The rising demand for greater bandwidth and increased flexibility in modern telecommunication systems has lead to increased research activities in the field of Millimetre Wave-Photonics. The combination of an optical access network and the radio propagation of high data-rate signals provides a solution to meet these demands. Such structures are also known as Radio Over Fibre systems. They implement the optical Millimetre Wave generation in a central station and the transmission of radio waves via a remote antenna unit to the radio cell. The expected data rate is very high, due to the fact that both the optical and the radio-link provide a large transmission bandwidth. This dissertation concerns the investigation of a new and simple method for the flexible generation of Millimetre Waves for application in Radio Over Fibre systems. The method is based on the heterodyne detection of two optical waves in a photo detector. By externally amplitude modulating the optical wave, different sidebands are generated. Two of these sidebands are selected and amplified by the non-linear effect of stimulated Brillouin scattering. As a gain medium, a standard single mode fibre is used. According to the theoretical investigation, very good carrier performances are possible with this method, and a computer simulation shows little degradation to the signals during their propagation in the system. The measured results are in strong agreement with the theoretical analysis. Experimental results show that the system can be fully utilised as a Radio Over Fibre system. The thesis is divided into five main parts: Introduction – Theory – Simulation – Experiment – Conclusion. In the Introduction, an overview of the current methods of Millimetre Wave generation, Radio Over Fibre and the nonlinear effect of Brillouin scattering is given. In the theoretical section, a differential equation system which mathematically describes the system is derived and also solved numerically. With a proof of the concept set-up, the simulated results are compared with the experimental data. In the last section the work is concluded and future tasks are discussed.
Table of Contents
1 Introduction
1.1 The Merging of Optical and Wireless Telecommunications
1.2 Motivation and Scope
1.3 Outline
2 Millimetre Wave Applications
2.1 Closed Requests
2.2 Public Request
2.2.1 Current Commercial Applications of mm-Waves
2.2.2 Future Mobile Communication Systems – Radio Over Fibre
2.2.2.1 Properties of Radio Over Fibre
2.2.2.2 Radio Over Fibre Applications
3 Millimetre Wave Generation Systems
3.1 Electrical Millimetre Wave Generation
3.1.1 Yttrium-Iron-Garnet Oscillators
3.1.2 Gunn Oscillators
3.1.3 Electron Tubes
3.1.4 Frequency Multiplier
3.2 Optical Millimetre Wave Generation
3.2.1 Direct Detection
3.2.2 Heterodyne Detection
3.2.2.1 Mode Locking
3.2.2.2 Injection Locking
3.2.2.3 Optical Phase Locking
3.2.2.4 Optical Frequency Multiplying by Modulation
3.2.2.5 Unconventional Millimetre Wave Generation Techniques
4 Stimulated Brillouin Scattering
4.1 Basics
4.2 Intensity Equations
4.3 Gain Characteristics
4.4 Threshold
4.5 Gain Bandwidth Broadening
4.6 Amplification Processes
5 Backround Theory on Stimulated Brillouin Scattering
5.1 Basics
5.2 Derivation of the Differential Equation System
5.2.1 The Nonlinear Wave Equation
5.2.2 Investigation of the Wave Equation for the Density Modulation
5.2.3 The Complete Differential Equation System
5.3 Analysis
6 Simulations
6.1 Shooting Method
6.2 Results
6.2.1 Simulation of Basic Brillouin Interactions
6.2.2 Diverse Amplification Scenarios
6.2.2.1 Different Pump Powers and Constant Signal Power
6.2.2.2 Different Signal Powers and Constant Pump Power
6.2.3 Optimum Fibre Length
6.2.4 Simulation of Pump Power Drifts
6.2.5 Simulation of Frequency Detuning
6.2.6 Conclusion
7 Experimental Verifications
7.1 Frequency Comb Generation
7.2 Brillouin Amplification Properties
7.2.1 Different Pump Powers at Constant Signal Power
7.2.2 Different Signal Powers at Constant Pump Power
7.3 Heterodyne Detection of Optical Frequency Components
7.3.1 Chromatic Dispersion Effects
7.3.2 Optic-Electric Conversion
7.3.3 Spectral Properties
7.3.4 Noise Measurements
7.3.4.1 Phase Noise
7.3.4.2 Amplitude Noise
7.3.5 Stability
7.3.5.1 Short Term Power Fluctuations
7.3.5.2 Long Term Power Fluctuations
7.4 Conclusion
8 Carrier Modulation
8.1 Set-up
8.2 Modulation Format
8.2.1 Time Domain
8.2.2 Frequency Domain
8.3 Modulation Results Back to Back
8.4 Modulation Results after Radio Propagation
8.5 Conclusion
9 Limitations
9.1 Bias Drift
9.2 Polarisation Penalties
9.3 Stabilisation
9.4 Modulation Bandwidth
9.5 Brillouin Amplifier Noise
9.6 Location of Pump Sources
10 Conclusion
11 Future Work
12 References
13 Authors Publications
14 Appendix
Research Objectives and Topics
The primary objective of this dissertation is to investigate a new, simple, and flexible method for generating millimetre waves for use in Radio Over Fibre (ROF) systems. The research aims to develop a system that achieves high performance through a heterodyne detection principle, specifically leveraging the non-linear effect of stimulated Brillouin scattering for signal amplification, while addressing challenges like signal bandwidth and system stability.
- Investigation of millimetre wave generation using stimulated Brillouin scattering in optical fibres.
- Development of a theoretical framework and differential equation system to model the Brillouin-based amplification process.
- Simulation and experimental verification of frequency comb generation and heterodyne detection techniques.
- Evaluation of system performance, including modulation, noise properties, and stability in ROF applications.
Excerpt from the Book
3.2.2.1 Mode Locking
Classical lasers are based on an optical cavity, which consists of two mirrors and an active gain medium inside it. One of the mirrors is partially transparent. The laser beam is coupled out through this mirror. The emitted spectrum is determined by the length of the cavity and the gain medium. In practice there are several frequencies which can be generated by such a set-up. Therefore, the emitted output spectrum is described by a frequency comb consisting of closely separated frequency components (modes). The axial distribution of the longitudinal modes depends on the number of half-wavelengths along the axis of the cavity.
Random fluctuations and non-linear effects in the cavity affect the amplitudes, phases and frequencies of the resonator modes. If frequency spacing and phases are fixed to a certain value the modes have a special relationship to each other. This status is called Locking of laser modes or Mode Locking.
Summary of Chapters
1 Introduction: Provides an overview of the merging of optical and wireless telecommunications and outlines the thesis structure.
2 Millimetre Wave Applications: Discusses current commercial and non-public applications of Extremely High Frequencies (EHF) and the concept of Radio Over Fibre.
3 Millimetre Wave Generation Systems: Reviews state-of-the-art electrical and optical generation techniques, focusing on heterodyne detection.
4 Stimulated Brillouin Scattering: Explains the fundamental properties of Stimulated Brillouin Scattering (SBS) as a non-linear effect useful for optical amplification.
5 Backround Theory on Stimulated Brillouin Scattering: Derives a complex differential equation system to mathematically describe the SBS-based amplification of multiple frequency components.
6 Simulations: Presents numerical simulations based on the derived equations to optimize the experimental set-up and analyze system performance under various scenarios.
7 Experimental Verifications: Validates the theoretical and simulated models through practical experimental results, detailing the frequency comb generation and amplification performance.
8 Carrier Modulation: Describes the implementation of a modulation scheme to facilitate data transmission using the generated millimetre wave carrier.
9 Limitations: Analyzes the practical constraints of the proposed system, such as bias drift and polarization-related power penalties.
10 Conclusion: Summarizes the thesis findings and confirms the effectiveness of the proposed Brillouin-based millimetre wave generation method.
11 Future Work: Suggests improvements and further research directions, including fully automatic system control and the exploration of different fibre types.
Keywords
Millimetre Waves, Radio Over Fibre, Stimulated Brillouin Scattering, Optical Generation, Heterodyne Detection, Brillouin Gain, Frequency Comb, Phase Noise, Optical Amplification, Nonlinear Wave Equation, Millimetre Wave Photonics, System Stability, Data Modulation, Fibre Optics, Differential Equation System
Frequently Asked Questions
What is the primary focus of this research?
The research focuses on investigating a new and simple method for generating millimetre waves for use in Radio Over Fibre systems, utilizing stimulated Brillouin scattering for signal amplification.
What are the core technical themes covered in this dissertation?
The work explores millimetre wave generation techniques, the non-linear properties of optical fibres, differential equation modeling of signal amplification, and experimental verification of modulation and transmission.
What is the ultimate goal of the proposed system?
The goal is to provide a flexible and high-performance carrier generation method that simplifies the architecture of remote antenna units in modern broadband radio communication networks.
Which scientific methods are employed?
The research uses both analytical modeling—deriving differential equation systems for non-linear optical wave interactions—and extensive numerical simulations, validated by experimental laboratory setups.
What is addressed in the main part of the thesis?
The main body covers the theoretical background of stimulated Brillouin scattering, the derivation of the governing equations for wave interaction, detailed numerical simulations for system optimization, and a multi-part experimental verification including frequency comb generation and data transmission.
Which keywords best characterize this work?
Key terms include Millimetre Waves, Radio Over Fibre, Stimulated Brillouin Scattering, Heterodyne Detection, and System Stability.
How does stimulated Brillouin scattering (SBS) contribute to the system?
SBS acts as a high-gain, narrow-band optical amplifier, which is critical for selectively amplifying specific sidebands in the generated frequency comb to create the desired millimetre wave signal.
What role does the Mach-Zehnder Modulator (MZM) play?
The MZM is used to generate the optical frequency comb through non-linear amplitude modulation, which serves as the basis for the subsequent heterodyne detection and millimetre wave generation.
- Citation du texte
- Dr. Markus Junker (Auteur), 2008, Investigation of Millimetre Wave Generation by Stimulated Brillouin Scattering for Radio Over Fibre Applications, Munich, GRIN Verlag, https://www.grin.com/document/118120
