Thermally insensitive fibre delay-line interferometer operating at room temperature

Abstract

Due to the unique properties of fibre interferometers, such as low cost, easy alignment, and light weight, they have become popular in scientific research. However, optical fibre is susceptible to environmental noise, including pressure perturbations, acoustic vibrations, and temperature variations. In some specialized applications, such as frequency references, optical fibre sensing, and metrology, a stable fibre interferometer is strongly required. Various methods have been proposed to reduce fibre interferometers’ sensitivities, especially the sensitivity to temperature. These methods generally have not achieved zero thermal sensitivity at room temperature and sometimes require cryogenic environment. The motivation behind this project is to build a thermally insensitive fibre interferometer that operates at room temperature without the need for a vacuum environment or an expensive thermal control system. In this thesis, a compensated delay-line fibre interferometer structure is proposed to achieve zero thermal sensitivity. One arm is constructed with a long length of hollow-core fibre (HCF) with low thermal sensitivity to provide optical delay, while the other arm is made of a short standard single-mode fibre (SSMF) with high thermal sensitivity to compensate for the thermally induced phase accumulation in the HCF arm. Simulation results in COMSOL with different winding patterns show that the thermal stability of this design is 1000 times better than that of SMF-based interferometers. A prototype of the compensated Mach-Zehnder fibre interferometer is fabricated and characterized. The results demonstrate that the coating of HCF significantly affects the thermal response. By employing thinly-coated HCF and single mode fibre (SMF), the first generation of the thermally insensitive fibre interferometer is fabricated and characterized. It crosses zero sensitivity at 28°C, and the zero-crossing temperature can be tuned by adjusting the length of SMF in the compensation arm. Furthermore, it exhibits 1000 times better thermal stability than SSMF within a ±1°C range. Unfortunately, the thinly-coated HCF was found to suffer from random breaks when coiled over extended period of time. The second generation of the interferometer benefited from mechanically stable HCF with thicker coating, effect of which was offset by using larger silica glass jacket. The long-term frequency instability (∆f/f) of this thermally insensitive interferometer reaches 10-12 at averaging time of 1000 seconds. Additionally, by reducing the Fresnel reflections at the SSMF-HCF interfaces, the Mach-Zehnder interferometer was upgraded to a Michelson interferometer with Faraday rotator mirrors (FRM), which is polarization insensitive and provide double the delay length. This represented the third compensated interferometer generation. Lastly, the application of laser stabilization to the thermally insensitive fibre interferometer was demonstrated, showing over an order-of-magnitude improvement in the long-term stability of the laser frequency when compared to a free-running laser. we believe this low-cost and high-performance thermally insensitive fibre interferometer will be of interest in various scientific research applications.

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Identifiers

ORCID for Bo Shi: orcid.org/0000-0003-0895-3052

ORCID for Radan Slavik: orcid.org/0000-0002-9336-4262

ORCID for David Richardson: orcid.org/0000-0002-7751-1058

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