Simplified Calibration and Laser Nonlinearity Error Analysis for Peak-Locking TDLAS Systems

Tunable diode laser absorption spectroscopy (TDLAS) has been widely applied in industrial gas detection, but traditional calibration methods face challenges including high standard gas consumption, exorbitant costs, and heavy calibration workloads—particularly under wide temperature and pressure ranges, where variations in laser parameters can introduce significant errors. This study proposes a simplified calibration method for peak-locking TDLAS systems based on wavelength modulation spectroscopy (WMS). First, a coupling relationship model between the modulation index (m) and amplitude modulation (AM) is established, and an iterative calibration method is developed to synchronously adjust the laser operating current and temperature, enabling both m = 2.2 and AM = 0.1 to converge to their target values simultaneously, thereby eliminating the need for per-device temperature–pressure–concentration ternary calibration. Subsequently, the effects of quadratic nonlinearity in the laser power–current (P-I) and wavenumber–current (v-I) curves on detection accuracy are systematically analyzed for both 2f/1f and 4f/1f WMS representation methods. The simulation results obtained within a temperature range of 300 K ± 100 K and a pressure range of 1 atm ± 0.2 atm show that, for 2f/1f-WMS, a v-I nonlinearity rate of ±5% yields a maximum detection error of no more than 0.11% (with the error peak occurring in the 40–65% concentration range and no zero offset), while a P-I nonlinearity rate of ±5% results in a maximum error of no more than 0.82%, nearly independent of concentration. By comparison, the 4f/1f-WMS scheme effectively suppresses the zero offset induced by P-I nonlinearity, achieves lower detection errors, and requires only span calibration for each device. The proposed method significantly reduces the calibration workload and cost of TDLAS systems while ensuring detection accuracy.