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Mode Cleaner Cavities

Author: the photonics expert Dr. Rüdiger Paschotta (RP)

Definition: optical cavities used as filters for improving the beam quality of laser beams

Categories: article belongs to category optical resonators optical resonators, article belongs to category photonic devices photonic devices

DOI: 10.61835/lib Cite the article: BibTex plain text HTML Link to this page! LinkedIn

A high-power laser beam with a single optical frequency can be fed through an optical cavity (resonator) to improve its beam quality. One exploits the fact that in general the optical frequencies of higher-order resonator modes do not coincide with the frequencies of the fundamental (Gaussian) modes. (This is caused by the Gouy phase shift in the resonator.) When the laser frequency is locked to a fundamental mode resonance, the higher-order modes of the resonator are not in resonance and are therefore strongly attenuated in transmission. The transmitted beam can thus have a strongly improved beam quality. At the same time, there is some cleanup of the optical spectrum because only the light in resonance is fully transmitted, while other optical frequency components are attenuated.

For a high power efficiency, the mode matching of the input beam to the fundamental mode of the resonator has to be optimized.

Strictly, the term mode cleaner is somewhat imprecise, since such a device “cleans” beam profiles rather than modes.

Resonant mode cleaner cavities are used e.g. in large-scale interferometers for gravitational wave detection, where the required very high beam quality is difficult to achieve directly from the laser source due to thermal lensing and similar effects in high-power laser systems.

In comparison with nonresonant mode cleaners, mode cleaner cavities can achieve a much better cleanup of a beam profile and can provide spectral filtering at the same time. Also, they can be operated at much higher optical powers. However, they work only with (quasi-)monochromatic radiation, require careful mode matching, and are more difficult to design and set up.

Designing Mode Cleaners

The software RP Resonator is a particularly flexible tool for calculating all kinds of resonator mode properties. With a little script code (which we are happy to provide), you can also calculate the complete function of a mode cleaner cavity.

More to Learn

Bibliography

[1]	A. Araya et al., “Optical mode cleaner with suspended mirrors”, Appl. Opt. 36 (7), 1446 (1997); https://doi.org/10.1364/AO.36.001446
[2]	B. Willke et al., “Spatial and temporal filtering of a 10-W Nd:YAG laser with a Fabry–Pérot ring-cavity premode cleaner”, Opt. Lett. 23 (21), 1704 (1998); https://doi.org/10.1364/OL.23.001704

(Suggest additional literature!)

Questions and Comments from Users

2024-04-18

Reference [2] describes an experiment where they lock the cavity fundamental mode to the laser using PDH and cavity length control, but you describe the case where the laser is locked to the cavity instead. They are different techniques, but is the outcome effectively the same?

The author's answer:

Yes, fundamentally it does not matter whether you lock the resonance through the laser frequency or the resonance frequency of the resonator.

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