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Über dieses Buch

This book is subdivided into three main Parts. The common spirit in these parts is to provide, at the beginning of each, a comprehensive introduction into the subject treated, followed by specific aspects pertaining to the modelling and/or measuring particularities arlsmg from the investigation of photonic devices for telecommunications. Some of the devices treated here can be considered as widely known and well established. Others are rather new and their potential for applications is not yet fully exploited. The methods to model and measure photonic in this book and the comparison of results obtained devices and structures outlined by applying such methods are likely to interest both the engineer investigating the of a device in a system and the engineer looking for new ways to explore behaviour the possibilities offered by emerging devices. Many authors have contributed to this book. There are two main reasons for this. in photonic device research, modelling First, the book addresses two broad fields and measurements, for which a vast knowledge exists in many research groups that was not integrated in a book before. Second, a significant number of laboratories decided to closely co-operate in order to gain additional information on merits and drawbacks of their own methods for simulation and experimentation of devices as compared to the methods used by their colleagues in other laboratories. The outcome are new aspects and approaches that would not have been investigated in the absence of a framework for a co-operative programme.



Photonic Waveguide Structures


1. Mode solvers and related methods

To determine propagation constants and spatial optical field distributions of eigenmodes of integrated optical waveguide structures is a fundamental problem of their modelling. The knowledge of (generally complex) propagation constants, or the phase and attenuation constants, and spatial field distributions of guided modes of the waveguide structure helps make important decisions, e. g., whether the waveguide can be used for a particular application. In particular, the field distribution shows whether a waveguide is prone to radiation loss due to irregularities introduced in the fabrication process. Bends of homogeneous waveguides can be analysed by eigenmode solvers; the spatial field distribution of modes furnishes a physical insight into the radiation loss.
R. Pregla, A. Sudbø, Ph. Sewell

2. Beam propagation methods

In the research of optical devices and circuits, a principal theoretical problem is to calculate how a lightwave propagates in an optical medium having an arbitrary refractive-index distribution. Optical waveguide devices are usually very long compared to their transversal dimensions: the ratio is typically of the order of a thousand. Therefore, a rigorous analysis is difficult or not possible at all.
R. Pregla, W. von Reden, H. J. W. M. Hoekstra, H. V. Baghdasaryan

3. Benchmark tests and modelling tasks

In this chapter we describe benchmark tests and modelling tasks to mutually compare the performance of various beam propagation methods and other software developed and/or currently in use in various laboratories. Two sets of tasks for testing the behaviour of beam propagation methods applied to both lossless and lossy (absorbing) waveguide structures are described in the next section. Theoretical background required to understand the latter task is presented in Section 3.2. Then, an important problem of modelling light propagation in waveguide tapers, both symmetric and asymmetric, is discussed in Section 3.3. Comparative modelling of an example of optical waveguide devices containing thin metal films that can support propagation of surface plasmon waves is described in Section 3.4. In the last section, results of truly bi-directional modelling of light propagation in a very deeply etched Bragg waveguide grating filter are presented. The work described in this chapter was organised within the framework of the Action COST 240.
H.-P. Nolting, J. Haes, S. Helfert

4. Methods for waveguide characterisation

Several methods have been devised and used over the years for the determination of waveguide parameters; a few are related to, or derived from, techniques used for optical fibre characterisation, but many are significantly different from their fibre counterparts, or have no counterpart at all in the field of fibres. This difference comes from several reasons, namely from the wide variety of optical materials, refractive index and index differences, dispersion, geometrical shape and symmetry properties, and fabrication techniques which are commonplace in the field of integrated optics. This is in contrast with the far more circumscribed range for fibres, which exhibit (nearly perfect) cylindrical symmetry, negligible attenuation, small refractive index difference, well-known material properties and (obvious but very important) flexibility and availability in long lengths.
C. De Bernardi, A. Küng, O. Leminger

5. Comparison of experimental results

Round robins represent excellent opportunities to compare results obtained by several variations of the same measuring method, and in particular to compare the performances of competing measuring methods. In fact, it is the same measured item which circulates in partners’ laboratories, who afterwards meet to discuss the results and try to find the reasons for possible differences and refine experimental procedures. The field of possible measurements relating to waveguides and passive devices is far too vast for us to go into it thoroughly here. We have thus restricted ourselves to some measurements of a basic nature, connected for the most part with theories developed in previous chapters. Three chips have circulated. The first, made of InGaAsP/InP, comprising straight waveguides and integrated mirrors, was supplied by the Institute of Quantum Electronics of the Swiss Federal Institute of Technology in Zurich (ETHZ), the second, a glass chip with straight waveguides obtained by ion exchange, was supplied by CSELT (Centre Studi E Laboratori Telecomunicazioni S.P.A. Torino, Italy), the third, an InP/InGaAsP/InP heterostructure, by the Laboratory of Telecommunication and Remote Sensing Technology of The Delft University of Technology (TUD), The Netherlands.
Ph. Robert

Semiconductor Distributed Feedback Laser Diodes


6. Introductory physics

In this chapter it is attempted to give the reader an easy-to-follow introduction to the operation of single frequency laser diodes and their characteristics and to provide the basic background that is considered necessary to understand the following chapters on modelling and characterisation of DFB laser diodes. To this end, the structure of a laser diode is first described in section 2 and the fundamental physical laws governing the behaviour of the component are explained in section 3. The subsequent sections then introduce briefly the chapters 7 to 9 and deal with common problems encountered in laser modelling and characterisation (section 4) and general approximations used in laser diode modelling (section 5). Section 6, finally, summarises the work that was done on standardisation of laser parameters in the framework of COST 240.
G. Morthier, R. Baets

7. Modelling of DFB laser diodes

The behaviour of DFB lasers is so complex that it cannot be described analytically. Neither can the design of these lasers rely on a number of simple analytical formulae as is the case for FP lasers. The main cause of this complex behaviour is the strong dependence of the distributed feedback, and hence of the facet loss, on local refractive index and carrier density variations. The main consequences of it are changes in side mode rejection or stability and significant contributions to FMresponse and harmonic distortion, which are not easy to predict.
G. Morthier, A. Lowery

8. Measurements on DFB lasers

The measurement of the standard laser performance is generally assumed to be quite easy and to require no particular care. The case of semiconductor lasers is, however, more complicated. Under standard working conditions, semiconductor lasers have a material gain coefficient much higher than the other lasers and a dependence of the refractive index on the gain. These features, together with the low Q factor of a typical semiconductor-laser cavity make these devices very sensitive to external reflections. This reflects on an intrinsic difficulty in performing reliable measurements.
R. Paoletti, P. Spano

9. Parameter extraction

“In order to satisfactorily model a system, reliable parameters which describe the system must first be derived from measurement”.
R. Schatz, D. McDonald, H. Hillmer

Nonlinear Effects in Semiconductor Optical Amplifiers: Four-Wave Mixing


10. Why and how to study four-wave mixing?

Photonic networks are the backbone of todays global communication systems. In order to satisfy the ever growing demand for more bandwidth, the capacity of present days core networks has to be increased continuously. Optical fibres possess excellent transmission properties and serve to transmit high data capacity over long distances. In order to use the bandwidth of an optical fibre effectively, optical multiplexing techniques, namely wavelength division multiplexing (WDM) and time division multiplexing (TDM) techniques, are applied. However, wherever optical data are to be routed or switched, i. e. where the respective channels have to be converted in wavelength (WDMsystems) or to be (de)multiplexed in the temporal domain (TDM-systems) signal processors are needed. In fact, signal processing, which is currently mainly performed via the optic-electronic-optic pathway, is the bottleneck in the current high capacity communication networks. In order to overcome the disadvantages that are imposed by the limited speed of electronics, alloptical solutions are of interest. One method for high capacity all-optical data processing is four-wave mixing (FWM) in semiconductor optical amplifiers (SO As). FWM in SO As can be used for frequency conversion of optical data signals [1], dispersion compensation using mid-span spectral inversion [2], and time division demultiplexing of high bit rate optical data signals [3, 4]. Besides that, FWM can be used as spectroscopic tool to extract the dynamic SO A parameters [5].
S. Diez

11. Theory of four-wave mixing

The present chapter is devoted to presenting the theory of four-wave mixing in semiconductor optical amplifiers (SOAs). At first, a model describing the dynamics in terms of rate equations is developed. The rate equations are derived from density matrix equations, and after coupling with the propagation equation for the field envelope and its phase, a rather general description of the four-wave mixing is arrived at. In the limit of zero scattering loss, the model leads to a very simple description of the effects of four-wave mixing. In the succeeding sections we will concentrate on the advantages and draw-backs of some analytical models based on the coupled-mode theory and used to calculate the FWM performance of SOAs. In addition, the effects of amplified spontaneous emission (ASE) noise are considered. The theoretical models presented here should be of interest for all those performing experimental FWM investigations. Particular attention is given to the validity of the assumptions used in the models.
K. Obermann, A. Mecozzi, J. Mørk

12. Measurement techniques and results

Before measuring one has to answer a few important questions such as: Which set-up to use? What are the possible methods? One of the goals of this chapter is to help to answer these questions by showing and comparing measurements. FWM performance of SOAs has been measured with different set-ups and techniques in different laboratories.
F. Girardin, T. Ducellier, S. Diez

13. Related topics

As already pointed out in Ch. 10, studying Four-Wave Mixing (FWM) in Semiconductor Optical Amplifiers (SOAs) can be of interest for numerous reasons. In Ch. 11 and 12, FWM among continuous waves was treated both theoretically and experimentally. These investigations helped to develop a better understanding of the underlying physical processes involved in FWM. Moreover, it became possible to compare different techniques to measure the conversion efficiency as well as the signal-to-background ratio. From an application point of view, the presented investigations are of interest for frequency conversion and optical phase conjugation arrangements. However, there are many more aspects of FWM in SOAs that have not been covered in the previous chapters. Ch. 13 aims to serve as an introduction to some related topics involving theoretical and experimental FWM observations. The results that are presented in this chapter are not meant to cover all related aspects. Still, they were also obtained within the activities of our Working Group and we regard them to be a valuable addition to the results presented before.
S. Diez, K. Obermann, S. Scotti, D. Marcenac, F. Girardin


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