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Optimization of ultra-high speed electrooptic modulators using the finite element method

Haxha, S. (2004). Optimization of ultra-high speed electrooptic modulators using the finite element method. (Unpublished Doctoral thesis, City, University of London)


Ultra-high-speed optical modulators are amongst the most promising and useful components in optical communications systems. External ultra-high-speed optical modulators of multi-gigahertz bandwidth with a high optical power handling capacity are key components in current optical communications systems and valuable for future optical signal processing technology. In recent years, optical fibre communication networks have been experiencing a very rapid development, driven by the explosive growth of internet technology, mobile phones, video phones, video conferencing, video- on-demand, and e-commerce. Therefore, it is essential to create advanced tools to design new ultra-high-speed optical modulators which fulfil the requirements of such high capacity transmission systems.

The recent advances in light wave technology indicate a crucial need for the accurate design, characterization and optimization of modem optoelectronic devices such as electroptical modulators, using rigorous and efficient computational modelling methods. The finite element method is the most versatile and popular numerical approach for the solution of various engineering problems. The vector H- field finite element method provides the most accurate and efficient computational numerical technique for the analysis of different uniform optical waveguide problems involving isotropic, anisotropic, and nonlinear waveguide materials.

A numerical approach using quasi-TEM analysis based on the efficient finite element method is developed to investigate the microwave properties of the electrooptic modulators, LiNbCL and GaAs, respectively. The combination of the vector //-field finite element method and the Least Square Boundary Residual (LSBR) has been employed successfully to investigate the polarization conversion phenomenon in a
deeply-etched GaAs/AlGaAs semiconductor electrooptic modulator has been investigated and physically justified.

The potential distribution, capacitance calculation, electric and magnetic field distribution, microwave index, characteristic impedance, optical field confinement, conductor loss and the dielectric loss for different regions of the substrate, half-wave voltage length, bandwidth, driving power and optical loss are investigated thoroughly. Simultaneous matching between the microwave effective index and the optical and the characteristic impedance are achieved. The work shows for the first time, that conductor loss, dielectric loss and the mismatch between optical and microwave carrier and characteristic impedance for both LN and GaAs electrooptic modulators operating beyond 40 GHz, will play a significant role in the determination of the overall speed of these modulators.

The effect of various imperfect fabrication parameters of GaAs electrooptic modulators has been thoroughly investigated. It is confirmed that the use of full vectorial simulation techniques such as the FEM (Finite Element Method) and LSBR (Least-Square Boundary Residual) are very important in order to account for, and to avoid problems arising as a result of such unexpected and also unwanted polarization conversion effects in electrooptic semiconductor modulators. It has been confirmed for the first time that for the new semiconductor electrooptic modulator designs and fabrication methods, it is essential to take in account these parameters in order to avoid unwanted polarization conversion, which can negatively impact the performance of the device.

Publication Type: Thesis (Doctoral)
Subjects: T Technology > TJ Mechanical engineering and machinery
T Technology > TK Electrical engineering. Electronics Nuclear engineering
Departments: School of Science & Technology > Engineering > Electrical & Electronic Engineering
School of Science & Technology > School of Science & Technology Doctoral Theses
Doctoral Theses
[thumbnail of Haxha thesis 2004 PDF-A.pdf]
Text - Accepted Version
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