Note on Fabry-Perot etalon :

Figure 7.8(I) Schematic drawing of a Fabry-Perot etalon (left); the reflecting surfaces are inside and have a separation of l . The transmitted light is given a symbol T while R is the reflected portion. Figure taken from http://en.wikipedia.org/ wiki/Fabry%E2%80%93P%C3%A9rot_interferometer

The etalon is a pair of partially-silvered mirrors separated by a fixed distance and filled with a medium of known refractive index (Figure 7.8(I)). The silvered portions of the mirrors face each other. The etalon is designed in such a way that the ( rays of the) incident light beam will interfere with the ( corresponding rays of the ) multiply-reflected light beams producing a pattern of constructive interference. Other harmonics are reflected away and the transmitted light beam is at practically at isolated (discrete) wavelengths. In reality, transmittivity of an etalon attains a peak at the designed wavelength and is small at others, but may not strictly reduce to zero. The transmission through an etalon is also a strong function of the reflectivity of the two surfaces. For the configuration shown in Figure 7.8(I), the phase difference between rays T₁ and T₂ can be shown to be

For reflectance of the inner surfaces, transmission through the etalon is given by the formula

Conditions of maximum transmission for a given wavelength can now be established as a function of wavelength and the angle of incidence. Specifically, for integer values

we get

Transmission is a maximum under these conditions. In practice, we may have broadened spectrum centered around the desirable value of the wavelength entering the etalon. Passage through the etalon would eliminate most wavelengths and a light beam with sharp spectral peaks would be obtained. Among these, one of them could be most prominent, depending on the transmission function.

An etalon is also used to improve the coherence of a laser. The beam from a laser may appear to have a single color but contains, generally, several frequencies corresponding to the modes of oscillation in the laser resonator. The etalon may be used to separate modes or limit the number of modes. Using interference for mode selection, the desired frequency of oscillation alone passes through. Thus, the coherence of the light output from the etalon is higher than that of the laser.