Photothermal spectroscopy of CO2 in an intracavity mode-locked fiber laser configuration

A novel configuration of a photothermal gas sensor is demonstrated. Photothermal-induced gas refractive index (RI) modulation is probed by a simple, mode-locked (ML) ring cavity fiber laser, operating in the 1.55 mu m wavelength region. The measured gas sample is placed in an open-path section of the ML laser and the RI variations directly translate to its optical path-length change, which is easily detectable as pulse repetition frequency deviations. Wavelength modulation spectroscopy (WMS) technique was used along with a custom-built FM demodulator simplifying the signal retrieval and acquisition. Normalized noise equivalent coefficient calculated for the sensor was 1 x 10(-5)cm(-1 )W Hz(-1/2). (C) 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement