In comparison to traditional bulk spectrometers, on-chip spectrometers possess advantages such as lower costs and enhanced portability. It's worth noting that in practical applications, it's not always necessary to analyze the entire continuous wavelength band. Instead, the focus is often on several specific spectral sub-bands. Professor Daoxin Dai's team from the College of Optical Science and Engineering, Zhejiang University, has developed a customized multi-band silicon photonic spectrometer. By flexibly combining multiple Bragg filters and cascading precision micro-ring filters, the number of the spectral sub-band, central wavelength, bandwidth, and resolution can all be tailored flexibly. Importantly, by streamlining excess spectral segments, the chip size can be significantly reduced, thereby improving manufacturing yield, and decreasing calibration complexity and measurement speed. The system fully leverages the design flexibility of photonic integrated circuits, highlighting the innovative concept of chip-based spectrometers tailored to meet specific application needs. It provides a flexible and efficient custom spectral analysis solution for various domains such as multi-target gas analysis. Relevant research results were recently published in Photonics Research, Volume 12, No. 5, 2024. [ Long Zhang, Xiaolin Yi, Dajian Liu, Shihan Hong, Gaopeng Wang, Hengzhen Cao, Yaocheng Shi, Daoxin Dai. Silicon photonic spectrometer with multiple customized wavelength bands[J]. Photonics Research, 2024, 12(5): 1016 ]

Spectrometers, as widely used scientific detection instruments, play a pivotal role in fields like pharmaceutical analysis, environmental monitoring, gas sensing, and aerospace. Recent advancements in silicon photonic integration technology have made it possible to realize low-cost, portable chip-based spectrometers. Spectral resolution and spectral range, as key performance indicators of spectral analysis systems, are constantly hitting new limits. However, achieving a broad spectral range and high resolution simultaneously often leads to complications such as complex system structure, large system size, high system power consumption, poor chip yield, and low measuring efficiency.

Fig. 1. The present silicon photonic spectrometer with multiple customized wavelength-bands. (a) The 3D view; (b) The top view; (c) The principle.

Fig. 1 shows the schematic diagram and working principle of the customized multi-band silicon-based spectrometer. This proposed spectrometer employs a combination of wideband filter for coarse filtering and narrowband filter for fine filtering. The unknown spectrum is first transmitted through the wideband filter, where it undergoes coarse filtering, separating the spectrum into several sub-band channels. Subsequently, the filtered spectral signals are transmitted to their respective microring filters for precise filtering. By utilizing thermal tuning, a complete scan of each individual channel is achieved. Finally, the monitoring signals from all channels are combined to reconstruct the entire measured spectrum.

Fig. 2. Design for the photonic filters. (a) Triangular-corrugations MWG wideband filter; (b) Rectangular-corrugations MWG wideband filter; (c)-(e) longitudinal apodization for the triangular-corrugations MWG; (f)-(h) longitudinal apodization for the rectangular-corrugations MWG; (i) mode (de)multiplexer based on an adiabatic dual-core taper; (j) bent coupling based narrowband filter.

The design for the optical filter structure used in the spectrometer is shown in Fig. 2. Here, the broadband filter employs a multimode Bragg grating design and is tailored based on the desired wavelength range to select specific spectral bands. The narrowband filter uses a high-Q bent coupling micro-ring resonator. By introducing a widened waveguide design, it ensures a large free spectral range while maintaining high spectral resolution.

Fig. 3. Calculated wavelength dependence of the designed dual-core adiabatic taper for (a) the 1310 nm wavelength band, (b) the 1550 nm wavelength band, and (c) the 1930 nm wavelength band; Calculated spectral responses of the transmissions at the drop ports for the designed MWGs at the channels of (d) 1310 nm, (e) 1560/1570 nm, and (f) 1930 nm. Calculated spectral responses for the drop/through ports of designed microring resonators at the channels of (g) 1310 nm, (h) 1560/1570 nm, and (f) 1930 nm.

In this work, we present a 4-channel spectrometer operating at 1310 nm, 1560/1570 nm, and 1930 nm, targeting gas sensing applications for HF, CO, H₂S, and CO₂. Fig. 3 shows the spectral responses of the adiabatic dual-core mode demultiplexers and the wide/narrowband filters. Simulation results demonstrate that the mode multiplexer has an insertion loss of less than 0.02 dB and crosstalk below -35 dB. The multimode Bragg grating wideband filter exhibits an excellent flat-top response with a sidelobe suppression ratio greater than 20 dB. The high-Q micro-ring narrowband filters have 3-dB bandwidths of 0.1 nm, 0.1 nm, and 0.35 nm respectively, thereby providing a strong guarantee for high spectral resolution.

Fig. 4. Microscope images of the fabricated multiband-customized spectrometer.

Fig. 5. Retrieved spectrum for a given spectrum with a single peak when using the present on-chip spectrometer as well as a commercial OSA. (a) 1309.65, (b) 1557, (c) 1569, (d) 1931.94. Normalized retrieved spectrum with double peak input around 1550 nm. (e) (1559.58, 1559.66) nm, (1557.58, 1562.58) nm, (1569.65, 1569.73) nm, and (1567.58, 1572.58) nm. (i) Measured results for the ultra-wide window complex spectrum ranging from 1307 nm to 1935 nm.

Fig. 4 shows the microscope image of the fabricated on-chip spectrometer. Fig. 5 shows the measurement results of different input spectra. Figs. 5(a) - 5(d) indicate that the on-chip spectrometer has high consistency with commercially available spectrometers of equivalent resolution. According to the dual-peak measurements in Figs. 5(e) - 5(h), the spectrometer has a resolution exceeding 0.08 nm. Fig. 5(i) indicates that the spectrometer can effectively distinguish complex spectra, enabling multi-spectral measurements across a super wide spectral range (1307-1935nm).