Optical amplifier system

Abstract

High-power fiber amplifier systems require means to avoid the unexpected occurrence of stimulated Brillouin scattering (SBS), which can cause optical loss and, if sufficiently large, damage the system. A standard active technique used to suppress SBS is to modulate the seed beam with a high-frequency linewidth expansion signal, such as white noise, to increase the spectral linewidth of the seed beam. This significantly increases the threshold power at which SBS becomes a problem by greatly reducing the coherence length. The problem associated with the reduction in coherence length is that the tolerance for variations in optical path length is significantly reduced. A solution offered to this problem is to delay the linewidth expansion signal by a predetermined amount to compensate for differences in optical path length.

Description

Technical Field

[0001] [Background Art] High-power fiber amplifier systems require means to avoid the unexpected occurrence of stimulated Brillouin scattering (SBS) that can cause optical losses or, if large enough, damage the system.

[0002] There are known techniques for suppressing SBS in fiber components, lasers, and amplifiers. These can be classified into either passive or active techniques.

[0003] Passive techniques rely on careful design of the fiber structure or selection of appropriate dopants. Examples of these techniques include core radius modulation to adjust the longitudinal acoustic frequency, introduction of an optical isolator, introduction of a selected dopant into the fiber core or cladding, and periodic modulation of the strain within the fiber.

[0004] The standard active technique used to suppress SBS is to modulate the seed beam with a high-frequency linewidth broadening signal to increase the spectral linewidth of the seed beam. To generate an electronic linewidth broadening signal, an electronic white noise source is commonly used, but other techniques such as Fourier synthesis of an appropriate high-frequency (RF) linewidth broadening signal as described in European Patent Application Publication No. 0730190 may also be used.

[0005] Power scaling amplifiers need to use coherent beam combining (CBC) techniques where the outputs of a number of parallel fiber amplifiers are combined to increase the output power of the laser system. Each of the standard CBC techniques, spectral, tiled aperture, and filled aperture, uses active SBS suppression.

[0006] A prior art system implementing this approach is shown in FIG. 1.

[0007] The fiber optic amplifier system 1 comprises a seed laser 2, a primary phase modulator 3, an RF noise source 4, a fan out 5, a set of preamplifiers 6, a set of secondary phase modulators 7, a set of power amplifiers 8, a controller 9, and a coherent beam combiner 10.

[0008] Seed laser 2 generates a low-power, narrow-linewidth seed beam. The seed beam is modulated by primary phase modulator 3 for applying RF noise from noise source 4. As a result of the modulation, the modulated seed beam has an increased linewidth. The modulated seed beam is divided by fan-out 5 into a set of optical beamlets, each carried by a separate fiber. A set of preamplifiers 6 amplifies each optical beamlet in the set. After preamplification, the set of optical beamlets is modulated by a set of secondary phase modulators 7 under the control of controller 9. The secondary phase modulators are used for three purposes: (1) to compensate for low-frequency wavelength-scale path length changes as a result of thermal changes in refractive index in the fiber amplifier, for example; (2) to perform phase correction for other reasons, for example, to enable phase locking in the target (in-loop target (TIL)) function; and (3) to perform electron beam steering by adjusting the phase across the entire coupled beam.

[0009] After exiting the fiber tip of each fiber and typically after passing through an end cap or collimator, the set of optical beamlets is coherently coupled by a coherent beam combiner 10. The coherent beam combiner 10 may be based on a tiled configuration, a filled configuration, or a mixed configuration.

[0010] The output of the coherent beam combiner 10 is a free-space beam 11. The pick-off mirror 12 is configured to direct a portion of the free-space beam 11 towards a diagnostic system 13 that outputs the characteristics of the beam 11 to the controller 9. The controller 9 uses the output of the diagnostic system 13 to adjust the secondary phase modulator 7 to ensure that the beam 11 has a coherent phase across its entire beam width.

[0011] Seed beam coherence length L Cs It is given by the following equation.

[0012]

number

[0013] Here, c is the speed of light and Δv is the line width.

[0014] The tolerance for any variation in optical path length is generally

[0015]

number

[0016] This is considered to be the case. The optical path length of each beamlet between the fan-out 5 and the coherent beam combiner 10 is often several tens of meters, and the expected variation between path lengths is usually less than a few meters. In a system with a maximum optical path length difference of 1 m, the maximum linewidth of the seed laser is in the range of 93.6 MHz.

[0017] Seed lasers that supply seed beams with linewidths much smaller than 93.6 MHz are readily available. For example, a seed laser that provides a 5 kHz seed beam linewidth has a coherence length L of 18.722 km in its seed beam. Cs This L CsSince the value of is much larger than the expected variation in optical path length, the seed beam remains coherent unless modulated by the primary phase modulator 3. However, the modulated seed beam output from the primary phase modulator 3 can typically have a bandwidth in the range of 20 GHz to 30 GHz (particular consideration must be given to linewidths greater than twice the Brillouin frequency shift of approximately 13 GHz in fused silica). This greatly increases the threshold power when SBS becomes a problem by drastically reducing the coherence length. A 30 GHz bandwidth has a coherence length of approximately 3 millimeters.

[0018] As a result of the reduction in coherence length, the acceptable range of variation in optical path length decreases significantly. In the 30 GHz bandwidth, L Cs teeth

[0019]

number

[0020] This reduces the overall length. In other words, each fiber channel providing the optical path between the fan-out 5 and the combiner 10 must have the same overall length within a tolerance of 0.6 mm. This requirement presents significant challenges in manufacturing and measurement. Typically, the length of each fiber channel is determined, for example, by physically measuring the length of the fiber when the construction process is performed, or by using a time-domain reflectivity method to directly measure the optical path length of each amplifier, or by measuring the time of flight of short optical pulses passing through each amplifier. This is followed by an optical path length equalization process, for example, by removing or adding fibers to the amplifier, or by increasing or decreasing the free-space distance between the seed laser and where beam coupling takes place.

[0021] In U.S. Patent No. 11588556B1, a different approach is taken. In this patent, an optical heterodyne technique is used to adjust the timing of the RF SBS suppression signal to correct for the optical path length mismatch. This makes it unnecessary to measure and cut the fiber carrier, but requires significantly more complex electronic and optical solutions, and thus may be less reliable and less scalable compared to the solution of FIG. 1 for real-world applications. SUMMARY OF THE INVENTION

[0022] According to a first aspect of the present invention, (i) an optical fanout adapted to receive a coherent optical beam and divide it into a set of multiple optical beams, (ii) a set of optical fiber transmission lines, each transmission line of the set of optical fiber transmission lines being adapted to carry a different optical beam of the set of multiple optical beams; each optical fiber transmission line providing a different optical path length from the others of the set, (iii) each optical fiber transmission line comprising a separate optical fiber amplifier configured to amplify the optical beam carried by the transmission line, (iv) a coherent beam combiner arranged to combine the set of multiple optical beams output from the set of optical fiber transmission lines to provide a combined output beam that is substantially spatially and temporally coherent, (v) a radio frequency induced Brillouin scattering (RF SBS) suppression signal generator means (generator), the generator being configured to generate a set of RF SBS suppression signals (signals), each signal of the set being time-offset from each of the other signals of the set, (vi) a set of electro-optic phase modulators, each electro-optic phase modulator being configured to phase-modulate a different one of the signals of the set of signals onto a different one of the set of multiple beams, characterized in that it comprises an optical fiber amplifier system, An optical fiber amplifier system is provided where the values of the time offsets of the respective signals are selected to compensate for the different optical path lengths of the fiber transmission lines such that the signals applied to each of the beams are substantially temporally aligned when the beams are combined by a coherent beam combiner.

[0023] Since the time offset can compensate for fluctuations in the path length of the optical fiber transmission line on a scale from millimeters to meters, it is not necessary to accurately cut each optical fiber to the same length or subsequently make the fiber lengths substantially equal by adding or removing fiber.

[0024] In one embodiment, the RF SBS suppression signal generator comprises (a) an RF signal source configured to output an RF signal, (b) a splitter for splitting the RF signal to supply a set of RF SBS suppression signals, and (c) a bank of true time delays, with each true time delay of the bank of true time delays adapted to apply the true time delay to a separate RF SBS suppression signal. Each true time delay device may be implemented separately by one or more of a delay line and / or true time delay chips.

[0025] The typical narrow linewidth SBS threshold of a fiber amplifier has been measured to be on the order of 100 W. Operation at 2 kW, which is 20 times the narrow linewidth SBS threshold, requires an increase in the linewidth of the input signal by 20 times the Brillouin linewidth, i.e., up to 10 GHz - 20 GHz. Therefore, it is preferred that each beam of the set of multiple beams output from the electro-optic phase modulator has a bandwidth of 20 GHz or more.

[0026] The spectral expansion process using a properly filtered RF region white noise power supply is known to be effective in suppressing SBS. Therefore, the RF signal source may comprise one or more of a white noise source (WNS) or a pseudo-random RF noise source, such as a pseudo-random binary sequence (PBRS). If so, a splitter may be configured to split the RF white noise signal or the pseudo-random RF noise signal.

[0027] However, the RF SBS suppression signal does not need to be random or pseudo-random. Instead, a signal suitable for RF SBS suppression can be generated by combining appropriate selections of different frequencies and utilizing intermodulation effects to yield a fuller spectrum.

[0028] In fact, this process can be used as an alternative to using true time delays, or as an alternative method to obtain the desired time delay. In other words, the generator can be configured to synthesize and generate each signal in a set individually with the desired time delay.

[0029] Thus, the generator may comprise means for generating a set of frequencies, a set of signal generators each adapted to receive the set of frequencies, and each signal generator adapted to apply weighting to each RF frequency (for example, amplitude and / or phase) and to combine the weighted frequencies to generate a separate signal from a set of time-offset signals.

[0030] Synthesizing and generating time delays has the advantage that the values ​​of each time delay can be easily adjusted during operation, for example, to compensate for variations in the parameters of each physical path.

[0031] The system may include a seed laser to supply a coherent light beam. The coherent light beam may have a linewidth in the range of 5 kHz. It should be understood that seed lasers with other linewidths may also be used.

[0032] The combined output beam is preferably a free-space beam. The system includes a diagnostic system, which may include sensors positioned to receive at least a portion of the combined output beam, and which are configured to output one or more signals indicating the characteristics of the combined output beam (e.g., the degree of spatial or temporal coherence).

[0033] The system may include a controller adapted to receive signals from a diagnostic system and output electrical control signals to a generator. The generator may be adapted to use the control signals to modify one or more time offsets of the RF SBS suppression signals relative to others, for example, to improve the degree of spatial and / or temporal coherence of the output beam from a coherent beam combiner.

[0034] The amplifier system may further comprise means for generating a set of second-order phase-modulated signals and means for compensating for nanometer-scale changes in optical path length by modulating each separate signal of the set of second-order phase-modulated signals onto different beams of a set of beams for, for example, one or more of the following: (1) for low-frequency wavelength-scale path length changes resulting from thermal changes in refractive index in a fiber amplifier; (2) for other reasons, such as phase correction to enable phase locking at a target (TIL); and (3) for electron beam steering by adjusting the phase across the entire beam.

[0035] The controller can be adapted to generate a set of second-order phase-modulated signals.

[0036] A set of secondary phase-modulated signals is modulated across multiple beam sets by a bank of secondary phase modulators. Alternatively, though less preferred, a set of secondary phase-modulated signals may also be applied to multiple beam sets by a (first) set of electro-optic phase modulators.

[0037] The need for SBS suppression is most important in the case of high-power amplifiers. Therefore, the amplifier system can be adapted for normal operation with multiple light beams, each with a beam power of 200W or more.

[0038] The present invention may also be expressed in relation to a method. That is, according to a second aspect of the present invention, a method for suppressing stimulated Brillouin scattering of an optical amplifier is provided. This delicious, (i) Generate a set of RF SBS suppression signals (signals), and each signal in the set is time-offset from each of the other signals in the set. (ii) Modulating each signal in the set into separate beams of a set of multiple beams, and each beam propagating along separate optical fiber transmission lines of a set of optical fiber transmission lines, (iii) comprising coupling multiple sets of optical beams output from a set of optical fiber transmission lines in order to supply a coupled output beam that is substantially coherent in both space and time, Here, the time offset values ​​for each signal are selected to compensate for the difference in optical path lengths between the optical fiber transmission lines so that the signals applied to each of the optical beams are substantially time-matched when the multiple optical beams are combined.

[0039] Next, the present invention will be described illustratively with reference to the following figures. [Brief explanation of the drawing]

[0040] [Figure 1] A schematic diagram of a conventional high-power fiber amplifier system. [Figure 2] Schematic diagram of a fiber optic amplifier system. [Figure 3]A schematic diagram of a modified embodiment of a fiber amplifier system. [Figure 4] A schematic diagram of the signal generator means in the modified embodiment shown in Figure 3. [Modes for carrying out the invention]

[0041] Referring to Figure 2, a fiber optic amplifier system 100 is shown, comprising a seed laser 101 adapted to supply a continuous wave (CW) seed beam BS carried by an optical fiber link 102 to an optical fan-out 103 that substantially evenly divides the seed beam BS into N beamlets B1 to BN with respect to N fiber channels 104, one for each channel 104.

[0042] The system 100 further comprises a set of fiber optic preamplifiers 105, one for each channel 104; a first set of phase modulators 106, one for each channel 104; a second set of phase modulators 107, one for each channel 104; a set of optical power fiber amplifiers 108, one for each channel 104; a controller 109; a coherent beam combiner 110; a diagnostic system 111; a pick-off mirror 112; and a signal generator means 120.

[0043] The seed laser 101 is configured to supply a seed beam BS with a linewidth that provides a coherence length significantly exceeding the maximum range of expected optical path length differences between the fan-out 103 and the fiber channel 104 between the fan-out 103 and the coherent beam combiner 110. A laser with a linewidth of 5 kHz is suitable for this purpose.

[0044] Each preamplifier 105 amplifies different beamlets B1-BN to compensate for power losses resulting from the splitting of the seed beam BS by the fan-out 103, and to improve the efficiency of the power amplifier 108. Therefore, the preamplifiers 105 are preferred but optional.

[0045] Following pre-amplification, each beamlet B1-BN is modulated by one of the phase modulators 106(1)-106(N) to apply a linewidth expansion signal received from the signal generator means 120. Each expanded beamlet output from each phase modulator 106(1)-106(N) has a frequency linewidth ranging from 20 GHz to 30 GHz.

[0046] The beamlets of each channel 104 are coherently coupled by a coherent beam combiner 110. Typically, collimators are provided at the ends of each fiber channel 104 to collimate each beamlet before it is coupled by the coherent beam combiner 110. The coherent beam combiner 110 can be coupled based on a tiled, packed, or mixed configuration, each known to those skilled in the art. The output of the coherent beam combiner 110 is a free-space beam 113.

[0047] A pick-off mirror 112 is positioned to redirect a portion of the free-space beam 113 towards a diagnostic system 111. The diagnostic system 111 includes one or more electro-optic sensors for receiving the redirected beam portion from the pick-off mirror 112, and processing means for characterizing the characteristics of the received beam, including its temporal and spatial coherence across its entire beam width, using techniques well known to those skilled in the art. The electrical signals representing the beam characteristics are output by the diagnostic system 111 to a controller 109.

[0048] Controller 109 uses signals from diagnostic system 111 to generate a set of secondary phase-modulated signals. Each phase modulator 107 in the secondary set of phase modulators is adapted to receive separate secondary phase-modulated signals in the set for one or more of the three purposes described in relation to the prior art system in Figure 1: (1) to compensate for low-frequency path length differences (on an optical wavelength scale) between beams resulting from changes in the refractive index of the fiber amplifier; (2) to perform phase correction for other reasons, e.g., to enable phase locking at the target (TIL); and (3) to perform electron beam steering by adjusting the phase across the entire beam.

[0049] The signal generator means 120 comprises a high frequency (e.g., GHz), a noise source 121, an RF splitter 122, a bank of true time delay devices 123, and a set of electrical outputs 124(1) to 124(N), each output 124(1) to 124(N) connected via a separate electrical channel 114 to a separate modulator 106 of a first set of phase modulators 106(1) to 106(N).

[0050] The noise source 121 generates high-frequency (RF) noise signals for N electrical channels E1 to EN, which are split by an RF splitter 122 (implemented, for example, by one or more RF amplifiers and RF splitters). The noise source may be a white noise source implemented, for example, by a digital signal processor. The RF noise signals for each electrical channel E1 to EN undergo a true time delay by a separate true time delay device 123. The delayed noise signals carried by the set of electrical channels E1 to EN combine to form a set of linewidth expansion signals. Each signal in the set is output to a separate output unit 124(1) to 124(N) to be received by a separate modulator 106 of a first set of modulators 106(1) to 106(N), and to be applied to a separate beamlet B1 to BN.

[0051] Each true time delay device 123 is configured separately to provide a time delay of a value δt selected to compensate for the different optical path lengths of the fiber channel 104 between the phase modulator 106 and the coherent beam combiner 110, such that the linewidth expansion signals modulated to each beamlet B1-BN are substantially time-matched at the time the beamlets B1-BN are combined. Therefore, the value of δt provided by each true time delay device 123 may differ from that of others in the bank.

[0052] For example, if optical channel 104(1) is determined to be the longest of the N optical channels, then each of the true time delay devices 123 associated with the other optical channels 104(2) to 104(N) is configured to provide an independent true time delay to the RF noise signal carried by each of the electrical channels E2 to EN, which are sized to compensate for the degree to which the associated optical channel is shorter than optical channel 104(1).

[0053] Setting the required time delay length for each channel E1-EN can be done during the manufacturing of the amplifier system 100 as part of the calibration process.

[0054] One or more of the optical fiber channels 104 may be sufficiently similar in length that their associated time delay values ​​are identical. This value may be zero.

[0055] The true time delay device 123 can be implemented by a dedicated true time delay line or a true time delay chip. Alternatively, true time delay can be implemented by different physical lengths of the electrical channels between the splitter 122 and the modulator 106.

[0056] The process of presetting the time delay may involve measuring the length of each optical channel and then using these measurements to preset the time delay value for each channel E1 to EN.

[0057] Measuring the length of each optical channel can be done in various ways, including, for example, by directly measuring the optical length of each channel using time-domain reflectivity, or by measuring the time of flight of short light pulses passing through each channel.

[0058] Next, the measurements are used to determine the time delay value for each line. In this case, each true time delay device 123 is pre-configured to obtain the determined time delay for its respective channel.

[0059] Figure 3 shows a fiber amplifier system 200 using a deformed signal generator means 130 comprising a bank of Fourier composite signal (FSS) generators 131(1) to 131(N). Each FSS 131 is adapted to generate one of a separate set of linewidth-expanded signals for output to a first set of modulators 107.

[0060] Referring to Figure 4, the deformation signal generator means 130 further comprises a bank of frequency sources 132. Each frequency source 132 is adapted to output a different electrical signal at a different frequency.

[0061] Each FSS generator 131 is adapted to receive all frequencies from the signal source 132 and combine (sum) those frequencies to supply a separate linewidth expansion signal. In other words, each linewidth expansion signal in the set is synthesized by a separate Fourier series from a separate FSS generator 131.

[0062] Each frequency supplied by the bank of frequency generators 132 is a positive prime multiple of the selected fundamental frequency. The fundamental frequency does not need to be a prime number itself. The bank of frequency generators supplies each FFS generator 131 with a set of input frequencies that are not harmonically closely related, i.e., have no common factors, in order to ensure that the frequency spectral components of the resulting linewidth expansion signal are sufficiently spread out to avoid coherent intermodulation terms.

[0063] Since the linewidth of the generated spectrum is proportional to the signal frequency and modulation amplitude, higher prime frequencies, for example, above 2000, are preferred over lower prime frequencies. For example, a 20 GHz wide optical spectrum with a fundamental frequency of 1 MHz can be obtained by combining frequencies of 2081 MHz, 2377 MHz, 2851 MHz, and 3001 MHz (where 2081, 2377, 2851, and 3001 are each multiples of 1). Further information on this embodiment can be found, for example, in European Patent Application Publication No. 0730190.

[0064] In addition, each FSS generator 131 is configured to apply separate weights to amplitude and phase, respectively, for each frequency before combination. Each FSS generator 131 may hold its own weight values, or its weight values ​​may be provided by the controller 9. The set of weights applied by each FSS generator 131 is based on the equivalent real-time delay value required for the signal to be generated. Methods for implementing weights to generate a particular time delay value will be known to those skilled in the art.

[0065] Similarly, although not very efficient from a processing standpoint, all FSS generators 131 are activated independently and sequentially by the controller 9, and the delay in the start times between the generators 131 corresponds to the required real-time delay.

[0066] Calibration of the time delay values ​​associated with each channel 104 is initiated before system 100 is first put into use. Initially, systems 100, 200 are operated without generating and / or modulating line-expanding signals to beamlets B1-BN to calibrate the signals to a second set of phase modulators 107 to compensate for low-frequency optical wavelength-scale path length differences. A portion of the beam 113 output from the coherent beam combiner 110 is redirected to the diagnostic system 111 by the pick-off mirror 112. The characteristics of the beam 113, particularly its coherence, are output to the controller 109, which uses its output to adjust one or more of the second set of phase modulators 107. For example, the diagnostic system 111 may be adapted to output one or more error values ​​corresponding to the degree to which the beam 113 is incoherent at one or more points. The controller 9 adjusts each of the electrical modulation signals applied by the secondary modulator 107 to minimize the error value, for example, based on the hill-climbing method.

[0067] Following this first stage of calibration, a set of line-expanding signals is modulated into beamlets by a first set of modulators 106 without any time delay that has not yet been applied. Differences in the optical path lengths of channels 104 between the first modulators 106 and the coherent beam combiner 110 result in a temporal misalignment of the line-expanding signals between channels 104, which manifests as an increase in the incoherence of the free-space beam 113 when the beams are combined by the coherent beam combiner 110. This is represented by an increase in the error value output to the controller 109 by the diagnostic system 111.

[0068] In the system 100 in Figure 2, which utilizes true time delay, the true time delay value of each TDD 123 is adjusted, for example, according to the hill-climbing method, until the error value falls below a preferred threshold.

[0069] In the system 200 shown in Figures 3 and 4, calibration is performed by adjusting one or more of the weighting values ​​applied independently by each generator 123. The adjustment can be automated by transmitting control signals that transmit weighting information from the controller 9 to each generator 123 via control lines.

[0070] The seed beam BS does not need to be a continuous wave, as long as it has a pulse length longer than the time it takes for amplifier 108 to be switched on.

[0071] The functions of the first and second sets of modulators 106 and 107 can instead be undertaken by a single set of modulators.

[0072] Passive SBS suppression techniques may be used additionally within the laser systems 100 and 200 described above.

[0073] Alternative techniques to those described may be used or become known in the future for generating a set of time-delayed RF SBS suppression signals.

Claims

1. (i) an optical fan-out adapted to receive a coherent light beam and split it into a set of multiple light beams, (ii) A set of optical fiber transmission lines, wherein each transmission line in the set of optical fiber transmission lines is adapted to carry a separate light beam from the set of multiple light beams, and each optical fiber transmission line has a different optical path length from the others in the set of optical fiber transmission lines. (iii) Each optical fiber transmission line comprises a separate optical fiber amplifier configured to amplify the coherent light beam carried by the optical fiber transmission line, (iv) A coherent beam combiner arranged to combine the set of optical beams output from the set of optical fiber transmission lines in order to supply a combined output beam that is substantially coherent in both space and time, (v) In an optical fiber amplifier system comprising a high-frequency stimulated Brillouin scattering (RF SBS) suppression signal generator means (generator), The generator is configured to generate a set of RF SBS suppression signals (signals), and each signal in the set of signals is time-offset by a predetermined value from each of the other signals in the set. (vi) A set of electro-optic phase modulators, each electro-optic phase modulator configured to phase-modulate one separate signal from the set of signals to a separate beam from the set of multiple light beams, An optical fiber amplifier system in which a preset value for the time offset of each signal is selected to compensate for different optical path lengths of the optical fiber transmission line such that the signals applied to each of the optical beams at the time the optical beams are combined by the coherent beam combiner are substantially time-matched.

2. The generator, (a) An RF signal source configured to output an RF signal, (b) A divider for dividing the RF signal in order to supply the set of signals, (c) a bank of true time delays, wherein each true time delay in the bank of true time delays is adapted to apply a true time delay to separate signals of the set of signals, according to claim 1.

3. The optical fiber amplifier system according to claim 1, wherein the generator comprises means for generating a set of frequencies and a set of signal generators adapted to receive the set of frequencies, each signal generator being adapted to combine the weighted frequencies in order to apply weight to each RF tone and to generate a signal of the set of signals having a different time offset from the other signals of the set of signals.

4. The optical fiber amplifier system according to any one of claims 1 to 3, further comprising a seed laser for supplying the coherent optical beam.

5. A method for suppressing stimulated Brillouin scattering in an optical amplifier, (i) Generate a set of RF SBS suppression signals (signals), wherein each signal in the set of signals is time-offset from each of the other signals in the set of signals. (ii) Modulating each signal of the set of signals into separate light beams of a set of multiple light beams, where each beam propagates along a separate optical fiber transmission line of a set of optical fiber transmission lines, (iii) comprising coupling the set of optical beams output from the set of optical fiber transmission lines in order to supply a coupled output beam that is substantially coherent in both space and time, A method in which the time offset value of each signal is selected to compensate for differences in the optical path lengths of the optical fiber transmission line such that the signals applied to each of the plurality of optical beams at the time the plurality of optical beams are combined are substantially time-matched.

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