Chirp Analysis for Pulsed CO2 Laser Oscillators

Abstract

For many applications that utilize single frequency output of pulsed CO2 lasers, the shift in frequency during the pulse must be minimized. For example, in CO2 coherent Doppler lidar backscatter measurements of atmospheric wind speeds, the intrapulse chirp must be less than 200 kHz in order to achieve a wind speed resolution of 1 m/sec.1 Experimental studies in CO2 hybrid TEA lasers show a characteristic U-shape curve of frequency versus time, first falling during the early part of the pulse and then rising throughout the remainder of the pulse. Measured values of chirp range from 0.5 to 5 MHz for 3 μsec duration pulses. Analysis and experiments by Willetts and Harris2 have shown that the early chirp is due to decay of the plasma electron density in the laser gas and the late chirp is due to the gas density decrease near the optical axis of the cavity caused by nonuniform gas heating induced by the radial intensity profile of the laser mode structure.