An efficient, compact, and versatile fiber double scrambler for high precision radial velocity instruments

TL;DR

This paper introduces a compact, efficient fiber double-scrambler that enhances radial velocity measurement precision by stabilizing the spectrograph profile and reducing sensitivity to input variations, suitable for high-resolution astronomical instruments.

Contribution

The paper presents a novel fiber double-scrambler design using a high-index ball lens, achieving high scrambling gains and high efficiency with simplified alignment, outperforming previous systems.

Findings

Achieved scrambling gains over 10,000 with 87% throughput.

Further increased scrambling gain to over 20,000 with additional fiber.

Demonstrated effectiveness for high-precision radial velocity instruments.

Abstract

We present the design and test results of a compact optical fiber double-scrambler for high-resolution Doppler radial velocity instruments. This device consists of a single optic: a high-index $n$$\sim$2 ball lens that exchanges the near and far fields between two fibers. When used in conjunction with octagonal fibers, this device yields very high scrambling gains and greatly desensitizes the fiber output from any input illumination variations, thereby stabilizing the instrument profile of the spectrograph and improving the Doppler measurement precision. The system is also highly insensitive to input pupil variations, isolating the spectrograph from telescope illumination variations and seeing changes. By selecting the appropriate glass and lens diameter the highest efficiency is achieved when the fibers are practically in contact with the lens surface, greatly simplifying the alignment process when compared to classical double-scrambler systems. This prototype double-scrambler has demonstrated significant performance gains over previous systems, achieving scrambling gains in excess of 10,000 with a throughput of $\sim$87% using uncoated Polymicro octagonal fibers. Adding a circular fiber to the fiber train further increases the scrambling gain to $>$20,000, limited by laboratory measurement error. While this fiber system is designed for the Habitable-zone Planet Finder spectrograph, it is more generally applicable to other instruments in the visible and near-infrared. Given the simplicity and low cost, this fiber scrambler could also easily be multiplexed for large multi-object instruments.