Fabrication quality analysis of a fiber optic refractive index sensor created by CO2 laser machining.

This study investigates the CO2 laser-stripped partial cladding of silica-based optic fibers with a core diameter of 400 µm, which enables them to sense the refractive index of the surrounding environment. However, inappropriate treatments during the machining process can generate a number of defects in the optic fiber sensors. Therefore, the quality of optic fiber sensors fabricated using CO2 laser machining must be analyzed. The results show that analysis of the fiber core size after machining can provide preliminary defect detection, and qualitative analysis of the optical transmission defects can be used to identify imperfections that are difficult to observe through size analysis. To more precisely and quantitatively detect fabrication defects, we included a tensile test and numerical aperture measurements in this study. After a series of quality inspections, we proposed improvements to the existing CO2 laser machining parameters, namely, a vertical scanning pathway, 4 W of power, and a feed rate of 9.45 cm/s. Using these improved parameters, we created optical fiber sensors with a core diameter of approximately 400 µm, no obvious optical transmission defects, a numerical aperture of 0.52 ± 0.019, a 0.886 Weibull modulus, and a 1.186 Weibull-shaped parameter. Finally, we used the optical fiber sensor fabricated using the improved parameters to measure the refractive indices of various solutions. The results show that a refractive-index resolution of 1.8 × 10(-4) RIU (linear fitting R2 = 0.954) was achieved for sucrose solutions with refractive indices ranging between 1.333 and 1.383. We also adopted the particle plasmon resonance sensing scheme using the fabricated optical fibers. The results provided additional information, specifically, a superior sensor resolution of 5.73 × 10(-5) RIU, and greater linearity at R2 = 0.999.

Photonic crystal fiber Mach-Zehnder interferometer for refractive index sensing.

We report on a refractive index sensor using a photonic crystal fiber (PCF) interferometer which was realized by fusion splicing a short section of PCF (Blaze Photonics, LMA-10) between two standard single mode fibers. The fully collapsed air holes of the PCF at the spice regions allow the coupling of PCF core and cladding modes that makes a Mach-Zehnder interferometer. The transmission spectrum exhibits sinusoidal interference pattern which shifts differently when the cladding/core surface of the PCF is immersed with different RI of the surrounding medium. Experimental results using wavelength-shift interrogation for sensing different concentrations of sucrose solution show that a resolution of 1.62 × 10(-4)-8.88 × 10(-4) RIU or 1.02 × 10(-4)-9.04 × 10(-4) RIU (sensing length for 3.50 or 5.00 cm, respectively) was achieved for refractive indices in the range of 1.333 to 1.422, suggesting that the PCF interferometer are attractive for chemical, biological, biochemical sensing with aqueous solutions, as well as for civil engineering and environmental monitoring applications.

Shrinkage of picosecond laser beam by plasma reflection in super-resolution near-field structure of SiN/Sb/SiN thin film.

The transmittive and reflective Z-scan technique is used with a 10 Hz, frequency doubled, Q-switched, and mode-locked Nd:YAG laser to verify that the reflectivity of the super-resolution near-field structure of an SiN/Sb/SiN thin film increases as incident intensity decreases. This intensity-dependent reflection, called nonlinear reflection, reflects a TEM(00) mode laser beam more strongly at its periphery than at its center and so shrinks the transmitted laser beam. The observed nonlinear reflection is attributed to laser-induced change of carrier densities in Sb, to justify quantitatively the experimental results.

Formation of amyloid fibrils from ß-amylase.

Fibril formation has been considered a significant feature of amyloid proteins. However, it has been proposed that fibril formation is a common property of many proteins under appropriate conditions. We studied the fibril formation of ß-amylase, a non-amyloid protein rich in α-helical structure, because the secondary structure of ß-amylase is similar to that of prions. With the conditions for the fibril formation of prions, ß-amylase proteins were converted into amyloid fibrils. The features of ß-amylase proteins and fibrils are compared to prion proteins and fibrils. Furthermore, the cause of neurotoxicity in amyloid diseases is discussed.

Insights into structural properties of denatured human prion 121-230 at melting temperature studied by replica exchange molecular dynamics.

Misfolding and aggregation of the prion protein (PrP) is responsible for the development of fatal transmissible neurodegenerative diseases. PrP undergoes structural conversion from a natively folded state into a misfolded state, resulting in aggregated amyloid fibrils. Partial unfolding has been recognized as an essential step in fibrillation, especially at the middle point of unfolding. To study the possible aggregation-prone states, we characterized the structure of the C-terminal globular domain of human prion (huPrP) 121-230 near extended conformation at melting temperature by replica exchange molecular dynamics (REMD) simulation, as the REMD method is the most suited generalized-ensemble algorithm that performs random walk in energy space and promotes a system to escape from local energy traps. Our results revealed that denatured huPrP is partially folded with α-helical structure at melting temperature. The simulation results provide further insight into the unfolding of prion, which is essential in pathogenesis of prion diseases.

Poly(9,9-dioctylfluorene-alt-thiophene)/graphene nanocomposite: effects of graphene on optoelectronic characteristics.

Nanocomposite comprising Poly(9,9-dioctylfluorene-alt-thiophene) (PDOFT) and graphene sheets was synthesized for the purpose of investigating its enhanced optoelectronic performance caused by the graphene. UV/Vis and photoluminescence spectra of PDOFT/graphene nanocomposite show that the existence of graphene does not alter the optical characteristics wavelength of PDOFT, and both solution samples and thin films emit green light. Meanwhile, the insignificant change in PL quantum efficiency indicates that graphene does not have significant effects on the transfer of excitation energy, the occurrence of self-quenching and the formation of excimer. However, the electric conductivity increases with an increase in the amount of graphene. The ionization potential(HOMO) and the electron affinity(LUMO) measured from the cyclic voltammetry lead to the determination of the optical band gap (Eg(chem)). When used to fabricate an optoelectronic device, the threshold voltage decreases with an increase in the graphene content until an excessive amount of graphene causes an unbalance on the electron and hole mobilities. The device fabricated with PDOFT/graphene with a 5% graphene content has a maximum luminescence of 5908 cd/m2 at a voltage of 9.5 V. The corresponding power efficiency at this maximum luminescence is 0.38 cd/A which is much higher than the device fabricated with pristine PDOFT. From the Admittance spectroscopy of the device, the electron mobility has been proved to increase with the graphene content.

Kramers-Kronig relation between nonlinear absorption and refraction of C(60) and C(70).

Using the Z-scan technique with 532 nm 16 picosecond laser pulses, we observe reverse saturable absorption and positive nonlinear refraction of toluene solutions of both C(60) and C(70). By deducting the positive Kerr nonlinear refraction of the solvent, we notice that the solute molecules contribute to nonlinear refraction of opposite signs: positive for C(60) and negative for C(70). Attributing nonlinear absorption and refraction of both solutes to cascading one-photon excitations, we illustrate that they satisfy the Kramers-Kronig relation. Accordingly, we attest the signs and magnitudes of nonlinear refraction for both solutes at 532 nm by Kramers-Kronig transform of the corresponding nonlinear absorption spectra.

A multi-D-shaped optical fiber for refractive index sensing.

A novel class of multi-D-shaped optical fiber suited for refractive index measurements is presented. The multi-D-shaped optical fiber was constructed by forming several D-sections in a multimode optical fiber at localized regions with femtosecond laser pulses. The total number of D-shaped zones fabricated could range from three to seven. Each D-shaped zone covered a sensor volume of 100 µm depth, 250 µm width, and 1 mm length. The mean roughness of the core surface obtained by the AFM images was 231.7 nm, which is relatively smooth. Results of the tensile test indicated that the fibers have sufficient mechanical strength to resist damage from further processing. The multi-D-shaped optical fiber as a high sensitive refractive-index sensor to detect changes in the surrounding refractive index was studied. The results for different concentrations of sucrose solution show that a resolution of 1.27 × 10(-3)-3.13 × 10(-4) RIU is achieved for refractive indices in the range of 1.333 to 1.403, suggesting that the multi-D-shaped fibers are attractive for chemical, biological, and biochemical sensing with aqueous solutions.

Feasibility of fiber Bragg grating and long-period fiber grating sensors under different environmental conditions.

This paper presents the feasibility of utilizing fiber Bragg grating (FBG) and long-period fiber grating (LPFG) sensors for nondestructive evaluation (NDE) of infrastructures using Portland cement concretes and asphalt mixtures for temperature, strain, and liquid-level monitoring. The use of hybrid FBG and LPFG sensors is aimed at utilizing the advantages of two kinds of fiber grating to implement NDE for monitoring strains or displacements, temperatures, and water-levels of infrastructures such as bridges, pavements, or reservoirs for under different environmental conditions. Temperature fluctuation and stability tests were examined using FBG and LPFG sensors bonded on the surface of asphalt and concrete specimens. Random walk coefficient (RWC) and bias stability (BS) were used for the first time to indicate the stability performance of fiber grating sensors. The random walk coefficients of temperature variations between FBG (or LPFG) sensor and a thermocouple were found in the range of -0.7499 °C/ [square root]h to -1.3548 °C/ [square root]h. In addition, the bias stability for temperature variations, during the fluctuation and stability tests with FBG (or LPFG) sensors were within the range of 0.01 °C/h with a 15-18 h time cluster to 0.09 °C/h with a 3-4 h time cluster. This shows that the performance of FBG or LPFG sensors is comparable with that of conventional high-resolution thermocouple sensors under rugged conditions. The strain measurement for infrastructure materials was conducted using a packaged FBG sensor bonded on the surface of an asphalt specimen under indirect tensile loading conditions. A finite element modeling (FEM) was applied to compare experimental results of indirect tensile FBG strain measurements. For a comparative analysis between experiment and simulation, the FEM numerical results agreed with those from FBG strain measurements. The results of the liquid-level sensing tests show the LPFG-based sensor could discriminate five stationary liquid-levels and exhibits at least 1,050-mm liquid-level measurement capacity. Thus, the hybrid FBG and LPFG sensors reported here could benefit the NDE development and applications for infrastructure health monitoring such as strain, temperature and liquid-level measurements.

An optical fiber viscometer based on long-period fiber grating technology and capillary tube mechanism.

This work addresses the development and assessment of a fiber optical viscometer using a simple and low-cost long-period fiber grating (LPFG) level sensor and a capillary tube mechanism. Previous studies of optical viscosity sensors were conducted by using different optical sensing methods. The proposed optical viscometer consists of an LPFG sensor, a temperature-controlled chamber, and a cone-shaped reservoir where gravitational force could cause fluid to flow through the capillary tube. We focused on the use of LPFGs as level sensors and the wavelength shifts were not used to quantify the viscosity values of asphalt binders. When the LPFG sensor was immersed in the constant volume (100 mL) AC-20 asphalt binder, a wavelength shift was observed and acquired using LabVIEW software and GPIB controller. The time spent between empty and 100 mL was calculated to determine the discharge time. We simultaneously measured the LPFG-induced discharge time and the transmission spectra both in hot air and AC-20 asphalt binder at five different temperatures, 60, 80, 100, 135, and 170 Celsius. An electromechanical rotational viscometer was also used to measure the viscosities, 0.15-213.80 Pa·s, of the same asphalt binder at the above five temperatures. A non-linear regression analysis was performed to convert LPFG-induced discharge time into viscosities. Comparative analysis shows that the LPFG-induced discharge time agreed well with the viscosities obtained from the rotational viscometer.

Error analysis and measurement uncertainty for a fiber grating strain-temperature sensor.

A fiber grating sensor capable of distinguishing between temperature and strain, using a reference and a dual-wavelength fiber Bragg grating, is presented. Error analysis and measurement uncertainty for this sensor are studied theoretically and experimentally. The measured root mean squared errors for temperature T and strain ε were estimated to be 0.13 °C and 6 µÎµ, respectively. The maximum errors for temperature and strain were calculated as 0.00155 T + 2.90 × 10(-6) ε and 3.59 × 10(-5) ε + 0.01887 T, respectively. Using the estimation of expanded uncertainty at 95% confidence level with a coverage factor of k = 2.205, temperature and strain measurement uncertainties were evaluated as 2.60 °C and 32.05 µÎµ, respectively. For the first time, to our knowledge, we have demonstrated the feasibility of estimating the measurement uncertainty for simultaneous strain-temperature sensing with such a fiber grating sensor.

Solute migration caused by excited state absorptions.

Using the Z-scan technique, we find that migration of chloroaluminum phthalocyanine in liquid ethanol can be induced by the absorption of a 19 ps laser pulse with energy exceeding a threshold but not by that of a 2.8 ns pulse depositing more energy at the solute molecules. Considering each solute molecule as an oscillator confined within a potential well, we explain, in accordance with the five-energy-band model, that solute molecules excited by a 19 ps pulse retain more translational excess energy to overcome the potential well barrier compared with those excited by a 2.8 ns pulse of equal energy. Therefore, they are more likely to migrate out of the laser beam center, weakening the solution's absorption that we detect in the Z-scan measurements. Furthermore, we theoretically infer that the 19 ps pulse-induced solute migration tends to be nonquasistatic and experimentally verify that it cannot be attributed to the Soret effect, a quasistatic process.

Chemical Sensing Sensitivity of Long-Period Grating Sensor Enhanced by Colloidal Gold Nanoparticles.

A simple and effective method is proposed to improve spectral sensitivity anddetection limit of long period gratings for refractive index or chemical sensing, where thegrating surface is modified by a monolayer of colloidal gold nanoparticles. Thetransmission spectra and optical properties of gold nanospheres vary with the differentrefractive index of the environment near the surface of gold nanospheres. The sensorresponse of gold colloids increases linearly with solvents of increasing refractive index.The results for the measurement of sucrose and sodium chloride solutions are reported,which show that this type of sensor can provide a limiting resolution of ~10-3 to ~10-4 forrefractive indices in the range of 1.34 to 1.39 and a noticeable increase in detection limit ofrefractive index to external medium.

Negative nonlinear refraction obtained with ultrashort laser pulses.

We present nonlinear refraction results for liquids methanol and acetic acid obtained with the Z-scan technique and 28 femtosecond (fs) 800 nm laser pulses. In contrast to the positive lensing effect obtained previously with picosecond and nanosecond laser pulses, a negative lensing effect is observed. The associated mechanism features the third-order polarization arising from the nonlinear response of the molecular skeletal motion that is driven into resonance through its electrostatic coupling to the valence electron cloud distorted by the fs laser field.