In Fabry - Perot interferometer, the distance between the partially reflecting mirrors are varied by using coarse and finely adjustable translation stage driven by micro-meters. One beam splitter is fixed and the other is mounted on the translation stage through a kinematic mount. This two axis kinematic mount is used to correct the parallelism between beamsplitters.
The Fabry - Perot design contains plane surfaces that are partially reflecting so that multiple rays of light are responsible for the creation of the observed interference patterns. For high resolution spectroscopy, where a resolution in the range of MHz to GHz is required, a Fabry - Perot interferometer (FP) is used. The FP consists of two plane mirrors mounted accurately parallel to each another, with an optical spacing of 'd' between them.
The enclosed air gap generally varies from several millimeters to centimeters, when the device is used interferometrically. If the gap can be mechanically varied by moving one of the beam splitters, then the device is referred to as an interferometer. It’s transmission spectrum as a function of wavelength exhibits peaks of transmission corresponding to resonances of the etalon. Fabry - Perot interferometers are widely used in telecommunication, lasers and spectroscopy for controlling and measuring the wavelength of light.
The wavelength of laser is calculated by,
λ = ( 2d / N ) Δ
The Spacing of the Etalon is given by,
t = nD2λ / χn2
where n is the order, ‘D ’ is the distance between the etalon and the screen, ‘λ’ is the wavelength of light used.
χn2 = χm+n2 - χm2
χm+n2 is the square of the radius of (m+n)th ring and χm2 that of mth ring.
The Free Spectral Range of the Etalon is given by,
FSR = c / 2t,
Where c is the speed of light in air and t is the spacing of the Etalon
The finesse is a measure of the interferometer's ability to resolve closely spaced spectral lines.
The finesse F is defined by the following equation.
F = π √R / ( 1-R )
Where, R is the reflectivity of surfaces