Few-MHz bandwidth tunable optical filter based on a fiber-ring resonator

TL;DR

This paper introduces a fiber-ring resonator-based optical filter with tunable, ultra-narrow bandwidth below 2 MHz, high extinction, and multiple operational modes, suitable for precise frequency selection.

Contribution

It presents a novel fiber-ring resonator design using commercial couplers enabling tunable, multi-mode optical filtering with sub-natural linewidth resolution.

Findings

Bandwidth less than 2 MHz

Extinction exceeds 20 dB

Multiple operational modes

Abstract

We present a fiber-ring resonator that realizes an ultra-narrowband, high-extinction, low-loss, tunable optical filter. It consists of a pair of commercial variable ratio directional couplers that allow precise adjustment of the filter bandwidth and its on-resonance transmission. This design also grants access to multiple modes of operation, such as a simultaneous band-stop and band-pass filter. Our characterization reveals a bandwidth of less than 2 MHz, together with an extinction exceeding 20 dB. The tunability of the filter properties establishes our device as a versatile platform for selective frequency filtering with sub-natural atomic linewidth resolution.