Fiber-tip photothermal transducer with gold-coated multi-beam interferometric cavity for high sensitivity gas detection

The power-boosted laser transmitted via a fiber tip sinusoidally excites the target trace molecules, generating a photothermal (PT) effect. The Gaussian-distributed plane wave heats the medium adjacent to the Fresnel reflection surface. Meanwhile, a continuous probe light traverses the heating field, and the periodic temperature change then modulates the phase of the probe light. A multi-beam interferometer formed by aligning the Fresnel reflection surface to a gold-coated high-reflection surface possesses high resolution and sensitivity, significantly enhancing the detection performance. A well-established theoretical model of instantaneous PT transduction and optical phase modulation is employed to obtain the optimized interferometric configuration. To validate the effective interferometric phase transformation within the cavity, a comparison is made between the simple silicon cavity and the gold-coated cavity. The limitation of detection of such extrinsic fiber-tip PT sensors indicates one magnitude lower as compared to the conventional PT interferometric gas sensor with such robust and compact sensing designs. This work lays a solid foundation for future research on gas laser phase modulators and nonlinear laser–matter interactions.