Research On The Film Design And The Thermal Effects Of Strong Laser Reflector At1.315μm

The high reflective membrane appropriate for COIL was obtained using TFCALE simulation designsoftware, based on the design principle of high reflective membrane, the traditional design method ofquarter-wave stack adopted. The electric field intensity distribution in the multilayer films under the laserirradiation was given. In the chemical laser, the reflector needs reflection in many wavelengths. Becausethe infrared light at1.135μm emitted from COIL is tuned or guided by visible light, the design of doublestacks was employed. The high reflective membrane appropriate for COIL with the structure ofSi/（LH）₁¹⁰（LH）₂⁴/Air was designed on the Si substrate, with Si, SiO₂as high and low refractive indexmaterials, where H and L are the high refractive index membrane layer （Si layer）, low refractive indexlayer （SiO₂layer） and the structure（LH）101refers to the1.315μm stack and the structure（LH）₂⁴refersto the0.6328μm stack. The reflectivity of COIL on the center output wavelength1.315μm achieved over99.999%according to theory simulation, which satisfy the need to the mega watt chemical laser cavitysurface medium membrane; The reflectivity of harmonic wave on the wavelength0.6328μm achieved over99.2%, which exceeds the requirement to harmonic wave reflectivity of95%.Heating effect has an important effect on the strong laser reflector performance. Heat conductiondifferential equations in different coordinates are deduced by heat transfer theory knowledge. Mathematicexpressions of the laser damage threshold and the temperature distribution of multilayer membrane weregot. Green’s function method is applied to calculate the silicon substrate temperature distribution under theirradiation of the1.315μm Gaussian beam. The temperature field distribution curves under different thelaser energy Gaussian were drawn. The silicon substrate thermal deformation was calculated by the axial thermal deformation formula. Thermal deformation curves under different the laser energy were drawn.Finally, according to the formula of laser damage threshold of the film, the laser damage threshold of thetwo layers of SiO₂and Si was calculated, and the damage threshold curve with the laser irradiation timechanging was drawn. The temperature field distribution and thermal deformation after membrane depositedwere analyzed in theory from the results of the silicon substrate temperature field distribution and thermaldeformation. The heating effect of the multilayer membranes was concluded, thus the damage mechanismof heating effect of the high energy laser reflector was obtained. At last, the damage mechanism of theheating effect of high energy laser reflector was summarized, which provides the basis for finding newmaterials suitable for high energy laser reflector and improving the laser output power.