Real-time wavelength-calibrated laser heterodyne radiometer based on an all-fiber unbalanced Mach-Zehnder interferometer

Laser heterodyne spectroscopy is facilitating groundbreaking advances across multiple fields, including planetary atmospheric exploration, terrestrial greenhouse gas monitoring, wind field measurements, isotopic ratio analysis, and industrial gas emission monitoring. Nevertheless, Laser heterodyne radiometers (LHRs) lack effective wavelength calibration methods, which hinders their application in scenarios requiring miniaturization, high stability, and high precision. In this paper, what we believe to be a novel all-fiber LHR capable of real-time wavelength calibration is presented. The wavelength calibration scheme for this LHR was implemented using an all-fiber unbalanced Mach-Zehnder interferometer (MZI) combined with a corresponding wavelength calibration algorithm. Key performance-limiting factors of unbalanced MZI were experimentally analyzed. The criteria for determining the optimal optical path difference of the unbalanced MZI were established for the first time specifically for LHR applications. With a 22 cm unbalanced arm design, the system achieved a calibration resolution of 0.01623 cm^-1 and demonstrated a calibration uncertainty of 2 × 10^-5cm^-1 at an averaging time of 1s. Its performance was validated through absorption spectra measurements conducted in a gas cell. Field measurements of atmospheric CO₂ absorption spectra were performed with the developed real-time wavelength-calibrated LHR. The unbalanced MZI wavelength calibration scheme not only provides a high-precision, environmentally robust frequency scale for laser heterodyne absorption spectroscopy but also exhibits great potential to serve as a reliable calibration solution for other high-resolution spectroscopic techniques.