Chao Geng

Experimental demonstration of using divergence cost-function in SPGD algorithm for coherent beam combining with tip/tilt control

A novel approach of tip/tilt control by using divergence cost function in stochastic parallel gradient descent (SPGD) algorithm for coherent beam combining (CBC) is proposed and demonstrated experimentally in a seven-channel 2-W fiber amplifier array with both phase-locking and tip/tilt control, for the first time to our best knowledge. Compared with the conventional power-in-the-bucket (PIB) cost function for SPGD optimization, the tip/tilt control using divergence cost function ensures wider correction range, automatic switching control of program, and freedom of cameras intensity-saturation. Homemade piezoelectric-ring phase-modulator (PZT PM) and adaptive fiber-optics collimator (AFOC) are developed to correct piston- and tip/tilt-type aberrations, respectively. The PIB cost function is employed for phase-locking via maximization of SPGD optimization, while the divergence cost function is used for tip/tilt control via minimization. An average of 432-μrad of divergence metrics in open loop has decreased to 89-μrad when tip/tilt control implemented. In CBC, the power in the full width at half maximum (FWHM) of the main lobe increases by 32 times, and the phase residual error is less than λ/15.

1.5 kW Incoherent Beam Combining of Four Fiber Lasers Using Adaptive Fiber-Optics Collimators

We demonstrate the incoherent beam combining (BC) of four high-power single-mode fiber lasers. Adaptive fiber-optics collimators (AFOC) of 0.5 kW-level are developed to correct the tip/tilt-type phase errors among beamlets. The maximum deflection angle of the AFOC is about 0.29 mrad. A stochastic parallel gradient descent algorithm is employed as the tip/tilt control strategy. The tip/tilt control makes four separate laser beams overlap with each other in the far-field, and the average of normalized power-in-bucket increases from 0.171 in open loop to 0.798 in closed loop when 1.5-kW total output power is achieved. The precise tracking ability and control accuracy of this BC system are investigated. Experimental results expose that thermal-blooming effect disturbs BC seriously when the total output power is up to kW-level, which could be mitigated by inducing air flow near the laser output.

Co-Aperture Transceiving of Two Combined Beams Based on Adaptive Fiber Coupling Control

We demonstrate the reception and transmission of fiber laser beams using two-element adaptive fiber-optics collimator (AFOC) array under simulated turbulences. Coupling efficiency of laser beam from free space into polarization maintaining fibers is optimized with tip/tilt aberration correction. According to the principle of optical reciprocity, tip/tilt errors of the phase-conjugated collimating laser beams are compensated simultaneously. The average of normalized coupling efficiency of one AFOC increases from 0.61 without control to 0.85 under control and 0.57 to 0.85 for another. The average of diffraction-pattern centroids deviation of two outgoing combined beams in the far-field drops from 17.5 μrad to 2.35 μrad, and mean-square-error value drops from 6.59 to 1.35 μrad. Control bandwidth of 30 Hz for tip/tilt correction is achieved with computer-based stochastic parallel gradient descent controller with update rate of 1.15 kHz. Results indicate the potential value of the AFOC array in fiber coupling, laser communication and beam projection applications.

Combining module based on coherent polarization beam combining

A multiaperture receiver with a phased array is an effective approach to overcome the effect of the random optical disturbance in coherent free-space laser communications, in which one of the key technologies is how to efficiently combine the multiple laser beams received by the phased array antenna. A combining module based on coherent polarization beam combining (CPBC), which can combine multiple laser beams to one laser beam with high combining efficiency and output a linearly polarized beam, is proposed in this paper. The principle of the combining module is introduced, the coherent polarization combining efficiency of CPBC is analyzed, and the performance of the combining module is evaluated. Moreover, the feasibility and the expansibility of the proposed combining module are validated in experiments of CPBC based on active phase-locking.

Coherent Polarization Beam Combining Approach Based on Polarization Controlling in Fiber Devices

A novel, scalable coherent polarization beam combining (CPBC) approach based on polarization controlling in fiber devices has been proposed and experimentally verified. By actively controlling the polarization state of the combined beam, the proposed CPBC approach can combine imbalanced input beams with arbitrary power ratios efficiently. A series of experiments are carried out to validate the feasibility and the flexibility of the proposed CPBC approach in the number and power ratio of the input beams. We believe that the proposed CPBC approach, in this letter, has great potential in certain applications of combination, where the numbers of the individual beams are not binary, the individual beams are power imbalanced, or the optical powers of the individual beams are fluctuant.

Research Progress of Multi-aperture Laser Transceiving Control for Beam Combining Applications in IOE, CAS

Nowadays, the development of fiber laser beam combining faces new challenges during propagating through the real long-range atmosphere. Aberrations in such transmission systems include turbulenceinduced dynamic aberrations located at the path from the fiber laser array to the target, besides the inherent phase errors like phase noises and tip/tilt errors. Existing techniques, e.g., target-in-the-loop and delayed stochastic parallel gradient descent, are difficult to deal with the fast-changing turbulence-induced tip/tilt aberrations. But correcting these aberrations is critical for obtaining combined laser beams on the target with the best beam quality. In this paper, research progress of multi-aperture laser transceiving control for beam combining applications in IOE, CAS is presented. These novel techniques presented here provide efficient ways to achieve tip/tilt control for the beam coupling from space to fiber and the outgoing laser beams in the beam combining applications.

Research Progress of Fiber-based Coherent Polarization Beam Combining for Free-Space Optical Communications in IOE, CAS

Multi-aperture receiver with phased array is an effective approach to overcome the atmospheric turbulence effect on the performance of the fiber-based free-space optical (FSO) communications, where how to combine the multiple beams received by the sub-apertures efficiently is one of the key techniques. In this paper, we report on the research progress of the fiber-based coherent polarization beam combining (CPBC) in IOE, CAS, which is a promising beam combining solution for coherent FSO communications employing the multi-aperture receiver. Phase-locking control and polarization-transforming control were proposed to combine linearly polarized beams with orthogonal polarizations into one linearly polarized beam efficiently, and three fiber-based CPBC schemes were proposed and experimentally validated.

Multi-aperture all-fiber active coherent beam combining for free-space optical communication receivers

Multi-aperture receiver with optical combining architecture is an effective approach to overcome the turbulent atmosphere effect on the performance of the free-space optical (FSO) communications, in which how to combine the multiple laser beams received by the sub-apertures efficiently is one of the key technologies. In this paper, we focus on the combining module based on fiber couplers, and propose the all-fiber coherent beam combining (CBC) with two architectures by using active phase locking. To validate the feasibility of the proposed combining module, corresponding experiments and simulations on the CBC of four laser beams are carried out. The experimental results show that the phase differences among the input beams can be compensated and the combining efficiency can be stably promoted by active phase locking in CBC with both of the two architectures. The simulation results show that the combining efficiency fluctuates when turbulent atmosphere is considered, and the effectiveness of the combining module decreases as the turbulence increases. We believe that the combining module proposed in this paper has great potential, and the results can provide significant advices for researchers when building such a multi-aperture receiver with optical combining architecture for FSO commutation systems.

Experimental Demonstration of Coherent Combining With Tip/Tilt Control Based on Adaptive Space-to-Fiber Laser Beam Coupling

A novel approach of tip/tilt control through optimizing laser beam coupling efficiencies from space to polarization-maintaining-fibers with stochastic parallel gradient descent (SPGD) algorithm for coherent beam combining (CBC) applications is proposed and demonstrated experimentally using a seven-element adaptive fiber-optics collimator (AFOC) array for the first time to the best of our knowledge. Compared with the normal target-in-the-loop SPGD method with just a single cost function, the tip/tilt control here is independent of the phase-locking control and parallel for each cell of the AFOC array. Such characteristic gives the AFOC array capacities of real-time correction of fast-changing turbulence induced tip/tilt-type phase errors. Piezoelectric-ceramic-ring fiber-optic phase compensator and AFOC are developed to correct the piston- and tip/tilt-type aberrations, respectively. The power-in-the-bucket (PIB) metric is used for phase-locking. Parallel tip/tilt control is implemented via maximizing the received power in each subaperture with fixed two-channel SPGD control. The average of normalized coupling efficiencies of seven AFOCs increases from 0.76 without tip/tilt control to 0.94 under control. In CBC, the PIB metric increases by 4.6 times, and the phase residual error is less than λ/15.

Performance analysis of adaptive fiber laser array propagating in atmosphere with correction of high order aberrations in sub-aperture

Recently developed adaptive fiber laser array technique provides a promising way incorporating aberrations correction with laser beams transmission. Existing researches are focused on sub-aperture low order aberrations (pistons and tips/tilts) compensation and got excellent correction results for weak and moderate turbulence in short range. While such results are not adequate for future laser applications which face longer range and stronger turbulence. So sub-aperture high aberrations compensation is necessary. Relationship between corrigible orders of sub-aperture aberrations and far-field metrics as power-in-the-bucket (PIB) and Strehl ratio is investigated with numeric simulation in this paper. Numerical investigation results shows that increment in array number won’t result in effective improvement of the far-field metric if sub-aperture size is fixed. Low order aberrations compensation in sub-apertures gets its best performances only when turbulence strength is weak. Pistons compensation becomes invalid and higher order aberrations compensation is necessary when turbulence gets strong enough. Cost functions of the adaptive fiber laser array with high order aberrations correction in sub-apertures are defined and the optimum corrigible orders are discussed. Results shows that high order (less than first ten Zernike orders) compensation is acceptable where balance between increment of the far-field metric and the cost and complexity of the system could be reached.