Fast beam steering is useful in applications including free space optical switching and communications. Quasi-static beam steering, where the beam rapidly switches between discrete fixed directions, presents special challenges. Here we demonstrate a fast quasi-static ‘pinwheel’ scanner by conformally mapping linear blazed gratings into curved structures fabricated onto annular sections of a rotary disk. We use Matlab and Zemax to model the effects of the conformallymapped grating on the emitted optical beam. We show a specific two-dimensional (2D) ‘pinwheel’ scanner design with 56 gratings, each deflecting 1.31 μm incident light by 11.3° in one of four directions with 75% optical efficiency. The element was fabricated by optical grey-scale lithography on a 95 mm diameter substrate, coated with gold, and mounted onto the spindle of a 3.5” format 7200 rpm magnetic disk drive. We characterize the optical beam steering efficiency, pointing, and stability, and demonstrate microsecond switching speed of a single mode fiber signal.
Liquid crystal spatial light modulators (LC-SLMs) are usually polarization sensitive optical elements. In this paper, we propose a polarization-independent beam steering system to overcome the polarization problem of conventional liquid crystal devices by employing two polarization-dependent LC-SLMs, a polarizing beam splitter and a half-wave plate. In this system, two one-dimensional LC-SLMs are aligned orthogonally to deflect the beam in azimuthal and elevation, respectively. This system enables LC-SLMs to work in any polarization state of incident light, and can realize continuous two-dimensional laser beam pointing. Properties of polarization-independence as well as two-dimensional beam steering were mathematically and experimentally verified with a good agreement. Using the well aligned beam steering system, linearly polarized beams in different polarization angle are deflected with high accuracy and efficiency. The measured angular deviations are less than 5 micro-radians to show a high-accuracy beam steering of the system. This polarization-independent beam steering scheme is useful in the applications of nonmechanical laser communication, Lidar, and other LC-based devices.
Liquid crystal optical phased array (LC-OPA) has been considered with great potential on the non-mechanical laser deflector because it is fabricated using photolithographic patterning technology which has been well advanced by the electronics and display industry. As a vital application of LC-OPA, free space laser communication has demonstrated its merits on communication bandwidth. Before data communication, ATP (acquisition, tracking and pointing) process costs relatively long time to result in a bottle-neck of free space laser communication. Meanwhile, dynamic real time accurate tracking is sensitive to keep a stable communication link. The electro-optic medium liquid crystal with low driving voltage can be used as the laser beam deflector. This paper presents a fast-track method using liquid crystal optical phased array as the beam deflector, CCD as a beacon light detector. PID (Proportion Integration Differentiation) loop algorithm is introduced as the controlling algorithm to generate the corresponding steering angle. To achieve the goal of fast and accurate tracking, theoretical analysis and experimental verification are demonstrated that PID closed-loop system can suppress the attitude random vibration. Meanwhile, theoretical analysis shows that tracking accuracy can be less than 6.5μrad, with a relative agreement with experimental results which is obtained after 10 adjustments that the tracking accuracy is less than12.6μrad.
To realize a non-mechanical laser beam deflector with a wide steering range, improved high order grating method (i-HOG) was theoretically proposed on the promising optical phased array device using the material of liquid crystal. In this paper, experimental prototype and results are presented to verify the performance on the wider steering range when the method of i-HOG was applied on the corresponding device. To satisfy the requirement on much deeper phase retardation, the optical film of liquid crystal was redesigned and fabricated as well as driving hardware. Comparing with the conventional methods where the modulo of phase delay is 2π, liquid crystal optical phased array using the method of i-HOG can succeed a double steering range. Meanwhile, experimental results also show better diffraction efficiency.
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.
Liquid crystal optical phased array (LC-OPA) has been reported to be one of the promising methods to realize nonmechanical laser beam steering in free-space laser communication. The nonmechanical beam steering mechanism of LC-OPA in a free-space laser communication process is demonstrated. To analyze the steering performance of LC-OPA for a beam arriving from arbitrary directions, a theoretical model based on two-dimensional liquid crystal orient model and backward propagation method is proposed. In addition the phase characteristics are discussed both in normal and oblique incident cases, and a linear relationship between angle-of-arrival and detected incident angle is also derived. All of the theoretical results are verified by the following experiments with a good agreement.
To achieve a wider scanning range of liquid crystal optical phased array (LC-OPA), in this paper, a novel method of improved high order grating (i-HOG) is proposed in one device without introducing any other devices. The method of i-HOG breaks through the traditional ideas of modulo 𝟐𝛑 phase and takes the fringe effect into account to have a multi order extension. Subsequently, the method is verified by numerical simulation showing that it realizes a scanning range of wider than 20 degrees and even wider.
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