Interference fading suppression with a multi-subcarrier pulse in a distributed acoustic sensor.

A low-complexity multi-subcarrier pulse generation scheme is proposed to suppress the interference fading in a phase-sensitive optical time-domain reflectometer (Φ-OTDR) based distributed acoustic sensor (DAS) with heterodyne coherent detection. The multi-subcarrier pulse is generated in the digital domain based on the proper clipping operation of a sine signal. The localization and recovery of the disturbance signal are realized by the spectrum extraction and rotated vector sum (SERVS) method. The experimental results show that the occurrences of interference fading can be significantly reduced. The intensity fluctuation is reduced from ∼75 dB to ∼25 dB. Multiple disturbance signals are successfully demodulated to verify the effectiveness of the proposed method.

A IR-Femtosecond Laser Hybrid Sensor to Measure the Thermal Expansion and Thermo-Optical Coefficient of Silica-Based FBG at High Temperatures.

In this paper, a hybrid sensor was fabricated using a IR-femtosecond laser to measure the thermal expansion and thermo-optical coefficient of silica-based fiber Bragg gratings (FBGs). The hybrid sensor was composed of an inline fiber Fabry-Perot interferometer (FFPI) cavity and a type-II FBG. Experiment results showed that the type-II FBG had three high reflectivity resonances in the wavelength ranging from 1100 to 1600 nm, showing the peaks in 1.1, 1.3 and 1.5 µm, respectively. The thermal expansion and thermo-optical coefficient (1.3 µm, 1.5 µm) of silica-based FBG, under temperatures ranging from 30 to 1100 °C, had been simultaneously calculated by measuring the wavelength of the type-II FBG and FFPI cavity length.

Optical vector network analyzer based on double-sideband modulation.

We report an optical vector network analyzer (OVNA) based on double-sideband (DSB) modulation using a dual-parallel Mach-Zehnder modulator. The device under test (DUT) is measured twice with different modulation schemes. By post-processing the measurement results, the response of the DUT can be obtained accurately. Since DSB modulation is used in our approach, the measurement range is doubled compared with conventional single-sideband (SSB) modulation-based OVNA. Moreover, the measurement accuracy is improved by eliminating the even-order sidebands. The key advantage of the proposed scheme is that the measurement of a DUT with bandpass response can also be simply realized, which is a big challenge for the SSB-based OVNA. The proposed method is theoretically and experimentally demonstrated.

Mid-infrared supercontinuum generation in silica photonic crystal fibers.

A mid-infrared supercontinuum (SC) light source, which has important applications in many fields, has been extensively investigated in soft glass fibers. However, the poor instinct properties of soft glass fibers and the development of ultrashort pulse lasers left an opportunity for mid-infrared SC generation in silica fiber. Until now, silica fiber has been the commonly used medium for SC generation due to its outstanding properties. In this paper, mid-infrared SC generation in short silica photonic crystal fibers (PCFs) is investigated theoretically and systematically. In the case of a 1550�nm pump, the soliton self-frequency shift effect is utilized to extend the long wavelength edge of SC. Adopting a fiber that has a zero dispersion wavelength away from the pump pulse is a benefit for the suppression of blue spectral component and energy distribution in the long wavelength band. In the case of a 1950�nm pump, the generation of a red-shifted dispersive wave is an efficient way to extend the long wavelength edge of SC. Additionally, the coherence for femtosecond pulse pumping is discussed in this paper. Finally, the long wavelength edge of SC is beyond 3000�nm when a 1950�nm femtosecond pump pulse propagates in a PCF with negative dispersive slope around the pump pulse.

Mass fabrication of homogeneously Yb-doped silica nanoparticles and their spectroscopic properties.

A large number of homogeneously Yb-doped silica nanoparticles were continually fabricated in a vapor synthesis route, in which the Yb doping level can be well controlled by varying either the heating temperature or the carrier gas flow rate of the Yb precursor. The sizes, shapes, and morphologies of the nanoparticles were examined, and no crystallites and no Yb2O3 clusters were observed in the nanoparticles. These nanoparticles exhibit a clear Yb3+-derived absorption at around 973-975 nm and a dependence of the emission intensity and decay time on the doping level, much different from that of sintered pellets.

Facile catalytic combustion of rice husk and burning temperature dependence of the ashes.

In this work, it was discovered and demonstrated that the combustion of rice husk is a catalytic process by the thermoanalytical technique. The catalyst involves the oxides of such transition metals as Mn, Fe, and Cu, which are mainly formed in the initial stage of rice husk combustion and remain in the rice husk ash as an impurity. Mn(2+) ions of various concentrations were reloaded into the HCl-washed husk for cocombustion. As a result, the complete combustion temperature of the husk was decreased exponentially depending on the Mn(2+) concentration. By the facile Mn loading technique using a 0.5 M solution, the combustion temperature can be decreased by approximately 100 degrees C, and the resulting ashes themselves can be a good catalyst in the complete combustion of many other organic compounds. The physicochemical properties and amorphous structure of the ashes from both the raw and HCl-washed husks were found to be strongly dependent on the burning temperature. A decreased complete rice husk combustion temperature can be beneficial in preparing porous amorphous silica with high surface area, high densification, and small Si-O-Si band angles.

Aligned silver nanorod arrays for surface-enhanced Raman scattering.

Silver nanorods were prepared by a seed-mediated growth approach, and self-assembled into two-dimensional ordered arrays on glass substrates. The polarization-dependent optical responses of the rods were measured, which indicated ordered alignment. These arrays were evaluated as potential surface-enhanced Raman spectroscopy (SERS) substrates using trans-bis(4-pyridyl)ethylene molecules. The SERS signals were observed to be enhanced with the increase of the aspect ratio of the Ag nanorod, and this was mainly attributed to the local field enhancement. The lateral arrangement of the Ag nanorod arrays was also partially responsible for the SERS enhancement.

A mesoporous template route to the low-temperature preparation of efficient green light emitting Zn2SiO4:Mn phosphors.

Green light emitting Zn2SiO4:Mn phosphors have been prepared via a low-temperature solid-state reaction using mesoporous silica SBA-15 template. This mesoporous silica template method features low-temperature formation of phosphors and easy doping. The structure and morphology of the phosphors were characterized by XRD, SEM, TEM, and N2 adsorption/desorption techniques, which confirmed the single crystallinity, ordered mesostructure, closed pore channels, and elongated ropelike morphology. The luminescent properties were examined by photoluminescence spectroscopy at room temperature, and the results of fluorescence decay time measurements show non-single-exponential decay behavior and a decrease of the decay time with an increase of the Mn concentration.