MIR spiral fourier transform spectrometer

Abstract

In recent years, on-chip integrated spectrometers have attracted much attention due to their broad application prospects in the fields of environmental monitoring, biological analysis, and gas detection. Since many chemical bonds have unique mid-infrared fingerprints, the mid-infrared band plays an important role in sample detection. However, traditional infrared spectrometers require larger optical components and high-precision mechanical components to implement. Manufacture and adjustment of these elements and components require considerable time and cost, and make the spectrometer large in size and weight. But the emergence of silicon photonics provides a new idea for the miniaturisation of devices. Silicon photonics is a technology that uses silicon wafer technology to fabricate micron-scale photonic devices and realise photonic chips. It has many advantages, including low loss, high speed, easy fabrication and integration. These advantages make silicon photonics a useful technology for miniaturised mid-infrared spectrometers. Compared with traditional spectrometers, miniature infrared spectrometers have the advantages of small size, light weight, fast response and low power consumption. They are portable and can perform real-time non-destructive measurements, which can make infrared spectral analysis more convenient, efficient and flexible. In this thesis, based on the working principle of the thermo-optic effect, three silicon waveguide based thermo-optic Fourier transform spectrometers (FTS) are proposed. The devices are designed and demonstrated on a silicon-on-insulator (SOI) wafer platform. Among them, the first and second designs are based on a single Mach-Zehnder interferometer (MZI) structure that uses heaters in both arms of the MZI to perform thermo-optic modulation of the optical path difference. Their working centre wavelengths are 1.55 μm and 3.8 μm respectively. The third design combines thermo-optic tuning with the design concept of ”digital” FTS, to create a higher resolution spectrometer with a 3.8 μm centre wavelength. The resolution of the spectrometer is improved by integrating thermo-optic switches that can select between multiple fixed path lengths, in order to increase the range of optical path difference adjustability in the MZI. All three designs have been experimentally confirmed to successfully retrieve the spectra of single-wavelength laser sources in the working wavelength range. The resolution can reach 9 nm/(6.2 cm−1) and 4 nm/(2.7 cm−1) respectively. Finally, a new design for compact waveguide spirals will be introduced. This new design increases the coupling length between two waveguides adjacent to each other, by introducing a difference in the widths of the two waveguides. In this way, for the same length of the spiral waveguide, the gap between adjacent waveguides can be reduced without introducing additional coupling loss. This paves the way for further reduction of both the size and power consumption of future spectrometers.

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Identifiers

ORCID for Milos Nedeljkovic: orcid.org/0000-0002-9170-7911

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