Optical multi-beam forming method based on a liquid crystal optical phased array

Beam steering devices have wide applications in both military and civil fields. The ultimate goal for such devices is to reduce their size, weight, and power consumption. However, the laser source in these devices is spatially separate from the phase shifter, resulting in large size, complex packaging, and low coupling efficiency. To solve these problems, a novel electrically controlled beam steering chip based on coherently coupled vertical cavity surface emitting laser (VCSEL) array directly integrated with liquid crystal optical phased array (LCOPA) is proposed in this paper. Implant-defined in-phase coherently coupled VCSEL arrays (CCVAs) with uniform near-field are designed and fabricated first to act as the coherent laser source for the chip. Then, taking advantage of the CCVA planar structure, the LCOPA is integrated directly on the CCVA by conventional process. The coherent light generated by the in-phase CCVA is uniformly and normally incident into the LCOPA and is electrically steered by the LCOPA. One-dimensional beam steering is achieved by two proof-of-concept integrated chips. The chips based on a 4 × 4 square CCVA and a 16-element hexagonal CCVA offer a field of view of 2.21° and 6.06°, respectively. Independent control of the CCVA and LCOPA guarantees a relatively high wavelength stability and power stability. Theoretical calculations are also performed, which are consistent with the experiments.

Non-mechanical beam steering is required for holographic displays, free-space optical communication, and chip-scale LiDAR. Optical phased arrays (OPAs), which allow for inertia-free, high-speed beam control via electronic phase control, are an important research topic. The present study investigates the primary material platform for on-chip OPAs: Liquid crystal OPAs (LC-OPAs) employ electrically tunable refractive indices for low-voltage operation; lithium niobate OPAs (LN-OPAs) utilize high electro-optic coefficients for high-speed, low-power consumption, and large-bandwidth operation; and silicon-based OPAs (Si-OPAs) apply mature photonic integration to achieve high integration density and GHz-range steering. The paper thoroughly examines OPA basics, recent material-specific advancements, performance benchmarks, outstanding issues, and future prospects.

Optical phased arrays (OPAs) are a key technology in solid-state beam scanning, capable of achieving beam steering and scanning without relying on any mechanical moving parts. However, their application in the field of scanning is constrained by two significant limitations: a narrow scanning field of view (FOV) and long scanning periods. To address these limitations, we propose an omnidirectional scanning model combining a liquid crystal optical phased array (LC-OPA) and a conical mirror. The experimental results demonstrate that by generating annular beams with the LC-OPA and combining them with a passive conical mirror, a scanning FOV of 360°×2.1° was achieved. The scanning error was less than 0.04°, and the scanning period was reduced to 1/4800 that of a two-dimensional grating scanning system, and no mechanical moving parts were involved.

2-D optical phased array (OPA) antenna based on a liquid crystal on silicon (LCOS) spatial light modulator is introduced. Diffraction efficiency of the 1-D OPA for fine angle beam steering and 2-D OPA for aberration correction is characterized experimentally and by computer simulation. Excellent agreement between the simulation, measurement and empirical model is observed. Large aperture transmitter for long distance free space communication is analyzed by wave optics simulation. The system performance for beam steering and correction for wavefront aberration in large aperture transmitter is discussed.

The liquid crystal optical phased array (LCOPA), with low SWaP (size, weight and power consumption), has been widely researched in laser communication, laser radar, laser guidance, laser weapons and many other application scenarios. As the number of laser paths increases, the laser power density also increases, such as multi-beam control (“one-to-many”) and laser beam combining ("many-to-one"), it is imperative to study the high power threshold of LCOPA. To solve the heat problem induced by high power laser of LCOPA, in this paper, we proposed a transparent high power threshold of LCOPA with micro-channel heat sinks. A high thermal conductivity and transparent (to $1064 \mathrm{~nm}$ light) Boron compounds material was used to compose the micro-channel heat sinks. Heat on LCOPA was carried away through the circulation of the index matching fluid in micro-channel. Several typical factors (micro-channel structure parameter, cooling index matching fluid rate and substrate material parameter) are analyzed to evaluate the influence of phase distortion when a relative high power laser is pumped into the LCOPA. Typically, the optical turbulence caused by micro-channel heat sinks with cooling fluid was considered in simulation. When the incident laser power is 500 W, and the total absorption is 5 %, the flow rate is $1.5 \mathrm{~m} / \mathrm{s}$, the laser induced beam quality deterioration on the LC layer is less than 15 % under the condition of the width and depth of micro channels are $250 \mu \mathrm{m}$.

A practical numerical model for a liquid crystal cell is set up based on the geometrical structure of liquid crystal optical phased arrays. Model parameters include width and space of electrodes, thickness of liquid crystal layer, alignment layers and glass substrates, pre-tilted angles, dielectric constants, elastic constants and so on. According to electrostatic field theory and Frank-Oseen elastic continuum theory, 2D electric potential distribution and 2D director distribution are calculated by means of the finite difference method on non-uniform grids. The influence of cell sizes on director distribution is analyzed. The fringe field effect between electrodes is also discussed.

Beam steering characteristics of transmission liquid crystal optical phased array(LC-OPA) were measured using ultra precision electronic autocollimator. A continuous beam steering with a constant angular resolution in the order of 20 μrad is obtained experimentally from 0° to 6° based on the method of variable period grating (VPG).Meanwhile, the angular repeatability of less than 4 μrad (RMS) has been achieved.

For laser beam steering, a pure optical-electric system without mechanical or inertial parts is used. The new device is a Liquid Crystal Optical Phased Array (LCOPA). Supplying electric field to liquid crystal cell with electrodes, the extraordinary refractive index changes, which results in phase shift that modulates the direction of incident beam. The advantages of this device are small size, less energy consuming, programmable and addressable control. A one dimensional device is designed and tested. The resolution of the steering angle is 20 μrad, the range is ±2o. The effect of flyback region and phase valley on diffraction efficiency is also discussed briefly.

High-precision beam array scanning system based on Liquid Crystal Optical Phased Array and its zero-order leakage elimination

On the basis of Coherent Beam Combination(CBC) based on Array of Liquid Crystal Optical Phased Arrays(LCOPA array), two major contributions are made in this article. Firstly, grating lobes and side lobes of combined beam are analyzed. Furthermore, according to interference theory the methods to suppress grating lobes and side lobes are put forward. Secondly, a new beam quality factor Q(θ 0 ) is proposed to evaluate the beam quality of combined beam and several influence factors are discussed. These analysis results help to obtain combined beam with better beam quality.

In one-dimensional (1D) liquid crystal optical phased array (LCOPA) device, liquid crystal molecular is controlled by discrete strip-shaped electrodes. The size of electrodes influences directly the phase modulation characteristics and far field diffraction efficiency. The model of LCOPA device was set up and the director distribution of liquid crystal in electric field was simulated. The influence of electrode size on phase modulation and diffraction efficiency was analyzed. The results showed that diffraction efficiency increases monotonously with electrode space decreasing. However, when electrode space decreases to less that 1μm, the increment of diffraction efficiency is very small and less than 1%. This means that 1μm of electrode space is small enough for the design of LCOPA devices.

Three novel multiple-beam forming methods were proposed in this paper for liquid crystal optical phased array. These three methods called sub-aperture method, array division multiplexing method, iterative Fourier transform pattern approximation method could form multiple beams simultaneously for multi-target tracking in lidar and uninterrupted communication among multiple satellites. Principles of these methods were discussed in this paper. Simulations and experiment results were given to verify the feasibility of these methods.

The small number of pixels in the current linear mode avalanche photodiode (LM-APD) array limits its three-dimensional (3D) imaging resolution. We use an optical phased array-based beam array subdivided pixel method to improve the 3D imaging resolution, using an optical phased array to generate a beam array with the same number of pixels as the LM-APD array and matching positions and controlling each sub-beam in the beam array to scan in the field of view of the corresponding pixel. The sub-beam divergence angle in the beam array is smaller than the instantaneous field of view angle of a single pixel in the LM-APD array. The sub-beam scanning in a single pixel’s field of view realizes the multiple acquisition of the target 3D information by the LM-APD array, thus improving the resolution of the LM-APD array. The distribution of the beam array in the far field is simulated, and the main performance parameters of 3D imaging are analyzed. Finally, a liquid crystal phase modulator is used as an optical phased array device to conduct experiments on a target 20 m away, and the results prove that our method can improve the resolution from 4 × 4 to 8 × 8, which can be improved at least four times.

Research on high- precision beam pointing based on liquid crystal optical phased arrays

基于液晶相控阵的聚焦式相干合成建模及性能