Etalons are narrowband wavelength filters. They offer the advantages of transmission. Because etalons feature low wavefront distortion, low insertion loss, easy tenability etc, they are widely applied in fiber communications such as WDM networks, tunable filter, wavelength locker, DWDM system etc.
An etalon is a very simple device and it is theory basis is the interference of multiple beams, it consists of two partially transmitting mirrors that are sparated by a distanced to from a reflective cavity. When a plane wave incident at angle α as in figure 1 below, the transmission of the etalon is given by the following formula:
A1 and A2 and R1 or R2 are reflective mirror’s absorbance and reflectivites:
δ is the internal phase shift per traversal:
n is the cavity refractive index, d is the cavity thickness, λ is the wavelength of incident wave andθ is the inclination of the inclination of the direction of the coming radiation to the normal of the mirror.
The transmission curve is shown if Figure 2:
Figure1: The Device of An Etalon Figure2: transmission peaks an Etalon
The frequency separation between adjacent peaks is termed the free spectral range (FSR). The full width a half maximum of peaks is termed FWHM. It is given by the following formula:
The finesse (F) is given by:
The finesse (F) indicates sharp extent of interference stripe, the factor that affects finesse is the reflectivity of the mirrors, surface figure and parallelism.
Total finesse is given by:
Here finesse_R is decided by reflectivity, Finesse_P is decided by surface and parallelism.
Because FSR if decided by the internal phase shift per traversal δ, we change FSR through tuingδ o, according to
The following three ways that we suggest to turn an etalon:
(1) Angle or tit-tune the etalon to change θ, the way is widely in tunable filter, wavelength locker etc.
(2) Change the cavity refractive index and the way is widely used in WDM system etc.
(3) To rune the optical path length nd through controlling temperature.
We offer two types of etalons for the filter communication; Air-spaced Etalons and Solid Etalons.
Air-spaced etalons are two extremely parallel plates polished to very tight specification with an air gap between them. Using a ULE or fused silica as a gasket that is optically contacted between the two plates creates the air gap. Designed with special partial reflecting etalon coating and AR coatings result in improved transmission. The mechanical design is robust and our etalon has great thermal stability. Out air-spaced etalons are available in a wide range of FSR values from 1500 GHz to 10 GHz.
Solid etalon are two parallel plates but with a special coating to create the cavity. High quality cavities result in higher transmission a compact size and lower cost. These etalons have the potential for a high damage threshold and are particularly good for intercavity use. Our solid etalons are available in a wide range of FSR’s, from 1500 GHz to 10 GHz.
Fused silica, optical-grade silicon, Zerodur
Typical Free Spectral Range (FSR)
25 GHz / 50 GHz / 100 GHz (Other FSR available)
20-10 scratch and dig
10-5 scratch and dig
Angle of Incidence
Single Surface Reflectivity
Diameter D (mm)
1 mm x 1 mm (minimum)
Typical Lead Time