Mathematical and numerical modelling of dispersion-managed solitons, autosolitons and self-similar optical pulses

This thesis presents theoretical investigation of three topics concerned with nonlinear optical pulse propagation in optical fibres. The techniques used are mathematical analysis and numerical modelling. Firstly, dispersion-managed (DM) solitons in fibre lines employing a weak dispersion map are analysed by means of a perturbation approach. In the case of small dispersion map strengths the average pulse dynamics is described by a perturbation approach (NLS) equation. Applying a perturbation theory, based on the Inverse Scattering Transform method, an analytic expression for the envelope of the DM soliton is derived. This expression correctly predicts the power enhancement arising from the dispersion management.Secondly, autosoliton transmission in DM fibre systems with periodical in-line deployment of nonlinear optical loop mirrors (NOLMs) is investigated. The use of in-line NOLMs is addressed as a general technique for all-optical passive 2R regeneration of return-to-zero data in high speed transmission system with strong dispersion management. By system optimisation, the feasibility of ultra-long single-channel and wavelength-division multiplexed data transmission at bit-rates ³ 40 Gbit s-1 in standard fibre-based systems is demonstrated. The tolerance limits of the results are defined.Thirdly, solutions of the NLS equation with gain and normal dispersion, that describes optical pulse propagation in an amplifying medium, are examined. A self-similar parabolic solution in the energy-containing core of the pulse is matched through Painlevé functions to the linear low-amplitude tails. The analysis provides a full description of the features of high-power pulses generated in an amplifying medium.

Exploitation of fibre characteristics in temperature sensing and radio-over-fibre applications

The development of sensing devices is one of the instrumentation fields that has grown rapidly in the last decade. Corresponding to the swift advance in the development of microelectronic sensors, optical fibre sensors are widely investigated because of their advantageous properties over the electronics sensors such as their wavelength multiplexing capability and high sensitivity to temperature, pressure, strain, vibration and acoustic emission. Moreover, optical fibre sensors are more attractive than the electronics sensors as they can perform distributed sensing, in terms of covering a reasonably large area using a single piece of fibre. Apart from being a responsive element in the sensing field, optical fibre possesses good assets in generating, distributing, processing and transmitting signals in the future broadband information network. These assets include wide bandwidth, high capacity and low loss that grant mobility and flexibility for wireless access systems. Among these core technologies, the fibre optic signal processing and transmission of optical and radio frequency signals have been the subjects of study in this thesis. Based on the intrinsic properties of single-mode optical fibre, this thesis aims to exploit the fibre characteristics such as thermal sensitivity, birefringence, dispersion and nonlinearity, in the applications of temperature sensing and radio-over-fibre systems. By exploiting the fibre thermal sensitivity, a fully distributed temperature sensing system consisting of an apodised chirped fibre Bragg grating has been implemented. The proposed system has proven to be efficient in characterising grating and providing the information of temperature variation, location and width of the heat source applied in the area under test.To exploit the fibre birefringence, a fibre delay line filter using a single high-birefringence optical fibre structure has been presented. The proposed filter can be reconfigured and programmed by adjusting the input azimuth of launched light, as well as the strength and direction of the applied coupling, to meet the requirements of signal processing for different purposes in microwave photonic and optical filtering applications. To exploit the fibre dispersion and nonlinearity, experimental investigations have been carried out to study their joint effect in high power double-sideband and single-sideband modulated links with the presence of fibre loss. The experimental results have been theoretically verified based on the in-house implementation of the split-step Fourier method applied to the generalised nonlinear Schrödinger equation. Further simulation study on the inter-modulation distortion in two-tone signal transmission has also been presented so as to show the effect of nonlinearity of one channel on the other. In addition to the experimental work, numerical simulations have also been carried out in all the proposed systems, to ensure that all the aspects concerned are comprehensively investigated.

Nonlinearly regenerated long haul data transmission

This thesis experimentally examines the use of different techniques for optical fibre transmission over ultra long haul distances. Its format firstly examines the use of dispersion management as a means of achieving long haul communications. Secondly, examining the use concatenated NOLMs for DM autosoliton ultra long haul propagation, by comparing their performance with a generic system without NOLMs. Thirdly, timing jitter in concatenated NOLM system is examined and compared to the generic system and lastly issues of OTDM amplitude non-uniformity from channel to channel in a saturable absorber, specifically a NOLM, are raised. Transmission at a rate of 40Gbit/s is studied in an all-Raman amplified standard fibre link with amplifier spacing of the order of 80km. We demonstrate in this thesis that the detrimental effects associated with high power Raman amplification can be minimized by dispersion map optimization. As a result, a transmission distance of 1600 km (2000km including dispersion compensating fibre) has been achieved in standard single mode fibre. The use of concatenated NOLMs to provide a stable propagation regime has been proposed theoretically. In this thesis, the observation experimentally of autosoliton propagation is shown for the first time in a dispersion managed optical transmission system. The system is based on a strong dispersion map with large amplifier spacing. Operation at transmission rates of 10, 40 and 80Gbit/s is demonstrated. With an insertion of a stabilizing element to the NOLM, the transmission of a 10 and 20Gbit/s data stream was extended and demonstrated experimentally. Error-free propagation over 100 and 20 thousand kilometres has been achieved at 10 and 20Gbit/s respectively, with terrestrial amplifier spacing. The monitor of timing jitter is of importance to all optical systems. Evolution of timing jitter in a DM autosoliton system has been studied in this thesis and analyzed at bit ranges from 10Gbit/s to 80Gbit/s. Non-linear guiding by in-line regenerators considerably changes the dynamics of jitter accumulation. As transmission systems require higher data rates, the use of OTDM will become more prolific. The dynamics of switching and transmission of an optical signal comprising individual OTDM channels of unequal amplitudes in a dispersion-managed link with in-line non-linear fibre loop mirrors is investigated.

Multispectral imaging of the ocular fundus using light emitting diode illumination

We present an imaging system based on light emitting diode (LED) illumination that produces multispectral optical images of the human ocular fundus. It uses a conventional fundus camera equipped with a high power LED light source and a highly sensitive electron-multiplying charge coupled device camera. It is able to take pictures at a series of wavelengths in rapid succession at short exposure times, thereby eliminating the image shift introduced by natural eye movements (saccades). In contrast with snapshot systems the images retain full spatial resolution. The system is not suitable for applications where the full spectral resolution is required as it uses discrete wavebands for illumination. This is not a problem in retinal imaging where the use of selected wavelengths is common. The modular nature of the light source allows new wavelengths to be introduced easily and at low cost. The use of wavelength-specific LEDs as a source is preferable to white light illumination and subsequent filtering of the remitted light as it minimizes the total light exposure of the subject. The system is controlled via a graphical user interface that enables flexible control of intensity, duration, and sequencing of sources in synchrony with the camera. Our initial experiments indicate that the system can acquire multispectral image sequences of the human retina at exposure times of 0.05 s in the range of 500-620 nm with mean signal to noise ratio of 17 dB (min 11, std 4.5), making it suitable for quantitative analysis with application to the diagnosis and screening of eye diseases such as diabetic retinopathy and age-related macular degeneration.

Dissipative nonlinear structures in optical fiber systems III : dissipative solitons via nonlinear mapping

In the third and final talk on dissipative structures in fiber applications, we discuss mathematical techniques that can be used to characterize modern laser systems that consist of several discrete elements. In particular, we use a nonlinear mapping technique to evaluate high power laser systems where significant changes in the pulse evolution per cavity round trip is observed. We demonstrate that dissipative soliton solutions might be effectively described using this Poincaré mapping approach.

Short-circuit and ground fault analyses and location in VSC-based DC network cables

The application of high-power voltage-source converters (VSCs) to multiterminal dc networks is attracting research interest. The development of VSC-based dc networks is constrained by the lack of operational experience, the immaturity of appropriate protective devices, and the lack of appropriate fault analysis techniques. VSCs are vulnerable to dc-cable short-circuit and ground faults due to the high discharge current from the dc-link capacitance. However, faults occurring along the interconnecting dc cables are most likely to threaten system operation. In this paper, cable faults in VSC-based dc networks are analyzed in detail with the identification and definition of the most serious stages of the fault that need to be avoided. A fault location method is proposed because this is a prerequisite for an effective design of a fault protection scheme. It is demonstrated that it is relatively easy to evaluate the distance to a short-circuit fault using voltage reference comparison. For the more difficult challenge of locating ground faults, a method of estimating both the ground resistance and the distance to the fault is proposed by analyzing the initial stage of the fault transient. Analysis of the proposed method is provided and is based on simulation results, with a range of fault resistances, distances, and operational conditions considered.

A study of the relationship between exit, voice, loyalty and neglect and commitment in India

Drawing on exit, voice, loyalty and neglect (EVLN) literature, this study examines direct and interactive associations between organizational-level commitment and team-level commitment and the use of EVLN by managers in India. The study is based on a survey of 200 managers and supervisors from seven Indian firms. The findings on the use of voice are consistent with the past research in Western countries, but challenge the prevailing assumption about the use of voice in high power distance societies. The results also indicate that team-level commitment moderates the association between organizational-level commitment and the use of EVLN.

Fiber optical parametric amplifier performance in a 1-Tb/s DWDM communication system

We have reduced signal-signal four-wave mixing crosstalk in a fiber optical parametric amplifier (OPA) by using a short nonlinear fiber for the gain medium and a high-power pump. This allowed us to obtain less than 1 dB penalty for amplification of 26 dense wavelength-division multiplexed (WDM) channels modulated at 43.7Gb/s return to zero-differential phase-shift keying, with the OPA placed between transmitter and receiver. We then used the same OPA in several different roles for a long-haul transmission system. We did not insert the OPA within the loop, but investigated this role indirectly by using equivalent results for small numbers of loop recirculations. We found that standard erbium-doped fiber amplifiers currently hold an advantage over this OPA, which becomes negligible for long distances. This paper shows that at this time OPAs can handle amplification of WDM traffic in excess of 1 Tb/s with little degradation. It also indicates that with further improvements, fiber OPAs could be a contender for wideband amplification in future optical communication networks.

In-fiber linear polarizer based on UV-inscribed 45° tilted grating in polarization maintaining fiber

We report an in-fiber linear polarizer structured by UV-inscribing a 45° tilted fiber grating (TFG) into polarization maintaining (PM) fiber along its principal axis. The polarization extinction ratio (PER) achieved by a 48 mm long 45° TFG has reached 46 dB at 1550 nm and the overall PER is >40 dB over a 50 nm wavelength range. Such 45° TFG based polarizers have many advantages over conventional products, including low loss, low cost, simple fabrication process, and no physical modification to the fiber, thus offering high stability and capable of handling high power.

Dissipative nonlinear structures in optical fiber systems III:dissipative solitons via nonlinear mapping

In the third and final talk on dissipative structures in fiber applications, we discuss mathematical techniques that can be used to characterize modern laser systems that consist of several discrete elements. In particular, we use a nonlinear mapping technique to evaluate high power laser systems where significant changes in the pulse evolution per cavity round trip is observed. We demonstrate that dissipative soliton solutions might be effectively described using this Poincaré mapping approach.

Theory of parabolic pulse generation in tapered fiber

We examine similarities and differences between high-power parabolic pulse generation in an active medium and in tapered fiber with decreasing normal dispersion. Using a realistic tapered fiber design, we demonstrate the possibility of parabolic pulse generation without an external pump and determine the limitations of this approach. © 2007 Optical Society of America.

Efficient yellow-green light generation at 561 nm by frequency-doubling of a QD-FBG laser diode in a PPLN waveguide

A compact high-power yellow-green continuous wave (CW) laser source based on second-harmonic generation (SHG) in a 5% MgO doped periodically poled congruent lithium niobate (PPLN) waveguide crystal pumped by a quantum-dot fiber Bragg grating (QD-FBG) laser diode is demonstrated. A frequency-doubled power of 90.11 mW at the wavelength of 560.68 nm with a conversion efficiency of 52.4% is reported. To the best of our knowledge, this represents the highest output power and conversion efficiency achieved to date in this spectral region from a diode-pumped PPLN waveguide crystal, which could prove extremely valuable for the deployment of such a source in a wide range of biomedical applications.

Analysis and improvement of performance in LCL filter-based PWM rectifier/inverter application using hybrid damping approach

A hybrid passive-active damping solution with improved system stability margin and enhanced dynamic performance is proposed for high power grid interactive converters. In grid connected active rectifier/inverter application, line side LCL filter improves the high frequency attenuation and makes the converter compatible with the stringent grid power quality regulations. Passive damping though offers a simple and reliable solution but it reduces overall converter efficiency. Active damping solutions do not increase the system losses but can guarantee the stable operation up to a certain speed of dynamic response which is limited by the maximum bandwidth of the current controller. This paper examines this limit and introduces a concept of hybrid passive-active damping solution with improved stability margin and high dynamic performance for line side LCL filter based active rectifier/inverter applications. A detailed design, analysis of the hybrid approach and trade-off between system losses and dynamic performance in grid connected applications are reported. Simulation and experimental results from a 10 kVA prototype demonstrate the effectiveness of the proposed solution. An analytical study on system stability and dynamic response with the variations of various controller and passive filter parameters is presented.

Effect of methane concentration in hydrogen plasma on hydrogen impurity incorporation in thick large-grained polycrystalline diamond films

We investigate the impact of methane concentration in hydrogen plasma on the growth of large-grained polycrystalline diamond (PCD) films and its hydrogen impurity incorporation. The diamond samples were produced using high CH4 concentration in H2 plasma and high power up to 4350 W and high pressure (either 105 or 110 Torr) in a microwave plasma chemical vapor deposition (MPCVD) system. The thickness of the free-standing diamond films varies from 165 µm to 430 µm. Scanning electron microscopy (SEM), micro-Raman spectroscopy and Fourier-transform infrared (FTIR) spectroscopy were used to characterize the morphology, crystalline and optical quality of the diamond samples, and bonded hydrogen impurity in the diamond films, respectively. Under the conditions employed here, when methane concentration in the gas phase increases from 3.75% to 7.5%, the growth rate of the PCD films rises from around 3.0 µm/h up to 8.5 µm/h, and the optical active bonded hydrogen impurity content also increases more than one times, especially the two CVD diamond specific H related infrared absorption peaks at 2818 and 2828 cm−1 rise strongly; while the crystalline and optical quality of the MCD films decreases significantly, namely structural defects and non-diamond carbon phase content also increases a lot with increasing of methane concentration. Based on the results, the relationship between methane concentration and diamond growth rate and hydrogen impurity incorporation including the form of bonded infrared active hydrogen impurity in CVD diamonds was analyzed and discussed. The effect of substrate temperature on diamond growth was also briefly discussed. The experimental findings indicate that bonded hydrogen impurity in CVD diamond films mainly comes from methane rather than hydrogen in the gas source, and thus can provide experimental evidence for the theoretical study of the standard methyl species dominated growth mechanism of CVD diamonds grown with methane/hydrogen mixtures.