Nonlinear Modal Propagation Analysis Method For Multimode Interference Coupler Evaluation
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Date
2015-06
Authors
TAJALDINI, MEHDI
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Abstract
This thesis investigates a novel nonlinear modal propagation analysis (NMPA) method to evaluate a multimode interference (MMI) coupler, which is based on modal expansion, propagation, interferences, and interactions in a nonlinear regime, that uses in an optical power splitter, switch and gates. The presented investigation of modal propagation in a nonlinear regime demonstrates the appearance of self-phase modulation, cross-phase modulation, wave mixing and energy exchange between guided modes. Additionally, a conversion of the modal envelope from sinusoidal to Gaussian in the propagation direction is a significant effect that this method produces. The NMPA method is applied to a silicon-on-insulator (SOI) MMI as high refractive index waveguide to demonstrate the appearance of self-focusing phenomena in MMI waveguide. A compact nonlinear power splitter is proposed based on the NMPA method for the first time. This power splitter uses 11 parallel-fold images in its output channels that are on 104.64 μm of the MMI length. The device is optimized based on the insertion loss and the power imbalance (i.e., opposite of uniformity), which are critical parameters; then, simulations are performed based on the intensity distribution. The results show a 0.256 and 0.04 dB power imbalance and insertion loss, respectively. A compact optical switch is thus proposed based on the NMPA method. The desired length is calculated after an analysis of different lengths to be 80μm, and the switching performance gain (SPG) and the insertion loss are measured to be 6.9552 and 5.1321 dB in the bar state, and 6.1402 and 6.5292 dB in the cross state, respectively, which produced an optimum switch. Simulations show that the obtained switch is compact and efficient. The
proposed nonlinear modal propagation analysis also enables the introduction of an MMI coupler that can operate as any type of logic gate. A compact 3×3 MMI coupler designed for use with the NMPA method is proposed as a conventional single-structure MMI to function as any logic gate. This novel design can potentially be applied in optical logic gates for Boolean logic signal processing devices. Logic gate operations are optimized via the output width, where the contrast ratio between the ON and OFF states determines the efficiency of the optical gates, while the minimum and maximum contrast ratios belong to XNOR and NAND gates and are 22 dB and
12 dB, respectively.
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Keywords
Nonlinear Modal Propagation Analysis Method , For Multimode Interference Coupler Evaluation