A dye ring laser is stabilized to a D2a Doppler-free feature of sodium vapor using a LabVIEW®-based, phase-sensitive servo. Locking precision and stability, at better than ±1 MHz, are suitable for Na lidar applications. This performance was achieved with improved digital filtering and new approaches to the problem. The inverse (type II) Chebyshev discrete filter employed demonstrates superior filtering and computational efficiency plus improved flexibility. New approaches include the determination of optimum modulation frequency, laser-tuning sensitivity, and bandwidth requirements via spectral analyses of the noise spectrum, derivative scan, and modulated spectrum. This practice guides a user in selecting the system operation parameters and negotiating the trade-offs involved when expanding the filter's passband. Allan deviation plots provide a quantitative description of the short- and long-term frequency excursions. A comparison of Allan deviation plots before and after locking shows a substantial improvement in stability throughout time scales from 0.10 to 10 s.
Resonance fluorescence Doppler lidars using Doppler shift and spectral broadening effects are the principal instruments to simultaneously measure wind and temperature in the middle atmosphere. Such lidars demand high accuracy, precision, and stability of the laser optical frequency. Current resonance Doppler lidars suffer various problems in frequency stabilization that limit their locking precision and stability. We have addressed these problems by developing a LabVIEW®-based laser frequency locking system. This new system utilizes wavelength-modulation and phase-sensitive-detection techniques in conjunction with a proportional-integral-derivative feedback servo loop. It achieves better than ±1-MHz locking precision and stability over 1 h. The system also remains locked throughout a series of abrupt disturbance tests. Owing to its high locking precision, immunity to electronic and laser noise, reliability, and flexibility in adapting for various systems, we believe that this new system represents a marked improvement in resonance Doppler lidar technology.