Infrared laser technology over the last decades has led to an increasing demand for optical detectors with high sensitivity and a wide operative spectral range suitable for spectroscopic applications. In this work, we report on the performance of a custom quartz tuning fork used as a sensitive and broadband infrared photodetector for absorption spectroscopy. The photodetection process is based on light impacting on the tuning fork and creating a local temperature increase that generates a strain field. This light-induced, thermoelastic conversion produces an electrical signal proportional to the absorbed light intensity due to quartz piezoelectricity. A finite-element-method analysis was used to relate the energy release with the induced thermal distribution. To efficiently exploit the photo-induced thermoelastic effects in the low-absorbance spectral region of quartz also, chromium/gold layers, acting as opaque surface, have been deposited on the quartz surface. To demonstrate the flat response as photodetectors, a custom tuning fork, having a fundamental resonance frequency of 9.78 kHz and quality factor of 11 500 at atmospheric pressure, was employed as photodetector in a tunable diode laser absorption spectroscopy setup and tested with five different lasers with emission wavelength in the 1.65–10.34 μm range. A spectrally flat responsivity of ∼2.2 kV/W was demonstrated, corresponding to a noise-equivalent power of 1.5 nW/√Hz, without employing any thermoelectrical cooling systems. Finally, a heterodyne detection scheme was implemented in the tunable diode laser absorption spectroscopy setup to retrieve the resonance properties of the quartz tuning fork together with the gas concentration in a single, fast measurement.
We report on a study of light-induced thermo-elastic effects occurring in quartz tuning forks (QTFs) when exploited as near-infrared light detectors in a tunable diode laser absorption spectroscopy sensor setup. Our analysis showed that when the residual laser beam transmitted by the absorption cell is focused on the QTF surface area where the maximum strain field occurs, the QTF signal-to-noise ratio (SNR) is proportional to the strain itself and to the QTF accumulation time. The SNR was also evaluated when the pressure surrounding the QTF was lowered from 700 Torr to 5 Torr, resulting in an enhancement factor of ∽4 at the lowest pressure. At 5 torr, the QTF employed as light detector showed an SNR ∽6.5 times higher than that obtained by using a commercially available amplified photodetector.
A palm-sized methane (CH4) tunable diode laser absorption spectroscopy (TDLAS) sensor is reported, in which a quartz tuning fork (QTF) is used as a thermal detector, working together with a mini-multi-pass cell (mini-MPC) to compose a gas detection module (GDM) with a compact dimension of 78 mm × 40 mm × 40 mm. A 1.65 µm near-infrared distributed feedback (DFB) laser is installed in the sensor for CH4 detection. A minimum detection limit (MDL) of 52 ppb is achieved at an integration time of 300 ms, corresponding to a normalized noise equivalent absorption coefficient (NNEA) of 2.1×10−8 cm−1W/Hz1/2. A seven-day continuous monitoring of atmospheric CH4 concentration is implemented to verify the sensor’s long-term stability.
<p>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 <sup>[1-2]</sup>. In this work, we report on the performance of a custom quartz tuning fork (QTF), having a fundamental resonance frequency of 9.78 kHz and quality factor of 11500 at atmospheric pressure, which was used as a sensitive and broadband infrared photodetector for laser absorption spectroscopy <sup>[3]</sup>. Fourier infrared spectrometer was used to characterize the infrared absorption capacity of quartz material at the wavelength of 1-20 &#956;m. Wide spectral response capability of the used QTF detector was investigated based on tunable diode absorption spectroscopy using lasers operating at five different wavelengths (1.6-10.35 &#956;m). A spectrally flat responsivity of ~2.2 kV/W was demonstrated, corresponding to a noise-equivalent power of 1.5 nW/Hz<sup>1/2</sup>, without employing any thermoelectrical cooling systems. In order to compensate for the drift of inherent characteristics (resonance frequency and quality factors) of the QTF detector, a heterodyne detection scheme was implemented to retrieve the resonance properties of the QTF detector together with the gas concentration in a single, fast measurement <sup>[4]</sup>. Experimental details including theoretical simulation and application demonstration will be discussed and presented.</p><p><strong>Acknowledgments</strong></p><p>The authors acknowledge financial support from National Key R&D Program of China (No. 2019YFE0118200), THORLABS GmbH, within PolySense, a joint-research laboratory, and the National Natural Science Foundation of China (Nos. 62075119 and 61805132).</p><p><strong>References</strong></p><p>[1] L. Dong, F. K. Tittel, C. Li, N. P. Sanchez, H. Wu, C. Zheng, Y. Yu, A. Sampaolo, and R. J. Griffin, Opt. Express <strong>24 </strong>(2016) A528-A535.</p><p>[2] S. Dello Russo, A. Zifarelli, P. Patimisco, A. Sampaolo, T. Wei, H. Wu, L. Dong, and V. Spagnolo, Opt. Express <strong>28</strong> (2020) 19074-19084.</p><p>[3] T. Wei, A. Zifarelli, S. Dello Russo, H. Wu, G. Menduni, P. Patimisco, A. Sampaolo, V. Spagnolo, L. Dong, Appl. Phys. Rev. <strong>8 </strong>(2021) 041409.</p><p>[4] H. Wu, L. Dong, H. Zheng, Y. Yu, W. Ma, L. Zhang, W. Yin, L. Xiao, S. Jia, and F. K. Tittel, Nat. Commun. <strong>8 </strong>(2017) 15331.</p>
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