Commercially available infrared photodetectors can be categorized in thermal detectors, i.e. The Lambert-Beere law rules the light absorption through a gas sample, but the sensor capability in detecting small intensity variation of the transmitted light are substantially determined by the performances of the optical detector employed. Laser absorption spectroscopy is a well-established technique for gas sensing, widely used in environmental monitoring 1, industrial process control 2, diagnostic and biomedical applications 3. The detection limits achieved for the QTF were comparable or even lower down to one order of magnitude with respect to market-available photodetectors. Once identified the most performant resonator, this QTF was implemented in a TDLAS setup and it was combined with laser diodes, interband- and quantum-cascade laser sources emitting from 1 μm to 10.5 μm and targeting different gas spacies. The role of the strain field, accumulation time and working pressure of the quartz resonator in this Light-Induced Thermo-Elastic Spectroscopy (LITES) approach was then evaluated for a whole set of tuning forks. In this work we present an extensive study on quartz tuning forks (QTFs) used as photodetectors, exploiting the opto-thermo-elastic energy conversion arising from the laser radiation-QTF interaction. It is proved that it is possible to construct a miniature quartz tuning-fork temperature sensor with high sensitivity and small non-linearity.The main limitations of tunable diode laser absorption spectroscopy (TDLAS) sensors are represented by the high cost, limited detection bandwidth and low adaptability of photodetectors to work in harsh environments. The temperature sensor based on quartz tuning-fork thermo-sensitive resonator is mounted in a standard holder for use from −30 degree C to 160 degree C with an accuracy of 0.05 degree C and a high resolution of 0.001 degree C. Using ZYtw-cut can bring the temperature coefficient up to 70ppm/(degree C) and make power consumption lower. The results obtained from the experimental investigations show the method which synthesizes many factors to design quartz tuning-fork temperature sensor. Precise temperature versus frequency analyses of resonators have been carried out and Least Mean Squared (LMS) curve fit algorithm is applied to compute the temperature values quantitatively. This type of sensor designed with a new ZYtw-cut is proved that working at flexural vibration mode was better than at the others modes. In order to optimize the design of quartz tuning fork temperature sensor, finite element method (FEM) is applied to model optimum tuning fork geometry, tine tip and tine surface electrode shape and thickness with a resonance frequency close to 32 kHz and a series resistance value of 40 kΩ as design targets.
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