Optical regeneration has the potential to significantly increase the reach of long-haul transmission systems. In this thesis, wavelength-preserving polarization-insensitive all-optical 3R regeneration is investigated and demonstrated for 10 and 40 Gb/s signals. The all-optical regenerator utilizes a self-pulsating laser for clock recovery, cross-phase modulation (XPM) based spectral broadening in a highly nonlinear fiber (HNLF) and offset filtering for retiming, and self-phase modulation based spectral broadening in a HNLF and offset filtering for reshaping. Raman amplification is used to increase the XPM-based spectral broadening and thus allow a design that meets the tradeoffs involved in simultaneously achieving good retiming and reshaping performance. The regenerator is shown to reduce amplitude noise and timing jitter while not causing a BER penalty. To fully validate the regeneration scheme, the cascadability is demonstrated using a recirculating loop. For a 10 Gb/s signal, with a regenerator spacing of 240 km, a return-to-zero, on-off-keyed (RZ-OOK) signal was transmitted over 18,000 km (75 loops) with a power penalty of 1.6 dB at a BER of 1E-9 compared to the back-to-back case. For a 40 Gb/s signal, with a regenerator spacing of 80 km, a RZ-OOK signal was transmitted over 8,000 km (100 loops) with a power penalty of 1.2 dB. In addition, all-optical 3R regeneration is demonstrated using a multimode quantum-dot Fabry Petot laser with ultra-low timing jitter. / Thesis (Ph.D, Electrical & Computer Engineering) -- Queen's University, 2009-10-19 14:11:53.826

Thesis (Ph.D.) - University of St Andrews, April 2009.

Thesis (Ph. D.)--Ohio State University, 2006. / Title from first page of PDF file. Includes bibliographical references (p. 129-133).

Thesis (Ph. D.)--University of Texas at Austin, 2006. / Vita. Includes bibliographical references.

In this thesis we ran a series of 2D simulations of femtosecond laser pulses filamenting in air using the FDTD method, a saturable Lorentz oscillator model of air [1], and two separate models of plasma: a Drude model where the plasma density is static in space, and a particle-in-cell model where plasma is free to migrate throughout the simulation space. By comparing matched pairs of simulations, which varied in pulse size, duration, and intensity, we can gauge the impact plasma dynamics has upon the evolution of a filamenting laser pulse. From these tests we determine that, while there are some visible differences between dynamic and static plasmas, plasma dynamics do not significantly alter the evolution of the pulse.

This doctoral dissertation focuses on the nonlinear optical response of water vapour as well as some solids at terahertz (THz) frequencies. In this study, the propagation of broadband single-cycle THz pulses through a medium with the third-order nonlinear optical response is theoretically investigated. Also, a technique to measure the nonlinear response of transparent materials based on the time-domain THz spectroscopy is developed, which provides frequency dispersion curves of the nonlinear Kerr coefficient (n₂). A numerical model is used to simulate the THz pulse propagation. This model takes into account non-paraxial effects, self-focusing, and diffraction, as well as dispersion, in both the linear and nonlinear optical regimes. The contribution of non-instantaneous Kerr-type nonlinearity to the overall instantaneous and delayed Kerr effect at the THz frequencies is investigated. It is shown how increasing the nonlinear relaxation time and its dispersion modifies the THz pulse after the propagation through a transparent medium. The effect of linear dispersion on self-action during pulse propagation is also discussed. Moreover, the nonlinear spectroscopy of water vapour at THz frequencies is reported. Atmospheric water vapour has a rich spectrum with several strong resonances at frequencies below 3 THz, falling within the range of operation of most existing THz sources. An extremely large nonlinear response to THz radiation is observed at the positions of these resonances. Using the optical Kerr model for the nonlinear response, a minimum nonlinear refractive index of the order of 10² m²/W is estimated. The results provide insight into the energy levels of the water molecule and give a more accurate picture of its response to electromagnetic radiation, paving the way to more accurate THz spectroscopy, imaging, and sensing systems, and thereby facilitating future emerging THz technologies. Finally, the nonlinear response of solids at THz frequencies is studied. It has been shown that a phonon-induced THz Kerr effect can result in a larger nonlinear refractive index than the nonlinear refractive index at the visible or near-infrared range (optical Kerr effect). This pronounced nonlinear optical behavior is verified using a time-domain characterization approach. The results indicate a large delay occurred to the THz fields as they transmit through some of the material samples. In the frequency domain, the induced nonlinear phase shift of the intense THz field is shown to be relatively large of the order of 0.1 rad. From the phase information, the nonlinear phase is extracted by which the dispersion profile of n₂ is obtained.

No description available.

This thesis explores the theoretical foundations of cross-phase modulation (XPM) between optical fields in the N-configuration atom. This is the process by which the refractive index experienced by one field can be modulated by controlling the intensity of another. The electro-optical version of this effect was first discovered by John Kerr in 1875 and found applications in photonics as a means of very rapidly modulating the phase and intensity of electromagnetic fields. Due to recent advances in experimental techniques there has been growing interest in generating nonlinear optical interactions in coherently prepared atomic ensembles. The use of coherently prepared media brings the possibility of achieving a much larger cross-phase modulation than is possible using classical materials. This is particularly useful when trying to create large optical nonlinearities between low-intensity electromagnetic fields. Much of the current research into cross-phase modulation is directed towards realising potential applications in the emerging field of quantum information processing. Above all, the possibility of constructing an all-optical quantum computer has been at the heart of much research and controversy in the field. In this thesis the theory of steady-state, transient and pulsed cross-phase modulation is developed. Moreover, care has been taken to relate all research back to experimentally feasible situations. As such, the relevance of the theory is justified by consideration of the situation present in rubidium-87. Due to the close relationship between XPM in the N-configuration atom and electromagnetically induced transparency in the Lambda-atom, many similarities and insights act as link between these two fields. Indeed, it is frequently demonstrated that the key to understanding the various properties of XPM in the N-configuration atom is by comparison with the situation in the corresponding Lambda-atom equivalent.

We present a time-resolved Kerr microscope, capable of measuring the magnetization dynamics of samples grown on transparent, double-side-polished substrates.The magnetization is excited by a current pulse, using a coplanar waveguide placed beneath the samples. The Kerr rotation is detected with the stroboscopic pump-probe technique, using a probing laser, synchronized with the current pulse.We report benchmark measurements of the time-resolved Kerr instrument for magnetization dynamics in thin permalloy and FePd films. The experimental results for ferromagnetic resonance peaks have been compared with the values predicted by Kittel. / Vi presenterar ett tidsupplöst Kerr-mikroskop, kapabel till att mäta magnetiseringsdynamiken hos magnetiska prov tillverkade på transparenta, dubbelsdigt polerade substrat. Magnetiseringen exiteras med en strömpuls via en koplanär vågguide placerad under provet. Kerr-rotationen detekteras med hjälp av en stroboskopisk ”pump-probe” teknik som använder en ”probing” laser synchroniserad med strömpulsen. Vi rapporterar prestandatest av det tidsupplösta kerr-instrumentet för magnetiseringsdynamiken i tunna permalloy - och FePd - filmer. De experimentella resultaten för de ferromagnetiska resonans-peakarna har jämförts med beräknade värden från Kittel.

The ability of a ferromagnet to maintain its magnetic state in the absence of an external magnetic field has made ferromagnetic materials an important subject of study in physics since the end of the 19th century. Moreover, ferromagnetic materials are the cornerstone for data storage systems such as magnetic tapes, magnetic disk drives and magnetic random access memory. The discovery of the Giant Magneto Resistance (GMR) in 1988 suggested that, since the magnetic state of the electrical conductor has an important effect upon the current flow, there may also be an inverse influence of the current upon the magnetization. In this effect, predicted in 1989 [1] by Slonczewski and called Spin Transfer Torque, angular momentum transferred by a spin polarized current can exert a torque on the magnetization of a ferromagnetic material, changing the local magnetization and stimulating the precession of the magnetic moments, generating microwave signals. This provides a new method of manipulating magnetization without applying an external field. Large polarized currents lead to spin transfer effects which are the driving force for the magnetic dynamics of devices known as Spin Transfer Oscillators (STO). In this new kind of nano-device the emission of microwaves is stimulated by a DC electrical current and measured as a change in the output voltage due the GMR effect. The specific characteristics of these devices such as working frequency and DC current ranges, microwave emission linewidth, and maximum emission power among others, are given by the design and size of the device,and the nature of the magnetic oscillations producing the emission. Among the multiple types of STO that now exist , I have focused my research upon three of them: Spin Transfer Vortex Oscillators (STVO), Single Layer Spin Transfer Oscillators (SL-STO) and Orthogonal Pseudo Spin Valves. Within STVOs and SL-STOs we can nucleate what is called a magnetic vortex. A magnetic vortex is a curling of the in-plane of a magnetic layer with its centre pointing out of the magnetization plane. The gyration of this vortex due to STT produces a microwave emission < 1GHz with a greater emission power than that produced by the precession of magnetic moments in STOs. The phase-locked synchronisation of multiple vortices is expected to exhibit enhanced microwaved power and phase stability compared to a single vortex device, providing a solution to the drawbacks of the STO in the low frequency regime. On the other hand, Orthogonal Pseudo Spin Valves promote the nucleation of magnetic dissipative solitons, also called magnetic droplets. This type of magnetic structure has an opposite out of plane magnetization to the layer that contains it. Compared to the microwave emission of magnetic vortices , magnetic droplets have a higher frequency range and emission power. However, their nucleation is subject to large external fields being applied to the sample. In this thesis, I electrically characterized these devices and applied magnetic imaging techniques in order to go further in the understanding of the spatial features and dynamic behaviour of these magnetic structures. It is not possible to acquire this knowledge by only using electrical characterization. Understanding the magnetization dynamics in these devices is crucial for the design of STO based devices while imaging studies are required to prove the existence of these magnetic structures, as in case of the magnetic droplet. In chapter 2 I will introduce the background concepts of magnetism that are relevant to this thesis. I will go from the basics principles of ferromagnetism, its quantum mechanical treatment, and the theory that explain the dynamics of the magnetisation. I will also present the state of the art in experimental research in the field of spin transfer oscillators. My aim is to give the basic background needed to understand the results presented in this thesis. In chapter 3 I will introduce the two main experimental techniques used for imaging the magnetisation of the devices presented: Holography with Extended Reference by Autocorrelation Linear Differential Operator (HERALDO) and Time Resolved Scanning Kerr Microscopy (TRSKM). I will revise the theoretical background concepts and the development of the techniques in order to demostrate the uniqueness of each technique and how they were used in this thesis. It is interesting to note that while MOKE is a well-known and widely-used technique, far fewer laboratories in the world area able to perform time resolved measurements using MOKE, with the University of Exeter being one of them. Furthermore, HERALDO is a novel technique that is used for the first time to image magnetic structures within multilayer systems in this thesis, which is a milestone in the development of the techinque. In chapter 4 I present an investigation of the magnetization dynamics of a SL-STO. Electrical transport measurements provided an initial characterization of the device. We then used HERALDO for the first time to investigate the magnetization dynamics in an intermediate layer of a multilayer stack. We present time averaged measurements of the magnetisation of a magnetic vortex formed underneath a nano contact (NC) positioned on top of the multilayer, using a combination of x-ray holography and x-ray magnetic circular dichroism. In chapter 5 I present the first direct measurement at the time of a magnetic dissipative droplet, using holography with extended reference autocorrelation by linear differential operator (HERALDO). I studied the out of plane magnetisation of the free layer under a NC within an orthogonal pseudo spin salve. In chapter 6 I present and study STVO devices with pairs of NCs of 100 nm diameter and centre-to-centre separation D = 200 to 1100 nm, by a combination of electrical measurements and time-resolved scanning Kerr microscopy (TRSKM). It will be shown that the dynamic behaviour of vortices and anti vortices changes when the distances between the NCs within the devices is changed.