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Vector finite element optimisation of compact spot-size converters in photonics integrated circuits

Ladele, E. O. (2003). Vector finite element optimisation of compact spot-size converters in photonics integrated circuits. (Unpublished Doctoral thesis, City, University of London)


It is well known that one of the major problems of integrated optical systems is the efficient coupling of photonic devices such as semiconductor lasers, amplifiers, modulators, or switches to a single mode fiber (SMF) in such a way that little or no power loss occurs. A well confined beam is needed in order to optimize the performance of a wide range of these photonic devices, because up to 90% of the optical power can be lost due to a large mismatch between their small non-circular spot-size and a SMF with a larger and circular spot-size when they are butt-coupled. Over the last 10 years several attempts have been made to close the gap and reduce such a high loss when coupling a photonic integrated circuit (PIC) to SMF. Among these is the use of a microlens or lensed fiber to enhance the coupling efficiency. However, the disadvantage of this approach is that associated sub-micron alignment tolerances lead to very high packaging costs. For a small business network, such a large cost is preventing the rapid extension of fiber-to-the-home (FTTH).

This makes the problem of optical coupling a big challenge to optoelectronics researchers worldwide as huge efforts were made to expand the narrow spot-size within a PIC, such that efficient coupling to a SMF with a large spot-size can be made. Monolithically integrated spot-size converters (SSCs) have been reported recently as being used to enhance optical coupling without deteriorating alignment tolerances and majority of the expanded SSCs do incorporate tapered structures, operating very close to the modal cut-off, to expand their spot-size.

In this thesis, some compact SSC designs have been carried out using the twin rib (TR), multimode interference (MMI) and silicon-on-insulator (SOI) waveguides to improve the coupling efficiency. The TR and SOI do require a tapered section in their mode of operation to expand the spot-size whereas the MMI does not need a tapered section.

Some numerical techniques have been employed in this thesis as tools in the design, analysis and optimization of the above guided-wave photonics devices. The robust, versatile and accurate full-vector finite element method (FVFEM) is the backbone of all the numerical techniques, as it has been used to obtain the modal solutions of the waveguide sections of the photonic devices throughout this thesis.

The FVFEM has been used in conjunction with the Least squares boundary residual (LSBR) method in the novel compact design, analysis and optimisation of 3-Core multimode waveguide as a device for improving power coupling efficiency. The transmission and reflection coefficients of the guided-waves are obtained as well. In a similar manner, the FVFEM is also used in conjunction with the finite element-based full-vector beam propagation method (FVBPM) to study the propagation of the guided-waves along the longitudinal z-direction of tapered devices for the TR and SOI waveguides. In the analysis, the propagating power, the radiation loss and the spot-size are obtained for these PICs. Tapered spot-size converters, with various high- index SOI waveguides, which consists of secondary polymeric cover, are investigated in this work. Mode beating phenomenon was observed and explained. Also the An characterisation SOI was carried in this work because of the high-index contrast of the SOI materials which is a vital information for any design Engineer since the operations depend heavily on the materials as well as the geometry of the device.

The robust PML boundary conditions have been used to stem down unwanted radiations during propagation and the Pade approximation has been employed to take care of the waves propagating at wide angles to the z-axis. The incorporated popular overlap integral (OI) has been used in the determination of the coupling efficiency of the devices, which in the case of TR is 95%, and SOI is 99.25%.

Publication Type: Thesis (Doctoral)
Subjects: Q Science > QA Mathematics
T Technology > TA Engineering (General). Civil engineering (General)
Departments: School of Science & Technology > Engineering
School of Science & Technology > School of Science & Technology Doctoral Theses
Doctoral Theses
[thumbnail of Ladele thesis 2006 PDF-A.pdf]
Text - Accepted Version
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