The Design, Characterisation and Athermalisation of Vernier Widely Tunable Semiconductor Lasers

Abstract

The early 21st century has seen a surge in demand for communications technologies, particularly with the internet having become a near essential utility in recent years. The rise of high-resolution video streaming, among other online services exacerbated by the COVID-19 pandemic, has lead to an ever-increasing demand for high-speed internet at the consumer level. This can be facilitated by implementing optical communications technologies such as wavelength division multiplexing (WDM) in local networks, to replace the existing lower bandwidth electrical architecture. However, widescale roll-out presents a challenge in reducing the overall cost and complexity of optical components in the deployed systems. Tunable laser diodes play the key role of optical transmitters in such networks, where they can supply a configurable optical signal to suit the network needs. In addition, conventional systems make use of arrays of unique devices operating at different centre wavelengths in order to cover the desired optical bandwidth. However, widely tunable lasers such as the sampled-grating distributed Bragg reflector (SG-DBR), can lower the spatial footprint of the system by being able to cover a full optical band from a single laser, but at the cost of operational complexity. As a result, efficient and effective characterisation becomes a crucial element in their successful operation. Present research by the Trinity College Dublin Semiconductor Photonics Group has been into designing high-order surface-etched slotted grating lasers, which offer a simpler fabrication over low-order conventional buried-grating designs. Such devices can be monolithically integrated with other photonic elements as part of photonic integrated circuits (PICs). A widely tunable Vernier laser has also been developed, making use of a pair of very high-order surface gratings and an operational principle similar to the SG-DBR, capable of quasi-continuous tuning over 60 nm in the C-band with a side mode suppression ratio (SMSR) >30 dB. Wavelength tuning is achieved via current-induced self-heating into three independent tuning sections. The work presented in this thesis aims to gain a deeper understanding of this three-current characterisation space. An analytical mode mapping technique based on the 2D scattering matrix method (SMM) is presented, capable of rapidly emulating experimental tuning behaviour that is otherwise very time-consuming to measure. Additionally, the existing designs are further developed by introducing sampled surface gratings to the originally single period structures. These new designs are produced with the aid of a genetic algorithm optimisation technique. A significant challenge in the stable operation of these lasers is their inherent temperature sensitivity. Changes in ambient temperature, whether from self-heating or external sources, can produce a substantial detuning effect of around 0.1 nm/K in indium phosphide (InP) based devices. This demands the usage of a thermoelectric cooler (TEC) to monitor and control ambient temperature conditions. However, conventional TECs such as Peltier modules are relatively power inefficient and limit the lifetimes of laser packages, providing a motivation to achieve TEC-free operation as these laser packages get deployed on a wider scale. Ongoing research by our group is into developing control schemes to stabilise the laser emission in response to ambient temperature feedback, a process known as athermalisation. By manually adjusting the tuning currents in response to ambient temperature changes, athermalisation is demonstrated in multiple channels across the C-band using the Vernier laser. A wavelength stability of ?0.004 nm (?0.5 GHz) and SMSR >30 dB could be achieved over a temperature range of 20-70 ?C, through quasi-continuous tuning across multiple discrete modes. A method of automating this process is also proposed. By implementing a mode-segmentation technique based on the watershed algorithm, the boundaries of single-mode operation could be identified and tracked, allowing for fast, targeted wavelength characterisation in the three-current space. A preliminary measurement using this technique demonstrated a stability of ?0.03 nm (?3.75 GHz) and SMSR >35 dB over 20-32 ?C, while traversing between modes.