Improvements of phase linearity and phase flicker of phase-only LCoS devices for holographic applications

Author:

Yuan Tong, Mike Pivnenko, and Daping Chu

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.

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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