Diploma thesis

The main task of my diploma thesis was, the description and implementation of one semiconductor optical amplifier(SOA) module for "Lucent Technologies", as an expansion of the simulation tool DICSi (Digital Communication System Simulation). The work was done in the programming language MATLAB. The programming language "C" was used for time consuming program parts.

In the diploma thesis a simple and a complicated but more exact mathematical model for a semiconductor optical amplifier was introduced. The simple model take into account the repletion of the amplifiers, the self phase modulation because of temporal changes of the amplification and the noise of the amplifiers. The more exact model taken also into account the place and the temperature dependence of the amplification, the multiple reflection at his front and rear side, the inner amplifier losses as well the contribution of spontaneous emission to the repletion of the amplifier.

Two modules for semiconductor optical amplifier were implemented for a simple and a resonant SOA in MATLAB as well as in C. The programming of so-called MEX files (C-program parts which are callable directly in MATLAB) was a successful point of the diploma thesis, because compared with the MATLAB programs we have reached an improvement of the computing times with a factor approx. 20.

An approximative solution was used for the check of the implemented amplifier models. A comparation between simulation and measuring also was pursued. The simulation results were compared with measurement results of the experiment at the technical University of Eindhoven (July 1996). Similar results could by proved with simulations.

Also a quite number of simulation distances with several SOA's were examined. One of this was the demonstration distance (for " CeBit 97 " ) between Kassel-Hannover .

DICSi

DiCSi is a simulation tool under MATLAB with numerous modules, and is suitable for simulation of digitals communication systems. A simulation model can be created with several modules. Every simulation model can be developed in a graphical window by a block diagram. The needed modules are selected with the help of the keyboard or by simple mouse click. The modules for DiCSi are usually MATLAB functions but the user can program his own modules also in C or FORTRAN. An essential advantage of DiCSi is that all predefined MATLAB functions and the powerful graphical interface are available. DiCSi modules can process arbitrary long signals.

An example of a block diagram produced with DiCSi:

Optical amplifiers

The transmission length of a optical communication system is limited by the dispersion and attenuation of the fibres on the transmission distance. A possibility to increase the transmission length is the application of so-called repeaters. A repeater consists of two parts: a receiver part which transform the optical signal into an electrical signal and one transmission part which amplifies the electrical signal and transform it back into an optical signal. The use of repeaters can be very cost intensive. A second possibility for increasing the transmission length is the use of optical amplifiers. These can directly amplify the light without previous transformation into an electrical signal. The principle of optical amplifiers is similarly like that one of a laser; the light is amplified by spontaneous emission.

Some kinds of optical amplifiers are: the semiconductor optical amplifier (SOA), the Raman amplifier, the Brillouin amplifier, the Erbium remunerative booster the Praseodym booster and others. Base of the Raman and Brillouin amplifiers are the two nonlinear non elastic spreading effects: Raman and Brillouin spread. EDFA amplifiers are ideal components of a optical communication system for 1550 nm wavelength. Many applications (video transmission and LAN) require a wavelength near to 1330 nm (in the proximity of the zero dispersion point). For the transmission by wavelengths near to 1300 nm they are two possibilities for amplification of the optical signal: a Praseodym amplifier or a semiconductor optical amplifier.

The principles of semiconductor optical amplifiers:

A semiconductor optical amplifier consists of an active semiconductor which is embedded between two mirrors with the reflection factors R1 and R2. Semiconductor optical amplifiers can be divided up into two types:

- ideal (non resonant) amplifiers

- resonant amplifiers

We speak about ideal amplifiers if the reflection factors one front and rear side R1 = R2 = 0. Otherwise if R1 != 0 and R2!= 0 we speak about resonant amplifiers. In this case the mirrors cause a feedback of the output signal to the input which influences the gain of the amplifier. In practice a non resonant amplifier doesn't exist. However we speaks about ideal amplifiers if the reflection factors are less then 10^(-4).

The applications of optical amplifiers are large. Optical amplifiers are used for e.g. for power amplification on the transmission side or as preamplifiers on the receiver side. For compensation of attenuation loss through the fibres optical amplifiers are used as "in line" amplifiers. Because of the bidirectional work of optical amplifiers they are used in local networks, so-called "local area network" amplifiers.

Test route Kassel-Hannover

Some examinations for the optical communication distance "Kassel-Hanover" takes into account the practice's desired case; minimal configuration of the amplifiers. As an example we show the case, in which all the in line amplifiers have the same maximal amplification. The simulation was done for a wavelength 1312 nm with gauss pulses of the duration of 40 ps, extinction ratio of 10 % and peak power of 25 mW. The noise figure for each in line amplifier was 8,5 dB, "Henry" factor was 5 and the maximum laser chirp was 40 GHz. The sampling frequency of 5,12 THz was used for a realistic modelling of the noise.

Different distance configurations were examined: a configuration with 4, 5 and 6 amplifiers. Below we show the DiCSi block diagram for a configuration with 5 amplifiers. For each in line amplifier we have chosen a maximum amplification of 12 dB. At every connecting side of the amplifiers we have chosen a 3 dB loss.

The simulated power over the distance Kassel-Hannover:

Simulated eye patterns before and after the receiver:

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