Liquid Crystal Optical Phased Array Research Articles

Research Progress on Liquid Crystal Optical Phased Array Technology for Laser Communication (Invited)

Research Progress on Liquid Crystal Optical Phased Array Technology for Laser Communication (Invited)

Research of Non-mechanical Beam Scanning Technology

Laser beam scanning technology has been widely used in various fields such as LIDAR, photoelectric detection and space optical communication, etc. Mechanical scanning has problems such as inflexibility and low scanning rate. This paper introduces the basic principles of acoustic-optical scanning, electro-optical scanning, optical phased array and MEMS scanning, and analyses the advantages and disadvantages of several scanning methods and the main application scenarios, and finds that the current optical scanning technology is developing towards large scanning angle, high accuracy, fast response and miniaturization, among which the liquid crystal optical phased array and MEMS scanning technology are relatively mature. The scanning angle of the LCD optical phased array can reach ±10°, and the scanning angle of the MEMS can reach 6.6°*4.4°, which has a broad development prospect.

Four-access, 80 mm aperture all phase-controlled liquid crystal laser antenna

High speed, wide range, and high precise beam steering technology are fundamental to free-space optical communication (FSO). Liquid crystal optical phased array (LC-OPA) is capable of achieving non-mechanical beam steering. This paper demonstrates a parallel, four-communication access beam deflection antenna based on LC-OPA, providing a large aperture, wide steering range, and high pointing precision. This antenna is composed of a coarse tracking subsystem using cascaded liquid crystal polarized grating (LCPG) and a fine tracking subsystem based on LC-OPA. The optical aperture is 80 mm × 80 mm, arranged in 2 × 2 subapertures, each serving a single wavelength beam. Besides, a cascaded control platform based on field programmable gate array is proposed to realize low-latency response. This optical antenna achieves a steering resolution of 20 μrad and covers a range of ±12 deg in two dimensions, laying the foundation for simultaneous access of multiple terminals in FSO communication network.

All-Solid-State Beam Steering via Integrated Optical Phased Array Technology.

Light detection and ranging (LiDAR), combining traditional radar technology with modern laser technology, has much potential for applications in navigation, mapping, and so on. Benefiting from the superior performance, an all-solid-state beam steering realized by integrated optical phased array (OPA) is one of the key components in the LiDAR system. In this review, we first introduce the basic principle of OPA for beam steering. Then, we briefly review the detailed advances of different solutions such as micro-electromechanical system OPA, liquid crystal OPA, and metasurface OPA, where our main focus was on the recent progress of OPA in photonic integrated chips. Finally, we summarize the different solutions and discuss the challenges and perspectives of all-solid-state beam steering for LiDAR.

Fast, closed-loop iterative system-on-chip of deflection efficiency enhancement for a liquid crystal optical phased array.

To implement a liquid crystal optical phased array (LC-OPA) on a practical free-space laser communication terminal, there are two essential parameters: insertion loss and the closed-loop bandwidth required to meet the dynamic linking condition of the acquisition-tracking-pointing sub-system. Real-time hardware platforms and deflection efficiency optimization algorithms have been suggested since the invention of LC-OPA. In this paper, the so-called ZYNQ platform, a field-programmable-gate-array-based heterogeneous system-on-chip (SoC), is utilized to keep real-time response and accelerate data generation, such as beam steering, beamforming, beam enhancement, etc. In addition, a novel, to the best of our knowledge, optimization algorithm is proposed on the concept of dimension reduction of the number of objective variables. After deploying on this heterogeneous SoC platform, numerical simulations and experimental results both verify that, compared to the conventional PC-based system, the integrated SoC platform offers 15.8 times faster iterative speed, a rapid convergence rate, and excellent robustness, yet with less usage of power, physical size, and monetary cost. The efficiency enhancement process costs only a few seconds at any angle, laying the foundation for practical in-line applications.

Wide range retina-like scanning based on liquid crystal optical phased array

A retina-like scanning method based on liquid crystal optical phased array (LCOPA) is proposed to improve field of view and efficiency. The functions of human retina, lens and fovea gaze control are achieved with optimized designed scanning strategy. Beam splitting method and angular magnification system are used to meet the requirements of the efficient scanning and large FOV. The models are verified by several experiments and the results show that the scanning performance well with the flexible retina-like strategy, especially for eccentric fovea. The scanning FOV is magnified 2.2 times and the average scanning error is less than 5%.

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

In order to improve the scanning efficiency of lidar, a set of high precision scanning system with 40 × 40 sub-beams is established, in which a liquid crystal optical phased array is used to load the phase calculated by Gerchberg–Saxton(GS) algorithm to split the incident plane wave. Technically, the imperfections of GS algorithm often lead to problems such as sidelobes and uneven intensity of output spots, which eventually result in poor pointing accuracy of sub-beams and low scanning resolution of lidar. To solve those problems, we propose an optical method rather than the classical algorithm optimization methods, which restricts the preset sub-beams to the diffraction limit to unify the actual sub-spot morphology and ensure high pointing accuracy. In this way, the standard deviation of pointing error eventually decreases from 42 μrad to 16 μrad. In the aspect of eliminating zero-order leakage, the zero-order light leakage cannot be separated by the conventional tilt method in the case that the spatial spectrum span of the generated beam array exceeds the cut-off frequency. Concerning this issue, a novel zero-order leakage elimination method is proposed to avoid zero-order leakage by splicing two half beam array of adjacent diffraction orders on both sides of the cut-off frequency. Meanwhile, the intensity correction is carried out to achieve leakage avoidance without damaging the intensity uniformity of the beam array.

Controllable liquid crystal diffractive sail and its potential applications

This paper introduces a new type of diffractive solar sail composed of liquid crystal optical phased arrays, which control the thrust through electric modulation. The main feature of the proposed liquid crystal diffractive sail is its capability of generating a continuously variable and actively controllable tangential radiation pressure at the sun-facing attitude. The principles governing the generation of radiation pressure by electric modulation of the liquid crystal optical phased array under a broad spectrum of solar radiation are derived by considering a periodical voltage distribution analogous to a reflection step-shaped binary phase grating. Then the reliability of the theoretical model of radiation pressure is verified through comparison with finite-difference time-domain (FDTD) simulation results. A sun-facing liquid crystal diffractive sail may be utilized to replace the reflective sail in space missions by taking the advantage that no active attitude control of the sail surface towards sunlight is required. We also study the attitude control strategy of the proposed diffractive sail along its normal axis, allowing for the direction of tangential radiation pressure to be varied without changing its sun-facing attitude.

Quantitative error analysis for non-mechanical phase-controlled beam steering based on symmetrical radial sub-aperture coherence algorithm

ABSTRACT Based on the fundamental of traditional Sub-aperture coherence (SAC) algorithm, Radial Sub-aperture coherence (RSAC) and Symmetrical Radial Sub-aperture coherence (SRSAC) have been designed to achieve non-mechanical beam steering with ultra-high precision and stability by utilising two-dimensional Liquid Crystal Optical Phased Array (LCOPA). Both of them inherit the original advantages, that is, to eliminate the local pointing accuracy defects and greatly improve the angular resolution of the beam steering system. Meanwhile, the steering angle error caused by the aperture variation and the alignment error can be also effectively restricted by SRSAC. In this paper, the essential mathematical framework of SRSAC and its affiliated local error elimination method was established to supplement the theoretical support of these phase generation algorithms. By dissecting the generation mechanism of each pointing error branch, the theoretical formulae with great universality are finally proposed, which provide visualised and necessary references for the specific application in other beam control situations.

基于液晶相控阵和体全息光栅的激光多目标指示技术

基于液晶相控阵和体全息光栅的激光多目标指示技术

Two-dimensional symmetrical radial sub-aperture coherence and the local precision defect elimination method for high-precision beam steering.

Sub-aperture coherence (SAC) is a classical phase control method for high-precision beam steering using liquid crystal optical phased arrays (LCOPA). On this basis, radial sub-aperture coherence (RSAC) and symmetrical radial sub-aperture coherence (SRSAC) were proposed, which guarantee the stability of steering angles when the beam aperture and incident position fluctuate. In this article, the pre-existing one-dimensional SRSAC was firstly extended to a more universal 2D phase generation algorithm. Meanwhile, for the intractable problem of local precision defects caused by the basic two-dimensional variable period grating (2D-VPG) algorithm, we tracked their locations accurately and designed a targeted elimination method carefully. So these remarkable error peaks could be thoroughly removed by using 2D-SRSAC optimized by the local precision defect elimination method. Since then, all the excellent performance of 1D-SRSAC can be perfectly transplanted to 2D, which makes the non-mechanical beam steering technology using LCOPA more mature and competitive in the applications required ultra-high precision.

Aperture scalable liquid crystal optically duplicated array of phased array

To achieve non-mechanical laser beam steering in the scenario of long distance propagation such as free-space laser communication between satellites, large aperture size is an inevitable issue to be considered to narrow the divergence angle of the output beam. Liquid crystal optical phased array, to be one of the solutions of non-mechanical beam steering, has already shown its obvious potential to achieve a relative large aperture on the order of centimeter. To achieve even larger, its driving matrix becomes squared larger. In this paper, we proposed a novel architecture to realize an optical phased array with a scalable aperture. Meanwhile the driving matrix is almost not increased. It provides the feature of a cascade system with a device of spatial phased modulation and an array of duplicating units. Each unit of the duplicating array is consist of a polarization beam splitter and a half wave plate to have the same output optical field distribution as the input beam whose phase front is modulated by a small size spatial modulator. Not only the property of beam deflection is numerically simulated and experimentally verified, but also the property of divergence angle compression and grating lobes limitation are evaluated. Meanwhile, due to the high precision of the experimental alignment, the non-mechanical beam deflection property is still maintained no matter how many the duplicated unit number is. The relative standard derivation steering error is 0.025.

Liquid crystal-optical phased arrays (LC-OPA)-based optical beam steering with microradian resolution enabled by double gratings.

We demonstrate an optical beam steering setup based on a liquid crystal-optical phased array (LC-OPA) with high angular resolution and low beam divergence, due to the use of double-grating configuration to enhance the corresponding angular resolution of LC-OPA. The introduction of two nonparallel blazed gratings with a special included angle can achieve multiple diffractions for the incident light, leading to the realization of angular compression. Numerical simulation results show the angular compression ratio can be optimized by selecting the appropriate grating constant and the included angle of double-grating. Experimental results verify that the steered angle of the incident light can be compressed so that the angular resolution of the steered beam can correspondingly be improved more than six times. In addition, when the beam is multiply diffracted within the double-grating configuration, the divergence angle becomes smaller as the beam size is enlarged. Therefore, both the microradian steering resolution and the narrow beam divergence can be simultaneously obtained with our proposed setup. The efficiency of the steering where the beam diffracted four times within the double-grating configuration is 80%.

Nonmechanical two-user tracking method of space-polarization division using the liquid crystal optical phased array

We propose a two-user tracking method called space-polarization division to enable simultaneously multiple terminals tracking in the field of free space laser communications. This method is based on the nonmechanical beam steering device of liquid crystal optical phased array, which is capable of continuously laser beam pointing and multiple beams generating. An optical architecture of two-user access is presented to show a one-to-two communication link between the relay satellite terminal and user terminals. The two-user tracking strategy is modeled numerically and experimentally verified to show the feasible and agile two-user tracking with a good accuracy less than 50 microradians which meet the requirements of coarse tracking.

Ultra-compact electrically controlled beam steering chip based on coherently coupled VCSEL array directly integrated with 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.

Radial sub-aperture coherence method used to achieve beam steering with high precision and stability.

Sub-aperture coherence (SAC) algorithm, which is based on the classical phase modulation method called variable period grating (VPG), was usually used to control liquid crystal optical phased arrays (LCOPA) to achieve agile beam steering with high precision. However, the beam steering angle of SAC is severely affected by the beam aperture, which limits the generality of the algorithm distinctly. In this article, two kinds of new phase modulation method have been proposed to solve this problem, which were named as radial sub-aperture coherence (RSAC) and symmetrical radial sub-aperture coherence (SRSAC). By using RSAC, the holistic drift of steering angle, which is caused by the variation of beam aperture, can be effectively avoided. In addition, a series of equidistant steering points with ultra-high precision can be obtained. Upon this basis, SRSAC greatly enhances the steering angle's stability in the presence of system alignment error and relative vibration. Thus, the practicability of LCOPA for beam steering can be improved effectively.

基于液晶光学相控阵的光束扫描研究进展

基于液晶光学相控阵的光束扫描研究进展

The realizable method for large diameter liquid crystal optical phased array and the analysis of its far-field characteristics

The optical aperture of the antenna is an important technical indicator of the liquid crystal optical phased array. Based on the multi-subarray parallel driving and two-level device cascade method (PAPA), in this paper, an improved i-PAPA method was proposed. Large area phased beam control is realized on a single phased array antenna by subdivision of the COM electrodes, and it has the advantages of single device operation and low insertion loss. Through numerical simulation, the results show that the antenna near field phase has continuous distribution; when the point angle varies from 0 degrees to +6 degrees, the far-field diffraction efficiency drops smoothly and monotonously as the point angle increases, the diffraction efficiency is greater than 48%; When the point angle varies from 0 degrees to +3 degrees, the diffraction the efficiency is greater than 80%.

Arbitrary multiple beam forming by two cascaded liquid crystal optical phased arrays.

A new method for non-mechanical laser beam splitting and steering is demonstrated. Two cascaded liquid crystal optical phased arrays (LC-OPAs) controllably modulate the amplitude and phase of an incident laser beam to realize the near-field wavefronts of multiple simultaneous beams with arbitrary directions. Diffraction between the two arrays is avoided by precise 4-f imaging from one LC-OPA to the other (array resolution 1×1920). In the method of cascaded amplitude and phase (CAP) devices, numerical simulation results show the characteristics of amplitude and phase modulation profiles, as well as the far-field intensity patterns. Both the numerical and experimental results clearly demonstrate the capabilities of fast multi-beam forming with high efficiency (>85%, 4 beams) and accuracy (deviation <90μrad).

Theoretical analysis on power stability and switch time of the non-mechanical beam steering using liquid crystal optical phased array

ABSTRACTFocusing on the coherent optics of phased array, a dynamic theoretical model is built to analyse the process of laser beam steering using the device of liquid crystal optical phased array. Based on the dynamics of electrical field induced liquid crystal molecule rotation and the mechanism of phased control non-mechanical steering, switching time is redefined according to two curves representing receiving power on the current direction and targeting direction. Meanwhile, including the far field distribution, power stability and pointing direction, two cases with large steering step and tiny steering step are evaluated. The steering time between two angles is minished to be less than the switch-off decay time. In addition, the instability on the process of beam steering is obvious although the steering step is only half of beam width. To overcome this instability, tinier steering step is suggested although it is also limited by the driving algorithm and driving circuits.