The proliferation of Internet-of-Things has promoted a wide variety of emerging applications that require compact, lightweight, and low-cost optical spectrometers. While substantial progresses have been made in the miniaturization of spectrometers, most of them are with a major focus on the technical side but tend to feature a lower technology readiness level for manufacturability. More importantly, in spite of the advancement in miniaturized spectrometers, their performance and the metrics of real-life applications have seldomly been connected but are highly important. This review paper shows the market trend for chip-scale spectrometers and analyzes the key metrics that are required to adopt miniaturized spectrometers in real-life applications. Recent progress addressing the challenges of miniaturization of spectrometers is summarized, paying a special attention to the CMOS-compatible fabrication platform that shows a clear pathway to massive production. Insights for ways forward are also presented.

We present an ultra-compact single-shot spectrometer on silicon platform for sparse spectrum reconstruction. It consists of 32 stratified waveguide filters (SWFs) with diverse transmission spectra for sampling the unknown spectrum of the input signal and a specially designed ultra-compact structure for splitting the incident signal into those 32 filters with low power imbalance. Each SWF has a footprint less than 1 µm × 30 µm, while the 1 × 32 splitter and 32 filters in total occupy an area of about 35 µm × 260 µm, which to the best of our knowledge, is the smallest footprint spectrometer realized on silicon photonic platform. Experimental characteristics of the fabricated spectrometer demonstrate a broad operating bandwidth of 180 nm centered at 1550 nm and narrowband peaks with 0.45 nm Full-Width-Half-Maximum (FWHM) can be clearly resolved. This concept can also be implemented using other material platforms for operation in optical spectral bands of interest for various applications.

There has been a rapidly growing demand for low-cost, integrated single-shot spectrometers to be embedded in portable intelligent devices. Even though significant progress has been made in this area, two major problems are still remaining, namely the high temperature sensitivity and poor bandwidth-resolution ratio (BRR) that can’t meet the requirement of most applications. In this work, we present an integrated single-shot spectrometer relying on a silicon photonic circuit that has a footprint less than 3mm², but could achieve broad operation bandwidth about 100 nm and high resolution up to 0.1 nm (with a BRR ~ 1000). Moreover, for the first time, we demonstrate an integrated spectrometer that could operate within a wide temperature range (between 10 and 70 degrees Celsius) without additional power consumption for temperature management.

We present a programmable silicon photonic integrated circuit (PIC) that can be configured to show nonlinear nonreciprocal transmission at high optical input power. Nonreciprocal transmission in PICs is of fundamental importance in various fields. Despite diverse approaches to generate nonreciprocal transmission, the research on efficient control of this effect is still scarce. The silicon PIC presented here has programmable linear and nonlinear behavior using integrated phase shifters. In the nonlinear regime (high optical power), the device can be configured to be either reciprocal or nonreciprocal between opposite propagation directions with over 30 dB extinction ratio and only 1.5 dB insertion loss. More importantly, the high/low transmission direction can be dynamically reconfigured. Furthermore, nonreciprocal transmission based on nonlinearities usually requires the optical field in both propagation directions to be high, in order to induce a large extinction ratio. For our circuit, only the forward-propagating light needs to have high power to enjoy low-loss transmission while the backward propagating light will always suffer a high rejection. Besides this nonreciprocal behavior, the circuit also offers the ability for all-optical functions, such as switching, optical compute gates, or optical flip-flops, thanks to its unique controllable nonlinear behavior. This work can trigger new research efforts in nonreciprocal photonics circuits.