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Precision Photonics 2009 Catalog - 36217 Etalon Basics

Etalons for Wavelength Locking

A This F T

Etalons are often used for stabi -

L .

lizing a laser’s frequency, or wave -length. In a typical “wavelength locker” that uses the side-fringe-lock- ing technique, a portion of the laser’s

n , L ,

output is transmitted through an etalon so that the center wavelength is halfway down the side of a transmission peak. The technique takes advantage of the almost linear slope on either side of the transmis- sion peak where frequency variations will be linearly converted into amplitude variations. To eliminate spurious laser amplitude noise, a reference signal from the laser is removed from the transmitted etalon signal. The re-sulting clean signal can then be processed by a feedback control system to adjust

R .
Ln

the frequency of the laser. For an exam -ple of our wavelength-locker designs, see pages 4–5. Etalons are also used as wave -length references, interleav-ers, and periodic optical filters.

R The , m q =0Ëš), , is
Note: the transmitted beam of a higher finesse de-vice is the result of interference between beams that travel many more times through the device. Thus, the physical limitation to the finesse is set by the overall beam overlap of the etalon as limited by the deviations of the surfaces from plane parallelism, coating losses, and beam characteristics (angle of incidence, divergence, beam diameter, etc).

with .

FSR R=25% R=50% R=90% FWHM

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Normalized Transmission 0.1 0.9 1 0.8 0.7 0.6 0.5 0.4 0.3 0.2 190.02 190.04 190.06 190.08 190.1 190.12

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Optical F r equency (THz )

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The Airy functions or transmitted intensities of etalons of various reflectivities as a function of frequency. The higher the reflectivity, the higher the finesse, and the narrower the FWHM.

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Etalon Design Basics

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