Architecture and mechanism of the light-harvesting apparatus of purple bacteria

Photosynthetic organisms fuel their metabolism with light energy and have developed for this purpose an efficient apparatus for harvesting sunlight. The atomic structure of the apparatus, as it evolved in purple bacteria, has been constructed through a combination of x-ray crystallography, electron microscopy, and modeling. The detailed structure and overall architecture reveals a hierarchical aggregate of pigments that utilizes, as shown through femtosecond spectroscopy and quantum physics, elegant and efficient mechanisms for primary light absorption and transfer of electronic excitation toward the photosynthetic reaction center.

Time-resolved spectroscopy of the metal-to-metal charge transfer excited state in dinuclear cyano-bridged mixed-valence complexes

Visible pump-probe spectroscopy has been used to identify and characterize short-lived metal-to-metal charge transfer (MMCT) excited states in a group of cyano-bridged mixed-valence complexes of the formula [(LCoNCMII)-N-III(CN)(5)](-), where L is a pentadentate macrocyclic pentaamine (L-14) or triamine-dithiaether (L-14S) and M is Fe or Ru. Nanosecond pump-probe spectroscopy on frozen solutions of [(LCoNCFeII)-Co-14-N-III(CN)(5)](-) and [(LCoNCFeII)-Co-14S-N-III(CN)(5)](-) at 11 K enabled the construction of difference transient absorption spectra that featured a rise in absorbance in the region of 350-400 nm consistent with the generation of the ferricyanide chromophore of the photoexcited complex. The MMCT excited state of the Ru analogue [(LCoNCRuII)-Co-14-N-III(CN)(5)](-) was too short-lived to allow its detection. Femtosecond pump-probe spectroscopy on aqueous solutions of [(LCoNCFeII)-Co-14-N-III(CN)(5)](-) and [(LCoNCFeII)-Co-14S-N-III(CN)(5)](-) at room temperature enabled the lifetimes of their Co-II-Fe-III MMCT excited states to be determined as 0.8 and 1.3 ps, respectively.

Experimental investigations of the machinability of Ni50.6Ti49.4 alloy

This paper reports an investigation of the machinability of a Ni50.6Ti49.4 alloy by two machining methods: electrical discharge machining and femtosecond laser machining. The electrical discharge wire cutting used resulted in an average surface roughness of similar to 1.2 mu m and a heat-affected layer of 150 mu m depth. In the laser machining, an ultrashort pulse laser with a width of 150 A was used to minimize the effect of laser-generated heat on the surface integrity. This resulted in a much smaller surface roughness of similar to 0.4 mm and a heat-affected layer of only 50 mu m. The two machining methods were compared as regards machined surface integrity.

Optical precursors and Beer's law violations; non-exponential propagation losses in water

We develop a model for exponential decay of broadband pulses, and examine its implications for experiments on optical precursors. One of the signature features of Brillouin precursors is attenuation with a less rapid decay than that predicted by Beer's Law. Depending on the pulse parameters and the model that is adopted for the dielectric properties of the medium, the limiting z-dependence of the loss has been described as z(-1/2), z(-1/3), exponential, or, in more detailed descriptions, some combination of the above. Experimental results in the search for precursors are examined in light of the different models, and a stringent test for sub-exponential decay is applied to data on propagation of 500 femtosecond pulses through 1-5 meters of water. (C) 2005 Optical Society of America.

Equiangular spiral micro-structured waveguides in lithium niobate

Buried, micro-structured waveguides with an equiangular spiral geometry, which can be formed in a lithium niobate crystal by direct femtosecond laser writing, are analysed with the full-vectorial finite element method. The guiding properties of such waveguides are presented.

Highly thermally responsive hybrid in-fiber slab waveguide grating structure with a plastic core and a silica cladding

A 1.2(height)×125(depth)×500(length) micro-slot was engraved along a fiber Bragg grating by chemically assisted femtosecond laser processing. By filling epoxy and UV-curing, waveguide with plastic-core and silica-cladding was created, presenting high thermal responding coefficient of 211pm/°C.

Microchanneled chirped fiber Bragg grating formed by femto-second laser aided chemical etching for refractive index and temperature measurements

In this work, a microchanneled chirped fiber Bragg grating (MCFBG) is proposed and fabricated through the femtosecond laser-assisted chemical etching. The microchannel (~550 µm) gives access to the external index liquid, thus inducing refractive index (RI) sensitivity to the structure. In the experiment, the transmission bands induced by the reduced effective index in the microchannel region were used to sense the surrounding RI and temperature changes. The experimental results show good agreement with the theoretical analysis. The proposed MCFBG offers enhanced RI sensitivity without degrading the robustness of the device showing good application potential as bio-chemical sensors.

Advanced UV inscribed fibre grating structures and applications in optical sensing and laser systems

This thesis presents detailed investigation of UV inscribed fibre grating based devices and novel developments in the applications of such devices in optical sensing and fibre laser systems. The major contribution of this PhD programme includes the systematic study on fabrication, spectral characteristics and applications of different types of UV written in-fibre gratings such as Type I and IA Fibre Bragg Gratings (FBGs), Chirped Fibre Bragg Gratings (CFBGs) and Tilted Fibre Gratings (TFGs) with small, large and 45º tilted structures inscribed in normal silica fibre. Three fabrication techniques including holographic, phase-mask and blank beam exposure scanning, which were employed to fabricate a range of gratings in standard single mode fibre, are fully discussed. The thesis reports the creation of smart structures with self-sensing capability by embedding FBG-array sensors in Al matrix composite. In another part of this study, we have demonstrated the particular significant improvements made in sensitising standard FBGs to the chemical surrounding medium by inducing microstructure to the grating by femtosecond (fs) patterning assisted chemical etching technique. Also, a major work is presented for the investigation on the structures, inscription methods and spectral Polarisation Dependent Loss (PDL) and thermal characteristics of different angle TFGs. Finally, a very novel application in realising stable single polarisation and multiwavelength switchable Erbium Doped Fibre Lasers (EDFLs) using intracavity polarisation selective filters based on TFG devices with tilted structures at small, large and exact 45° angles forms another important contribution of this thesis.

Highly thermally responsive hybrid in-fiber sSlab waveguide grating structure with a plastic core and a silica cladding

A 1.2(height)×125(depth)×500(length) micro-slot was engraved along a fiber Bragg grating by chemically assisted femtosecond laser processing. By filling epoxy and UV-curing, waveguide with plastic-core and silica-cladding was created, presenting high thermal responding coefficient of 211pm/°C.

Point-by-point inscription of first-order fiber Bragg grating for C-band applications

The influence of the fiber geometry on the point-by-point inscription of fiber Bragg gratings using a femtosecond laser is highlighted. Fiber Bragg gratings with high spectral quality and strong first-order Bragg resonances within the C-band are achieved by optimizing the inscription process. Large birefringence (1.2×10-4) and high degree of polarizationdependent index modulation are observed in these gratings. Potential applications of these gratings in resonators are further illustrated.

Refractometer based on a liquid core FBG device

A 1.2X500µm slot was engraved across a fiber Bragg grating (FBG) using femtosecond laser patterning and chemical etching. liquid core FBGs were constructed and their sensitivity to refractive index of up to 10-6/pm was measured.

Optical fibre sensors with applications in gas and biological sensing

This thesis describes the study of various grating based optical fibre sensors for applications in refractive index sensing. The sensitivity of these sensors has been studied and in some cases enhanced using novel techniques. The major areas of development are as follows. The sensitivity of long period gratings (LPGs) to surrounding medium refractive index (SRI) for various periods was investigated. The most sensitive period of LPG was found to be around 160 µm and this was due to the core mode coupling to a single cladding mode but phase matching at two wavelength locations, creating two attenuation peaks, close to the waveguide dispersion turning point. Large angle tilted fibre gratings (TFGs) have similar behaviour to LPGs, in that they couple to the co-propagating cladding modes. The tilted structure of the index modulation within the core of the fibre gives rise to a polarisation dependency, differing the large angle TFG from a LPG. Since the large angle TFG couple to the cladding mode they are SRI sensitive, the sensitivity to SRI can be further increased through cladding etching using HF acid. The thinning of the cladding layer caused a reordering of the cladding modes and shifted to more SRI sensitive cladding modes as the investigation discovered. In a SRI range of 1.36 to 1.40 a sensitivity of 506.9 nm/URI was achieved for the etched large angle TFG, which is greater than the dual resonance LPG. UV inscribed LPGs were coated with sol-gel materials with high RIs. The high RI of the coating caused an increase in cladding mode effective index which in turn caused an increase in the LPG sensitivity to SRI. LPGs of various periods of LPG were coated with sol-gel TiO2 and the optimal thickness was found to vary for each period. By coating of the already highly SRI sensitive 160µm period LPG (which is a dual resonance) with a sol-gel TiO2, the SRI sensitivity was further increased with a peak value of 1458 nm/URI, which was an almost 3 fold increase compared to the uncoated LPG. LPGs were also inscribed using a femtosecond laser which produced a highly focused index change which was no uniform throughout the core of the optical fibre. The inscription technique gave rise to a large polarisation sensitivity and the ability to couple to multiple azimuthal cladding mode sets, not seen with uniform UV inscribed gratings. Through coupling of the core mode to multiple sets of cladding modes, attenuation peaks with opposite wavelength shifts for increasing SRI was observed. Through combining this opposite wavelength shifts, a SRI sensitivity was achieved greater than any single observed attenuations peak. The maximum SRI achieved was 1680 nm/URI for a femtosecond inscribed LPG of period 400 µm. Three different types of surface plasmon resonance (SPR) sensors with a multilayer metal top coating were investigated in D shape optical fibre. The sensors could be separated into two types, utilized a pre UV inscribed tilted Bragg grating and the other employed a post UV exposure to generate surface relief grating structure. This surface perturbation aided the out coupling of light from the core but also changed the sensing mechanism from SPR to localised surface plasmon resonance (LSPR). This greatly increased the SRI sensitivity, compared to the SPR sensors; with the gold coated top layer surface relief sensor producing the largest SRI sensitivity of 2111.5nm/URI was achieved. While, the platinum and silver coated top layer surface relief sensors also gave high SRI sensitivities but also the ability to produce resonances in air (not previously seen with the SPR sensors). These properties were employed in two applications. The silver and platinum surface relief devices were used as gas sensors and were shown to be capable of detecting the minute RI change of different gases. The calculated maximum sensitivities produced were 1882.1dB/URI and 1493.5nm/URI for silver and platinum, respectively. Using a DFB laser and power meter a cheap alternative approach was investigated which showed the ability of the sensors to distinguish between different gases and flow rates of those gases. The gold surface relief sensor was coated in a with a bio compound called an aptamer and it was able to detect various concentrations of a biological compound called Thrombin, ranging from 1mM to as low as 10fM. A solution of 2M NaCl was found to give the best stripping results for Thrombin from the aptamer and showed the reusability of the sensor. The association and disassociation constants were calculated to be 1.0638×106Ms-1 and 0.2482s-1, respectively, showing the high affinity of the Aptamer to thrombin. This supports existing working stating that aptamers could be alternative to enzymes for chemical detection and also helps to explain the low detection limit of the gold surface relief sensor.

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.

Long period grating directional bend sensor based on asymmetric index modification of cladding

A long period grating (LPG) written in a standard optical fibre was modified by using a femtosecond laser to induce an asymmetric change in the cladding's refractive index. This device produced blue and red wavelength shifts depending on the orientation of applied curvature, with maximum sensitivities of -1.6 nm m and +3.8 nm m, suggesting that this type of LPG may be useful as a shape sensor.

Inscription of a 300-nm-period nanostructure in a pure fused silica

Very recently, using tightly-focused femtosecond near-IR pulses, periodical sub-micron structures have been recorded [1,2]. Such microfabrication utilizes the multi-photon approach, which allows the inscription inside various non-photosensitive optical materials. The combination of multi-photon excitation with the point-by-point technique offers the great potential of creating non-uniform chirped gratings by controlling the rate of femtosecond pulses or the sample translation speed.