Nonlinear loop mirror-based all-optical signal processing in fiber-optic communications

All-optical data processing is expected to play a major role in future optical communications. The fiber nonlinear optical loop mirror (NOLM) is a valuable tool in optical signal processing applications. This paper presents an overview of our recent advances in developing NOLM-based all-optical processing techniques for application in fiber-optic communications. The use of in-line NOLMs as a general technique for all-optical passive 2R (reamplification, reshaping) regeneration of return-to-zero (RZ) on-off keyed signals in both high-speed, ultralong-distance transmission systems and terrestrial photonic networks is reviewed. In this context, a theoretical model enabling the description of the stable propagation of carrier pulses with periodic all-optical self-regeneration in fiber systems with in-line deployment of nonlinear optical devices is presented. A novel, simple pulse processing scheme using nonlinear broadening in normal dispersion fiber and loop mirror intensity filtering is described, and its employment is demonstrated as an optical decision element at a RZ receiver as well as an in-line device to realize a transmission technique of periodic all-optical RZ-nonreturn-to-zero-like format conversion. The important issue of phase-preserving regeneration of phase-encoded signals is also addressed by presenting a new design of NOLM based on distributed Raman amplification in the loop fiber. © 2008 Elsevier Inc. All rights reserved.

Spatially-multiplexed fibre-optic Bragg grating strain and temperature sensor system based on interferometric wavelength-shift detection

A prototype fibre-optic system using interferometric wavelength-shift detection, capable of multiplexing up to 32 fibre-optic Bragg grating strain and temperature sensors with identical characteristics, has been demonstrated. This system is based on a spatially multiplexed scheme for use with fibre-based low-coherence interferometric sensors, reported previously. Four fibre-optic Bragg grating channels using the same fibre grating have been demonstrated for measuring quasi-static strain and temperature.

All optical networking beyond 10 Gbits/s OTDM networks based on electro-optic modulators and fibre ring lasers

This thesis examines options for high capacity all optical networks. Specifically optical time division multiplexed (OTDM) networks based on electro-optic modulators are investigated experimentally, whilst comparisons with alternative approaches are carried out. It is intended that the thesis will form the basis of comparison between optical time division multiplexed networks and the more mature approach of wavelength division multiplexed networks. Following an introduction to optical networking concepts, the required component technologies are discussed. In particular various optical pulse sources are described with the demanding restrictions of optical multiplexing in mind. This is followed by a discussion of the construction of multiplexers and demultiplexers, including favoured techniques for high speed clock recovery. Theoretical treatments of the performance of Mach Zehnder and electroabsorption modulators support the design criteria that are established for the construction of simple optical time division multiplexed systems. Having established appropriate end terminals for an optical network, the thesis examines transmission issues associated with high speed RZ data signals. Propagation of RZ signals over both installed (standard fibre) and newly commissioned fibre routes are considered in turn. In the case of standard fibre systems, the use of dispersion compensation is summarised, and the application of mid span spectral inversion experimentally investigated. For green field sites, soliton like propagation of high speed data signals is demonstrated. In this case the particular restrictions of high speed soliton systems are discussed and experimentally investigated, namely the increasing impact of timing jitter and the downward pressure on repeater spacings due to the constraint of the average soliton model. These issues are each addressed through investigations of active soliton control for OTDM systems and through investigations of novel fibre types respectively. Finally the particularly remarkable networking potential of optical time division multiplexed systems is established, and infinite node cascadability using soliton control is demonstrated. A final comparison of the various technologies for optical multiplexing is presented in the conclusions, where the relative merits of the technologies for optical networking emerges as the key differentiator between technologies.

Interrogation of fibre optic sensors

The aim of the research work described in this thesis was to investigate the interrogation of fibre optic sensors using "off the shelf optical components and equipment developed mainly for the telecommunications industry. This provides a cost effective way of bringing fibre optic sensor systems to within the price range of their electro-mechanical counterparts. The research work focuses on the use of an arrayed waveguide grating, an acousto-optic tuneable filter and low-coherence interferometry to measure dynamic strain and displacement using fibre Bragg grating and interferometric sensors. Based on the intrinsic properties of arrayed waveguide gratings and acousto-optic tuneable filters used in conjunction with interferometry, fibre Bragg gratings and interferometric sensors a number of novel fibre optic sensor interrogation systems have been realised. Special single mode fibre, namely, high-birefringence fibre has been employed to implement a dual-beam interrogating interferometer. The first interrogation scheme is based on an optical channel monitor, which is an arrayed waveguide grating with integral photo-detectors providing a number of amplified electrical outputs. It is used to interrogate fibre Bragg grating and interferometric sensors. Using the properties of polarisation maintainability in high-birefringent fibre an interrogating interferometer was realised by winding a length of the fibre around a piezoelectric modulator generating a low-frequency carrier signal. The system was used to interrogate both fibre Bragg grating and interferometric sensors. Finally, the use of an acousto-optic tuneable filter is employed to interrogate fibre Bragg gratings. The device is used to generate a very high frequency carrier signal at the output of an optical interferometer.

Measurements of stress-optic coefficient in polymer optical fibers

We have systematically measured the differential stress-optic coefficient, ?C, in a number of poly(methyl methacrylate) (PMMA) fibers drawn with different stress, ranging from 2 up to 27 MPa. ?C was determined in transverse illumination by measuring the dependence of birefringence on additional axial stress applied to the fiber. Our results show that ?C in PMMA fibers has a negative sign and ranges from -4.5 to -4.5×10-12 Pa-1, depending on the drawing stress. Increase of the drawing stress results in greater initial fiber birefringence and lower ?C.

Optic flow in human vision: MEG reveals a foveo-fugal bias in V1, specialization for spiral space in hMSTs, and global motion sensitivity in the IPS

Abstract We recorded MEG responses from 17 participants viewing random-dot patterns simulating global optic flow components (expansion, contraction, rotation, deformation, and translation) and a random motion control condition. Theta-band (3–7 Hz), MEG signal power was greater for expansion than the other optic flow components in a region concentrated along the calcarine sulcus, indicating an ecologically valid, foveo-fugal bias for unidirectional motion sensors in V1. When the responses to the optic flow components were combined, a decrease in MEG beta-band (17–23 Hz) power was found in regions extending beyond the calcarine sulcus to the posterior parietal lobe (inferior to IPS), indicating the importance of structured motion in this region. However, only one cortical area, within or near the V5/hMT+ complex, responded to all three spiral-space components (expansion, contraction, and rotation) and showed no selectivity for global translation or deformation: we term this area hMSTs. This is the first demonstration of an exclusive region for spiral space in the human brain and suggests a functional role better suited to preliminary analysis of ego-motion than surface pose, which would involve deformation. We also observed that the rotation condition activated the cerebellum, suggesting its involvement in visually mediated control of postural adjustment.

Genetics of optic atrophy: recent progress in understanding

This article considers the clinical symptoms associated with hereditary optic atrophy and reviews recent progress in our understanding the genetics of the disorder. The major genes linked to optic atrophy are identified and how defects in these genes could lead to the optic disc pathology is discussed.

Large diameter optic nerve axon fibres in Alzheimer's disease

A variety of visual symptoms have been associated with Alzheimer's disease (AD). These include delays in flash visual evoked potentials which indicate a disruption of the integrity of the visual pathway. Examination of the visual cortex has revealed the presence of both senile plaques and neurofibrillary tangles. The purpose of this study was to determine whether there were differences in the number and/or size of optic nerve axons between AD patients and non-demented age-matched controls. Five optic nerves from AD patients and five from age-matched controls were embedded in epon resin and 1 micron sections prepared on a Reichert ultramicrotome. The sections were then stained in toluidine blue and examined at x400 magnification. The numbers of axons were counted in photographs of three fields taken at random from each section. To evaluate the axon diameters, 70 axons were chosen at random from each patient and measured using a calibrated eyepiece graticule. The total axon counts revealed no significant differences between the AD optic nerves and the age-matched controls. However, the frequency distribution of axon diameters was significantly different in the two groups. In particular, there were fewer larger diameter axons in patients with AD as previously reported. Degeneration of the large diameter axons suggests involvement of the magnocellular as opposed to the parvocellular pathways. Hence, there could be differences in visual performance of AD patients compared with normals which could be important in clinical diagnosis.

The incidence of corpora amylacea (CA) in the optic nerve of patients with Alzheimer's disease

Corpora amylacea (CA) are spherical or ovoid bodies 50-50 microns in diameter. They have been described in normal elderly brain as well as in a number of neurodegenerative disorders. In this study, the incidence of CA in the optic nerves of Alzheimer's disease (AD) patients was compared with normal elderly controls. Samples of optic nerves (MRC Brain Bank, Institute of Psychiatry) were taken from 12 AD patients (age range 69-94 years) and 18 controls (43-82 years). Optic nerves were fixed in 2% buffered glutaraldehyde, post-fixed in osmium tetroxide, embedded in epoxy resin and then sectioned to a thickness of 2 microns. Sections were stained with toluidine blue. CA were present in all of the optic nerves examined. In addition, a number of similarly stained but more irregularly shaped bodies were present. Fewer CA were found in the optic nerves of AD patients compared with controls. By contrast, the number or irregularly shaped bodies was increased in AD. In AD, there may be a preferential decline in the large diameter fibres which may mediate the M-cell pathway. Hence, the decline in the incidence of CA in AD may be associated with a reduction in these fibres. It is also possible that the irregualrly shaped bodies are a degeneration product of the CA.

Modified hydrogel matrices in fibre optic sensors

Common problems encountered in clinical sensing are those of non-biocompatibility, and slow response time of the device. The latter, also applying to chemical sensors, is possibly due to a lack of understanding of polymer support or membrane properties and hence failure to optimise membranes chosen for specific sensor applications. Hydrogels can be described as polymers which swell in water. In addition to this, the presence of water in the polymer matrix offers some control of biocompatibility. They thus provide a medium through which rapid transport of a sensed species to an incorporated reagent could occur. This work considers the feasibility of such a system, leading to the design and construction of an optical sensor test bed. The development of suitable membrane systems and of suitable coating techniques in order to apply them to the fibre optics is described. Initial results obtained from hydrogel coatings implied that the refractive index change in the polymer matrix, due to a change in water content with pH is the major factor contributing to the sensor response. However the presence of the colourimetric reagent was also altering the output signal obtained. An analysis of factors contributing to the overall response, such as colour change and membrane composition were made on both the test bed, via optical response, and on whole membranes via measurement of water content change. The investigation of coatings with low equilibrium water contents, of less than 10% was carried out and in fact a clearer signal response from the test bed was noted. Again these membranes were suprisingly responding via refractive index change, with the reagent playing a primary role in obtaining a sensible or non-random response, although not in a colourimetric fashion. A photographic study of these coatings revealed some clues as to the physical nature of these coatings and hence partially explained this phenomenon. A study of the transport properties of the most successful membrane, on a coated wire electrode and also on the fibre optic test bed, in a series of test environments, indicated that the reagent was possibly acting as an ion exchanger and hence having a major influence on transport and therefore sensor characteristics.

The temperature dependence of the electro-optic kerr effect in solutions

A novel Kerr cell of greater optical path length and temperature stability has been designed and built. The Kerr effect experimental has been substantially automated using an Apple IIe computer. Software has been written allowing the computer to partially control the Kerr effect equipment and to acquire and analyse the relevant data.The temperature dependent electro-optic Kerr effect of 2-methyl-4-nitroaniline, p-nitroaniline, nitrobenzene, aniline, and toluene as solutions in 1, 4-dioxane has been studied. The Kerr effect measurements combined with dipole moment, depolarisation ratio, dielectric, and electronic polarisation measurements have been used to calculate the first hyperpolarisability of the solute.Although first hyperpolarisabilities for the compounds studied have been measured in various physical states using a variety of experimental techniques, it is gratifying to find that the values presented in this thesis have a linear relationship with values reported by other workers.

The combination of smart hydrogels and fibre optic sensor technology for analyte detection

The subject of investigation of the present research is the use of smart hydrogels with fibre optic sensor technology. The aim was to develop a costeffective sensor platform for the detection of water in hydrocarbon media, and of dissolved inorganic analytes, namely potassium, calcium and aluminium. The fibre optic sensors in this work depend upon the use of hydrogels to either entrap chemotropic agents or to respond to external environmental changes, by changing their inherent properties, such as refractive index (RI). A review of current fibre optic technology for sensing outlined that the main principles utilised are either the measurement of signal loss or a change in wavelength of the light transmitted through the system. The signal loss principle relies on changing the conditions required for total internal reflection to occur. Hydrogels are cross-linked polymer networks that swell but do not dissolve in aqueous environments. Smart hydrogels are synthetic materials that exhibit additional properties to those inherent in their structure. In order to control the non-inherent properties, the hydrogels were fabricated with the addition of chemotropic agents. For the detection of water, hydrogels of low refractive index were synthesized using fluorinated monomers. Sulfonated monomers were used for their extreme hydrophilicity as a means of water sensing through an RI change. To enhance the sensing capability of the hydrogel, chemotropic agents, such as pH indicators and cobalt salts, were used. The system comprises of the smart hydrogel coated onto an exposed section of the fibre optic core, connected to the interrogation system measuring the difference in the signal. Information obtained was analysed using a purpose designed software. The developed sensor platform showed that an increase in the target species caused an increase in the signal lost from the sensor system, allowing for a detection of the target species. The system has potential applications in areas such as clinical point of care, water detection in fuels and the detection of dissolved ions in the water industry.

Fibre Bragg grating sensor interrogation using an acousto-optic tunable filter and low-coherence interferometry

We describe how an acousto-optic tunable filter can be used to both demultiplex the signals from multiple fibre Bragg grating sensors and simultaneously provide wide bandwidth signal demodulation in a system using interferometric wavelength shift detection. In an experimental demonstration, the approach provided a noise limited strain resolution of 24.9 n epsilon Hz(-1/ 2) at 15 Hz.