Crystals refers to solid state materials with a repeating pattern of molecular structure, forming a three dimensional lattice.
Optical crystals are mainly grown in laboratories and therefore are called synthetic crystals, in opposition to natural crystals that can be found directly in the nature.
Each type of crystal shows specific optical properties, that leads to active modification of light that is passing through them.
These above characteristics are the reason why they are useful in many different photonics applications.
Two main optical crystals types
Two main type of light modification can be sort out of optical crystals that correspond to two main crystal types:
Laser crystals, are synthetic crystals specially design to be used in Laser. They are an optical gain medium, meaning crystals will amplify light going through it. Due to their good thermal dissipation characteristics, they are able to cope with the high power generated by lasers.
Laser crystals are widely used in diode pumping lasers, as an amplifier of the diode output light.
Most current laser crystals includes : nd:YAG, Ti:Sapphire and Er:YAG.
Naming of the crystal follow below pattern :
Where dd stand for the dopant and XX for the crystal material, for example Er:YAG is a Erbium doped Yttrium Aluminium Garnet crystal.
Unlike laser crystals, non-linear crystals are not provided optical gains, but rather change the characteristics of light. For exemple they may change light frequency.
Most common Non-linear crystals are KTP, BBO, LBO.