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.

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 7-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 (TIL) 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 (PZT PC) 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 sub-aperture 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.

We demonstrate the coherent beam combining (CBC) of a 350-W single-frequency fiber amplifier array with simultaneous tilt-tip control and phase-locking control. An adaptive fiber-optics collimator is employed to perform the tilt-tip control, and the phase-locking control is realized by the phase modulator. The fiber array in the CBC system consisted of two high-power fiber amplifiers, each with output power of about 175 W. Stochastic parallel gradient descent algorithm is employed in both tilt-tip and phase-locking control. The tilt-tip control makes two separate laser beams overlap with each other in the far field, while the phase-locking control compensates the piston phase differences between the fiber lasers. Normalized power in bucket increases from 0.27 in open loop to 0.99 in closed loop. The fringe contrast is improved from 0% to more than 97% correspondingly. The tilt-tip control bandwidth is higher than 5.4 Hz when the tilt-tip mean square error is 10 μrad.

The adaptive coupling of laser beam from space into single-mode fiber (SMF) plays an important role in the fiber-based free space optical communication (FSOC) technology. This paper investigates a kind of adaptive SMF coupling system based on a novel corrector named adaptive fiber coupler (AFC), and an improved control algorithm named precise-delayed stochastic parallel gradient descent (PD-SPGD). Compared to the SPGD algorithm used previously, PD-SPGD can compensate for the inherent response delay of the controlled system by setting a precise time delay between the disturbed voltages and the performance metrics. The experimental results show that this new algorithm increases the iteration rate from 3 kHz to 8 kHz and decreases the convergence time from 6.3 ms to 2.5 ms when the static angular errors was corrected. As a result, the control bandwidth of the sinusoidal angular jitters is more than doubled. At the end, a preliminary atmospheric compensation experiment over a 520 m near-ground propagation path is conducted to verify the effectiveness of the method mentioned above.

Coherent beam combination of fiber laser phased array is a promising technique to achieve high-brightness laser output. In this paper, we focus on central-lobe energy enhancement of the combined beam in the far-field by theoretically and experimentally investigating the relationship between the power-in-the-bucket (PIB) of the combined beam in the far-field and the beamlets amplitude modulation in the near-field. We employ a 19-elements fiber laser phased array integrating adaptive fiber-optic collimators with a beamlet filling factor of 0.903 and achieve phase-locking and beamlet pointing correction among fiber lasers using the SPGD algorithm. The amplitude modulation applied to the tiled sub-apertures is used to control the shape of the emissive array's amplitude envelope to further change the PIB of the combined beam under the same total output power. According to theoretical analysis, the fiber laser phased array achieves PIB = 0.51 for a zero truncation ratio (Tr), which is increased to PIB = 0.57 for Tr = 1.3. Experimentally, we observe PIB = 0.49 for Tr = 0 and PIB= 0.56 for Tr = 1.3. The experimental results are in good agreement with the theoretical analysis, suggesting that the amplitude modulation is helpful for further improving the energy density of central-lobe.

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.