Title: Detecting liquid in external cavity of multiple laser self-mixing interference

Abstract

The resonant cavity of a laser usually emits the light after amplification. The multiple laser self-mixing interference contains both the resonant cavity inside the laser and the external cavity outside the laser. The liquid sample is placed in the external cavity consisted of the external reflector and the output facet of the laser. The emitted light of the laser goes through the sample for several times provided that the external reflector is a mirror. The light is absorbed or scattered by the liquid or some matter in the liquid. For trace liquid, the absorption or scattering might be too weak for detection, thus, the multiple reflection of the light in the external cavity will enhance the absorption or scattering of the liquid sample. The multiple laser self-mixing interference will record the absorbed or scattered by the liquid via the reflected lights, thus the trace liquid can be detected via the characteristic of the multiple laser self-mixing interference. Since the original light in the resonant cavity is quite stronger than the reflected light, the reflected light’s phase is modulated by the external reflector to make the reflected light more obvious in the multiple laser self-mixing interference. Moreover, the reflected lights will be also amplified by the resonant cavity and the gain material so that the work is helpful for detecting trace liquid. In general, the multiple reflections in the external cavity will enhance the absorption and scattering of the liquid, besides, the resonant cavity including the gain material will make the multiple laser self-mixing interference more obvious for detection.