Objective Fast and accurate beam scanning is the key technology in free space laser application. An optical phased array (OPA) based on electro-optics or thermo-optics overcomes the limitation of mechanical steering and can achieve noninertial beam steering with flexible beam pointing. It has demonstrated applications in free-space laser, such as light detection and ranging (LIDAR), free-space optical (FSO) communication, and optical imaging. The spatial light modulator (SLM) and microelectromechanical mirror array (MEMS) have realized tens of kHz and 1-MHz beam steering, respectively. Chip-scale OPA can achieve ultrawide beam steering and it has been demonstrated at kHz to GHz. However, chip-scale OPA suffers serious loss and cannot be used in remote detection. The optical fiber phased array (OFPA) based on a lithium niobate (LiNbO3) phase modulator can achieve beam directional and fast steering at GHz while realizing high power laser synthesis output, but the phase noises make each beam phase fluctuate strongly, which seriously affects the output beam quality and cannot guarantee its steering angle accuracy. Compared with the coherent combination of high power fiber lasers, the fast beam steering of OFPA not only involves phase control compensation but also ensures the accuracy of beam steering angle and higher steering speed, which improves the difficulty of phase control.
Methods Based on LiNbO3 phase modulators and the stochastic parallel gradient descent (SPGD) algorithm, the phase noises of 1×16 channel OFPAs are compensated, and the multibeam fast steering is achieved by the “steering after correction” method. First, a narrow-linewidth (<200 kHz at 1550 nm) laser is divided into several sub-beams through the polarization-maintaining (PM) fiber splitters. After the modulation by the electro-optic phase modulators and power amplification by the fiber amplifiers, these sub-beams are outputted by the fiber array, which is arranged and combined by PM single-mode bare fiber. Second, pinhole samples of the on-axis far-field intensity with a silicon photodetector are used as the performance metric of the SPGD algorithm. The phase compensation voltage is obtained using the phase control system and loaded on each phase modulator to realize the beam optimization. Third, according to Eq. (7), the steering voltages are calculated and loaded onto each phase modulator to realize beam steering. Finally, the multibeam steering refers to that the main lobe of coherent combined beam is steering in the field of view (FOV) between its adjacent sub-beams, and other sub-beams follow it to steer to rapidly increase and cover the full FOV. For 1×16 channel OFPA, the total FOV is 10.65°, when sixteen coherent sub-beams are taken.
Results and Discussions The phase control system developed in this study effectively achieves phase noise compensation, the performance metric (on-axis intensity) increases from 0.43 to 0.94, the quality of coherent combined beams is improved (Fig. 6), and the convergence time of the SPGD algorithm is 1.2 ms [Fig. 7(a)]. Using peak-to-side lobe ratio (PSLR) as the quality evaluation index of beam optimization, the PSLR is 24.7 dB after phase noise compensation, which is close to the theoretical limit of 26.4 dB (Fig. 8). In this paper, the beam steering angle is set to be -0.30°, -0.20°, -0.10°, 0.10°, 0.20°, and 0.30°. The quality of the coherent combined beam after steering maintains a good intensity distribution state (Fig. 9). The results show that the actual steering angles are -0.31°, -0.20°, -0.11°, 0.12°, 0.22°, and 0.29°. Three factors lead to errors: 1) the half-wave voltage of each LiNbO3 is different from the theoretical value; 2) the actual output voltage of the phase control system has errors compared with the theory values; 3) owing to the processing error, the spacing parameters of adjacent bare fibers in the fiber array are inaccurate. The pixel size of the short-wave infrared camera (SWIC) is 15 μm×15 μm, and its FOV is 0.008°; the maximum error of the experimental results is 0.02°, which shows that the systematic error caused by the resolution of the SWIC cannot be ignored. The beam steering speed of the system (defined by the switching speed of the beam between any two angles) measured by a single point detector is 500 kHz.
Conclusions An experimental system containing an OFPA of 1×16 channels is built, and the experiments for coherent beam combining and multibeam steering are conducted. Experimental results show that the algorithm is of high efficiency, taking only 10 μs in a single iteration. Besides, the PSLR reaches 24.7 dB, with the theoretical limit being 26.4 dB. The steering range is in good consistency with the theoretical prediction range. The OFPA developed in this study permits high-quality coherent beam combining and allows high-speed (500 kHz) beam steering, and the scan angle range is -0.70°--0.70°. Finally, the feasibility of fast multibeam steering of 1×16 channel OFPA is verified. In the future, the bandwidth of the phase control circuit system will be further improved, and higher precision beam steering will be achieved, which will lay a foundation for the applications of laser detection and imaging.
