1 Department of Photonics Engineering, Technical University of Denmark2 Department of Micro- and Nanotechnology, Technical University of Denmark
This work concerns modeling and chracterization of non ampligying silica-on-silicon optical components for wavelength division mulitplexed networks. Emphasis is placed on optical couplers and how they can be used as building blocks for devices with a larger complexity. It has been investigated how to construct wavelength flattened and process tolerant couplers. A thorough comparison between directional couplers, multi mode interference couplers and interferometer-based couplers has been performed. Numerically all these architectures have the ability to obtain similar wavelength-flatness, but the multi mode interference couplers have superior proces tolerance. The measured characteristics of mulit mode interference couplers deviate from the simulations, showing an unexpected imbalance and large polarization sensitivity. This can be explained by a sligthly non-uniform index distribution across the multi mode interference slab. Accordingly the process tolerance of multi mode interference couplers is inferior to what has previously been assumed. The directional couplers show better agreement between simulations and measurements, but a qualitative comparison shows that a rectangular step-index model is not an adequate description of the waveguides. A simple application for an optical couplers is as a 980/1550 nm mulitmplexer for erbium doped wavguide amplifiers. A numerical analysis shows that a directional coupler has acceptable specifications, whereas a mulit mode interference coupler does not. The wavelength flatness can be improved by suing a new point-symmetric interferometer structure. The proposed multiplexer fulfills commercial specifications over a wavelength range from 1500 nm to 1600 nm. Furthermore the spectral and dispersion response of a 1x4 interferometer based multiplexer/demultiplexer with single-stage all-pass filters in the arms is modeled. The all-pass filters ensure that the demultiplexer has excellent transmission, but add a substantial, detrimental cubic dispersion over the channel passband. This limits the number of demultiplexers that can be cascaded without incurring a significant power penalty. The dispersion can be removed by adding a three-stage all-pass filter on the input arm. The above mentioned silica-on-silicon components have been fabricated using a conventional method where the waveguides are defined and fabricated using cleanroom processing. Waveguides can also be fabricated by an alternative ultraviolet-writing method. It is shown experimentally that it is possible to describe the waveguiding properties of ultraviolet-written waveguides by a rectangular step-index profile. The model facilitates development of new ultraviolet-written components.