Hollow core fibre based Fabry-Perot interferometers with high finesse

Abstract

A typical Fabry-Perot interferometer (FP) is formed by enclosing an optical path with two highly reflective mirrors, forcing light to travel many times in between these two mirrors. This gives rise to many resonant peaks in the transmission spectrum, making the FP element useful in many applications such as sensing and stabilised lasers. In many of these applications, the performance improves when the transmission peaks spectral width is reduced. To achieve this, FP needs to have long optical length or highly-reflective mirrors. Currently, there are two main FPs implementations. The first one is based on free-space light propagation in which the distance between the mirrors (and thus the FP optical lengths) are usually limited to less than 50 cm. To achieve narrow transmission spectral peak width, this requires extremely high mirrors reflectivity and associated finesse (>10⁵ ), which makes such FPs highly sensitive to alignment. An alternative FP implementation uses single-mode optical fibre (SMF-FPs). SMF-FPs can have long lengths (e.g., 100’s of meters) and can be very compact and lightweight. Although the finesse of SMF-FPs does not reach that achievable in free-space FPs (due to the fibre transmission loss), their long length enables achieving narrow transmission peaks with similar to that achieved in the high-finesse free-space FPs. Unfortunately, SMF-FPs have several drawbacks that make free-space FPs the preferred approach for many applications, despite free-space FPs larger size and the alignment challenges. The two primary drawbacks are the large sensitivity to temperature variations and unwanted nonlinear effects like stimulated Brillion scattering. The nonlinear effect can be relatively prominent in FPs, where the intra-cavity power is strongly enhanced by the resonant effect. Both of these parasitic effects are mainly due to the interaction of light with the silica glass material in SMFs. For example, the thermal sensitivity of optical length is dominated (95%) by thermally-induced changes in the refractive index of silica glass (thermooptic effect) with the thermally-induced fibre length (thermal expansion effect) change providing the other 5%. As light in hollow core fibres (HCFs) travels in air, the unwanted light-glass interaction observed in SMFs is strongly supressed, making HCFs ideal medium for FPs. However, up to date, high finesse HCF based FPs (HCF-FPs) were studied in the literature only with very short HCFs lengths (several cm), limiting their use for many applications. Besides this, all reports have dealt with open (unsealed) HCFs with no discussion on their long-term applications, which is expected to be limited due to HCF degradation when opened to atmosphere. Finally, the performance limitation of HCFFPs, e.g., what is the maximum achievable finesse, have not been studied yet. In this Thesis, I focus on long length and high finesse HCF-FPs and their applications. Firstly, we developed a new method based on an incoherent source and RF spectrum analysis to characterise long-length FPs. The method characterises the beating signal of the incoherent optical comb obtained by transmitting incoherent light through the tested FP. It enabled us to monitor finesse and free spectral range during alignment of HCF-FPs, including measurement of long-length FPs. Secondly, we built HCF-FPs with free-space coupling. We characterized several HCF-FPs of different length to explore experimentally finesse limits. The results showed that the coupling loss between the forward and backward propagating light in HCFs reflected off the mirror can be as low as 0.0028 dB. In turn this enables a finesse of up to 5 000. Experimentally, we I achieved a finesse in excess of 2 500 (limited by the available mirrors), which is a value 20 times larger than reported before for HCF-FP. Thirdly, we fabricated two all-fibred HCF-FPs in collaboration with our colleagues in the Czech Technical University in Prague. The two components had length of 5 m and 23 m, respectively, and finesse in excess of 120 over the entire C band. This represented the first demonstration of all fibred long (>1 m) HCF-FPs with such a high finesse. We have tested the 23 m HCF-FP almost 3 years later after it was fabricated, with no observable degradation in its performance. This shows the first HCF-FP that can be used in long term applications. Subsequently, we demonstrated two applications based on the fabricated HCF-FPs. The first one uses the HCF-FP to characterise HCF attenuation. The other uses the all-fibred HCF-FP for microwave photonics filtering. Finally, we further improve already-low HCF’s thermal stability by proposing and demonstrating a new method. It is based on coiling the HCF on a temperature insensitive spool. In our proof-of-principle demonstration, we achieved HCF thermal sensitivity reduction 3 times, achieving thermal sensitivity as low as 0.13 ppm/^oC. This up to date demonstrates the most thermally stable fibre FP at room temperature.

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

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