Probe-Type QEPAS Gas Sensor Based on Graded-Index Fiber Collimator

The quartz-enhanced photoacoustic spectroscopy (QEPAS) gas sensor exhibited high sensitivity and excellent gas selectivity without requiring a long optical path cell, offering cost-effectiveness and ease of miniaturization compared to tunable diode laser absorption spectroscopy (TDLAS) and cavity enhance absorption spectroscopy (CEAS) technologies. However, its volume was constrained by the dimensions of the laser collimator and the multi-reflection optical path, limiting further size reduction. This study employed a fusion splice method to combine single-mode fiber, coreless fiber, and graded-index fiber, developing a graded-index fiber collimator specifically tailored for QEPAS. The collimator featured a diameter of only 125 μm while achieving an effective collimation distance exceeding 20 mm. This allowed mechanical integration into the acoustic micro-resonator (MR) using a multi-tube nested coupling method, forming a probe-type structure with dual-MR parallel off-axis alignment and compact dimensions of 16 mm × 3.5 mm. This design significantly reduced the sensor volume while maintaining high sensitivity. Experimental results showed that, using water vapor as the target gas with a laser light source at a wavelength of 1392.53 nm and an average optical power of 10 mW, the amplitude of the second harmonic photoacoustic signal generated by this probe was nearly 26 times higher than that of the bare tuning fork off-axis structure and 1.72 times higher than the conventional gradient-index lens collimation single-MR off-axis structure. The probe’s minimum detectable limit for water vapor reached 0.73 ppm. Its compact design enhances adaptability to various measurement scenarios, offering considerable application value.