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Computer modelling of electro-optic modulators

Anwar, N. (2005). Computer modelling of electro-optic modulators. (Unpublished Doctoral thesis, City, University of London)


An electro-optic modulator is an essential part of an optical communications system. An ideal modulator would have low drive voltage, large bandwidth, small size, low insertion loss, a very high extinction ratio, very low chirp and low power consumption. However, there is a trade-off between some of these characteristics, and consequently the modulator is designed with a compromise. It is due to these complex devices that computer modelling has become a necessary and integral part of device design, which is made easier and more economically acceptable by the larger capacities of today’s computers.

In this work, a finite element method-based approach is used to design and develop computer programs in order to simulate the major characteristics of electro-optic modulators (i.e. low drive voltage, a large bandwidth, small size, low insertion loss) and fabrication variables and combine these separate computer programs into one complete suite to model electro-optic waveguide modulators for a good and complete design process for these devices. The devices examined in this work are the TkLiNbCfi directional coupler-based modulator, the Ti:LiNb03 Mach-Zehnder interferometric modulator and a GaAs/AlGaAs interferometric-based electro-optic modulator.

These computer programs give the optical mode for each of the devices examined in this work both before and after application of an electric field, giving exact values for the refractive and any change in refractive index with applied electric field. In this way it is the drive voltage and size of the device that can be adjusted and examined in order to obtain an optimum design. The electrode design is important to modulators particularly bandwidth but is also essential for decreasing optical loss without increasing the drive voltage. A program was designed and developed using a perturbational approach in order to model this optical loss. For the TiiLiNbCfi directional coupler based modulator, the light coupling process between the two adjacent waveguides as well as the power transfer efficiency were also modelled using a FEM LSBR-based approach. However, the directional coupler based devices are slower than their MZI-based counterparts. A TkLiNbCfi MZI-based modulator was also studied with both channel and ridge waveguides. It was found that a ridge waveguide is optimally beneficial only if the FWHM (width and height) of the mode fit into the ridge. The bending waveguides at the input and output of the Ti:TiNb03 MZI-based modulator were modelled using a FEM BPM-based approach. This approach yields accurate results for structures where the cross-section of the device is changing. A computer program was also developed to incorporate the RF characteristics of the device in order to find the bandwidth of the device for perfect velocity and impedance matching conditions. Furthermore, smaller studies were undertaken for a ThLiNbCfi MZI-based modulator for electrical sensing applications as well as for a GaAs/AlGaAs interferomtric-based modulator.

The main work of this thesis was to improve these device structures in order to provide optimal results. The suite of computer programs yielded results very close and similar to that examined in the literature.

Most importantly, the computer modelling approach to device design yields accurate results for both simple and more complex devices structures and is a cheaper long-term approach to device design.

Publication Type: Thesis (Doctoral)
Subjects: Q Science > QA Mathematics > QA75 Electronic computers. Computer science
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 Anwar thesis 2005 PDF-A.pdf]
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