Zhou Pu

Optimal Truncation of Element Beam in a Coherent Fiber Laser Array

The beam quality of a coherent fiber laser array often suffers from the low fill factor of the Gaussian laser array. One simple and effective approach to improve the fill factor is to truncate the array element properly. An analytical expression for far-field intensity distribution of a truncated coherent fiber laser array is derived. Optimal truncation of the element beam in different coherent fiber laser arrays is obtained by using energy encircled in the far-field central-lobe as the beam quality criterion. By optimal truncation, energy encircled in the central-lobe can be 97% compared with the ideal case. The shift in optimal truncation parameter in the case of phase noise is also analyzed.

Numerical and experimental study on coherent beam combining of fibre amplifiers using simulated annealing algorithm

We present the numerical and experimental study on the coherent beam combining of fibre amplifiers by means of simulated annealing (SA) algorithm. The feasibility is validated by the Monte Carlo simulation of correcting static phase distortion using SA algorithm. The performance of SA algorithm under time-varying phase noise is numerically studied by dynamic simulation. It is revealed that the influence of phase noise on the performance of SA algorithm gets stronger with an increase in amplitude or frequency of phase noise; and the laser array that contains more lasers will be more affected from phase noise. The performance of SA algorithm for coherent beam combining is also compared with a widely used stochastic optimization algorithm, i.e., the stochastic parallel gradient descent (SPGD) algorithm. In a proof-of-concept experiment we demonstrate the coherent beam combining of two 1083 nm fibre amplifiers with a total output power of 12 W and 93% combining efficiency. The contrast of the far-field coherently combined beam profiles is calculated to be as high as 95%.

Coherent Beam Combining of Two Fiber Amplifiers Seeded by Multi-Wavelength Fiber Laser

Multi-wavelength seed laser can mitigate stimulated Brillouin scattering (SBS) and improve the output power of the narrow-linewidth fiber amplifier. In this present study, coherent combining of two fiber amplifiers seeded by a multi-wavelength laser is proposed and demonstrated using stochastic parallel gradient descent (SPGD) algorithm. The long-exposure visibility of the far field interference pattern is increased from 0.15 to 0.97 when the system evolves from open-loop to closed-loop. The feasibility of coherent combining of fiber amplifiers seeded by multi-wavelength seed laser is validated.

Coherent Beam Combining of Three Watt-Level Fiber Amplifiers Using a DSP-Based Stochastic Parallel Gradient Descent Algorithm

We present an experimental study on coherent beam combining of three watt-level fiber amplifiers using a stochastic parallel gradient descent (SPGD) algorithm. Phase controlling is performed by running the SPGD algorithm on a digital-signal-processor (DSP) chip with a voltage updating rate of 16500 times per second. Energy encircled in the target pinhole is 2.62 times more than that in an open loop. The combining efficiency is as high as 87%.

Coherent Beam Combination of Three Fiber Amplifiers with Multi-dithering Technique

Coherent beam combination of three W-level fiber amplifiers with multi-dithering technique is demonstrated. The multi-dithering technique is used for phase control in two channels. In the experiment, two channels are modulated by sine wave with 70 kHz and 100 kHz respectively, and two regular commercial DSP lock-in amplifiers and an industrial computer are used for electric signal processing in the feedback loop. The fringe contrast is advanced from 12% to 81%, and 78% coherent combination efficiency is obtained when the feedback loop is closed.

Coherent beam combination of adaptive fiber laser array with tilt-tip and phase-locking control

We present an experimental study on tilt-tip (TT) and phase-locking (PL) control in a coherent beam combination (CBC) system of adaptive fiber laser array. The TT control is performed using the adaptive fiber-optics collimator (AFOC), and the PL control is realized by the phase modulator (PM). Cascaded and simultaneous controls of TT and PL using stochastic parallel gradient descent (SPGD) algorithm are investigated in this paper. Two-fiber-laser-, four-fiber-laser-, and six-fiber-laser-arrays are employed to study the TT and PL control. In the cascaded control system, only one high-speed CMOS camera is used to collect beam data and a computer is used as the controller. In a simultaneous control system one high-speed CMOS camera and one photonic detector (PD) are employed, and a computer and a control circuit based on field programmable gate array (FPGA) are used as the controllers. Experimental results reveal that both cascaded and simultaneous controls of TT using AFOC and PL using PM in fiber laser array are feasible and effective. Cascaded control is more effective in static control situation and simultaneous control can be applied to the dynamic control system directly The control signals of simultaneous PL and TT disturb each other obviously and TT and PL control may compete with each other, so the control effect is limited.

Propagation of phase-locked truncated Gaussian beam array in turbulent atmosphere

Truncation manipulation is a simple but effective way to improve the intensity distribution properties of the phase-locked Gaussian beam array at the receiving plane. In this paper, the analytical expression for the propagation of the phase-locked truncated Gaussian beam array in a turbulent atmosphere is obtained based on the extended Huygens–Fresnel principle. Power in the diffraction-limited bucket is introduced as the beam quality factor to evaluate the influence of different truncation parameters. The dependence of optimal truncation ratio on the number of beamlets, the intensity of turbulence, propagation distance and laser wavelength is calculated and discussed. It is revealed that the optimal truncation ratio is larger for the laser array that contains more lasers, and the optimal truncation ratio will shift to a larger value with an increase in propagation distance and decrease in intensity of atmosphere turbulence. The optimal truncation ratio is independent of laser wavelength.

Synchronization and Coherent Combining of Two Pulsed Fiber Ring Lasers Based on Direct Phase Modulation

We demonstrate a scalable architecture for coherent combining of pulsed fiber ring lasers based on mutual injection and direct phase modulation. By direct phase modulation in the common arm of two ring lasers, synchronous pulsed lasers can be generated and coherent combining of the two synchronous lasers is obtained. Two pulsed fiber ring lasers are coherently combined with 0.55 μJ pulse energy and 10 μs pulse duration at a repetition rate of 27.5 kHz. Experimental results show that the two fiber ring lasers are phase locked with an invariable phase difference of π and have good temporal synchronization and spatial coherence. The combining efficiency of the two pulsed fiber laser reaches 90% and the fringe contrast is larger than 40%. Neither active phase control nor polarization control is used in our experiment and this method can be extended to combine more beams and higher repetition rate scaling up to higher power.

Beam Quality and Power Scalability of Various Multicore Fiber Lasers

Beam quality and power scalability analyses for various multicore fiber lasers (MFL) are presented. In-phase complex amplitude and corresponding far-field intensity distribution of each type of MFL is calculated using the finite-difference time-domain method. It is revealed that the hexagonal ring-type MFL has the best performance in power handling and maintaining beam quality. Further calculation indicates that power encircled in the diffraction-bucket of L-core MFL scales with no more than 0.4L, which challenges the practical worthiness of fielding MFL with a large number of cores.

Propagation of Coherent Gaussian Schell-Model Beam Array in a Misaligned Optical System

Based on a generalized Collins formula, the analytical formula for the propagation property of coherent Gaussian Schell-model (GSM) beam array through a misaligned optical system is derived. As numerical examples, the propagation of a coherent GSM beam array in a typical misaligned optical system with a thin lens is evaluated. The influence of different misalignment parameters is calculated and the normalized-intensity distribution is graphically illustrated.