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Improvements of phase linearity and phase flicker of phase-only LCoS devices for holographic applications
Author:
Abstract
Significant phase distortion corrections were achieved by optimizing the digital driving patterns of phase-only liquid crystal on silicon devices for digital holographic applications. Nearly perfect phase linearity and phase flicker of 0.09% over 256 addressed phase levels in respect to the total modulation range of 2𝜋 were realized, enabling a meaningful increase of phase levels from 8 bits (256 levels) to 9 bits (512 levels). Tests were carried out to evaluate the qualities of optically reconstructed holographic images with reduced phase flicker and optimized phase linearity, and an increase of 17.7% in the root-mean-square contrast was demonstrated.
© 2019 Optical Society of America
1. INTRODUCTION
Digital holography [1,2] is an emerging technique in which a digital hologram that contains the entire wavefront of an object is recorded, and the object is reconstructed by using a computer. It has a variety of fascinating applications, ranging from holographic displays [3–6], optical tweezers [7,8], digital holographic microscopy (DHM) [9] to wavelength selective switches(WSS)[10],etc.
Phase-only liquid crystal on silicon (LCoS) spatial light modulators (SLMs) are acting as the optical engines among these applications. Unlike their rival, digital micro-mirror device(DMD) SLMs, which have poor light efficiency and are only capable of modulating light at a binary manner, phase-only LCoS SLMs have the advantages of high efficiency, small pixel size, high resolution, and multilevel phase modulation [11].
However, despite these attractive merits, LCoS devices are also facing the challenge of the loss in image quality and resolution,due to the nonlinearity and the instability of phase modulation depth.
Ideally, a linear phase response with a range of at least from 0 to 2 for the operating wavelength is desirable. However,the orientation of the liquid crystal (LC) molecules does not follow a linear behavior in relation to the applied voltage, which results inanonlinearcontrolofthephasedepth.Forcommercial LCoS devices, the linearization process can be done by gamma correctioninsoftware.
Phase flicker, which can be profound in digital driving LCoS devices, introduces instability in the applied phase, causing the reduction in resolution. It is the temporal fluctuation caused by the competition between the change of the electri cal driving force and LC molecule relaxation, resulting in a fast-changing phase shift error. Drifting around the desired phasevaluemakesitdifficulttoresolvetheadjacentphaselevels, and hence reduces the phase and depth resolution that can be achieved.
Some current approaches for the improvement of phase flicker include the following: Martínez et al. reasonably reduced the flicker by using a high-frequency driving sequence for fast compensation [12]; García-Márquez et al. demonstrated a reduction of up to 80% of the flicker initial value by increasing the LC viscosity, and hence the damping force, through cool ing the LCoS device down to 8C [13]; Yang et al. reduced the peak-to-peak flicker to around 0.03 for an 8-bit modulation device by analyzing the speed of phase change introduced by the pulses in the pulse width modulation (PWM) driving waveforms [14]. However, these methods either still suffer fromthepresenceofarelatively large flicker, or the LCoS device can only work at a low temperature with reduced switching speed.
This work goes back to the molecular level behavior of the LC molecules and attempts to improve the phase linearity and the phase flicker from the point of view of the fundamental mechanism. A novel method is proposed to linearize the phase modulation depth and minimize the phase flicker at the same time by optimizing the driving patterns, based on the existing digital drivingmethod.
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Improvements of phase linearity and phase flicker of phase-only LCoS devices for holographic applications
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This paper describes the fundamentals of phase-only liquid crystal on silicon (LCOS) technology, which have not been previously discussed in detail. This technology is widely utilized in high efficiency applications for real-time holography and diffractive optics. The paper begins with a brief introduction on the developmental trajectory of phase-only LCOS technology, followed by the correct selection of liquid crystal (LC) materials and corresponding electro-optic effects in such devices. Attention is focused on the essential requirements of the physical aspects of the LC layer as well as the indispensable parameters for the response time of the device.Furthermore, the basic functionalities embedded in the complementary metal oxide semiconductor(CMOS) silicon backplane for phase-only LCOS devices are illustrated, including two typical addressing schemes. Finally, the application of phase-only LCOS devices in real-time holography will be introduced in association with the use of cutting-edge computer-generated holograms. Light: Science & Applications (2014) 3, e213; doi:10.1038/lsa.2014.94; published online 24 October 2014 Keywords: electro-optic effect; liquid crystal material; liquid crystal on silicon device; real-time holography
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Abstract: Sub-millisecond response time with a refresh rate higher than 2000 frames per second (fps) and no degradation of the contrast ratio or diffraction efficiency is demonstrated in working liquid crystal on silicon (LCOS) spatial light modulators (SLMs) with 8-bit grey levels of amplitude and phase modulations. This makes possible to achieve an information bandwidth of about 190 Gb s 1 with a 4k LCOS operating at 10-bit phase modulation levels. The normalised contrast stays at almost the unit level for a frame rate up to 1700 fps and at higher than 0.9 for 2500 fps. The diffraction efficiency stays above -1.0 dB for a frame rate up to 2400 fps. Such a fast response allows us to eliminate image blurring in replaying a fast movie. Published by The Optical Society under the terms of the Creative Commons Attribution 4.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
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