Speckle-Based Spectrometer

Spectrometers are widely used tools in chemical and biological sensing, material analysis and light source characterization. The traditional spectrometer uses a grating/prism to disperse light, and the spectral resolution scales with the optical path length from the grating to the detector, imposing a trade-off between device size and resolution. We employed multiple scattering of light in a random photonic structure to build a chip-scale spectrometer. The probe signal diffuses through a scattering medium generating wavelength-dependent speckle patterns which can be used to recover the input spectrum after calibration. By fabricating the scattering structure on-chip, we efficiently channel the scattered light to the detectors and engineer the disorder to enhance sensitivity. We also proposed and demonstrated that a single multimode fiber can function as a high-resolution, low-loss spectrometer. It utilizes the speckle pattern from a multimode fiber for spectral reconstruction, and the resolution is competitive with the top commercially available grating-based spectrometers. This compact, light-weight and low-cost spectrometer has been applied to high-precision optical-frequency-comb spectroscopy.

A chip-scale random spectrometer. (a) Scanning electron micrograph of the random spectrometer fabricated on a silicon-on-insulator (SOI) wafer. The probe signal is coupled to the semi-circular random structure via a waveguide at the bottom of the semicircle. The light then diffuses through the random structure and eventually reaches the defect waveguides. (b) Optical image of light intensity distribution across an array of detection channels, which is used to identify the input spectrum. (c) Reconstructed spectrum of two narrow spectral lines separated by 0.75 nm.