high-contrast grating mems optical phase-shifters for two...

High-contrast grating MEMS optical phase-shifters for two-dimensional free-space beam steering

Mischa Megensa, Byung-Wook Yoob, Trevor Chana, Weijian Yangb, Tianbo Sunb, Connie J. Chang-Hasnainb, Ming C. Wub, and David A. Horsleya,*

aDepartment of Mechanical and Aerospace Engineering, University of California at Davis, Davis, California 95616, United States; bDepartment of Electrical Engineering and Computer Sciences,

University of California, Berkeley, CA 94720, United States

ABSTRACT

We report an optical phased array (OPA) for two-dimensional free-space beam steering. The array is composed of tunable MEMS all-pass filters (APFs) based on polysilicon high contrast grating (HCG) mirrors. The cavity length of each APF is voltage controlled via an electrostatically-actuated HCG top mirror and a fixed DBR bottom mirror. The HCG mirrors are composed of only a single layer of polysilicon, achieving >99% reflectivity through the use of a subwavelength grating patterned into the polysilicon surface. Conventional metal-coated MEMS mirrors must be thick (1-50 μm) to prevent warpage arising from thermal and residual stress. The single material construction used here results in a high degree of flatness even in a thin 350 nm HCG mirror. Relative to beamsteering systems based on a single rotating MEMS mirror, which are typically limited to bandwidths below 50 kHz, the MEMS OPA described here has the advantage of greatly reduced mass and therefore achieves a bandwidth over 500 kHz. The APF structure affords large (~2π) phase shift at a small displacement (< 50 nm), an order-of-magnitude smaller than the displacement required in a single-mirror phase-shifter design. Precise control of each all-pass-filter is achieved through an interferometric phase measurement system, and beam steering is demonstrated using binary phase patterns.

Keywords: optical phased arrays, beam steering, micro electro mechanical systems, high contrast gratings, Gires-Tournois etalon, all-pass filters.

1. INTRODUCTION Optical phased arrays (OPAs)1,2 are versatile beam steering devices suitable for a variety of applications including LIDAR, free-space optical communication, 3D holographic displays, and high-resolution 3D imaging.3,4 An OPA consists of a two-dimensional (2D) array of phase shifters which impose a desired phase profile on an incoming beam of light. The constructive interference of the outgoing light waves forms the desired beam. An OPA is usually much faster than a single steering mirror, as individual phase shifters are much smaller and more nimble than a large scanning mirror.

The dominant OPA technology is based on liquid crystal phase shifters, which have been studied extensively since their initial demonstration using liquid crystal television panels.5,6 However, liquid crystals have limited operating speed because it takes tens of milliseconds for an electric field to reorient the molecules of the liquid crystal. More recently, micro-electromechanical systems (MEMS) have been used to produce OPAs.7,8 A typical MEMS-based phase shifter is realized by a “piston” mirror which is displaced to provide the desired phase shift. To increase the speed of such MEMS-based beam steering, we recently introduced phased arrays based on high contrast gratings (HCG’s), rather than mirrors.9–11 Unlike earlier multi-layer MEMS mirrors, these grating mirrors are made of a single dielectric layer, achieving high reflectivity (~99.9%) over a broad optical bandwidth.9,12 The HCG’s single-material construction results in greater manufacturability since residual stress is easily controlled when depositing the single polysilicon layer and the process eliminates the need for non-CMOS compatible metals such as gold. The HCG-OPA has the potential to operate at high optical power without warping due to mismatch in coefficient of thermal expansion and without the thermal damage that plagues mirrors composed of low melting-point metals (e.g. Au and Al). The thin, open structure of the grating mirrors greatly reduces its mass, and this increases the operating bandwidth of the OPA.

* [email protected]; phone: 1-530-341-3236, fax: 1-530-752-4158

Invited Paper

High Contrast Metastructures III, edited by Connie J. Chang-Hasnain, David Fattal, Fumio Koyama, Weimin Zhou, Proc. of SPIE Vol. 8995, 89950Q · © 2014 SPIE

CCC code: 0277-786X/14/$18 · doi: 10.1117/12.2045341

Proc. of SPIE Vol. 8995 89950Q-1

Downloaded From: http://proceedings.spiedigitallibrary.org/ on 07/21/2014 Terms of Use: http://spiedl.org/terms

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much reduced actu

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of our HCG-DBR, open symbols),uation voltage.

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resonant frequd in. Since the the all-pass filass filter provi

R all-pass filter p, with similar res

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gths from 1480 n

we compare itse 6. The HCGuency. For the displacement v

lter mirror decrides a larger ph

phase shifter (resonant frequency

f mirrors usingages for a selece modest 33% rners of the fier vertical lines

nm to 1530 nm, i

s phase shift w-only mirror isconventional Hvaries quadratireases as the ethase shift for a

ed, filled symboly. The all-pass fi

g a binary pattection of patternfill factor of

eld (for the ches), or diagonal

in 10 nm

with that achis comparable iHCG mirror, thically with volttalon tunes throa considerably

s) with the phasefilter achieves a

ern of phase shns are shown ithe present arreckerboard patlly (for a diago

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Figure 7. pattern be

Video 8. scan is av

A novel mideveloped fosuspended abactuated by athe mirror panumber of wlarge voltageactuation vol

aller deflectiond pattern. In fahe beam raster

A selection of below each panel

Beam steering. Ivailable at:

cro-electromecor fast beam stbove a distribuapplying a volair is poised cl

wavelengths. Ae-controlled phltage. We demo

n angles can beact, any beam pscanning over

beam patterns. Tshows the arran

Images for multi

chanical systemteering. The 8uted Bragg refltage between lose to resonan

A small mirror hase shift. In tonstrate beam

e obtained by aposition inside the square is a

The dashed rectanngement of mirro

iple beam positi

4. COms optical ph×8 array of phflector mirror a grating mirronce, where thedisplacement

this way we acsteering with a

applying a morthe dashed sq

available online

ngle in the first pors switched on o

ons are superimp

ONCLUSIOhase shifter bahase shifters cosubstrate usingor and the sub

e round trip opthen suffices tchieve a widean 8×8 array of

re coarsely struquare can be ree.

panel outlines thor off (π or zero

posed here. A vi

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bstrate. The miptical path lengto pull the mir operating banf these all-pass

uctured patternached by apply

he maximum steephase shift).

ideo of the beam

all-pass filter hweight high co

ngs. The mirroirror separationgth is slightly lrrors through rndwidth of oves-filter mirrors.

, as illustrated ying a suitable

ering range. The

m making a raster

has been succontrast grating

ors are electrosn is designed slonger than anresonance, leader 500 kHz, fo

with the e pattern.

e

r

cessfully g mirrors statically such that n integral ding to a or a low

http://dx.doi.org/10.1117/12.2045341.1



ACKNOWLEDGEMENTS This work was supported by the DARPA SWEEPER program (Grant No. HR0011-10-2-0002). Devices were fabricated at the Marvell Nanofabrication Laboratory at UC Berkeley.

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