Enhanced free space beam capture by improved optical tapers

TL;DR

This paper introduces an improved optical taper design that significantly enhances beam capture stability in free space optical systems, especially for quantum key distribution, by increasing angular tolerance without sacrificing bandwidth.

Contribution

The authors develop a novel optical taper geometry that offers up to four times greater angular tolerance compared to traditional lenses, enabling more stable free space optical communication.

Findings

Optical tapers can increase photodiode effective area without bandwidth loss.

The new design improves spatial stability in free space QKD systems.

Enhanced tolerance reduces beam-jitter effects in atmospheric conditions.

Abstract

In our continuous variable quantum key distribution (QKD) scheme, the homodyne detection set-up requires balancing the intensity of an incident beam between two photodiodes. Realistic lens systems are insufficient to provide a spatially stable focus in the presence of large spatial beam-jitter caused by atmospheric transmission. We therefore present an improved geometry for optical tapers which offer up to four times the angular tolerance of a lens. The effective area of a photodiode can thus be increased, without decreasing its bandwidth. This makes them suitable for use in our free space QKD experiment and in free space optical communication in general.