55 research outputs found
Resonant Scanning Design and Control for Fast Spatial Sampling
Two-dimensional, resonant scanners have been utilized in a large variety of
imaging modules due to their compact form, low power consumption, large angular
range, and high speed. However, resonant scanners have problems with
non-optimal and inflexible scanning patterns and inherent phase uncertainty,
which limit practical applications. Here we propose methods for optimized
design and control of the scanning trajectory of two-dimensional resonant
scanners under various physical constraints, including high frame-rate and
limited actuation amplitude. First, we propose an analytical design rule for
uniform spatial sampling. We demonstrate theoretically and experimentally that
by including non-repeating scanning patterns, the proposed designs outperform
previous designs in terms of scanning range and fill factor. Second, we show
that we can create flexible scanning patterns that allow focusing on
user-defined Regions-of-Interest (RoI) by modulation of the scanning
parameters. The scanning parameters are found by an optimization algorithm. In
simulations, we demonstrate the benefits of these designs with standard metrics
and higher-level computer vision tasks (LiDAR odometry and 3D object
detection). Finally, we experimentally implement and verify both unmodulated
and modulated scanning modes using a two-dimensional, resonant MEMS scanner.
Central to the implementations is high bandwidth monitoring of the phase of the
angular scans in both dimensions. This task is carried out with a
position-sensitive photodetector combined with high-bandwidth electronics,
enabling fast spatial sampling at ~ 100Hz frame-rate.Comment: 16 pages, 11 figure
Miniature photonic-crystal hydrophone optimized for ocean acoustics
This work reports on an optical hydrophone that is insensitive to hydrostatic
pressure, yet capable of measuring acoustic pressures as low as the background
noise in the ocean in a frequency range of 1 Hz to 100 kHz. The miniature
hydrophone consists of a Fabry-Perot interferometer made of a photonic-crystal
reflector interrogated with a single-mode fiber, and is compatible with
existing fiber-optic technologies. Three sensors with different acoustic power
ranges placed within a sub-wavelength sized hydrophone head allow a high
dynamic range in the excess of 160 dB with a low harmonic distortion of better
than -30 dB. A method for suppressing cross coupling between sensors in the
same hydrophone head is also proposed. A prototype was fabricated, assembled,
and tested. The sensitivity was measured from 100 Hz to 100 kHz, demonstrating
a minimum detectable pressure down to 12 {\mu}Pa (1-Hz noise bandwidth), a
flatband wider than 10 kHz, and very low distortion
Experimentally realized in situ backpropagation for deep learning in nanophotonic neural networks
Neural networks are widely deployed models across many scientific disciplines
and commercial endeavors ranging from edge computing and sensing to large-scale
signal processing in data centers. The most efficient and well-entrenched
method to train such networks is backpropagation, or reverse-mode automatic
differentiation. To counter an exponentially increasing energy budget in the
artificial intelligence sector, there has been recent interest in analog
implementations of neural networks, specifically nanophotonic neural networks
for which no analog backpropagation demonstration exists. We design
mass-manufacturable silicon photonic neural networks that alternately cascade
our custom designed "photonic mesh" accelerator with digitally implemented
nonlinearities. These reconfigurable photonic meshes program computationally
intensive arbitrary matrix multiplication by setting physical voltages that
tune the interference of optically encoded input data propagating through
integrated Mach-Zehnder interferometer networks. Here, using our packaged
photonic chip, we demonstrate in situ backpropagation for the first time to
solve classification tasks and evaluate a new protocol to keep the entire
gradient measurement and update of physical device voltages in the analog
domain, improving on past theoretical proposals. Our method is made possible by
introducing three changes to typical photonic meshes: (1) measurements at
optical "grating tap" monitors, (2) bidirectional optical signal propagation
automated by fiber switch, and (3) universal generation and readout of optical
amplitude and phase. After training, our classification achieves accuracies
similar to digital equivalents even in presence of systematic error. Our
findings suggest a new training paradigm for photonics-accelerated artificial
intelligence based entirely on a physical analog of the popular backpropagation
technique.Comment: 23 pages, 10 figure
Immersion graded index optics: theory, design, and prototypes
Immersion optics enable creation of systems with improved optical concentration and coupling by taking advantage of the fact that the luminance of light is proportional to the square of the refractive index in a lossless optical system. Immersion graded index optical concentrators, that do not need to track the source, are described in terms of theory, simulations, and experiments. We introduce a generalized design guide equation which follows the Pareto function and can be used to create various immersion graded index optics depending on the application requirements of concentration, refractive index, height, and efficiency. We present glass and polymer fabrication techniques for creating broadband transparent graded index materials with large refractive index ranges, (refractive index ratio)2 of ~2, going many fold beyond what is seen in nature or the optics industry. The prototypes demonstrate 3x optical concentration with over 90% efficiency. We report via functional prototypes that graded-index-lens concentrators perform close to the theoretical maximum limit and we introduce simple, inexpensive, design-flexible, and scalable fabrication techniques for their implementation
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