Daniel Thomas Cassidy
Professor Emeritus, Engineering Physics

Devices based on III-V materials such as AlGaAs and InGaAsP semiconductor diode lasers are used in optical communication systems. Many of the operating characteristics of the devices are not well understood even though the devices are used extensively in the telecommunications industry. It is the object of the research programme to study the physics and applications of III-V materials and devices and to use the accumulated knowledge and experience to explain the operating characteristics and to find new applications.

Much of the work uses two technologies that were and are being developed in the research group: degree of polarization (DOP) of luminescence and short-external-cavity (SXC) modules. These technologies enhance the ability of the research group to make contributions.

With the DOP technique, the strain fields owing to, e.g., growth, processing, metallisation, die bonding, and single dislocations can be mapped quickly. There are few, if any, research groups that have this capability. Thus, the effects of strain on device performance can be determined and means to improve device performance suggested.

The luminescence from quantum wells is highly polarized. We have shown that the DOP of luminescence from quantum wells is a sensitive measure of the physical characteristics of the quantum well. Thus quantum wells can be characterized by the DOP of luminescence and the information gained by the characterization may be used to understand aspects of growth and performance of quantum well devices.

With the SXC technology, the spectral output of diode lasers can be controlled (i.e., the laser can be forced to run single mode on any of the modes near the gain peak). This allows for investigation of the wavelength dependence of ,e.g., the modulation bandwidth of diode lasers, the line width of lasers, and the nonlinear gain parameter. SXC lasers can also be used to construct sensors and to perform spectroscopy. The sensor work typically involves development of a system including source, optics, modulation/demodulation scheme, detection, signal processing electronics and algorithm, as necessary and application of the system to a relevant problem. We use a custom designed diffractive optical element as the feedback element for the external cavity lasers. The spectral properties of devices has been a continuing interest and recently has led to work on gain-coupled DFB lasers.

We design, fabricate, characterize and use lasers with broad gain peaks for operation over extended spectral intervals. The work on design, fabrication, and understanding of broad gain peak lasers is interesting for the physics of operation that is obtained and for the development of sources.