Xiaolin Dong

Coherent beam combination of 1.08 kW fiber amplifier array using single frequency dithering technique.

Coherent beam combination of a 1.08 kW fiber amplifier array has been demonstrated for the first time, to our knowledge. In the experiment, nine fiber amplifiers are tiled into a 3×3 array, and the output power of each amplifier is approximately 120 W. A single frequency dithering algorithm is used to compensate the phase noises between the elements, which runs on a signal processor based on field programmable gate array for phase control on the fiber amplifiers. When the phase control system goes into closed loop, the fringe contrast of the far-field intensity pattern is improved to more than 85%, and the residual phase error is less than λ/15.

Phase locking of a 275W high power all-fiber amplifier seeded by two categories of multi-tone lasers

Multi-tone radiation is a promising technique to suppress stimulated Brillouin scattering (SBS) effects and improve the ultimate output power of the fiber laser/amplifier. Coherent beam combining of fiber lasers/ amplifiers is another feasible way to increase the output laser power from single gain medium while simultaneously maintaining good beam quality. In this paper, we combine the multi-tone driven all-fiber amplifiers and active phase compensation to demonstrate high power phase locking for coherent beam combining. First, we present the theory of coherent beam combining of multi-tone lasers. Second, we optimize the all-fiber power amplifier oscillator power-amplifier (MOPA) system with high scalability and flexibility based on compact, high efficiency Yb-doped fiber amplifier chains. Then, two categories of multi-tone master oscillators are used to driven the amplifier chains. The first category is two coupled single-frequency lasers and the second is a frequency modulated single-frequency laser. The output powers are all boosted to 275 W without any distinct SBS. Last, phase locking of the amplifier chains are implemented using stochastic parallel gradient descent (SPGD) algorithm. Scaling this configuration to a higher power involves increasing the power per chain and adding the number of amplifier channels.

A 275-W Multitone Driven All-Fiber Amplifier Seeded by a Phase-Modulated Single-Frequency Laser for Coherent Beam Combining

We present what we believe to be a new approach of multitone driven amplifier for power scaling and coherent beam combining. A novel multitone seed is generated by a sine wave phase-modulated single-frequency laser and used for stimulated Brillouin scattering suppression of the amplifier. The theory for coherent beam combining of multitone lasers is presented for the first time. We optimize the all-fiber master-oscillator power-amplifier system with high scalability and flexibility based on compact, high efficiency Yb-doped fiber amplifier chain. With this amplifier system, we demonstrated a 275-W high-power output when driven by the phase-modulated single-frequency laser, while the ultimate output power is only 120 W when driven by the single-frequency laser. Scaling this amplifier to a higher power can be expected by adding pumps as the output power is limited by the pumps.

29.2 W, 20 kHz linewidth all fiber single-frequency MOPA laser

We demonstrate a high power all fiber single-frequency MOPA laser. The center wavelength of the single-frequency fiber laser seed is 1064 nm with a linewidth narrower than 20 kHz. A two-stage power amplification configuration is employed. A single-frequency fiber laser with an output power of 29.2 W is obtained, and the optical to optical efficiency is 78%.

122-W high-power single-frequency MOPA fiber laser in all-fiber format

We demonstrate a high-power single-frequency master oscillator power amplifier (MOPA) fiber laser. The central wavelength of the single-frequency fiber laser seed is 1 063.8 nm, with a linewidth narrower than 20 kHz and output power of 120 mW. By using two-stage amplification, a single-frequency fiber laser with an output power of 122 W is obtained, and the optical-optical conversion efficiency is 72%. No significant amplified spontaneous emission (ASE) or stimulated Brillouin scattering (SBS) is observed. The output power can be further increased by launching more pump power.

Effect of the polarization-state fluctuation on coherent beam combining of fiber lasers

In this paper, we studied theoretically influence of the polarization-state fluctuation on coherent beam combining (CBC) of fiber lasers. We investigated far-field intensity distribution of the coherently combined fiber laser beams when the polarization-state varies with time and computed the visibility of the beam profile. The CBC of fiber laser arrays with time-varying polarization-state was mathematically modeled and the effect of polarization-state fluctuation was numerically evaluated. The dependence of visibility on the number of fiber laser was computed. Numerical results showed that the polarization-state fluctuation has a serious influence of coherent beam combining of fiber lasers in the case of small number, nevertheless, if the number of laser is larger than 10, the minimal visibility can be as high as more than 80% in the case of random time-varying polarization-state fluctuation.

Experimental study of the SBS effect in multitone-driven narrow-linewidth high-power all-fiber amplifiers

Theoretical research on stimulated Brillouin scattering (SBS) suppression shows that the SBS threshold in multitone-driven narrow-linewidth high-power amplifiers can be enhanced. In this paper, a two-stage amplification all-fiber amplifier system is set up to investigate SBS effect in multitone-driven high-power amplifiers. In one case the system is seeded with a 1064 nm single frequency laser with output power of 43 mW. In another case the system is seeded with a narrow-linewidth laser with center walvelength around 1064 nm and output power of 45 mW. In the last case the system is seeded with both the aforementioned lasers. Experimental results are in excellent agreement with the theoretical predictions. The SBS threshold in the last case is highest in those three cases.

22 W single mode tm-doped fiber MOPA in all-fiber format

We demonstrate an all-fiberized laser in main oscillator power amplification (MOPA) configuration with 22 W output power and a central wavelength of 1950 nm. The laser is pumped by pigtailed laser diode with a central wavelength of 793 nm. Strictly single-mode can be obtained and the optical to optical efficiency of the fiber laser is 40% with respect to the pump power. No power-roll is observed and the power can be scaled straightforwardly by using more pump sources.

126-W all-fiberized single-mode laser from an 11-μm small-core fiber

We demonstrate a 126-W all-fiberized single-mode laser from an 11-μm small-core fiber. The active fiber is a strictly single-mode fiber that produces pure single-mode. The optical to optical efficiency of the fiber laser is 68.2% with ASE suppressed by a factor of ∼35 dB and no power-roll. The output power is only limited by the available pump source, scaling the pump power may achieve much higher output power using the 11 μm small-core fiber.

Experimental study of SBS suppression with phase-modulation in all-fiber amplifier

A cascaded all-fiber amplifier was set up to investigate stimulated Brillouin scattering (SBS) suppression with phase-modulation (PM) in our experiment. With a modulation index of π / 2 , the SBS threshold power of the modulated amplifier can be enhanced with a factor of 3.5 compared with the unmodulated single frequency amplifier. In high power level, we demonstrate a 275 W all-fiber amplifier, where the SBS threshold enhancement factor is larger than 2.2 compared with the unmodulated case with SBS threshold power of 126 W. Further enhancing the output power is limited by available gain fiber and pump powers in this case.