Narrow linewidth external cavity diode lasers: combining integrated photonics and frequency locking techniques
Creators: name: Palmer, Sonya
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 400908
subject: Microelectronics
classificationCode: 400909
subject: Photonic and electro-optical devices, sensors and systems (excl. communications)

description: Over half a century ago the invention of the atomic clock revolutionized the concept of keeping time, by tying this measurement to the fundamental property of the vibrations of a caesium atom. This level of accuracy in timekeeping has been an essential part of the development of modern-day technologies. Satellite navigation systems like GPS, network synchronisation across the internet and timestamping transactions for financial institutions, all rely on the precise and accurate nature of atomic clocks. The best most accurate clocks are those hosted by large government and scientific institutions, as they require significant amounts of money, physical space, and specialised expertise to maintain. However, limitations in the current technology, for example in the GPS network difficulty to maintain accuracy in dense urban environments, and the push for future applications, such as GPS independent on-board navigation systems for autonomous vehicles, has spurred new interest in overhauling our current atomic clock timekeeping systems. Specifically, these applications are driving the need for compact, portable, and robust field deployable clock devices, something that cannot be achieved with the current standard, laboratory-based, atomic clock architecture. Additionally, one must achieve these aims, without sacrificing the accuracy and precision of the clock and if possible, improving it. For improvements in timekeeping accuracy scientists and researchers are already working with atomic clocks in the optical rather microwave spectrum, as the higher frequency vibrations of optical atomic clocks inherently offer greater fidelity.

One of the biggest aspects underpinning optical atomic clocks and the ability to make them smaller, without compromising accuracy, is the consideration of compact frequency stable and spectrally narrow (narrow linewidth) lasers, operating at the frequencies of atomic clock transitions. Traditionally the mainstay of technology has been external cavity diode lasers, typically further stabilised in active frequency locking schemes. However, as these schemes are comprised of bulk optical components, they do not readily lend themselves to miniaturisation or field deployable robustness. This thesis considers and demonstrates how lasers for optical atomic clocks could benefit from an integrated photonic approach. Where integrated photonic components, such as micro-ring resonators, are used instead of bulk optical cavities, to achieve the same lasing requirements. 

The first two chapters, position this research within the greater context and introduces background material on optical atomic clocks and semiconductor lasers which established the specific requirements for compact, robust, accurate timekeeping. Chapter 3 severs as an investigation into how photonic integrated chip fabrication technology can be used to improve existing bulk external cavity diode lasers. A chip-scale modulator incorporated into a laser cavity is demonstrated to provide sufficiently fast feedback in Pound-Drever-Hall locking to achieve a locked laser linewidth of 1 Hz. Critically this was achieved without overall increases to the size, weight and power, and using integrated photonic fabrication tool, providing a compelling first step towards the case for fully integrated devices. Chapter 4 investigates the design and fabrication of high-quality factor micro-ring resonators, for the purposes of on-chip narrow linewidth lasing. Both in-house and foundry approaches are considered and contrasted as to their benefits. Ultimately a low-confinement, high-aspect ratio foundry-based platform was able to yield rings with quality factors of 2.5 million for blue wavelengths and 27 million for near-IR red wavelengths. Chapter 5 then demonstrates how micro-rings fabricated in Chapter 4 can be used to produce a stabilised, single mode laser output, when self-injection locked to a Fabry-Perot diode. The linewidth of self-injection locked laser is reduced to the kHz level, matching the performance of a free-running commercial, bulk optical, external cavity diode laser. The results of Chapter 5 demonstrate how self-injection locking with integrated photonic micro-ring resonators will pave a path toward accurate, compact, robust, narrow linewidth lasers, for use in the next generation of optical atomic clocks.