Haotong 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 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.

Ultra-directional forward scattering by individual core-shell nanoparticles

We study the angular scattering properties of individual core-shell nanoparticles that support simultaneously both electric and optically-induced magnetic resonances of different orders. In contrast to the approach to suppress the backward scattering and enhance the forward scattering relying on overlapping electric and magnetic dipoles, we reveal that the directionality of the forward scattering can be further improved through the interferences of higher order electric and magnetic modes. Since the major contributing electric and magnetic responses can be tuned to close magnitudes, ultra-directional forward scattering can be achieved by single nanoparticles without compromising the feature of backward scattering suppression, which may offer new opportunities for nanoantennas, photovoltaic devices, bio-sensing and many other interdisciplinary researches.

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.

Generation of flat-top beam with phase-only liquid crystal spatial light modulators

A system containing two-phase-only liquid crystal spatial light modulators (LC-SLMs) is designed to convert a quasi-Gaussian beam into a flat-top beam. The first LC-SLM redistributes the intensity and the second LC-SLM re-collimates the reshaped beam. The phase distributions of the LC-SLMs are derived based on energy conservation and the constant optical path length principle. The influences of deviations of the beam waist and beam shape from the assumed value are analyzed in detail. Experimental results show that approximately 71.5% of the incident power is enclosed in a region with less than 6% rms power variation. The performance is compared quantitatively with the theoretical design. The far-field intensity distribution and the phase distribution of the reshaped beam show that the wavefront after reshaping is maintained.

Average intensity and spreading of a Lorentz beam propagating in a turbulent atmosphere

The propagation of a Lorentz beam in a turbulent atmosphere is studied. By expanding a Lorentz distribution in terms of orthogonal Hermite–Gauss beams, an analytical formula for the average intensity distribution at the receiving plane is obtained on the basis of the extended Huygens–Fresnel principle. It is found that the intensity pattern evolves from a Lorentz-shaped spot into a Gaussian-shaped spot under the isotropic influence of the turbulence.

Near-diffraction-limited flattop laser beam adaptively generated by stochastic parallel gradient descent algorithm.

We demonstrate the adaptive generation of a near-diffraction-limited flattop laser beam in the near field based on the stochastic parallel gradient descent algorithm and dual-phase-only liquid crystal spatial light modulators (LC-SLMs). One LC-SLM redistributes the intensity, and the other compensates the wavefront of the output beam. The experimental results show that approximately 69% of the power is enclosed in a region with less than 6% rms intensity variation. The 5mm diameter near-diffraction-limited output beam retains a flattop intensity distribution without significant diffraction peaks for a working distance of more than 30 cm.