Alexandrite laser is a solid state
laser in which Chromium ions (Cr+3), at the amount of 0.01-0.4
%, are embedded in BeAl2O4 crystal.
It has energy level structure similar to the energy level structure
of Ruby laser.
Alexandrite laser was operated for the first time as a three level laser in 1973 at a wavelength of 680 [nm].
A few years later, it was found that at longer wavelengths the Alexandrite laser can be operated as a four level laser, which can be tune over a range of wavelengths: 720-800 [nm].
It was the first tunable solid state laser to reach the market.
A model of energy level diagram which explain the tunability can be seen in figure 6.16.