Dual-wavelength laser frequency locking for the direct-detect wind lidar

Abstract

The frequency stability of the laser transmitter is essential for wind measurement of the direct-detect wind lidar. Given that the external frequency actuator can only achieve green radiation stability and is technically complicated, here, a simple and robust method is presented – locking the laser to iodine absorption lines with an automated PID controller. The frequency stability is better than 200 kHz over 3 hours. Both the fundamental and second-harmonic frequencies are stabilized, essential for seed injection and frequency monitoring. The frequency stability performance of the laser using thermal tuning can be improved to a significant extent by the improved PID control. Two kinds of cw single-frequency lasers with different tuning actuators are applied in the frequency locking system. The root Allan variance is kept below 3.5 × 10⁻¹¹ around τ = 0.003 s, and long-term stability of 2 × 10⁻¹⁰ for integration up to 3000 s was obtained. This locking technique is simpler and requires less laser power than locking to Doppler-free lines.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (NSFC) projects No. 40427001 and No. 40505003.