# Output coupling from x-ray free-electron laser cavities with intracavity beam splitters

## TL;DR

This paper proposes and analyzes novel intracavity x-ray beam splitters made of diamond crystals for efficient, flexible, and high-efficiency photon outcoupling from XFEL cavities, surpassing traditional permeable mirror methods.

## Contribution

It introduces alternative intracavity beam splitter schemes using diamond crystals, enabling adjustable and high-efficiency photon outcoupling in XFELs.

## Key findings

- Diamond crystal beam splitters achieve near 100% outcoupling efficiency.
- The schemes allow for prompt variation of outcoupling from zero to nearly full power.
- Extensions to multi-beam outcoupling are feasible.

## Abstract

Permeable mirrors are typically used for coupling photons out of laser cavities. A similar approach was proposed for output coupling photons from the cavities of x-ray free-electron laser (XFEL) oscillators. One of the Bragg-reflecting crystal mirrors is thin, just a few extinction length, and is used as a permeable mirror. However, this method is very often limited to extractions of only a few tenths of the intracavity power. Other cavity-based XFELs, such as the high-gain regenerative amplifier XFEL, require much higher outcoupling efficiency. Here, alternative schemes are proposed and analyzed for coupling x-ray photons out of XFEL cavities using intracavity Bragg-reflecting, x-ray-transparent diamond crystal beam splitters. The intracavity beam splitters are efficient and flexible in terms of the amount of the power they are capable of coupling out of the cavity, an amount that can be varied promptly from zero to close to 100%. The schemes can be readily extended to multi-beam outcoupling.