Extended-range high-accuracy wavelength calibration beyond gas absorption lines using Mach–Zehnder interferometer and Fabry–Pérot cavity

The real-time wavelength calibration of a tunable laser (TL) typically relies on a combination of gas absorption lines and auxiliary interferometers. However, owing to the limited spectral coverage of gas absorption lines, the calibration accuracy deteriorates in regions outside their coverage. To address this limitation, this study proposed a relative wavelength calibration method that integrated a Mach–Zehnder interferometer (MZI) with a Fabry–Pérot (FP) cavity. In this method, the transmission peaks of the FP cavity were used to extend the absolute wavelength reference provided by the gas absorption lines over a broader spectral range. Simultaneously, the stable free spectral range of the FP cavity was utilized to correct the nonlinear phase-frequency mapping in the MZI caused by material dispersion. Furthermore, a complex Morlet wavelet transform was employed to extract the instantaneous phase of the MZI signal with high precision, thereby enhancing the accuracy of the relative calibration. The experimental results demonstrated that the absolute wavelength uncertainty was maintained below 0.4 pm over the extended 40 nm spectral range. For a frequency difference of 40 MHz, the corresponding relative wavelength measurement error was less than 0.5 MHz (approximately 4 fm at 1550 nm). This study provides an effective solution for high-precision wavelength calibration of TL over extended spectral ranges, demonstrating its strong practical value and promising prospects for broad applications.