The off-axis parabolic mirror is a mirror that does not contain a part of the axis of symmetry from the rotationally symmetric parabolic mirror. Because it can produce high-quality parallel beams without center occlusion in a simple shape, it has gradually become a widely used optical part in recent years, and its processing technology belongs to the field of optical processing technology.
If the target is placed at the focal point of the paraboloid, part of the incident light will be blocked, so only a part of the paraboloid mirror is used to let the beam incident off-axis, which is called off-axis paraboloid mirror. With it, the incident light will not be blocked by the target. Its effective focal length EFL is different from the focal length PFL of the mother paraboloid mirror, and the EFL should be taken as the focal length when calculating the diffraction limit.
Many off-axis parabolic mirrors are made of copper or aluminum as the base by cutting with a diamond cutter, with gold plated surface and a SiO2 protective layer. The reason for this is that it is easy to process, especially with short focal length. The reflectivity of the gold film almost does not change with the angle and polarization of the incident light in the wavelength range we use (700nm-900nm). The reflectivity of the mirror coated with dielectric film varies greatly with the incident angle.
The surface of the off-axis parabolic mirror is not as smooth as polished glass, and the surface roughness is less than 10 nm, according to which the scattering loss can be estimated as (4 π σ/λ) 2<3%, in formula σ Is rms roughness, λ Is the laser wavelength. These high-frequency (spatial) scattering will spread in all directions due to large divergence angle when focusing, and will not affect the focal spot. Another obvious feature of the off-axis parabolic mirror is the knife mark that can be observed with the naked eye. The spatial frequency is about 250/mm, similar to the grating, which will produce a focal line near the central focal spot.
Surface flatness or surface finish is an important indicator. There are three main evaluation methods, namely RMS, Ra and PV. The premise for comparing surface finish is to use the same set of evaluation methods. At present, the main off-axis parabolic mirror technology may ensure good spot uniformity in terahertz and infrared bands, while for visible bands, special surface treatment is required, and we can achieve the RMS level of only 20nm.
