Establishment of a Mass Concrete Strength-Monitoring Method Using Barium Titanate-Bismuth Ferrite/Polyvinylidene Fluoride Nanocomposite Piezoelectric Sensors with Temperature Stability.

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.

Single-mode large-mode-area laser fiber with ultralow numerical aperture and high beam quality.

By using the chelate precursor doping technique, we report on an ytterbium-doped aluminophosphosilicate (APS) large-mode-area fiber with ultralow numerical aperture of 0.036 and effective fundamental mode area of ∼550 µm2. With a bend diameter of 600 mm, the bending loss of fundamental mode LP01 was measured to be <10-3 dB/m, in agreement with the corresponding simulation results, while that of higher order mode LP11 is >100 dB/m at 1080 nm. Measured in an all-fiber oscillator laser cavity, 592 W single-mode laser output was obtained at 1079.64 nm with high-beam quality M2 of 1.12. The results indicate that the chelate precursor doping technique is a competitive method for ultralow numerical aperture fiber fabrication, which is very suitable for developing single-mode seed lasers for high power laser systems.

5kW GTWave fiber amplifier directly pumped by commercial 976nm laser diodes.

With home-made fiber perform and special fiber drawing & coating technique, a new-type of (3 + 1) GTWave fiber theoretically designed for bi-directional pump method, was successfully fabricated and justified of integrating multi-kW pump energy from commercial 976nm laser diodes. This (3 + 1) GTWave fiber amplifier demonstrated uniform absorption of pump light and easy thermal management characteristics along the whole fiber length. This amplifier is capable of simultaneously aggregating 5.19kW pump power at 976nm and finally generating 5.07kW laser output at 1066.5nm with an optical-to-optical efficiency of 74.5%, the first publically-reported multi-kW GTWave fiber directly pumped with commercial 976nm laser diodes to the best of our knowledge. No power roll-over was found at 5kW level and further power scaling can be expected with more pump power. The results indicate that GTWave fiber is a competitive integrated fiber device to collect enough pump energy from low-cost commercial laser diodes for multi-kW fiber laser development.

Ytterbium-doped large-mode-area silica fiber fabricated by using chelate precursor doping technique.

We reported on a highly effective chelate precursor doping technique for Yb-doped large-mode-area (LMA) fiber manufacture. By accurately controlling the evaporation temperature and flow rate of carrier gas, the chelate precursor doping technique is capable of making Yb-doped LMA silica fiber with good uniformity free of center dip, low numerical aperture of ~0.056, large preform core size of 4.46 mm, and appropriate cladding absorption of 1.17 dB/m at 976.4 nm. Based on a single-end-pump all-fiber oscillator laser setup, the laser output at 1080 nm reached 700 W with slope efficiency of 54.2%.

Multiring large-mode-area delivery fiber for high power.

The study presents a novel design of multiring delivery fiber with large mode area for high power. Using a FiberCAD method, we investigated a fiber whose core is surrounded by alternative low- and high-index rings. Based on our calculation, the effective area is 400 µm2 at 1.08 µm, larger than the ~280 µm2 of conventional step-index fiber (20/400). The macrobending loss at 1.08 µm is estimated to be 1×10(-3) dB/m, approximately one-third that of conventional step-index fiber (20/400). The single-mode operation can be achieved by the macrobending loss contrast between the fundamental mode (<1 dB/m) and high-order mode (>100 dB/m). The results indicate that multiring delivery fiber fabricated by modified chemical vapor deposition process is a promising candidate for high-power transmission.

1.23 µm emission of Er/Pr-doped water-free fluorotellurite glasses.

To investigate the relatively unexplored 1.2 µm region, we identified a near-infrared emission at around 1.23 µm from Er3+/Pr3+-codoped water-free fluorotellurite glass with a composition of 60TeO2-30ZnF2-10NaF (TZNF60, mol. %). Under the condition of pumping with the 488 nm optical parametric oscillator (OPO) laser system, the directly measured lifetime (τ(f)) at 1.23 µm in Er/Pr-codoped fluorotellurite glasses is about 111.2 µs, much longer than that of Er-doped fluorotellurite glass (80.1 µs). The stimulated emission cross section (σ(em)) and quantum efficiency (η) for Er3+:4S3/2→4I11/2 transition are greatly enhanced when appropriate Pr3+ ions are incorporated. These advances arise partially from the absence of the hydroxyl (OH) group and low phonon energy with the addition of a large amount of fluorides into oxide-based host glasses. With high quantum efficiency (56.2%) and a large stimulated cross section (4.03×10(-21) cm2), Er3+/Pr3+-codoped TZNF60 glass is regarded as promising material for the development of optical amplification and laser operation at the relatively unexplored 1.2 µm region.

High contrast ballistic imaging using femtosecond optical Kerr gate of tellurite glass.

We investigated the ballistic imaging technique using femtosecond optical Kerr gate of a tellurite glass. High contrast images of an object hidden behind turbid media were obtained. Compared to the conventional femtosecond optical Kerr gate using fused quartz, the optical Kerr gate using tellurite glass has more capacity to acquire high quality images of the object hidden behind a high optical density turbid medium. The experimental results indicated that the tellurite glass is a good candidate as the optical Kerr material for the ballistic imaging technique due to its large optical nonlinearity.

Intense red upconversion emission of Yb/Tm/Ho triply-doped tellurite glasses.

By conventional melting and quenching methods, 3Yb2O3-0.2Tm2O3-xHo2O3 (wt%, x=0.2~1.2) was doped into an easily fiberized tellurite glass with composition of 78TeO2-10ZnO-12Na2O (mol%) to form YTH-TZN78 glasses. Under 976 nm excitation, the direct sensitizing effect of Yb ions (Yb→Ho) and indirect sensitizing and self-depopulating effects of Tm ions (Yb→Tm→Ho) were found to present intense red upconversion emission at 657 nm (Red, Ho:5F5→5I8) and were responsible for the absence of the usually observed 484 nm emission (Blue, Tm:1G4→3H36). Regardless of the dopant concentration of Ho ions, the intensity of the red emission at 657 nm (Red, Ho:5F5→5I8) is about three times stronger than that of the green one at 543 nm (Green, Ho:5S2→5I8). For this certain red emission at 657 nm, 0.4 wt% Ho2O3-doped YTH-TZN78 glass was found to present the highest emission intensity and is therefore determined as a promising active tellurite glass for red fiber laser development.

Intense 2.7 µm emission of Er3+-doped water-free fluorotellurite glasses.

By physical and chemical dehydration techniques, a group of Er3+-doped water-free fluorotellurite glasses with a composition of 60TeO2-30ZnF2-10NaF(TZNF60,mol%)+x Er2O3(wt%,x=0~1.5) were fabricated. Under 978 nm excitation, the 4I(11/2)→4I(13/2) emission of Er3+ ions in TZNF60-glass was investigated: τ(f) is of 1.07~1.93 ms and emission bandwidth is about 163 nm at 2.71 µm, which benefits from the absence of OH groups and the decreased phonon energy with the addition of fluorides. In contrast, 1.25Er-TZNF60 glass is proposed to be a promising material for mid-infrared fiber lasers at around 2.7 µm.

Tunable third-harmonic generation in a solid-core tellurite glass fiber.

A solid-core tellurite glass fiber with 1.8 dB/m loss at 1.55 µm was made by using the built-in casting preform fabrication method and rod-in-tube fiber drawing technique. Pumping a 10 cm fiber piece with picosecond pulses of 3-5×10(12) W/cm(2), 0.1% of the fundamental power limited by the coherence length of 0.3-5 µm was converted into visible third-harmonic power tunable over a broad near-IR wavelength ranging from 1500 to 1680 nm. Frequency conversion from the mid-IR to near-IR was found to be even more efficient due to the longer coherence lengths of 12-20 µm in the wavelength range of 2200-2500 nm.

Fabrication and characterization of a water-free mid-infrared fluorotellurite glass.

Using a physical and chemical dehydration technique and a high-pressure, ultradry O2 atmosphere in a semiclosed steel-chamber furnace, we fabricated a group of fluorotellurite glasses with a composition of (90-x)TeO2-xZnF2-10Na2O (mol.%, x=0-30). For x=30, no OH absorption was observed in the range of 0.38-6.1 µm. This is the first report of a water-free mid-IR fluorotellurite glass, to our knowledge, offering the common advantages of a robust oxide glass and an IR-transparent fluoride one. Besides optimized linear transmittance and absorption, the nonlinear refractive indices and Raman gain coefficients are reduced. These results are discussed in the context of mid-IR high-power laser generation and transmission.

Third-order nonlinear optical properties of GeS(2)-Sb(2)S(3)-CdS chalcogenide glasses.

The third-order nonlinear optical properties of GeS(2)-Sb(2)S(3)-CdS chalcogenide glasses were investigated utilizing the Z-scan and femtosecond time-resolved optical Kerr effect (OKE) methods at the wavelength of 800 nm, respectively. The compositional dependences were analyzed and the influencing factors including the linear refractive index, the concentration of lone electron pairs, the optical bandgap and the amount of weak covalent/ homopolar bonds were discussed. A glass, i.e. 76GeS(2)·19Sb(2)S()·5CdS, with large nonlinear refrative index (n(2) = 5.63 × 10(-14) cm(2)/W), low nonlinear absorption coefficient (ß = 0.88 cm/GW) and minimum figure of merit (FOM = 2ßλ/n(2) = 2.51) was finally prepared. The electronic contribution in weak heterpolar covalent and homopolar bonds are responsible for large n(2) in chalcogenide glass, and the Sheik-Bahae rule combining the Moss rule are proved to be an effective guidance for estimating the third-order nonlinearities and further optimizing the compositions in chalcogenide glasses.

Intense green upconversion emission in Tb3+/Yb3+ codoped alumino-germano-silicate optical fibers.

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

Solid-core tellurite glass fiber for infrared and nonlinear applications.

By optimizing glass composition and using a multistage dehydration process, a ternary 80TeO(2)-10ZnO-10Na(2)O glass is obtained that shows excellent transparency in the wavelength range from 0.38 microm up to 6.10 microm. Based on this optimized composition, we report on the fabrication of a single-mode solid-core tellurite glass fiber with large mode area of 103 microm(2) and low loss of 0.24-0.7 dB/m at 1550 nm. By using the continuous-wave self-phase modulation method, the non-resonant nonlinear refractive index n(2) and the effective nonlinear parameter gamma of this made tellurite glass fiber were estimated to be 3.8x10(-19) m(2)/W and 10.6 W(-1) x km(-1) at 1550 nm, respectively.

Tellurium-enhanced nonresonant third-order optical nonlinearity in a germanosilicate optical fiber.

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, n2, was measured to be 5.52 x 10(-20) m2/W, which is 2 to 3 times that of the undoped germanosilicate glass fiber. Polyhedron structures TeO3 and TeO4 with GeO4 and SiO4 are believed responsible for lower phonon energy, more nonbridging oxygens, and a larger hyperpolarization, leading to the observed higher nonresonant optical nonlinearity.

Multistability evolution and hysteresis phenomena of dissipative solitons in a passively mode-locked fiber laser with large normal cavity dispersion.

We report on the experimental observations of multiple dissipative solitons in a passively mode-locked fiber laser with large normal cavity dispersion. The dynamic evolution of solitons as a function of the pump power is demonstrated, alternately evolving on the stable and unstable states. The proposed laser produces the multiple solitons of up to ten for the pump power of about 406 mW. Multistability and hysteresis phenomena observed in this report are qualitatively distinct from those observed in the large net-anomalous-dispersion conventional-soliton fiber lasers. The experimental results suggest that the accumulation of excessive pulse chirps together with the nonlinear polarization effect play key roles in the multistability operation of dissipative solitons.

All-optical switching application of germano-silicate optical fiber incorporated with Ag nanocrystals.

Ag-nanocrystal-incorporated germano-silicate optical fiber with high resonant nonlinearity was fabricated by using modified chemical vapor deposition and solution doping techniques. An all-optical signal switching application based on cross-phase modulation was demonstrated in the cascaded long-period fiber gratings. Pumped with 499 nm argon laser, all-optical signal gating of the pi-phase shift was achieved with low pump intensities of less than 7.64 GW/cm2.

Large temperature sensitivity of Sagnac loop interferometer based on the birefringent holey fiber filled with metal indium.

The large temperature sensitivity of the Sagnac loop interferometer based on the birefringent holey fiber filled with metal indium was experimentally demonstrated. The temperature sensitivities of the wavelength shift of the interferometer and the birefringence the fiber with indium were measured to be -6.3 nm/K and -3.3x10(-6) /K, respectively. The large temperature sensitivity of the fiber was explained by introduction of the fiber birefringence change originated from the large thermal expansion property of the metal indium at the elevated temperature.

Identical-dual-bandpass sampled fiber Bragg grating and its application to ultranarrow filters.

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(-3) pm (corresponding to approximately 0.1 MHz), which is less than the bandwidth of the conventional SFBG filters by a factor of 10(2)-10(4). The proposed grating and filter can find potential applications with slow light and dual-wavelength single-longitudinal-mode fiber lasers.

Tunable multiwavelength fiber laser using a comb filter based on erbium-ytterbium co-doped polarization maintaining fiber loop mirror.

A tunable comb filter based on a fiber loop mirror setup, which incorporates a piece of pumped erbium-ytterbium co-doped polarization maintaining fiber, is newly presented. It is accomplished by controlling the pump power or adjusting a polarization controller in the loop mirror, which results from the fact that the effective birefringence of the erbium-ytterbium co-doped polarization maintaining fiber depends on the pump power and polarization state of the traversing signal. By using the proposed comb filter, a continuously tunable multiwavelength fiber ring laser in the L-band is successfully demonstrated.