Mode Cleaner Cavities
A high-power laser beam with a single optical frequency can be fed through an optical cavity (resonator) in order to improve its beam quality. One exploits the fact that in general the 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. For 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, but they work only with (quasi-)monochromatic radiation, require careful mode matching, and are more difficult to design and set up.
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|||A. Araya et al., “Optical mode cleaner with suspended mirrors”, Appl. Opt. 36 (7), 1446 (1997), doi:10.1364/AO.36.001446|
|||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), doi:10.1364/OL.23.001704|