Scientists at the Chinese University of Hong Kong have developed an approach to chip-scale spectrometry that offers both high spectral resolution and optical bandwidth. The researchers designed and developed an integrated spectrometer by tailoring the dispersion of mode splitting in a photonic molecule to identify the spectral information at different free-spectral ranges (FSR). The chip-scale miniaturization of spectrometers allows the rapid detection of spectral information in portable devices. The chip-scale integration of optical spectrometers could offer new opportunities for in situ biochemical analysis, remote sensing, and intelligent health care, as well as for wearables. Further, integrated spectrometers — which are built with all-solid-state photonic integrated circuits — are potentially low-cost and stand up well to vibrations. However, the miniaturization of these spectrometers has often led to a trade-off between spectral resolution and working bandwidth. High spectral resolution requires a long optical path length to support sufficient spectral decorrelation, which results in a smaller FSR. To surmount this issue, the researchers based their spectrometer on a pair of identical, tunable micro-ring resonators (MRR) and used dispersion-engineering on the coupled resonators. Due to strong intercavity coupling in the resonators, each resonant mode is split into a symmetric and an anti-symmetric mode, exhibiting a behavior that resembles the energy-level splitting in a two-level molecule. By engineering the dispersion of this “photonic molecule,” the mode-splitting strength of the resonators is made to vary throughout the whole bandwidth, which spans multiple FSRs. When the two MRRs are simultaneously tuned, each wavelength channel produces a distinct scanning trace, making it possible to reconstruct any unknown input spectrum. Different FSRs are identified from the dispersive mode splitting, leading to spectrum reconstruction with ultrahigh resolution over an ultrabroad bandwidth. (a) Artist’s depiction of the integrated spectrometer using a dispersion-engineered photonic molecule. (b) The structure consists of two identical, tunable micro-ring resonators. The unknown input spectrum is scanned via thermo-optical tuning to generate an output signal. The goal is to restore the spectral information from the signal with a calibrated transmission matrix. (c) For a single resonator, the information at different free spectral ranges (FSR) is indistinguishable. If a pair of resonators is strongly coupled, then each resonance will split into a symmetric mode and an anti-symmetric mode, which resembles a two-level molecule. The splitting strength is proportional to the coupling strength between resonators. Consequently, by tailoring the dispersion, the splitting strength will vary over multiple FSRs. (d) For two wavelengths spaced by an integral multiple of FSRs (λ2 = λ1 + m·FSR), their power scanning traces can be identified from the distinct peak spacing induced by the dispersive mode splitting. In this way, all wavelength channels are decorrelated, making it possible to reconstruct the spectrum beyond the FSR limit. Courtesy of Hongnan Xu et al. According to the researchers, the phenomenon of mode splitting in coupled resonators is analogous to the energy level splitting in a molecule with two atoms. To demonstrate their approach, the researchers used the photonic molecule scheme to retrieve numerous test spectra with diverse, complex features. The experimental results showed a high spectral resolution of 40 pm across an ultrabroad bandwidth of 100 nm, yielding a wavelength-channel capacity of 2501 and far exceeding the narrow FSR. A single spatial channel within a small footprint of about 60 × 60 μm2 supports the wavelength-channel capacity; to the best of the researchers’ knowledge, this represents the highest channel-to-footprint ratio and spectral-to-spatial ratio demonstrated to date. Also, the researchers found that they could maintain high reconstruction precision even with the presence of thermal noises. The spectrometer’s design features a simple configuration and a small size, so it can be densely packed with other devices. Further, it is compatible with mainstream nanophotonic fabrication technology, the researchers said. The research was published in Light: Science & Applications (www.doi.org/10.1038/s41377-023-01102-9).
