Large-mode enhancement cavities

Henning Carstens,Alexander Apolonski,Jan Kaster,Ioachim Pupeza,Volodymyr Pervak,Ferenc Krausz,Simon Holzberger,Ernst Fill,Johannes Weitenberg

doi:10.1364/oe.21.011606

Abstract

In passive enhancement cavities the achievable power level is limited by mirror damage. Here, we address the design of robust optical resonators with large spot sizes on all mirrors, a measure that promises to mitigate this limitation by decreasing both the intensity and the thermal gradient on the mirror surfaces. We introduce a misalignment sensitivity metric to evaluate the robustness of resonator designs. We identify the standard bow-tie resonator operated close to the inner stability edge as the most robust large-mode cavity and implement this cavity with two spherical mirrors with 600 mm radius of curvature, two plane mirrors and a round trip length of 1.2 m, demonstrating a stable power enhancement of near-infrared laser light by a factor of 2000. Beam radii of 5.7 mm × 2.6 mm (sagittal × tangential 1/e(2) intensity radius) on all mirrors are obtained. We propose a simple all-reflective ellipticity compensation scheme. This will enable a significant increase of the attainable power and intensity levels in enhancement cavities.

Highlights

Passive optical resonators can be efficiently excited by light emitted from a single-frequency or modelocked laser as its coherence allows for a constant phase relationship of the incident field with the field inside the resonator
Compared to the cavity described in Ref. [4], which can be considered as a benchmark for intensity-related power scaling limitations with a beam radius on the mirrors of 1 mm, a spot area increase by a factor of 8 and 15 was achieved with the ACM and the standard bow-tie (SBT) cavity, respectively
This metric enables the comparison of various cavity designs. It identifies the standard bow-tie (SBT) ring resonator consisting of two spherical mirrors and several additional folding mirrors, operated close to the inner stability edge, as the most robust approach to large-mode enhancement cavity (EC)

Summary

Introduction

Passive optical resonators can be efficiently excited by light emitted from a single-frequency or modelocked laser as its coherence allows for a constant phase relationship of the incident field with the field inside the resonator. The resonant enhancement of pulsed radiation is made possible by the comb-like structure of the spectrum emitted by a modelocked laser which allows coupling each individual comb line to a cavity resonance. In the time domain this means that the cavity round trip time is equal to a multiple of the pulse repetition period. This makes ECs suitable for the enhancement of pulse trains with repetition rates between ∼10 MHz and several GHz. Recently, average powers on the order of a few kW [3,4,5] have been reached with near-infrared intracavity femtosecond pulses around 100 MHz with peak intensities exceeding 1014 W/cm. For a 80 MHz enhancement-cavity with 200 fs pulses the maximum obtainable average power was determined to be 18 kW [4]

Methods

Results

Conclusion