Frequency calibration for Rayleigh Doppler wind lidar

Precise frequency calibration is critical to maintain the accuracy of wind retrieval from a Rayleigh Doppler wind lidar. The frequency of the laser is locked at the cross-point of the transmission functions of double-edge channels by tuning the cavity length of the Fabry-Perot interferometer (FPI) continuously. However, the non-linearity due to hysteresis in the PZT will introduce systematic errors. To overcome this drawback, a new method of frequency calibration and frequency locking is proposed and demonstrated. If the relative frequency drift between the laser and the FPI is less than 100 MHz, the frequency deviation is monitored and then compensated in the data processing. Once the relative frequency drift is large than 100 MHz, the frequency locking point is reset. In their procedure, the cavity length is shrunk in two steps: first tune the voltage under the preset point, then increase the voltage along the rising edge of the hysteresis loop gradually until the frequency locking is achieved again. In the comparison experiments, the new method is adopted. The maximum (average) wind speed deviation between the results from Doppler lidar and radiosonde is 6.22 m/s (1.12 m/s) in the altitude from 15 km to 30 km.