Seed source for plasma compression in the long wavelength infrared

Two color laser pulses are used to form an air plasma and generate broadband infrared radiation suitable as a seed for backward Raman amplification of CO2 laser pulses. Broadband radiation in the atmospheric window from 8 to 14 μm is observed. The infrared radiation is characterized using a long wavelength grating spectrometer specially designed to accept an ionizing laser filament at its input plane. The long wavelength infrared yield is greatly enhanced by chirping the drive pulse, but this affects the spectrum as well. Unidirectional pulse propagation simulations suggest that this is due in part to the dependence of the nonlinear refractive index on the pulse duration.