Telecom Optics
C-lens | Birefringent wedges | Etalons | Bandpass filter
Etalons
Etalons are narrowband wavelength filters. They offer the advantages of high transmission. Because etalons feature low wavefront distortion, low insertion loss, easy tunability etc, they are widely applied in fiber communications such as WDM networks, tunable filter, wavelength locker, DWDM systems etc.
An etalon is a very simple device and is theory basis is the interference of multiple beams. It consists of two partially transmitting mirrors that are sparated by a distance d to form a reflective cavity. When a plane wave incident at angle a as in figure 1 below, the transmission of the etalon is given by the following formula.
Here:
A1 or A2 and R1 or
R2 are reflective mirror's absorbance and reflectivites:
d is the internal phase shift per traversal: 
n is the cavity refractive index, d is the cavity thickness, l is the wavelength of incident wave and q is the inclination of the inclination of the direction of the incoming radiation to the normal of the mirror.
The transmission curve is shown in Figure 2:
Figure 1:The Device of An Etalon
Figure 2:transmission peaks an etalon
The frequency separation between adjacent peaks is termed the Free Spectral Range(FSR).The full width at half maxmum 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 figure and parallelism.
Because FSR is decided by the
internal phase shift per traversal d,
we change FSR through tuing do
According to 
,The following three ways that we
suggest to turn an etalon:
(1)Angle or tilt-tune the etalon to change q, the way is widely used in tunable filter, wavelength locker etc..
(2)Change the cavity refractive index and the way is widely used in WDM system etc.
(3)To tune the optical path length nd through controlling temperature,
We offer two types of etalons for the fiber communication: Air-spaced Etalons and Solid Etalons .
Air-Spaced 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 coatings and AR coatings results in improved transmission. The mechanical design is robust and our etalon has great thermal stability. Our Air-spaced Etalons are available in a wide range of FSR values from 1500 GHz to 10 GHz.
Solid Etalons
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.
Specifications:
MATERIAL: ULE RT FUSED SILICA
Parallelism: <0.5 are Seconds
Surface flatness: l/20@632.8 nm
Surface Quality: 10/5
Diameter Tolerance:
+0.00 mm, -0.20 mm
Thickness Tolerance: +/- 0.001mm
Reflectivity: 40%
Diameter D(mm): 12.7
Clear Aperture(mm): 80%
Order Information:
Part
No. |
Type |
FSR |
ETA1205 |
Air-Spaced |
50GHz |
ETA1210 |
100GHz |
|
ETS1205 |
Solid
Etalon |
50GHz |
ETS1210 |
100GHz |