Investigation of Millimetre Wave Generation by stimulated Brillouin scattering for Radio Over Fibre Applications

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 in 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 effects 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 conclude and future tasks are discus

Semiconductor Laser Dynamics

This is a collection of 18 papers, two of which are reviews and seven are invited feature papers, that together form the Photonics Special Issue “Semiconductor Laser Dynamics: Fundamentals and Applications”, published in 2020. This collection is edited by Daan Lenstra, an internationally recognized specialist in the field for 40 years

Optical code division multiple access systems in AlGaInAs/InP

The rise of photonic integration makes optical code division multiple access (OCDMA) worth revisiting due to its promising role in future all-optical networks. OCDMA has the potential to exploit the surplus bandwidth of optical fibres and to carry over to the optical domain the benefits seen CDMA radio communication systems, such as the effective sharing of the spectrum for multiple network subscribers, and resistance to jamming and eavesdropping. One of the major requirements for the deployment of OCDMA in networks is integration. This thesis presents a research study of integrated OCDMA systems using the AlGaInAs/InP semiconductor material system. This material is considered due to its useful intrinsic properties such as thermal stability, strong electron confinement, and low threshold, making it suitable for fabricating optoelectronic devices. Two bespoke OCDMA systems are considered for integration: coherent temporal phase coding (TPC), and incoherent wavelength-hopping time-spreading (WHTS) OCDMA systems. TPC systems are excellent for high speed communications due to their static en/decoding enabling features. In this research, a 2×2 asymmetric Mach Zehnder interferometer (AMZI) is used to generate a 2-bit phase code, allowing multiplexing for up to four users. A semiconductor mode-locked ring laser is also embedded in the circuit, and using a synchronous mode-locking method, adequate signal en/decoding is achieved. WHTS systems on the other hand fully exploit the spectral and temporal space available in networks by assigning each user with a unique wavelength-time hop sequence for en/decoding data signals. Here, a mode-locked laser array is used with intracavity distributed Bragg reflectors (DBRs) for spectrally tuning each laser, and a 4:1 multimode interference coupler is used to combine the laser signals into a single channel for amplification, modulation and transmission. The integrated system is fully characterised and synchronisation experiments are performed to show the potential for its use in high speed multi-user networks. Mode-locked lasers play an important role in many OCDMA implementations due to their wide spectrum and discrete temporal properties, which can be easily exploited during data en/decoding. Various mode-locked laser devices have been studied during this research with additional embedded components such as intracavity DBRs and phase controllers for precise tuning of the wavelength and pulse repetition frequency. However, the noisy nature of passively operating mode-locked lasers make them prone to high jitter, which can result in high bit error rates. Synchronisation schemes are thereby explored in order to temporally stabilise the pulse oscillations to make them suitable for use in long haul transmission systems. This includes synchronous and hybrid mode-locking, as well as a passive technique using an optical fibre loop to provide phase feedback, which is shown to promote ultralow RF linewidths in mode-locked lasers

Phase noise reduction techniques for RF signal generator

EThOS - Electronic Theses Online ServiceGBUnited Kingdo

X-Band LLRF Developments for High Power CLIC Test Stands and Waveguide Interferometry for Phase Stabilisation

This thesis describes the upgrade of the first high power X-band RF test for high gradient accelerating structures at CERN, as required for the e+ e- collider research program; Compact Linear Collider, CLIC. Significant improvements to the control system and operation of the first test stand, Xbox-1, are implemented. The design and commissioning of the new Low Level Radio Frequency, LLRF, system is described in detail. The upgrade also encompasses software, interlock systems, timing, safety and control. The new LLRF requires an up-convertor to convert an input signal at 187.4 MHz to 11.806 GHz. The most common method is a phase locked loop, PLL, an alternative method was envisioned which uses single side-band up-convertor. This necessitated the design and manufacture of a custom cavity filter. The up-convertor and PLL are compared and both are implemented in the new LLRF. The new LLRF system is implemented at Xbox1 and used to RF condition a 50 MW CPI klystron, the final output power was 45 MW for a 50 ns RF pulse length. The phase and amplitude of the LLRF, TWT and klystron are characterised with both the PLL and up-convertor. The klystron phase stability was studied using a sensitivity analysis. The waveguide network between the klystron and the accelerating structures is approximately 30 m. This network is subject to environmental phase changes which affect the phase stability of the RF arriving at the structures. A single path inteferometer was designed which will allow a phase measurement pulse at a secondary frequency to be injected into the waveguide network interleaved with klystron pulses. The interferometer is commissioned in the lab and low power measurements validate its operation. The system is then integrated into the high power network at Xbox1 and used to measure phase shifts in the waveguide network which are correlated with temperature

Reconfigurable multi-carrier transmitters and their application in next generation optical networks

With the advent of new series of Internet services and applications, future networks will have to go beyond basic Internet connectivity and encompass diverse services including connected sensors, smart devices, vehicles, and homes. Today’s telecommunication systems are static, with pre-provisioned links requiring an expensive and time-consuming reconfiguration process. Hence, future networks need to be flexible and programmable, allowing for resources to be directed, where the demand exists, thus improving network efficiency. A cost-effective solution is to utilise the legacy fibre infrastructure more efficiently, by reducing the size of the guard bands and allowing closer optical carrier spacing, thereby increasing the overall spectral efficiency. However, such a scheme imposes stringent transmitter requirements such as frequency stability, which would not be met with the incumbent laser-array based transmitters. An attractive alternative would be to employ an optical frequency comb (OFC), which generates multiple phase correlated carriers with precise frequency separation. The reconfigurability of such a multi-carrier transmitter would enable tuning of channel spacing, number of carriers and emission wavelengths, according to the dynamic network demands. This research thesis presents the work carried out, in the physical layer, towards realising reconfigurability of an optical multi-carrier transmitter system. The work focuses on an externally injected gain-switched laser-based OFC (EI-GSL), which is a particular type of multi-carrier source. Apart from the detailed characterisation of GSL OFCs, advances to the state of the art are achieved via comb expansion, investigating new demultiplexing methods and system implementations. Firstly, two novel broadband GS-OFC generation techniques are proposed and experimentally demonstrated. Subsequently, two flexible and compact demultiplexing solutions, based on micro-ring resonators and laser based active demultiplexers are investigated. Finally, the application of a reconfigurable multi-carrier transmitter, employed in access and data centre networks, as well as analog-radio over fibre (A-RoF) distribution systems, is experimentally demonstrated

Development of high-performance quantum dot mode-locked optical frequency comb

This PhD thesis focus on the development of high-performance optical frequency combs (OFCs) generated by two-section passively mode-locked lasers (MLLs) based on novel optimised InAs quantum dot (QD) structures grown on GaAs substrates. Throughout the thesis, several important aspects are covered: the epitaxial structures, the device designs, the fabrication process, the characterisation of the fabricated laser devices and the evaluation of their performance. To gain a deep level comprehension of the mode-locking mechanisms in two-section QD MLLs, a detailed study is presented on a series of QD MLLs with different saturable absorber (SA) to gain section length ratios (from 1: 3 to 1: 7) in either ridged-waveguide structure or tapered waveguide structure. The effect of temperature on different device configurations is experimentally examined. And the data transmission capability of the QD MLLs is systematically investigated in different scenarios. In this thesis, an ultra-stable 25.5 GHz QD mode-locked OFC source emitted solely from the QD ground state from 20 °C to a world record 120 °C with only 0.07 GHz tone spacing variation has been demonstrated. Meanwhile, a passively QD MLL with 100 GHz fundamental repetition rate is developed for the first time, enabling 128 Gbit s−1 λ−1 PAM4 optical transmission and 64 Gbit s−1 λ−1 NRZ optical transmission through 5-km SSMF and 2-m free-space, respectively. All of the studies aim to prove that our two-section passively InAs QD MLLs can be used as simple, compact, easy-to-operate, and power-efficient multi-wavelength OFC sources for future high-speed and large-capacity optical communications

Feed-forward linearisation of a directly modulated semiconductor laser and broadband millimetre-wave wireless over fibre systems.

This thesis is concerned with reduction of non-linear distortion in a directly modulated uncooled semiconductor laser using feed-forward compensation and investigating the performance of broadband millimetre-wave wireless over fibre systems. One of the key elements that determine the performance in a fibre optic link is the linearity of the optical source. Direct modulation of an uncooled semiconductor laser diode is a simple and cost effective solution. However, the distortion and noise generated by the laser limit the achievable dynamic range and performance in a system. Feed-forward linearisation is demonstrated at 5 GHz, the highest operating frequency reported, with 26 dB third order intermodulation distortion suppression and simultaneous noise reduction leading to enhanced spurious free dynamic range of 107 dB (1Hz). The effectiveness of feed-forward in a multi-channel system is investigated. Laser non-linearity can cause spectral re-growth and interchannel distortion that can completely mask the adjacent channel. A significant 11 dB interchannel distortion suppression and 10.5 dB power advantage is obtained compared to the non-linearised case. These results suggest that feed-forward linearisation arrangement can make a practical multi-channel or multi-operator wireless over fibre system. In the second part of this thesis the first experimental transmission of wireless data over fibre with remote millimetre-wave local oscillator delivery using a bi-directional semiconductor optical amplifier in a full duplex system with 2.2 km coarse wavelength division multiplexing fibre ring architecture is demonstrated. The use of bi-directional SOAs in place of unidirectional erbium doped fibre amplifier or unidirectional SOAs, together with the use of CWDM and optical distribution of the local oscillator signal allow substantial reduction in overall complexity and cost. Successful transmission of data over 12.8 km fibre is achieved with clear and well defined constellations and eye diagrams as well as 10.5% and 7.8 % error vector magnitude values for the downlink and uplink directions, respectively. The thesis also presents an implementation and performance of a millimetre-wave gigabit wireless over fibre system. CWDM devices such as uncooled laser diodes and passive components are used for the first time in a gigabit system allowing cost savings compared to dense WDM. This makes such solutions more attractive for millimetre-wave access systems. Optically modulated gigabit wireless data signals to and from the base stations are distributed at 5 GHz and up-converted using a remotely delivered LO source. Eye diagrams and bit error rate are measured to assess the system performance

Characterisation of optimum devices and parameters for enhanced optical frequency comb generation

The Internet has become an irreplaceable aspect of our daily life. It is used every day by billions of people around the world for various functions such as business, study, and entertainment. Hence, an unabated rise in the demand for higher and faster data traffic has been experienced through the last few decades. This demand for bandwidth is further fuelled by the introduction of bandwidth intensive applications such as ultra-high-definition video streaming, real time online gaming and cloud services making the realization of higher capacity and performance optical networks a necessity. Today’s telecommunication systems are static, with pre-provisioned links requiring an expensive and time-consuming reconfiguration process. The state-of-the-art approach (wavelength division multiplexing - WDM), entailing multiple lasers emitting differing wavelengths (each modulated) multiplexed together (on a 50 GHz grid), cannot meet the growing demands. Hence, future networks need to be flexible and programmable, allowing for resources to be directed, where the demand exists, thus improving network efficiency. A cost-effective solution is to utilise the legacy fibre infrastructure more efficiently by reducing the size of the guard bands and allowing closer optical carrier spacing, thereby increasing the overall spectral efficiency. However, such a scheme imposes a stringent transmitter requirement in terms of wavelength stability, noise properties and cost-efficiency, which would not be met with the incumbent laser-array based transmitters. An attractive alternative would be to employ an optical frequency comb (OFC), which generates multiple phase-correlated optical carriers with a precise frequency separation. The reconfigurability of such a multi-carrier transmitter would enable tuning of channel spacing, number of carriers and emission wavelengths, according to the dynamic network demands. This thesis focusses on the externally injected gain-switched laser-based OFC (GSL-OFC) technique. Advances to the state of the art are achieved via a detailed static and dynamic characterisation of lasers, which is then used for enhancing the comb generation process. Specifically, initial efforts are devoted to the use of different laser structures for OFC generation. This aspect is then furthered by incorporating the concept of photonic integration to reduce the cost, power consumption and footprint of the multi-carrier transmitter. Self and externally seeded photonic integrated circuits are used to generate combs that are then fully characterized to verify their employability in optical networks