Air Spaced UV Etalons
These
Fabry-Perot air-spaced etalons are made from two polished plates of UV synthetic
fused silica optically contacted using precision polished spacers.
The air gap is precisely controlled so that reflections from opposite
faces are in phase, and therefore reinforced, at the wavelength of interest.
They are used for the selection of laser line output and the elimination
of unwanted nearby lines. Air-spaced
etalons are designed for a particular free spectral range (FSR) which represents
the distance between adjacent peak transmittance wavelengths λo/λ.
FSR is normally specified as a frequency interval in GHz.
The finesse (FSR/FWHM) of an etalon is controlled by the reflectance of
the surfaces with higher reflectance providing a better finesse but accordingly
reduced transmittance.
Air-spaced
etalons can be produced with an air gap ranging from 25 μm to 1 mm
corresponding to FSR from 6000 GHz down to 150 GHz.
Coatings are applied to the surfaces of the etalon to produce controlled
reflectance from 30% to 97%, thereby providing finesse values ranging from 2.5
to 100.
Excimer
lasers benefit from the use of etalons. FSR
is independent of wavelength but the reflective coatings are wavelength
dependent. Several different laser
wavelengths in the UV can be accommodated including: 193, 248 and 308 nm.
Usual
dimensions are 25.4 mm diameter with a clear aperture of 22mm but other sizes
can be supplied on request. These
air-spaced etalons are normally supplied mounted in a metal ring of 38.1mm
outside diameter.