New computational method to estimate the effect of roughness on scattering loss and its implementation in a hybrid heterojunction optical modulator

Abstract

In the past year, there has been an advancement in the development of optical active and passive Silicon-On-Insulator for Photonic Integrated Circuits (PICs) applications. Following the continuous miniaturisation trend in this technology the study of the loss performance of these devices also become an attractive subject. Among the many components silicon based optical modulators are specifically important for low loss and high bandwidth short reach interconnects. The general aim of this thesis is to design and computationally investigate the performance of a multilayer hybrid silicon modulator. The design involves a heterojunction structure that operates based on plasma dispersion effect taking the effect of surface roughness and scattering loss into account. For this purpose a novel numerical approach is developed to estimate scattering loss due to the roughness in general waveguide structures and also the proposed silicon based optical waveguide modulator in this work. The applicability can be however extended towards wider range of optical waveguide based devices including multilayer configurations. The method is based on 2D Fourier transform technique that is widely used in Magneto Resonance Imaging technique.

Firstly, the effect of three forms of roughness is investigated in a general strip waveguide structure; isotropic, anisotropic and the mixture of isotropic and anisotropic. In each case the generated sidewall roughness is implemented in various SOI high contrast refractive index waveguides. The waveguide dimensions have been chosen to cover a large variety of waveguide sizes to evaluate the accuracy of the modelling technique. Two SOI waveguide samples have been fabricated. The first sample is 100nm, 1200nm and 1400nm in width and 500nm in height which are single mode in Mid Infra Red (MIR) region. The second sample contains waveguide with 220nm height and 330nm widths that is single mode in Near Infra Read (NIR) wavelength. Other dimensions are chosen from several published works. The calculated losses using FDTD show good agreement with all measured fabricated waveguide and the referred experimental works in the literature. The three dimensional model successfully explains the scattering loss dependence on the width of a high aspect ratio waveguide when the result is compared with a published work. While the measurement shows the loss reduces from 32dB/cm to 0.8dB/cm, the simulation results varies from 44dB/cm to 1dB/cm. The interdependence of scattering loss is also investigated against other theoretical approaches when the correlation length varies from 0 to 1000nm. The relatively low aspect ratio waveguide is chosen to have a fixed dimension of 220nm by 330nm and 5nm value.

In order to study the scattering loss caused by roughness the roughness model is applied in the proposed modulator structure. The design involves a multimode strip like p doped silicon material wrapped by transparent ZnO as a naturally n doped active material. This forms a pn heterojunction that is implemented in one arm of a Mach Zehnder interferometer. The switch is designed to operate at 1.55µm and in depletion mode to avoid minority carrier life time effect in switching speed. The calculated capacitance switching speed of a pulse were less than 1 pf=cm and 90ps respectively. The resistivity v is higher compared to a general form of heterojunction due to the relativity larger ZnO/Si contact area of the device. The phase shifter is implemented in Mach Zehnder structure to change from phase modulation to amplitude modulation using a MMI structure. The calculated extinction ratio was as high as 23.7dB with the insertion loss of 2.5dB. Further simulation results shows that the 100nm change in the ZnO thickness can alter the effective index of refraction and loss performance of the devices. In an ideal situation, as the thickness increases from 50nm to 150nm the loss changes from 2 to 8dB/cm for TE mode. The involvement of sidewall roughness results higher insertion loss by at least by 0.2dB when the rms of the sidewall roughness increases by 7nm. As the ZnO coating thickness increases, the roughness effect is counterbalanced by almost 50%.

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Identifiers

ORCID for Harold Chong: orcid.org/0000-0002-7110-5761

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