Optical cavity

An optical resonator, an arrangement of mirrors, which serves to reflect light as often as possible. In certain arrangements, this due to interference in the resonator forms a standing wave when the optical path length of the resonator is a multiple of half the wavelength of the incident light. Such devices are also referred to as standing wave resonators. There also exist a setup with no standing wave, such as ring resonators.

Depending on the geometry of the mirror structure stable and unstable resonators are distinguished. In an unstable resonator can escape a ray of light after a few reflections from the building as he always runs back in a stable in the resonator itself. Possible arrangements are confocal (both mirror radii equal to the cavity length ), planar ( both mirrors are flat ), concentric (both mirror radii correspond to half the cavity length ) or semikonfokale resonators ( a mirror plane and the focus of the other, the spherical mirror ).

The simplest and most important design is Fabry -Perot resonator, consisting of two parallel planar mirrors at a distance. The resonance condition for the formation of standing waves is then, so it must match a multiple of the half wavelength between the mirrors.

The distance between two resonance frequencies is called free spectral range (FSR, Free Spectral Range of eng. )

The transmission of the resonator, ie, the ratio of irradiated and the escaping intensity is frequency dependent and is described by the Airy formula. The transmission maxima are even sharper, the better to reflect the mirror.

The half-width of the peaks is

The thereby occurring factor is called the finesse and is the key indicator for resonators that specifies the spectral resolution. The finesse depends neglecting the losses in the resonator only on the reflectivity of the mirror from:

Depending on the mirrors used can have values of about 10 up to several 100,000 finesse.

Application

Laser resonators

An optical resonator is an essential part of almost any laser. Here, he serves on the one determining the direction of the induced emission: only along the resonator emitted photons through multiple times in it back and forth and therefore stimulate primarily running in this direction further emission. Secondly, each photon must at little amplifying active media being used both with multi-pass to stimulate further emissions to meet the laser condition. The laser resonator is used - possibly in conjunction with other components - even for frequency and mode selection.

The increased by multiple reflections inside the cavity compared to the out-coupled laser light intensity, giving the non-linear optics. An example of the frequency doubler in the resonator, the efficiency increases with the square of the field strength.

In laser diodes, the outer surfaces of the semiconductor material form in the simplest case, even the resonator, as occurs due to the widely different refractive index of semiconductors and around here always some reflection on. Therefore, no external mirrors are required. The optical path length of the resonator, and thus the wavelength of the emitted light, in this case on the temperature or the current flowing through the flow of material (affects the refractive index) can be controlled. However, there are diode lasers that use an external resonator for wavelength selection, called ECDL (English: external cavity diode laser ). In this case, laser diodes with non-reflective surfaces are often used in order to eliminate the intrinsic resonator effect described above.

Spectroscopy and wavelength selection

Mechanically and thermally stabilized in particular optical resonators are used as the optical frequency reference for spectroscopy, and the frequency stabilization of lasers, there is also the method of the cavity ring-down spectroscopy.

In quantum optics, the interaction of atoms is studied with the light field in resonators extremely high finesse, the so-called cavity quantum electrodynamics.

Each optical resonator is suitable due to constructive interference for wavelength selection. The selection (contrast ratio ) is higher, the higher the reflectivity of the mirror. By suitable arrangements of several reflecting layers, interference filters can be realized.

Modern absorption spectrometer for the study of gases and liquids are also working with adjustable optical resonators by count back the changes in the intensity curve in the absorption at certain wavelengths.