Local gratings due to angular hole burning in a photorefractive polymer

We investigate the processes involved in writing real-time holographic gratings in a photorefractive polymer (PRP) that incorporates an azo-dye. In such systems there may be gratings due to mechanisms associated with trans–cis isomerization (angular hole burning (AHB) and/or angular redistribution), which appear in addition to those arising from the photorefractive (PR) effect. The work presented here helps to understand the interactions which may occur between these different gratings. The formation of local gratings due to mechanisms associated with photoisomerization is studied, in a new PRP based on the photoconductor poly(N-vinylcarbazole):2, 4, 7-trinitro-9-fluorenone, plasticized with N-ethylcarbazole. The polymer includes the azo-dye 4-nitro-4'-pentyloxy-azobenzene and we observe both PR and photoisomerization gratings. The gratings are shown to be both polarization-sensitive and reversible. The presence of the photoisomerization gratings (which diffract almost as strongly as the PR gratings) significantly affects the field-dependent diffractive behaviour of the composite. A measurement of the lifetime of the cis state is made (τcis = 38 s) using photoinduced dichroism. This is close to the decay time constant of the local gratings (τdecay = 42 s), and it is suggested that the local grating mechanism is AHB of the azo-dye. This is the first time (to the knowledge of the authors) that a local grating due to AHB has been demonstrated in a PRP.

Strong and fast phase modulation for quantitative analysis of photorefractive gratings

A new approach for studying photorefractive gratings in two-wave mixing experiments by a phase modulation technique is presented. The introduction of a large-amplitude, high-frequency sinusoidal phase modulation in one of the input beams blurs the interference pattern and provides powerful harmonic signals for accurate measurements of the grating diffraction efficiency eta and the output phase shift rho between the transmitted and diffracted waves. The blurring of the light fringes can be used to suppress the higher spatial harmonics of the grating, allowing a space-charge field with sinusoidal profile to be recorded. Although the presence of such a strong phase modulation affects the beam coupling in a rather complicated way, it is shown that for the special case of equal intensity input beams, the effect of the phase modulation on eta and rho is reduced to a weakening of the coupling strength. The potentialities of the technique are illustrated in a study of refractive-index waves excited by running interference patterns in a Bi12TiO20 crystal. Expressions for the diffraction efficiency and the output phase shift are derived and used to match numerically calculated curves to the experimental data. The theoretical model is supported by the very good data fitting and allows the computation of important material parameters.

Light-induced relief gratings and a mechanism of metastable light-induced expansion in chalcogenide glasses

We report on a metastable light-induced volume expansion in Ge25+xGa10-xS65 glasses under irradiation with band gap (UV) light, which can result in recording of relief gratings on their surface in the case of irradiation with two interfering beams. We propose a mechanism for the expansion, which is based on the light-induced change in the polarizability of secondary (van der Waals type) bonds and the effect of this change on primary (covalent type) bonds of the glass. The effect is suggested to be due to an interference of electrons, which belong to a chalcogen atom and participate in the formation of secondary and primary bonds, respectively. We suggest that a minimum point of the Lennard-Jones potential, which corresponds to the equilibrium position of a chalcogen atom is shifted in the course of irradiation to a larger interatomic distance. This shift causes a volume expansion and allows a diffusion of chalcogen atoms into the irradiated area. We show that light-induced polymerization of the glass network is an important attribute of the light-induced volume expansion.

Photoinduced effect in Ga-Ge-S based thin films

Glassy films of Ga10Ge25S65 with 4 mu m thickness were deposited on quartz substrates by electron beam evaporation. Photoexpansion (PE) (photoinduced increase in volume) and photobleaching (PB) (blue shift of the bandgap) effects have been examined. The exposed areas have been analyzed using perfilometer and an expansion of 1.7 mu m (Delta V/V approximate to 30%) is observed for composition Ga10Ge25S65 exposed during 180 min and 3 mW/cm(2) power density. The optical absorption edge measured for the film Ge25Ga10S65 above and below the bandgap show that the blue shift of the gap by below bandgap photon illumination is considerable higher (Delta E-g = 440 meV) than Delta E-g induced by above bandgap illumination (Delta E-g = 190 meV). The distribution of the refraction index profile showed a negative change of the refraction index in the irradiated samples (Delta n = -0.6). The morphology was examined using a scanning electron microscopy (SEM). The chemical compositions measured using an energy dispersive analyzer (EDX) indicate an increase of the oxygen atoms into the irradiated area. Using a Lloyd's mirror setup for continuous wave holography it was possible to record holographic gratings using the photoinduced effects that occur in them. Diffraction efficiency up to 25% was achieved for the recorded gratings and atomic force microscopy images are presented. (c) 2005 Elsevier B.V. All rights reserved.

Characterization of integrated Bragg gratings in silicon-on-insulator

Silicon-on-insulator (SOI) is rapidly emerging as a very promising material platform for integrated photonics. As it combines the potential for optoelectronic integration with the low-cost and large volume manufacturing capabilities and they are already accumulate a huge amount of applications in areas like sensing, quantum optics, optical telecommunications and metrology. One of the main limitations of current technology is that waveguide propagation losses are still much higher than in standard glass-based platform because of many reasons such as bends, surface roughness and the very strong optical confinement provided by SOI. Such high loss prevents the fabrication of efficient optical resonators and complex devices severely limiting the current potential of the SOI platform. The project in the first part deals with the simple waveguides loss problem and trying to link that with the polarization problem and the loss based on Fabry-Perot Technique. The second part of the thesis deals with the Bragg Grating characterization from again the point of view of the polarization effect which leads to a better stop-band use filters. To a better comprehension a brief review on the basics of the SOI and the integrated Bragg grating ends up with the fabrication techniques and some of its applications will be presented in both parts, until the end of both the third and the fourth chapters to some results which hopefully make its precedent explanations easier to deal with.

A moiré deflectometer for antimatter

The precise measurement of forces is one way to obtain deep insight into the fundamental interactions present in nature. In the context of neutral antimatter, the gravitational interaction is of high interest, potentially revealing new forces that violate the weak equivalence principle. Here we report on a successful extension of a tool from atom optics—the moiré deflectometer—for a measurement of the acceleration of slow antiprotons. The setup consists of two identical transmission gratings and a spatially resolving emulsion detector for antiproton annihilations. Absolute referencing of the observed antimatter pattern with a photon pattern experiencing no deflection allows the direct inference of forces present. The concept is also straightforwardly applicable to antihydrogen measurements as pursued by the AEgIS collaboration. The combination of these very different techniques from high energy and atomic physics opens a very promising route to the direct detection of the gravitational acceleration of neutral antimatter.

Discriminating depth in stereo surfaces: dipper effect found, and lost

Measurement of detection and discrimination thresholds yields information about visual signal processing. For luminance contrast, we are 2 - 3 times more sensitive to a small increase in the contrast of a weak 'pedestal' grating, than when the pedestal is absent. This is the 'dipper effect' - a reliable improvement whose interpretation remains controversial. Analogies between luminance and depth (disparity) processing have attracted interest in the existence of a 'disparity dipper' - are thresholds for disparity, or disparity modulation (corrugated surfaces), facilitated by the presence of a weak pedestal? Lunn and Morgan (1997 Journal of the Optical Society of America A 14 360 - 371) found no dipper for disparity-modulated gratings, but technical limitations (8-bit greyscale) might have prevented the necessary measurement of very small disparity thresholds. We used a true 14-bit greyscale to render small disparities accurately, and measured 2AFC discrimination thresholds for disparity modulation (0.6 cycle deg-1) of a random texture at various pedestal levels. Which interval contained greater modulation of depth? In the first experiment, a clear dipper was found. Thresholds were about 2X1 lower with weak pedestals than without. But here the phase of modulation (0° or 180°) was randomised from trial to trial. In a noisy signal-detection framework, this creates uncertainty that is reduced by the pedestal, thus improving performance. When the uncertainty was eliminated by keeping phase constant within sessions, the dipper effect disappeared, confirming Lunn and Morgan's result. The absence of a dipper, coupled with shallow psychometric slopes, suggests that the visual response to small disparities is essentially linear, with no threshold-like nonlinearity.

Advanced fibre gratings and their applications

This thesis describes a detailed study of advanced fibre grating devices using Bragg (FBG) and long-period (LPG) structures and their applications in optical communications and sensing. The major contributions presented in this thesis are summarised below. One of the most important contributions from the research work presented in this thesis is a systematic theoretical study of many distinguishing structures of fibre gratings. Starting from the Maxwell equations, the coupled-mode equations for both FBG and LPG were derived and the mode-overlap factor was analytically discussed. Computing simulation programmes utilising matrix transform method based on the models built upon the coupled-mode equations were developed, enabling simulations of spectral response in terms of reflectivity, bandwidth, sidelobes and dispersion of gratings of different structures including uniform and chirped, phase-shifted, Moiré, sampled Bragg gratings, phase-shifted and cascaded long-period gratings. Although the majority of these structures were modelled numerically, analytical expressions for some complex structures were developed with a clear physical picture. Several apodisation functions were proposed to improve sidelobe suppression, which guided effective production of practical devices for demanding applications. Fibre grating fabrication is the other major part involved in the Ph.D. programme. Both the holographic and scan-phase-mask methods were employed to fabricate Bragg and long-period gratings of standard and novel structures. Significant improvements were particularly made in the scan-phase-mask method to enable the arbitrarily tailoring of the spectral response of grating devices. Two specific techniques - slow-shifting and fast-dithering the phase-mask implemented by a computer controlled piezo - were developed to write high quality phase-shifted, sampled and apodised gratings. A large number of LabVIEW programmes were constructed to implement standard and novel fabrication techniques. In addition, some fundamental studies of grating growth in relating to the UV exposure and hydrogenation induced index were carried out. In particular, Type IIa gratings in non-hydrogenated B/Ge co-doped fibres and a re-generated grating in hydrogenated B/Ge fibre were investigated, showing a significant observation of thermal coefficient reduction. Optical sensing applications utilising fibre grating devices form the third major part of the research work presented in this thesis. Several experiments of novel sensing and sensing-demodulating were implemented. For the first time, an intensity and wavelength dual-coding interrogation technique was demonstrated showing significantly enhanced capacity of grating sensor multiplexing. Based on the mode-splitting measurement, instead of using conventional wavelength-shifting detection technique, successful demonstrations were also made for optical load and bend sensing of ultra-high sensitivity employing LPG structures. In addition, edge-filters and low-loss high-rejection bandpass filters of 50nm stop-band were fabricated for application in optical sensing and high-speed telecommunication systems

Fabrication and applications of fibre Bragg gratings

The consequences of fabricating Bragg gratings in various fibres, with or without hydrogen loading, and with varying laser power levels are explored. Three new techniques for fabricating chirped gratings are presented. Beams with dissimilar wavefront curvatures are interfered to give chirped gratings. With the same aim techniques of writing gratings on tapered fibres and on deformed fibres are also covered. With these techniques, a wide variety of gratings has been fabricated from the 'superbroad' (with bandwidths of up to 180 nm), small to medium bandwidth gratings with linear chirp profiles and quadratic chirped gratings. It is demonstrated that chirped grating can be concatenated to form all-fibre Fabry-Perot and Moiré resonators. These are further concatenated with chirped gratings to produce filters with narrow passbands and very broad stopbands. A number of other applications are also addressed. The use of chirped fibre gratings for dispersion compensation and femtosecond chirped pulse amplification is demonstrated. Chirped gratings are used as dispersive elements in modelocked fibre lasers producing ultrashort pulses. A chirped fibre grating Fabry-Perot transmission filter is used in a continuous wave laser that exhibits eleven simultaneously lasing wavelengths. Finally, the use of grating-coupler devices as variable reflectivity mirrors for laser optimisation and gain clamping is considered.

Thermal response of Bragg gratings in PMMA microstructured optical fibers

We report on the thermal characteristics of Bragg gratings fabricated in polymer optical fibers. We have observed a permanent shift in the grating wavelength at room temperature which occurs when the grating has been heated above a threshold temperature. This threshold temperature is dependent on the thermal history of the grating, and we attribute the effect to a shrinking of the fiber. This effect can be avoided by annealing the fiber before grating inscription, resulting in a linear response with temperature and an increased linear operating temperature range of the grating.

Vector bending sensors based on fibre Bragg gratings inscribed by infrared femtosecond laser

A novel, direction-sensitive bending sensor based on an asymmetric fiber Bragg grating (FBG) inscribed by an infrared femtosecond laser was demonstrated. The technique is based on tight transverse confinement of the femto-inscribed structures and can be directly applied in conventional, untreated singlemode fibers. The FBG structure was inscribed by an amplified, titanium sapphire laser system. The grating cross-section was elongated along the direction of the laser beam with the transverse dimensions of approximately 1 by 2 μm. It was suggested that the sensitivity of the device can be improved by inscribing smaller spatial features and by implementing more complex grating designs aimed at maximizing the effect of strain.

Single- and dual-wavelength switchable erbium-doped fiber ring laser based on intracavity polarization selective tilted fiber gratings

We propose and demonstrate a single- and dual-wavelength switchable erbium-doped fiber laser (EDFL) by utilizing intracavity polarization selective filters based on tilted fiber gratings (TFGs). In the cavity, one 45° TFG functions as an in-fiber polarizer and the other 77° TFG is used as a fiber polarization dependent loss (PDL) filter. The combined polarization effect from these two TFGs enables the laser to switch between the single- and the dual-wavelength operation with a single-polarization state at room temperature. The laser output at each wavelength shows an optical signal-to-noise ratio (OSNR) of >60 dB, a side mode suppression ratio (SMSR) of >50 dB, and a polarization extinction ratio of ~35 dB. The proposed EDFL can give stable output under laboratory conditions.

Photorefractive subharmonics - a beam-coupling effect?

Using the so-called ac field technique, we investigate experimentally the influence of optical beam coupling on the generation of subharmonic gratings in a photorefractive sillenite crystal. By the use of two different recording configurations, we are able to distinguish between effects caused by material nonlinearities and effects caused by optical beam coupling.

Photorefractive subharmonics - a beam-coupling effect?

Using the so-called ac field technique, we investigate experimentally the influence of optical beam coupling on the generation of subharmonic gratings in a photorefractive sillenite crystal. By the use of two different recording configurations, we are able to distinguish between effects caused by material nonlinearities and effects caused by optical beam coupling.

Contrast sensitivity for complex and random gratings

This thesis studied the effect of (i) the number of grating components and (ii) parameter randomisation on root-mean-square (r.m.s.) contrast sensitivity and spatial integration. The effectiveness of spatial integration without external spatial noise depended on the number of equally spaced orientation components in the sum of gratings. The critical area marking the saturation of spatial integration was found to decrease when the number of components increased from 1 to 5-6 but increased again at 8-16 components. The critical area behaved similarly as a function of the number of grating components when stimuli consisted of 3, 6 or 16 components with different orientations and/or phases embedded in spatial noise. Spatial integration seemed to depend on the global Fourier structure of the stimulus. Spatial integration was similar for sums of two vertical cosine or sine gratings with various Michelson contrasts in noise. The critical area for a grating sum was found to be a sum of logarithmic critical areas for the component gratings weighted by their relative Michelson contrasts. The human visual system was modelled as a simple image processor where the visual stimuli is first low-pass filtered by the optical modulation transfer function of the human eye and secondly high-pass filtered, up to the spatial cut-off frequency determined by the lowest neural sampling density, by the neural modulation transfer function of the visual pathways. The internal noise is then added before signal interpretation occurs in the brain. The detection is mediated by a local spatially windowed matched filter. The model was extended to include complex stimuli and its applicability to the data was found to be successful. The shape of spatial integration function was similar for non-randomised and randomised simple and complex gratings. However, orientation and/or phase randomised reduced r.m.s contrast sensitivity by a factor of 2. The effect of parameter randomisation on spatial integration was modelled under the assumption that human observers change the observer strategy from cross-correlation (i.e., a matched filter) to auto-correlation detection when uncertainty is introduced to the task. The model described the data accurately.