Holmarc's Fabry-Perot etalon is made of plate beamsplitters separated by 3mm distance. The beamsplitters are made by coating thin aluminum films on optically polished N-BK7 substrates. The reflecting surfaces are kept inside the assembly. The etalon has optical resonating properties similar to that of laser. A translucent screen is provided with vernier and needle to take the readings of the fringes directly.
Etalons transmit light as a series of periodic frequencies and their narrow bandwidth makes them well-suited for wavelength selection, measurement and line-narrowing. The properties of an etalon are largely determined by its thickness, index of refraction and the reflectivity of it’s surfaces. The finesse is a measure of the interferometer's ability to resolve closely spaced spectral lines.
An etalon is an optical interferometer in which a beam of light undergoes multiple reflections between two reflecting surfaces and it’s resulting optical transmission (or reflection) is periodic in wavelength. In other words, an etalon is a narrow band wavelength filter.
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
Fig. Schematic Diagram
Fig. Fabry Perot etalon produces sharp interference peeks