Lu, Ying; (2023) Development of high-performance, cost-effective quantum dot lasers for data-centre and Si photonics applications. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Photonic technologies have been considered new methods to achieve high bandwidth data communication and transmission. Si-photonics was proposed to address the discrepancy between bulky photonic devices and advanced electronics and create high-density integrated photonics. One of the challenges is integrating all the components necessary for full-functionality photonic integrated circuits (PIC). Great efforts have been devoted to overcoming the inherent limitations of Group-IV materials to provide sufficient gain, efficient modulation and sensitive detections. Making Si the host material for efficient light emission poses the most stringent requirements and is the primary missing component in the Si-photonics platform. Incorporating III-V materials with the Si photonics platform and quantum dot (QD) structure is a promising solution to the problem of a fully-integrated and high-functioning PIC. High-performance QD lasers on III-V substrate or epitaxially on silicon have been developed in the last few decades with low threshold current density, low-temperature sensitivity, great reliability and large injection efficiency. Moreover, from the dynamic aspect, the intrinsic frequency of direct modulated laser and noise intensity is important for its applications in a data centre. QD is considered an alternative to quantum wells (QWs); however, the demonstrated QD laser has not fulfilled initial expectations, mainly due to its high gain compression and low differential gain. Another feature that needs to be noticed is feedback sensitivity, as the properties of semiconductor lasers are greatly degraded by reflection from external reflectors, such as the fibre connects and facets of integrated devices. QD devices are predicted to have stronger feedback resistance due to their large damping and small linewidth enhancement factor (LEF). These properties have attracted much research, and high-performance QD devices have been developed. In this thesis, we comprehensively investigated QD laser performance and applied our QD laser in the optical module instead of the commercial QW distributed feedback (DFB) laser. The background of Si photonics, the development of QD devices, and the fundamentals of QD lasers are presented in Chapter 1. The basic static and dynamic performances are demonstrated in Chapters 2 and 3. The GaAs-based QD laser provides a low threshold, high-temperature stability, and low noise operation with a limited small signal bandwidth. Chapter 4 provides a comprehensive study of the feedback resistance of the QD laser. The onset of coherence collapse is determined as -14 dB, verified by the static optical and electrical spectra and small signal response. Based on previous measurements, the QD laser is proven to be a high-performance, low-cost candidate for the Si-photonics module. In Chapter 5, the QD laser is used in practical applications, including a large signal transmission system with and without feedback and a commercial optical module. Although the intrinsic bandwidth of the QD laser is limited to around 5GHz due to the large damping and unoptimised capacitance, 30 Gbps data transmission has been demonstrated by a directly modulated QD laser. Large, high-speed signal modulation is achieved due to its high gain compression factor. Regarding the laser with intentional feedback, there is little degradation in the eye diagram under the whole feedback level up to -8dB. We also replaced the commercial QW DFB laser in 100G data-centre reach (DR)-1 optical module with our QD Fabry Perot (FP) laser without an isolator which gives a clear eye diagram under 53 Gbps 4-level pulse amplitude modulation (PAM4) with an extinction ratio (ER) of 4.7 dB. In conclusion, this thesis verifies the feasibility of adopting the QD laser as a light source for the Si-photonics module. The QD laser is selected over other lasers because of its low threshold, high-temperature stability and maximum operating temperature, and strong tolerance to unintentional feedback. This is the first project to measure critical feedback levels with different characteristics and to theoretically analyse the inconsistent value. More importantly, this thesis’ most original contribution is investigating the commercial applications of QD lasers in a Si-photonics module in an isolator-free state. In summary, the QD laser has been proven to be a feasible solution for the next-generation optical system.

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