Semiconductors containing so-called DX centers exhibit at low temperatures an ability to modify locally a refractive index under illumination. This modification persists due to the metastable character of the centers. As a result at low temperatures persistent photoeffects are observed for the materials. This property suggests the potential application of the materials in optoelectronic devices and an optical memory based on holography. It has been shown that at low temperatures the centers behave like deep defects whereas after photoionization they behave like shallow dopants. This twofold nature also accompanies different localization at the crystal lattice. As a result at low temperatures return to the "dark" ground state is not possible unless the system possesses enough energy. This energy is called the capture barrier. Changes in the refractive index depend on the height of the capture barrier and temperature. Estimation of the barrier is therefore of prime importance for materials possessing DX centers. It was found that a gallium dopant in Cd1-xMnxTe exhibits DX-like behaviour. In this paper we report on the capture barrier of gallium doped Cd0.99Mn0.01Te. In order to determine the barrier persistent photoconductivity (PPC) decay kinetics were investigated. The kinetics are governed by the transition from a shallow donor state to the deep DX state. The measurements were run within a temperature range from 77K
This paper focuses on optical fiber laser-based wide range temperature measurement applications, under circumstances
where different strain values were applied to the chirped fiber Bragg grating at a fixed temperature and the uniform type IIA fiber Bragg grating was used temperature-tuned. This grating forms the end reflector in the laser cavity feedback configuration. Erbium doped fiber was used as the laser gain medium and was pumped by using light from a 1480 nm laser diode. The response of the sensor was achieved over the tunable-temperature range from 70°C to 500°C when a fixed strain value of 1000 με was applied to the chirped Bragg grating, with a root mean square error of 10°C, using linear fitting over the above measurement range.
For the first time the term data diffraction is introduced, with examples drawn from the algorithm known as phase coherent data-scatter (PCDS) that produces identifiable visual patterns for different types of signal degradation in optical telecommunications. The main signal degradation factors that affect the performance of optical fibers include attenuation, rise-times and dispersion. The theory behind data-scatter is introduced including comprehensive explanations of the theoretical conceptual components of this technique such as centroids, exchange operation, coherence, closeness and projection radius. The various issues of assessing the quality of digital signals are outlined using a simulation study. The authors for the study of optical telecommunications issues have extended the functionality of data-scatter. This approach shows considerable promise. The utility of the data-entropy based 'quality budget method' for optoelectronic system engineering is revisited using an information theory based approach for optical telecommunications. Proposals for the implementation of pattern recognition algorithms to analyse the repeatable patterns within data-scatter are discussed. The paper concludes with brief considerations into the advantages of linking the new data-scatter and data-entropy approaches in digital fiber systems for performance quantification and assessment.
An extensive experimental study into the relationships between tensiotrace features and surface tension of alcohols and bifunctional liquids has produced a series of empirical relationships. The use of this 'inside the rainbow' studies for pendant drops is known as optical tensiography. A series of empirical relationships discovered will enable the experimental measurement of surface tension without the correction factors that have been used since the development of the drop volume/weight method over a century ago for a restricted range of liquids. This approach offers potentially important applications in surface science and it is also suggested how these new relationships will be tested using theoretical models developed by the authors in the ongoing work. This paper provides the first experimental investigation into the commencement of the tensiotrace, a position at which optical coupling begins, which reveals measurement possibilities.
A fiber laser-based sensor system has been developed to measure wide range of temperatures (22 - 500°C) and strain (0 - 1200με) using a normal and a chirped grating as optical feedback elements and Er3+-doped fiber as gain medium.
The paper critically assesses and illustrates the use of the data entropy budget method in both product and systems engineering based on the experience of developing an optoelectronic instrument known as the tensiograph. The design of such a system involving optoelectronic, electronic, thermal, mechanical, chemical and data processing noise components presents difficult engineering problem from the complex of noise spectrum contributions. This project provides perhaps an important case study for optical engineers because it was developed over a period of 15 years. The design history recorded in the data entropy-time graph, shows clearly the step-wise improvements achieved from the various engineering efforts. The present 11-bits information content of the instrument, with impressive signal-to-noise ratio exceeding 1000:1, was developed from prototype with less than 3-bit resolution. The paper concludes with an assessment of the relevance of this method to optical engineering in which a diverse number of technologies are frequently integrated in products and systems. Finally, the role of data entropy methods in third level education is then briefly considered with very clear lessons drawn from the foregoing concrete example offered by this case study.
The progress of a PhD in Optical Instrumentation is examined as a case study of an example of postgraduate training in the area of optical instrumentation. The work itself is concerned with the further development of the Fibre Optic Resipratory Plethysmograph (FORP), in order to improve respiratory detection and permit cardiac response detection. Issues relating to skill mix development are examined.
The work which we report on here makes use of a new (patented)
technique for measuring the tensile and viscosity properties of
any liquid. One modality uses a laser-derived beam of light
directed into a drop as it builds up on a drop-head, grows and
eventually falls off through gravity. The light is reflected
through the drop, and a trace is built up of its intensity over
time. The trace has been found to have very good discrimination
potential for various classes of liquid. Other sensing modalities
can be used, -- multiple simultaneous optical and near infrared
wavelengths, ultraviolet, ultrasound. In the studies reported on
here, we use the ultrasound modality. Further background on this
new technology for the fingerprinting of liquid content and
composition can be found in McMillan et al. (1992, 1998, 2000).
The Fiber Optic Respiratory Plethysmograph (FORP) is a non-invasive instrument for respiratory and heart monitoring in humans, based on the design of the Respiratory Inductive Plethysmograph (RIP. The FORP uses two sensors at thoracic and abdominal levels that measure circumferences rather than the cross sectional areas measured by the RIP. Each sensor is made of a specifically looped optical fiber that responds to elongation with variation in light transmission, via the macrobending loss effect. The design and construction of the original version of the FORP has been reported previously 1. This paper presents the results of a new figure-of-eight construction for the fiber loops. The resulting system shows greater signal range, increased linearity and less hysteresis than former constructions. Results are presented detailing the calibration of the respiratory measurements against a spirometer using a range of calibration protocols, one based on isovolume breathing and others on a Least Mean Square (LMS) fit. Additional improvements to signal processing and the increased sensitivity of the new design now make it practicable to detect torso motion arising from cardiac activity. This paper also presents results showing the simultaneous monitoring of respiratory and cardiac signals, using only the FORP transducers.
The multianalyzer is a powerful amplitude modulated fiber optic sensor which is perhaps quite typical of so many sensor innovations in that it is a technology looking for an application. Consequently, a series of collaborations with fruit juice, brewing, distilling, biotechnology and polymer industries were made with the objective of identifying potential applications of the multianalyzer. An assessment of these interactions is made for each of the industrial fields explored, by giving for each, just one positive result from the work. The results are then critically assessed. While these studies have illustrated the universal nature of the technology, in every case, lessons have been drawn of a general nature. This experience in particular underlined the difficulty in acceptance of a fiber based technology in industrial process monitoring, against the backdrop of the conservative practice of industry with long established instrumentation. The hard won experience of this product development has shown the vital important of technologists understanding the difference between the marketing concepts of features, benefits and advantages. Three categories of conclusions are drawn, the technical, the commercial, and finally, conclusions drawn from generalizations of the project by the Kingston partners based on their own independent experience in sensor development involving industrial and medical collaborations.
We report here the adaptation of an existing optical respiration monitor, the fiber optic respiratory plethysmograph (FORP), to act as a gating tool to compensate for respiratory motion that occurs during scans in magnetic resonance (MR) scanners. The instrument is based on the principle of bending losses in optical fibers, and features a user-friendly graphical interface written in Visual Basic for Windows. One of the novelties of the system is the use of an optical feedback channel to instruct the subject to inhale, exhale or hold their breath. Results are presented that demonstrate that the instrument is useful in helping a test subject reproduce accurate breath holds.
Invasive techniques used in the measurement of respiration are often regarded as cumbersome, distressing to the subject and in some cases impractical for prolonged use. A current non- invasive alternative is the respiratory inductive plethysmograph (RIP), widely used in the clinical environment. The RIP, however, is expensive and prone to electromagnetic interference (EMI) and is therefore unsuitable in such environments as the interior of a magnetic resonance imaging (MRI) scanner. This research focuses on the development and construction of an all-optical analogue of the RIP, the fiber optic respiratory plethysmograph (FORP). A number of calibration techniques, both linear and non-linear, have been investigated to determine the relationship between the ribcage and abdomen during breathing. An evaluation of these techniques has provided an efficient method of respiratory volume monitoring. The results of testing the device using the various calibration methods are presented.
The authors have previously reported the use of liquid crystals as transducing elements in novel configurations. The use of liquid crystals in these devices relies on the temperature dependence of their alignment and relaxation times when activated in an LCD, as opposed to their more conventional usage when it is their color change with temperature that is utilized. The liquid crystals in these latter instances have been thermotropic, and have not been activated in an electrical fashion. This paper reports the further development of a sensor which uses the temperature dependence of the activation voltage of the liquid crystals in an LCD. The device is self-referenced in that two time-multiplexed signals are produced when the LCD is alternately activated and inactivated. The output signal is a ratio of these two signals and the division is performed using a look-up table. The sensitivity of the system has been increased and additional features added to detect both damage to the probe head/optical fiber and excursion beyond the specific temperature range.
The development of a stable self-oscillatory system based on the alignment response times of liquid crystals is reported. A simple phenomenological mathematical model has been developed and solved numerically which produces results in accord with the observations, namely that the frequency of oscillation increases as the temperature increases. The model is extended and solved numerically to describe a mixture of two liquid crystal species.
We report the development of a novel hybrid optoelectronic system for temperature measurement based on the alignment response time of liquid crystals in a liquid crystal display (LCD). The LCD is incorporated as a transducer which is fully optically powered into a self-resonant system whose resonant frequency depends on the liquid crystal viscosity and hence on temperature. The frequency dependence on temperature is characterised. and the suitability of the system as a temperature sensor is discussed.
KEYWORDS: LCDs, Light emitting diodes, Liquid crystals, Control systems, Photovoltaics, Optical fibers, Solar cells, Temperature metrology, LED displays, Photodiodes
A novel method is reported by which the alignment response time of liquid crystals contained in a display may be measured by a relatively simple and inexpensive system. The method has the advantage that the output is in the frequency domain and is strongly intensity independent. The dependence of the resonance frequency of the system with temperature is measured, and it is envisaged that the system can be further miniaturized to serve as a compact optical temperature sensor.
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