Dublin Institute of Technology
School of Electronic & Communications Engineering

Dissertation submitted in fulfilment of the requirements for candidature of the degree of
Doctor of Philosophy

Investigation of Millimetre Wave Generation by Stimulated Brillouin Scattering for Radio Over Fibre Applications

Markus Junker

Table of Contents

Abstract ... I

Declaration ... II

Acknowledgements ... III

List of Abbreviations ... IV

Table of Contents ... VII

List of Symbols ... X

List of Figures ... XIV

1 Introduction ... 1

1.1 The Merging of Optical and Wireless Telecommunications ... 1
1.2 Motivation and Scope ... 3
1.3 Outline ... 4

2 Millimetre Wave Applications ... 6

2.1 Closed Requests ... 6
2.2 Public Request ... 7
2.2.1 Current Commercial Applications of mm-Waves ... 9
2.2.2 Future Mobile Communication Systems – Radio Over Fibre ... 11
2.2.2.1 Properties of Radio Over Fibre ... 11
2.2.2.2 Radio Over Fibre Applications ... 13

3 Millimetre Wave Generation Systems ... 16

3.1 Electrical Millimetre Wave Generation ... 16
3.1.1 Yttrium-Iron-Garnet Oscillators ... 17
3.1.2 Gunn Oscillators ... 17
3.1.3 Electron Tubes ... 18
3.1.4 Frequency Multiplier ... 19
3.2 Optical Millimetre Wave Generation ... 19
3.2.1 Direct Detection ... 20
3.2.2 Heterodyne Detection ... 21
3.2.2.1 Mode Locking ... 24
3.2.2.2 Injection Locking ... 26
3.2.2.3 Optical Phase Locking ... 27
3.2.2.4 Optical Frequency Multiplying by Modulation ... 28
3.2.2.5 Unconventional Millimetre Wave Generation Techniques ... 31

4 Stimulated Brillouin Scattering ... 32

4.1 Basics ... 32
4.2 Intensity Equations ... 35
4.3 Gain Characteristics ... 36
4.4 Threshold ... 38
4.5 Gain Bandwidth Broadening ... 39
4.6 Amplification Processes ... 42

5 Backround Theory on Stimulated Brillouin Scattering ... 46

5.1 Basics ... 46
5.2 Derivation of the Differential Equation System ... 48
5.2.1 The Nonlinear Wave Equation ... 50
5.2.2 Investigation of the Wave Equation for the Density Modulation ... 55
5.2.3 The Complete Differential Equation System ... 58
5.3 Analysis ... 61

6 Simulations ... 65

6.1 Shooting Method ... 65
6.2 Results ... 66
6.2.1 Simulation of Basic Brillouin Interactions ... 66
6.2.2 Diverse Amplification Scenarios ... 69
6.2.2.1 Different Pump Powers and Constant Signal Power ... 69
6.2.2.2 Different Signal Powers and Constant Pump Power ... 72
6.2.3 Optimum Fibre Length ... 74
6.2.4 Simulation of Pump Power Drifts ... 76
6.2.5 Simulation of Frequency Detuning ... 78
6.2.6 Conclusion ... 79

7 Experimental Verifications ... 81

7.1 Frequency Comb Generation ... 82
7.2 Brillouin Amplification Properties ... 87
7.2.1 Different Pump Powers at Constant Signal Power ... 88
7.2.2 Different Signal Powers at Constant Pump Power ... 90
7.3 Heterodyne Detection of Optical Frequency Components ... 93
7.3.1 Chromatic Dispersion Effects ... 95
7.3.2 Optic-Electric Conversion ... 97
7.3.3 Spectral Properties ... 98
7.3.4 Noise Measurements ... 102
7.3.4.1 Phase Noise ... 102
7.3.4.2 Amplitude Noise ... 107
7.3.5 Stability ... 107
7.3.5.1 Short Term Power Fluctuations ... 108
7.3.5.2 Long Term Power Fluctuations ... 110
7.4 Conclusion ... 111

8 Carrier Modulation ... 113

8.1 Set-up ... 114
8.2 Modulation Format ... 117
8.2.1 Time Domain ... 117
8.2.2 Frequency Domain ... 118
8.3 Modulation Results Back to Back ... 119
8.4 Modulation Results after Radio Propagation ... 121
8.5 Conclusion ... 122

9 Limitations ... 124

9.1 Bias Drift ... 124
9.2 Polarisation Penalties ... 124
9.3 Stabilisation ... 125
9.4 Modulation Bandwidth ... 127
9.5 Brillouin Amplifier Noise ... 127
9.6 Location of Pump Sources ... 128

10 Conclusion ... 129

11 Future Work ... 131

12 References ... 133

13 Authors Publications ... 153

14 Appendix ... 158

1 Introduction

1.1 The Merging of Optical and Wireless Telecommunications

Modern mobile broadband radio communication systems take centre stage in many research institutes all over the world. Since the first experiments in sending radio signals between two war ships in the English Channel over a distance of 119 km by Marconi in 1899 [1] the 20th century was shaped by new investigations in the field of cabled, wireless and particularly optical telecommunications. Since 1921 radio signals of wavelengths less than 200 m have been used for long distance data transmission and the first public mobile network was put into operation in 1946 in America. Due to the invention of electronic circuits and cellular structures the proliferation of mobile phones was made possible. Before the global standardisation of the mobile network in the 1990’s creating the current GSM system, many countries used different standards for mobile communication systems which were incompatible with one other. The 3rd mobile generation provides a user bandwidth of up to 2 Mbit/s as the state of the art [2].

Along with the rapid development of the wireless communication systems, the optical telecommunication sector also flourished. The foundation was made in 1960 when the first laser was invented by Theodor Maiman [3] and the first optical fibre in 1966. Although the losses of the fibres first produced were extremely high, it was now possible to generate and transmit coherent light. In 1970 Kapron illustrated a big decrease of the attenuation in optical fibres down to 20 dB/km [4] and lasers operating continuously at room temperatures were invented by Hayashi [5]. The current range of optical fibre types for different applications has become very large. Initially, the Standard Single Mode Fibre (SSMF) was produced with a natural attenuation of approximately 0.2 dB/km at 1550 nm, followed by Dispersion Shifted Fibres (DSF), Highly Non-linear Fibres (HNLF), Photonic Crystal Fibres and others. Moreover, the development of application-based lasers has also seen rapid progress which includes Distributed Feedback Laser diodes, Fibre Lasers, Vertical-Cavity Surface-Emitting Lasers and Fabry-Perot Lasers.

The theoretical bandwidth capacity of a SSMF is around 25 THz represented by the S-band, the 3rd optical window and the L-band [6]. The present optical communication systems have many advantages over electronical systems. They include reduced size, weight and cost, low dispersion, low and constant attenuation over the entire modulation frequency range, extremely wide bandwidth and high information transfer capacity. The current data rate for a multi channel transmission system is 20.4 Tbit/s and 1 Tbit/s over a distance of 240 km and 2375 km, respectively, as the state of the art [7], [8]. Recently, an RZ-DQPSK signal with a data rate of 25.6 TBit/s was generated and successfully detected [9]. On the other hand, wireless telecommunications can circumvent restrictions of the optical communications such as reduced flexibility and burying costs.

However, besides speech, several other applications such as data and video transmission are becoming more important in wireless communications. Hence, wireless local area computer networks (WLAN) as well as mobile radio systems have a growing demand for higher bandwidths [10]. While the cabled network systems transmission rates reach tens of Gbit/s (Gigabit Ethernet) the wireless computer networks have transmission rates of only tens of Mbit/s (54 Mbit/s for IEEE 802.11). Future trends demonstrate new applications such as video broadcasting in congested areas and airports or traffic information systems at traffic nodes. In order to meet these requirements, wireless communication systems need higher data rates of several Gbit/s to keep up with the wired network techniques [11]. A way to increase the bandwidth significantly is the use of Millimetre Waves as carrier frequencies. The Millimetre Wave range lies between 30 GHz and 300 GHz. The same frequency range is also known as extreme high frequency (EHF). This domain is followed by the terahertz region.

The bandwidth allocated to wireless links which operate in the lower frequency ranges such as Microwaves¹ is insufficient because these frequency bands are already used by many systems. For example, the Worldwide Interoperability for Microwave Access (WiMAX) works at frequencies up to 11 GHz for mobile radio networks. The Japanese and U.S. governments have allocated 5 and 7 GHz bandwidths, respectively, to the 60 GHz band wireless communication system. These bands are subdivided into plural different wireless communication systems. On the other hand, Millimetre Waves especially the range above 100 GHz and higher, are rarely employed by radio stations or industrial services with the exception of radio astronomy applications. Therefore, it is important to investigate the applicability of Millimetre Waves in wireless communications in order to increase the data rate. The frequency region in the mm-Waves range particularly above 100 GHz remains undeveloped, mainly due to technical difficulties associated with conventional electronic systems. As the frequency is increased, the generation, modulation and amplification of electronic signals is more complicated due to the characteristics of semiconductor devices. Furthermore, the electrical channel is affected by a very high transmission loss even in case of short transmission distances. These electronic systems limitations can be overcome by combining the wireless link with photonic techniques. The merging of the optical and the electrical domain creates an extreme highly efficient base. It consists of the low-loss high-data rate optical transmission link on one hand, and of the flexible, user-friendly and convenient wireless communication system on the other. Such a combination is also called Microwave Photonics (MWP) or Millimetre Wave Photonics (mmWP) depending on the frequency range that is used.

1.2 Motivation and Scope

The integration of wireless and optical networks is a potential solution for increasing capacity and mobility as well as decreasing costs in the access network. The Radio Over Fibre (ROF)² technology with its use of Millimetre Waves (mmW) represents one of the best solutions. The attenuation of electromagnetic waves with frequencies from 30 GHz during their propagation in the atmosphere rises up to 60 GHz. Furthermore, there are attenuation maxima at the resonant frequencies of H2O and O2 molecules in the higher Gigahertz range [12]. Hence, the transmission distance reaches only few hundreds meters or less. This property affects the planning of future cellular communication systems. The cell diameter is adapted to the propagation conditions, i.e. the higher the attenuation of the waves the smaller the diameter of the radio cells [10]. The amount of cells has to increase to guarantee full network coverage.

To make a virtue of necessity, the advantage of using high frequencies such as mm-Waves is the following. As the propagation distance via air is short [15] the same carrier frequency can be reused in the neighbouring clusters. Because of this and the fact that the electromagnetic spectrum in this frequency range is fairly unoccupied, it offers an enormously large transferable bandwidth and a high number of usable frequency bands.

Another advantage of electromagnetic waves in the mm-Wave range is that they do not penetrate into the human skin deeper than a few millimetres, whereas the penetration depth for the current mobile applications lies in the range of centimetres. In the event that electromagnetic waves are found to have some adverse biological effect other than the established thermal effects, then it can be negligible if mm- Waves are used. With the use of frequencies in the Millimetre Wave range the antenna dimensions can be very small. Hence, small antenna arrays for multiple input, multiple output (MIMO) systems can be designed to achieve a data rate enhancement [16]. The state of the art of data rates in wireless communications lie in the Giga bit range. At 10.8 GHz and in the 60 GHz band, a data transmission rate of 1.25 Gbit/s was verified [17], [18]. Hirata et al. demonstrated a 3 Gbit/s transmission link at 120 GHz [11] and a 10 Gbit/s data rate at 125 GHz [19]. Recently, a 15 Gbit/s, 10 Gbit/s and 5 Gbit/s data transmission over a 1, 2 and 5 metre radio link was performed [20].

The main objective of this work is to analyse and investigate a new and simple method for the generation of Millimetre Waves for Radio Over Fibre Systems. The increased demand for mm-Wave generation techniques has made them more attractive. This dissertation presents a new mm-Wave generation method including some obvious applications. This includes a signal generator or a base station in ROF system. The method relies on a very simple principle, achieves an excellent system performance and is variable in its application.

1.3 Outline

The thesis is divided into 11 Chapters as is detailed below. The following chapter describes typical applications of Millimetre Waves. For purposes of classification, an allocation into private (astronomy, military etc.) and public (radio communication, RADAR etc.) domains has been conducted. Special attention is given to the application of mm-Waves for Radio Over Fibre systems and is described in more detail.

Chapter 3 gives an overview of the state of the art electrical and optical techniques for the generation of Millimetre Waves. The main objective in this chapter is to discuss the heterodyne superposition of two optical phase correlated signals in a photo detector.

Since “stimulated Brillouin scattering” is applied in the system, an overview of the important properties of the non-linear effect is provided in Chapter 4, where characteristics such as gain, its bandwidth, the threshold and several amplification scenarios are presented.

In Chapter 5, the proposed Millimetre Wave generation method is briefly described followed by a detailed theoretical investigation of the system. Based on the nonlinear wave equation and the wave equation for the density modulation, a complex differential equation system is derived and analysed. This equation system is the basis for a simulation presented in Chapter 6. Several scenarios are calculated in order to optimise the experimental set-up. Furthermore, calculations concerning the optimum fibre length, as well as power and frequency drifts are illustrated in Chapter 6.

The experiment, which is verified in Chapter 7, confirms the predictions of Chapters 4 - 6. Several scenarios of Brillouin amplification are carried out in order to compare theory and experiment. Furthermore, Millimetre Wave signals at different grades of up conversion are presented. Properties such as line width, power, and stability are analysed.

In order to modulate the carrier signal with data, the set-up has been modified in Chapter 8. Thus, a simple implementation of modulation into the set-up can be provided. An error free transmission of a 1 Gbit/s data signal for a “back to back” case is shown, as well as a successful realisation of radio propagation between two antennas.

In Chapter 9, the limitations of the presented mm-Wave generation technique are described in order to analyse the method from all points of view.

A general summation of the results is provided in Chapter 10, while a forecast for future work is described in Chapter 11.

2 Millimetre Wave Applications

The worldwide development in wireless and fibre-optic telecommunications is a result of the availability of high quality signal generators and signal processing systems and the requirement of higher bandwidths and low cost system set-ups. While available mm-Wave generation systems are verified in Chapter 3, the application aspects of the wide field of Millimetre Waves are demonstrated here.

A classification of the main applications can be made if a distinction is made between commercial public telecommunication systems for private and business demands and non-public applications such as military, meteorological or aerospace related systems.

2.1 Closed Requests

This Chapter is based on [21] and should give an overview of the non-public applications of Extremely High Frequencies (EHF) in the range from 30 GHz to 275 GHz. It should be noted, that frequency bands in the EHF domain are indeed allocated but the use and the density of channels are much smaller than in the Very High Frequency (VHF; 30 MHz – 300 MHz) and the Ultra High Frequency band (UHF; 300 MHz – 3 GHz) for instance. Frequencies exceeding 275 GHz are not allocated to any radio channel.

Around 30 GHz the radio astronomy systems search for radio waves and radiation from outer space. Furthermore, the space radio communication research applies mm-Waves for investigations of space and natural phenomenon’s on board spacecraft by passive sensors (radiometer). The data transport of technical and scientific measurement results made by spacecrafts is also completed in this frequency range. The 34 GHz band is used for the transmission of data for sea and ashore surveying.

Other uses of mmWaves in this range are e.g. speed controls, traffic counts, security services and distance detection. In the higher 40 GHz range a radio amateur band for satellite communications is provided. In addition, a digital point to multipoint line-of-sight (LOS) radio system with radio link antennas is located at a certain height in the stratosphere (High Altitude Platform Station). Frequencies of about 55 GHz are used for satellite radio between two satellites for speech and data transmission. For applications such as the satellite position and speed control the 66–71 GHz band is used. For traffic telematics such as distance control electromagnetic waves with frequencies of around 77 GHz are applied. Packet characterisation techniques and the evaluation of plastic package are planned at frequencies around 79 GHz. It allows impedance characterisation of enclosures, failure analysis and fault localisation [22]. The allocation of applications presented in this chapter reaches frequencies of up to 300 GHz

All verified applications are recurrent at higher frequencies up to the upper border of the mm-Wave range (300 GHz).

[...]

1 The Microwave range starts at 0.3 GHz and ends at 30 GHz. The Millimetre Wave range lies between 30 GHz and 300 GHz.
2 Another expression for ROF is Hybrid Fibre Radio (HFR) [13],[14]