The Development of a Frequency Control System of a Seeded Laser for DGV Application
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For an effective Doppler Global Velocimeter (DGV), there is a requirement to accurately record and tune the frequency content of the laser used. The laser used for this experiment was an ND:YAG. Adjusting the mean frequency of the ND:YAG is accomplished by controlling the seed laser diode output, which also narrows the bandwidth of the laser to below 20 MHz. The exact frequency of operation is critical for the operation of the system. Standard interferometry techniques that measure laser frequency content, such as Fabre-Perot and grating based systems, are not able to provide an adequate spectrum resolution for the 9 ns pulse duration of the ND:YAG laser. A method was developed that employs a CCD line camera and a laser reference cell to effectively and cost efficiently solve this problem. The hardware and software for this real time monitoring system were developed and used with a real time feedback loop to stabilize the laser operating frequency at a specified value. The receiving optics of this DGV system were upgraded with 12 bit CCD cameras and a temperature controlled laser reference cell to decrease the uncertainty to the velocity measurement from over 4 m/s to less than 1 m/s. Recommendations to the effectiveness of the system and future improvements are provided.