# Fiber cavities with integrated mode matching optics

## TL;DR

This paper presents a new all-fiber mode matching technique for fiber-based Fabry-Pérot cavities, significantly improving coupling efficiency and versatility for various applications.

## Contribution

The authors introduce a novel fiber assembly combining graded-index and multimode fibers to enhance mode matching in FFPCs, achieving 90% efficiency at long cavity lengths.

## Key findings

- Achieved 90% mode matching efficiency for a 400 μm cavity.
- Significantly improved mode matching compared to conventional methods.
- Versatile approach adaptable to various single mode fibers.

## Abstract

In fiber based Fabry-P\'{e}rot Cavities (FFPCs), limited spatial mode matching between the cavity mode and input/output modes has been the main hindrance for many applications. We have demonstrated a versatile mode matching method for FFPCs. Our novel design employs an assembly of a graded-index and large core multimode fiber directly spliced to a single mode fiber. This all-fiber assembly transforms the propagating mode of the single mode fiber to match with the mode of a FFPC. As a result, we have measured a mode matching of 90\% for a cavity length of $\sim$400 $\mu m$. This is a significant improvement compared to conventional FFPCs coupled with just a single mode fiber, especially at long cavity lengths. Adjusting the parameters of the assembly, the fundamental cavity mode can be matched with the mode of almost any single mode fiber, making this approach highly versatile and integrable.