Enhancement resonator including non-spherical mirrors

Sensoren, Geräte und Komponenten

Ref.-Nr.: 1202-4590-WT

An Enhancement Cavity (EC) with at least one toroidal and/or cylindrical mirror is described. This allows for an arbitrary beam ellipticity of the eigenmode. In particular at a stability edge, where the ellipticity diverges with purely spherical optics, a round beam can be obtained.


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.

Figure 1:
a) Schematic of a large-mode bowtie cavity, b) Beam profile along the optical axis of a 125-MHz bow-tie cavity (2400 mm length, 600 mm radius of curvature mirrors) with (solid line) and without (dotted line) compensation using the technique described above.


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)

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