Multipass Faraday rotators and isolators

TL;DR

The paper introduces a multipass method to enhance Faraday rotation in weak-effect materials, enabling compact optical isolators with improved material options across UV and IR ranges.

Contribution

It presents a novel multipass approach using Herriott-type cells to achieve required polarization rotation in weak Faraday effect materials, expanding the range of usable materials for optical isolators.

Findings

Achieved 45° polarization rotation in fused silica using multipass technique.

Demonstrated a compact proof-of-principle experiment at 532 nm.

Enabled potential for new optical isolator designs with weak Faraday materials.

Abstract

Faraday isolators are usually limited to Faraday materials with strong Verdet constants. We present a method to reach the 45{\deg} polarization rotation angle needed for optical isolators with materials exhibiting a weak Faraday effect. The Faraday effect is enhanced by passing the incident radiation multiple times through the Faraday medium while the rotation angle accumulates after each pass. Materials having excellent thermos-optical properties in the ultraviolet and mid-infrared range become available for optical isolators. Herriott-type multipass cells offer a simple and compact way to realize the desired propagation length in usual optical materials of standard sizes. A proof-of-principle experiment was carried out, demonstrating polarization rotation of a 532 nm laser beam by an angle of 45{\deg} in anti-reflection-coated fused silica surrounded by a standard neodymium ring magnet.