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Integrated Optical Frequency Comb Generation For Photonic Terahertz Synthesis

Tough, Euan John; (2024) Integrated Optical Frequency Comb Generation For Photonic Terahertz Synthesis. Doctoral thesis (Ph.D), UCL (University College London). Green open access

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Abstract

The research described in this thesis addresses the design and characterisation of an optical frequency comb generator (OFCG) for the purpose of photonic terahertz (THz) synthesis. Using optical heterodyning techniques, filtered comb lines separated by THz frequencies can be mixed on an appropriately fast uni-travelling carrier (UTC) photodiode to emit THz radiation. Implementation of this technique with an inherently phase coherent OFCG enables coherent THz synthesis to be achieved with high spectral purity. This work explored a novel realisation of photonically integrated OFCG fabricated on an indium phosphide (InP) photonic integrated circuit (PIC) using generic foundry fabrication methods. PICs enable the miniaturisation of complex photonic systems to a form-factor compatible with portable and mobile applications. The OFCG architecture, based on an amplified recirculating loop, consists of cascaded electro-optic phase modulators embedded within a waveguide loop with length corresponding to the comb-line frequency spacing. Through recirculation, the modulation is enhanced to generate many phase-coherent, evenly spaced optical comb lines. The InP integration platform allows immediate optical amplification of the modulated signal by embedded semiconductor optical amplifiers, enabling expansion of the comb across broad optical bandwidths by compensating for loop losses. The performance of the integrated OFCG is assessed with a photonic circuit model, justifying design choices of the fabricated device, identifying optimal resonant drive criteria and highlighting limitations in the system. Following this, supplementary electronic and RF interfaces are modelled and designed to maximise the delivery of various drive signals to the PIC and optimise comb generation. The performance of fabricated devices operating at 6.6 GHz and 10 GHz round-trip loop frequencies are characterised by comb span and phase noise properties, achieving promising comb bandwidths and phase coherence for photonic THz synthesis. Finally, phase noise characteristics of filtered comb lines are measured at increasing separations, culminating in the generation of a 100 GHz low-THz signal using a packaged W-band UTC, indicating high spectral purity with phase noise below -90 dBc/Hz at 10 kHz offset frequency.

Type: Thesis (Doctoral)
Qualification: Ph.D
Title: Integrated Optical Frequency Comb Generation For Photonic Terahertz Synthesis
Open access status: An open access version is available from UCL Discovery
Language: English
Additional information: Copyright © The Author 2023. Original content in this thesis is licensed under the terms of the Creative Commons Attribution-NonCommercial 4.0 International (CC BY-NC 4.0) Licence (https://creativecommons.org/licenses/by-nc/4.0/). Any third-party copyright material present remains the property of its respective owner(s) and is licensed under its existing terms. Access may initially be restricted at the author’s request.
Keywords: Photonics, Integrated Photonics, Optical Frequency Comb Generator, Terahertz, Photonic Terahertz Synthesis
UCL classification: UCL
UCL > Provost and Vice Provost Offices > UCL BEAMS
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Engineering Science
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Engineering Science > Dept of Electronic and Electrical Eng
URI: https://discovery.ucl.ac.uk/id/eprint/10186591
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