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Etalons


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:

23.JPG

Here:

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:

Passive Optics-the device of an etalon.png           Passive Optics-tramsmission peaks an etalon.png

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:

                                     23-2.JPG

The finesse (F) is given by: 23-3.JPG


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:

                                    23-4.JPG


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

                                       23-5.JPG


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:

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 etalons:

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.


Capability:

Attribute

Commercial

High Precision

Operating Wavelength

1520-1620nm

Material

Fused silica, optical-grade silicon, Zerodur

Typical Free Spectral Range (FSR)

25 GHz / 50 GHz / 100 GHz (Other FSR available)

FSR Tolerance

25 GHz

±0.0025 GHz

25 GHz

±0.0015 GHz

50 GHz

±0.005 GHz

50 GHz

±0.003 GHz

            100GHz

±0.025 GHz

 

100 GHz

 

±0.015 GHz

Surface Quality

20-10 scratch and dig

10-5 scratch and dig

Angle of Incidence

Single Surface Reflectivity

>90%

Operating Temperature

User specified

Diameter D (mm)

1 mm x 1 mm (minimum)

Typical Lead Time

5-8 weeks

 

 

 

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