Wide band integrated spectrometers

Abstract

Near-infrared (NIR) spectroscopy is an attractive technology for the detection of molecules. It enables compounds to be identified rapidly. One main issue plaguing NIR sensing is that most equipment is bulky and is often laboratory-based. Hitherto, miniaturized counterparts are facing several technological challenges: low power requirement, suitable optical filter for field use, uncooled detector operation at elevated temperature and the utilization of an appropriate packaging that confers physical and chemical protection. Because of these limitations, samples have to be brought back from the field for analysis. Often, it is challenging to survey samples with fast deterioration. To address the current limitations, novel designs field operable miniaturized MIR spectrometers are proposed in this thesis. The scope of this thesis covers the design and simulation results of integrated spectrometers capable of operating over wide wavelength bands. In particular, the tailored designs of the concave grating for interferometric filters are discussed. Interferometric filters are arguably considered to be superior to other kinds of filters for integrated spectroscopy. The ubiquitous presence of Micro-opto-mechanical-systems (MEMS) and integrated photonics is poised to have significant impacts on many fields, including spectroscopy. In this Ph.D. thesis, we strive to combine integrated optics with MEMS on the same platform to create novel system-on-chips (SoC). The targeted SoC devices are two different types of spectrometers that can cover a wide range of operational wavelengths and provide solutions for reducing cost by cutting the need for a single array or multiple array of photodetectors. This Ph.D. exploit the benefits obtained by the marrying these two technologies toward miniaturizing subsystems tailored for tunable spectrometers and sensing spectrometers while achieving state-of-the-art performance. We propose the design and simulation results of two tunable MOEMS spectrometers in a monochromator set-up and propose an in-house fabrication process to minimize the fabrication steps. Some envisioned benefits include cost reductions resulting in a wider access to hand-held spectrometers, and high-volume fabrication through batch processing. The focus of this research is to design and optimize with simulations that account for fabrication variations two different types of spectrometers. The first design is a continuously tunable MEMS-based optical spectrometer in a monochromator set-up. The novel integrated micro-opto-mechanical-system spectrometer consists of a tailored concave grating design to be fabricated in a planar waveguide that is connected to a rotational electrostatic actuator, which enables angular tuning of the grating. The spectrometer covers a wide operational wavelength range (>100 nm), covering partially the E and fully the S, C, L-band (1416.6 nm - 1696.6 nm), and requires a single photodetector to acquire the spectrum. The spectrometer is designed to exhibit simulated low optical losses throughout the range of motion. The spectrum can be acquired at a frequency of 1.76 kHz. The simulated acquired spectrum features an average insertion loss of -1.8 dB and a crosstalk better than -70 dB with a resolution as low as 1.62 nm. The entire device covers an area of 4 mm x 4 mm and is based on a thick silicon-oninsulator platform. A fabrication process to implement the device is proposed with minimal steps and prototypes of the static optical system were fabricated but not characterized experimentally. The second proposed design is a novel integrated optical spectrometer is able to operate over four different optical bands in the infrared that cover over 900 nm of aggregated bandwidth. The device, named integrated optical four bands spectrometer (IOFBS), consists of a single planar concave grating with 4 inputs waveguides, each corresponding to a different wavelength band, and 39 output channels that can be implemented on a silicon nitride platform. The Inputs Waveguides (IWGs) are optimized so that the echelle grating works in different diffraction orders to create constructive interference at the fixed output waveguides. The grating facets are engineered to maximize the diffraction efficiency of the beam launched from any of the four IWGs. The IOFBS works in the near infrared, the O-band, part of the S&E bands and the L-band. The simulated spectra feature an average insertion loss of -1.69 dB across the four bands and a crosstalk better than -32 dB with a 3-dB resolution as low as 0.37 nm and a channel spacing of ~2.1 nm. The entire device covers an area of 5 mm x 4 mm. This work led to significant knowledge on developing new designs that take advantage of miniaturization techniques to realize complex microsystems for spectroscopic applications. These proposed approaches could be a promising route toward developing highly integrated optical-MEMS systems for numerous potential applications.

Date	18 Jun 2024
Original language	American English
Awarding Institution	École de technologie supérieure
Supervisor	Michaël Ménard (Supervisor) & Frédéric Nabki (Co-supervisor)