Laser beams with high peak power and narrow line output are fundamental for non-linear conversion. Novel intracavity techniques which improve the spectral brightness of a high average power, degenerate, periodically-poled Lithium Niobate (PPLN) optical parametric oscillator (OPO) are reported. The OPO was pumped with a modulated, high repetition rate Q-switched Nd:YAG laser, which produced an average power of 20 W in a near-diffraction limited beam. The broadband full width half maximum (FWHM) of the OPO was measured to be 120 nm, with an average power output of 7.7 W. Using an air-spaced etalon in combination with a quartz birefringent filter as line-narrowing elements, this decreased to a line-narrowed average power of 4.5 W, with a full width half maximum (FWHM) of 3 nm. Experimental results indicate that the power loss from the OPO is proportional to the residual loss of the line-narrowing elements in the OPO cavity.
A comparison is made between the backscattering Mueller matrices of two bead-blasted aluminium samples at both 632.8nm HeNe and 10.6micrometers CO2 lasers wavelengths. The samples are bead-blasted at two blasting pressures to vary the rms surface roughness and slope. These surfaces are characterized using a mechanical profilometer. Mueller matrices are determined with the detector fixed close to direct backscatter and the target angle varied between 0 degrees and 80 degrees. Results are presented of the angular variation of the major diagonal elements and off-diagonal elements m12 and m21 with increasing roughness to wavelength ratio. The copolarized scattering intensities and copolarized ratios derived from the Mueller matrices show similarities to microwave radar sea echo.
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