Highly sensitive detection of ammonia based on light-induced thermoelastic spectroscopy with hollow waveguide

In this paper, an ammonia (NH3) sensor based on light-induced thermoelastic spectroscopy (LITES) with hollow waveguide (HWG) is proposed for the first time. HWG has many advantages, such as small size, simple structure, high system stability and fast filling speed. The sensor used HWG with a length of 22 cm and an inner diameter of 4 mm as the light transmission medium and gas chamber. The interior of the HWG is covered with silver (Ag) and silver iodide (AgI). The reflectivity is increased and the transmission losses is reduced in the infrared region of the HWG. A diode laser with an output wavelength of 1.53 mu m was employed, which can cover the two NH3 absorption lines of 6534.6 cm-1 and 6533.4 cm-1. The amplitude of the second harmonic signal (2f) showed a good linear response to the gas concentration. The experiments demonstrated that the NH3 sensor's minimum detection limit (MDL) under this system was 9.896 parts per million (ppm).