Optical phased arrays (OPAs) with fast response time are of great interest for various applications such as displays, free space optical communications, and lidar. Existing liquid crystal OPAs have millisecond response time and small beam steering angle. Here, we report on a novel 32×32 MEMS OPA with fast response time (<4 microseconds), large field of view (±2°), and narrow beam divergence (0.1°). The OPA is composed of high-contrast grating (HCG) mirrors which function as phase shifters. Relative to beam steering 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 OPA is fabricated using deep UV lithography to create submicron mechanical springs and electrical interconnects, enabling a high (85%) fill-factor. Each HCG mirror is composed of only a single layer of polysilicon and achieves >99% reflectivity through the use of a subwavelength grating patterned into the mirror’s 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 400 nm HCG mirror. Beam steering is demonstrated using binary phase patterns and is accomplished with the help of a closed-loop phase control system based on a phase-shifting interferometer that provides in-situ measurement of the phase shift of each mirror in the array.
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.
A novel 8x8 optical phased array based on high-contrast grating (HCG) all-pass filters (APFs) is experimentally demonstrated with high speed beam steering. Highly efficient phase tuning is achieved by micro-electro-mechanical
actuation of the HCG to tune the cavity length of the APFs. Using APF phase-shifters allows a large phase shift with an
actuation range of only tens of nanometers. The ultrathin HCG further ensures a high tuning speed (0.626 MHz). Both one-dimensional and two-dimensional HCGs are demonstrated as the actuation mirrors of the APF arrays with high beam steering performance.
We present a single crystalline silicon optical phased array using high-contrast-gratings (HCG) for fast two dimensional
beamforming and beamsteering at 0.5 MHz. Since there are various applications for beamforming and beamsteering
such as 3D imaging, optical communications, and light detection and ranging (LIDAR), it is great interest to develop
ultrafast optical phased arrays. However, the beamsteering speed of optical phased arrays using liquid crystal and
electro-wetting are typically limited to tens of milliseconds. Optical phased arrays using micro-electro-mechanical
systems (MEMS) technologies can operate in the submegahertz range, but generally require metal coatings. The metal
coating unfortunately cause bending of mirrors due to thermally induced stress.
The novel MEMS-based optical phased array presented here consists of electrostatically driven 8 × 8 HCG pixels
fabricated on a silicon-on-insulator (SOI) wafer. The HCG mirror is designed to have 99.9% reflectivity at 1550 nm
wavelength without any reflective coating. The size of the HCG mirror is 20 × 20 μm<sup>2</sup> and the mass is only 140 pg,
much lighter than traditional MEMS mirrors. Our 8 × 8 optical phased array has a total field of view of ±10° × 10° and a
beam width of 2°. The maximum phase shift regarding the actuation gap defined by a 2 μm buried oxide layer of a SOI
wafer is 1.7π at 20 V.
Large arrays of periodic nanostructures are widely used for plasmonic applications, including ultrasensitive
particle sensing, optical nanoantennas, and optical computing; however, current fabrication
processes (e.g., e-beam lithography and nanoimprint lithography) remain time consuming and expensive.
Previously, researchers have utilized double casting methods to effectively fabricate large-scale arrays of
microscale features. Despite significant progress, employing such techniques at the nanoscale has
remained a challenge due to cracking and incomplete transfer of the nanofeatures. To overcome these
issues, here we present a double casting methodology for fabricating large-scale arrays of nanostructures.
We demonstrate this technique by creating large (0.5 cm × 1 cm) arrays of 150 nm nanoholes and 150 nm
nanopillars from one silicon master template with nanopillars. To preclude cracking and incomplete
transfer problems, a hard-PDMS/soft-PDMS (h-PDMS/s-PDMS) composite stamp was used to replicate the
features from: (i) the silicon template, and (ii) the resulting PDMS template. Our double casting technique
can be employed repeatedly to create positive and negative copies of the original silicon template as
desired. By drastically reducing the cost, time, and labor associated with creating separate silicon
templates for large arrays of different nanostructures, this methodology will enable rapid prototyping for
diverse applications in nanotechnological fields.
Sensors constructed with single-crystal PMN-PT, i.e. Pb(Mg<sub>1/3</sub>Nb<sub>2/3</sub>)O3-PbTiO<sub>3</sub> or PMN, are developed in this paper for
structural health monitoring of composite plates. To determine the potential of PMN-PT for this application, glass/epoxy
composite specimens were created containing an embedded delamination-starter. Two different piezoelectric materials
were bonded to the surface of each specimen: PMN-PT, the test material, was placed on one side of the specimen, while
a traditional material, PZT-4, was placed on the other. A comparison of the ability of both materials to transmit and
receive an ultrasonic pulse was conducted, with the received signal detected by both a second surface-bonded transducer
constructed of the same material, as well as a laser Doppler vibrometer (LDV) analyzing the same location. The optimal
frequency range of both sets of transducers is discussed and a comparison is presented of the experimental results to
theory. The specimens will be fatigued until failure with further data collected every 3,000 cycles to characterize the
ability of each material to detect the growing delamination in the composite structure. This additional information will be
made available during the conference.
Optical filters based on resonant gratings have spectral characteristics that are lithographically defined. Nanoimprint lithography is a relatively new method for producing large area gratings with sub-micron features. Computational modeling using rigorous coupled-wave analysis allows gratings to be designed to yield sharp reflectance maxima and minima. Combining these gratings with microfluidic channels and micromechanical actuators produced using micro electromechanical systems (MEMS) technology forms the basis for producing tunable filters and other wavelength selective elements. These devices achieve tunable optical characteristics by varying the index of refraction on the surface of the grating. Coating the grating surface with water creates a 33% change in the resonant wavelength whereas bringing a grating into contact with a quartz surface shifts the resonant wavelength from 558 nm to 879 nm, a fractional change of 58%. The reflectivity at a single wavelength can be varied by approximately a factor of three. Future applications of these devices may include tunable filters or optical modulators.
A project targeted at developing a low-cost fiber optic interrogator system for fiber Bragg grating (FBG) sensors has
been completed, and has resulted in a stand-alone system that can be used in multiple applications. The interrogator
system, tailored as a potential solution for embedded strain sensing in composite wind turbine blades, was recently tested
and its performance validated at the Infrastructure Assurance & Non-Destructive Inspection (NDI) department at Sandia
National Laboratories (SNL). The test specimen used to test the system consisted of a single fiber optic cable with six
FBG sensors embedded in a 36-ply fiberglass composite specimen. The FBG sensors were installed around a series of
known engineered flaws. Six foil type resistive strain gauges were bonded to the composite specimen surface and co-located
with the six embedded FBG sensors. The fiber optic interrogator was used to sample the FBG sensors and an
independent data acquisition system was used to sample the foil strain gauges. The test specimen was subjected to a
series of static loads and the results from both the foil strain gauges and the FBG sensors were compared. Results from
the analysis show a good correlation between the embedded FBG sensors and the foil strain gauges.
We present a fabrication method to realize three dimensional (3D) isotropic homogeneous negative index material (3DNIMs)
using a low cost and massively parallel manufacturable and self-assembly technique. The construction of self-assembled
3D-NIM array was realized through two dimensional (2-D) planar microfabrication techniques exploiting the
as-deposited residual stress imbalance between a bi-layer consisting of e-beam evaporated metal (chromium) and a
structural layer of low stress silicon nitride deposited by LPCVD on a p-doped silicon substrate. A periodic continuation
of a single rectangular unit cell consisting of split-ring resonators (SRR) and wires were fabricated to generate a 3D
assembly by orienting them along all three Cartesian axes. The thin chromium and silicon nitride bi-layer is formed as
hinges. The strain mismatch between the two layers at the hinge curls the structural layer containing the SRR upwards.
The self-assembled out-of-plane angular position depends on the thickness and material composing the bi-layer. This
built-in stress-actuated assembly method is suitable for applications requiring a thin dielectric layer for the SRR and/or
Metal films perforated with periodically-spaced subwavelength diameter holes have been shown to transmit light with greater efficiency than predicted by classical models for evanescent propagation. This transmission mechanism is caused either by the coupling of light to surface plasmon polariton modes on the surfaces of the metal film or by diffraction patterns in the lateral evanescent modes of electromagnetic field. Regardless of the root cause, this characteristic performance leads to electric field enhancement at the apertures in the metal, an effect that holds promise for nanoscale optical sensors. In particular, the propagation of these modes is very sensitive to changes in the index of refraction on either surface of the metal. This paper will describe our work on patterned metal films which are interrogated using infrared (IR) radiation. These metal gratings are fabricated using a surface micromachining process, allowing MEMS actuators to be integrated alongside the optically-active surface. The integration of MEMS structures with subwavelength optical structures can be used to create structures whose optical properties are modulated by changes in the position of a MEMS element, resulting in mechanical sensors and tunable optical filters. We will describe structures in which small changes in the separation between the metal film and a dielectric substrate result in large changes in the optical transmission and reflection spectra.
The purpose of this study was to evaluate the performance of a bimorph deformable mirror from AOptix, inserted into an adaptive optics system designed for <i>in-vivo</i> retinal imaging at high resolution. We wanted to determine its suitability as a wavefront corrector for vision science and ophthalmological instrumentation. We presented results obtained in a closed-loop system, and compared them with previous open-loop performance measurements. Our goal was to obtain precise wavefront reconstruction with rapid convergence of the control algorithm. The quality of the reconstruction was expressed in terms of root-mean-squared wavefront residual error (RMS), and number of frames required to perform compensation. Our instrument used a Hartmann-Shack sensor for the wavefront measurements. We also determined the precision and ability of the deformable mirror to compensate the most common types of aberrations present in the human eye (defocus, cylinder, astigmatism and coma), and the quality of its correction, in terms of maximum amplitude of the corrected wavefront. In addition to wavefront correction, we had also used the closed-loop system to generate an arbitrary aberration pattern by entering the desired Hartmann-Shack centroid locations as input to the AO controller. These centroid locations were computed in Matlab for a user-defined aberration pattern, allowing us to test the ability of the DM to generate and compensate for various aberrations. We conclude that this device, in combination with another DM based on Micro-Electro Mechanical Systems (MEMS) technology, may provide better compensation of the higher-order ocular wavefront aberrations of the human eye
We report on an Optical Add/Drop Multiplexer designed from a single Arrayed Waveguide Grating and a MEMS optical switch. Three different MEMS switches were tested, the most promising being a potentially integrable binary actuator array.
The wave front corrector is one of the three key elements in adaptive optics, along with the wave front sensor and the control computer. Low cost, compact deformable mirrors are increasingly available. We have tested the AOptix bimorph deformable mirror, originally developed for ultra-high bandwidth laser communication systems, to determine its suitability for vision science applications, where cornea and lens introduce optical aberrations. Measurements of the dynamic response of the mirror to a step input were obtained using a commercial Laser Doppler Vibrometer (LDV). A computer-controlled Twyman-Green interferometer was constructed to allow the surface height of the deformable mirror to be measured using Phase-Shifting Interferometry as a function of various control voltages. A simple open-loop control method was used to compute the control voltages required to generate aberration mode shapes described by the Zernike polynomials. Using this method, the ability of the deformable mirror to generate each mode shape was characterized by measuring the maximum amplitude and RMS error of each Zernike mode shape up to the fifth radial order. The maximum deformation amplitude was found to diminish with the square of the radial order of the Zernike mode, with a measured deformation of 8 microns and 1.5 microns achieved at the second-order and fifth-order Zernike modes, respectively. This deformation amplitude appears to be sufficient to allow the mirror to correct for aberrations up to the fifth order in the human eye.