Abstract
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.
Highlights
Trace gas sensing finds applications in numerous fields, such as environmental monitoring,1 industrial process control,2 and medical diagnosis.3 In these applications, a fast and accurate measurement of trace gas concentrations is often required
We demonstrated that a quartz tuning fork (QTF) can operate as a narrow-bandwidth ($1 Hz), fast-response, broadband, high-responsivity infrared photodetector, suitable for tunable laserbased absorption spectroscopy
The spectral responsivity as well as the noise-equivalent power were measured as a function of wavelength and compared with the most performant infrared detectors available on the market
Summary
Trace gas sensing finds applications in numerous fields, such as environmental monitoring, industrial process control, and medical diagnosis. In these applications, a fast and accurate measurement of trace gas concentrations is often required. The commercialization of distributedfeedback (DFB) laser diodes as well as of heterostructure-based lasers (interband and quantum cascade lasers) has revolutionized the field of spectroscopy in the near- and mid-IR range, providing a large quantity of sources capable of covering the whole infrared range in a short time These sources provide narrow linewidth as well as narrow spectral tunability, mostly limiting their use for targeting a single, interference-free absorption line. A modulation of the optical power causes periodic heating/cooling, which in turn generates a modulation of accumulated charges on the QTF surface due to quartz piezoelectricity This technique combined with the TDLAS approach, known as light-induced thermoelastic spectroscopy (LITES), has been explored in the last two years. The spectral dependence of two main figures of merit, namely, the responsivity and the noise-equivalent power, of the QTF photodetector will be measured by realizing a TDLAS setup with five interchangeable singlemode DFB laser sources covering the infrared range, from 1.6 to 10.5 lm. The LITES technique will be combined with a heterodyne detection scheme to simultaneously measure the resonance properties of the QTF as well as the target gas concentration in the absorption cell
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