A highly sensitive LITES sensor based on a multi-pass cell with dense spot pattern and a novel quartz tuning fork with low frequency

A highly sensitive light-induced thermoelectric spectroscopy (LITES) sensor based on a multi-pass cell (MPC) with dense spot pattern and a novel quartz tuning fork (QTF) with low resonance frequency is reported in this manuscript. An erbi-um-doped fiber amplifier (EDFA) was employed to amplify the output optical power so that the signal level was further enhanced. The optical path length (OPL) and the ratio of optical path length to volume (RLV) of the MPC is 37.7 m and 13.8 cm^-2, respectively. A commercial QTF and a self-designed trapezoidal-tip QTF with low frequency of 9461.83 Hz were used as the detectors of the sensor, respectively. The target gas selected to test the performance of the system was acetylene (C₂H₂). When the optical power was constant at 1000 mW, the minimum detection limit (MDL) of the C₂H₂-LITES sensor can be achieved 48.3 ppb when using the commercial QTF and 24.6 ppb when using the trapezoid-al-tip QTF. An improvement of the detection performance by a factor of 1.96 was achieved after replacing the commercial QTF with the trapezoidal-tip QTF.