Chirp-modulated light-induced thermoelastic spectroscopy for simultaneous precise measurement of resonant frequency and gas concentration

This paper demonstrates chirp-modulated light-induced thermoelastic spectroscopy (LITES), enabling simultaneous precise measurement of quartz tuning fork (QTF) resonant frequency and gas concentration. The injection current of the laser is modulated using the chirped signal. The chirped laser beam is injected into a multi-pass cell and the output beam is then focused onto the root of the QTF. Adopting the chirped signal as the reference signal, a lock-in amplifier is used to obtain the first harmonic (1 <i>f</i>) signal. The 1 <i>f</i> signal can be well-fitted with the Lorentz profile. Based on the peak position and area of the 1 <i>f</i> signal, the QTF resonant frequency and gas concentration can be obtained respectively. Compared with the traditional electric excitation method, the deviation of the calculated resonant frequency is only 0.02 Hz. This technique is able to prevent QTF frequency drift from affecting precision measurements, thereby guaranteeing the long-term stability of the LITES system.