Comparing optical transmission link for digital broadcast signals

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Kovács Gábor
Department of Broadband Infocommunications and Electromagnetic Theory

The subject of my thesis is the comparison of digital broadcast signal transmission via various optical links. This topic requires the knowledge of the basic properties of optical telecommunication and broadcast systems. The goal of my work was to create a test network, where the quality of signal transmission can be measured. The analysis can be done real-time by instruments or by off-line computer signal processing. In this work I used the off-line method based on Matlab Communication Toolbox for signal processing.

The thesis can be divided three main parts. In the first part I introduce the necessary optical devices, required in my experimental setup: the laser diode, the photo diode, and fibers. The second part is about the broadcast systems and modulation methods, specifically the quadrature amplitude modulation and direct modulation. In the last part I present the operation of the script, which I have written in Matlab and used for signal analysis. I analysed four experimental setups with the script. In the first setup single mode fiber with DFB laser was used 1537 nm wavelength. The second setup uses a VCSEL laser and multimode fiber. The fiber has graded index profile, and the laser operated a 837 nm wavelength. In the third setup the same laser, but polimer optical fiber was used. In the last measurement I used again the single mode fiber and another DFB laser, at 1312 nm. The script demodulates the QAM signals, and plots the results in four diagrams. The first two diagrams contain the Signal to Noise Ratio versus fiberlength, and carrier frequency respectively. The third and the fourth figure presents the Bit Error Rate versus fiberlength and carrier frequency.


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