By means of numerical simulations, the thesis aims at improvements in the understanding of light propagation in
dielectric optical waveguides, with emphasis on nonreciprocal integrated magnetooptic devices. The results include:
Proposal, implementation, and assessment of the WMM mode solver (Wave Matching Method)
For waveguides with piecewise constant, rectangular permittivity profiles, the calculation of guided modes can be based
on a local expansion into factorizing harmonic or exponential trial functions. A least squares expression for the
mismatch in the continuity conditions at dielectric boundaries connects the fields on neighbouring regions. Minimization
of this error allows to compute propagation constants and mode fields. The procedure has been implemented both for
semivectorial and fully vectorial mode analysis. The piecewise defined trial fields are well suited to deal with field
discontinuities or discontinuous derivatives. Numerical assessment shows excellent agreement with accepted previous
results from other methods. The WMM turns out to be effective especially for structures described by only a few
rectangles. It yields semianalytical mode field representations which are not restricted to a computational window. The
fields are therefore perfectly suited for further processing, e.g. in the framework of various kinds of perturbation theory.
Perturbational geometry tolerancing procedure
Shifting the location of a dielectric boundary in the cross section of a waveguide with piecewise constant refractive
index profile results in a permittivity perturbation in a layer along the discontinuity line. On the basis of these thin layer
perturbations, perturbational expressions for the derivatives of the propagation constants with respect to geometry
parameters are discussed. The approach provides direct access to wavelength dependences. Comparison with
rigorously calculated data shows that the accuracy is sufficient to yield reasonable tolerance estimates for realistic
integrated optical devices, at almost no extra computational cost. This perturbational approach allows to establish and
to quantify guidelines for geometry tolerant devices.
Numerical assessment of nonreciprocal wave propagation
The coefficients of coupled mode theory for the magnetooptic permittivity contribution allow a classification of the
influences of gyrotropy on guided wave propagation. For mirror symmetric waveguides, one identifies the dominant
effects of TE phase shift, TM phase shift, and TE/TM polarization conversion, for polar, equatorial, and longitudinal
magnetooptic configurations, respectively. Layered equatorial magnetooptic profiles lead to the well known phase
shifters for TM modes. Analogously, sliced asymmetric polar magnetooptic profiles yield phase shifts for TE polarized
modes. Simulations of rib waveguides with a magnetooptic domain lattice predict effects of the same order of magnitude
as the phase shift for TM modes. Phase matching as a condition for complete polarization conversion in longitudinally
magnetized waveguides can be realized with selected geometries of raised strip waveguides or embedded square
waveguides. Based on coupled mode theory for hybrid fundamental modes, the analysis of the performance of such
devices in an isolator setting includes birefringence, optical absorption, and an explicit perturbational evaluation of
fabrication tolerances. A magnetooptic waveguide which is magnetized at a tilted angle may perform as a unidirectional
polarization converter. The term specifies a device that converts TE to TM light for one direction of propagation, while it
maintains the polarization for the opposite direction. A double layer setup with two magnetooptic films of opposite
Faraday rotation is proposed and simulated.
Designs of three waveguide couplers for applications as isolators/circulators and polarization splitters
Three-guide couplers with multimode central waveguides allow for a remote coupling between the outer waveguides.
While the power transfer is a truly multimode interference process, one can identify two different regimes where either
two or three supermodes dominate the coupling behaviour. Numerical simulations show reasonable agreement between
the main coupling features in planar an three dimensional devices. The specific form of the relevant modes suggests the
design of integrated optical isolators and circulators. Both planar and three dimensional concepts are investigated. A
radiatively coupled waveguide polarization splitter should be designed such that the entire dynamic range of the coupling
length variations is exploited. This is easily possible with a three dimensional raised strip configuration. Combination of
two magnetooptic unidirectional polarization converters and two radiatively coupled waveguide based polarization
splitters leads to a concept for a polarization independent integrated four port circulator device. The simulation predicts
a total length of about three millimeters.
| Identifer | oai:union.ndltd.org:uni-osnabrueck.de/oai:repositorium.ub.uni-osnabrueck.de:urn:nbn:de:gbv:700-2000090888 |
| Date | 08 September 2000 |
| Creators | Lohmeyer, Manfred |
| Contributors | Prof. Dr. Peter Hertel, Prof. Dr. Horst Dötsch |
| Source Sets | Universität Osnabrück |
| Language | English |
| Detected Language | English |
| Type | doc-type:doctoralThesis |
| Format | application/zip, application/gzip |
| Rights | http://rightsstatements.org/vocab/InC/1.0/ |
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