Speckle-Based Spectrometer

The conventional spectrometers rely on one-to-one spectral-to-spatial mapping. Such mapping is necessary for a wavelength demultiplexer but not a spectrometer. Complex spectral to spatial mapping has been explored with disordered photonic crystals, thin scattering media, and random polychromat, for spectrometer application. The disorder-induced scattering of light produces wavelength-dependent speckle patterns which can be used as fingerprints to identify unknown spectra. We recently utilized multiple scattering of light in a random photonic chip to build a compact on-chip spectrometer. The probe signal diffuses through a scattering medium generating wavelength-dependent speckle patterns which are used to recover the input spectrum after calibration. In contrast to single scattering or diffraction from a thin disordered material that gives a linear scaling of spectral resolution with dimension L, the multiple scattering in a lossless diffusive medium makes the scaling quadratic, thus the resolution increases more rapidly with L. By fabricating the scattering structure on-chip, we can efficiently channel the scattered light to the detectors and engineer the disordered medium to reduce out-of-plane scattering. We obtain sub-nanometer (0.75 nm) resolution (at a wavelength of 1500 nm) with a very small (25 μm radius) footprint [1].