  Optical spectrum analysis is the cornerstone of spectroscopic sensing,
optical network performance monitoring, and hyperspectral imaging. While
conventional high-performance spectrometers used to perform such analysis are
often large benchtop instruments, on-chip spectrometers have recently emerged
as a promising alternative with apparent Size, Weight, and Power (SWaP)
advantages. Existing on-chip spectrometer designs, however, are limited in
spectral channel count and signal-to-noise ratio (SNR). Here we demonstrate a
transformative on-chip digital Fourier transform (dFT) spectrometer that can
acquire high-resolution spectra via time- domain modulation of a reconfigurable
Mach-Zehnder interferometer. The device, fabricated and packaged using
industry-standard silicon photonics technology, claims the multiplex advantage
to dramatically boost SNR and unprecedented scalability capable of addressing
exponentially increasing numbers of spectral channels. We further implemented
machine learning regularization techniques to spectrum reconstruction and
achieved significant noise suppression and spectral resolution enhancement
beyond the classical Rayleigh criterion.
