Results and Discussions The far-field normalized light intensity (NLI) distribution of the double-beam is obtained using MATLAB simulation (Fig. 3, Fig. 4). The phase depression between adjacent pixels results in diffraction side lobes. The maximum NLI of the diffraction side lobes generated using the composite phase, sub-aperture, and IFT methods are 14%, 8%, and 5%, respectively. The IFT method can suppress the diffraction side lobes more effectively than the composite phase and sub-aperture methods. The camera records the far-field light intensity distribution of the double-beam during the experiment (Fig. 5). The experimentally measured far-field light intensity distribution is consistent with the simulated far-field light intensity distribution. The beam tracks and aims the double-target moving at different speeds. The trajectories of the double-target positions approximately coincide with those of their corresponding spot position estimations when the double-target moves at a speed of 2 mm/s (Fig. 9), indicating that the synchronicity of beam tracking and aiming is better. As the moving speed of the double-target increases, the lag between the trajectories becomes increasingly evident (Fig. 10, Fig. 11), and the error of beam tracking and aiming also gradually increases. In addition, both the measured root mean square error (RMSE) and estimated RMSE progressively increases (Table 1).
Acquisition, tracking, and pointing (ATP) technology is a core technology for establishing stable physical links in the field of laser communications. Research on ATP technology focuses on beam deflection. Traditional beam deflection techniques are typically implemented using mechanical devices such as gimbals and mechanical mirrors. However, mechanical devices have the disadvantages of large mass, high energy consumption, and mechanical inertia that result in a slow response to beam deflection and unstable beam control. Therefore, new nonmechanical beam deflection devices have been widely used in recent years, such as acousto-optic modulators, electro-optic modulators, and liquid crystal spatial light modulators (LCSLMs). LCSLMs can overcome the defects of mechanical inertia; therefore, they are widely used for beam tracking. In addition, LCSLMs can overcome the defects of traditional mechanical beam deflection techniques, which require multiple devices to achieve multibeam deflection. The deflection direction of multiple beams can be simultaneously controlled using a single LCSLM. The methods for generating multibeams based on the LCSLM mainly include the composite phase, sub-aperture, and iterative Fourier transform (IFT) methods. Currently, double-target tracking can be achieved using the spatial polarization division method that employs a polarizing beam splitter to generate two beams with perpendicular polarization directions to track two targets. However, this method requires two LCSLMs. To use a single LCSLM to deflect multiple beams for synchronously tracking multiple targets, a scheme of synchronous beam tracking and aiming for multiple targets is proposed by combining LCSLM-based beam tracking technology and the multibeam generation method. This scheme is expected to be applicable to the multibeam tracking mechanism of laser communication networks. We also construct an experimental system of synchronous beam tracking for double-target that can communicate with two mobile target terminals.
In this experiment, a camera was employed as the position detector, and an LCSLM was employed as the beam deflection device. The tracking system mainly consisted of an LCSLM, a laser, collimator, polarizer, nonpolarizing beam splitter (NPBS), an angle magnifier, a camera, and stepper motor. First, the stepper motor controlled the movement of the two targets within the field of view. The two targets were imaged in the camera using scattered light, and the images were processed using the feature matching method to obtain the initial and current positions of the two targets. Subsequently, the offsets of the initial and current positions were converted into pre-deflection angles that were substituted into the grating equation to calculate the period of the corresponding blazed grating. Next, a phase grayscale map was generated using the sub-aperture method and loaded onto the phase screen of the LCSLM. Finally, the NPBS spatially divided the beam into two mutually perpendicular beams, one of which was incident perpendicular to the LCSLM. The LCSLM controlled the deflection direction of the double-beam based on the phase grayscale map, enabling passive tracking and aiming of two mobile targets.
The results show that the double-target tracking system can achieve synchronous tracking of two targets using a single LCSLM to deflect two beams. Moreover, the double-target tracking algorithm is also applicable to beam tracking and aiming for multiple targets that verifies the feasibility of the multitarget synchronous beam tracking and aiming scheme. This tracking system can achieve beam tracking of a target within a field of view of ±57.9 mrad. The tracking error of the system is less than 20 μrad that meets the tracking error requirement. This multitarget tracking scheme has promising applications in the multibeam tracking mechanism of laser communication networks. However, improved synchronicity of beam tracking and aiming can be obtained using the filter prediction technique in the tracking algorithm.
