Integrated Single-Resonator Spectrometer beyond the Free-Spectral-Range Limit

The chip-scale miniaturization of optical spectrometers may enable many potential applications, such as wearable health monitoring, field deployable biochemical sensing, dense hyperspectral imaging, and portable optical coherence tomography. However, the widespread use of integrated spectrometers is hampered by an inherent trade-off between resolutions and bandwidths. Here, a ground-breaking design strategy is proposed to overcome the bottleneck. The most noteworthy finding in this work is that, by simultaneously leveraging temporal and spatial decorrelations, a single micro-ring resonator (MRR) can serve as a spectrometer with an ultra-high-resolution across an ultrabroad bandwidth far exceeding the narrow free spectral range (FSR). The structure is based on a tunable MRR that supports TE0 and TE1 transverse modes. When tuning the MRR, the unknown spectrum is scanned by dual-mode resonances in a synchronized manner. The recorded signal is formed by "splicing" the responses of TE0 and TE1. Due to the intermodal dispersion, all of the wavelength channels in the transmission matrix are sufficiently decorrelated beyond the FSR limit by cross-referring two matrix halves; thus, any arbitrary spectra can be retrieved by solving a linear inverse problem with preconditioned iterative optimizations. Experimental results demonstrate an ultrahigh resolution of <80 pm across an ultrabroad working bandwidth of >100 nm, which yields an ultralarge-wavelength channel capacity of 1250. The device footprint is also as compact as 20 x 35 mu m2. These results represent the smallest-resolution-footprint product (approximate to 0.056 pm center dot mm2), the highest bandwidth-to-footprint ratio (approximate to 0.14 nm mu m-2), and the highest channel-to-footprint ratio (approximate to 1.79 ever demonstrated.