Yanxing Ma

Coherent Beam Combining of Fiber Amplifiers Using Stochastic Parallel Gradient Descent Algorithm and Its Application

We present theoretical and experimental research on coherent beam combining of fiber amplifiers using stochastic parallel gradient descent (SPGD) algorithm. The feasibility of coherent beam combining using SPGD algorithm is detailed analytically. Numerical simulation is accomplished to explore the scaling potential to higher number of laser beams. Experimental investigation on coherent beam combining of two and three W-level fiber amplifiers is demonstrated. Several application fields, i.e., atmosphere distortion compensating, beam steering, and beam shaping based on coherent beam combining using SPGD algorithm are proposed.

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.

Average spreading of a Gaussian beam array in non-Kolmogorov turbulence

By using the rms beam width as the parameter to characterize the spreading property, the average spreading of Gaussian beam array propagates in non-Kolmogorov turbulence is studied. Numerical examples reveal that the beam width depends on the exponent value alpha, and the beam width in non-Kolmogorov turbulence is different from that in Kolmogorov turbulence with alpha=3.667. The beam width also depends on the distance between adjacent beamlets, the inner and outer scale parameters of the non-Kolmogorov turbulence. The beam array spreads more significantly with smaller inner scale and larger outer scale parameters.

Coherent beam combination of three two-tone fiber amplifiers using stochastic parallel gradient descent algorithm

Multitone radiation is a promising technique to mitigate stimulated Brillouin scattering effects in narrow-linewidth fiber amplifiers. We demonstrate coherent beam combination of three two-tone fiber amplifiers using a stochastic parallel gradient descent (SPGD) algorithm. Phase control on the fiber amplifiers are performed by running the SPGD algorithm on a digital signal processor. The contrast of far-field intensity pattern of a coherently combined beam is more than 85%. Experimental results validate that a single-frequency seed laser is not indispensable for coherent beam combination in master oscillator power amplifier configuration.

Coherent beam combination with single frequency dithering technique

The single frequency dithering technique for coherent beam combination with one single modulation frequency and without a reference beam is presented for the first time to our knowledge. Coherent beam combination of four fiber amplifiers is successfully demonstrated by using the single-dithering technique. When the phase control system is in the closed loop, the fringe contrast of far-field intensity pattern is improved by more than 85% from 8% in open loop, and the residual phase error is less than lambda/20. Experimental results reveal that the single-dithering technique has great potential in scaling to a large number of beamlets with a simple phase control system and low cost.

Review on recent progress on mid-infrared fiber lasers

We present review on recent progress on mid-infrared fiber lasers. The relevant glass fiber host materials and different kinds of doped rare-earth ions are introduced. Lasers operating from 1.9 to 3 μm in the rare-earth ions Tm3+, Ho3+, Er3+, and Dy3+ in the recent two years are discussed. Three interesting research issues, i.e., supercontinuum generation based on mid-infrared fiber laser, mid-infrared Raman fiber laser, and coherent beam combining in the mid-infrared wavelength band are classified and analyzed. Some discussions and prospective predictions are proposed at the end of this paper.

Power scaling analysis of tandem-pumped Yb-doped fiber lasers and amplifiers

Taking the improved SRS threshold formula into consideration, the power scaling of tandem-pumped Yb-doped silica fiber lasers and amplifiers is analyzed by new models. The results show that the power scaling of tandem-pumped Yb-doped fiber lasers and amplifiers is primarily limited by optical damage, SRS and thermal lens, while the pump brightness induced limitation is almost removed. It is also found that tandem-pumped Yb-doped fiber lasers and amplifiers, based upon state-of-art fiber technology, have the potential to achieve a power limit of 70.7 kW with a core diameter of 63.4 μm, and in the case of a strict single-mode fiber, the power limit is about 13.3 kW with a core numerical aperture of 0.03.

Coherent beam combination of two-dimensional high power fiber amplifier array using stochastic parallel gradient descent algorithm

We demonstrate coherent beam combination of two-dimensional high power fiber amplifier array using stochastic parallel gradient descent (SPGD) algorithm. Four polarization-maintained fiber amplifiers are tiled side by side into a 2×2 laser array with a fill factor of 54% in the near-field. Phase control on the fiber amplifiers are performed by running SPGD algorithm on a digital dignal processor with updating rate of 50 000 Hz/channel. Coherent beam combination of the four fiber amplifiers with a total output power of 60.1 W using SPGD algorithm is demonstrated. Beam quality of the combined beam is computed to be BQ<1.4.

Near-diffraction-limited annular flattop beam shaping with dual phase only liquid crystal spatial light modulators

We demonstrate the annular flattop beam shaping technique with dual phase only liquid crystal spatial light modulators (LC-SLM) based on the refractive laser beam shaping systems. One LC-SLM redistributes the intensity distribution, and the other restores the initial underlying wave front. Differing from the conventional annular beam shaping technique, the wave front of the output beam can be maintained. The influences of deviations of beam waist and beam shape on the output beam profile are discussed in detail. Experimental results show that approximate 71% of the power is enclosed in a region with less than 7% rms intensity variation. The 4.1mm diameter near-diffraction-limited beam retains an annular flattop intensity distribution without significant diffraction peaks for a working distance of more than 24cm in the near field.

Average intensity of a partially coherent rectangular flat-topped laser array propagating in a turbulent atmosphere

The propagation of a partially coherent rectangular flat-topped laser array in a turbulent atmosphere is studied. An analytical expression for the average intensity distribution at the receiving plane is obtained based on an extended Huygens-Fresnel principle. The effect of correlation length, intensity of turbulence, laser numbers, and beam orders on the beam quality in a target plane is studied by numerical examples.