Novel architectures and fabrication techniques for distributed feedback lasers for communication

Salmond, Ben 2025. Novel architectures and fabrication techniques for distributed feedback lasers for communication. PhD Thesis, Cardiff University. Item availability restricted.

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Abstract

This thesis describes the development of laterally coupled (LC) distributed feedback (DFB) lasers on InP-based material. The first part of this work demonstrates the design, fabrication and measurement of high-order LC-DFBs using I-line photolithography. Measurements of devices revealed the successful demonstration of single-mode lasing, with a side mode suppression ratio (SMSR) of 25 dB observed. Some lateral mode instability, however, was noted as a result of broadening of the central ridge portion of the grating structures. The second part of this work was concerned with the comparison of varying the grating order LC-DFB laser diodes on both commercially grown quantum well (QW) material and InAs/InAlGaAs quantum dot (QD) material grown using metal organic chemical vapour deposition (MOCVD). A comparison of 3rd and 11th order LCDFB devices revealed higher spectral purity in the high-order grating structures, achieving side mode suppression values exceeding 50 dB. 11th order devices were also compared on the two material systems. Sub-threshold gain measurements using the Cassidy method suggested that the QD material had an optical gain width almost 20nm broader than the QW material. Spectral measurements of both devices also exhibited a significantly wider region of DFB mode operation for the QD device, by virtue of the broadened gain spectrum in the QD material system. The final portion of this work investigated the use of direct milling of 1st order grating structures using bismuth and gold focused ion beams (FIBs). Two types of structures were investigated, one where the gratings were milled directly on the ridge waveguide (RWG) and one where they were milled adjacent to the RWG. Various milling strategies were examined and it was found that off-ridge grating structures were simpler to define using this technique, with the concentric scanning routine offering the most desirable grating structures. Modelling work predicted that the fabricated gratings would provide coupling strength values exceeding 70 cm−1 with a milled depth of 76 nm. This would be sufficient to achieve kL = 2 for cavity lengths as low as 300 μm.

Item Type:	Thesis (PhD)
Date Type:	Completion
Status:	Unpublished
Schools:	Schools > Physics and Astronomy
Subjects:	Q Science > QC Physics
Uncontrolled Keywords:	semiconductor laser diodes, distributed feedback lasers, indium phosphide device processing, quantum dot lasers
Funders:	EPSRC
Date of First Compliant Deposit:	18 August 2025
Last Modified:	18 Aug 2025 14:17
URI:	https://orca.cardiff.ac.uk/id/eprint/180472

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