Compact and versatile caesium thermal photonic-cells based on microstructured hollow core fibres for frequency and time references

Abstract

The recent developments in fabrication of hollow core Photonic crystal fibres (HC-PCFs), especially Anti-resonant fibres (ARFs), made this class of fibre very attractive for gas-phase applications, which ultimately led to the concept of photonic reference cell. This device is a fully-connectorised, stand-alone, HC-PCF filled with a gas specimen. Its application spans from instrument calibration, quantum optics to high-resolution spectroscopy and metrology.

For time and frequency applications, the use of alkali metals with narrow atomic features (in the kHz range) is a requirement for ultra-stable reference signals. However, the coherence time of atoms is strongly dependent on the vapour dynamics and photonic cell geometry. This dependency limits the use of alkali vapour in miniaturised reference cells, such as photonic cells, because of the short coherence time that leads to atomic features in the MHz range, for reference cells with cross-section of 100 μm, operating at room temperature. The diffusion of the vapour can be tailored by adding a mixture of inert gas (buffer gas), with resulting atomic features between kHz and sub-kHz range. This is a very well-known method widely used in standard reference cells.

This thesis reports on the advances in HC-PCFs technology for time and frequency reference applications. Firstly, I present a method to interconnect solid core fibres and HC-PCFs with dissimilar core sizes by employing a mode field adapter (MFA) to mediate the joint. I model and demonstrate the use of such MFAs for a photonic cell, fully-connectorised to SMFs. Secondly, I demonstrate for the first time spectroscopy of coherent population trapping (CPT) dark resonance in Kagome-lattice ARFs filled with Cs. CPT is a quantum effect very useful in frequency reference applications as it can provide an indirect measurement of the Cs clock transition (~9.2 GHz). I model and demonstrate world record line-width narrowing of CPT dark resonance in HC-PCFs and micro-metre scale capillaries filled with thermal Cs-vapour, with experimental observation of line-widths below 50 kHz for a Kagome-lattice ARF with a hollow core diameter of 125 μm.

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Identifiers

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

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