Advances in cost-effective integrated spectrometers

Optical spectrometer is one of the most essential instruments in numerous fields, including chemical engineering, materials analysis, astronomical science, medical diagnosis, and biological sensing. Conventional high-performance spectrometers based on bulky and costly systems can no longer meet the requirements of various emerging application scenarios where the portability, cost, robustness, and power-consumption are paramount metrics, such as portable or wearable sensing devices for healthcare, food safety monitoring, smartphone-based spectrometers, drone-based remote sensing, and space exploration. During the past decades, substantial progresses have been made by both academia and industry in miniaturizing spectrometers while maintaining adequate performance. However, while some work has indeed advanced the state-of-the-art in miniaturized spectrometers, they may suffer from insufficient performance and low technology readiness levels in connection with commercialization.

In a recent review paper published in Light Science & Application, researchers from Nanjing University of Aeronautics and Astronautics, University of Cambridge, and University of California at San Diego summarized the recent progresses on miniaturized spectrometers with a special focus lied on the integrated spectrometers based on CMOS-compatible integration platforms that hold great promises for massive fabrication at low cost. The paper started with a brief market analysis of miniaturized spectrometers, showing the rapid growth of miniaturized spectrometers, especially the chip-size integrated spectrometers which is predicted have a disruptive increase in market volume from less than 2 million US dollars in 2019 to over 1.6 billion US dollars in 2024. Nevertheless, in general, the footprint reduction of a spectrometer consequentially give rise to certain performance degradation regarding its operation bandwidth, resolution, measuring speed, dynamic range, or signal-to-noise ratio, making it crucial to customize the spectrometer design for specified application scenarios. Hence, taking the most popular biomedical sensing and industrial chemical detection as examples, the authors summarized a set of key figures of merits in terms of resolution, bandwidth, etc., to establish the performance benchmarks for developing the next-generation integrated spectrometers in different fields.

Afterward, the paper dived into technological side that two main categories of integrated spectrometers based on the wavelength de-multiplexing (WdM) and wavelength multiplexing (WM) are discussed successively: “As suggested by their names, the WdM spectrometers need to de-multiplex, or in other words, split the incident signals’ spectra, either spatially or temporally, and measure the intensity of individual channel. On-chip WdM spectrometers are typically implemented by dispersive structures. Alternatively, WdM spectrometers can be implemented by using an array of narrowband filters or a single tunable narrowband filter, whose spectral responses determine the spectral contents arriving at the detector. In contrast, WM spectrometers do not require to split the spectral contents of the spectrum, they typically pro-modulate the entire spectrum and reconstruct it by signal processing using specific algorithms. Dependent on the modulation principle, they can be further divided into Fourier Transform Spectrometers, or Computational Spectrometers.” Among them, computational spectrometers have recently drawn extensive study, which utilizes an array of propagation channels with distinct spectral responses to sample the entire spectrum simultaneously. However,  “even if the computational spectrometers exhibit distinct advantages in terms of footprint, dynamic range, and measurement time, but the nature of using much less equations to solve many unknown values determines it is only good to reconstruct sparse spectra, which refer to smooth spectra or spectra with only a few non-zero components. While for dense spectra that contain rapidly changing features such a series of dips and peaks, the number of filters have to been significantly increased in order to maintain satisfying performance.” they added.

“To date, silicon photonics is still the most promising platform due to its unique CMOS compatibility. Considering that currently demonstrated integrated spectrometers are still unsatisfiable to the market requirements, we foresee four possible directions for developing the next-generation spectrometers on silicon photonics, involving spectrometers based on active path configuration, spectrometers with programmability, spectrometer systems with parallelism, and hybrid integration techniques for spectrometers, respectively. We expect to see expanding integration of chip-scale spectrometers into consumer productions within the next few years, providing cost-effective and reliable services to users worldwide.” the authors forecast.

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