What Is An Etalon?

A Fabry-Perot etalon is an array of two flat and parallel optical surfaces that are separated by a gap. Constructive interference at the gap results in transmission peaks. The gap size and etalon reflectivity determine the spectral characteristics of the transmission peaks. High finesse etalon filters can be used as very precise wavelength references for data communications applications.

Fabry-Perot Interferometers

An interferometer is an optical instrument that consists of two parallel surfaces with semi-transparent, highly reflecting coatings, separated by a fixed distance. When the distance between the plates can be varied, the instrument is called a Fabry-Perot interferometer and it can selectively transmit certain wavelengths while rejecting others. The free spectral range varies inversely with the spacing between the plates, t, which is often adjusted to optimize its performance. The Fabry-Perot interferometer has several uses, including as an optical filter.

A Fabry-Perot interferometer can be used to stabilize the frequency of a laser (which often fluctuates due to mechanical vibrations or temperature changes) by locking it to a mode of the cavity. It can also be used to prolong the interaction length in laser absorption spectrometry techniques such as cavity ring-down.

Etalons as Optical Filters

Etalons are often used as optical filters to select and stabilize wavelengths of interest. For example, telecommunications networks employ banks of small tuned solid etalons in fused silica or diamond to suppress ghost signals and other stray light. An Etalon® has a very narrow bandpass. Its spectrum depends on the monochromatic wavelength used to illuminate it, with the spectral profile forming an imperfect bell-shaped curve with extended wings. This imposes a limit on the resolution of any spectrometer using it, even if the etalon is free from defects.

The quality of the wavefront passing through the etalon can be improved by reducing its cavity width. This can be accomplished in a number of ways, but manufacturing, economic and mechanical constraints prevent the cavity from being arbitrarily thin. Moreover, reducing the cavity width tends to broaden the etalon’s bandpass in an inversely proportional way. The best solution is to use a pre-filter placed between the etalons with low transmission and high rejection of out-of-band light to reduce ghost signals (see Beckers 1998). The problem of ghost signals is further reduced by angular tuning the etalons to shift their peak wavelengths.

Etalons As Wavelength Lockers

Etalons are a common device for wavelength locking in DWDM systems. They consist of a pair of partially-reflecting mirrors separated by an air gap, with the forward transmission signal from the etalon varying periodically as a function of wavelength. The output of the etalon can be used to stabilize the wavelength of laser light input to a transmitter, thus enabling the transmitter to achieve the desired optical frequency.

A high slope of the etalon’s transmission peak versus frequency, known as the locking slope, is a key property of the device. A large slope enables a much larger feedback signal to be generated for a given deviation of the transmitter’s output wavelength from the desired lock wavelength. A large lock slope also improves the stability of the resulting signal. The etalon is typically constructed from a material such as fused silica that does not change the polarization of the transmitted beam. The temperature of the etalon is controlled by a closed-loop temperature controller to make the device insensitive to ambient environmental changes that could otherwise detune the laser.

Etalons As Optical Power Monitors

An etalon may be used to monitor optical power. These devices are called optical power monitors (OPM) and are typically permanently integrated into some systems such as lasers or optical fiber communication components. They are not to be confused with optical channel monitors, which can measure the optical powers of the individual wavelength channels of a wavelength division multiplexing system.

A typical OPM consists of a miniature, hermetic package with an input and output port for insertion into the optical fibers. It contains a stabilized tap and a photodiode to measure the incoming optical power. The device has a small optical insertion loss and polarization-dependent loss, and it is very robust against vibrations or mechanical shock. An OPM can also be equipped with a display to make it easy to use by untrained personnel. It is often used in conjunction with erbium-doped fiber amplifiers (EDFA) to provide power stability feedback and prevent damage due to aging pump diodes or variations in the input optical power level.

Conclusion

Etalons are Fabry-Perot interferometers consisting of two flat and parallel optical surfaces seperated by an air or solid gap. The etalon’s characteristic transmission function depends on the interaction of multiple reflections between the high reflector plates and can be tuned by changing the distance between them.

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