(117b) Non-Contact Gas Spectroscopy Exploting Tuning Fork-Based Light Detectors

In the past decade, the rapid development of infrared laser technology has led to an increasing demand for photodetectors with high sensitivity and a wide operative spectral range suitable for spectroscopic applications. In this work, we report on the study of light-induced thermo-elastic effects occurring in quartz tuning forks (QTFs) when exploited as light detectors in Tunable Diode Laser Absorption Spectroscopy (TDLAS) spectrometers for non-contact gas sensing [1]. The induced photothermal processes and the temperature distribution following the absorption of laser beam upon the crystal quartz was studied by using finite-element-analysis with COMSOL Multiphysics. The electromagnetic energy release and the induced thermal distribution were related to the absorbance curve of the quartz crystal, shown in Fig.1a. In the spectral region with high absorption, the radiation travels few tens of micrometers in the quartz crystal, while in the spectral region with low absorption, the radiation is trapped at the interface between the chromium film and the highly reflective gold layer [2].

Figure 1: (a) Transmittance spectrum of the QTF quartz measured using a Fourier transform spectrometer (continuous line). A 750 µm-thick quartz slab was employed for the measurement. (b) Responsivity as a function of the wavelength for three models of photovoltaic Vigo detectors, namely, PVI-4 (red line), PVI-6 (blue line), and PVI-10.6 (green line) HgCdTe, and for Thorlabs PDA10CF InGaAs Amplified Photodetector (black line), as provided by the device manufacturers. Responsivity of the QTF (black squares) employed as a photodetector.

The spectral response of the QTF-based photodetector was investigated by using a custom QTF with a resonance frequency of 9.78 kHz and quality factor of 11500 at atmospheric pressure. Five interchangeable laser sources operating at different wavelengths from 1.6 up to 10.35 µm were employed within TDLAS sensors. A spectrally flat responsivity of 2.2 kV/W was demonstrated (see Fig.1b), corresponding to a noise-equivalent power of 1.5 nW/Hz^1/2, without employing any thermoelectrical cooling system. This tuning fork-based sensing configuration pave the way to low cost, modular spectrometers capable to perform non-contact analysis of continuous flow gas streams with highly variable composition and with a sensitivity in the part-per-billion range.

References:

[1] S. Dello Russo et al., Opt. Express 28, 19074-19084 (2020).

[2] T. Wei et al., Appl. Phys. Rev. 8, 041409 (2021).