Pseudo-random binary sequency phase modulation in high power Yb-doped fiber amplifiers

Abstract

We present experimental and theoretical studies on the stimulated Brillouin scattering (SBS) threshold in fiber amplifiers seeded with a spectrally broadened single-frequency laser source. An electro-optic phase modulator is driven with various pseudo-random binary sequence (PRBS) patterns to highlight the unique characteristics of this linewidth broadening technique and its facility in SBS mitigation. Theoretical predictions show a variation in SBS suppression based on PRBS pattern and modulation frequency. These predictions are experimentally investigated in a kilowatt level monolithic fiber amplifier operating with near diffraction-limited beam quality. We also show Rayleigh scattering and other sources of back reflected light in phase modulated signals can seed the SBS process and significantly reduce the nonlinear threshold.

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

“…3. The estimated result of the Brillouin gain spectrum bandwidth is expectedly less than the Brillouin spontaneous bandwidth (~40-50 MHz) due to gain narrowing 1,7 . Figure 6(b) compares the measurement results of the amplifier SBS threshold when PRBS signal generator at eleven different spectral line spacings.…”

“…In these equations, optical fibers are seeded with phase-modulated light, and the three-wave interaction among the pump (A L ), the Stokes (A S ), and acoustic (ρ) fields is described as following: www.nature.com/scientificreports www.nature.com/scientificreports/ here γ e is the electrostrictive constant, Г B is the Brillouin bandwidth, Ω B is the resonant acoustic frequency, c is the speed of light, n is the refractive index of fiber, ω is the optical frequency (ω ≈ ω L ≈ ω S ), ω L is the frequency of the pump light, which is the signal light of amplifier, ω S is the frequency of the Stokes light, ρ 0 is the mean density of the fiber medium, ε 0 is the vacuum permittivity, and f is a Gaussian random variable which initiates SBS. These equations can be solved via the initial conditions of the phase modulated pump wave 7,11,12 . When the optical spectrum of the pump laser is modulated to generate a series of equidistant spectral lines, the effective Brillouin gain G eff is expressed as 17 where L eff is the effective length of the fiber; g 0 = γ e 2 ω 2 /ρ 0 nc 3 ν S Г B ; ω L,j = ω L,0 + jΔυ, ω S,j = ω S,0 + jΔυ; ω L,0 is the central frequency of the pump light, Ω a is the frequency of the acoustic wave resonant with the laser mode oscillating at ω L,0 ; I L,m is the intensity of the pump light.…”

“…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 .…”

A review of recent progress in holmium-doped silica fibre sources

“…The Brillouin gain bandwidth scales as 1/k 2 which results in an increased susceptibility of 2 lm sources to inhomogeneous broadening techniques; temperature gradients, tension gradients, amplitude and phase modulation [10]. These techniques are relied upon for suppression of SBS in ytterbium fibre lasers [11][12][13] and have been successfully demonstrated in thulium-doped fibre lasers at 2 lm [14]. Similar advantages will be present in holmium-doped fibre lasers.…”

780W narrow linewidth all fiber laser with sinusoidal phase modulation

“…For example, Nufern company got narrow linewidth fiber laser output with linewidth of 3.5GHz and power of 1kW by white noise phase modulation in 2010 [7] . In 2013, American Air Force Research Laboratory realized narrow linewidth fiber laser output with power of 1kW by pseudorandom binary sequence phase modulation [8] . In China, Shanghai Institute of Optics and Fine Mechanics got single frequency fiber laser output with linewidth of 70KHz and power of 128W in 2010 [9] .…”