Fine pixel size and high-resolution liquid crystal on silicon (LCoS) backplanes have been developed by various companies and research groups since 1973. The development of LCoS is not only beneficial for full high definition displays but also to spatial light modulation. The high-quality and well-calibrated panels can project computer generated hologram (CGH) designs faithfully for phase-only holography, which can be widely utilized in 2D/3D holographic video projectors and components for optical telecommunications. As a result, we start by summarizing the current status of high-resolution panels, followed by addressing issues related to the driving frequency (i.e., liquid crystal response time and hardware interface). LCoS panel qualities were evaluated based on the following four characteristics: phase linearity control, phase precision, phase stability, and phase accuracy.

https://ir.nctu.edu.tw:443/bitstream/11536/148510/1/6036c0acb1e0f4f2c5da40b60411593f.pdf

Pursuing high quality phase-only liquid crystal on silicon (LCoS) devices

Non-mechanical beam steerers with lightweight, compact, high-efficiency, high-precision, and/or large-angle are pivotal for light detection and ranging (LiDAR) of autonomous vehicles, eye-tracking for near-eye displays, microscopy, optical tweezers, and high-precision three-dimensional (3D) printing. However, even the most matured optical phased array can only provide quasi-continuous, efficient beam steering within a small angle range. A telescope module with an angle magnification function can be coupled to enlarge the steering range or precision. But obtaining a compact, low-cost, lightweight, high-quality telescope module with conventional optics remains challenging. Patterned liquid crystal-based planar optical elements offer great design freedom for manipulating the phase profile of light in 2D space. Owing to the advantages of high efficiency, thinness, low cost, easy processing, flexibility, and response to environmental stimuli, a plethora of high-quality optical devices have been demonstrated. Here, a miniature planar telescope mediated by liquid crystal polymers is proposed to offer angle magnification independent of incident spatial location. It consists of two cascaded liquid crystal planar optical elements, each performing a predefined mathematical transformation. By this concept, planar optical elements are fabricated using a new exposure method and assembled into planar telescopes with different magnification factors. Within the incident field range, over 84.6% optical efficiency is achieved with small wavefront distortion. Such a miniature planar telescope shows the potential of cascaded liquid crystal planar optical elements for realizing functionalities that cannot be fulfilled by single optical elements, and enables lightweight, low loss, passive optical transmitters for widespread applications.

/pdf/miniature-planar-telescopes-for-efficient-wide-angle-high-2jmxnxwzhm.pdf

Miniature planar telescopes for efficient, wide-angle, high-precision beam steering

A slim beam deflector that satisfies both a large steering angle and a large area can be very useful in various applications. However, a smaller electrode pitch for a large steering angle and enlargement of its area are trade-off relations due to the limited number of control channels in an electrically tunable beam deflector system. For a large steering angle in the active area where actual diffraction occurs, an indium tin oxide electrode of 2 µm pitch was implemented through a stepper lithography. The via-hole process was developed to expand the reduced active area due to the small electrode pitch. We developed a beam deflector with 7200 controllable channels in an active area of 14.4mm×14.4mm. The maximum steering angle is 7.643° at a wavelength of 532 nm.

Large-area liquid crystal beam deflector with wide steering angle.

Phase compensation is a critical step for the optical measuring system using spatial light modulator (SLM) The wavefront distortion from SLM is mainly caused by the phase modulation non-linearity and non-uniformity of SLM’s physical structure and environmental conditions A phase modulation characteristic calibration and compensation method for liquid crystal on silicon spatial light modulator (LCoS-SLM) with a Twyman-Green interferometer is illustrated in this study A method using two sequences of phase maps is proposed to calibrate the non-uniformity character over the whole aperture of LCoS-SLM at pixel level A phase compensation matrix is calculated to correct the actual phase modulation of the LCoS-SLM and ensure that the designed wavefront could be achieved Compared with previously known compensation methods, the proposed method could obtain the phase modulation characteristic curve of each pixel on the LCoS-SLM, rather than a mono look-up table (LUT) curve or multi-LUT curves corresponding to an array of blocks over the whole aperture of the LCoS-SLM The experiment results show that the phase compensation precision could reach a peak-valley value of 0061λ in wavefront and this method can be applied in generating freeform wave front for precise optical performance

https://www.mdpi.com/1424-8220/21/3/967/pdf

Phase Compensation of the Non-Uniformity of the Liquid Crystal on Silicon Spatial Light Modulator at Pixel Level

Optical beam steering using liquid-based devices

In order to calibrate the spatial phase response nonuniformity of liquid crystal on silicon (LCoS), we propose to use a Twyman–Green interferometer to characterize the wavefront distortion, due to the inherent curvature of the device. During the characterization, both the residual carrier frequency introduced by the Fourier transform evaluation method and the lens aberration are error sources. For the tilted phase error introduced by residual carrier frequency, the least mean square fitting method is used to obtain the tilted phase error. Meanwhile, we use Zernike polynomials fitting based on plane mirror calibration to mitigate the lens aberration. For a typical LCoS with 1×12,288 pixels after calibration, the peak-to-valley value of the inherent wavefront distortion is approximately 0.25λ at 1550 nm, leading to a half-suppression of wavefront distortion. All efforts can suppress the root mean squares value of the inherent wavefront distortion to approximately λ/34.

Precise calibration of spatial phase response nonuniformity arising in liquid crystal on silicon.

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%.

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

We demonstrate a small angle measurement setup enabled by the double-grating configuration, where the multiple diffractions are used to magnify the deflection angle of the optical beam. Such small angle measurement setup has characteristics of high sensitivity and compact size. The use of two unparallel blazed gratings with a special included angle can realize multiple diffractions for the incident light, leading to the realization of deflection angle amplification. Experimental results verify that small angle can be accurately characterized and the angular resolution of measurement can be improved more than 40 times by inserting the double-grating configuration into the conventional auto-collimation angle measurement setup. Therefore, the micro-radian angle can be accurately measured with our proposed compact characterization setup.

Double-grating with multiple diffractions enabled small angle measurement.

The invention discloses a blazed grating based beam deflection angle amplifying unit. The blazed grating based beam deflection angle amplifying unit includes beam deflection angle amplifying subunits;each beam deflection angle amplifying subunit includes two blazed gratings; if an incident angle of a beam incident on the blazed grating is i0, the diffraction angle of the diffracted light containing the main energy of the beam is Theta 0; an included angle between the two blazed gratings is Theta0-i0, so that the incident beam can be diffracted back and forth between the two blazed gratings for multiple times, and the incident angle and the diffraction angle of the incident light are identical in the two blazed gratings. The incident angle i0 and the diffraction angle Theta0 are adjusted,so that the deflection amount of the diffracted light is larger than the deflection amount of the incident light when the incident light is deflected, and multiple diffraction amplifications of the deflection amount of the incident light can be realized. The method can achieve multiple diffraction amplifications of the deflection amount of the incident light.

Blazed grating based beam deflection angle amplifying unit

The invention discloses an optical signal to noise ratio monitoring device and a monitoring method based on spatial light interference; the device comprises an optical shaping module, a liquid crystalmodulation module, a probing module, a liquid crystal control terminal and a computing module, wherein the liquid crystal modulation module comprises a beam splitter, a plane mirror, a transmission grating, a first lens and a liquid crystal spatial light modulation unit; the optical shaping module is used for sequentially performing a process of adjusting polarization state, collimation, polarization transmission, and beam expansion on the input signal light; the liquid crystal modulation module is used for dividing the optically shaped signal light into reference light and measurement lightso that a corresponding phase delay is applied to components of different frequencies in the measurement light; the reference light and the measurement light are reflected back to a mirror to form a probe light to be incident on the probing module; and the probing module is used for measuring the optical power of the probe light. According to the device and the method in the invention, the filtering characteristics of an interferometer can be improved while the advantages of the interferometric method for monitoring the optical signal to noise ratio are preserved, thereby improving the measurement accuracy of the optical signal to noise ratio.

Siyi Qin

Papers

Precise calibration of spatial phase response nonuniformity arising in liquid crystal on silicon.

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

Double-grating with multiple diffractions enabled small angle measurement.

Blazed grating based beam deflection angle amplifying unit

Optical signal to noise ratio monitoring device and monitoring method based on spatial light interference