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  • Open Access

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

Gabriele Maron, Anton Bölian, Xin-Xin Hu, Luke Masters, Arno Rauschenbeutel*, and Jürgen Volz†

  • *Contact author: arno.rauschenbeutel@hu-berlin.de
  • †Contact author: juergen.volz@hu-berlin.de
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Phys. Rev. Applied 26, 024029 – Published 12 August, 2026

DOI: https://doi.org/10.1103/yw1q-f6f4

Abstract

We present a fiber-ring resonator that realizes an ultranarrowband, 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.

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Phys. Rev. Applied 26, 024029– Published 12 August, 2026
  • Received 25 October 2025
  • Revised 28 April 2026
  • Accepted 10 July 2026
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