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ABSTRACT: A stable and tunable thulium-doped "all-fiber" laser offering a narrow linewidth has been created specifically to act as a compact and simple laser source for gaseous CO<sub>2</sub> detection. This has been done through a careful design to match the laser output wavelengths to the CO<sub>2</sub> absorption lines at 1.875 and 1.997 μm, respectively. A sustainable output power of 11 mW over a tuning range of 7 nm has been obtained by using a combination of a high-reflective fiber Bragg grating with a low-reflective broadband mirror, fabricated at the end of the fiber through silver film deposition. The tuning was achieved using the relaxation-compression mechanism of the fiber Bragg grating, which formed an integral part of the laser resonant cavity. A fiber Bragg grating at 1.548 μm was utilized as a wavelength reference to monitor the tuning of the laser output over the 2 μm wavelength range with a simple and inexpensive interrogator, to avoid the use of an expensive optical spectrum analyzer and to facilitate "in-the-field" operation. This "all-fiber" laser resonator has been shown to be superior in terms of laser tuning range, output power, and linewidth compared to that created with a fiber Bragg grating pair, which was limited by the nonuniform strain transfer to both fiber Bragg gratings.
Applied Optics 10/2012; 51(29):7011-5. · 1.41 Impact Factor
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ABSTRACT: Photodiode-array-based spectrometers are increasingly being used in a wide variety of applications. However, the signal measured by this type of instrument often is not what is anticipated by the user and is often subject to contamination from stray light. This paper describes an efficient and low-cost stray light correction approach based on a relatively simple system using a monochromator-based source. The paper further discusses the limitations of using a monochromator instead of a laser, as used by previous researchers, and its impact on the quality of the stray light correction. The reliability and robustness of the stray light correction matrix generated have been studied and are also reported.
Applied Optics 09/2011; 50(26):5130-8. · 1.41 Impact Factor
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ABSTRACT: Basaltic glasses (hyaloclastite) are a widespread habitat for life in volcanic environments, yet their interior physical conditions are poorly characterized. We investigated the characteristics of exposed weathered basaltic glass from a surface outcrop in Iceland, using microprobes capable of continuous sensing, to determine whether the physical conditions in the rock interior are hospitable to microbial life. The material provided thermal protection from freeze-thaw and rapid temperature fluctuations, similar to data reported for other rock types. Water activity experiments showed that at moisture contents less than 13% wet weight, the glass and its weathering product, palagonite, had a water activity below levels suitable for bacterial growth. In pore spaces, however, these higher moisture conditions might be maintained for many days after a precipitation event. Gas exchange between the rock interior and exterior was rapid (< 10 min) when the rocks were dry, but when saturated with water, equilibration took many hours. During this period, we demonstrated the potential for low oxygen conditions within the rock caused by respiratory stimulation of the heterotrophic community within. These conditions might exist within subglacial environments during the formation of the rocks or in micro-environments in the interior of exposed rocks. The experiments showed that microbial communities at the site studied here could potentially be active for 39% of the year, if the depth of the community within the outcrop maintains a balance between access to liquid water and adequate protection from freezing. In the absence of precipitation, the interior of weathered basaltic glass is an extreme and life-limiting environment for microorganisms on Earth and other planets.
Astrobiology 09/2011; 11(7):651-64. · 2.15 Impact Factor
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02/2011; , ISBN: 978-953-307-829-8
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ABSTRACT: Temperature and nonlinearity effects are two important factors that limit the use of photodiode array spectrometers. Usually the spectrometer is calibrated at a known temperature against a reference source of a particular spectral radiance, and then it is used at different temperatures to measure sources of different spectral radiances. These factors are expected to be problematic for nontemperature-stabilized instruments used for in-the-field experiments, where the radiant power of the site changes continuously with the sun tilt. This paper describes the effect of ambient temperature on a nontemperature-stabilized linear photodiode array spectrometer over the temperature range from 5 °C to 40 °C. The nonlinearity effects on both signal amplification and different levels of radiant power have also been studied and are presented in this paper.
Applied Optics 02/2011; 50(6):866-75. · 1.41 Impact Factor
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ABSTRACT: Bragg gratings written in H<sub>2</sub>-free and H<sub>2</sub>-loaded SMF-28 fiber by use of infrared (IR) femtosecond laser radiation with a phase mask have been investigated in terms of their axial and cross-sectional morphology. It was found that Type I-IR and Type II-IR refractive index changes existed simultaneously in a single grating (which is termed a hybrid grating in this paper) written in either H<sub>2</sub>-free or H<sub>2</sub>-loaded fiber. The proportion of the two types of refractive index changes is a key factor in determining the thermal stability of the grating and spectral properties such as cladding mode coupling and polarization dependent loss (PDL). High-pressure molecular hydrogen loading (H<sub>2</sub> loading) can dramatically enhance the efficiency of grating inscription while preventing the laser-induced damage, due to the enhanced color center formation and the suppressed multiphoton ionization (MPI). The H<sub>2</sub> -loaded fiber Bragg gratings thus exhibit spectral properties and annealing behavior which is distinguishable from the H<sub>2</sub>-free gratings. The findings of this work are of particular importance in the design of optical fiber sensors which can operate at very high temperatures for prolonged periods and thus can have important uses in industry.
IEEE Sensors Journal 12/2010; · 1.52 Impact Factor
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ABSTRACT: The use of an unidirectional auxiliary pump at approximately 1600 nm in conjunction with a 980 nm primary pump for Ytterbium (Yb(3+))-sensitized-Thulium (Tm(3+))-doped single mode silica fiber (YTDF) is found to be very effective to activate the most significant resonance energy transfer from Yb(3+) to Tm(3+), in order to obtain significant emission in the near-infrared. The resulting laser performance of the YTDF at 1874 nm is reported here. The influence of the Tm(3+)/Yb(3+) concentration, their relative proportions and the host glass composition on the lasing efficiency has also been investigated to optimize the fiber parameters for maximum laser output power.
Optics Express 03/2010; 18(5):5068-74. · 3.59 Impact Factor
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ABSTRACT: This study addresses the direct calibration of optical fiber strain sensors used for structural monitoring and is carried out in situ . The behavior of fiber-Bragg-grating-based sensor systems when attached to metal bars, in a manner representative of their use as reinforcement bars in structures, was examined and their response calibrated. To ensure the validity of the measurements, this was done using an extensometer with a further calibration against the response of electrical resistance strain gauges, often conventionally used, for comparison. The results show a repeatable calibration generating a suitable geometric factor of extension to strain for these sensors, to enable accurate strain data to be obtained when the fiber-optic sensor system is in use in structural monitoring applications.
IEEE Sensors Journal 12/2009; · 1.52 Impact Factor
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ABSTRACT: Results are reported from a study carried out using a series of Bragg grating-based optical fiber sensors written into a very short length (60 mm) optical fiber network and integrated into carbon fiber polymer reinforcement (CFPR) rod. Such rods are used as reinforcements in concrete structures and in tests were subjected to strain through a series of cycles of pulling tests, with applied forces of up to 30 kN. The results show that effective strain measurements can be obtained from the different sensors mounted along the rod. Additionally, the tests show that close agreement with the results obtained from the calibrated force applied by the pulling machine and from a conventional resistive strain gauge mounted on the rod itself is obtained. Calculations from strain to shear stress show a relatively uniform stress distribution along the bar anchor used. The results give confidence to results from various methods of in situ monitoring of strains on such CFRP rods when used in different engineering structures.
IEEE Sensors Journal 12/2009; · 1.52 Impact Factor
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ABSTRACT: We use the phase mask method to investigate both experimentally and theoretically the temporal thermal response of Type II-IR fiber Bragg gratings inscribed by a femtosecond laser. A fast testing system is developed to measure the thermal response time by means of periodic CO2 laser irradiation, which creates a rapid temperature change environment. The temporal thermal response is found to be independent of the heat power and the heat direction, although the grating produced destroys the axial symmetry of the fiber. The measured values of the temporal thermal response are approximately 230 ms for heating and approximately 275 ms for cooling, which different from the simulation results obtained from a lumped system equation. The causes of such differences are investigated in detail.
Applied Optics 07/2009; 48(16):3001-7. · 1.41 Impact Factor
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ABSTRACT: The paper reports results obtained from an unusual situation in structural monitoring - a "test-to-destruction," measuring strain using a series of fiber Bragg gratings (FBGs) sensors on a decommissioned concrete Railway Bridge in Ornskoldsvik, Sweden. The study formed part of the EU "Sustainable Bridges" Project. To undertake the study, FBG-based sensors were chosen and embedded in carbon fiber reinforced polymers (CFRPs) composite rods incorporated into grooves specially created in the concrete structure and interrogated using a compact system based on wavelength-division multiplexing (WDM). Throughout the study, the FBG sensors were continuously monitored, allowing the incremental increases in the strain to be seen and through the yield point of the carbon composite reinforcement. The sensors were able to follow the resulting induced changes in strain of over a range in excess of 4000 muepsiv just prior to failure of the bridge and representing extreme levels of strain for such a mechanical structure. The measurements made with the FBG-based system were found to be in agreement with the changes expected in the structure (together with the embedded reinforcement), produced by the loading applied. The study has demonstrated the successful use of FBG-based technology premounted in "smart" carbon fiber composite reinforcements used in a test of this type on a typical bridge structure of a type found widely across the world.
IEEE Sensors Journal 01/2009; · 1.52 Impact Factor
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ABSTRACT: This paper describes a thorough investigation of the NPL Vega BB3500 furnace to improve the quality and duration of the melt plateaux of a Re–C eutectic cell. The results of this investigation have shown that the position of the cell within the furnace is the most critical factor for optimizing fixed-point performance. It was found that better performance could be obtained from a furnace with uniform ring resistance than with highly non-uniform resistance profiles, but this effect was relatively small. When high resistance (hot) rings were deliberately introduced on either side of the cell, at a distance corresponding to several rings beyond the position of the cell, considerable improvement in the plateaux was observed. Nevertheless, the cell (or the cell performance or even plateaux) is very sensitive to the positioning of these hot rings. A new cell holder was designed and introduced. Raising the cell on supporting rings provided a radiative gap that increased the thermal uniformity of the cell itself. Carbon–carbon composite material (CC sheet) wrapped loosely around the cell further improved the uniformity of the cell wall and hence the plateau quality. Following these changes, the plateau duration was increased by a factor of 5.
Metrologia 12/2008; 46(1):33. · 1.75 Impact Factor
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ABSTRACT: A long period grating (LPG) coated with polyvinyl alcohol has been developed and evaluated through exposure to a range of relative humidity (RH) levels (from 33%RH to 97%RH). The conditions for the creation of the sensor probes are described and the response of the device in terms of changes in the resonance loss of several of the spectral loss bands has been monitored. In order to make comparative measurements, a noncoated LPG was exposed to various known refractive index (RI) conditions and the grating response was monitored, analyzed and the results evaluated. This is then related to the change in RH when the sensor is exposed to various RH levels, as the coating RI changes, leading to the change in resonance loss. The PVA coated LPG sensor is showing a relatively fast response time of ~ 80 s to reach 97%RH from 33%RH and inducing ~ 2 dB change in transmission of resonance loss band.
IEEE Sensors Journal 08/2008; · 1.52 Impact Factor
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ABSTRACT: A small palm-sized, reference spectrometer, mounted on a remote-controlled model helicopter is being developed and tested
by the National Physical Laboratory (NPL) in conjunction with City University, London. The developed system will be used as
a key element for field vicarious calibration of optical earth observation systems in the visible-near infrared (VNIR) region.
The spectrometer is hand held, low weight, and uses a photodiode array. It has good stray light rejection and wide spectral
coverage, allowing simultaneous measurements from 400 to 900nm. The spectrometer is traceable to NPL’s primary standard cryogenic
radiometer via a high-temperature metal-carbon eutectic fixed-point blackbody. Once the fixed-point temperature has been determined
(using filter radiometry), the eutectic provides a high emissivity and high stability source of known spectral radiance over
the emitted spectral range. All wavelength channels of the spectrometer can be calibrated simultaneously using the eutectic
transition without the need for additional instrumentation. The spectrometer itself has been characterized for stray light
performance and wavelength accuracy. Its long-term and transportation stability has been proven in an experiment that determined
the “World’s Bluest Sky”—a process that involved 56 flights, covering 100,000km in 72days. This vicarious calibration methodology
using a eutectic standard is presented alongside the preliminary results of an evaluation study of the spectrometer characteristics.
International Journal of Thermophysics 11/2007; 28(6):2041-2048. · 0.95 Impact Factor
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ABSTRACT: A novel Er-doped silica fiber, with heavy Er doping, was specially developed for application to a single frequency fiber laser. Two high temperature-sustainable fiber Bragg gratings, written into Bi-Ge codoped photosensitive fiber, were chosen for the application and spliced to the specialist Er doped silica fiber to form a compact, linear cavity, fiber laser. The fiber laser retained single mode oscillation over a wide temperature range, from room temperature to 400 degrees C. The wavelength of the laser output could be tuned smoothly, without mode hopping being observed, when the temperature was changed. A narrow linewidth of less than 1 kHz was measured at the output of fiber laser and this indicates the potential of the fibre laser sensing system with extremely high sensitivity and resolution over this wide range.
Optics Express 02/2007; 15(2):363-70. · 3.59 Impact Factor
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ABSTRACT: A parallel multiplexed temperature sensor scheme using a Bragg grating-based fiber laser approach has been developed and evaluated. Multiple laser cavities were formed as the active gain media of the system using a common broadband chirped fiber Bragg grating (CFBG) and several normal FBGs, which were used as optical feedback elements, in conjunction with different lengths of erbium-doped fibers (EDFs). These gain media were externally pumped by light from a 1480-nm laser diode (LD) through a 1480-nm 1 times 4 splitter. Normal FBGs were used as the wavelength-selective and sensing elements of the laser system. Simultaneous laser action at three different wavelengths corresponding to channels 1, 3, and 4, respectively, was obtained using this scheme. The temperature was measured over the range from room temperature (27 degC) to a maximum of 540 degC, which shows the potential of the scheme for quasi-distributed sensor applications
IEEE Sensors Journal 09/2006; · 1.52 Impact Factor
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ABSTRACT: An optical fiber-based sensing scheme for the simultaneous measurement of strain and temperature over a wider range has been demonstrated by writing a grating with very high reflectivity in a 10-cm-long specially fabricated antimony-erbium-germanium (Sb-Er-Ge)-doped silica fiber. The scheme exploits the grating sensitivity to both strain and temperature in association with the temperature-dependent peak power ratio of the two fluorescence peaks around 1535 and 1552 nm of the amplified spontaneous emission due to <sup>4</sup>I<sub>13</sub>2/→<sup>4</sup>I<sub>15</sub>2/ transition arising from the presence of erbium ions in the fiber core, using a 980-nm laser diode as a pumping source. The sensor created using this fiber can be used for the simultaneous measurement of strain and temperature over the wide ranges of 0-2000 με and 20°C-600°C, with root-mean-square errors of 36 με and 2.8°C, respectively.
IEEE Sensors Journal 01/2006; · 1.52 Impact Factor
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ABSTRACT: A fiber-optic-based humidity sensor has been fabricated using a fiber Bragg grating (FBG) coated with a moisture-sensitive polymer. The sensing concept exploits the inherent characteristics of the FBG and is based on the strain effect induced in the Bragg grating through the swelling of the polymer coating. A direct indication of the humidity level is given by the shift of the Bragg wavelength caused by the expansion of the sensing material. The FBG sensor used in this work has an approximate coating thickness of 33 μm and was exposed to different humidity levels at room temperature. The sensitivity of the sensor was estimated to be about 4.5 pm/%RH at a wavelength of 1535 nm, this being obtained through a process of linear regression. The resulting uncertainty in the measurement is ±4%RH and the response time of the sensor and the moisture expansion coefficient of polyimide were obtained from a series of experimental investigations and cross compared with the results of previous work.
IEEE Sensors Journal 11/2005; · 1.52 Impact Factor
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ABSTRACT: Bragg gratings were fabricated in an Sn-Er-Ge-codoped silica fiber with a phase mask and ultraviolet radiation from a 248-nm KrF excimer laser. The photosensitivity of the fiber was examined by studying the initial growth rate of the gratings written into it. The thermal stability of the gratings was investigated and modeled in terms of both the refractive-index modulation and the effective refractive index of the fiber core. It was shown that the temperature-induced irreversible shift in the Bragg wavelength could not be predicted by the isothermal decay of the refractive-index modulation. Finally, the potential of the gratings written into the fiber is discussed in terms of their use in high-temperature-sensing applications.
Journal of the Optical Society of America A 09/2004; 21(8):1503-11. · 1.56 Impact Factor
