# Transmission matrix of a multimode fiber: In-line vs off-axis holography

## TL;DR

This paper compares two holography techniques for measuring light transmission in multimode fibers, showing each has strengths for different imaging tasks.

## Contribution

The study experimentally compares in-line and off-axis holography for transmission matrix measurement in multimode fibers.

## Key findings

- In-line holography provides better focus quality for single-mode targets.
- Off-axis holography is more effective for complex-shaped target projections.
- A trade-off exists between the two techniques for wavefront shaping.

## Abstract

Multimode fibers (MMFs) hold great promise for a variety of imaging applications. However, controlling the complex propagation of light through thousands of modes in MMFs remains a challenging task. To enable precise wavefront control, the MMF transmission matrix (TM) must be accurately measured. In this paper, we compare in-line and off-axis holography as phase retrieval techniques for TM measurement within a single optical setup. The quality of wavefront shaping is analyzed for different target shapes, including single-mode and complex free-form projections. Our findings reveal a trade-off in wavefront shaping: while in-line holography achieves superior focus quality for single-mode targets, off-axis holography excels in projecting complex-shaped targets. These experimental findings provide a basis for choosing and optimizing holographic techniques in MMF-based imaging setups.