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

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

Rate-flexible optical CDMA networks based on coherent modulation formats

In this paper, we propose, design, and assess the performance of optical coherent access networks based on optical code-division multiple-access (OCDMA) based on three distinct coherent modulation formats, namely, BPSK, QPSK, and 16-QAM. We design a flexible network architecture based on these coherent modulation formats to provide either single data rate transmissions or multiple data rate transmissions. In this way, the network supports users with the same transmission rate, or group of users transmitting at different data rates. We derive new close-form analytical expressions for both single and multiple rate use cases to assess the performance of the coherent OCDMA network. Results shown that the multi-rate data traffic from the users can coexist almost transparently in terms of network performance. The flexible OCDMA network might be a prospective alternative for implementing coherent systems for different service provision according to traffic demand requirements while supporting with high capacity, security, and simultaneous numbers of subscribers. Keywords: BPSK, QPSK, 16-QAM, coherent network, BER, OCDM

Multi-channel SPR biosensor based on PCF for multi-analyte sensing applications

This paper presents a theoretical investigation of a novel holey fiber (Photonic Crystal Fiber (PCF)) multi-channel biosensor based on surface plasmon resonance (SPR). The large gold coated micro fluidic channels and elliptical air hole design of our proposed biosensor aided by a high refractive index over layer in two channels enables operation in two modes; multi analyte sensing and self-referencing mode. Loss spectra, dispersion and detection capability of our proposed biosensor for the two fundamental modes (HE x 11 and HE y 11 ) have been elucidated using a Finite Element Method (FEM) and Perfectly Matching Layers (PML)

Design of Environmental Biosensor Based on Photonic Crystal Fiber with Bends Using Finite Element Method

Copyright © 2015 The Author(s). In this paper, a biosensor based on photonic crystal fiber (PCF) is proposed and designed using Full-Vectorial Finite Element Method (FVFEM). The proposed PCF sensor consists of three concentric circles surrounding the core. The key optical sensor characteristics such as sensitivity, the field profiles and real part of the refractive index of the proposed PCF structure are investigated by employing the FVFEM. The proposed sensor can be deployed for environmental sensing when the PCF active region is filled with either analytes such as liquids or gas. By careful selection of the design parameters such as the radius of the sensing circle, the diameter of air holes in the core region and hole to hole spacing, Λ, the sensitivity analytes is determined. Our simulation results show that, the electric field distribution is primary localized in the third concentric circle with a radius of 16 μm. Effects of PCF bending on the sensitivity is also studied and reported

Comparative analysis of long-haul system based on SSB modulation utilising dual parallel Mach–Zehnder modulators

In this paper, we have proposed a long-haul optical transmission system, based on a single sideband (SSB) modulation scheme. Analytical and simulation models have been developed, optimised and demonstrated for the proposed SSB system configurations. The SSB modulation scheme was proposed to overcome dispersion in the fibre. We have shown that the related link losses can be minimized by increasing the quality of the optical signal at the modulation. We have optimised the radio over fibre configuration scheme based on dual parallel dual drive Mach–Zehnder Modulator, thereby increasing transmission length of the fibre. With the proposed SSB, by suppressing some of the harmonics and cancelling one of the sidebands, we have halved the RF power fading and interference. The developed analytical (theoretical/mathematical) model agrees very well with the simulation results using two (both) different commercial simulation tools. The optical signal is boosted while minimizing the number of repeaters. We report a SSB configuration, compensation and amplification with individual spans of 150 km, by extending the length of the link up to 3250 km. The proposed system configuration exhibits high performance with less complexity and lower cost

Pulse oximetry optical sensor using oxygen-bound haemoglobin

In this paper we report a unique approach to measuring oxygen saturation levels by utilising the wavelength of the haemoglobin instead of the conventional absorption difference. Two experiments are set up to measure the wavelength of the haemoglobin bound to oxygen at different oxygen saturation levels with the help of a spectrometer. We report a unique low cost and robust wavelength monitoring SpO2 sensor that measures the SpO2 by using the colour of the blood and not the absorption difference of oxyhaemoglobin and deoxyhaemoglobin. With use of a spectrometer, we show that the wavelength of the oxygen-bound haemoglobin has a relation to the oxygen saturation level. The proposed device is designed and experimentally implemented with a colour sensor to measure the SpO2 level of the blood

Improved design for SOI based evanescently coupled multilayer spot-size converter

We report an improved version of a spot-size converter (SSC) consisting of a silicon nanowire evanescently coupled to a phase-matched Poly-Si multilayer structure. With wider transversal dimensions the multilayer structure expands the mode significantly thus increasing the coupling efficiency with the conventional single-mode fiber. Detailed optimization process of a 17-layer based SSC is discussed and its coupling efficiency with a high-NA fiber of radius 2 μm is obtained as 98% providing only 0.087 dB loss. Vertical alignment tolerance between the optimized SSC and a high-NA fiber of radius 2 μm is also shown. This novel design does not consist of a taper and can be fabricated by using CMOS compatible process. It has a short device length and more relaxed alignment tolerances with the fiber. Full-vectorial and computationally efficient finite element method and the least squares boundary residual method have been used for the analysis and optimization of the proposed structure