Definition：
High quality mirrors used in laser resonators and other optical devices.
The laser resonator contains a mirror, which needs to meet some requirements,
especially the following:
Low reflection loss (for high mirrors), or a specified transmittance over a given
wavelength range (for output couplers)
High surface optical quality (very flat over a large area，avoid wavefront distortion that
degrades the beam quality)
Can withstand high light intensity, with high damage threshold, avoid laser-induced
damage (especially in q-switched lasers)
In most cases, a multi-layer dielectric reflector (usually a quarter wavelength reflector) is
used as the laser reflector. (The specific preparation details of dielectric laser mirrors are
given in dielectric coatings.)Typically, one laser mirror is used as an output coupler with
high transmittance to the laser radiation, while others are highly reflective (reflectivity
>99.8%). Some mirrors may use dichromatic mirrors so that the pump light can enter the
gain medium of the end-pumped laser. (For quasi three level lasers, because the pump
light and laser wavelength are very similar, it is necessary to use this dichromatic mirror.)
The laser mirror can also reflect light from outside the laser resonator. For example, a pair
of tilted mirrors is often used, each reflecting at an Angle close to 90° to reflect the laser
beam to other devices. The adjustment price of the inclined mirror usually has two or three
adjusting screws, which can adjust the starting point and direction of the beam.
Mirror substrate
Typically, laser mirrors are prepared on a glass substrate (for example, BK7 or fused
quartz), but the mirror coating can also be directly coated on the laser crystal, for example,
a monolithic laser. Usually the mirror substrate is elliptical in shape with a size of 25.4mm
or 12.7mm and a thickness of 6mm. Even for high mirrors, the properties of the substrate
are very important, especially the surface quality and high rigidity, low thermal expansion
coefficient, high thermal conductivity (avoiding thermal expansion in high power lasers).
For partially transmitted reflectors, high optical uniformity (avoiding beam distortion of
transmitted light), low absorption and scattering losses are also required.
