Microfabrication of waveguide-based devices for quantum optics

Abstract

Quantum optics and photonics play a key part of the field of quantum technology. For these technologies to be of practical use, scale must be realised in both complexity and manufacture. To this end new and improved components are required. The development of these new components is the core of this thesis, with a specific focus on the integration of Bragg gratings into silica waveguide devices.
The process used to fabricate these structures, small-spot direct UV writing, is detailed and modelled to build a better understanding of the platform. The small-spot direct UV writing technique is applied to the fabrication of fibre Bragg gratings and proven to be a highly capable technique with great flexibility. This new approach to the fabrication of fibre Bragg gratings is shown to be able to, in one process, write in excess of fifty gratings in a single length of fibre. Such grating structures can yield large data sets for detailed statically analysis. This has allowed for thorough characterisation of the writing technique which showed good agreement with the modelling. Using this characterisation, filters for single photon sources are fabricated, achieving suppression in the stop band of over 50 dB. Other high performance grating devices were also fabricated, such as matched pairs of narrowband ultra-long gratings with 3 dB bandwidths of less than 50 pm. Though the silica waveguide platform offers low optical loss any loss critically degrades the performance of quantum systems. This work aimed to reduce the losses in the platform from coupling between optical fibre and direct UV written integrated waveguides. In order to understand the optical modes of both types of waveguides a bespoke, automated, beam profiling system was built. The system was then used to adjust the fabrication parameters of the integrated waveguides such that their modes matched the fibre. The resultant predicted coupling losses achieved were close to 0.1 dB. In order to directly measure the broadband coupling losses accurately a new technique was developed whereby Bragg gratings were written into both the fibre and the integrated waveguide. By comparing the response of the gratings before and after the coupling between waveguides, coupling loss were measured to an accuracy of ∼0.1 dB. Finally, the use Bragg gratings to enhance the efficiency of superconducting single-photon nanowire detectors was investigated. This was done by using the gratings to form of cavities around the detectors. Testing of these cavity structures showed a potential improvement in efficiency of between 5% and 50%.

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ORCID for Peter Smith: orcid.org/0000-0003-0319-718X

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