An optical cavity, or an optical resonator, may be described as an arrangement of mirrors that produce a standing light wave resonator. Optical cavities, along with their optical gain, are an integral part of lasers, optical parametric oscillators, and light interferometers.
The simplest form of optical cavity consists of two or more mirrors that are arranged in such a way that light is made to propagate in a closed path. The study of an optical cavity, or resonator, is a vast field, hence is treated as a separate optical subject for study. The standing wave patterns produced in an optical cavity are called modes.
Types of Optical Cavity and Modes
There are two basic types of optical resonator modes. These are: the standing wave resonator, and the travelling wave resonator. The travelling wave resonator is also referred to as the ring resonator.
- Longitudinal or Standing Wave Resonator – The light waves in this type of optical cavity differ in their frequencies
- Travelling, or Ring, Wave Resonator – The light waves produced in this type of optical cavity differ on both frequency and the intensity pattern
Common types of resonators are flat and spherical mirror resonators. These types differ mainly in the focal lengths and the distance between the mirrors.
- Flat or Plane-Parallel Optical Cavity –This type of optical cavity consists of two opposing flat mirrors. This is also referred to as the Fabry-Perot Cavity. Despite the simplicity of this optical cavity, it is not used for large-scale laser systems, due to the difficulty in alignment.
- Spherical Cavity – This type of optical cavity exhibits optical resonance when the size of the sphere, refractive index, or the optical wavelength is varied.
Design of Optical Cavities
Flat mirrors are not preferred in building optical cavities. This is due to the difficulty in aligning them. The geometry of the cavity must be such that the light beam remains stable. Some other factors that are considered while designing an optical cavity are Q factor and beam width.
Applications of Optical Cavities
Some of the application areas of optical cavities are listed below:
- Laser resonators
- Multipass optical delay lines
- Optical interferometers
- Laser oscillators