Design And Implementation Of Light Source System For Sodium Fluorescent Lidar Based On Optical Phase Locked Loop

Sodium fluorescence lidar is one of the most effective ways to detect the temperature,wind field and atmospheric momentum in the top of the atmosphere.It is of great significance to study the efficient and stable sodium fluorescence lidar light source for exploring climate change,forecasting space weather,atmospheric chemical changes and other scientific fields.Only the sodium fluorescence lidar emitting three narrow linewidth 589 nm lasers can detect the atmospheric temperature and wind field at the top of the mesosphere with high resolution at the same time,and the stability of the three lasers is directly related to the accuracy of the detection data.Therefore,this paper studies the light source of sodium fluorescent laser radar,designs a narrow linewidth 589nm light source output module with long-term stable frequency,and adopts an optical phase-locked loop as a three-wavelength frequency shift module of sodium laser radar,which realizes the fast switching output of three-wavelength 589 nm laser.This design scheme provides a new design idea for the light source system of sodium fluorescence lidar.The 589 nm laser generation technology and semiconductor laser frequency stabilization technology are studied.Using solid-state laser sum-frequency technology,narrow linewidth 1064nm laser and 1319 nm laser output 589 nm laser through PPLN crystal sum-frequency,and the optical path is connected by all fiber,which improves the stability of 589 nm laser output.A 589 nm laser frequency stabilization module was designed by using the saturated absorption frequency stabilization technology.A tunable 1064 nm laser driver was used to scan the frequency of the 1064nm laser in a small range,and the scanning results indirectly affected the 589 nm laser,and the saturated absorption spectrum of sodium atoms was obtained.The saturated absorption signal was demodulated by a frequency quasi-servo system to generate a DC error signal,which was applied to the 1064 nm laser,and the 589 nm laser was locked on the₍2（6） resonance step line of sodium atoms for a long time.A heterodyne optical phase-locked loop module with wide frequency shift range,high frequency stability and flexible operation is designed.The frequency synthesizer ADF4111 with high performance and editable function is used as the frequency discriminator of the optical phase-locked loop.By modifying the value of n-division,the frequency of the beat signal is divided by n,and the frequency shift from the laser is realized arbitrarily in the range of 80 MHz to 1200MHz.The error signal output by the frequency discriminator is divided into positive and negative error signals,and the continuous positive and negative frequency shift from the laser is realized by selectively controlling the error signal output by single chip microcomputer.An acousto-optic frequency shifter is added in the optical phase-locked loop module to realize the three-wavelength output from the laser.The response time of optical phase-locked loop is analyzed,and it is determined that the integration time of PID circuit in the loop is the main factor affecting the fast locking from laser frequency.The integration time of PID circuit is optimized,and the response speed of the loop is accelerated,and the three-wavelength switching time from laser output is reduced to 5 ms A new scheme of optical phase-locked loop as a three-wavelength frequency shift module of sodium fluorescence laser radar is realized.After the frequency of the 589 nm laser is stabilized,the power of the 1319 nm laser and the three-wavelength laser output from the laser are amplified to 5 W and 10 W respectively,and then the two lasers are summed,and finally 4.7 W three-wavelength 589 nm lasers（589.15968 nm,589.896 nm and 589.15824 nm）are obtained.