Scaling on-chip photonic neural processors using arbitrarily
  programmable wave propagation
Tatsuhiro Onodera Martin M. Stein Benjamin A. Ash Mandar M. Sohoni Melissa Bosch Ryotatsu Yanagimoto Marc Jankowski Timothy P. McKenna Tianyu Wang Gennady Shvets Maxim R. Shcherbakov Logan G. Wright Peter L. McMahon
On-chip photonic processors for neural networks have potential benefits in
both speed and energy efficiency but have not yet reached the scale at which
they can outperform electronic processors. The dominant paradigm for designing
on-chip photonics is to make networks of relatively bulky discrete components
connected by one-dimensional waveguides. A far more compact alternative is to
avoid explicitly defining any components and instead sculpt the continuous
substrate of the photonic processor to directly perform the computation using
waves freely propagating in two dimensions. We propose and demonstrate a device
whose refractive index as a function of space, $n(x,z)$, can be rapidly
reprogrammed, allowing arbitrary control over the wave propagation in the
device. Our device, a 2D-programmable waveguide, combines photoconductive gain
with the electro-optic effect to achieve massively parallel modulation of the
refractive index of a slab waveguide, with an index modulation depth of
$10^{-3}$ and approximately $10^4$ programmable degrees of freedom. We used a
prototype device with a functional area of $12\,\text{mm}^2$ to perform
neural-network inference with up to 49-dimensional input vectors in a single
pass, achieving 96% accuracy on vowel classification and 86% accuracy on $7
\times 7$-pixel MNIST handwritten-digit classification. This is a scale beyond
that of previous photonic chips relying on discrete components, illustrating
the benefit of the continuous-waves paradigm. In principle, with large enough
chip area, the reprogrammability of the device's refractive index distribution
enables the reconfigurable realization of any passive, linear photonic circuit
or device. This promises the development of more compact and versatile photonic
systems for a wide range of applications, including optical processing, smart
sensing, spectroscopy, and optical communications.