A Mid-Infrared Optical Parametric Oscillator Gas Sensor Sized by Absorption Cross-Section and Detection Limit

This paper presents a computed design for a singly resonant optical parametric oscillator producing tunable 3.3 micrometre radiation for trace methane sensing in a multipass cell. The sizing follows from applying the relevant intensity, thermal, nonlinear and geometric bounds at a single stated operating point, with every number produced by a deposited computation. The headline result is a signal wavelength of 1.57 micrometres paired with the 3.3 micrometre idler, a 50 m multipass path, and a methane detection limit near 8 parts per billion for a noise-equivalent absorbance of 1e-4 in one second, corresponding to 2.0e+11 molecules per cubic centimetre. The binding constraint is identified explicitly: parasitic absorption and photorefraction in the crystal at the circulating signal power, and mode hops during tuning scans. The design is SIMULATED, NOT MEASURED: no hardware was built for this study, and all performance figures are predictions from stated assumptions, offered as a basis for a build decision and a measurement plan.