Abstract
We describe a novel method for providing geometrical on-axis access to an optical enhancement resonator through an on-axis aperture at one of its mirrors. A superposition of transverse modes in a stable degenerate resonator is used to form a field distribution which avoids the aperture and therefore exhibits small loss. Upon propagation in the resonator the modes acquire a different phase, and an on-axis intensity maximum is formed at a different position. We call this a quasi-imaging resonator, because it is related to imaging in the sense that a hole in the field distribution, exacted by the aperture, is reproduced after a resonator round trip.
Highlights
The many round trips the radiation takes depending on the resonator loss lead to an increased sensitivity that is used in metrology [1, 2], and to an enhanced intensity that is e.g. used for nonlinear processes like frequency conversion or processes with a small cross section like Thomson back-scattering at electrons [3]
We described a new concept providing a geometrical on-axis access to a high-finesse enhancement resonator, which we call quasi-imaging
Eigen-modes of the quasiimaging resonator can be constructed as a combination of simultaneously resonant modes, which avoid the obstacle and exhibit small loss
Summary
For many applications an access along the optical axis of the resonator is necessary to couple radiation in or out This can often be achieved with dichroic mirrors. Geometrical output coupling through an opening in a resonator mirror can be advantageous compared to alternative methods: there is no additional optical element in the resonator and no additional dispersion, nonlinearity, bandwidth limitation or polarization discrimination for the fundamental radiation, and the harmonics are not angularly dispersed. Field distributions with an on-axis intensity maximum at a different position can be excited This basic concept has been proposed in active resonators for free electron lasers, where holes in resonator mirrors allow to introduce the electron beam in a collinear fashion [6].
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