High-sensitivity, large dynamic range, auto-calibration methane optical sensor using a short confocal Fabry–Perot cavity

We present a methane sensor based on a short confocal Fabry–Perot cavity. The sensor uses not only the transmitted signals from the back cavity mirror to obtain high sensitivity but also the reflected signals from the front cavity mirror to extend efficient dynamic range and to cancel emitter-amplitude variations. Thus a minimum detectable absorbance of 4.1 × 10−5 has been achieved and the linear concentration dynamic range can span four orders of magnitude when the end mirrors of the cavity have a moderately high reflectivity of 99%. Moreover, a new type of signal processing providing further auto-calibration is employed to make the useful signal independent on the coupling coefficient of the cavity and the gain of the amplifier. The correlative theory is also given in detail. The technique based on confocal cavity has the same absorption path length with similar conventional cavity techniques that employ a nonconfocal cavity, whereas it is more convenient to align without deliberate mode matching. The prototype system has been demonstrated with an ultimate sensitivity of 2.9 ppm-m at 1.63 μm and a large linear concentration response ranging from 50 to 5 × 105 ppm (0.005–50%).