The difference between F-P cavity and coaxial spherical cavity

According to the different sources of loss, the loss of the resonator is divided into five categories: transmission loss of cavity mirror, absorption and scattering loss of working material, geometric deflection loss, diffraction loss and accessory loss.

Among the five types of losses, geometric loss is to consider whether the light will escape out of the cavity from the perspective of geometric optics, and if it escapes outside the cavity, it will cause a relatively large geometric loss.

Flat cavity or F-P cavity

For a flat cavity or F-P cavity with two plane mirrors, only light rays that are strictly perpendicular to the cavity will not escape out of the cavity when propagating back and forth in the cavity, while any light that is inconsistent with the normal direction of the cavity will escape out of the cavity after a finite number of round trips. Obviously, the geometric loss of the F-P cavity is very sensitive to the parameters of the resonator. If the parallelism of the two mirrors is slightly off, in fact any light will escape out of the cavity very quickly, and the geometric loss of the cavity will increase dramatically.

The resonator is unstable

The phenomenon of a sharp increase in geometric loss due to the change of cavity parameters is called resonator instability. The instability of the resonator causes the output power of the laser to drop rapidly and stop the laser oscillation. In most cases. A stable resonator is the prerequisite for ensuring the stable output of the laser. In most cases, we need a stable resonator to guarantee a stable and efficient laser output of the laser.

Coaxial spherical cavity

The cavity mirror can be replaced with a concave mirror, which can be changed to a coaxial spherical cavity. The coaxial spherical cavity is an optical resonator composed of coaxial spherical mirrors, which has better stability than the flat cavity, in which any paraxial rays (rays with a small angle to the optical axis) can be well confined to the resonator and will not escape out of the cavity. Because the spherical surface is easy to process, the mirror is generally made into a spherical surface, and we call the resonator composed of this coaxial spherical surface a coaxial spherical cavity.