Abstract
Spectral pattern recognition is used to measure temperature and generate calibrated wavelength/frequency combs using a single silicon waveguide ring resonator. The ring generates two incommensurate interleaving TE and TM spectral combs that shift independently with temperature to create a spectral pattern that is unique at every temperature. Following an initial calibration, the ring temperature can be determined by recognizing the spectral resonance pattern, and as a consequence, the wavelength of every resonance is also known. Two methods of pattern-based temperature retrieval are presented. In the first method, the ring is locked to a previously determined temperature set-point defined by the coincidence of only two specific TE and TM cavity modes. Based on a prior calibration at the set-point, the ring temperature and hence all resonance wavelengths are then known and the resulting comb can be used as a wavelength calibration reference. In this configuration, all reference comb wavelengths have been reproduced within a 5 pm accuracy across an 80 nm range by using an on-chip micro-heater to tune the ring. For more general photonic thermometry, a spectral correlation algorithm is developed to recognize a resonance pattern across a 30 nm wide spectral window and thereby determine ring temperature continuously to 50 mK accuracy. The correlation method is extended to simultaneously determine temperature and to identify and correct for wavelength calibration errors in the interrogating light source. The temperature and comb wavelength accuracy is limited primarily by the linewidth of the ring resonances, with accuracy and resolution scaling with the ring quality factor.
Highlights
Silicon photonic ring resonators are integrated optical filters with a transmission spectrum consisting of a comb of resonance peaks with a quasi-periodic wavelength spacing, or free spectral range (FSR), determined by the ring path length L and wavelength-dependent effective index Neff [1,2]
By using the correlation method developed to reassess the ring temperature of both the reference and recovered comb spectra used here, we find that the calibration spectrum yields a temperature of 27.18 (0.05) °C while the recovered spectrum at I = 20 mA gives a temperature of 27.13 (0.05) °C
The accuracy and resolution of pattern recognition transduction is determined by the linewidths of the ring resonances
Summary
Silicon photonic ring resonators are integrated optical filters with a transmission spectrum consisting of a comb of resonance peaks with a quasi-periodic wavelength spacing, or free spectral range (FSR), determined by the ring path length L and wavelength-dependent effective index Neff [1,2]. These ring resonators were not designed for narrow linewidth, and the primary limitation on the accuracy of the set-point arises from the resonance width (approximately 80 pm full width at half maximum (FWHM) for the TE mode, and 400 pm for the TM modes). This first experiment shows that a known calibrated temperature point and calibrated wavelength comb can be recovered on demand by tuning the ring resonator temperature to a set point defined only by the coincidence of two specific TE and TM mode resonances. For a future optimized ring design, the linewidths can be reduced by an order of magnitude by using smaller waveguide-to-ring coupling strengths
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