Dispersion turning point-enhanced photothermal interferometry gas sensor with an optical microfiber interferometer

Photothermal interferometry (PTI) spectroscopy is a background-free and ultrasensitive gas or chemical sensing method. To date, most works have focused on applying different techniques to improve the sensitivity of PTI gas sensors via enhancing the photothermal (PT) phase modulation. Herein, we develop an all-fiber dispersion turning point (DTP)-enhanced PTI gas sensor with a taper-based mode interferometer in which the PT phase detection sensitivity of the interferometer is enhanced in a millimeter-long single fiber PTI configuration, which has not been studied yet. The design is demonstrated by conducting the measurement of acetylene with a 3-mmlong, 2.29-mu m-diamter microfiber interferometer. A lower detection limit of 965 parts per billion (ppb) acetylene is achieved when the probe wavelength is operating near DTP, showing 16 times enhancement. Besides, theoretical analysis shows that the sensitivity of phase detection would be enormously enhanced when the dispersion factor approaching zero. By optimal design of the taper-based mode interferometer, together with thinner fiber diameter and longer fiber length, gas detection may be further improved by double enhancement of both PT phase generation and phase detection. With advantages of high sensitivity, compact size and low cost, the proposed all-fiber DTP-enhanced PTI gas sensor is potentially promising for ultra-sensitive trace chemical or biochemical sensing.