Enhancement resonator including non-spherical mirrors

By operating an EC at a stability edge, large illuminated areas on all optics can be obtained simultaneously, which circumvents the problem of intensity-related damage. However, the beam ellipticity, i.e. the ratio of beam radius in the tangential to the one in the sagittal plane, diverges due to the non-zero angle of incidence on spherically curved mirrors. In the patent, a purely reflective scheme for compensating the ellipticity is suggested. A typical implementation of the scheme comprises two spherically curved mirrors for focusing and two weakly curved cylindrical mirrors compensating for the astigmatism caused by the spherical mirrors.

• Only reflective optics are used, which mitigates intensity and thermal related degradation effects.
• Symmetry with respect to the focal plane can be preserved, i.e. the focus position in the sagittal and tangential planes is the same (no astigmatism)
• The smaller beam radius of the elliptical beam can be increased, which increases the illuminated area on the optics.
• Typically, a bow-tie resonator at the inner stability edge is used. Here, the beam size is (almost) equal on all optics and the alignment sensitivity is low.

Henning Carstens, Simon Holzberger, Jan Kaster, Johannes Weitenberg, Volodymyr Pervak, Alexander Apolonski, Ernst Fill, Ferenc Krausz, and Ioachim Pupeza, "Large-mode enhancement cavities," Opt. Express 21, 11606-11617 (2013)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-9-11606