2.5 kW Narrow Linewidth Linearly Polarized All-Fiber MOPA With Cascaded Phase-Modulation to Suppress SBS Induced Self-Pulsing

Abstract

As a major limitation for power scaling of high power narrow linewidth fiber master oscillator power amplifiers (MOPAs), Stimulated Brillouin Scattering (SBS) induced selfpulsing in polarization maintaining (PM) fiber amplifiers is well characterized and analyzed in this paper by comparing different white noise signal (WNS) phase-modulated modes in experiments. It is found that the self-pulsing effect is not observed in the PM-amplifier with single-frequency laser seed injection, and cascaded WNS modulation provides superior self-pulsing suppression than single WNS modulation with similar output linewidth. Moreover, the experimental results indicate that the self-pulsing threshold can hardly be predicted only by the output linewidth or the defined SBS threshold in a WNS phase modulated fiber amplifier system. As self-pulsing is originated from the spectral spikes in WNS modulated system, we theoretically analyzed characteristics of these spikes in different phase-modulation modes. It indicates the spectral peak intensity can be reduced by cascaded modulation, for which self-pulsing can be suppressed. The theoretical predictions agree well with the experimental results. At the same time, in order to suppress the mode instability effect, a plum blossom shaped bending mode selection device is used in this high-power narrow linewidth fiber amplifier system. Finally, a 32 GHz cascaded WNSs modulated, over than 2.5 kW linearly polarized all-fiber amplifier with a slope efficiency of 86.7% is demonstrated. The polarization extinction ratio (PER) is measured larger than 14 dB and the beam quality factor M 2 maintains lower than 1.3 in the power scaling process.

Enhancing the Efficacy of Noise Modulation for SBS Suppression in High Power, Narrow Linewidth Fiber Lasers by the Incorporation of Sinusoidal Modulation

“…The tailoring of laser line-shape with the proposed DMSN was initially investigated through simulations based on phase modulation. Transfer function of the phase modulator was used to incorporate the proposed modulation scheme and the spectra of filtered white noise was utilized to synthesize instances of noise voltage [29]. Sinusoid voltage is added to the noise to analyze the laser line-shape evolution for the proposed DMSN scheme with increasing sinusoidal power (1 GHz tone) as shown in Fig.…”

“…Improved power scaling using multiple modulation blocks in cascade has been demonstrated previously in [27] using nonpolarization maintaining (non-PM) architecture and in [28,29] with fully polarization maintaining (PM) architectures. In [28] the authors use cascaded sinusoidal modulation and demonstrate 3-dB spectral linewidth and output powers of 6 GHz and 477 W (single-stage), 18.5 GHz and 1040 W (twostage) and 45 GHz and 1890 W (three-stage modulation) respectively.…”

“…In [28] the authors use cascaded sinusoidal modulation and demonstrate 3-dB spectral linewidth and output powers of 6 GHz and 477 W (single-stage), 18.5 GHz and 1040 W (twostage) and 45 GHz and 1890 W (three-stage modulation) respectively. Similarly, in [29] cascaded WNS modulation is used with 3 dB spectral linewidth and output power of 12.8 GHz and 1230 W (single-stage), 20 GHz and 1790 W (two-stage) and 32 GHz and 2620 W (also two-stage). It is important to note that the increase in output power corresponds to a significant increase in linewidth after each stage of the cascade.…”

Towards Ultimate High-power Scaling: Coherent Beam Combing of Fiber Lasers

“…Numerous theoretical and experimental research on mitigating the SBS effect in fiber lasers and amplifiers has been conducted to prevent instability and physical damage to the laser system [36][37][38][39][40][41][42][43][44][45][46][47][48][49][50][51][52][53]. As a case in point, in 2020, Wang et al demonstrated a 2.5 kW narrow linewidth linearly polarized MOPA by efficient suppressing of SBS-induced self-pulsing (SP) via implementing cascade phase modulation [54]. In the same year, they presented the first 3 kW all-fiber narrow linewidth linearly polarized MOPA via a bi-directional pumping scheme [55].…”

Towards Ultimate High-Power Scaling: Coherent Beam Combining of Fiber Lasers

“…Numerous theoretical and experimental studies on mitigating the SBS effect in fiber lasers and amplifiers has been conducted to prevent instability and physical damage to the laser system [36][37][38][39][40][41][42][43][44][45][46][47][48][49][50][51][52][53]. As a case in point, in 2020, Wang et al demonstrated a 2.5 kW narrow linewidth linearly polarized MOPA by efficient suppressing of SBS-induced self-pulsing (SP) via implementing cascade phase modulation [54]. In the same year, they presented the first 3 kW all-fiber narrow linewidth linearly polarized MOPA via a bi-directional pumping scheme [55].…”