Am / FM seed for nonlinear spectrally compressed fiber amplifier

Abstract

To provide a fiber amplifier that provides frequency modulation and amplitude modulation of a seed beam to increase beam power and reduce a beam width.SOLUTION: A fiber amplifier system includes: an optical source providing an optical seed beam; and a FM electro-optic modulator (EOM) that frequency-modulates the seed beam to broaden its spectral linewidth. The system also includes an AM EOM that amplitude-modulates the seed beam to provide an amplitude-modulated seed beam that is synchronized with the frequency-modulated seed beam. The system also includes a non-linear fiber amplifier receiving the AM and FM modulated seed beam, where the amplitude-modulated seed beam causes self-phase modulation in the fiber amplifier that phase-modulates the seed beam as it is being amplified by the fiber amplifier that acts to cancel the spectral linewidth broadening caused by the frequency modulation.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] Government Provisions

[0001] This invention was made with Government support under Contract No. FA9451-18-C-0101 awarded by the Air Force Research Laboratory. The Government has certain rights in this invention.

[0002] background Field

[0002] This disclosure relates generally to fiber laser amplifiers having high power and narrow linewidth, and more specifically to a fiber laser amplifier system that includes an electro-optic modulator (EOM) that applies a frequency modulation (FM) signal to a seed beam to broaden its linewidth and also applies an amplitude modulation (AM) signal to the seed beam that is synchronized with the FM signal, and the modulated seed beam is amplified by a nonlinear fiber amplifier whereby self-phase modulation, which phase modulates the seed beam as it propagates through the amplifier, cancels the frequency modulation of the beam and restores the spectrum of the original seed beam. [Background technology]

[0003] Consider

[0003] High power laser amplifiers have many applications, including industrial, commercial, and military. For these and other applications, laser amplifier designers are continually researching ways to increase the power of laser amplifiers. One known type of laser amplifier is the fiber laser amplifier, which uses a doped fiber that receives a seed beam and a pump beam that amplifies the seed beam to produce a high power laser beam, the fiber having an active core diameter of about 10-20 μm or larger.

[0004]

[0004] Improvements in the design of fiber laser amplifiers have increased the output power of a fiber so that it approaches its practical power and beam quality limits. To further increase the output power of a fiber amplifier, some fiber laser systems use multiple fiber laser amplifiers, which combine multiple amplified beams in some fashion. ) to produce high power. The design challenge for this type of fiber laser amplifier system is to combine the multiple beams from the multiple fiber amplifiers in a way that they provide a single beam output and be able to focus that beam to a small focal spot. Focusing the combined beam to a small spot at long distances (far field) limits the quality of the beam.

[0005]

[0005] In one known multi-fiber amplifier design, a master oscillator (MO) generates a seed beam that is split into multiple fiber seed beams, each of which has a common wavelength, and each fiber beam is amplified. The amplified fiber seed beam is then collimated and sent to a diffractive optical element (DOE), which combines the multiple coherent fiber beams into one output beam. The DOE has a periodic structure formed within the element such that when the individual fiber beams, each with a slightly different angular orientation, are redirected by the periodic structure, all of the beams diffract in the same direction from the DOE. Each fiber beam is provided to a phase modulator, which controls the phase of the beam so that the phases of all the fiber beams remain coherent. However, bandwidth limitations and phase alignment issues remain. Pairing errors limit the number of fiber beams that can be coherently combined and therefore limit the output power of the laser.

[0006]

[0006] In another known multi-fiber amplifier design, multiple master oscillators (MOs) generate multiple fiber seed beams at multiple wavelengths, and each fiber seed beam is amplified. The amplified fiber seed beams are then collimated and sent to a diffraction grating or other wavelength-selective element, which combines the multiple fiber beams of different wavelengths into one output beam. The diffraction grating has a periodic structure formed into the element so that the individual fiber beams, each with slightly different wavelengths and angular directions, all diffract from the diffraction grating in the same direction when they are redirected by the periodic structure. However, bandwidth limitations limit the number of fiber beams that can be wavelength-combined, and therefore limit the output power of the laser.

[0007]

[0007] To overcome these limitations and further increase the power of the laser beam, multiple master oscillators can be provided to generate multiple seed beams of various wavelengths, in which each of the seed beams of individual wavelengths is split into multiple fiber seed beams, and the fiber seed beams of each group have the same wavelength and are mutually coherent. The multiple coherent fiber seed beams of each wavelength of each group are first coherently combined by a DOE, and then the coherently combined beams of each group are sent to a spectral beam combining (SBC) grating at slightly different angles, which diffracts the beams in the same direction into one combined beam of multiple wavelengths. The SBC grating also includes a periodic structure for combining multiple beams of different wavelengths.

[0008]

[0008] To improve the quality of the beam, it is often desirable for the output beam from a fiber amplifier to have a narrow linewidth, i.e., a narrow frequency range. However, providing both high power and narrow linewidth has traditionally been a challenge in the art, as high power and narrow linewidth are typically mutually exclusive, since high power requires a wide beam linewidth. More specifically, stimulated Brillouin scattering (SBS), i.e., the nonlinear backscattering of the beam as it propagates along the fiber amplifier, increases with narrow linewidth over a small frequency range, which acts to reduce the power of the beam. However, the wider the beam linewidth, the more difficult it is to coherently combine or spectrally combine multiple beams from multiple fibers into one beam through known beam combining techniques. In particular, dispersion effects from SBC gratings require the linewidth of the beam being amplified to be narrow, where spectral dispersion causes the spectral components of the beam to be diffracted at various angles. In other words, for SBC, the spectral brightness of the seed beam directly limits the theoretical brightness of the combined beam output.

[0009]

[0009] For coherent beam combining (CBC), the spectral brightness of the seed beam limits the efficiency of combining, since the group delay and dispersion between the amplifiers are not perfectly matched. Typically, the source spectral brightness is limited by SBS, and the seed beam source to the fiber amplifier must be frequency modulated to reduce the peak SBS gain and achieve the desired output power. Frequency modulation spectral broadening limits the achievable spectral brightness from one fiber amplifier and therefore limits the system output.

[0010] To overcome these limitations, fiber amplifier designers typically use one or more phase modulators prior to the amplification stage of the fiber amplifier to reduce linewidth through frequency modulation. However, when frequency modulation is applied to a beam before it is amplified by the fiber amplifier, the broadening of the spectral components of the beam is preserved throughout the amplifier, and The result is an amplified beam of low spectral brightness.

[0011]

[0011] U.S. Patent No. 9,036,252, issued May 19, 2015 to Goodno et al. and entitled "Nonlinear Spectrally Narrowed Fiber Amplifier" is a commonly assigned US Pat. discloses a fiber laser amplifier system having high power and narrow linewidth for enhanced spectral brightness, which is incorporated herein by reference. The fiber amplifier system disclosed in the '252 patent includes a seed source providing an optical seed beam and a harmonic phase modulator that receives the seed beam and an RF drive signal and frequency modulates the seed beam with the drive signal to remove optical power from the main or zeroth frequency of the seed beam and place the power into a sideband frequency bounded by the frequency of the drive signal. A dispersive element receives the frequency modulated seed beam and provides temporal amplitude modulation of the seed beam. A nonlinear fiber amplifier receives the frequency and amplitude modulated seed beam from the dispersive element and amplifies the seed beam, where self-phase modulation (SPM) caused by the frequency modulation and nonlinearity of the fiber amplifier combine to remove optical power from the sideband frequencies and place it back into the zeroth frequency.

[0012]

[0012] As generally described above, the '252 fiber amplifier system frequency modulates a seed beam and then amplitude modulates the frequency modulated seed beam using dispersion, where the amplitude modulation drives the self-phase modulation caused by the nonlinearity of the fiber amplifier, so that the spectrum of the beam is reduced as it is being amplified, producing a high power output beam with a narrow linewidth. While this technique can effectively provide high power and narrow linewidth beams as described above, relying on dispersion to provide amplitude modulation of the seed beam is limited because amplitude modulation does not match exactly with frequency modulation, which limits the efficiency of nonlinear spectral compression in fiber amplifiers at high modulation depths. More specifically, for low modulation depths and highly nonlinear fiber amplifiers, spectral compression in fiber amplifiers is effective. However, when the nonlinearity in the fiber amplifier is low, more dispersion is required to achieve a higher amplitude modulation depth of the beam. However, when the dispersion is large, the shape of the amplitude modulation line broadening does not exactly match that of the frequency modulation line broadening, i.e., the amplitude modulation waveform is not a perfect sine wave, so nonlinear spectral compression is inefficient and a lot of power remains in the sidebands, which limits the amount of linewidth that can be reduced. Thus, there is a trade-off between the efficiency of spectral compression and high SBS suppression. [Brief description of the drawings]

[0013] [Figure 1] Figure 1 is a schematic block diagram of the input section of a fiber laser amplifier system that includes separate EOMs for providing frequency modulation and amplitude modulation of the seed beam. [Diagram 2] Figure 2 is a schematic block diagram of the input section of a fiber laser amplifier system that includes one EOM to provide both frequency modulation and amplitude modulation of the seed beam. [Diagram 3] Figure 3 is a schematic block diagram of the input section of a fiber laser amplifier system using CBC with matching B integrals between multiple fiber amplifiers, including one EOM to provide both frequency and amplitude modulation of the seed beam. [Figure 4] Figure 4 is a schematic block diagram of the input section of a fiber laser amplifier system that includes one EOM to provide both frequency and amplitude modulation of the seed beam and uses CBC with mismatched B integrals between the multiple fiber amplifiers. [Diagram 5] Figure 5 is a schematic block diagram of a fiber laser amplifier system with multiple channels, where each channel contains one EOM to provide both frequency modulation and amplitude modulation of the seed beam and uses an SBC. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014]

[0018] The following description of embodiments of the present disclosure relating to a fiber laser amplifier providing frequency and amplitude modulation of a seed beam to increase beam power and reduce beam width is merely exemplary in nature and is not intended to limit the disclosure or its applications or uses.

[0015]

[0019] As mentioned above, in fiber laser amplifiers, the tradeoff between high power and narrow linewidth limits the spectral brightness. To overcome this tradeoff, this disclosure proposes to broaden the linewidth of the seed beam by using both frequency and amplitude modulation, and then to spectrally compress the linewidth of the amplified beam close to that of the original unbroadened seed beam using self-phase modulation induced by the nonlinearity of high-power fiber amplifiers.

[0016]

[0020] FIG. 1 is a schematic block diagram of a portion of a fiber laser amplifier system 10, including a master oscillator 12 that generates a seed beam having a particular wavelength on line 14. The seed beam is provided to an auxiliary RF electro-optic modulator (EOM) 16, which is controlled by an auxiliary RF driver 18 to provide frequency modulation. The frequency modulation provided by the EOM 16 represents a conventional technique for providing frequency modulation broadening, such as white noise or pseudorandom bit sequence (PRBS), and may not be required or desired in some amplifier systems. It is noted that the EOM 16 may be located at any suitable location in the system 10 before the seed beam is amplified. The laser field E1(t) following the auxiliary EOM 16 has the form:

[0017]

number

[0018]

[0021] As can be seen from equation (1), the amplitude of the laser field is constant in time and its phase varies in time as a function φ(t) imposed by the EOM 16.

[0019]

[0022] The modulated seed beam from EOM 16 is then sent to FM EOM 20, which also receives an RF drive signal f(t) from RF driver 22. EOM 20 imposes the RF drive signal on the phase of the optical seed beam to vary the frequency of the seed beam in time, providing frequency modulation. The frequency modulated field output from EOM 20 has the form

[0020]

number

[0021] In the above, it is assumed that the drive signal f(t) is zero-mean (time average) and normalized to 1, and β is the frequency modulation depth in radians.

[0022]

[0023] Frequency modulation provides a time-dependent change in the phase of the seed beam, which broadens the linewidth of the beam, and the broadened linewidth provides SPS suppression. In one non-limiting example for purposes of this discussion, the RF drive signal provided by driver 22 is , single-tone sine wave signal f(t) = sin(ω m t) Wherein, ω m t / 2π is the modulation frequency, which can be 32 GHz, which is twice the Stokes frequency shift due to SBS in fused silica fibers. However, it should be noted that other high frequency sinusoidal drive signals can be used in various applications. More generally, the drive signal f(t) does not necessarily have to be sinusoidal, and in fact can be of any functional form, including, for example, a PRBS format or a shaped noise spectrum.

[0023]

[0024] The frequency modulation provided by the EOM 20 produces an optical seed beam that includes a broadened spectral width determined by the functional form f(t) of the drive signal and the modulation depth β. In the non-limiting example described herein, the spectral content of the seed beam includes frequency sidebands spaced apart at 32 GHz. The modulation depth β of the RF drive signal from the driver 22 is selected according to the desired spectral width, with a higher modulation depth signal producing a wider linewidth. For example, in the non-limiting example described herein, the modulation depth β of the drive signal can be selected to eliminate all of the power from the zeroth order frequency of the seed beam in the EOM 20. Alternatively, the modulation depth β of the drive signal can be selected to produce equal amplitude power at the zeroth and + / -1st order sideband frequencies of the seed beam in the EOM 20. Alternatively, the modulation depth β of the drive signal can be selected to produce multiple sidebands of the seed beam in the EOM 20.

[0024]

[0025] The frequency modulated seed beam is then sent to the AM EOM 24, which receives an RF drive signal from an RF driver 26 that provides amplitude modulation of the seed beam, i.e., varies the power of the seed beam in time, and the EOM 2 imposes the drive signal on the amplitude of the optical seed beam to provide the amplitude modulation. The RF driver 26 is synchronized with the RF driver 22 via a common underlying drive signal f(t) to provide an AM / FM field output from the EOM 24 of the following form:

[0025]

number

[0026] In the above, parameter B is the nonlinear phase shift (in radians) due to the self-phase modulation associated with the fiber amplifier 28 seeded by the AM / FM source; in other words, the amplified high-power beam emerging from the fiber amplifier 28 experiences a nonlinear phase shift of parameter B.

[0027] In the absence of frequency modulation, the amplitude modulation of the seed beam (1-(β / B)f(t)) 1 / 2 will provide a slight broadening of the linewidth of the seed beam. As is evident from an examination of equation (3), the amplitude modulation is synchronized with the frequency modulation, so that the amplitude peaks are aligned with the phase valleys. Because the EOM24 directly provides amplitude modulation of the beam and does not rely on dispersion to provide the amplitude modulation as in the '252 patent, the amplitude modulation term (1-(β / B)f(t)) 1 / 2 and frequency modulation terms e iβf(t) are precisely matched even for high modulation depth β and / or low amplifier nonlinearity B. It is possible to do so.

[0028]

[0027] It is also clear from equation (3) that it is not necessary to perform the frequency modulation of the seed beam before the amplitude modulation of the seed beam, if the order of the EOM 20 and EOM 24 can be reversed. Furthermore, the FM EOM 20 and the AM EOM 24 can be combined into one device. An embodiment is illustrated by the fiber amplifier system 40 of FIG. 2, in which similar components to the system 10 are indicated by the same reference numbers. In the system 40, the EOM 20 and the EOM 24 are combined into one AM / FM EOM 41, which receives a synchronized drive signal from an RF driver 44 and simultaneously imposes amplitude and frequency modulation on the seed beam. The EOM 42 can be any device suitable for the purposes described herein, such as a broadband low-loss LiNbO3 electro-optic dual-drive Mach-Zehnder interferometric intensity modulator commercially available from EOspace™.

[0029]

[0028] The amplitude and frequency modulated seed beam is then sent to a nonlinear fiber amplifier 28, which may be a number of fiber amplification stages, each stage including a pump beam and a length of doped fiber, such as a fiber doped with Ytterbium (Yb) and having a core of 10-20 μm, and the amplified output beam is provided to a fiber 30. The amplitude and frequency modulation are synchronized according to equation (3), which provides optimal spectral compression of the amplified beam for high power and narrow linewidth for a given nonlinear parameter B of the fiber amplifier 28. The combined amplitude and frequency modulated seed beam is matched to the nonlinearity of the fiber amplifier 28, so that the spectral linewidth is broadened when the seed beam is sent to the amplifier 28. Due to the nonlinear Kerr effect in the fiber amplifier 28, the power-dependent refractive index of the fiber causes a large phase shift in the optical beam at high powers, so that the interaction of the amplitude-modulated power fluctuations in the seed beam produces a synchronous phase shift of the beam in the fiber amplifier 28. The time-dependent nonlinear phase caused by this self-phase modulation is given by

[0030]

number

[0031]

[0029] Therefore, the amplified field emerging from fiber amplifier 28 is:

[0032]

number

[0033]

[0030] Equation (5) shows that the phase shift SPM(t) caused by the nonlinear self-phase modulation cancels the frequency modulation βf(t) previously provided by the EOM 20. The only remaining phase term is the constant global phase shift (constant g The seed beam propagates through the fiber amplifier 28. When amplified, the nonlinear Kerr effect causes self-phase modulation in the amplifier 28, which shifts the power of the beam back to the original linewidth associated with the field E1(t), providing a high-power beam with a narrow linewidth at the output of the fiber amplifier.

[0034] By canceling the frequency modulation of the optical signal through this effect, the spectrum of the original beam E1(t) is transformed at the output of the amplifier 28 into a residual amplitude modulation term (1-(β / B)f(t)) 1 / 2 The spectrum is almost completely recovered, with only a small amount of line broadening resulting from the induced self-phase modulation. The change in spectrum between the input and output of the amplifier 28 (fields E3(t) and E4(t), respectively) reduces the spectral overlap of the backscattered SBS from various positions in a length of the fiber amplifier 28. This increases the SBS threshold compared to the seed spectrum without modulation. In other words, the broadening of the spectral linewidth of the seed beam, represented by field E3(t), when the seed beam is frequency modulated reduces the backscattering of light that overlaps spectrally with the linewidth of the amplified beam, represented by field E4(t). If the optical power is spectrally compressed by the accumulated self-phase modulation as the beam propagates through the fiber amplifier 28, SBS increases, but it is limited by the reduction in spectral brightness early in the beam's propagation.

[0035]

[0032] As mentioned above, the seed beam is first modulated to broaden its spectral linewidth, and then, as the seed beam is amplified and nonlinear phase accumulates, the power is spectrally compressed back to the original linewidth associated with the field E1(t). The backscattered SBS Stokes light from any point in the fiber amplifier 28 represents the local spectrum at that point. The forward propagating beam through most of the fiber amplifier 28 has very little spectral overlap with the backscattered returning wave near the output end of the fiber amplifier 28, so the SBS gain is much lower than without AM / FM modulation. This increases the threshold for SBS, allowing a higher spectral brightness output than the prior art for frequency modulation without self-phase modulation compression. Furthermore, for the non-limiting case of sinusoidal modulation, a judicious choice of modulation frequency of twice the SBS Stokes shift, i.e., 32 GHz, can eliminate much of the self-seeding effect, since it reduces the SBS threshold.

[0036]

[0033] To ensure maximum compression efficiency to the original spectral linewidth associated with the input field E1(t), the magnitude of the amplitude modulation is optimally set to the value described by equation (3): (1-(β / B)f(t)) 1 / 2 This can be done by either changing the modulation depth of the amplitude modulation drive voltage, or by adding a passive delivery fiber after the amplifier 28, which will increase the B integral, or by varying the power of the amplifier 28, which will change the B integral proportionally.

[0037] An example set of modulation parameters useful for illustrating the suppression of SBS is described below. The fiber amplifier 28 may be a 2 kW fiber amplifier with a typical associated B integral of B=10 radians. The modulated RF drive signal is f(t) = sin(ω mt) where ω m / 2π=32 GHz. By choosing a frequency modulation depth of β=2.4 radians, the spectral linewidth of the FM field E2(t) is ∼2βω m / 2π=150 GHz. The EOM24 imposes a synchronous amplitude modulation described by equation (3), such that the AM / FM field becomes:

[0038]

number

[0039]

[0035] The resulting power fluctuations are sinusoidal with a peak-to-peak modulation depth of ∼48% relative to the unmodulated continuous wave power level. Upon amplification in the nonlinear fiber amplifier 28, the accumulated SPM cancels out the imposed frequency modulation, so that the output field is simply:

[0040]

number

[0041]

[0036] The spectral linewidth of the amplified output field is very similar to that of the original input field E1(t). Calculations show that by reducing the spectral brightness of the seed beam over most of the length of the fiber amplifier 28, the expected SBS threshold for this AM / FM configuration will be increased by a factor of ∼2x compared to the unmodulated case. This allows for an output power of ∼2x higher spectral brightness than would otherwise be achievable.

[0042]

[0037] The above-mentioned fiber laser amplifier systems 10 and 40 can be part of any suitable fiber amplifier system, and one skilled in the art will understand how to arrange the various components in accordance with the description herein. For example, if the fiber amplifier system 10 or 40 is part of a coherent beam combining (CBC) fiber amplifier system that includes multiple parallel fiber amplifiers 28 with the same (matched) B integral, the frequency modulated seed beam is split into multiple channels after the EOM 24 or 42. Each channel also includes a phase actuator. If the fiber amplifiers 28 of each channel are not matched in terms of B integral, the seed beam is split between the EOM 16 and the EOM 20 or 42, and the component downstream of the EOM 16 is duplicated for each channel. If the fiber amplifier system 10 or 40 is part of a spectral beam combining (CBC) fiber amplifier system, there will be several fiber amplifier systems 10 or 40, each operating at a different wavelength and having no common components. These fiber amplifier systems are further described below.

[0043]

[0038] FIG. 3 is a schematic block diagram of a fiber laser amplifier system 50, which includes one EOM for providing both frequency and amplitude modulation of the seed beam similar to system 40, where like components are identified with the same reference numerals, and which uses CBC with matched B integrals between fiber amplifiers 28. System 50 includes a beam splitter 52 after EOM 42, which splits the modulated seed beam into multiple channels 54. The modulated seed beam of each channel 54 is sent to a phase actuator 56, which controls the phase of the modulated seed beam of each channel 54 so that they are in phase with each other. The phase-controlled modulated seed beam is then amplified by amplifiers 28 of each channel 54, and the multiple amplified beams are combined by beam combining optics 58, which includes appropriate optics and gratings for CBC of beams having a common wavelength, and the combined ... The beam is output therefrom.

[0044]

[0039] Figure 4 is a schematic block diagram of a fiber laser amplifier system 60 that includes one EOM for providing both frequency and amplitude modulation of the seed beam similar to systems 40 and 50, where like components are identified with the same reference numerals, and uses CBCs with mismatched B integrals between fiber amplifiers 28. System 60 includes a beam splitter 52 that splits the seed beam before it is modulated by EOM 42, and the split seed beam is sent to a number of channels 62. The split beam of each channel 62 is sent to its own EOM 42, and the modulated seed beam is sent to a phase actuator 56 that controls the phase of the modulated seed beam of each channel 54 so that they are in phase with each other. The phase-controlled modulated seed beams are then amplified by amplifiers 28 in each channel 54, and the multiple amplified beams are then combined by beam combining optics 58 and output therefrom as a combined beam.

[0045] 5 is a schematic block diagram of a fiber laser amplifier system 70 including multiple channels 72, each channel 72 having one of the laser amplifier systems 40 and 50, where like components are identified with the same reference numerals, and each MO 12 of each channel 72 operating at a separate wavelength suitable for SBC. The amplified beams from the amplifiers 28 of each channel 72 are combined by beam combining optics 74, which produces a combined beam using appropriate gratings and optics for SBC of the beams having different wavelengths.

[0046]

[0041] The foregoing discussion discloses and describes merely example embodiments of the present disclosure. Those skilled in the art will readily recognize from such description, and from the accompanying drawings and claims, that various changes, modifications, and variations can be made without departing from the spirit and scope of the present disclosure, as defined in the following claims.

Claims

1. 1. A fiber amplifier system comprising: an optical source providing an optical seed beam; a frequency modulation (FM) electro-optic modulator (EOM) responsive to the seed beam and a first RF drive signal, the FM EOM frequency modulating the seed beam with the first drive signal to broaden a spectral linewidth of the seed beam; an amplitude modulated (AM) EOM responsive to the seed beam and a second RF drive signal, the AM EOM amplitude modulating the seed beam with the second drive signal to provide an amplitude modulated seed beam synchronous with the frequency modulated seed beam; a nonlinear fiber amplifier for receiving the AM and FM modulated seed beam and amplifying the seed beam, wherein the amplitude modulated seed beam causes self-phase modulation in the fiber amplifier, phase modulating the seed beam as it is being amplified by the fiber amplifier and acting to counteract spectral line broadening caused by FM EOM.

2. 2. The system of claim 1, wherein the FM EOM and the AM EOM are separate modulation devices.

3. 2. The system of claim 1, wherein the FM EOM and the AM EOM are a combined modulation device.

4. 4. The system of claim 3, wherein the second drive signal is a modified version of the first drive signal that has been phase shifted and amplified for the amplitude modulation.

5. 2. The system of claim 1, wherein the first drive signal and the second drive signal are single tone sinusoidal signals.

6. 6. The system of claim 5, wherein an amplitude of the first drive signal is selected to remove most of the power from a zeroth frequency of the seed beam by the FM EOM.

7. 6. The system of claim 5, wherein an amplitude of the first drive signal is selected to generate equal amplitude power at the 0th and + / -1st order frequencies of the seed beam by the FM EOM.

8. 6. The system of claim 5, wherein the first drive signal has a frequency of 32 GHz.

9. 10. The system of claim 1, further comprising an auxiliary EOM that frequency modulates the seed beam to provide frequency modulation broadening.

10. 2. The system of claim 1, wherein the optical source is a master oscillator.

11. 10. The system of claim 1, wherein the fiber amplifier system is part of a coherent beam combining (CBC) fiber amplifier system or a spectral beam combining (SBC) fiber amplifier system.

12. 1. A fiber amplifier system comprising: an optical source providing an optical seed beam; an auxiliary electro-optic modulator (EOM) for frequency modulating the seed beam to provide frequency modulated broadening; a combined frequency modulation (FM) and amplitude modulation (AM) EOM responsive to the seed beam, a first RF drive signal, and a second RF drive signal, the FM and AM EOM frequency modulating the seed beam with the first drive signal to broaden a spectral linewidth of the seed beam, and amplitude modulating the seed beam with the second drive signal to provide an amplitude modulated seed beam synchronous with the frequency modulated seed beam; a nonlinear fiber amplifier that receives the AM and FM modulated seed beam and amplifies the seed beam, the amplitude modulated seed beam causing self-phase modulation in the fiber amplifier, phase modulating the seed beam as it is being amplified by the fiber amplifier and serving to counteract spectral line broadening caused by FM and AM EOM; 1. A fiber amplifier system comprising:

13. 13. The system of claim 12, wherein the second drive signal is a modified version of the first drive signal that has been phase shifted and amplified for the amplitude modulation.

14. 13. The system of claim 12, wherein the first drive signal and the second drive signal are single tone sinusoidal signals.

15. 15. The system of claim 14, A system in which the amplitude of the first drive signal is selected to remove most of the power from a zeroth frequency of the seed beam by the FM and AM EOM.

16. 15. The system of claim 14, wherein an amplitude of the first drive signal is selected to produce equal amplitude power at the 0th and + / -1st order frequencies of the seed beam in the FM and AM EOMs.

17. 1. A method for amplifying an optical seed beam, comprising the steps of: frequency modulating the seed beam with a first RF drive signal to broaden its spectral linewidth; frequency modulating the seed beam with a second RF drive signal to provide an amplitude modulated seed beam synchronous with the frequency modulated seed beam; amplifying the frequency-modulated and amplitude-modulated seed beam in a nonlinear fiber amplifier such that the amplitude-modulated seed beam produces self-phase modulation in the fiber amplifier, modulating the seed beam as it is being amplified by the fiber amplifier and serving to cancel out the spectral linewidth affected by the frequency modulation; The method includes:

18. 20. The method of claim 17, wherein the first drive signal and the second drive signal are single tone sinusoidal signals.

19. 20. The method of claim 18, wherein the amplitude of the first drive signal varies during the frequency modulation. wherein the zeroth frequency is selected to remove most of the power from the zeroth frequency of the seed beam.

20. 20. The method of claim 18, A method according to claim 1, wherein an amplitude of the first drive signal is selected to produce equal amplitude power at the 0th and + / -1st order frequencies of the seed beam during the frequency modulation.

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