Patent Publication Number: US-10763635-B2

Title: Low noise FM fiber laser amplifier

Description:
BACKGROUND 
     Field 
     This disclosure relates generally to a fiber laser amplifier that provides active reduction of frequency modulation (FM) to amplitude modulation (AM) conversion and, more particularly, to a fiber laser amplifier that reduces FM to AM conversion in the amplifier by actively controlling a parameter that operates to co-align the peaks or nulls of the spectral transmission caused by one or more optical components in the amplifier with the center wavelength of the seed beam. 
     Discussion 
     High power laser amplifiers have many applications, including industrial, commercial, military, etc. Designers of laser amplifiers are continuously investigating ways to increase the power of the laser amplifier for these and other applications. One known type of laser amplifier is a fiber laser amplifier that employs a doped fiber that receives a seed beam and a pump beam that amplifies the seed beam and generates the high power laser beam, where the fiber has an active core diameter of about 10-20 μm or larger. Fiber laser amplifiers are useful as energy sources for directed energy weapons because of their high efficiency, high power scalability and excellent beam quality. 
     Improvements in fiber laser amplifier designs have increased the output power of the fiber to approach its practical power and beam quality limit. To further increase the output power of a fiber amplifier some fiber laser systems employ multiple fiber laser amplifiers that combine the amplified beams in some fashion to generate higher powers. A design challenge for fiber laser amplifier systems of this type is to combine the beams from a plurality of fiber amplifiers in a manner so that the beams provide a single beam output such that the beam can be focused to a small focal spot. Focusing the combined beam to a small spot at a long distance (far-field) defines the quality of the beam. 
     In one known multiple fiber amplifier design called coherent beam combining (CBC), a master oscillator (MO) generates a seed beam that is split into a plurality of fiber seed beams each having a common wavelength, where each fiber beam is amplified. The amplified fiber seed beams are then directed to a diffractive optical element (DOE) that combines the coherent fiber beams into a single output beam. The DOE has a periodic structure formed into the element so that when the individual fiber beams each having a slightly different angular direction are redirected by the periodic structure all of the beams diffract from the DOE in the same direction. Each fiber beam is provided to a phase modulator that controls the phase of the beam so that the phase of all the fiber beams is maintained coherent. However, limitations on phase control bandwidth and wavefront errors limit the number of fiber beams that can be coherently combined, thus limiting the output power of the laser. 
     In another known multiple fiber amplifier design called spectral beam combining (SBC), a plurality of master oscillators (MOs) generate a plurality of fiber seed beams at a plurality of wavelengths, where each fiber seed beam is amplified. The amplified fiber seed beams are then directed to a diffraction grating, or other wavelength-selective element, that combines the different wavelength fiber beams into a single output beam. The diffraction grating has a periodic structure formed into the element so that when the individual fiber beams each having a slightly different wavelength and angular direction are redirected by the periodic structure all of the beams diffract from the diffraction grating in the same direction. However, limitations on spectral brightness limit the number of fiber beams that can be wavelength-combined, thus limiting the output power of the laser. 
     To overcome these limitations and further increase the laser beam power, multiple master oscillators can be provided to generate seed beams at different wavelengths, where each of the individual wavelength seed beams is split into a number of fiber seed beams and where each group of fiber seed beams has the same wavelength and are mutually coherent. Each group of the coherent fiber seed beams at a respective wavelength are first coherently combined by a DOE, and then each group of coherently combined beams are directed to an SBC grating at slightly different angles that diffracts the beams in the same direction as a single combined beam of multiple wavelengths. The SBC grating also includes a periodic structure for combining the beams at the different wavelengths. 
     When high power, single-mode light is amplified or propagates through long lengths of fiber, a host of non-linear effects can arise because of the fiber Kerr non-linearity that act to degrade the optical coherence or spectral purity of the beam. The most apparent manifestation of the Kerr non-linearity is typically self-phase modulation (SPM), which is parameterized by the B—integral, i.e., the non-linear phase shift, and which can degrade beam coherence by converting low levels of uncontrolled amplitude modulation (AM) into phase noise. This non-linear effect can limit the efficiency of CBC or the beam quality of SBC, hence reducing the performance of the laser system. Specifically, there is a loss of spectral purity or a loss of optical coherence. 
     To avoid or reduce these effects, it is generally desirable to limit the amount of AM, also known as relative intensity noise (RIN), propagating in the seed beam that seeds the fiber amplifier. Techniques that broaden the spectrum of the seed beam to provide frequency modulation without providing amplitude modulation can be implemented in a fiber amplifier, where if the seed beam is only frequency modulated, then the Kerr non-linearities will not create problems, i.e., no time dependent non-linear phase shifts of the seed beam. However, this results in spectral beam broadening, which could reduce beam quality during SBC. 
     Generally, to maintain the degradation of the beam quality defined by the Strehl ratio to be below 1% in a beam combined fiber laser weapon system, it is desirable to maintain non-linear SPM phase fluctuations B*RIN&lt;0.1 radians. For a typical 1.5-2 kW fiber amplifier with a B—integral of 10 radians, this implies a requirement to maintain RIN&lt;1%. Therefore, it is industry standard practice to employ FM seed beam sources having little or no AM, i.e., with constant power versus time. However, a number of effects have been observed that still partially convert FM into uncontrolled AM, where it can cause non-linear degradation through SPM. These effects include polarization mixing, chromatic dispersion, spectral filtering, or generally, any multi-path interference (MPI) effects. A typical signature of an MPI effect in a fiber component or fiber-based system is a spectrally dependent transmission that exhibits a periodic modulation pattern. 
     It is known in the art that for FM line-widths that are significantly smaller than the free spectral range (FSR) of the spectral modulation, the magnitude of the FM to AM conversion can vary significantly depending on the relative wavelengths of the seed beam and the spectral transmission peak. The FM to AM conversion in the fiber will be minimized when the beam wavelength is aligned with a peak or null of the transmission spectrum, and will be maximized when the beam wavelength is between a peak and a null. This is because the instantaneous frequency of the FM signal is changing in time, and thus its transmission amplitude will change in the time leading to the time-varying output power, i.e., AM. The FM to AM conversion will be minimized when the spectral transmission over the beam bandwidth is as uniform as possible, which occurs near a transmission peak or null. Hence, there is a need for a fiber amplifier architecture that can amplify FM light with a minimum FM to AM conversion to yield a low-noise output despite the presence of components that exhibit non-uniform spectral transmission. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic block diagram of a fiber laser amplifier system that reduces FM to AM conversion using active control; 
         FIG. 2  is a schematic block diagram of a fiber laser amplifier system that reduces FM to AM conversion using active polarization control; 
         FIG. 3  is a schematic block diagram of a fiber laser amplifier system that reduces FM to AM conversion using active polarization control and provides seed beam SOP control; 
         FIG. 4  is a schematic block diagram of a fiber laser amplifier system that reduces FM to AM conversion using active temperature control; 
         FIG. 5  is a schematic block diagram of a fiber laser amplifier system that reduces FM to AM conversion using active temperature control and provides seed beam SOP control; 
         FIG. 6  is a schematic block diagram of a fiber laser amplifier system that reduces FM to AM conversion using active control for multiple optical components; 
         FIG. 7  is a schematic block diagram of a fiber laser amplifier system that reduces FM to AM conversion using a programmable spectral filter that flattens or equalizes the spectral transmission of the seed beam propagating through the amplifier system; 
         FIG. 8  is a schematic block diagram of a fiber laser amplifier system that reduces FM to AM conversion using a programmable spectral filter that flattens or equalizes the spectral transmission of the seed beam propagating through the amplifier system and provides seed beam SOP control; 
         FIG. 9  is a schematic block diagram of a fiber laser amplifier system that reduces FM to AM conversion using a programmable spectral filter that flattens or equalizes the spectral transmission of the seed beam propagating through the amplifier system and employs an optical spectrum analyzer for measuring the input and output power spectrums of the seed beam; 
         FIG. 10  is a schematic block diagram of a fiber laser amplifier system that reduces FM to AM conversion using a programmable spectral filter that flattens or equalizes the spectral transmission of the seed beam propagating through the amplifier system and includes a dispersion compensating fiber; and 
         FIG. 11  is a schematic block diagram of a fiber laser amplifier system that reduces FM to AM conversion using a programmable spectral filter that flattens or equalizes the spectral transmission of the seed beam propagating through the amplifier system and employs SBC. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The following discussion of the embodiments of the disclosure directed to a fiber laser amplifier system that employs various techniques for actively reducing FM to AM conversion is merely exemplary in nature, and is in no way intended to limit the disclosure or its applications or uses. 
     As will be discussed in detail below, the present disclosure proposes various techniques for reducing FM to AM conversion in a fiber laser amplifier system by actively controlling a certain parameter in the system. These techniques can generally be separated into two types. The first type includes co-aligning the peaks or nulls of the spectral transmission caused by one or more amplifier components with a center wavelength of the seed beam. FM line-widths for 1-2 kW fiber amplifiers are typically 20-30 GHz that are driven by the requirement to stay below the threshold for stimulated Brillouin scattering in the fiber, which is small in comparison to typical measured FSRs on the order of −100 GHz in various amplifier components. Hence, co-alignment of the center wavelength of the seed beam with the peaks or nulls of the transmission spectrum will minimize FM to AM conversion and will result in a low-noise FM fiber amplifier with reduced impairment from non-linear SPM noise. For example, since the spectral transmission of components within typical fiber amplifier systems can be polarization-dependent or temperature-dependent, the manual adjustment of the seed beam state of polarization (SOP) or component temperature can dramatically reduce the RIN on the amplified output beam. 
     The second type includes providing spectral equalization of the seed beam such as by employing a programmable spectral filter to compensate for spectral amplitude and phase distortions that initiate the FM to AM conversion. For spectral amplitude distortions, the spectral filter would be programmed to flatten the net spectral transmission profile through the fiber amplifier system. For spectral phase distortions, the spectral filter would be programmed to compensate for fiber group velocity dispersion (GVD) or other dispersive effects arising from amplification or component transmission spectrums. For static spectral phase correction, the spectral filter could also comprise a length of dispersion-compensating fiber used in series with the dynamic spectral filter, if this reduces the stroke required for compensation or improves precision. 
     Both of these types of techniques for reducing FM to AM conversion in a fiber laser amplifier system can be shown generally by fiber laser amplifier system  10  depicted in  FIG. 1 . The system  10  includes a master oscillator (MO)  12  that generates a seed beam on fiber  14  having a particular wavelength. The seed beam is provided to an RF electro-optical modulator (EOM)  16  that receives an RF signal to frequency modulate the seed beam for providing frequency modulation broadening, such as white noise or pseudo-random bit sequence (PRBS). It is noted that the EOM  16  can be at any suitable location in the system  10  before the seed beam is amplified. The broadened seed beam is then sent to an actuator  18  that controls some aspect or parameter of the seed beam, such as its phase, polarization, etc., for reducing FM to AM conversion as discussed herein. The actuated seed beam is then sent to an amplifier or optical component  20  that is intended to represent any of a number of optical components that may exist in the system  10  that provide a certain optical function depending on the particular application, and which may cause frequency modulation in the seed beam to be converted to amplitude modulation through SPM as discussed above. Suitable examples for the optical component  20  include, but are not limited to, a pump beam combiner, an optical splice, an optical isolator, a spectral filter, a length of optical fiber, a pre-amplifier stage, a mode field adapter, etc. It is noted that several of the components  20  can be employed. 
     The seed beam is then sent to a non-linear fiber amplifier  22 , which may be a plurality of fiber amplification stages each including a pump beam source and a length of doped fiber, such as a ytterbium (Yb) doped length of fiber having a 10-20 μm core, to amplify the seed beam and provide an amplified output beam  26 . The output beam  26  is then sent through a beam splitter  30  that splits off a small sample portion of the output beam  26  as a sample beam  32 . The sample beam  32  is sent to a detector  34  that determines the amount or magnitude of the particular parameter being monitored that identifies AM in the amplified beam  26 . The detector  34  generates a control metric that is sent to a controller  36  that controls the actuator  18  to adjust the parameter, thus optimizing the control metric and eliminating or reducing FM to AM conversion. 
       FIG. 2  is a schematic block diagram of a fiber laser amplifier system  40  that employs polarization control of the seed beam as one embodiment for the first type of technique for reducing FM to AM conversion that co-aligns the peaks or nulls of the spectral transmission caused by the component  20  with the center wavelength of the seed beam, where like elements to the system  10  are identified by the same reference number. In the amplifier system  40 , the actuator  18  is replaced with a polarization actuator  42 . More specifically, the sample beam  32  is not polarization filtered for a specific SOP, but unpolarized light in the sample beam  32  is detected and is used to adjust the polarization of the seed beam so that the non-uniform spectral transmission caused by the component  20  does not generate amplitude modulation. 
     In this embodiment, the sample beam  32  is detected by a high-speed photodetector  44  that generates a photocurrent having a time varying electrical AC signal that is proportional to optical power fluctuations caused by the amplitude modulation on the sample beam  32  and has a DC offset that is proportional to the optical power of the sample beam  32 . The photocurrent is filtered by a high-pass filter  46  to remove the DC offset. The filter  46  has a cut-off frequency that is less than the detection bandwidth of the photodetector  44  and higher than the desired feedback control loop rate. The AC signal that is passed by the filter  46  is rectified by a rectifier  48  to convert the negative parts of the signal to positive parts. The rectified signal is filtered and time-averaged by a low-pass filter  52  to provide a control metric signal that is proportional to the amplitude modulation or RIN power in the sample beam  32  that is provided to a polarization controller  50 . The low pass filter  52  has a cutoff frequency that is higher than the desired feedback control loop rate, but small enough to provide good signal-to-noise ratio for the control metric signal to enable stable feedback control. The polarization controller  50  provides a control signal to the polarization actuator  42  depending on the magnitude and dynamics of the control metric signal. The control metric signal changes or adjusts the polarization of the seed beam to cause the peaks or nulls of the spectral transmission created by the component  20  to be shifted to align with the center wavelength of the seed beam. In other words, by changing the polarization of the seed beam, the spectral transmission of the seed beam caused by the optical component  20  will be shifted, and by monitoring the magnitude of the amplitude modulation, the optimum polarization can be provided to the seed beam to minimize the amplitude modulation. 
     The system  40  does generate a low-noise FM amplified output laser beam, however, the output beam  26  will be indeterminately polarized based on the uncontrolled birefringence of the fiber components. This is acceptable for applications that do not require polarized seed beams, such as beam-combined laser architectures that are based on SBC using polarization-independent combining gratings. However, the laser system  40  may not be useful for applications that require polarized seed beams, such as beam-combined laser weapon architectures based on CBC or based on SBC using polarization-dependent gratings. 
       FIG. 3  is a schematic block diagram of a fiber laser amplifier system  60  that provides polarization control to reduce FM to AM conversion as discussed above and also provides polarization control of the seed beam to generate an output beam  58  that is polarized to a certain polarization state, where like elements to the system  40  are identified by the same reference number. The system  60  includes a polarization actuator  62  that adjusts the SOP of the seed beam to the desired polarization state for the particular application, which is not the same polarization control provided by the polarization actuator  42  in the system  40  that changed the polarization of the seed beam to align the peaks and nulls in the transmission spectrum caused the component  20  to the center frequency of the seed beam. The sample beam  32  is filtered by a polarization filter  64  that transmits optical power of the sample beam  32  in the desired polarization state. The polarization-filtered sample beam is detected by the high-speed photodetector  44  that generates a photocurrent having a time varying electrical AC signal that is proportional to optical power fluctuations caused by the amplitude modulation on the sample beam  32  and has a DC offset that is proportional to the optical power of the sample beam  32  that is in the desired polarization state. The SOP of the output beam  58  is controlled by providing the electrical signal from the photodetector  44  to a low-pass filter  66  that filters out the frequency fluctuations so that the DC offset identifying the magnitude of the sample beam  32  having the desired polarization remains. The filtered signal is provided to a polarization controller  68  that controls the polarization actuator  62  to adjust the polarization of the seed beam to maximize the amount of power that is provided by the polarization filter  64 . The cut-off frequency of the low-pass filter  66  is selected based on the desired loop rate of the polarization control loop, for example, 1-100 kHz, where the cut-off frequency of the high-pass filter  46  is selected to be greater than the cut-off frequency of the low-pass filter  66 , but less than the bandwidth of the photodetector  44 . 
     Because the polarization actuator  62  does control the polarization of the seed beam to the desired SOP it also alters the position of the peaks and nulls of the transmission spectrum caused by the optical component  20 , which could increase or decrease the amount of amplitude modulation the component  20  creates. In the system  60 , the polarization actuator  42  is moved downstream of the component  20  to provide the shift in the spectral transmission of the seed beam. As mentioned above, the control bandwidth or sampling frequency of the polarization control for reducing FM to AM conversion is selected to be much slower than the control bandwidth of the output beam polarization control to ensure the output SOP remains locked to the desired polarization. It is possible to move the polarization actuator  42  downstream of the component  20  in this embodiment and still be effective to reduce FM to AM conversion because the polarization actuator  62  responds to changes in the polarization made by the polarization actuator  42  to maintain the output SOP. Because the polarization actuator  62  changes the polarization of the seed beam in response to changes made by the polarization actuator  42 , the polarization of the seed beam is effectively being changed by the polarization actuator  42  only at locations between the actuator  62  and the actuator  42  in the optical train, i.e., the SOP is constant at all other locations. 
       FIG. 4  is a schematic block diagram of a fiber laser amplifier system  70  that employs active temperature control as another embodiment for the first type of technique for reducing FM to AM conversion that co-aligns the peaks or nulls of the spectral transmission caused by the component  20  with the center wavelength of the seed beam, where like elements to the system  40  are identified by the same reference number. The control metric signal that is proportional to the AM power in the sample beam  32  is provided to a temperature controller  72  that controls the temperature of a heater/cooler device  74  that is coupled to and operates to change the temperature of the optical component  20 , where the device  74  is the actuator  18  in this embodiment. Therefore, depending on the magnitude of the control metric signal from the low-pass filter  52 , the temperature controller  72  will increase or decrease the temperature of the component  20 , which causes its spectral transmission to be shifted so that the peaks or nulls are aligned with the center frequency of the seed beam. 
       FIG. 5  is a schematic block diagram of a fiber laser amplifier system  80  that provides temperature control of the optical component  20  to reduce FM to AM conversion as discussed above and also provides the polarized output beam  58  that is polarized to a desired polarization state, where like elements to the systems  60  and  70  are identified by the same reference number. In order to correct for the conversion of FM to AM, the same process used by the system  70  is employed to control the temperature of the optical component  20 . Changing the temperature of the component  20  can also change its birefringence, however these changes are sufficiently slow that they will appear adiabatic to the polarization controller  68  with typical bandwidths of 100 Hz or more, and can be dynamically compensated so that the amplified output beam  58  will simultaneously have a low-RIN and be properly polarized. Therefore, the polarization actuator  62  can operate quickly to change the polarization state of the seed beam to the desired polarization, where the correction for FM to AM conversion provided by the temperature controller  72  can operate more slowly. 
     The discussion above concerning the first type of technique for reducing FM to AM conversion that co-aligns the peaks or nulls of the spectral transmission caused by the component  20  with the center wavelength of the seed beam was specific to providing polarization control or temperature control. However, it is noted that these are merely examples of suitable techniques for aligning the peaks and nulls of the spectral transmission of the component  20  with the center frequency of the seed beam, where other techniques may be equally applicable. For example, it may be possible to control mechanical stress on the optical component  20  to change its birefringence, which could also operate to shift the non-uniform spectral transmission of the component  20  to align with the center wavelength of the seed beam. In this embodiment, the device  74  would be a device that applies mechanical stress to the optical component  20  and the controller  72  would be a stress controller. 
     As noted, a number of components in a fiber laser amplifier system could cause non-uniform spectral transmission produced by AM. Therefore, it may be desirable to provide AM reducing control to multiple or all of the various components that are in the amplification chain. This embodiment is illustrated in  FIG. 6  by fiber laser amplifier system  90 , where like elements to the system  80  are identified by the same reference number. The system  90  includes multiple optical components  20  positioned between the polarization actuator  62  and the fiber amplifier  22 , where each optical component  20  includes some type of actuation device  92 , such as a temperature actuator, polarization actuator, stress actuator, etc., as discussed above. A multi-dither control scheme, for example, a stochastic parallel gradient descent (SPGD) algorithm can be employed to simultaneously control multiple components to minimize FM to AM conversion. Such a controller is illustrated by multi-channel controller  94  that controls each of the actuators  92  to independently shift the spectral transmission caused by each of the components  20  to align the peaks or nulls with the center wavelength of the seed beam. 
     The second type of technique referred to above that reduces FM to AM conversion by providing spectral equalization, which is a more direct technique than the first type of technique, uses a programmable spectral filter as the actuator  18  in the system  10  that operates to flatten or equalize the net spectral transmission profile of the seed beam propagating through the fiber amplifier system  10 . If the spectral transmission of the seed beam through all of the optical components from the MO  12  through the fiber amplifier  22  is initially a transmission function T 1 (λ) without the programmable spectral filter, then by using a spectral filter programmed to produce an inverse transmission function T 2 (λ)=1/T 1 (λ), the net spectral transmission of the seed beam after it propagates through the system  10  will be T 1 (λ)T 2 (λ)=1, which should yield a low noise output beam. In general, both of the transmission functions T 1 (λ) and T 2 (λ) can be complex-valued functions, i.e., both spectral amplitude and phase can be non-uniform. Thus, the seed beam is pre-distorted with the inverse of the transmission function of the fiber amplifier system  10  so that the non-uniform spectral transmission of the seed beam changes its transmission function to its original transfer function as it propagates through the system  10  so that it does not include amplitude modulation. This approach has advantages over the first type of technique because it can, in principle, compensate for FM to AM conversion arising from multiple sources. In other words, this technique can correct arbitrary impairment to the spectral transmission and is not limited to periodic spectral transmission profiles. 
       FIG. 7  is a schematic block diagram of a fiber laser amplifier system  100  that employs spectral equalization control of the seed beam as a general depiction of this embodiment for the second type of technique for reducing FM to AM conversion, where like elements to the system  40  are identified by the same reference number. In this embodiment, the polarization actuator  42  is replaced with a programmable spectral filter  102  that provides pre-distortion of the seed beam as an inverse of the spectral distortion provided by the components in the amplification chain prior to the seed beam being sent to the fiber amplifier  22 . In this embodiment, the system  100  uses the photodetector  44 , the high-pass filter  46 , the rectifier  48  and the low-pass filter  52  in the manner discussed above to generate the control metric signal proportional to AM power in the sample beam  32  that identifies the non-uniform spectral transmission. A controller  104  receives the control metric signal from the low-pass filter  52  and controls the spectral filter  102  to provide the desired inverse transmission function that drives the metric signal to zero, and thus reduces the FM to AM conversion. 
     If, as discussed above, the fiber laser amplifier system needs to provide the polarized output beam  58  for a certain application, then polarization control can be provided in the same manner as discussed above. This embodiment is shown in  FIG. 8  as fiber laser amplifier system  106 , where like elements to the systems  60  and  100  are identified by the same reference number. 
     The discussion above provides feedback control to the spectral filter  102  using an advanced algorithm to obtain the spectral shape that minimizes AM. A simpler approach might involve a simple multi-variable controller based on adjusting one or more parameters associated with a Taylor expansion of the spectral transmission around the center wavelength of the seed beam. For example, a spectral transmission could be imposed on the seed beam that varies linearly with wavelength, where the control parameter is the slope of the spectral transmission curve that would serve to grossly compensate the lowest order spectral non-uniformity. Parameters could also be included to compensate quadratically varying spectral transmissions or even higher order terms. Typically, the lowest order term spectral phase that is of interest is quadratic that corresponds to group velocity dispersion (GVD) since linearly varying spectral phase simply corresponds to a constant time delay and does not impact FM to AM conversion. 
     In an alternate embodiment for the second type of technique, the detector  34  could be a spectral detector, such as a spectrometer or an optical spectrum analyzer, that provides measurements of both an output power spectrum S out (λ) of the sample beam  32  and an input power spectrum S in (λ) of the seed beam before it is sent to the programmable spectral filter  102 . This embodiment is shown as fiber laser amplifier system  110  in  FIG. 9 , where like elements to the system  100  are identified by the same reference number. The system  110  includes a switch  112  that receives the sample beam  32  having the output power spectrum S out (A) and the seed beam from the EOM  16  having the input power spectrum S in (A). The switch  112  toggles back and forth at a desired sampling rate between the input and output power spectrums and an optical spectrum analyzer  114  alternately measures the magnitude of those power spectrums. A controller  116  receives the measured magnitudes of the power spectrums, calculates the net system transmission function T 1 (λ)=S out (λ)/S in (λ) and controls the programmable spectral filter  102  to apply the inverse transmission function T 2 (λ)=1/T 1 (λ) to the seed beam. In an alternate embodiment, two optical spectrum analyzers could be employed, where each analyzer measures one of the power spectrums. In another alternate embodiment, for situations in which the input power spectrum S in (λ) is constant in time, the input power spectrum could be stored in the memory of the controller  104  and only the output power spectrum S out (λ) would be dynamically measured using a single optical spectrum analyzer. To avoid dynamic instability in a closed loop configuration, the controller  116  would typically be programmed to apply a small correction to the transmission function T(λ) upon each loop in the iteration. For example, for the nth iteration of the closed loop as:
 
 T   2   (n) (λ)= K*[T   2   (n−1) (λ)+ g* 1 /T   1   (n) (λ)],   (1)
 
where n denotes the nth loop iteration, g&lt;1 is a gain coefficient set to balance stability against loop convergence speed, i.e., dynamic control bandwidth, and K is a normalization constant set to maximize T 2 .
 
     It is noted that use of the optical spectrum analyzer  114  in the system  110  is only able to measure the spectral amplitude of the input power and the output power, but cannot provide FM to AM conversion compensation for the spectral phase. However, typically in these types of systems, the spectral phase is not time varying, and is thus fixed. Fiber dispersion can typically be compensated for with a one-time calibration measurement since it is not expected to change dynamically during normal use of the fiber laser amplifier system. Thus, active control to reduce FM to AM conversion may not be required for all system parameters. 
     To provide the compensation for fiber dispersion and provide spectral phase correction, a dispersion compensating fiber (DCF) could be used in the amplification chain before the fiber amplifier  22  to address static GVD and decrease the amount of spectral phase correction needed by the active control or provide the spectral phase control. This embodiment is shown in  FIG. 10  as fiber laser amplifier system  120 , where like components to the systems  10  and  100  are identified by the same reference number. In the system  120 , a DCF  122  is provided before the programmable spectral filter  102  that provides the spectral phase correction. Providing the DCF  122  as shown benefits the system  110  by providing static compensation for spectral phase that was not available from the spectrum analyzer  114 , but also benefits the system  100  by off-loading most of the spectral phase correction for FM to AM conversation to the DCF  122 , where additional spectral phase correction is provided by the controller  116 . 
     As discussed above, some laser amplifier systems employ a multichannel or SBC architecture. The use of a programmable spectral filter to reduce FM to AM conversion has particular application for these types of SBC architectures because a single programmable spectral filter can provide FM to AM conversion compensation for an entire array of spectrally beam combined fiber amplifiers. This not only enables higher per-fiber powers and long delivery fiber cabling, but also eliminates expensive components, such as electro-optic modulators and high frequency RF drive electronics, by allowing multiplexing of all seed beam wavelengths through a single modulator channel. 
     This embodiment is illustrated by fiber laser amplifier system  130  in  FIG. 11 , where like elements to the system  100  are identified by the same reference number. The system  130  includes a plurality of master oscillators  12 , where each master oscillator  12  generates a seed beam at a different wavelength A. All of the seed beams are provided to a wavelength division multiplexer  132  that combines the beams on to a single fiber  134 . Once all of the seed beams are on the same fiber, and are frequency modulated by the EOM  16 , the programmable spectral filter  102  can provide the spectral filtering in the manner discussed above to pre-distort all of the seed beams of different wavelengths propagating therethrough with the inverse transmission function. Once all of the seed beams have been pre-distorted, they are sent to a wavelength division demultiplexer  136  that separates the pre-distorted different wavelength seed beams onto separate fibers to be sent to separate fiber amplifiers  22 . Once the individual beams are amplified at their respective wavelengths, they are combined by SBC optics  138  to generate a high-power output beam  140 . The input power spectrum from the EOM  16  and the output power spectrum from the sample beam  32  are alternately measured by the optical spectrum analyzer  114  and the controller  116  calculates the net system transmission function in the manner discussed so that the controller  116  can control the programmable spectral filter  102 . It is noted that the spectrum analyzer can be replaced with the detection embodiment that employs the photodetector  44 , the high-pass filter  46 , the rectifier  48  and the low-pass filter  52 , as discussed above. 
     The foregoing discussion discloses and describes merely exemplary embodiments of the present disclosure. One skilled in the art will readily recognize from such discussion and from the accompanying drawings and claims that various changes, modifications and variations can be made therein without departing from the spirit and scope of the disclosure as defined in the following claims.