16  kW Yb fiber amplifier using chirped seed amplification for stimulated Brillouin scattering suppression

Abstract

In a high power fiber amplifier, a frequency-chirped seed interrupts the coherent interaction between the laser and Stokes waves, raising the threshold for stimulated Brillouin scattering (SBS). Moving the external mirror of a vertical cavity surface-emitting diode laser 0.2 μm in 10 μs can yield a frequency chirp of 5 × 10 17 Hz∕s at a nearly constant output power. Opto-electronic feedback loops can linearize the chirp, and stabilize the output power. The linear variation of phase with time allows multiple amplifiers to be coherently combined using a frequency shifter to compensate for static and dynamic path length differences. The seed bandwidth, as seen by the counter-propagating SBS, also increases linearly with fiber length, resulting in a nearly-length-independent SBS threshold. Experimental results at the 1.6 kW level with a 19 m delivery fiber are presented. A numerical simulation is also presented.

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

“…We model the SBS following previous treatments that solve the coupled first order equations representing propagation of the laser and Stokes wave, buildup of the acoustic wave, and initiation from a Langevin noise source representing thermal phonons throughout the length of the fiber [21]. Our code has been verified by comparison with experiments and previous theoretical work [9,12,14].…”

“…A discontinuous frequency chirp (consisting of a succession of 30-MHz jumps) and a low-pass-filtered white noise source have also been used to suppress SBS in a high-gain parametric fiber amplifier [11]. A chirp with a period much longer than the fiber transit time has been used in an amplifier with a 25 m final stage to achieve a fundamental-mode pump-limited output of 1.6 kW [12]. For coherent combining, this waveform has the advantage that path length differences can be compensated with an acousto-optic frequency shifter [13].…”

Fast and wide-band tuning single-mode microlaser based on fiber Fabry–Pérot microcavities

“…Therefore, the spatial resolution of an imaging system is inversely proportional to the chirp bandwidth, and the longest range of the distance measurement is limited by the coherence length which is determined by the linewidth of the laser. Besides, the laser seed with high chirp rate can significantly suppress the simultaneous stimulated Brillouin scattering (SBS) that is currently limiting the output power of narrow-linewidth fiber amplifiers [25,26]. It also can serve as optical beat sources of continuously tunable terahertz (THz) radiation.…”

“…However, DFB lasers have a low chirp of 10 14 Hz/s and a limited tuning range to GHz [30], and broadband ECDL to 5 THz has much lower chirp of 6 × 10 12 Hz/s [21]. Most VCSELs can achieve the linear chirp of 5 × 10 15 Hz/s [26], although a 100× faster chirp is obtained by moving the external mirror, the co-herent length is limited to 5 m due to the 40 MHz linewidth [25]. Compared with the traditional swept lasers, our FFPC microlaser demonstrated the most rapid chirp under THz tuning range with narrow linewidth at the same time.…”

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

“…As the combining beamlet in CBC system, the power scaling ability of high-brightness, Yb-doped, narrow-linewidth fiber amplifier is one of the significant aspects. Along with the development of output power scaling, the brightness of this type of fiber source is to be restricted by the dual effects of nonlinear stimulated Brillouin scattering (SBS) and thermal-induced mode instability (TMI) [9][10][11][12][13][14][15]. Specifically, SBS will induce the laser light transferring into the backward Stokes light so that the output power of the fiber amplifier will be limited and the whole amplification system will be fragile [16].…”