Frequency Spectral Measurements of a Pulsed, TE CO₂

Experimental measurements of the intra-pulse chirp and temporal coherence from a Joule class TE CO2 laser incorporating LAWS transmitter design features are presented. Digitized quadrature data (I and Q) from our ground-based coherent Doppler lidar system utilizing return signals off a hard target in the telescope far field are processed using fast Fourier transform and pulse pair techniques to obtain laser pulse frequency spectral components (offset frequency and spectral width) and high-resolution (∼ 50 ns/sample) frequency chirp profiles. Less than 300 kHz of frequency chirp is observed in the first 3·5 μs of the laser pulse which contains approximately 90% of the pulse energy. Spectral width of the laser pulse, including both chirp and transform limited components, are measured to be less than 300 kHz full width at half maximum.