Graphite Photoacoustic Detector Based Fiber-Optic Gas Sensing with 2f/1f Self-Calibration

A high-precision fiber-optic gas sensing system integrating a graphite photoacoustic (PA) detector with 2f/1f self-calibration is presented to reduce the influence of laser power fluctuations while retaining noncontact detection. A graphite flake and a microcantilever are copackaged within a brass housing to form a solid-state PA detector. A modulated laser beam passes through the target gas in a Herriott multipass cell and strikes the graphite flake; the absorbed optical power is converted into a PA response, which is subsequently detected by a fiber-optic cantilever microphone. The multipass cell provides an effective optical absorption path length of 13.5 m, enhancing the PA response by approximately 1 order of magnitude. The fundamental optical-power modulation is converted into a 1f PA response that serves as an optical-power reference, whereas the 2f PA response contains the CH4 absorption information. The 2f/1f ratio is used to reduce the influence of incident optical-power variations on concentration retrieval. For CH4 detection at 1650.91 nm, the system achieved a minimum detection limit of 6.8 ppb with an integration time of 100 s, corresponding to a normalized noise-equivalent absorption coefficient of 1.17 × 10-9 cm-1·W·Hz-1/2. Even under conditions of significant optical power fluctuation, the normalized concentration deviation remained within approximately 3%. Ambient-air measurements yield a CH4 concentration of approximately 1.84 ppm, demonstrating the applicability of the proposed system to practical methane monitoring.