Research on linewidth narrowing and frequency control technology of quantum cascade lasers based on optical feedback and optical phase-locked loops
Authors: GAO Jian , JIAO Kang , ZHAO Gang , YIN Runtao , YANG Jiaqi , YAN Xiaojuan , CHEN Wanning , MA Weiguang , JIA Suotang ,   State Key Laboratory of Quantum Optics and Quantum Optics Devices, Shanxi University, Taiyuan 030006, China,   Collaborative Innovation Center of Extreme Optics, Shanxi University, Taiyuan 030006, China,   Silesian College of Intelligent Science and Engineering, Yanshan University, Qinhuangdao 066000, China
Published: November 2024

The mid-infrared (MIR) spectral region, which corresponds to molecular vibrational and rotational energy level transitions, contains a wealth of molecular energy level information. By employing techniques such as cavity ring-down spectroscopy (CRDS), the MIR spectra can be precisely measured, thereby validating fundamental physical laws, the inversion of fundamental physical constants, and the detection of trace gases. However, technical noise from temperature fluctuations, mechanical vibrations, and current noise causes free-running quantum cascade laser (QCL) to suffer high-frequency noise, typically broadening the linewidth to the MHz range, thus reducing spectral resolution. Moreover, long-term drift in the laser frequency due to temperature and current fluctuations hinders high-precision spectroscopy, particularly for narrow-linewidth nonlinear spectroscopy, such as saturated absorption and multiphoton absorption spectroscopy. This work presents a method of combining optical feedback with an optical phase-locked loop (OPLL) for offset frequency locking, aiming to generate a mid-infrared (MIR) laser with excellent frequency characteristics. Strong optical feedback is employed to narrow the linewidth of the quantum cascade laser (QCL) acting as a slave laser, thereby alleviating the challenges associated with phase locking. The OPLL uses frequency-offset to lock the slave laser to the ultra-narrow laser. By adjusting the offset frequency, fine control of the slave laser is achieved. To ensure tight phase locking, the OPLL is based on the ADF4007, and combines a phase lead circuit to compensate for phase lag, effectively expanding the loop bandwidth of the system. In this work, the fundamental principles of the optical phase-locked loop are theoretically analyzed, and a basic model is established. The influence of loop bandwidth on locking performance is also investigated. Upon achieving phase locking using the combined optical feedback and OPLL system, the magnitude of the beat note of the two lasers is improved by 66 dBm, with phase noise suppressed to –81 dBc/Hz@2 kHz in the low-frequency region and -101 dBc/Hz@2MHz in the high-frequency region. The frequency noise power spectral density of both the master laser and slave laser is obtained via the error signal in the closed-loop system. Significant suppression of frequency noise is observed for the slave laser across both low- and high-frequency region, with suppression ratio reaching 86 dB at 100 Hz and 55 dB at 400 kHz. The frequency noise of the slave laser in the low-frequency domain is found to be comparable to that of the master laser. Based on the white noise response region in the frequency noise spectrum (from 200 Hz to 400 kHz), the locked slave laser linewidth is determined to be approximately 3 Hz, narrowing the initial MHz-level linewidth to match the Hz-level linewidth of the master laser. Finally, the locked laser is used to conduct cavity ring-down spectroscopy, achieving an improvement factor of 5 in the signal-to-noise ratio of the ringdown signal. This frequency-stabilized laser will be applied to high-precision spectroscopy for detecting radiocarbon isotopes in future.