (EDF)34. The first length of erbium doped fiber34has a single output that is connected to the input of a second optical isolator18. The second optical isolator18has a single output that is connected to a first input of a second optical coupler20. A second input of the optical coupler20is connected to the output of a second laser diode30. The optical coupler20has a single output that is connected to the input of a second length of erbium doped fiber36. The second length of erbium doped fiber36is connected to the input of a third optical isolator22. The third optical isolator22has a single output that is connected to the input of a second optical tap24. The second optical tap24has a first and a second output; the first output of the second optical tap24is connected to the output15of optical amplifier system40, while the second output of the second optical tap24is connected to the input of a second photodetector32. The outputs of the first and second photodetectors26and32are functionally connected to first and second inputs of an amplification controller38. The amplification controller38has first and second outputs, which are functionally connected to the inputs of the first and second laser diodes28and30respectively. The amplification controller38also has a control input42.

In some implementations, one or more wavelength dependent attenuators might be used to maintain a flat gain characteristic with respect to wavelength in the case where the optical amplifier does not have a flat gain characteristic with respect to wavelength. In some implementations, a GFF will only be sufficient to provide flat gain for a specific operating condition, such as a specific gain and/or power level. The location of a GFF in the signal path between the input11and the output15of the optical amplifier system40is an implementation specific detail. For example, in some implementations a wavelength dependent attenuator, such as a gain flattening filter (GFF) might be included between the output of the second optical isolator18and the second optical coupler20and between the output of the third optical isolator22and the second optical tap24.

In some implementations, one or more of the optical isolators14,18and22might not be included.

While the optical amplifier system40depicted inFIG. 7Aincludes two amplifier components, namely the first and second lengths of erbium doped fiber34and36, any number of amplifier components might be included in some implementations.

In some implementations, a single laser source, such as laser diode28or30might be used as a pump source for one or more doped amplifier fibers, such as the first and second lengths of erbium doped fiber34and36.

In some implementations, the first and second lengths of erbium doped fiber34and36, the first and second optical couplers16and20and the first and second laser diodes28and30might be replaced with one or more semiconductor optical amplifiers. In general, any type of optical amplifier might be used.

In operation, an optical signal that is to be amplified is applied to the input11of optical amplifier system40. The first optical tap12splits the input optical signal into a first and a second signal and passes the second signal to the first photodetector26and passes the first signal to the optical isolator14, which passes the first signal on to the first optical coupler16, and isolates the first optical coupler16from the first optical tap12. The first photodetector26measures the second signal and transmits the measurement information to the amplification controller38. The first optical coupler16couples the first signal and the pump source laser output from the first laser diode28, which is controlled by amplification controller38, and passes the first signal and the pump source laser output from the first laser diode28to the first length of erbium doped fiber34. The first length of erbium doped fiber34amplifies the first signal and passes the once amplified first signal to the second optical coupler20via the second optical isolator18, which provides isolation between the second optical coupler20and the first length of erbium doped fiber34. The amplification in the first length of erbium doped fiber34causes ASE, which will also be passed to the second optical coupler20via the second optical isolator18. The second optical coupler20couples the once amplified first signal, the ASE from the first length of erbium doped fiber34and the pump laser output from the second laser diode30, which is controlled by amplification controller38, and passes the once amplified first signal, the ASE from the first length of erbium doped fiber34and the pump source laser output from the second laser diode30to the second length of erbium doped fiber36. The second length of erbium doped fiber36amplifies the once amplified first signal and the ASE from the first length of erbium doped fiber34and passes the twice amplified first signal and the once amplified ASE from the first length of erbium doped fiber34to the second optical tap24via the third optical isolator22. Like the first length of erbium doped fiber34, the second length of erbium doped fiber36will also contribute ASE. This second ASE will also be passed on to the second optical tap24via the third optical isolator22. The second optical tap24receives an input signal, comprising the twice amplified input optical signal, the once amplified ASE from the first length of erbium doped fiber34and the second ASE from the second length of erbium doped fiber36, and splits this input signal into a third and a fourth signal. The second optical tap24passes the fourth signal to the second photodetector32and the third signal to the output15of the amplifier system40. The second photodetector32measures the fourth signal and transmits the measurement information to the amplification controller38.

The operation of the amplification controller38is setup through the control input42. The amplification controller38utilizes the measurement information from the first photodetector26and the second photodetector32to control the output of the first laser diode28and the second laser diode30. By controlling the output of the first and second laser diodes28and30, the amplification controller38can control the gain of the first and second lengths of erbium doped fiber34and36. As stated above, the output of the second length of erbium doped fiber36, and thus the output15of the amplifier system40, comprises a twice amplified first signal, and an ASE component, comprising the once amplified ASE from the first length of erbium doped fiber34and the ASE from the second length of erbium doped fiber36. Using operating characteristics of double-pump erbium doped fiber amplifier, which can be determined during a calibration, the power of the ASE component of the output can be calculated and the gain of the first and second lengths of erbium doped fiber34and36can be controlled such that the output signal of the optical amplifier system40is maintained at a power level equal to the sum of a target output signal power and the calculated power of the ASE component; thereby carrying out ASE compensated ASPC. In some implementations, the power of the ASE component of the output15may be calculated as a function of the gain of the optical amplifier50, wavelength and input signal power.

In some implementations, one or more variable optical attenuators (VOA) might be included in the signal path between the input11and the output15of the optical amplifier system40, such that the effective gain between the input11and the output15of the optical amplifier system40can be adjusted without changing the true gain of the first and second length of erbium doped fiber34and36. In these implementations, the attenuation of the one or more VOAs may be controlled by the amplification controller38. For example, in some implementations a VOA might be included between the output of the first optical isolator14and the input of the first optical coupler16. One skilled in the art will appreciate that the location of the one or more VOAs in the signal path of the optical amplifier40is an implementation specific detail. A specific example is shown inFIG. 7B.

FIG. 7Bis a block diagram of another example of an amplifier system40in accordance with an embodiment of the invention in which optical amplification of a double-pump erbium doped fiber amplifier is controlled by an amplification controller38that compensates for amplified spontaneous emissions (ASE) while performing automatic signal power control (ASPC). The amplifier system40shown inFIG. 7Bis the same as the amplifier system40shown inFIG. 7Awith the addition of a variable optical attenuator46between the second optical isolator18and the second optical coupler20. The variable optical attenuator46is functionally connected to the amplification controller38by VOA control signal47. The VOA46is provided in order to operate the amplifier system as a dynamic gain amplifier. Typically, the EDFAs34and36are operated at their flat-gain operating point, and the overall effective gain is then adjusted with the VOA46.

In operation, the optical amplifier system40shown inFIG. 7Boperates in the same manner as the optical amplifier system40shown inFIG. 7A, but rather than adjusting the gain of the optical amplifier system by adjusting the gain of the first and second lengths of erbium doped fiber34and36, as was done in the optical amplifier system40shown inFIG. 1, the amplification controller38controls the gain of the optical amplifier system40shown inFIG. 7Bby adjusting the attenuation of the variable optical attenuator46. Like the laser diodes28and30of the optical amplifier system40shown inFIG. 7A, the laser diodes28and30of the optical amplifier system40shown inFIG. 7Bare controlled by the amplification controller38. Typically, the amplification controller38will adjust the pump power put out by the laser diodes28and30as the input power of the amplifier system changes in order to maintain the true gain of the EDFAs34and36at a constant level.

In some implementations, the control signals of laser diodes acting as pump sources for EDFAs are held at a constant level or controlled by a controller other than the amplification controller38.

The location of a variable optical attenuator in the signal path between the input11and the output15of the optical amplifier system40is an implementation specific detail.

In some implementations, the signal path includes more than one variable optical attenuator.

In some implementations, the signal path includes one or more gain flattening filters.

InFIGS. 7A and 7B, the first laser diode28is used to provide pump laser light to the first EDF34while the second laser diode30is used to provide pump laser light to the second EDF36. However, in some implementations, a single laser diode is used to provide pump laser light to the first EDF34and the second EDF36. For example, in some implementations the first laser diode28and the second laser diode30are replaced with a single laser diode and a power splitter, which power splitter splits the output of the single laser diode to the second input of the first optical coupler16and the second input of the second optical coupler20. Alternatively, in some implementations, a single laser diode is used to provide pump laser light to the first EDF34and the left over pump laser light from the first EDF34is used to pump the second EDF36. For example, in some implementations, the second optical coupler20is not included in the optical amplifier and the first laser diode28and the second laser diode30are replaced with a single laser diode which provides pump laser light to the second input of the optical coupler16. The pump laser light from the single laser diode pumps the first EDF34and the left over pump laser light then pumps the second EDF36.

More generally, any type of optical amplifier and any mechanism for adjusting the amplification of the optical amplifier may be used.

What has been described is merely illustrative of the application of the principles of the invention. Other arrangements and methods can be implemented by those skilled in the art without departing from the spirit and scope of the present invention.