Correcting Fabrication-Induced Curvature in Micromirror-Based Spatial Light Modulators with a Microlens Array

TL;DR

The paper tackles curvature-induced phase errors in high-fill-factor piston-motion micromirror SLMs used for fast holography. It introduces a pitch-matched microlens array placed at a focal distance to focus light onto mirror centers, effectively flattening the phase and restoring a near-100% optical fill factor. Simulations and experiments show dramatic improvements: the phase fidelity metric PCC rises from ~0.11 to ~0.85 and a single holographic spot brightness increases by ~8×, approaching the performance of flat mirrors. This hybrid optical–electromechanical strategy enables scalable, high-speed, high-fidelity wavefront control for CGH-enabled applications such as VR/AR, holographic displays, and optogenetics.

Abstract

Computer generated holography requires high-speed spatial light modulators (SLMs) for dynamically patterning light in 3D. Piston-motion micromirror-based SLMs support high-speed ($\geq$ 10 kHz) phase modulation; however, fabricating micromirror arrays with sufficient fill factor necessary for high diffraction efficiency is challenging. In particular, the larger mirrors of high fill factor designs are susceptible to stress-induced curvature that significantly degrades optical performance. In this work, we introduce an optical compensation method using a pitch-matched microlens array (MLA) to focus light onto just the center of each mirror. Our approach thus avoids curvature-induced artifacts and improves optical fill factor to nearly 100$\%$, independent of the original mechanical fill factor. Through simulations and experiments on a fabricated micromirror array with bowed mirrors, we show that the Pearson correlation coefficient of the imparted phase profile is improved from 0.11 to 0.85 and the brightness of a holographically-generated single spot is enhanced by 8$\times$ with our microlens array in place. Our hybrid optical-electromechanical strategy thus provides a scalable path toward high-speed, high-fidelity wavefront control for applications such as adaptive optics, holographic displays, and optogenetics.

Figures (7)

• Figure 1: Micromirror compensation technique to improve modulation capability. (a) Ideal micromirrors pattern incident light to form point clouds, which can be used for downstream tasks such as neuromodulation. (b) Due to manufacturing issues, existing micromirror devices have curved surfaces, causing failures in point cloud production. (c) We present a simple and compact optical compensation method: by adding a pixel pitch-matched microlens array in front of the micromirror, we correct for mirror curvature. This results in accurate point cloud formation.
• Figure 2: Structure of the SLM and simulation of CGH point degradation due to micromirror curvature. (a–c) Schematics of a micromirror with flat (no curvature), mild curvature, and greater curvature, respectively. (d–f) Simulated wave propagation of the focused spot generated by the SLM for each curvature case. From left to right, the propagation distances are $z = 0$, 5, 10, and 15 mm. The colorbar indicates normalized intensity. (g) Mean intensity correlation between curved and flat mirrors as a function of radius of curvature. For each curvature, the propagated intensity patterns were compared with the corresponding flat-mirror reference at the same distance. The correlations across all propagation distances were averaged, and the standard deviation is shown as a shaded light-blue region. Scale bar in (d–f): 200µm.
• Figure 3: Concept of the MLA and phase flattening simulation. (a) Schematic illustration of phase flattening using a MLA. (b) Initial phase distribution of the micromirrors, each with unique random height displacements. (c) Distorted phase map from the curved mirror SLM. (d) Intensity distribution of the focal spot array at the SLM plane generated by the MLA with a focal length of 3 mm. (e) Phase profile after flattening by the MLA with a 3 mm focal length. (f, g) Same as (d, e), but using an MLA with a 1 mm focal length. The colorbars in (b), (c), (e), and (g) indicate phase in radians. The colorbars in (d) and (f) represent normalized intensity. Scalebars: 100µm.
• Figure 4: Diagram of the DHM setup combined with the CGH. HWP: half-wave plate, L1-4: lenses, P: pinhole, PBS: polarizing beam splitter, BS1-2: beam splitter, QWP: quarter-wave plate, OL: objective lens. The inset images indicated by red dashed lines show, from left to right, the MLA focusing the incident plane wave onto the curved micromirror array, the measured CGH point, and the SLM phase image
• Figure 5: Experimental phase measurement of the SLM using DHM. (a) Measured phase image of the SLM before flattening. (b) Phase image after flattening using the MLA. (c) Phase distribution within a single pitch, obtained by averaging the phase values of individual mirrors along the pink line in (a) and the light green line in (b). (d, e) Phase response of the SLM to single pixel actuation after MLA-based flattening. A voltage was applied only to the mirror enclosed by the yellow dashed square. (f) Phase modulation curve of a single pixel under continuous voltage variation. The colorbar indicates phase in radians. Scalebars: 200µm for (a, b), and 50µm for (d, e).