Ultra-high resolution and broadband chip-scale speckle enhanced Fourier-transform spectrometer.

Recent advancements in silicon photonics are enabling the development of chip-scale photonics devices for sensing and signal processing applications, among which on-chip spectrometers are of particular interest for precision wavelength monitoring and related applications. Most chip-scale spectrometers suffer from a resolution-bandwidth trade-off, thus limiting the uses of the device. Here we report on a novel passive, chip-scale, hybrid speckle-enhanced Fourier transform device that exhibits a two order-of-magnitude improvement in finesse (bandwidth/resolution) over the state-of-the art chip-scale speckle and Fourier transform spectrometers. In our proof-of-principle device, we demonstrate a spectral resolution of 140 MHz with 12-nm bandwidth for a finesse of 10 that can operate over a range of 1500-1600 nm. This chip-scale spectrometer structure implements a typical spatial heterodyne discrete Fourier transform interferometer network that is enhanced by speckle generated from the wafer substrate. This latter effect, which is extremely simple to invoke, superimposes the high wavelength resolution intrinsic to speckle generated from a strongly guiding waveguide with a more broadband but lower resolution discrete Fourier transform modality of the overarching waveguide structure. This hybrid approach signifies a new pathway for realizing chip-scale spectrometers capable of ultra-high resolution and broadband performance.