| dc.description.abstract | Over the past few decades, the world has experienced a revolution in information and communications technologies that has affected almost every aspect of modern life. The growth in internet capabilities and services has been immense and has led to new applications and economic activities. Today the relentless growth continues unabated. Every year the demand for global data traffic continues to grow at astonishing rates, often in excess of 25 % per annum. This has been made possible by advances in optical communications, allowing vast quantities of information to be carried as pulses of light along glass fibres. Such fibres allow multiple channels of information, carried by distinct wavelengths, to be transmitted simultaneously, a technology called wavelength division multiplexing (WDM). This technology allows massive parallelism along communications links, greatly reducing the cost of communications. Initially, optical technologies were limited to long haul links, but, as the demand for data exhausts the capabilities of electrical communications, optical technologies are finding applications in every part of the network. As optical technologies expand into lower value parts of the network low-cost optical components will need to be developed.
Semiconductor lasers are the crucial component for optical communications. Over the past number of years, the Semiconductor Photonics Group in the School of Physics at Trinity College Dublin have developed high-order surfaced etched slotted grating lasers. Such devices allow high performance single-frequency lasing while using a simpler fabrication procedure than is required in conventional low-order buried grating devices, potentially leading to lower cost devices.
Semiconductor lasers are highly sensitive to temperature, with the emission wavelength of InP based devices drifting at a rate of about 0.1 nm ℃-1. As WDM channel spacings can be as low as 0.1 nm, such drift would be catastrophic for these systems and temperature control to well within 1 ℃ is necessary. Conventionally, the laser temperature is controlled with thermoelectric coolers (TEC), which are power hungry devices, especially at elevated temperatures, and can consume several times the power required by the laser. As WDM technologies expand into more parts of the network, TEC power consumption will become problematic. Our group has been working towards techniques to remove TECs from the laser package without compromising the wavelength stability, a process known as athermalisation.
This thesis focuses on the thermal characterisation and modelling, the grating design and athermalisation of single-frequency slotted semiconductor lasers. Firstly, the spatial distribution of heat generation in these devices is investigated with the use of CCD-Thermoreflectance imaging, providing high resolution thermal images of the laser surface. A thermal model is developed, allowing determination of the temperature internal to the laser and showing good agreement with experimental measurements. In the second part, a simple technique is developed to athermalise the wavelength of a single-frequency slotted laser without the use of a TEC. With the use of an insulating submount the wavelength of a high temperature quantum dot distributed feedback (DFB) laser is continuously athermalised over a 74 ℃ temperature range using only 0.6 W of power. Good single mode performance with SMSR above 45 dB is observed throughout the athermal range. In the third part, a survey of the design space for high-order slotted gratings is investigated with scattering matrix method simulations, and a number of promising designs are identified. The tuning of two-section slotted lasers is discussed and an automated technique for athermalisation of a two-section laser is presented. | en |
