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Spectroscopic micro-computed tomography using photon counting detectors is a technology that promises to deliver material-specific images in pre-clinical research. Inherent to such applications is the need for a high spatial resolution, which can only be achieved with small focal spot sizes in the micrometer range. This limits the achievable x-ray fluxes and implies long acquisitions easily exceeding one hour, during which it is paramount to maintain a constant detector response. Given that photon-counting detectors are delicate systems, with each pixel hosting advanced analog and digital circuitry, this can represent a challenging task. In this contribution, we illustrate our findings on how to reduce image artifacts in computed tomography reconstructions under these conditions, using a Medipix3RX detector featuring a cadmium telluride sensor. We find that maintaining a constant temperature is a prerequisite to guarantee energy threshold stability. More importantly, we identify varying sensor leakage currents as a significant source to artifact formation. We show that these leakage currents can render the corresponding images unusable if the ambient temperature fluctuates, as caused by an air conditioning, for example. We conclude with demonstrating the necessity of an adjustable leakage current compensation.

With energy-resolving photon-counting detectors in grating-based x-ray phase-contrast imaging it is possible to reduce the dose needed and optimize the imaging chain towards better performance. The advantage of photon- counting detector’s linear energy response and absence of electronic noise in attenuation based imaging is known. The access to the energy information of the photons counted provides even further potential for optimization by applying energy weighting factors. We have evaluated energy weighting for grating-based phase-contrast imaging. Measurements with the hybrid photon-counting detector Dosepix were performed. The concept of energy binning implemented in the pixel electronics allows individual storing of the energy information of the incoming photons in 16 energy bins for each pixel. With this technique the full spectral information can be obtained pixel wise from one single acquisition. On the differential phase-contrast data taken, we applied different types of energy weighting factors. The results presented in this contribution demonstrate the advantages of energy-resolved photon-counting in differential phase-contrast imaging. Using a x-ray spectrum centred significantly above the interferometers design energy leads to poor image quality. But with the proposed method and detector the quality was enhanced by 2.8 times in signal-to-noise ratio squared. As this is proportional to dose, energy- resolved photon-counting might be valuable especially for medical applications.

This paper describes a new method for the immoblization of metalloporphyrin molecules for optical oxygen sensing. The porphyrin molecules have an acrylamide group attached allowing the porphyrin indicator to be covalently bound within a hydrogel polymer. Two hydrogel monomers: hydroxyethylmethylacrylamide (HEMA) and acrylamide have been investigated to assess their suitability as host polymers for the porphyrins. The photophysics of both the base and Platinum porphyrin: tetra porphyrin are discussed. Platinum tetra porphyrin bound in a hydrogel polymer using HEMA as the monomer unit was found to have a good sensitivity to oxygen.

In this work we have been developing fast response optical sensors for dissolved oxygen determination based on measuring the luminescence lifetime of a metalloporphyrin polymer. The sensor is produced by electropolymerization of the metalloporphyrin monomer units of platinum tetraphenylporphyrin (Pt-TPP), platinum octaethylporphyrin (Pt-OEP), palladium tetraphenylporphyrin (Pd-TPP) or palladium octaethylporphyrin (Pd-OEP). The polymerization process results in films which are in the region of micrometers thick. The Stern-Volmer quenching constants as determined from luminescence lifetime measurements for these sensors range from 0.90 (mg 1-1)-1) for Pt-TPP to 1.83 (mg 1-1)-1) for Pd-OEP. The response time of these sensors to a step change from an oxygen free to an oxygen saturated solution is in the millisecond region.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

A Quasi-distributed pH sensing system based on Optical Time Domain Reflectometry (OTDR) was developed to determine the spatial and intensity information from fluorescence signals coupled back into a single fiber. The evanescent wave due to a 488 nm light pulse from a N2 pumped dye laser was used to excite a pH sensitive fluorophore and the emission around 515 nm was detected. A 3dB Y-coupler was used to convey excitation light to the sensing sites and to deliver emission signals from these sites back to a filtered photomultiplier tube. Data collection was performed using a HP 54540A 500 MHz digital scope and analysis was carried out using a dedicated Pentium 166 MHz PC. A reproducible laboratory fabrication process was developed to produce sensing sites at discrete intervals along the length of the fiber. A polishing process carefully removed the cladding at each sensing site and photo- polymerization was then used to covalently bind the fluorophore fluorescein with a co-polymer directly onto the site. The results show the sensors performance over a range of pH4-pH10 with a pKa value of 6.3. The present system was chosen to have sites 10 m apart, however, based on the propagation rate of 5 ns m-1 for light in the fiber and 10 ns for the fluorescence lifetime, a resolution of approximately 1 m is possible.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Fibre optic sensors may be described as either 'intrinsic' or 'extrinsic' according to whether the fibre itself forms part of the sensing medium or merely acts as a waveguide [1], One considerable attraction of the intrinsic type is the possibility of developing a spatially extended, position sensitive sensor system.

This paper details the absorption and fluorescence spectra of sodium fluorescein in aqueous solution and sol-gel thin films as a function of pH. Our results show that the fluorescence spectrum is dependent not only on the microenvironment surrounding the fluorophore but also the concentration the probe in the sol-gel matrix. The pH sensitive range is also shown to be a function of the emission wavelength.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

This paper describes a new distributed optical fiber stress sensor system which consists of a piece of birefringent fiber with a mirror at one end and whose principle of operation is based on the frequency modulation continuous wave (FMCW) technique. The intensity and the location of an applied stress can be determined simultaneously by detecting the amplitude and the frequency of the beat signal which is produced by two forward-coupled mode beams. The system was found to have the advantages of large signal intensity, good signal contrast, 1 meter resolution and up to 100 meters measurement range.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

This paper discusses different configurations of polarization maintaining fiber Sagnac ring strain sensors based on FMCW technology. Each sensor consists of a frequency-modulated single mode laser, a 100 meter length of single mode birefringent fiber ring and PIN photodiode detector. In each case, two opposite propagating beams in the fiber are employed to sense the variations in length due to strain, although the generation of the beat signal is different in each case. The final sensor specifications are very similar as both exhibit high resolution, large dynamic measurement range, large signal intensity, good signal contrast and long sensing fiber.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Advances in fiber optic sensors and, in particular, the technique of electropolymerization have led to a development which allows for porphyrins, the sensing molecules, to be attached directly onto the tip of an optical fiber. The sensing element as a whole is very robust, since during the process of electropolymerization the molecule forms a matrix on the surface of the fiber-film, thus ensuring no leeching of the film constituents to the surrounding environment. Luminescence lifetime decays were obtained using a novel dedicated instrumentation system. The data was analyzed using high level languages incorporating mathematical modeling capabilities. The results of this work are reported as well as a detailed discussion of the electropolymerization process with respect to the porphyrins.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

A practical birefringent fiber Sagnac ring force sensor has been demonstrated. The sensor is based on the frequency modulation continuous wave technique and simply consists of a birefringent fiber ring. The intensity and the location of a stress applied to the fiber can be determined simultaneously by detecting the amplitude and the frequency of the beat signal which is produced by two forward-coupled mode beams in the Sagnac ring as a result of the applied stress. Similarly, the strain variation of one section of the fiber ring can also be measured by analyzing the phase shift of this beat signal. The experiments demonstrate that resolutions of 1 meter for distributed stress measurements and 4 microstrains for strain measurements can be obtained with this sensor system.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Chemical dye immobilization is very commonly used for optical fiber chemical sensors development. The dye may behave differently in aqueous state and in immobilized state. This paper describes the experimental results on absorption, emission and pH response of chemical indicator sodium fluorescein, both as dissolved in water and immobilized in sol-gel thin film, for optical fiber pH sensors development. It was found that in the sol-gel immobilized state, by altering the dye concentration and excitation/emission wavelength, a wide range of measurements from pH 0.5 to pH 10 is possible.

A novel reflectometric birefringent fiber remote strain sensor is reported. The sensor employs the principle of frequency modulation continuous wave interferometry and consists of a single length of single-mode birefringent fiber. The sensor is shown to have several advantages including a resolution of 2 microstrain resolution, a dynamic measurement range of 5000 microstrain, an environment-insensitive lead-in/lead-out signal section and a variable length strain sensing probe. The system is ideally suited to the measurement of absolute and relative strain.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Techniques for distributed optical fiber chemical sensor development were investigated and a model system for pH measurement was developed and, as a result, discrete, distributed signals were obtained. Fluorescein sodium was chosen as a pH indicator for this work because of its well known properties and high fluorescent intensity. A low temperature sol-gel glass manufacturing process was utilized to immobilize the indicator onto the optical fiber core. Thin (approximately 1 micrometer) porous glass films were deposited on the surface of the optical fiber core with indicator molecules entrapped in the matrix. An OTDR technique was employed to obtain the signal at specific positions along the fiber. A dye laser, pumped by a N2 laser, produced blue light pulses at 440 nm which were launched into a 1 multiplied by 2 optical fiber coupler. A fiber with eight sensitive sections was splice to a 50:50 coupler. The indicator molecules were excited by the blue light via the evanescent wave. Part of the fluorescent light from the indicator molecules was coupled back into the fiber and transmitted back to the coupler. A fast PMT tube was attached to the other arm of the 1 by 2 coupler to detect the fluorescent light. Results were obtained for solutions of various pH value. The system appears to have potential for applications in environmental and safety monitoring.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

This paper describes a novel optical fiber remote strain sensor which is based on the FMCW technique and consists of only one single piece of single-mode birefringent fiber. The lead-in and lead-out fibers which are simply separated from the strain sensing fiber by two short twisted fiber portions are insensitive to environmental changes so that the sensor can remotely detect the strain variation of a distant construction. Other advantages of the sensor, such as no fiber junctions, no feedback light, long sensing fiber, higher resolution (4 microstrain) and large dynamic measurement range (5000 microstrain) are demonstrated in this experiment.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

This paper describes a new birefringent fiber strain sensor which is based on the FMCW technique. The sensor comprises a single length of elliptical core birefringent fiber with a mirror attached at the far end and a mode coupler in the middle. The strain variation of the fiber sensing probe can be measured by detecting the phase shift of the beat signal which is produced by two forward-coupled polarization mode beams. The experimental results demonstrated that the sensor has many advantages, such as (1) high resolution (2 microstrain), (2) large dynamic measurement range (5000 microstrain), (3) large signal intensity and good signal contrast, (4) long and length-adjustable sensing probe, (5) long and environment-insensitive lead-in and lead-out fiber, (6) simple signal processing, and (7) low cost.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

We report a birefringent fiber remote strain sensor which is based on the FMCW technique and simply consists of two pieces of elliptical-core single-mode birefringent fiber. The first piece of fiber is used as the lead-in/lead-out fiber which is insensitive to environment, and the second one is used as a strain sensing fiber probe which is sensitive to the strain of itself with 2 microstrain resolution. The advantages of the sensor, such as large dynamic measurement range, long environment-insensitive leading-in/lead-out fiber and long strain sensing probe ar demonstrated in this experiment.

This paper describes a new Sagnac heterodyne interferometric birefringent fiber strain sensor which is based on the frequency modulation continuous wave technique and consists of a single 100 meter birefringent fiber ring. The strain variation of the fiber ring can be measured with 4 microstrain resolution and 5000 microstrain dynamic measurement range. Other advantages of the sensor, such as, simple configuration, simple signal processing and long sensing fiber are demonstrated in this experiment.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Techniques for distributed optical fiber chemical sensor development were investigated and a model system for pH measurement was developed and, as a result, discrete distribution signals were obtained. pH indicator fluorescein sodium was chosen for this work because of its well known properties and high fluorescent intensity. A low temperature sol-gel glass manufacturing process was utilized to immobilize the indicator onto the optical fiber core. Robust porous glass thin film (approximately 1 micrometers ) was grown on the surface of the optical fiber core with indicator molecules entrapped in the matrix. OTDR was employed to obtain the signal at specific positions along the fiber. A dye laser, pumped by a short pulse N2 laser, produced blue light pulses at 440 nm which were launched into a 1 X 2 optical fiber coupler. A fiber with four sensitive sections was spliced to a 50:50 coupler. The indicator molecules were excited by the blue light via the evanescent wave. Part of the fluorescent light from the indicator molecules was coupled back into the fiber and transmitted back to the coupler. A fast PMT tube was attached to the other arm of the 1 X 2 coupler to detect the fluorescent light. Results were obtained for solutions of various pH value. The system appears to have great potential due to the wide applications in chemical, biochemical, environmental and safety monitoring.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

It has been known for some time that the presence of NO plays an important part in the degradation of organic pollutants and in the formation of ozone and nitric acid in the troposphere. In recent years. there has been a significant increase in the concentration of atmospheric NO and aconsequent decrease in air quality. The authors believe that there is now a need for a computer- controlled, near-real time monitoring system for atmospheric gas samples taken from a large urbanarea. This paper describes a laser-based REMPI procedure which utilises a high power pulsed laser,atmospheric pressure ionisation chamber and data acquisition system and. with a detection limits aslow as I ppb. it may well form the basis of such a system. 1 INTRODUCTION Public interest in the quality of air is certainly not new - the citizens of ancient Rome frequentlycomplained to Senate regarding the indigenous aromas within the city! More recently. in the 1950s and1960s. many industrialised countries were subjected to severe winter smogs (smoke +fog) with theirassociated health hazards to the public and, as a result. legislation was introduced in Europe and theUSA. For example, the UK Clean Air Acts of 1956 and 1968 were designed to combat the emission ofparticulate and gaseous effluents due to the widespread combustion of coal. The visible impact of this

In this paper, we present two new approaches of distributed optical fiber force sensors. The first approach relies on the optical Kerr effect which utilizes two lasers launched into a Hi-Bi fiber from the same end. A He-Ne laser provides a CW probe beam which is launched into fiber to excite two polarization modes with equal intensities. A Nd:YAG laser is used to provide a pulsed pump beam which excites one polarization mode. The polarization state of the CW beam changes when the pump beam propagates through the fiber, or when a force is applied to the fiber. Information regarding the intensity of the force and its position can be obtained using a data acquisition and analysis system. The second approach is based on the FMCW technique. In this case, a frequency modulated laser beam is launched into a Hi-Bi fiber with one polarization mode and reflected from the far end of the fiber by a mirror. When a force is applied to the fiber at any position, mode coupling will occur. By detecting the beat frequency produced by the two coupled mode beams, the intensity and the position of the force can be found. In this paper, the experimental results obtained from the two sensor systems are presented.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

This paper reports the results of laser beam attenuation measurements for a u.v. transmitting fiber in terms of the pulse energy and wavelength. The investigation was carried out in view of the fact that all-silica u.v. fibers may have an important role to play in the evolution of new spectroscopic techniques such as Resonant Laser Ablation. The advantages and disadvantages of employing this type of fiber within a high power laser-based system must be clearly established by characterizing the attenuation properties of the fiber experimentally within the required operating ranges of wavelength and pulse energy (i.e. launch energy).© (1993) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The yield of singly- and multiply- charged ions of krypton and xenon is presented as a function of laser intensity and frequency. The measurements were performed using the second harmonic output of a well-characterized, tunable picosecond dye laser in the range 285 to 310 nm at laser intensities from lx10 12 to 10 14 W/cm 2 . Enhancement of the Kr + yield by two orders of magnitude by three-photon resonant, four-photon ionization is observed in the vicinity of the 4d'[5/2] 3 and the 4d[3/2) 1 intermediate states. A model incorporating line shifts and widths scaling linearly with intensity is in good agreement with the experimental results.