Generation of high-average-power ultra-broadband infrared radiation

Abstract

High-average-power ultra-broadband mid-IR radiation can be generated by illuminating a nonlinear medium with a multi-line laser radiation. Propagation of a multi-line pulsed CO2 laser beam in a nonlinear medium, e.g., gallium arsenide or chalcogenide, can generate directed broadband IR radiation in the atmospheric window (2–13 μm). A 3D laser code for propagation in a nonlinear medium has been developed to incorporate extreme spectral broadening resulting from the beating of several wavelengths. The code has the capability to treat coupled forward and backward-propagating waves, as well as transverse and full linear dispersion effects. Methods for enhancing the spectral broadening are proposed and analyzed. Grading the refractive index radially or using a cladding will tend to guide the CO2 radiation and extend the interaction distance, allowing for enhanced spectral broadening. Nonlinear coupling of the CO2 laser beam to a backward-propagating reflected beam can increase the rate of spectral broadening in the anomalous dispersion regime of a medium. Laser phase noise associated with the finite CO2 linewidths can significantly enhance the spectral broadening, as well. In a dispersive medium, laser phase noise results in laser intensity fluctuations. These intensity fluctuations result in spectral broadening due to the self-phase modulation mechanism. Finally, we present propagation through a chalcogenide fiber as an alternative for extreme spectral broadening of a frequency-doubled CO2 multi-line laser beam.