Juergen Schrage

Analysis of transverse mode competition of VCSELs based on a spatially independent model

A spatially independent model of vertical-cavity surface-emitting lasers (VCSELs) is built up in this paper by integrating the spatially dependent rate equations over the cross section of the active region of a VCSEL. The transverse modal structure is presented and discussed. This model is simulated numerically with the fourth-order Runge-Kutta method in a self-consistent way. Different kinds of injection currents such as a disk-contact and a ring-contact of injection current with different current parameters are employed in the investigations. The dependencies of the transverse mode competition on the current intensity, the current spread, and the geometrical parameters of the injection contact forms are thoroughly investigated and analyzed. The results are useful to design and control the transverse modal profile of a VCSEL.

Time domain simulation of optical multimode chip-to-chip interconnects

To increase the bandwidth of high-performance chip-to-chip interconnects optical on-board interconnects can be used. Since the design procedure of such optical interconnects has to be widely compatible with current computer aided board design processes, adequate simulation methods are required. In this paper an efficient and design process compatible method for simulating the transmission behavior of optical multimode chip-to-chip interconnects is presented. The approach is based on a time domain description where an optical multimode waveguide is represented by a multiport. The different transfer paths between the input- and output ports describe the transmission behavior of the entire waveguide. The transmission behavior of each individual path can be characterized by its step response, which can be computed by the aid of an extended ray tracing method. Due to some fundamental properties of these step responses, its piecewise approximation by simple exponential functions is possible. As a consequence the pulse responses of each transfer path can be determined analytically and they are also approximated by exponential functions. Finally this procedure enables the application of a semi-analytic recursive convolution method for the computation of the waveguide transmission behavior. The simulation procedure is illustrated and discussed by a set of examples.

Approach to model optical multimode interconnects for time-domain simulation

Next generation high-speed pc board interconnects will be based on integrated optical multimode waveguides with cross- sectional sizes comparable to those of electrical microstrip lines. The design of such interconnects requires appropriate simulation models of the multimode waveguides and the laser- and photo-diodes, as well. Since single-mode interconnects can be modeled very efficiently by well known numerical methods such as FEM and BPM, these methods are not applicable for optical multimode waveguides with more than 1000 propagating modes. Due to the numerical complexity only methods based on geometrical optics, called ray tracing, can be applied efficiently. This paper deals with a time domain modeling and simulation approach for analyzing the signal behavior of multimode waveguide based electrical-optical interconnection systems as a whole. Modeling of multimode waveguide components as well as macromodeling of laser- and photo-diodes is explained in detail. The modeling approaches are discussed by examples.

Optic-tactile robotics and medical applications

This paper gives an overview of new developed hardware and software framework for fiber-optical constructional sensory applied to robotics and medical applications. Different sensor types are shown and used together on an robotic arm and are also applied to human spine movement detection. The sensory bases on fiber-optical ribbons. After presenting the different sensors and both application areas the overall system is shown beginning with real-time measurement, processing and calibration, visualization and analysis of the movements. The control loop is closed to the actuators of the robot application. The correlation between different sensor segments will be mapped onto a Petri-net topology to handle interactions. The software suite which puts all parts together will be outlined afterwards.

Coupled Mode Analysis of Power Transport and Loss in Highly Multimodal Tapered Dielectric Waveguides for Coupling Applications

Power transport through the local guided modes and power loss by reflection and radiation in highly multimodal tapered dielectric waveguides is analyzed by a simplified coupled mode theory. Comparison with geometrical optics shows a very good agreement for long tapers. Application of tapered highly multimodal channel waveguides in PCBs is discussed.

Influence of surface roughness on the bandwidth of optical multimode waveguides analyzed by modal noise theory

Modal noise is an undesired modulation of the guided light intensity in a multimode waveguide. Applying the frequency correlation function the frequency dependence of this noise as well as the bandwidth of a multimode waveguide can be estimated. In this paper the existing model of the frequency correlation function for a waveguide with smoothed dielectric interfaces is enhanced to analyze the influence of surface roughness on the achievable bandwidth. This surface roughness is caused by the manufacturing process of the waveguides.

Self-Consistent Model and Numerical Analysis of VCSEL's Laser Diodes

A self-consistent model of vertical-cavity surface-emitting lasers (VCSELs) is presented in this paper, in which the carrier diffusion rate equation, the photon density rate equation, and the thermal conduction equation are considered simultaneously. The nonuniform heat flux density distribution in the active region due to the current-spreading effect is taken into account. The effects of temperature on gain and transparency carrier density are also included in this model. It is nonlinearly and self-consistently solved in the Matlab environment. The transient and lateral distribution characteristics of the carrier and photon densities and the junction temperature are investigated. Some interesting results are gotten and analyzed.

Evaluation of graded index glass waveguides for board-level WDM optical chip-to-chip communications

A Proof-of-Concept for a multi-channel WDM board-level optical communications link is under development. This paper is focusing on theoretical and experimental evaluation of thin-glass based nearly single mode graded index optical waveguides with regard to low loss in the 1310nm regime. Results from waveguide characterization will be reported. Waveguide modes are determined theoretically from the measured refractive index profiles. Towards improvement of the robustness of the coupling efficiency against misalignments, investigations on the use of tapered waveguide structures will be presented too.

Coupling behaviour of tapered highly multimodal dielectric waveguides as part of PCB-level optical interconnects

Optical interconnects replace electrical links increasingly at shorter distances. At printed circuit board (PCB) level highly multimodal polymer channel waveguides are the chosen approach to meet bandwidth-length and bandwidth-density requirements. One important challenge of board integrated waveguides is the coupling problem. The manufacturing process of PCBs leads to relatively high placement tolerances which cause poor optical coupling efficiency due to mechanical misalignment between separate components, e.g.: 1) Coupling between a VCSEL and the board integrated waveguides; 2) Coupling between waveguides in two separate boards. This paper deals with the deployment of tapered dielectric multimode waveguides for increasing the optical coupling robustness towards mechanical misalignments in these two coupling applications. A coupled mode approach for calculation of the mode coupling and power loss in a taper with decreasing width has been presented before [5]. In [6], the two above mentioned coupling applications for tapered dielectric waveguides have been dealt with, but only the coupling efficiency in case of longitudinal misalignment has been calculated. In this paper, results of advanced analysis of the two applications are presented. The coupling efficiency in case of transverse misalignment is simulated by a ray-optical approach. Furthermore the results of measurements of the coupling behaviour of board integrated tapered waveguides are presented. The results show that tapered multimodal dielectric waveguides have the capability to increase the coupling efficiency significantly if some conditions are fulfilled.

Numerical Investigation of the Effects of the Injection Current on the SHB-Effects of VCSEL's

A spatially independent model of vertical-cavity surface-emitting lasers (VCSELs) is built up in this paper by integrating the spatially dependent rate equations over the cross section of the active reigon of VCSELs. The LP cosine modes in a weakly guiding cylindrical waveguide are used to represent the transverse modes in the VCSELs cavity. This model is performed numerically with the 4th order Runge-Kutta method in a self-consistent way. The dynamic and steady state characteristics of the multi-transverse mode competitions are investigated in detail. Different kinds of injection currents such as a disk-contact and ring-contact injection current are employed in these numerically implementations. Some interesting results are obtained and analyzed detailedly.