A high sensitive methane QEPAS sensor based on self-designed trapezoidal-head quartz tuning fork and high power diode laser

Abstract

A high sensitive methane (CH ₄) sensor based on quartz-enhanced photoacoustic spectroscopy (QEPAS) using self-designed trapezoidal-head quartz tuning fork (QTF) and high power diode laser is reported for the first time in this paper. The trapezoidal-head QTF with low resonant frequency ( f ₀ ) of ∼ 9 kHz, serves as the detection element, enabling longer energy accumulation times. A diode laser with an output power of 10 mW is utilized as the excitation source. A Raman fiber amplifier (RFA) is employed to boost the diode laser power to 300 mW to increase the excitation intensity. Acoustic micro-resonators (AmRs) are designed and placed on both sides of the QTF to form an acoustic standing wave cavity, which increases the acoustic wave intensity and enhances the vibration amplitude of the QTF. Additionally, the long-term stability is analyzed by Allan deviation analysis. When the average time of the sensor system is increased to 150 s, the minimum detection limit (MDL) of the CH ₄-QEPAS sensor system can be improved to 15.5 ppb.