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

Table of Contents

Abstract

Declaration

Acknowledgements

List of Abbreviations

Table of Contents

List of Symbols

List of Figures

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 Modul ati on F ormat
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

Abstract

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.

Declaration

I certify that this thesis which I now submit for examination for the award of PhD, is entirely my own work and has not been taken from the work of others save and to the extent that such work has been cited and acknowledged within the text of my work.

This thesis was prepared according to the regulations for postgraduate study by research of the Dublin Institute of Technology and has not been submitted in whole or in part for an award in any other Institute or University.

The work reported on in this thesis conforms to the principles and requirements of the Institute's guidelines for ethics in research.

SignatureDate

Acknowledgements

First, I would like to thank my supervisor Prof. Dr. Thomas Schneider from the Hochschule für Telekommunikation, Leipzig for his inspiration to begin work on Ph.D., and for his support and help throughout. His advice and guidance was invaluable on many occasions.

Furthermore, I wish to express my gratitude to my supervisors Dr. Max Ammann, Dr. Andreas Schwarzbacher and Dr. Gerry Farrell from the School of Electronics and Communications Engineering at the Dublin Institute of Technology, for their support, and for the opportunity to work on the Ph.D..

I would also like to thank my colleagues from HfT Leipzig for all of the support that I received. They have been instrumental in providing access to building equipment, in conducting measurements and calculations, in providing explanations and reviews, and in technical discussions. I am indebted to Kai-Uwe Lauterbach, Ronny Henker, Jens Klinger, Andrzej Wiatrek and Steffen Neidhardt.

Additionally, I would like to thank the members of the academic administration of HfT Leipzig and the Deutsche Telekom AG for financial support.

I would also like to thank the research team of Prof. Dr. Ing. Schäffer of the Technical University Dresden for the use of technical equipment and for fruitful discussions.

Special thanks go to James Poborsa from the University of Toronto, who has read a major part of the dissertation and provided his comments and suggestions regarding the English translation.

Finally, I wish to thank Heidi, Clara and my parents for the moral support they have given me over the years.

List of Symbols

Abbildung in dieser Leseprobe nicht enthalten

List of Figures

Fig. 2-1 UHF/SHF/EHF radio spectrum and its frequency allocation [23]

Fig. 2-2 Millimetre Wave feeder concept [13]

Fig. 2-3 Possible Radio Over Fibre and Millimetre Wave applications

Fig. 2-4 ROF implementation into a satellite broadcast system

Fig. 3-1 Classification of possible mm-Wave generation techniques

Fig. 3-2 Drift velocity versus electrical field intensity in n-GaAs

Fig. 3-3 Direct and external modulation for direct reception links

Fig. 3-4 Heterodyne superposition

Fig. 3-5 Block diagram of the injection locking scheme with two slave lasers

Fig. 3-6 General set-up of an optical phase locked loop

Fig. 3-7 a: Mach Zehnder Modulator configuration and b: its operation characteristic

Fig. 3-8 Output spectrum of an MZM at different operation points (OP)

Fig. 4-1 Vector diagram analysis for the Brillouin scattering effect

Fig. 4-2 Dashed line - Lorentzian shaped Brillouin gain distribution versus Brillouin shiftfa; Solid line - measured Gaussian shaped gain spectrum

Fig. 4-3 Brillouin threshold in a 50.45 km SSMF; The inset shows the experimental set-up (LD: Laser Diode, C: Circulator, POW: Power Meter)

Fig. 4-4 Flat Brillouin gain characteristic

Fig. 4-5 10 GHz Brillouin gain bandwidth and its Anti-Stokes spectrum

Fig. 4-6 Simulated power of different amplified probe signals depending on the fibre length at altered probe signal powers

Fig. 4-7 Saturation effects in dependency of the probe signal

Fig. 4-8 Simulated power of different amplified probe signals depending on the fibre length at altered pump powers

Fig. 5-1 Pump, gain and sideband arrangement in the fibre as frequencies and vectors

[169]

Fig. 5-2 Normalised real (left) and imaginary part (right) of the Brillouin gain (FHWM=28 MHz) depending on the frequency difference (œS1,2 - wS1,2max) according to (5.95) and (5.96) [169]

Fig. 6-1 Principal system model

Fig. 6-2 Dashed line: Power of the signal wave as a function of the fibre length; Solid line: Phase of the signal vs. fibre length

Fig. 6-3 a: Power of the amplified signal wave (black dashed line) and of the pump wave (red solid line) along the fibre; b: Rise (derivative) of the pump wave

Fig. 6-4 Phase shift along the fibre

Fig. 6-5 Signal power at the fibre output and heterodyned signal power at different pump powers

Fig. 6-6 Gain at different pump powers; a: Related to the input power; b: Related to the output power

Fig. 6-7 Phase of the signal wave at the fibre output for different pump powers

Fig. 6-8 Fibre output power and power of the heterodyned signal for different input signal powers

Fig. 6-9 Gain for different signal powers. a: Related to the input power; b: Related to the output power

Fig. 6-10 a: Fibre output phase dependence on signal powers; b: Optimum signal input power vs. phase shift for a 50 km long fibre

Fig. 6-11 Order of amplified sidebands vs. optimum fibre length (black/solid line); Exponential growth fitting (red/dashed line)

Fig. 6-12 a: Power of the first (light grey curve) and second (dark grey curve) signal wave for the fibre output at drifting pump powers; b: Phase of both amplified signal waves at different pump powers

Fig. 6-13 Power (a) and phase (b) of the heterodyned signal wave for drifting pump powers

Fig. 6-14 Signal power at fibre output for different frequency detuning values

Fig. 6-15 Power (a) and phase (b) of the heterodyned signal under detuning impacts

Fig. 7-1 Experimental set-up for the generation of Millimetre Waves by Brillouin scattering

Fig. 7-2 Bessel coefficients versus modulation index

Fig. 7-3 a: Measured dependency of the power of the carrier and the sidebands on the HF voltage and b: bias voltage versus HF voltage. Grey/red lines: Upper OP; Black/dashed lines: Lower OP

Fig. 7-4 Optical frequency components at upper and lower operating points; a: SB 1­3; b: SB 4, 5; c: SB 6,7

Fig. 7-5 a: Amplification at different pump powers and a constant signal power (52.9pW); b: Comparison between simulation and experiment according to Table 7-3

Fig. 7-6 Amplification at different signal powers and a constant pump power (6 mW); Red/solid line: amplified signal; Black/dashed line: Signal at the fibre output. The inset numbers match to the order of the sidebands that correspond to the signal powers

Fig. 7-7 Comparison between simulation and experiment according to Table 7-4... 92 Fig. 7-8 a: 22.2 GHz signal spectrum; solid line: back to back; dashed line: after propagation in a 50.45 km SSMF; b: Phase noise of the 22.2 GHz signal back to back

Fig. 7-9 Power penalty due to chromatic dispersion vs. fibre length; a: - at 60 GHz frequency separation and b: frequency separation - at 50.45 km fibre length...

Fig. 7-10 Optic-electronic power conversion in theory and practice

Fig. 7-11 Spectral properties of the 5.71 GHz signal

Fig. 7-12 Comparison of experimental results and calculations

Fig. 7-13 Phase noise characteristics of mm-Wave signals according to the set-up shown in Appendix E5

Fig. 7-14 System noise figure (blue/dotted line), noise figure induced by frequency multiplying (black/solid line) and noise figure induced by SBS amplification (red/dashed line)

Fig. 7-15 Comparison of amplitude noise of a 17.142 GHz signal and the noise of the ESA

Fig. 7-16 Measurement points for short term fluctuation analysis

Fig. 7-17 5th order sidebands power fluctuations at the output of the MZM (point 4 in

Fig. 7-16)

Fig. 7-18 Long term stability of the 11.428 GHz signal

Fig. 8-1 Possible modulation scheme for the presented mm-Wave generation technique (FBG: fibre Bragg grating; C: Circulator; O/E: optic/electric converter)

Fig. 8-2 Principle operation of modulation set-up in respect to ROF

Fig. 8-3 a: Optical spectrum at the fibre input: Frequency comb without optical carrier (red/solid line) and with optical carrier (black dashed line); b: Optical spectrum at the circulator output

Fig. 8-4 32 GHz spectrum before (solid line) and after (dashed line) electrical amplification

Fig. 8-5 Eye diagram of the output signal of the pulse pattern generator at 1 Gbit/s

Fig. 8-6 a: 1 GBit/s modulated 32 GHz carrier signal. The spectrum is mirrored at the local frequency (310.4 MHz) of the ESA. b: 1 Gbit/s down converted into base band

Fig. 8-7 Eye diagram of 1 Gbit/s PRBS 23 signal back to back

Fig. 8-8 1 Gbit/s after radio propagation

Fig. 9-1 a: spectrum and b: phase noise of 25.9 GHz carrier generated by a DSSC modulated DFB LD as a signal laser with SBS amplification

Fig. 9-2 Brillouin threshold vs. fibre length

Fig. 9-3 Alternative location of pump sources

List of Abbreviations

Abbildung in dieser Leseprobe nicht enthalten

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 Microwaves^[1] 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 66­71 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).

2.2 Public Request

Fig. 2-1 gives an overview of the operation frequencies for different radio communication systems from 0.8 GHz to 70 GHz, including the ones treated below. The most popular are Global Systems for Mobile Communications (GSM) 900, 1800 and 1900 MHz, which provide data rates between 9.6 and 14.4 kbit/s. The cell diameters go from 10 m in buildings and 30 km in the countryside depending on the geographical environment. Higher bit rates can be achieved by channel packing (High Speed Circuit Switched Data - HSCSD). Due to allocation of several GSM channels the transmission rate increases up to 57.6 kbit/s if 4 timeslots are occupied.

The General Packet Radio Service (GPRS) provides the timeslot packing of all 8 GSM timeslots and achieves a theoretical bandwidth of 171.2 kbit/s. The packet based “high-speed transmission” system Enhanced Data rates for Global Evolution (EDGE) is an extension of GPRS and is three times as fast as GPRS. All demonstrated methods relying on the basis of GSM in a 200 kHz spectrum and are called the 2nd mobile generation.

Another radio system is the Digital European Cordless Telecommunications Standard (DECT). It works in a 20 MHz spectrum around 1.9 GHz and supplies narrow band indoor services and cordless telephony.

The next step in developing high-speed transmission links was represented by the 3rd generation with UMTS as the main application. UMTS uses carrier frequencies around 2 GHz and theoretically provides data rates up to 2 Mbit/s for the case of a user density of one mobile and a low mobility. Cell diameters go from 100 m for indoor applications to 10 km in the countryside. It can provide features like 2nd generation mobile systems but will also offer multimedia services like video telephony.

W-LAN (IEEE 802.11) uses frequencies around 2.4 and 5 GHz in an 80 MHz spectrum. The standard is designed for theoretical transmission rates from 2 Mbit/s to 600 Mbit/s indoor for up and downlink (IEEE 802.11a-n). The provided area can reach a diameter of 250 m.

Abbildung in dieser Leseprobe nicht enthalten

Fig. 2-1 UHF/SHF/EHF radio spectrum and its frequency allocation [23].

WiMAX is described by the IEEE 802 standard family and provides the user with data rates up to 100 Mbit/s. It is a standard for wireless networks (IEEE 802.16) and works with frequencies from 2 GHz to over 11 GHz. For data transmission between 2 GHz and 11 GHz no line-of-sight connection is necessary. Moreover, the antenna installation is easy and the modulation technique is Orthogonal Frequency Division Multiplexing (OFDM), which tolerates diversity reception and has a high spectral efficiency. The disadvantages are smaller antenna gains due to the high frequencies that are used and low transmission rates owing to lacking broadband frequency spectra. If WiMAX uses frequencies higher than 11 GHz a line-of-sight connection is required and hence mobile communication services are impossible [24].

HIPERLAN works around 5 GHz and offers data traffic up to 54 Mbit/s. The cell diameters are 30 m for indoor and 150 m for outdoor applications.

2.2.1 Current Commercial Applications of mm-Waves

While the UHF band has a dense allocation of many types of demonstrated applications the SHF and EHF band is very transparent in its spectrum. Millimetre Waves are only used in few commercial applications.

- The term RADAR (Radio Detection and Ranging systems) stands for detection and distance measurement. The basic principle relies on the transmitting of electromagnetic waves by directional antenna into air. In case the waves hit an object, a part of the energy is backscattered to the receiver. Object parameters can be investigated by the analysis of the received electromagnetic field [25].

Radar systems belong to one of the first natural applications in the area of mm- Waves due to its small antenna size and its inherent high resolution. The evolution of radar systems from X-band to 24 GHz, then 77 GHz, and to 100 GHz or 220 GHz have shown, that sub-millimetre distance resolution is possible. Recently a long range 7 km Radar system with a resolution of around l.5 m at a frequency of 94 GHz was achieved by Macfarlane et al. [26]. This high resolution has been achieved by a highly linear, low phase noise YIG oscillator^[3] in the frequency-multiplied transmitter. Nowadays Radar systems can be used for long distance applications such as tornado observations for instance [27] on the one hand or for short distance applications such as anti-collision radar between vehicles on the roadway and ground penetration (mine detection).

- Microwave Video Distribution Systems (MVDS) operate at frequencies of around 29 GHz and 40 GHz. They rely on the cell fragmentation into different sectors. A monitor control of the signal power level in each sector allows the system to control, adapt and improve the quality of signal reception as requested by the subscribers. By minimising the transmitting power the operational life of the transmitter increases and the responsiveness to other demands for signal transmissions is enhanced [28]. Sector monitoring in MVDS may be used in a variety of scenarios. A Millimetre Wave power detector can be used to shut down or reduce the power from a particular antenna sector. Moreover, it is beneficial in improving the stability of the transmitter and its component parts due to power reduction.

- Local Multipoint Distribution Service (LMDS) uses Millimetre Wave signals to transmit and receive data in the range between 26 GHz and 43 GHz having bandwidths of 0.1 up to 2 GHz. On the other hand, the area coverage is limited due to the line of sight constraints and large propagation losses [29]. The transmitter should be on the top of a tall building or on any other high platform. One transmitter covers a sector typically 60° - 90° wide. Hence, 4-6 transmitters are necessary to achieve a 360° coverage. LMDSs provide a point-to-multipoint broadcast downlink of 34-38 Mbit/s per transport stream to everybody located in the covered zone. This kind of data is typically television broadcasting, Internet applications and communications [30].

- The Mobile Broadband System (MBS) is a part of the “RACE” project that is supported by the European Union for the investigation of broadband networks. It belongs to the 4th mobile generation techniques and provides the terrestrial radio applications. The cellular network works in a frequency range between 40 GHz and 60 GHz and reaches data transmission rates up to 150 Mbit/s [31]. MBS provides voice, video, and high demanding data applications. The cell coverage range is 1 km for outdoor and 100 m for indoor applications. The broadband system is located in high traffic density service areas indoor as well as outdoor.

- The ETSI-BRAN High Performance Radio Access (Hiperaccess) standard is confined to specify radio communication systems that work with frequencies above 11 GHz. Hiperaccess supports voice and data services using frequencies between 11 GHz and 42 GHz with a large bandwidth of 2 GHz [32]. This provides “bandwidth on demand”, i.e. any appropriate data rate at any time. At least 25 Mbit/s can be offered at the user-network interface in up or downstream direction. These data rates are possible due to the large bandwidth and the application of the IP (Internet Protocol) and ATM (Asynchronous Transfer Mode) in the system [33].

Overall, the impact of Millimetre Waves to communication systems will rise in the future. The performance capabilities that come with short wavelengths enable many new applications, only a few of which have been covered here.

2.2.2 Future Mobile Communication Systems - Radio Over Fibre

As described in the introduction the merging of the wireless and fiberoptic worlds provides enormous potential regarding higher transmission rates and more local flexibility. Radio Over Fibre (ROF) represents such a combination. The main objective is to save costs while increasing the network coverage and performance.

The idea of ROF is not new. It was invented in the 1990s in order to simplify, or even replace, the mobile infrastructure. Commercial success was not obtained due to difficulties with the implementation of the feedback channel. For several years the diffusion process of broadband internet innovations such as “Video on Demand” has risen and thus the demand for a high download transmission speed for the customer has increased. This demand for high speed downlinks can be accommodated by Radio Over Fibre networks, whereas the narrowband uplink can be realised by conventional wireless cell systems or wireless LANs. Whereas recent investigations report a simple and impressive 150 Mbit/s transmission in the up link direction [34], the main research focus lies in the high speed downlink. Considering these aspects in this approach, investigations regarding the ROF-downlink have been made.

2.2.2.1 Properties of Radio Over Fibre

This aim can be achieved by centralising the system complexity to one location - the central station (CS). Due to this, the radio capacity can be allocated while considering the mobility of the subscriber. In a crowded city the requirements for a high capacity in the daytime exceeds that of the night. Since the demand in populated suburbs increases at night, the net capacity allocation can be adapted to the new situation. Moreover, congested areas can be provided with broadband radio access and employees can change their position within one office building without any restrictions.

In principal there are three feeder concepts for the realisation of ROF [13].

- The base band feeder concept with low transmission loss and a large bandwidth allows no complexity reduction of the remote antenna unit (RAU). NT: Network Terminal

- By using the intermediate frequency (IF) transmission concept some complexity of the RAU is shifted to the CS. However, up/down conversion to the Radio Frequency (RF) band and the amplification processes are still the functions of the RAU.

Abbildung in dieser Leseprobe nicht enthalten

- The Millimetre Wave feeder concept allows transferring the whole complexity to the CS. Thus, the line termination of the digital segment, modulation/demodulation, up/down conversion to RF, multiplexing/ demultiplexing can be performed by the CS. The RAU consists of an optic/electronic converter to transform the optical signal into the actual modulated Millimetre Wave carrier. An electrical power amplifier is used to increase the signal magnitude to the required output power. The Millimetre Wave feeder concept is demonstrated in Fig. 2-2.

Abbildung in dieser Leseprobe nicht enthalten

Fig. 2-2 Millimetre Wave feeder concept [13].

The Millimetre Wave feeder concept enables a significant reduction in maintenance owing to the fact that the antenna unit has a very simple set-up [35]. The antenna size is very small, low-priced and does not require an external power supply when passive electro-absorption modulators (EAM) [36] or even photo-diodes (PD) are used. The Millimetre Wave feeder consists of a low loss optical fibre that offers a large capacity and low production costs.

As described above, ROF uses mm-Waves as carrier frequencies. mm-Waves have the property that in the GHz range the transmission loss in the atmosphere increases with frequency. Additionally, at 24 GHz and 60 GHz the attenuation rises drastically due to the H2O and O2 absorption of the waves [15]. Furthermore, higher frequencies are much more affected by rain than waves in the lower frequency range [13]. This influences directly the cell diameter in the radio system. The planned cell sizes lie in the range of less than 100 m which corresponds to Pico cells. In standard mobile communication systems based on the cellular concept atmospheric loss is avoided. ROF uses this property, to reuse the carrier frequency in the next cluster. This increases the spectral efficiency and hence the transmission rate. The fact that the frequencies in the mm-Wave range have a lower penetration depth than signals with lower frequencies, has advantages for electromagnetic capability.

However, ROF can also be used for bridging the “last mile” in office buildings. The installation of optical fibres to each work place is a large cost factor in transmission networks. By offering high bandwidths via radio signals the users in overcrowded areas can be provided with any required data rates [36].

2.2.2.2 Radio Over Fibre Applications

In this chapter the variety of ROF techniques and hence the optical generation of Millimetre Waves is discussed. The properties - very high transmission rates and low cell diameters - offer great opportunities on the one hand and limit the applicability of the Radio Over Fibre technology on the other hand. Due to the fact, that cell diameters in the Pico cell range are small, planned applications in congested areas are preferred. Radio Over Fibre as a stand alone system can substitute present wireless communication systems such as GSM, UMTS, DECT, Wireless LANs, Microwave Multichannel/Video Distribution Systems (MMDS/MVDS), Local Multipoint Communication/ Distribution Service (LMCS/LMDS) etc.. Although applications are limited by the cell size, they increase in attractiveness due to their broadband transmission rate. ROF does not only provide users with mobile phone requirements. Moreover, it satisfies the demand of applications such as broadband internet access, multimedia applications of cell phones, computer networks and video on demand. A possible application scenario is demonstrated in Fig. 2-3.

Abbildung in dieser Leseprobe nicht enthalten

wireless link optical fibre RAU

Fig. 2-3 Possible Radio Over Fibre and Millimetre Wave applications.

Besides an application as a separate radio system the ROF technology can also be implemented into current mobile communication techniques such as WLAN or HIPERLAN for instance. Such systems operate mainly in the UHF range. Of course, the simple generation of ultra high frequencies in the mm-Wave range, which is the main object in this dissertation, can be adapted to lower frequency ranges. A further decrease of costs and complexity can be expected. The following scenarios of implementing ROF and hence a Millimetre Wave generation technique are reported in [13].

- The interconnection between terrestrial networks and WLAN (IEEE 802.11) can be realized by ROF. The radio base station corresponds to the central station (CS) where the complex technique is centralised. The WLAN access point reflects the RAU and consists only of an optic-electric conversion. By using Hybrid Fibre Radio (HFR) with an added repeater the indoor coverage of terrestrial systems can be enhanced.
- The application of HFR to the HIPERLAN/2 technology also allows a complexity reduction of the set-up. Thus, problems of carrying an RF or IF over cable from one device to the other would be avoided.
- Since LMDS operates in frequency bands above 30 GHz the attractiveness of ROF for LMDS has increased. The distribution service uses the digital base band (BB) and the analogue Optical Single Sideband (OSSB) as modulation techniques that are of interest for ROF. In respect to HFR the simple optical base band modulation has the advantage that the signal is conveyed through an optical fibre; hence a large amount of capacity is available. On the other hand, by using

BB modulation there are no possibilities for a central frequency allocation which is one interesting aspect of ROF. LMDS using OSSB modulation is more complex in its central station. But the access control consists again only of an optic-electronic converter and allows a complexity reduction as a typical property of ROF.

- Even Satellite Communication can be made more convenient for installation, use, and maintenance by applying ROF. The low loss optical fibre operates as the connection between service provider and the feeder station. The up/downlink channel for Digital Video Broadcasting (DVB) satellite TV uses carrier frequencies around 12 GHz and 18 GHz respectively. Fig. 2-4 demonstrates a possible feeder concept for the realisation of satellite video broadcasting. Note that polarisation is not considered.

A further possible implementation of ROF is the supplement of UMTS. Thus, hybrid fibre radio can also work as a connection link between the Radio Node Control (RNC) and the base stations. Due to the fact that there are no strict requirements on the dynamic range for UMTS an EAM could be used for the RAU realisation.

Abbildung in dieser Leseprobe nicht enthalten

Fig. 2-4 ROF implementation into a satellite broadcast system.

Abbildung in dieser Leseprobe nicht enthalten

3 Millimetre Wave Generation Systems

To classify all mm-Wave generation systems it is necessary to separate them into two fields: The electrical and the optical generation. A classification of the wide field of mm-Wave generation systems is shown in Fig. 3-1.

Generation of Millimetre Waves

Abbildung in dieser Leseprobe nicht enthalten

Fig. 3-1 Classification of possible mm-Wave generation techniques.

3.1 Electrical Millimetre Wave Generation

Although the electrical generation of extreme high frequencies has different applications than its optical counterpart, it is briefly explained for completeness. The propagation circumstances for electromagnetic waves in the mm-Wave range in cables, wave guides or the atmosphere are not suitable for long distance transmission. Hence, for ROF systems the oscillator has to be located directly at the transmitter. This would increase the complexity of the radio station enormously if electrical ways of carrier generation is chosen. A general drawback of electrical oscillators for telecommunication applications is the relatively small frequency tuning range.

Some common oscillators that operates in the mm-Wave range are presented here. Furthermore, a common technique for the generation of mm-Waves by frequency multiplication of signals to a higher level is explained.

3.1.1 Yttrium-Iron-Garnet Oscillators

Yttrium-Iron-Garnet (YIG) oscillators are Microwave oscillators which can be tuned over several octaves by an external magnetic field. A solid state cavity with a yttrium-iron garnet sphere as a frequency adjusting element is used. The sphere has an extremely high figure of merit and can be varied by applying an external magnetic field. A further enhancement of the frequency range can be achieved by gallium doping. The frequency range of YIG oscillators is between 0.5 and 25 GHz. Due to the high figure of merit YIG oscillators have a low phase noise property and a high adjustment linearity. To provide independency to temperature influences the cavity is located in a heated cover with a constant temperature of approximately 80 °C. Due to its relative large frequency tuning-range YIG oscillators are applied in spectrum analysers, microwave receivers or military devices [38]. Although the tuning range is very high, the 30 GHz threshold border can rarely be reached. However, YIG oscillators can be the basis for frequency multiplication systems [26], [39], [40].

3.1.2 Gunn Oscillators

The function of a Gunn-diode relies on the negative drift velocity as a function of the electrical field intensity. This causes a characteristic with a negative differential resistance. Gunn-diodes consist of n-doped GaAs. They do not have a pn-junction. Due to this the drift velocity decreases while the electrical field intensity increases as can be seen in Fig. 3-2. Owing to the negative resistance electrons accumulate and propagate in very fast packets through the Gunn element. If the Gunn element is

Abbildung in dieser Leseprobe nicht enthalten

Fig. 3-2 Drift velocity versus electrical field intensity in n-GaAs.

inserted into a resonator this property is employed for the generation of oscillations [41]. Gunn oscillators have output powers of 200 - 300 mW in the lower frequency range. For higher frequencies the emission power decreases to 0.29 mW at 289.74 GHz [42] or 1.6 mW at 329 GHz [43]. They have great potential regarding the emission frequency. Due to a high efficiency they can achieve oscillation frequencies from 1.5 GHz up to 500 GHz as the state of the art [43]. The phase noise is an important property of Gunn oscillators and is for instance: -113 dBc/Hz at 300 kHz offset from the carrier at a frequency of 60 GHz [44], -97 dBc/Hz at 100 kHz away from carrier at 24 GHz [45] or -110 dBc/Hz at 500 MHz off the oscillation frequency of 103 GHz and a power of 180 mW [46]. The limitations of Gunn oscillators lie in the low frequency tuning range and a high temperature-sensivity due to its extremely small dimensions [47].

3.1.3 Electron Tubes

Electron tubes have their main applications in Radar systems, the medical field, dielectric heaters or linear accelerators. This is due to the fact, that the transmitted signal has a high frequency and a high power at the same time. There was no reference found reporting on the phase noise measurements of electron tubes.

Table 3-1 Overview of Significant Properties of Electron Tubes (Powers correspond to Frequencies).

Abbildung in dieser Leseprobe nicht enthalten

Here, only a short overview of the main representatives of the wide field of so called “slow wave devices” such as planar tubes, velocity-modulated tubes and Gyrotrons is given. For detailed information see ref. [49].

3.1.4 Frequency Multiplier

A further way of generating extremely high frequencies is the multiplication of the frequency of a stable, linear and low phase noise, Microwave source. This could be a YIG or Gunn oscillator for example. It can be realised by applying the base frequency to an electronic semiconductor that is driven in the non-linear regime. This can be an Impact Ionisation Avalanche Transit Time Diode (IMPATT) [26], a Schottky diode [51], a point contact diode, a PIN-diode, a varactor diode, or a backward [49] diode for instance. Semiconductor diodes that use the charge storage effect of the forward directed pn-junctions for frequency multiplying achieve high efficiencies and high output powers [52], [54]. Due to the overmodulation harmonic waves are generated. All frequency components that are not useful for high frequency generation are suppressed by a band pass filter whereas the multiplied component is coupled out [55]. Frequency multiplication is a common application in laboratory equipment such as signal generators.

Although frequency multiplication is a convenient way for generating high frequency signals the method is limited by the following restrictions: since not only the frequency but also all properties of the base signal such as phase noise, fluctuations and other distortions are multiplied as well, the base signal has to be extremely pure with very good noise properties. Owing to the fact that harmonics have lower magnitudes than the fundamental, there is a significant loss of power.

3.2 Optical Millimetre Wave Generation

In the following the termini “heterodyne reception” respectively “detection” are used as synonyms for “generation”. This is due to the fact that generation relies on the optic-electrical conversion in a photo detector (PD). At the input of the PD the optical signal is “received” or “detected” and at the PD output the electrical signal is “generated”. More details about the relation of “receiving” and “generation” are given in section 3.2.2. In general optical mm-Wave generation techniques are most suitable for the field of Radio Over Fibre as described in the introduction. In contrast to the electrical signal generation mm-photonics transmit the data signal over large distances via an optical fibre to the point of use. Then the actual modulated mm- Wave carrier is generated and transmitted via air. For an overview of optical generation techniques see Fig. 3-1 on page 16.

3.2.1 Direct Detection

If the emitted light of LD is either direct or external modulated by a mm-Wave signal and detected by a PD (see Fig. 3-3) it can be classified as a directly received signal. If a direct reception is inevitable, an external modulation provides a much better performance due to a low RF-to-RF insertion loss. Furthermore, it allows suppression of the contribution of PD shot noise. Thus the noise figure of directly modulated links is substantially higher than externally modulated ones [56].

For external modulation MZMs or EAMs are useful since bandwidths up to 95 GHz can be expected [57]. This is one of the easiest ways of generating mm- Waves but there are many limitations on this technique. One restriction is a large influence of dispersion to the signal. Thus a 60 GHz signal is for instance smothered after 1 km SSMF because of dispersion [58]. This is due to the fact that the phases of the frequency components (carrier, upper/lower sideband) will change relative to each other along the fibre and interfere. Because of this, the application of direct detection for mm-Waves is not appropriate. However, a recently presented method proves that a 12.5 Gbit/s downlink can be realised in the 60 GHz band [59].

Abbildung in dieser Leseprobe nicht enthalten

Fig. 3-3 Direct and external modulation for direct reception links.

On the other hand, dispersion can be significantly reduced by adding a dispersion compensating fibre (DCF) with a length that corresponds to the dispersion of the link. Another method is the application of a chirped [60] or conventional fibre Bragg grating, together with an optical circulator [34] or specially designed FBGs, in order to filter the optical carrier and one signal sideband [61]. This would increase the system costs and the complexity.

3.2.2 Heterodyne Detection

Heterodyne detection relies on the beating between two waves E¡(t) and E2(t). The electric field of the one wave is given by

Abbildung in dieser Leseprobe nicht enthalten

(3.1)

and the field of the second wave can be written as

Abbildung in dieser Leseprobe nicht enthalten

(3.2)

The photodiode has a response to the intensity that is injected into the detector. The intensity depends on the square of the electric field [62]. That means the output signal is proportional to the square of the absolute value of the input. It can be written as [63]

Abbildung in dieser Leseprobe nicht enthalten

(3.3)

Abbildung in dieser Leseprobe nicht enthalten

(3.4)

The output has high frequency components (2ωι, 2ω2 and (ω2 + ω2)) and low frequency components (ω2 - ω2). If operation wavelengths around 1550 nm are considered, the corresponding frequencies lie around 192.9 THz. Due to the fact that the PD is unable to follow these high frequencies these components are filtered out. Owing to this non-linear operation the PD detects a signal containing the beat frequency that is ω^ = ω2 - ω2 and is described by

Abbildung in dieser Leseprobe nicht enthalten

(3.5)

In Fig. 3-4 the principle realisation of a heterodyne receiver is shown. The phase correlation of both signals impacts the phase of the heterodyned signal directly. In case where the phases are not controlled, stochastic phase fluctuations of each wave would lead to an increased phase noise of the heterodyned signal. Hence, the phase adjustment of both signals is one of the main objects that needs investigation.

Abbildung in dieser Leseprobe nicht enthalten

If either the field E¡(t) or E2(t) is modulated the information can be detected by a demodulator behind the PD. In the following the impact of an amplitude, frequency and phase modulation to the heterodyned signal is shown.

Amplitude Modulation:

A wave E¡(t) that is modulated in its amplitude by a signal

EmodAM(t) = Emod cos(œmad t+(pmod) can be written as

Abbildung in dieser Leseprobe nicht enthalten

(3.6)

For the heterodyne superposition of EAM(t) (3.6) and E2(t) (3.2) it follows that

Abbildung in dieser Leseprobe nicht enthalten

(3.7)

The calculations result in several terms containing DC components and frequencies at œmod and 2œmod. After neglecting all solutions that are not possible due to the performance of the PD such as 2ωι; 2ω2 or (ωι + ω2) for instance, the spectrum at the output of the PD can be described as

Abbildung in dieser Leseprobe nicht enthalten

(3 8)

with [Abbildung in dieser Leseprobe nicht enthalten] As can be seen in (3.8), the output signal at the

PD consists of the RF carrier and the upper and the lower sideband at ωRF + ωmod and

[Abbildung in dieser Leseprobe nicht enthalten] respectively.

Frequency and Phase Modulation:

The result of a frequency and a phase modulation is similar. The main difference lies in the fact, that the signal is impacted by a phase deviation or a frequency deviation. Furthermore, for a phase modulation all upper order sidebands have the opposite phase to the lower sideband of the same order [64].

A wave E1(t) that is modulated in its frequency or phase by a signal

[Abbildung in dieser Leseprobe nicht enthalten] can be written as

Abbildung in dieser Leseprobe nicht enthalten

(3.9)

with A(rn/f)mod as the frequency and phase deviation, respectively. For the heterodyne superposition of EPFM(t) (3.9) and E2(t) (3.2) it follows that

Abbildung in dieser Leseprobe nicht enthalten

(3 10)

Abbildung in dieser Leseprobe nicht enthalten

(3.10)

For the solution of a frequency/phase modulated signal it is necessary to transfer the cos(a + cos(b)) function in the first term of (3.10) to a Taylor row [65]. Each term of the row corresponds to the carrier and the sideband of the nth order. Theoretically the number of terms (sidebands) is unlimited. On the other hand, the power of the sidebands decrease in practice and can be neglected at higher orders. For simplification further calculations have been made with the second harmonic as a showcase. The heterodyned superposition of E2(t) with the second sideband is

Abbildung in dieser Leseprobe nicht enthalten

Again, the calculations bring several terms containing DC components and frequencies at 2œmod and 4œmod. After neglecting all solutions that are excluded due to the characteristic of the PD the spectrum of the superposition of the second harmonic of the frequency modulated signal E¡(t) and E2(t) can be described as

Abbildung in dieser Leseprobe nicht enthalten

(3.12)

In (3.12) it can be seen that the spectrum consists of the carrier roRF and the summation of roRF and 2romod for the second upper harmonic and the difference of roRF and 2romod for the second lower harmonic. Apart the term “7/8 E¡E2” the magnitude is determined by the frequency and the phase deviation A(œ/f)mod. Thus, it has been shown, that the modulated signal is unconverted to higher frequency. The phases act in all three modulation formats as well as the frequencies.

A special case of the heterodyne detection is known as the homodyne detection. In this case the frequency of the one wave E(t) fits exactly to the second wave E2(t). The o/e converter detects the mix frequency that is zero. Homodyne receivers are not used for the generation of mm-Waves due to a high system complexity [66] and a high impact of dispersion to the signals [62].

There are several methods for the generation of two phase correlated signals as a basis for the heterodyne detection technique that are described in the following subsection.

3.2.2.7 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. The longitudinal mode spacing Af corresponds to where nL is the group refractive index, LRes the separation distance of the resonator mirrors and c the speed of light. The number of half-wavelengths of the light for typical lasers is in the range of around 106 [67].

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 [67] or Mode Locking. Active mode locking (AML) can be realised by a modulation of the laser with a frequency fm that corresponds to the mode separation Af. Due to the stimulation of modes, the mode oscillations become dependent and their phases correlated. In contrast to AML passive mode locking (PML) is realised by a saturable absorber [68]. Although the practical realisation of AML is more complex than PML there are more control options of this method. A combination of AML and PML is represented by Hybrid Mode locking where lower modulation power as at AML is necessary [69]. On the other hand, Hybrid Mode locking and a special arrangement of the lasers increase the production costs [70], [71]. The high number of emitted modes is a further limiting factor of the technique due to dispersion effects. Although the optical filtering of two modes can generate dispersion independent signals there is a big decrease of power due to the screening procedure. Because the very complex mode locking analysis is not the focus of this thesis it is not investigated further at this point. For detailed information see [72].

Applications of mode locked lasers are a frequency modulated (FM) laser output, a specially scanning laser beam or an output of optical pulses with variable position for optical data transmission systems [70]. For mm-Wave generation techniques the generation of two phase correlated frequency components via mode locking takes a centre stage. For its realisation semiconductor laser diodes play a major role due to their compact set-up and a mode spacing in the GHz range.

There are efforts to investigate methods that generate only two locked modes (Dual-mode Laser) that are compact, efficient and emit signals that are also dispersion independent [73], [74].

In [75] an AML diode with an integrated high-mesa EAM was presented. Owing to an external modulation by an 80 GHz signal the AML falls into the locked mode and generates a frequency comb where two modes are dominant. After a heterodyne detection a 240 GHz signal with a power level of -11 dBm and a phase noise of - 86 dBc/Hz at 10 kHz offset from the carrier have been obtained. By using the same method the authors transmitted a 10 Gbit/s data signal at a carrier frequency of 125 GHz [76]. In [77] a 60 GHz carrier signal generation by using a dual laser which modes are selected by chirped gratings is described. Recently, due to the application of a mode locked fibre ring a 22.08 GHz signal with a spectral width of less than 1 Hz was recorded [78].

3.2.2.2 Injection Locking

In contrast to Mode Locked LDs in which the locking is realised by an electrical modulation the Injection Locking method is realised by an external optical signal [79]. A narrowband laser diode (master laser) is modulated by the desired mm-Wave signal. Due to this, the emission spectrum is broadened and sidebands are generated. If the frequency comb is injected into a second laser (slave laser) one sideband of the modulated master laser can lock the emitted signal of the slave laser to have the same phase as the sideband. An alternative is the application of two slave lasers that is schematised in Fig. 3-5. Each slave laser is locked by one sideband of the master laser.

Abbildung in dieser Leseprobe nicht enthalten

Fig. 3-5 Block diagram of the injection locking scheme with two slave lasers.

Hence, both lasers transmit two phase correlated signals with spectral separation that corresponds to the desired mm-wave signal. In [80] a 200 GHz signal and a power of 1.2 mW are predicted. The stimulated sidebands that are induced into the slave lasers are generated by a phase modulation. Noël et al. [81] presented an optical fibre link that uses a master/slave distributed feedback (DFB) arrangement to generate a 60 GHz carrier signal in an o/e converter. The authors show the transmission of a 120 Mbit/s Quadrature Phase Shift Keyed (QPSK) signal over a 100 km long SSMF and via air. A bidirectional transmission system using optical sideband injection locking was demonstrated in [82]. In that paper a 19 GHz carrier signal with a phase noise of -85 dBc/Hz at 10 kHz carrier offset and a 140 Mbit/s QPSK signal was generated and investigated.

Although many papers present different variations of Injection Locking with more or less good results the technique in general is complex in its set-up and strongly dependent on temperature influences. The wavelength detuning of master and slave spectrum has to be minimised by a temperature control of the laser and its environment. Thus, for optical injection locking a temperature change of a fraction of a degree leads to a fall out of lock in case of using standard DFB lasers [83].

3.2.2.3 Optical Phase Locking

A further method for the generation of two phase correlated frequency components is the heterodyne detection of the modes of two independent lasers in a PD [84]. The frequency separation corresponds to the desired mm-Wave signal as for all heterodyne methods. In case an uncorrelated reception occurs the detected signal is unstable. If an Optical Phase Locked Loop (OPLL) is applied to the emitted signals the phases can be varied and adapted to each other. This is done by a comparison of the beat signal of two lasers to the signal from a microwave reference oscillator with a phase noise a little lower than required for the generated carrier. The resulting phase difference signal is fed back to the slave laser that is thus forced to track the master laser [85], [86]. The principle of an OPLL is schematised in Fig. 3-6.

Abbildung in dieser Leseprobe nicht enthalten

Fig. 3-6 General set-up of an optical phase locked loop.

Owing to a relative high linewidth of semiconductor lasers (several MHz) the round- trip propagation delay should be very low (typical < 1 ns) [87].

Disadvantages of the application of optical phase locking for the generation of mm-Waves are the high system complexity and the feedback channel between control station and base station. It is required for the transmission of the phase information.

In [88] mm-Waves with a frequency of 110 GHz, 220 GHz and 330.566 GHz where obtained by a cascaded phase locking of three semiconductor lasers. A 140 Mbit/s amplitude shift keying (ASK) modulation over a 65 km uncompensated SSMF at 36 GHz was verified in [83]. The carrier signal has a phase noise of -93 dBc/Hz at 10 kHz carrier offset.

By applying three external-cavity lasers constructed from commercial laser diodes to a phase locked loop Andrew et al. [89] generated 33-40.5 GHz with a low
phase variance. A further millimetre wave generation technique by the optical phase locking of two laser diodes is realised by the application of an optical comb generator [90]. Thus, frequencies up to 100 GHz with a phase noise of -92.5 dBc/Hz at 10 kHz offset from the carrier have been observed.

3.2.2.4 Optical Frequency Multiplying by Modulation

A very elegant generation of two phase correlated signals can be achieved by the external modulation of laser diodes. An appropriate modulation scheme is represented by the exploitation of the non-linearities of external modulators such as Mach Zehnder (amplitude) Modulators (MZM).

A MZM consists of two wave guides that separate the incoming light as can be seen in Fig. 3-7 (a). This set-up results in an interferometer. Due to an applied voltage to a control electrode located at one wave guide, the phase is changed owing to a refractive index modulation. At the output the waves interfere with each other. If the phase difference of each wave part is equal to π the superposition is destructive hence, no light (logical “0”) is transmitted [56].

The other control electrode adjusts the operation point (OP) of the MZM via a bias voltage. The typical operation characteristic is illustrated in Fig. 3-7b. If the modulator is driven in the non-linear regime higher harmonics are generated. The operation in the lower quadratic OP leads to a double sideband suppressed carrier (DSSC) modulation whereas the operation in the upper quadratic OP leads to a generation of even order sidebands including the carrier [91]. A single sideband modulation (SSB) can be achieved by launching the modulation signal to the control electrode of both wave guides. Both signals have to have a phase shift of π.

Experimental verifications are illustrated in Fig. 3-8 to emphasise the flexibility of an MZM.

Abbildung in dieser Leseprobe nicht enthalten

Fig. 3-8 Output spectrum of an MZM at different operation points (OP).

- An application of Single Sideband modulation for Radio Over Fibre systems was described in [92] where a 155 Mbit/s Binary Phase Shift Keying (BPSK) was applied to a 38 GHz carrier signal. An error free transmission was observed over a distance of 50 km SSMF and 5 m radio propagation. A full- duplex mm-Wave link at 39 GHz carrier frequency was verified in [93] and in [94] SSB modulation was investigated under the aspect of dispersion penalties.
- Double Sideband Suppressed Carrier Modulation was used in [95] and in [96] for the generation of two phase correlated optical carriers while considering dispersion effects. Recently, a 2.5 Gbit/s ASK modulated 40 GHz carrier transmission over 44 km optical fibre was verified [97]. It was realised by a PRBS signal modulation of one sideband generated by a DSSC modulated optical carrier. There was no power penalty observed due to chromatic dispersion.
- A further mm-Wave generation method can be realised by Phase Modulation where the undesired frequency components are suppressed by
band pass filtering [98]. On the one hand, the number of generated sidebands is higher than at a conventional amplitude modulation. On the other hand, phase modulated optical carriers have the property that the odd order lower sidebands are in opposition to the corresponding upper sidebands [64]. Hence, odd order sidebands do not fulfil the requirements of a correlated status that is requested for heterodyne based ROF systems as a limiting factor.
- Whereas conventionally implemented modulators such as MZMs are based on Lithiumniob ate (LiNbO3) Electro Absorption Modulators gained in importance due to its fast modulation performance and low power requirements to modulation signals. EAMs are based on an absorption medium that passes light in dependency of the applied voltage. For this purpose effects of quantum mechanics are used [99]. An extension for a low chirp EAM modulator for ASK and Phase Shift Keying (PSK) is verified in [100] and an extensive investigation of a special designed EAM in the 60 GHz range for mm-Wave downlink systems is demonstrated in [101]. On the other hand, EAMs can be used as a photodetector as well since the absorbed light can be converted into an electrical signal. Due to this electro absorption modulators can be used in ROF systems as a modulator and a PD simultaneously. This leads to a further reduction of complexity in the base station [36], [102]. A comparison of EAMs and MZMs shows, that an EAM requires less modulation power for the generation of high order harmonics [103]. On the other hand, the MZMs achieve a high efficiency due to their cosines shaped dependency of the intensity to the applied voltage (see Fig. 3-7b).

A notable advantage of this generation method is that the generated optical carriers are totally phase correlated directly at the output of the modulator due to the fact, that they have the same emission source. There is neither a need for a phase control loop nor for any other unconventional super structural components in the set-up. On the other hand, the system is limited by the high frequency low noise modulation signal that is required for this method.

[...]

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^[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]

^[3] See chapter 3.1.1.