Seeding fiber amplifiers with piecewise parabolic phase modulation for high SBS thresholds and compact spectra

Abstract

We propose using piecewise parabolic phase modulation of the seed laser for suppressing stimulated Brillouin scattering (SBS) in a fiber amplifier. Simulations are run with a 9 m passive fiber. Compared with random phase modulation and 0-π pseudo-random phase modulation, the piecewise parabolic phase waveform yields a higher SBS threshold per unit bandwidth. If the bandwidth is defined as the range of frequencies containing 85% of the total power, the threshold for parabolic phase modulation is 1.4 times higher than the threshold for the five-or seven-bit pseudo-random modulation format. If the bandwidth is defined more tightly, e.g., the range of frequencies containing 95% of the total power, the threshold for parabolic phase modulation is three times higher. For both cases, achieving a bandwidth of 1.5 GHz requires a maximum phase shift of ~30 radians. All of the waveforms are compared on the basis of the bandwidth required of the phase moduator. The coherence functions are calculated in order to compare their suitability for coherent combining.

Effective Brillouin Gain Spectra Broadening for SBS Suppression Based on Pseudo Random Bit Sequence Phase Modulation in Fiber System

“…For the unfiltered case, the spectrum of laser wave is infinite extension as a sinc 2 shape (see Fig. 5(a)), and the concentration of energy will be lower than Gaussian or Lorentz shape [16]. The high-order sidelobes will cause the degradation of the coherence of the fiber system, thus reduce the combination efficiency for coherent combining systems [29].…”

Seeding High Brightness Fiber Amplifiers With Multi-Phase Coded Signal Modulation for SBS Effect Management

“…One conventional route seems to be feasible is to compare the SBS threshold at identical "spectral linewidth". In previous studies, different definitions of "spectral linewidth" of narrow-linewidth fiber amplifier, such as full-width at half-maximum (FWHM) [31], 85% fraction of total power [33] and root-meansquare (RMS) [34], have been proposed. However, different spectral distributions or spectral envelopes could be produced by using different modulation signals.…”

“…As for SBS effect, several suppressing techniques have been proposed, such as selecting large mode area, short active fibers [18,19], applying thermal or stress gradients [20][21][22][23], acoustic tailoring techniques [24,25], multilongitudinal-mode (MLM) fiber oscillator [26], multi-tone injection [27,28] and phase modulation [29][30][31][32][33][34]. Among all of them, phase modulation technique is a simple and robust approach to mitigate SBS effect and control the linewidth of fiber amplifiers to promise an excellent temporal coherence [35].…”

“…Among all of them, phase modulation technique is a simple and robust approach to mitigate SBS effect and control the linewidth of fiber amplifiers to promise an excellent temporal coherence [35]. Aiming on this approach, some typical modulation signals have been proposed and applied, which mainly includes sine-wave signal modulation [9,29], white noise signal modulation [30,31], pseudo-random bit sequence (PRBS) modulation [10,32], and piecewise parabolic signal (PPS) modulation [33] and multi-objective nonlinear optimized signal (MONOS) modulation [34]. Within these modulation signals, WNS and PRBS are the two prevail phase modulation techniques in multi-kilowatt level, narrowlinewidth fiber amplifiers [10,[30][31][32].…”

1.27 kW, 2.2 GHz pseudo-random binary sequence phase modulated fiber amplifier with Brillouin gain-spectrum overlap

“…Unfortunately, such linewidth may cause harmful effects for many fields, such as the beam quality degradation in spectral beam combining due to dispersion. To further suppress SBS at a much narrow level, a PRBS phase modulation technique has been theoretically and experimentally studied 7,8 . The advantage of PRBS has been proven in the long-distance optical communication systems 9 .…”