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Upland soils have been identified as a major CO 2 source induced by human activities, such as fertilizer applications. The aim of this study is to identify the characteristics of soil CO 2 emission and carbon balance in cropland ecosystems after continuous fertilizer applications over decades. The measurements of soil surface CO 2 fluxes throughout the years of 2009 and 2010 were carried out based on a fertilization experiment (from 1990) in a double cropping system rotated with winter wheat ( Triticum aestivum L.) and maize ( Zea mays L.) in upland soil in southern China. Four treatments were chosen from the experiment for this study: no-fertilizer application (SR), nitrogen–phosphorus–potassium chemical fertilizers (NPK), NPK plus pig manure (NPKM) and pig manure alone (M). Results showed that the mean value of soil CO 2 fluxes from 08:00 to 10:00 am could represent its daily mean value in summer period (June–August) and that from 09:00 am to 12:00 pm for the rest season of a year. Soil temperature and moisture combined together could explain 70–83% of variations of CO 2 emission. Annual cumulative soil CO 2 fluxes in the treatments with manure applications (8.2 ± 0.8 and 11.0 ± 1.2 t C ha −1 in 2009, and 7.9 ± 0.9 and 11.1 ± 1.2 t C ha −1 in 2010 in NPKM and M, respectively) were significantly higher than those in the treatments with non-manure addition (2.5 ± 0.2 and 3.4 ± 0.2 t C ha −1 in 2009, and 2.1 ± 0.2 and 3.7 ± 0.3 t C ha −1 in 2010 in SR and NPK, respectively). However, the treatments with manure applications represented a carbon sink in the soil (carbon output/input ratio

Data from the Pierre Auger Observatory are analyzed to search for anisotropies near the direction of the Galactic Centre at EeV energies. The exposure of the surface array in this part of the sky is already significantly larger than that of the fore-runner experiments. Our results do not support previous findings of localized excesses in the AGASA and SUGAR data. We set an upper bound on a point-like flux of cosmic rays arriving from the Galactic Centre which excludes several scenarios predicting sources of EeV neutrons from Sagittarius A. Also the events detected simultaneously by the surface and fluorescence detectors (the 'hybrid' data set), which have better pointing accuracy but are less numerous than those of the surface array alone, do not show any significant localized excess from this direction. © 2006 Elsevier B.V. All rights reserved.

An upper limit of 16% (at 95% c.l.) is derived for the photon fraction in cosmic rays with energies greater than 1019 eV, based on observations of the depth of shower maximum performed with the hybrid detector of the Pierre Auger Observatory. This is the first such limit on photons obtained by observing the fluorescence light profile of air showers. This upper limit confirms and improves on previous results from the Haverah Park and AGASA surface arrays. Additional data recorded with the Auger surface detectors for a subset of the event sample support the conclusion that a photon origin of the observed events is not favored. © 2006 Elsevier B.V. All rights reserved.

A novel, temperature-independent strain sensor is demonstrated using a chirped fibre grating in a tapered optical fibre. The effective bandwidth of the grating is uniquely dependent on strain and is essentially temperature-independent. The interrogation simply involves monitoring the back-reflected intensity from the grating. A strain resolution of 0.1% over a strain measurement range of 4066 mu epsilon has been experimentally achieved. >

We demonstrate the enhancement of optical bandwidth for modulators based on the quantum confined Stark effect using high barrier multiquantum well structures. This is achieved by carefully placing the Fabry-Perot mode in the wavelength region where the absorbing layer exhibits anomalous refractive index dispersion in the vicinity of the exciton peaks. The enhancement in both the exciton oscillator strength and the separation between the heavy hole and light hole in high barrier multiquantum well structures ultimately results in the enhancement of optical bandwidth. A simulation example shows an increase of 2.5 nm in optical bandwidth at the cost of a factor of three increase in electric field to obtain the same contrast ratio characteristics as a modulator fabricated with low barrier material.

In this paper, we propose a versatile and compact mechanism for tunable grating dispersion. A segmented piezoelectric stack with individually controlled segments was used to control the local strain at 21 positions along the grating. By step-wise monotonic increase or decrease of the strain along the grating a group-delay response close to that of a perfectly linearly chirped fiber Bragg grating (FBG) was obtained.

Two complimentary optical fiber strain sensors employing lock-in techniques are presented. The first system interrogates an array of long gauge length sensors, defined by broadband optical reflectors and employs multiplexing in the time domain. The second system operates over shorter gauge lengths using multiple narrowband reflectors and wavelength-division-multiplexing. The first system tracks minima in the amplitude response produced from the superposition of two sinusoidal subcarrier waves. The second uses an acousto-optic-tunable-filter (AOTF) to track the peak reflective wavelength of an array of Bragg gratings. Both systems are constructed using telecommunications components. Together, the systems may be used to examine both line-integrated strain (or temperature) over long gauge lengths and local strain at a number of selected discrete points of particular interest. Lock-in techniques using dithered signals are applicable to sensors having a transfer function containing at least one turning point. This may be a maximum or minimum when observed either in transmission or reflection. The sensor responds to the dither with an amplitude-modulated signal, which permits locking of the interrogation system to the turning point. This provides a real-time response and better noise performance than scanned measurements. High-resolution monitoring of time-varying strain is demonstrated using this method. The long gauge length system has demonstrated a resolution of 3 microstrain over discrete 5 m long sensing sections, with an interrogation time of 0.25 s. When multiplexed to interrogate an array of four sections, intersection crosstalk levels were below minus 50 dB. The short gauge length interrogation system has been demonstrated using both fiber Bragg gratings and an in-line Fabry-Perot cavity as the wavelength selective reflectors. A resolution below 1 microstrain was obtained using the gratings, whereas a resolution of 1.5 multiplied by 10-6 in optical path-length-difference was obtained when interrogating a Fabry-Perot cavity. Simultaneous monitoring of multiple Bragg gratings has also been demonstrated by multiplexing with different dither frequencies. The versatility and the high resolution make the lock-in systems ideal for smart structures applications.

A fibre Bragg grating pressure sensor with enhanced sensitivity has been demonstrated, using a grating housed in a glass bubble. This configuration increases the pressure sensitivity and effectively reduces the thermal crosstalk by a factor of 4. A pressure resolution of 0.5% over a full measurement range of 14 MPa has been achieved. This concept could lead to a range of sensors using fibre gratings with mechanical amplifiers to enhance sensitivity.

We discuss theoretically and experimentally a wavelength interrogation system for Bragg gratings using an acousto-optic tunable filter (AOTF). The measured resolution of 0.5µm in 5 Hz bandwidth agrees well with the theoretical analysis.

The paper discusses two complimentary optical fibre sensing techniques which have been researched for structural monitoring applications. The short-gauge-length sensor system is based on fibre gratings and has achieved a strain resolution below 1 microstrain. The long-gauge-length sensor system has achieved 100 micron spatial resolution using a new OTDR technique. Results are presented for surface-mounted and embedded sensors. Both sensors systems can be multiplexed to make more efficient use of the interrogating unit. Their system designs should be capable of being developed to meet real engineering applications.

The authors demonstrate for the first time an electronic lock-in system using an acousto-optic tunable filter (AOTF) to track the wavelength of a fibre Bragg grating. The change in the system output is proportional to the change in grating wavelength. Initial results show microstrain resolution at data acquisition rates of 10 Hz.

Fibre Bragg grating sensors are of considerable interest for a number of sensing applications [1-6]. More recently, chirped Bragg gratings are attracting research interest for strain sensing [2,3]. However, undesirable temperature sensitivity of the fibre grating sensor may complicate its use as a strain gauge. Morey et. al. have suggested that the temperature can be measured and compensated for using a second grating element contained within a different material and placed in series with the first grating [4]. We have also demonstrated that the problem can be resolved by arranging for the simultaneous measurement of strain and temperature [5], or by arranging for the thermal response to be cancelled [6]. Nevertheless, all of the above methods are difficult to implement when a large number of fibre gratings are involved in a multiplexed sensing system. In this paper we describe a novel temperature-independent strain sensor based on the use of a chirped fibre grating in a tapered optical fibre.

Enhanced photosensitivity is found in tin-codoped germanosilicate optical fibers. A photo-induced refractive index change (/spl sim/1.4/spl times/10/sup -3/) 3 times larger than that observed in pure germanosilicate fibers has been demonstrated. Unlike the technique of using boron-codoping to enhance the photosensitivity of germanosilicate fibers, tin-doping does not have a significant effect on fiber loss at the important telecommunication window of 1.55 /spl mu/m. High temperature stability of the gratings in tin-codoped germanosilicate fibers is also much over boron-codoped fibers. >

There has been a surge of interests in photosensitive fibre gratings in recent years chiefly due to their ease of fabrication and numerous application. Although very large photo-induced index changes have been achieved in pure germanosilicate fibres, an enhanced photosensitivity is desirable to enable gratings to be written with ease and with much cheaper laser source. Stronger photosensitivity also opens up many new applications. Boron-codoping in germanosilicate fibres has been reported to give a much enhanced photosensitivity comparing to that in pure germanosilicate fibres [1], but gratings in those fibres have a much poorer high temperature stability and the B-codoping can also give rise to excessive loss at the important telecommunication window of 1.55 μm. Some post-fabrication techniques have also been devised, i.e. H2/O2 flame-brushing of a germanium doped fibre [2] and Low-temperature hydrogen loading [3], but they are very time-consuming and there are also large induced losses [3]. In this paper, we report on enhanced photosensitivity in Sn-codoped germanosilicate fibres. The gratings in the Sn-doped fibres also have a much improved high temperature stability than those in B-codoped fibres. Unlike B-codoping, Sn- codoping does not introduce significant loss at the important telecommunication window of 1.55 μm.

The paper describes progress with two multiplexed methods which have been designed for strain sensing in optical fibres. Both methods depend, effectively, on monitoring changes in optical propagation velocity. They are therefore expected to be less affected by fibre bending and lateral pressure than systems based on polarimetry. Unlike complex interferometric systems, they do not require to reference fringe positions (the latter could have difficulty in fringe tracking under conditions of severe mechanical shock or vibration.

A new technique for the interrogation of interferometric-based fibre-optic sensors using an acousto-optic tunable filter (AOTF) is demonstrated. The scheme involves frequency shift keying (FSK) of the RF drive to the AOTF to track the 3hift of a single fringe peak from an interferometric sensor. Experimental results are presented for a Fizeau interferometer. The technique provides high accuracy, plus a large unambiguous tracking range free from interruptions.

We report on recent progress on developing a complimentary pair of optical-fibre-based sensor methods for structural monitoring, for potential application in aerospace composites or civil engineering structures. Particular emphasis is placed on a method for addressing arrays of grating sensors using an acousto-optic tunable filter (AOTF), and on a longer-gauge-length system based on optical-time-domain reflectometry (OTDR). The AOTF possesses the desired frequency-agile capability for random multiplexed access and has a wide tuning range. It is suitable for both dynamic and quasi-static strain sensing. The scheme involves frequency-shift-keying of the RF drive to an AOTF to track the wavelength changes of gratings. We are currently constructing a compact interrogation system based on the concept. For many sensing applications, it is desirable to have a system able to monitor the average strain over a longer length of the structure. This is being researched using a specially-designed high-resolution OTDR. It enables us to determine the optical path length between partially-reflected points (eg gratings) along the fibre. From the measured optical range of each individual discontinuity, changes in length in each intervening fibre section can be determined. In terms of distance, a resolution equivalent to 100µm over a 5m long fibre section has been achieved within a measurement time of 5sec. Work is being directed towards extending the number of sections monitored, and reducing the measurement time using improved processing algorithms, and to extend performance using communications components.

Spectral behaviour of strain and thermal sensitivities of two superimposed fibre gratings of two different Bragg wavelengths has been studied. This involves monitoring the Bragg wavelength as a function of strain and temperature on the gratings. The results show that the ratio of sensitivity at two different Bragg wavelengths (850 nm and 1300 nm) is different with strain and temperature, which can be used for simultaneous measurement of these parameters in fibre grating sensors.

An optical fiber sensor system to interrogate point sensors (Bragg gratings) and optical path length between point sensors is discussed. The paper describes interrogation schemes capable of measurement resolutions better than 100 microstrain based on simple optics, telecommunications electronics and sophisticated signal processing.

The spectral behaviour of two superimposed fibre gratings having different Bragg wavelengths (850 and 1300 nm) with respect to strain and temperature has been studied. The results show that the ratio of sensitivity at two Bragg wavelengths is dependent on strain and temperature, which can be used for simultaneous measurement of these parameters using a single sensing element.

We have demonstrated a new frequency-agile system for the interrogation of in-fibre Bragg grating sensors. The scheme involves frequency shift keying (FSK) of the RF drive to an acousto-optic tunable filter (AOTF) to track the wavelength shifts of a Bragg grating. Theoretical studies to derive the optimum frequency deviation for achieving maximum sensitivity are given, and experimental results are presented for temperature measurement. This technique is capable of rapid, random access and very wide tuning range, showing the potential for interrogating multiplexed arrays of Bragg-grating-based sensors

Laboratory prototypes of a novel pressure sensor have been produced using a hollow glass microsphere, bonded, in an on-axis position, to the end of a monomode optical fibre. The sphere surfaces form a low finesse Fabry-Perot interferometer. The construction of the probe is simple in concept, yet the sensing element is intrinsically hermetically sealed. Experimental trials, under the influence of hydraulic pressure have been carried out and show a good match with predicted behaviour. The observed shift in wavelength with pressure was -0.93 nm/MPa, two orders of magnitude higher than that we have measured with a in-fibre-grating sensor under similar conditions. The ratio of the pressure sensitivity to the temperature sensitivity for our microsphere sensor was more than two orders of magnitude better than the in-fibre-grating type, so therefore less compensation is necessary to correct for temperature changes. This new form of sensing probe has potential for many high-pressure sensing applications

A novel, in-line, polarimetric fibre interrogator for ‘white-light’ interferometry is described. It consists of two equal lengths of polarisation-maintaining fibre, spliced with their polarisation axes orthogonal. The interferometer path difference is thermally tuned over the free-space equivalent of ±180 μm to allow matching to a remote sensor interferometer.

Independent tuning of cascaded long period fibre gratings (LPFGs) is demonstrated. This involves applying a small controllable bend to each LPFBG, resulting in a synthetic spectral response due to the spectral overlap. This technique provides high spectral shaping flexibility coupled with high sensitivity and a wide tuning range.

A novel tunable fibre bandpass filter based on a linearly chirped fibre Bragg grating (LCFBG) has been demonstrated. The transmission peaks in the LCFBG stopband are electronically induced by controlling the strain distribution along the LCFBG using a piezoelectric stack. Transmission peaks with 0.58 nm bandwidth and 10.6 dB rejection ratio have been achieved with a tuning step of 0.28 nm over a range of 10 nm.

The authors describe novel dispersion variable fibre Bragg grating device using a segmented piezoelectric stack to combat chromatic dispersion. By applying appropriate voltages to the independently controlled constituent segments, it is shown that the dispersion can be varied from ~8770 to ~940 ps/nm with an increase in bandwidth from ~0.03 to ~0.32 nm.

Two different methods are demonstrated for simultaneously monitoring multiple fibre gratings using a single acousto-optic tunable filter. One method demultiplexes signals from the gratings optically, and the other uses electronic processing. Crosstalk levels below -52 dB have been observed for both methods.

A new technique for the interrogation of in-fibre Bragg grating sensors using an acousto-optic tunable filter (AOTF) is demonstrated. The scheme involves frequency shift keying (FSK) of the RF drive to the AOTF to track the wavelength shifts of a Bragg grating. Experimental results are presented for temperature measurement. This technique provides a frequency-agile system, capable of rapid, random access and very wide tuning range.

This work demonstrates the design of a novel multiquantum well optical modulator which functions as a reflection and/or transmission modulator at one wavelength whilst allowing three other wavelengths to be transmitted unmodified. The design technique optimizes the Fabry-Perot cavity and the cavity absorption for excellent modulator performance in both reflection and transmission modes. This technique simultaneously optimizes the cavity length for unmodified transmission wavelengths. The simulation shows that this type of modulator can be designed to give a contrast ratio larger than 10 dB for both modes, and exhibits 48% reflectance change and 63% transmittance change at the dual mode operating wavelength for an applied bias of 15 V. The transmittance of the three transmitted wavelengths is close to 100%.

A high pressure fibre optic sensor using an in-fibre grating has been constructed. The wavelength of peak reflection from an in-fibre grating is tracked as it is pressurised, and a wavelength shift of 0.22nm at 70MPa hydraulic pressure has been observed. This sensor is expected to be an attractive choice for ultrahigh-pressure monitoring.

A review of methods measuring the electric-field-dependent absorption coefficient in quantum confined structures is presented. The methods proposed are based on an approximate modeling of a thin film structure which takes into account the reflections at the air-semiconductor interfaces and interference effects within the structure. It is shown that determination of the absorption coefficient from the measured photocurrent and transmitted power is the most robust against measurement errors and easiest to implement for complete modulator structures.

We propose and investigate a modulator structure which acts as a conventional reflection modulator at one wavelength and allows several closely separated wavelengths to be transmitted unchanged. Modeling shows that this type of modulator can be designed with larger than 20 dB contrast ratio and lower than 2.5 dB insertion loss for the reflection wavelength, while having near unity transmission for the transmission wavelengths. The position of transmission peaks can be fully customized.

The performance of electroabsorption transmission modulators incorporating a fibre-matched externally tunable, asymmetric Fabry-Perot cavity is investigated, The theoretical analysis includes the effects of the optical power source characteristics, cavity finesse, and the power coupling loss for single fibre devices. The results show that for a laser source, fibre with standard numerical aperture of 0.2, fibre end reflectivity of 0.8, a cavity length less than four times of the working wavelength is required to obtain 6 dB contrast with an insertion loss less than 2 dB and a modulation change larger than 50%.