Integrated linear diffraction grating spectrometer

Thumbnail Image
Hnatiuk, Heather Joanne
Journal Title
Journal ISSN
Volume Title
The ever-increasing required capacity of telecommunications networks is driving research into new technology that can increase the network capacity with the lowest possible cost. One of the newest and most promising technologies is wavelength division multiplexing (WDM). This technology uses the concept of simultaneously transmitting many signals at different wavelengths down a single optical fiber. Optical signal transmission at four wavelengths, for example, quadruples the capacity of the current fiber infrastructure. The system, however, cannot be realized without a device that separates the signals at the receiver; i.e. a demultiplexer. Demultiplexers may use many different technologies (interference filter, or bulk grating for example). This thesis focuses on an integrated diffraction grating design. In this type of device, diffraction gratings are etched into slab waveguide layers. The incident light is guided hrough the slab and is reflected and diffracted by the grating. The diffraction process spatially separates the channels that were transmitted at different wavelengths. The goal of this thesis is to design, characterize and numerically model a novel integrated diffraction grating spectrometer. This design is to be used as a comparison to the performance of similar grating spectrometers using a different grating configuration. The guidelines used for design parameters of the spectrometer are defined by the concerns of the telecommunications industry. The device should have low insertion loss, low polarization dependent performance, high channel isolation, and use industry-defined channel spacings (and channel wavelengths). The designed integrated grating spectrometer uses a linear grating, to improve grating facet uniformity. The grating facets were designed to achieve large dispersion with a small device size. A concave mirror was used to collimate and focus the light. The spectrometer was fabricated and tested. It showed good channel spacing uniformity and output intensity profile shape. The insertion loss and polarization dependent performance were not sufficient for a practical telecommunications product however these problems may be solved with slightly different fabrication techniques. Simulations of the behaviour of the grating facets were done using finite-difference time-domain (FDTD) techniques. These results showed that the facet shape efficiently retro-reflected the incident light.