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Mass concrete is widely used in large-scale projects, including metro upper cover structures, water conservancy dams, and heavy equipment foundations, among others, necessitating the process of health monitoring in mass concrete construction. The development of reliable and simple strength-monitoring methods for mass concrete is challenging because the inner temperature of mass concrete is high and changes a lot. This study proposes a strength-monitoring approach for mass concrete using barium titanate–bismuth ferrite/polyvinylidene fluoride (BT–BFO/PVDF) nanocomposite piezoelectric sensors, wherein the new sensors are embedded as actuators and sensors in mass concrete. The stress wave generated by the BT–BFO/PVDF piezoelectric sensors is used to monitor the specimen's strength for 28 days. The piezoelectric voltage received by the sensors in mass concrete is analyzed. The experimental results indicate that the signal received by the BT–BFO/PVDF sensors is not easily affected by the internal temperature of mass concrete compared with that of the traditional PVDF piezoelectric sensors. The signal parameters sensitive to concrete strength variation and the change trend of concrete strength are closely related to the piezoelectric voltage. Therefore, the proposed approach using BT–BFO/PVDF nanocomposite piezoelectric sensors is efficient (error < 10%) in mass concrete monitoring. Moreover, the monitoring results do not need temperature compensation. The physical meaning of the obtained strength prediction formula is proposed. An experimental system based on PVDF dynamic strain-sensing characteristics is established. [ABSTRACT FROM AUTHOR]

High birefringence induced by rhombic air-hole photonic crystal fibers (PCFs) is numerically analyzed by using the finite-element method. The birefringence of a few kinds of PCFs was investigated with different parameters related to rhombic holes, including the rhombic-hole shape, size, and spacing. It was found that the birefringence of the proposed rhombic-hole PC F in this study is relatively larger than that of an elliptical-hole PCF with the same air-filling fraction (f = 0.0375) when the ratio of the rhombic-hole diagonal length is equal to the elliptical-hole ellipticity. OCIS codes: 060.0060, 060.5295.

We observed intense green upconversion emission from [Tb.sup.3+] centered at 546 nm due to transition [sup.5][D.sub.4] [right arrow] [sup.7][F.sub.5] in [Tb.sup.3+]/[Yb.sup.3+] codoped alumino-germano-silicate optical fibers by direct excitation of [Yb.sup.3+] ions with a laser diode at 976 nm. A two-photon cooperative energy transfer upconversion among a pair of [Yb.sup.3+] donor ions and a [Tb.sup.3+] acceptor ion are responsible for the observed green emission. Appropriately increasing the concentration of [Tb.sup.3+] relative to [Yb.sup.3+] was found to facilitate the energy transfer upconversion process from [Yb.sup.3+] to [Tb.sup.3+], while an excess of [Tb.sup.3+] ions inversely introduced more of the concentration quenching effect and phonon-assistant backward energy transfer from [Tb.sup.3+] to [Yb.sup.3+] and, therefore, damaged the observed green emission. The intensity of the observed green emission was found optimum for a concentration ratio of [Tb.sup.3+] to [Yb.sup.3+] of 2 : 1 in alumino-germano-silicate optical glass fibers. [c] 2010 Optical Society of America OCIS codes: 140.3613, 140.7300, 060.2290.

A tellurium-doped germanosilicate optical fiber was developed by modified chemical vapor deposition and solution doping techniques. Using the continuous-wave self-phase modulation method, the nonresonant nonlinear refractive index, [n.sub.2], was measured to be 5.52 x [10.sup.-20] [m.sup.2]/W, which is 2 to 3 times that of the undoped germanosilicate glass fiber. Polyhedron structures Te[O.sub.3] and Te[O.sub.4] with Ge[O.sub.4] and Si[O.sub.4] are believed responsible for lower phonon energy, more nonbridging oxygens, and a larger hyperpolarization, leading to the observed higher nonresonant optical nonlinearity. OCIS codes: 060.2280, 190.4370.

We have theoretically proposed and experimentally demonstrated a new kind of ultranarrow identical-dual-bandpass sampled fiber Bragg gratings (SFBGs) with a [pi] phase shift technique. The spacing of two bandpasses of the proposed grating can be flexibly adjusted by changing the sampled period, and any desired spacing can be achieved in principle. An experimental example shows that the transmission peaks of two narrow transmission-band are near 1549.1 and 1550.1 nm. Based on the proposed SFBG, an ultranarrow identical-dual-channel filter is designed. Two channels of the proposed filter have an equal bandwidth, an even strength, and the same group delay. The bandwidth of each channel of our filter is as small as 1 pm and up to [10.sup.-3] pm (corresponding to ~0.1 MHz), which is less than the bandwidth of the conventional SFBG filters by a factor of [10.sup.2]-[10.sup.4]. The proposed grating and filter can find potential applications with slow light and dual-wavelength single-longitudinal-mode fiber lasers. OCIS codes: 060.3735, 050.2770, 060.2340.

We demonstrated the controllable acquisition of optical gated spectra from a chirped supercontinuum (SC) pulse based on ultrafast transient lens (TrL) effect. Comparing with CS2, the gated spectra had much narrower spectral bandwidths using tellurite glass (Te glass) as the nonlinear medium due to its ultrafast nonlinear response. Experimental results showed that the chirp structure of the SC pulse measured by TrL method was quite accordant with that measured by femtosecond optical Kerr gate method. [ABSTRACT FROM AUTHOR]

By using the MCVD system and the all-gas-phase chelate precursor doping technique, we fabricated an Yb-doped aluminophosphosilicate fiber with 20 μm-core and 400 μm-clad in diameter, i.e., 20/400 Yb-APS fiber. The fiber core was doped with 1100 ppm Yb3+, 7200 ppm Al3+, and 8000 ppm P5+. With a molar ratio of Al/P close to 1:1, low refractive index difference and suitable numerical aperture was obtained for large-mode-area fiber design. Directly forward pumped by 976 nm laser diodes, 11-m-long 20/400 Yb-APS fiber presented 3.03 kW laser output at 1080 nm with a slope efficiency of 76.6% and beam quality M2 of ∼1.58. Up to this power level, no signs of instable beam profile, beam quality worsening, and laser power roll-over were obtained, indirectly justified no obvious mode instability in the whole laser setup. To directly characterize its power stability and photodarkening effect, the fiber laser was kept at ∼2.1 kW for over 500 min with power degradation less than 1.1%. These results indicated that the all-gas-phase chelate precursor doping technique is highly competitive for Yb-APS fiber fabrication toward high-power laser, and the fabricated fiber is very suitable for 2 kW-level or above commercial fiber laser development. [ABSTRACT FROM AUTHOR]

To incorporate sufficient pumping power, a large-scale Yb-doped aluminophosphosilicate (Al2O3 -P2O5-SiO2, APS) fiber with 30-μm core and 900-μ m inner clad in diameter, i.e., a 30/900 Yb-APS fiber, was theoretically designed and experimentally fabricated by using modified chemical vapor deposition system combining with chelate precursor doping technique. Analyzed by an electron probe microanalyzer, the dopant concentration of Yb2O3, Al2O 3, and P2O5 was estimated to be 0.14 mol%, 1.1 mol%, and 1.4 mol%, respectively. Based on an all-fiberized master oscillator power-amplifier laser setup directly forward pumped by commercial 976-nm laser diodes, 6.85-kW laser output at 1079.79 nm was achieved with an optical-to-optical efficiency of 67.1%, and the beam factor of $M^{2}$ was 2.38. The results indicate that large-scale Yb-APS fiber is quite competitive for high-power fiber laser development. [ABSTRACT FROM AUTHOR]

By a chelate precursor doping technique, we designed and fabricated an ytterbium/cerium codoped aluminosilicate fiber for noticeable suppression of photodarkening and demonstrated its laser performance. Cerium/ytterbium ions ratio of 0.5 was specially designed to mitigate photodarkening. Aluminum/fluorine ions ratio of 1 was devised to keep low numerical aperture of fiber core. The measurements showed the fiber core was homogeneously doped with ∼0.12 mol% Yb2O3, ∼0.05 mol% Ce2O3, ∼0.5 mol% Al2O3, and ∼0.2 mol% SiF4, respectively. Tested in a master oscillator power amplifier system, the fiber presented 1930 W laser output power with optical-to-optical efficiency of 79.2%. Stabilized at 1850 W for over 500 min, the output power presented a relatively small power degradation of <1.04%, directly justifying a strong photodarkening resistance. These results demonstrated that ytterbium/cerium codoped aluminosilicate fiber with appropriate concentration ratios of dopants could be an ideal choice for 2 kW-level commercial high power laser applications. [ABSTRACT FROM AUTHOR]

By using modified chemical vapor deposition system combined with chelate precursor doping technique, we report on the fabrication and characterization of Yb-doped aluminophosphosilicate (Al2O3-P2O 5-SiO2) laser fiber. Based on a master oscillator power amplifier laser setup and pumped directly by 976 nm laser diodes, 3.1 kW laser at ∼1064 nm was achieved with a slope efficiency of 78.4%. Benefiting from codoped Al and P, the laser output power showed no evidence of roll-over. The linear fitting of the output power versus the pump power shows the potentiality for further power scaling. The results indicate that chelate precursor doping technique is a competitive method for rare-earth-ion-doped fiber preform fabrication, and aluminophosphosilicate host material has potentiality to develop multi-kW level laser fibers. [ABSTRACT FROM PUBLISHER]

A distributed temperature sensor based on Rayleigh scattering Brillouin optical time domain analysis (Rayleigh-BOTDA) is proposed in this paper. The sensor uses Rayleigh backscattering effect of microwave modulated pulse base sidebands as probe wave and a high sensitive photon counting detector for Brillouin signal intensity detection. Compared with a conventional BOTDA system, the Rayleigh-BOTDA effectively suppresses polarization-induced signal fluctuation resulting in improved signal intensity. The experimental scheme presented is simplified by using a single laser with one-end access. The temperature accuracy of the new sensing system was demonstrated as 1 \ ^\circ\C on spatial resolution of 3 m. [ABSTRACT FROM PUBLISHER]

We report on the performance of double sideband (DSB) modulated probe wave in Brillouin optical time domain analysis (BOTDA) distributed fiber sensor. Compared to single sideband (SSB) modulation, along the sensing fiber the pump depletion of DSB modulation is remarkably suppressed in time domain and also has a relatively narrower Brillouin gain spectrum in frequency domain. Both the theoretical simulation and the experimental results demonstrate that the DSB modulation provides potentially longer sensing distance and higher accuracy in measurement than the SSB modulation in the BOTDA distributed fiber sensor system. © 2010 Wiley Periodicals, Inc. Microwave Opt Technol Lett 52:2713-2717, 2010; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.25566 [ABSTRACT FROM AUTHOR]

Abstract: Without introducing concentration quenching phenomenon, a few wt.% of Tb3+ and Yb3+ ions were doped into a group of easily-fiberized tellurite glasses characterized by loose polyhedron structures and rich interstitial positions. Intense green upconversion emission from Tb3+ ions centered at 539nm due to transition 5D4 → 7F5 was observed by direct excitation of Yb3+ ions with a laser diode at 976nm. Optimizing the concentration ratio of Tb3+/Yb3+, a tellurite glass with a composition of 80TeO2–10ZnO–10Na2O (mol%)+1.0wt.% Tb2O3 +3.0wt.% Yb2O3 was found to present the highest green light intensity and therefore is especially suitable for efficient green fiber laser development. [ABSTRACT FROM AUTHOR]

A modified chelate precursor doping technique using high‐purity materials and accurate gas flow control system is proposed to fabricate an efficient ytterbium‐doped large‐mode‐area silica fibre for laser development. This technique enables to make homogeneous large‐core preform with symmetrical shape, relatively low numerical aperture of ∼0.06 and large‐core size of over 4 mm. Homemade preform was drawn into standard fibre for testing and fully characterised by electron probe microanalysis and time‐of‐flight secondary ion mass spectrometry. On the basis of two‐stage master oscillator power amplifier system, 1 kW laser output with slope efficiency of 81.0% was realised at ∼1064 nm. [ABSTRACT FROM AUTHOR]

Abstract: We report for the first time the realization of the optical fiber doped with silicon nano-particles. Silica glass optical fibers incorporated with silicon nano-particles were fabricated by the modified chemical vapor deposition and the solution doping technique. We found that a broadband absorption appeared at 450–1300nm and a photoluminescence band appeared at 600–1100nm upon pumping with the Ar-ion laser, indicating that silicon nano-particles were successfully incorporated in the fiber core. We also estimated the third-order optical nonlinearity of the optical fibers by measuring the fringe shift obtained from the long-period gratings upon pumping with the Ar-ion laser from 477 to 512nm. Importantly the optical nonlinearity at 1550nm was found to be ∼1.5×10−15 m2/W and we believe that this large optical nonlinearity is caused by the exciton absorption of the silicon particles. [Copyright &y& Elsevier]

Abstract: A germano-silicate optical fiber codoped with Pb2+ and Al3+ was developed by modified chemical vapor deposition and solution doping techniques. The non-resonant third-order optical nonlinearity was measured to be 6.9105×10−20 m2/W by using the continuous-wave self-phase modulation method. The enhanced non-resonant nonlinearity was ascribed to the increasing number of non-bridging oxygens due to the incorporation of the glass modifiers Pb2+ and Al3+, the large molar refraction from heavy metal ions Pb2+, and the enhanced hyper-polarization from PbO doped into the core of the made fiber. [Copyright &y& Elsevier]

Silver nanocrystal-incorporated germano-silicate optical fiber was fabricated by modified chemical vapor deposition and solution doping techniques. By measuring the wavelength shift of the fringes obtained from the long-period fiber grating pair, upon pumping with an argon-ion laser at 499 nm, the resonant nonlinear refractive index around 1550 nm was estimated to be 5.4×10-16 m2/W. The observed high resonant optical nonlinearity of the silver nanocrystal-incorporated fiber is attributed to the contribution from the surface plasmon resonance of silver nanocrystals embedded in the fiber core. [ABSTRACT FROM AUTHOR]

Abstract: Compared with bismuth glasses and chalcogenide glasses, tellurite glasses have a wider transparency window. Especially, these glasses with excellent transparency in visible region could offer a larger dynamic range for the time-resolved simultaneous three-dimensional imaging based on the optical Kerr gate (OKG). We investigated the ultrafast nonlinear optical properties of a TeO2–ZnO–Na2O glass using a femtosecond optical Kerr gate at wavelength of 800nm. The nonlinear refraction n 2 and the ultrafast nonlinear response time of the tellurite glass were estimated to be 4.56×10–15 cm2/W and less than 200fs, respectively. We also demonstrated the time-resolved simultaneous three-dimensional imaging using the tellurite glass as the Kerr medium, which indicated that tellurite glasses may also be good candidates as the Kerr media for the time-resolved imaging based on the OKG. [Copyright &y& Elsevier]

Abstract: We propose a simple dual-wavelength Er-doped fiber laser configuration based on a dual-wavelength fiber Bragg grating written on the splice joint between two different fibers for wavelength-selective filter in the Sagnac loop interferometer. The wavelength separation between the adjacent lasing wavelengths is 1.12nm and the side-mode suppression ratio (SMSR) is over 55dB. The output power variation is less than 0.8dB over a two-minute period. Moreover, the lasing wavelength can be effectively tuned using the thermal heating method. [Copyright &y& Elsevier]