  Compressive sensing has been used to demonstrate scene reconstruction and
source localization in a wide variety of devices. To date, optical compressive
sensors have not been able to achieve significant volume reduction relative to
conventional optics of equivalent angular resolution. Here, we adapt
silicon-photonic optical phased array technology to demonstrate, to our
knowledge, the first application of compressive imaging in a
photonic-integrated device. Our novel sensor consists of an $8\times 8$ grid of
grating couplers with a spacing of $100~\mu$m. Path-matched waveguides route to
a single multimode interferometer (MMI), which mixes and randomizes the signals
into 64 outputs to be used for compressed sensing. Our device is fully passive,
having no need for phase shifters, as measurement matrix calibration makes the
measurements robust to phase errors. For testing, we use an Amplified
Spontaneous Emission (ASE) source with a bandwidth of 40 nm, centered at 1545
nm. We demonstrate simultaneous multi-point (2 sources demonstrated in this
work) brightness recovery and localization with better than 10 arcsecond
precision in a sub-millimeter thick form-factor. We achieve a single source
recovery rate higher than 99.9\% using 10 of the 64 outputs, and a 90\%
recovery rate with only 6 outputs, 10 times fewer than the 64 needed for
conventional imaging. This planar optical phased array compressive sensor is
well-suited for imaging sparse scenes in applications constrained by form
factor, volume, or high-cost detectors, with the potential to revolutionize
endoscopy, beam locators, and LIDAR.
