Resistance of fiber amplifier systems to nonlinear spectral broadening and decoherence.

Abstract

A method for reducing nonlinear frequency shift and suppressing stimulated Brillouin scattering (SBS) in a fiber laser amplifier system includes providing at least one seed beam having a specific wavelength and frequency-modulating the seed beam using an RF waveform, the RF waveform being a relatively slow waveform with a large modulation depth, to spectrally broaden the seed beam. The method also includes amplifying the frequency-modulated seed beam using an amplifier that has a large nonlinear phase shift and exhibits frequency modulation (FM) to amplitude modulation (AM) conversion, the modulation depth being significantly greater than the nonlinear phase shift of the amplifier.

Description

[Technical field]

[0001]

[0001] This disclosure relates generally to a method for reducing nonlinear frequency shift driven by frequency modulation (FM) to amplitude modulation (AM) conversion in a fiber laser amplifier system, and more specifically to a method for reducing nonlinear frequency shift driven by FM-to-AM conversion and suppressing stimulated Brillouin scattering (SBS) in a fiber laser amplifier system, which modulates a seed beam with a slow, high modulation depth RF waveform to broaden the beam width. [Background technology]

[0002]

[0002] 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. Fiber laser amplifiers are useful as energy sources for directed energy weapons due to their high efficiency, high power scalability, and excellent beam quality.

[0003]

[0003] Improvements in fiber laser amplifier design have increased the fiber output power to approach its practical power and beam quality limits. To further increase the fiber amplifier output power, some fiber laser systems use multiple fiber laser amplifiers and somehow combine the amplified beams to produce high laser beam power. The design challenge for this type of fiber laser amplifier system is to combine the multiple beams from the multiple fiber amplifiers in such a way that they provide one combined beam output that can be focused to a small focal spot. Focusing the combined beam to a small spot over a long distance (far field) defines the beam quality.

[0004]

[0004] In one known multiple fiber amplifier design, called coherent beam combining (CBC), a master oscillator (MO) generates a seed beam that is split into multiple split seed beams, each with a common wavelength, and each seed beam is amplified. The amplified seed beams are then directed to a diffractive optical element (DOE) or other optical system, which combines the coherent amplified beams into one output beam. The DOE has a periodic structure formed in the element, so that when the individual amplified beams, each with a slightly different angular orientation, are redirected by the periodic structure, all of the beams are diffracted in the same direction from the DOE. For each seed beam, an error signal derived from a sample of the amplified output beam is provided to a phase modulator, which controls the phase of the beam so that the phase of all seed beams remains coherent. The main requirement for a CBC fiber amplifier system is high beam coherence, which equates to a narrow spectral linewidth of the beam. For CBC laser systems using spectrally broad Ytterbium (Yb) doped fiber amplifiers, the fiber path lengths (time of flight) must be precisely matched to within a small fraction of the laser coherence length, which is a technical challenge to broaden linewidths: limitations on phase control bandwidth and wavefront errors limit the number of amplified beams that can be coherently combined, which limits the output power of the fiber laser amplifier system.

[0005]

[0005] In another known multiple fiber amplifier design, called Spectral Beam Combining (SBC), multiple master oscillators (MOs) generate multiple seed beams of different wavelengths, and each seed beam is amplified. The multiple amplified seed beams are then directed to a diffraction grating or other wavelength-selective element, which combines the fiber beams of different wavelengths into one combined output beam. The diffraction grating has a periodic structure formed in the grating, so that when the individual amplified beams, each with slightly different wavelengths and angular directions, are redirected by the periodic structure, all of the beams are diffracted in the same direction from the diffraction grating. The main requirement for an SBC system is a narrow spectral linewidth. For a CBC laser system using a spectrally broad Yb-doped fiber amplifier, the beam quality of the combined output beam can be degraded by the angular dispersion from the diffraction grating. By maintaining a narrow linewidth in the fiber amplifier, more wavelength channels can be packed into the amplifier tuning range, allowing scaling to higher powers. However, spectral brightness limitations limit the number of fiber beams that can be wavelength-combined, and therefore limit the output power of the fiber laser amplifier system.

[0006]

[0006] To overcome these limitations and further increase the laser beam power, multiple master oscillators can be provided to generate seed beams at various frequencies, where each of the seed beams at individual wavelengths is split into multiple seed beams, and the seed beams in each group have the same wavelength and are mutually coherent, thus providing a hybrid fiber laser amplifier system with CBC and SBC. Each group of coherent seed beams at each wavelength is first coherently combined by a DOE or other optical system so that the beams in the group propagate together as a group of coherently combined beams, and then each group of coherently combined beams is directed at a slightly different angle to the diffraction grating of the SBC, which diffracts the beams in the same direction as one combined beam of multiple wavelengths. The diffraction grating of the SBC also includes a periodic structure to combine the beams of different wavelengths.

[0007]

[0007] The power scaling of narrow-linewidth Yb-doped fiber amplifiers is currently limited by two distinct nonlinear optical defects, specifically stimulated Brillouin scattering (SBS) and self-phase modulation (SPM), with SBS being the primary nonlinear defect. SBS is a nonlinear effect in which the laser electric field creates a phase grating in the fiber core through electrical distortion, which reflects some small fraction of the forward-propagating beam. If the effective reflectivity of the grating becomes too large, the output power from the fiber amplifier is reduced, and the lost power is reflected back to upstream low-power components, eventually causing catastrophic damage. Since the SBS threshold is proportional to the spectral brightness (~power / linewidth), SBS limits the usable power from a single-frequency fiber laser to a few hundred watts. To increase the SBS threshold power, it is common to broaden the spectrum of the fiber laser's input seed into the multi-GHz domain using phase modulation, or eventually frequency modulation (FM). This reduces the optical coherence length and therefore the SBS gain. As the power of the Yb-doped fiber amplifier increases to several kW levels or the length of the delivery fiber increases, a wider FM linewidth is required to suppress SBS. Typically, the linewidth increases almost linearly with power, on the order of about (~) 10-20 GHz / kW for Yb-doped fiber amplifiers. Reducing the SBS-limited linewidth, i.e., increasing the brightness of the SBS-limited fiber spectrum, may enable the scaling of beam-combined fiber laser systems to higher powers.

[0008]

[0008] The source of the master oscillator seed beam for the laser chain is typically a single frequency laser source. The optical linewidth of the seed beam source is typically broadened by imposing FM on the seed beam using an electro-optic phase modulator (EOM) driven by an RF waveform to suppress SBS in the subsequent high power fiber amplifier chain. The broadened optical linewidth is typically proportional to the FM modulation depth, which is proportional to the amplitude of the voltage of the applied RF waveform. It should be noted that the term "high modulation depth RF waveform" as used herein refers to an RF waveform with a sufficiently high voltage that results in a high modulation depth phase shift when used to drive the EOM.

[0009]

[0009] There are many public documents that study the effects of various RF waveforms used to drive EOMs to apply FM to broaden the linewidth of a laser seed beam to suppress SBS. In general, the goal of known techniques is to raise the SBS threshold as high as possible for a given optical spectral linewidth. Two common RF waveforms used to drive EOMs to apply FM for SBS suppression are white noise and pseudorandom bit sequences (PRBS). Another technique applies a piecewise parabolic RF waveform to an EOM to integrate a seed beam source, whose optical frequency is repetitively chirped up and down in time. This technique predicts that a repetitively chirped seed beam source can provide ~1.9x better SBS suppression than a noise-modulated seed and ~1.2x better than a PRBS. Chirped seed beams have improved SBS performance due to their flat, wing-free spectrum and the ability to adjust the chirp rate and period to best match the effective SBS interaction length in the fiber. The compact chirped spectrum is a better fit for dense SBCs than PRBS or noise spectra, where angular dispersion from the synthetic grating results in broad spectral wings that degrade the synthetic beam quality.

[0010]

[0010] Optical defects, SPM, can degrade beam coherence by converting low levels of uncontrolled AM, parameterized by the B-integral, i.e., a nonlinear phase shift, into phase noise. This nonlinear effect can limit the efficiency of CBC or the beam quality of SBC, thus reducing the performance of the fiber laser system. In particular, there is a loss of spectral brightness and a loss of optical coherence. To avoid or reduce these effects, it is generally desirable to limit the amount of AM, also known as the relative intensity noise (RIN), that propagates in the seed beam that seeds the fiber amplifier. Techniques can be implemented in fiber amplifiers to broaden the spectrum of the seed beam to provide frequency modulation without providing amplitude modulation, in which case the Kerr nonlinearity that drives the SPM does not cause a problem if the seed beam is only frequency modulated, i.e., a non-time-dependent nonlinear phase shift of the seed beam. However, if AM is imposed, either deliberately or inadvertently, by FM-to-AM conversion, SPM can cause nonlinear spectral broadening of the beam emitted from the fiber amplifier, which can degrade the beam quality during SBC.

[0011]

[0011] In general, in coherently beam-combining fiber laser systems, it is desirable to maintain the nonlinear SPM phase variation such that B*RIN<0.1 radians in order to keep the beam quality degradation, as determined by the Strehl ratio, below 1%. For a typical 1.5-2 kW fiber amplifier with a B integral of 10 radians, this implies a requirement to maintain RIN<1%. To avoid nonlinear beam quality degradation from SPM in SBC fiber laser systems, any nonlinear frequency shifts over a small fraction of the linewidth of the original FM seed beam must be suppressed. Therefore, it is the industry standard to use FM seed beam sources with little or no AM, i.e., constant power vs. time. However, many effects have been observed that still partially convert FM into uncontrolled AM, which can cause nonlinear degradation through SPM. These effects include polarization mixing, chromatic dispersion, spectral filtering, or, in general, the effect of multipath interference (MPI). A typical signature of MPI effects in fiber components or fiber-based systems is a spectrally dependent transmission that exhibits a periodic modulation pattern.

[0012]

[0012] The effect of multipath interference in the components of a fiber amplifier system has been observed to partially convert FM to uncontrolled AM (FM-AM conversion), which can be the cause of nonlinear degradation through SPM. Many different physical effects are responsible for FM-AM conversion, including dispersion, etaloning at component faces or between splices, polarization mixing and spatial mode mixing. The effect of FM-AM conversion can be inferred from the measured spectral transmission of the fiber amplifier. If the measured spectral transmission is strongly frequency dependent, then a change in the instantaneous frequency of the laser with time due to the FM imposed on the seed beam will also change the transmission of the fiber amplifier, resulting in a time-dependent change in power (AM). Since FM-AM conversion is an interferometric phenomenon, it is difficult to eliminate at the component level. AM becomes large when the SBS-limited FM seed beam linewidth increases with power (typically ~10-20 GHz / kW). Since SPM scales as the product of AM and the fiber B-integral, which also scales with power (typically ~5 rad / kW for commercial fiber amplifiers), SPM-driven spectral broadening can grow fast with power and become the dominant mechanism limiting the combinability of multi-kW fibers.

[0013]

[0013] It is known in the art that for FM linewidths much smaller than the free spectral range (FSR) of the spectral modulation, the magnitude of the FM-AM conversion can vary greatly depending on the relative wavelength of the seed beam and the spectral conduction peak. FM-AM conversion in a fiber is minimized when the beam wavelength is aligned with a peak or null in the conduction spectrum, and is maximized when the beam wavelength is between the peak and null. This is because the instantaneous frequency of the FM signal varies with time, and therefore its conduction amplitude also varies with time, which gives rise to a time-varying output power, i.e., AM. The magnitude of the AM-FM conversion is minimized when the spectral conduction across the beam bandwidth is made as uniform as possible, which occurs near the peak or null of conduction. Summary of the Invention

[0014]

[0014] Various techniques in this technology for reducing SPM have been introduced. One such technique involves applying spectral shaping of the input seed beam to compensate for the non-uniform spectral conductance of the Yb-doped fiber amplifier. A spectral shaper is inserted into the low-power seed beam fiber to apply the inverse of the spectral conductance imposed by the fiber amplifier. This has been shown to eliminate AM and the resulting SPM-driven spectral broadening. Its drawback is the increased complexity and hardware due to the need to add spectral detectors, actuators, and active control systems. Another technique involves controlling the polarization of the input seed beam to partially compensate for the FM-AM conversion in the Yb-doped fiber amplifier. This works when the FM-AM conversion process is driven by the polarization mixing effect. However, it is less effective for other causes of FM-AM conversion and adds complexity by requiring additional detectors, actuators, and controllers. Despite these efforts, there is a need for techniques to mitigate SPM-driven nonlinear spectral broadening without the need for additional detectors, actuators, or control systems.

[0015]

[0015] A depolarized seed source may be desirable for SBC-based laser weapon systems because it offers additional SBS suppression compared to polarized seeds, thus allowing for narrower beam linewidths. Compared to polarized seed sources, depolarized seed beams are particularly susceptible to FM-AM conversion because any component in the fiber amplifier chain acting as a partial polarizer whose axis is not aligned with the axis of the depolarizer can cause mixing of previously orthogonal polarization states, resulting in large AM. [Brief description of the drawings]

[0016] [Figure 1] Figure 1 is a schematic block diagram of a fiber laser amplifier system that employs a slow and large modulation depth RF waveform to modulate a seed beam to provide increased beam linewidth, a method for reducing nonlinear frequency shifts and for suppressing SBS. [Diagram 2] Figure 2 is a schematic block diagram of an SBC fiber laser amplifier system that employs a slow and large modulation depth RF waveform to modulate the seed beam to provide increased beam linewidth, a method for reducing nonlinear frequency shifts and for suppressing SBS. [Diagram 3] Figure 3 is a schematic block diagram of a fiber laser amplifier system that uses a slow and large modulation depth RF waveform to modulate the seed beam, to provide increased beam linewidth and to provide depolarization of the seed beam using split-beam time delays, a method to reduce nonlinear frequency shifts and to suppress SBS. [Figure 4]Figure 4 is a schematic block diagram of a fiber laser amplifier system that uses a slow, large modulation depth RF waveform to modulate the seed beam to provide increased beam linewidth and to provide depolarization of the seed beam using a 45° spliced ​​polarization maintaining fiber, a method for reducing nonlinear frequency shifts and suppressing SBS. [Diagram 5] Figure 5 is a schematic block diagram of a CBC fiber laser amplifier system that uses active phase control to reduce decoherence resulting from nonlinear frequency shifts driven by FM-AM conversion. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017]

[0021] The following discussion of embodiments of this disclosure relating to a fiber laser amplifier system that uses a slow, high modulation depth RF waveform to modulate a seed beam to provide increased beam linewidth, various methods to reduce nonlinear frequency shifts, and suppress stimulated Brillouin scattering (SBS) is merely exemplary in nature and is in no way intended to limit this disclosure or its applications or uses.

[0018]

[0022] As will be discussed in detail later, this disclosure describes various architectures and techniques for reducing the effects of nonlinear imperfections in spectrally and coherently beam combined, multi-kW fiber laser amplifiers used, for example, for laser weapon systems. The architectures include replacing the fast (high frequency) electrical RF waveforms used in known systems to broaden the laser beam linewidth with slow (low frequency) high modulation depth RF waveforms. For known fast electrical RF waveforms, it has typically been desirable to set the frequency of the RF waveform relatively high based on the amount of optical linewidth required to suppress SBS, because the closer the frequency content of the electrical waveform is to the desired optical linewidth, the less the electrical waveform needs to be amplified to provide that linewidth. Thus, it is straightforward and technically simple to generate high frequency, low amplitude, i.e., low voltage, electrical waveforms to obtain the desired optical linewidth. Calculations have shown that low frequency, i.e., slow RF waveforms, provide resistance to the nonlinear spectral broadening that currently limits laser scaling. The AM dynamics resulting from FM-AM conversion tend to track the dynamics of the RF electrical waveform. Hence, the nonlinear frequency shift due to SPM driven by the AM dynamics also tends to track the dynamics of the RF electrical waveform. Thus, for a given optical linewidth, a fast RF waveform provides a nonlinear spectral broadening that adds multiples of the RF waveform to the output linewidth, which is significant compared to the original linewidth. For a slower but higher voltage, i.e., higher modulation depth, RF waveform that gives the same optical linewidth, the nonlinear spectral broadening is a multiple of the lower frequency, which is negligible compared to the original optical linewidth. Thus, for a fast RF waveform, the optical linewidth is broadened more than linearly as desired, and for a slow RF waveform, the optical linewidth does not change significantly.

[0019]

[0023] As discussed above, the inventive concept for mitigating SPM-driven spectral broadening is to replace the known high-speed, low-modulation-depth RF waveforms used for FM line broadening with slow, high-modulation-depth RF waveforms. The AM resulting from uncontrolled FM-to-AM conversion in fiber amplifiers generally tracks the dynamics of the applied RF. Hence, the nonlinear frequencies generated by SPM are small and the spectral broadening is minimized.

[0020]

[0024] This concept can be roughly parameterized to provide a rough approximation of the benefits. The fiber laser amplifier system is assumed to have a single frequency RF modulation source and a spectral conductance function that varies slowly across the laser beam optical linewidth. The variables referenced below are f as the RF modulation frequency. mod (Hz), β (radians) as the FM modulation depth, B (radians) as the nonlinear SPM phase shift imposed by the fiber amplifier, Δν (Hz) as the laser optical linewidth, and δν (Hz) as the nonlinear frequency shift due to SPM. SPM (Hz).

[0021]

[0025] The optical linewidth, ignoring certain pre-factors, is approximately δν = βf mod It is.

[0022]

[0026] With a slowly varying (large free spectral range) spectral transfer function, the AM frequency resulting from the FM-AM conversion varies with the applied RF modulation frequency f mod In the worst case scenario (100% AM), the maximum nonlinear frequency shift due to SPM is δν SPM = Bf mod It becomes.

[0023]

[0027] Therefore, the upper estimate is that the input spectrum is δν SPM / Δν = B / β That is, the width is nonlinearly widened by

[0024]

[0028] This means that for β>>B, the nonlinear spectral broadening should be small, which means that for a given SBS-limited optical linewidth, the nonlinear spectral broadening should be small at low frequencies f mod and suggests that a design with an RF modulation waveform that provides a large modulation depth β will provide the greatest resistance to nonlinear spectral broadening.

[0025]

[0029] 1 is a simplified block diagram of a fiber laser amplifier system 10 that provides FM line broadening in a manner that reduces the nonlinear frequency shift driven by FM-AM conversion and provides SBS suppression, as discussed above. The system 10 includes a single amplification channel 12 having a master oscillator (MO) 14 that generates a seed beam on a fiber 16 at a particular wavelength. The seed beam is provided to an RF electro-optic modulator (EOM) 18, which receives an RF waveform from an RF source 20 that modulates the seed beam to provide the spectral broadening, which RF source 20 includes an amplifier 22 for amplifying the RF waveform. However, instead of providing a high speed, low amplitude RF signal, such as white noise or pseudorandom bit sequence (PRBS), as is done in known fiber amplifier systems, the RF source 20 provides a low speed, such as 100 MHz, high modulation depth, such as 100 radians, RF waveform to achieve the desired optical line broadening, such as 100 radians x 100 MHz = 10 GHz. In another embodiment, the RF waveform can be, for example, a piecewise parabolic RF waveform that causes the instantaneous laser frequency to exhibit a periodic chirp, which produces an FM optical spectrum with less power in the spectral wings for a given optical linewidth. It should be noted that the EOM 18 can be located at any suitable location in the system 10 before the seed beam is amplified.

[0026]

[0030] The spectrally broadened seed beam is then sent to a nonlinear fiber amplifier 24, which may be a number of fiber amplification stages, each of which may include a pump beam source and a length of doped fiber, such as a length of ytterbium (Yb) doped fiber with a 10-20 μm core, to amplify the seed beam and provide an amplified beam. Each of the fiber amplification stages may include successive components (not shown), such as optical isolators, tap couplers, pump signal combiners, fiber pigtails, optical filters, and the like. These components and the doped fiber stages impose an FM-AM conversion on the spectrally broadened seed beam. The RF waveform has a large modulation depth β, and the amplifier 24 has a large nonlinear phase shift B>>1, and the system 10 is configured such that β>>B to avoid SPM driven spectral broadening. The amplified beam is provided on an output fiber 26.

[0027]

[0031] 2 is a simplified block diagram of an SBC fiber laser amplifier system 30 that provides FM line broadening in a manner that reduces the nonlinear frequency shift driven by FM-AM conversion and provides SBS suppression, as described above, with elements similar to system 10 identified with the same reference numerals. System 30 includes multiple system channels 12, each having an MO 14 that generates a seed beam on a fiber 16, with the MO 14 of different channels generating seed beams at different wavelengths. The amplified beams from each of the amplifiers 24 of the channels 12 are sent to an SBC combining optics 32 that includes a grating (not shown) having a periodic structure formed therein, such that when the individual amplified beams, each with slightly different wavelengths and angular orientations, are redirected by the periodic structure, all of the beams are diffracted from the grating into the same direction as the combined output beam 34.

[0028]

[0032] The above analysis is valid for polarized seed beams. For depolarized beams generated using known FM in combination with a depolarizer that induces a birefringence time delay of the order of the coherence time 1 / Δν or larger, the AM frequency produced by the subsequent polarization mixing (FM-to-AM mixing) is similar to the optical linewidth, and therefore the nonlinear frequency shift due to SPM is δν SPM = BΔν and the fractional spectral broadening is given by δν SPM / Δν = B is given by:

[0029]

[0033] Therefore, the spectral broadening of the depolarized beam is not expected to depend on the RF waveform, but only on the optical linewidth. Reducing the birefringence delay to values ​​smaller than the laser coherence time reduces the AM dynamics and therefore the spectral broadening, but at the cost of increasing the degree of polarization (DOP) of the laser beam, measured over timescales of ~10s of ns associated with SBS dynamics, which increases the SBS gain and requires a wider linewidth to suppress SBS. The reason for the increased DOP is that with low modulation depth RF waveforms and with birefringence delays smaller than the optical coherence time, there is a critical time window where the birefringence phase changes by an amount smaller than π, resulting in residual polarization.

[0030]

[0034] If a slow RF waveform with high modulation depth is used instead of a fast, low modulation depth waveform, the birefringence delay can be reduced to values ​​much smaller than the laser coherence time without increasing the DOP on the tens of nanoseconds time scale associated with SBS. The reason is that even with the small birefringence delay due to the large modulation depth, the polarization Stokes vector traverses a complete rotation in the Poincaire sphere, i.e., a full 2π birefringence phase shift, multiple times over the time window of about tens of nanoseconds associated with SBS.

[0031]

[0035] It should be noted that triangular frequency chirp waveforms exhibit a periodic change in the sign of the chirp, which corresponds to a periodic slowing down that is followed by a reversal in the direction of the state of polarization (SOP) dynamics. These SOP slowing downs and reversals occupy a window of twice the birefringence delay, so they should not affect SBS suppression as long as this delay is short compared to the SBS coherence time, which is about tens of ns. This condition can be easily satisfied for chirp rates of practical interest for SBS suppression, which are typically in the range of ~20-100 MHz.

[0032]

[0036] 3 is a schematic block diagram of a fiber laser amplifier system 60, such as one channel of an SBC fiber laser amplifier system, that uses methods to reduce the nonlinear frequency shift driven by FM-AM conversion using a slow RF waveform, as described above, and to provide depolarization of the seed beam using a depolarization assembly 62 before the seed beam is amplified, where similar elements to system 10 are identified with the same reference numerals. The broadened and polarized seed beam from EOM 18 is sent to the depolarization assembly 62, where the beam is split by a 50% beam splitter 64, and one split beam is sent to a time delay device 66, i.e., a length of fiber, to provide a birefringent time delayed beam, where the value of the time delay can be much smaller than the laser coherence time (~1 / optical linewidth) without increasing the DOP over the time scales relevant for SBS suppression due to the low frequency, high modulation depth RF modulation waveform. The delayed split beam and the other split beam from splitter 64 are combined by polarizing beam splitter 68 of assembly 62 so that the output state of polarization (SOP) of the combined beam exhibits temporal dynamics that are significantly smaller than the optical linewidth, but fast enough that, due to warping, on the time scales relevant for SBS, the beam appears completely depolarized, with the time-delayed beam oriented along one polarization axis and the other beam oriented along the orthogonal axis.

[0033]

[0037] The SOP temporal dynamics can be chosen anywhere from DC (~zero frequency) to the full optical linewidth by choosing a value of the birefringence delay ranging from zero to the laser coherence time. The birefringence delay can be chosen such that it is much smaller than the laser coherence time, but large enough that the SOP dynamics are fast compared to the time scales relevant for SBS (~tens of ns). The AM frequency created by polarization mixing (FM-AM conversion) after the fiber amplifier 24 follows the SOP dynamics. Thus, if the birefringence time delay is set to a value much smaller than the laser coherence time, the resulting AM dynamics will be much smaller than the optical linewidth, and the nonlinear frequency shift due to SPM will also be a small fraction of the optical linewidth. Hence, the nonlinear spectral broadening is reduced compared to a system where the birefringence time delay is set to a value similar to the laser coherence time.

[0034]

[0038] 4 is a schematic block diagram of a fiber laser amplifier system 70, such as one channel of an SBC fiber laser amplifier system, that uses methods to reduce the nonlinear frequency shift driven by FM-AM conversion and provide SBS suppression as described above, and to provide depolarization of the seed beam before it is amplified using a depolarization assembly 72, with elements similar to those in system 10 identified by the same reference numerals. In this embodiment, the depolarization assembly 72 includes a birefringent polarization-maintaining (PM) fiber 74 that is spliced ​​at 45 degrees to the input PM fiber, such that equal power is launched into each of the slow and fast axes of the fiber 74. The birefringence time delay is determined by multiplying the length of the PM fiber 74 by its birefringence, and is typically selected to be much smaller than the laser optical coherence time for reasons similar to those described above.

[0035]

[0039] The underlying physics of nonlinear SPM defects is the same for CBC and SBC, but the system impact is different. For CBC, the problem is not nonlinear spectral broadening per se, but rather nonlinear phase fluctuations that are faster than the system's ability to correct for phase changes that reduce the time-averaged coherence and therefore limit the CBC combining efficiency, which is 1-ΔΦ 2 where ΔΦ is the RMS phase variation in radians. For example, if the RMS nonlinear phase error is 0.1 radians, the CBC efficiency will decrease by 1%.

[0036]

[0040] Using slow RF waveforms does not directly affect the magnitude of the phase variation ΔΦ or the loss of coherence over timescales relevant for active servo-based phase locking (typically on the order of tens of kHz), where the amplified beam exhibits a similar RMS phase noise ΔΦ due to SPM, regardless of the speed of the RF signal. However, slow RF waveforms do not allow the expectation of direct time-domain compensation of the SPM to restore coherence and CBC efficiency, because the associated AM and SPM dynamics are slow and periodic, which makes active detection and feedback-based control feasible using relatively slow detection and slow controllers.

[0037]

[0041] These sub-GHz dynamics are well within the capabilities of modern EOM phase actuators and drive electronics. Since the SPM dynamics exhibit the same periodicity as the RF drive waveform, and are also essentially stationary if not, they can be detected and averaged over multiple cycles, and feedback control for compensation can be similarly slow, with the control bandwidth being driven by the rate of external changes to the fiber parameters, e.g., drift in power or polarization, which are typically in the Hz range and not due to the SPM dynamics themselves.

[0038]

[0042] FIG. 5 is a schematic block diagram of a CBC fiber amplifier system 80 in which SPM is directly sensed and compensated in the time domain. The system 80 includes an MO 82 that generates a seed beam that is fed to an FM EOM 84. The EOM 84 receives a low-speed RF waveform from a waveform generator 86 controlled by a low-speed clock signal provided by a clock 88 on a seed beam path 90, e.g., tens of MHz, that provides the periodicity for repeating the RF waveform used to broaden the seed beam linewidth, and the clock signal is also provided to a reference path. The waveform generator 86 may include a high-voltage RF amplifier to provide a high modulation depth RF waveform. The modulated seed beam is split by a splitter 94, and the split seed beam is fed to multiple CBC channels 96 and a reference path 98. The split and modulated seed beams are sent to a control EOM 100 for each channel 96, which may also act as a piston phase servo-actuator for the system 80, and then amplified to high power in a nonlinear fiber amplifier 102 where they are spectrally distorted, i.e., FM-AM conversion occurs. The amplified beams of each channel 96 are collimated in a collimating optic 104 and then combined with other amplified beams in a CBC combining optic 106 to produce an output laser beam 108. As discussed above, the CBC combining optic 106 may include a DOE or other suitable optic that combines a coherent amplified beam, where the DOE has a periodic structure formed within the element such that when the individual beams, each with slightly different angular orientations, are redirected by the periodic structure, all of the beams are diffracted from the DOE in the same direction. A sample of the combined output beam 108 is provided by a beam sampler 110 to a phase detector 112 along with a reference beam in path 98.Phase error signals detecting phase variations between the reference beam and the sample beam of each channel 96 are provided to separate synchronous detection and averaging devices 114 which average the phase error variations over a period of time determined by the clock signal on path 92, the clock 88 acting as a local oscillator for the RF mixer, and the averaged error signals are provided to separate feedback controllers 116 for each channel which control the EOM 100 to provide phase control.

[0039]

[0043] Numerous variations of the system 80 are possible, particularly those involving various electronic methods of phase locking and coherence measurement in a CBC fiber channel array environment. In particular, it may be possible to avoid the use of any RF class detection (reducing the bandwidth requirements from sub-GHz to about 10 times the disturbance frequency, which may be Hz) by simply using time-averaged coherence and related metrics to identify SPM and correct with simplified control schemes. Furthermore, it may be possible to sense and correct SPM dynamics using amplitude rather than phase detection and / or actuation, i.e., sensing synchronous output power fluctuations and applying corresponding phase inversion or AM to the seed input.

[0040]

[0044] Note that single-tone modulation may be an ideal RF drive waveform for CBC fiber amplifier applications. By using a single-tone RF, the abrupt transitions exhibited by the triangular frequency chirp are eliminated, which also eliminates the abrupt changes in the slope of the SPM. The SPM dynamics for a single-tone RF are limited to the fundamental and second harmonics of the RF frequency, thus relaxing the detection and triggering bandwidth requirements. For example, with an RF frequency of 33 MHz, the detection and triggering bandwidth can be less than 100 MHz. The number of control parameters is greatly reduced. Simply adjusting the RF phase and amplitude of the first one or two harmonics of the slow RF may be enough to provide a flattened phase profile in time.

[0041]

[0045] Furthermore, all-electronic path matching appears feasible using the slow RF techniques described above. By using a slow RF signal, uncontrolled variations between channels can be sensed and actively compensated for. Applying an RF time delay will then ensure that the laser at the output is coherence path matched.

[0042]

[0046] The foregoing discussion discloses and describes merely exemplary embodiments of the present disclosure, and those skilled in the art will readily recognize from such discussion and from the accompanying drawings and claims that various changes, modifications, and variations are possible therein without departing from the spirit and scope of the present disclosure, as defined in the following claims.

Claims

1. 1. A method for reducing nonlinear frequency shift and suppressing stimulated Brillouin scattering (SBS) in a fiber laser amplifier system, comprising: providing at least one seed beam having a particular wavelength; frequency modulating the at least one seed beam with an RF waveform to spectrally broaden the at least one seed beam, the RF waveform being a relatively low frequency waveform having a large modulation depth; amplifying the modulated seed beam with an amplifier having a large nonlinear phase shift and exhibiting frequency modulation (FM) to amplitude modulation (AM) conversion, the modulation depth being significantly greater than the nonlinear phase shift; The method includes:

2. 2. The method of claim 1 , wherein the step of frequency modulating the at least one seed beam with an RF waveform comprises the step of frequency modulating the at least one seed beam with a piecewise parabolic RF waveform.

3. 2. The method of claim 1, wherein the step of frequency modulating the at least one seed beam with an RF waveform comprises frequency modulating the at least one seed beam with a single tone RF waveform.

4. 4. The method of claim 3, wherein the frequency of the single-tone RF waveform is 100 MHz.

5. 2. The method of claim 1, further comprising splitting the frequency modulated seed beam into a plurality of frequency modulated seed beams, controlling the phase of each split seed beam with a phase controller, and amplifying the frequency modulated split seed beams with a plurality of amplifiers, each amplifier having a large nonlinear phase shift and exhibiting FM-AM conversion, the method further comprising providing the amplified beams to a coherent beam combining optics that combines the amplified seed beams.

6. 6. The method according to claim 5, further comprising the steps of: synchronizing the RF waveform with a clock signal; synchronously detecting a phase error of the split seed beam using the clock signal; and applying the detected phase error to the phase controller to lock the amplified beams in phase with each other.

7. 2. The method of claim 1, wherein the at least one seed beam is a plurality of seed beams having different wavelengths, each seed beam being frequency modulated by the RF waveform, and the modulated seed beams are amplified by a plurality of amplifiers, each amplifier having a large nonlinear phase shift and exhibiting FM-AM conversion, the method further comprising providing the amplified beams to a spectral beam combining optics that spectrally combines the amplified seed beams.

8. 2. The method according to claim 1, further comprising the step of depolarizing said at least one seed beam before it is amplified.

9. 9. A method according to claim 8, wherein the step of depolarizing the seed beam comprises the steps of splitting the seed beam into two seed beams, delaying one of the split seed beams, and combining the delayed split seed beam with the other split seed beam, the delay being set to a value significantly smaller than an optical coherence time of the frequency modulated seed beam.

10. 9. A method according to claim 8, wherein the step of depolarizing the seed beam comprises providing the seed beam into a birefringent polarization-maintaining (PM) fiber spliced ​​at 45 degrees to an input PM fiber such that equal power is launched into each of the slow and fast axes of the PM fiber to induce a birefringence delay, the birefringence delay being set to a value significantly smaller than the optical coherence time of the modulated seed beam by selecting the length and birefringence of the PM fiber.

11. 1. A method for reducing nonlinear frequency shift and suppressing stimulated Brillouin scattering (SBS) in a fiber laser amplifier system, comprising: providing at least one seed beam having a particular wavelength; frequency modulating the at least one seed beam with an RF waveform to spectrally broaden the at least one seed beam, the RF waveform being a relatively low frequency waveform having a large modulation depth; amplifying the frequency modulated seed beam with an amplifier; The method includes:

12. 12. The method of claim 11, wherein the step of frequency modulating the at least one seed beam with an RF waveform comprises frequency modulating the at least one seed beam with a piecewise parabolic RF waveform.

13. 12. The method of claim 11, further comprising splitting the frequency modulated seed beam into a plurality of frequency modulated seed beams, controlling the phase of each split seed beam with a phase controller, and amplifying the frequency modulated split seed beams with a plurality of amplifiers, each amplifier having a large nonlinear phase shift and exhibiting frequency modulation (FM) to amplitude modulation (AM) conversion, the method further comprising providing the amplified beams to a coherent beam combining optics that combines the amplified seed beams.

14. 12. The method of claim 11, wherein the at least one seed beam is a plurality of seed beams having different wavelengths, each seed beam being modulated by the RF waveform, and the modulated seed beams are amplified by a plurality of amplifiers, each amplifier having a large nonlinear phase shift and exhibiting frequency modulation (FM) to amplitude modulation (AM) conversion, the method further comprising providing the amplified beams to a spectral beam combining optics that spectrally combines the amplified seed beams.

15. 12. The method according to claim 11, further comprising the step of depolarizing the at least one seed beam before it is amplified.

16. 1. A system for reducing nonlinear frequency shift and suppressing stimulated Brillouin scattering (SBS) in a fiber laser amplifier system, comprising: means for providing at least one seed beam having a particular wavelength; means for frequency modulating the at least one seed beam with an RF waveform to spectrally broaden the at least one seed beam, the RF waveform being a relatively low frequency waveform having a large modulation depth; a means for amplifying the at least one frequency modulated seed beam, the means for amplifying having a large nonlinear phase shift and exhibiting a frequency modulation (FM) to amplitude modulation (AM) conversion, the modulation depth being significantly greater than the nonlinear phase shift; A system including:

17. 17. The system according to claim 16, further comprising means for splitting the frequency modulated seed beam into a plurality of split modulated seed beams and means for controlling the phases of the split seed beams, wherein the means for amplifying the frequency modulated seed beam amplifies all of the split seed beams, and the system further comprises coherent beam combining means for combining the amplified seed beams.

18. 18. The system according to claim 17, further comprising means for synchronizing the RF waveform with a clock signal and for synchronously detecting a phase error of the split seed beam using the clock signal and for applying the detected phase error to the means for controlling the phase so as to lock the amplified beams in phase with each other.

19. 17. The system according to claim 16, wherein the means for providing at least one seed beam provides a plurality of seed beams having different wavelengths, the means for frequency modulating the at least one seed beam frequency modulates each seed beam, and the means for amplifying the frequency modulated seed beams amplifies all of the seed beams, the system further comprising a spectral beam combining means for combining the amplified seed beams.

20. 17. The system according to claim 16, further comprising means for depolarizing said at least one seed beam before it is amplified.

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