Optical Knowledge

Mirrors have some basic properties:
Reflectance is the percentage of reflected light power. Usually it is related to wavelength and angle of incidence, but also to polarization direction.

Reflection phase is the phase shift of reflected light, that is, the phase change of light before and after light reflection. Phase shift is related to wavelength and polarization direction. If the phase changes of s polarization and p polarization are different (non-normal incidence), then even if the incident light is pure s or p polarization, the polarization state of the incident light will change after reflection.

Mirrors only work in a limited wavelength range, and the required reflectivity can only be obtained in this region. The width of this area is called the reflection bandwidth. The bandwidth value is related to the angle of incidence, polarization state and reflectivity tolerance. The range of angles of incidence is also limited, especially for dielectric mirrors.

There are other properties that need to be considered in many applications:
High surface quality is required in laser technology. Surface smoothness is often characterized in terms of wavelength, such as λ/10. Since surface defects are largely a random phenomenon, only their statistical characterization can be given. For small local defects, "scratches and pits" are often used to characterize them. This includes the maximum length of the scratch (tenths of a micron) and the maximum radius of the pit (hundredths of a micron). For example, a Scratch-Pit value of 20-10 indicates that the maximum length of the scratch is 2 microns and the maximum diameter of the pit is 0.1 micron.

Optical damage thresholds need to be considered when using high power lasers, especially pulsed lasers due to their high peak powers.