Three-dimensional force-tactile sensors based on embedded fiber Bragg gratings in anisotropic materials.

Three-dimensional force-tactile sensors have attracted much attention for their great potential in the applications of human-computer interaction and bionic intelligent robotics. Herein, a flexible haptic sensor based on dual fiber Bragg gratings (FBGs) embedded in a bionic anisotropic material is proposed for the detection of 3D forces. To achieve the discrimination of normal and tangential force angles and magnitudes, FBGs were orthogonally embedded in a flexible silicone cylinder for force determination. Fe3O4 nanoparticles were used as a modifying agent to induce anisotropic elasticity of the silicone structure to improve the angle detection resolution. The results show that the flexible tactile sensor can detect the angle and magnitude of the 3D force.

Compact Harmonic Vernier Sensor Based on an In-Fiber FPI with Three Reflector System for Simultaneous Gas Pressure and Temperature Measurement.

In this work, we proposed a sensitivity-enhanced temperature sensor, a compact harmonic Vernier sensor based on an in-fiber Fabry-Perot Interferometer (FPI), with three reflective interfaces for the measurement of gas temperature and pressure. FPI consists of air and silica cavities formulated by single-mode optical fiber (SMF) and several short hollow core fiber segments. One of the cavity lengths is deliberately made larger to excite several harmonics of the Vernier effect that have different sensitivity magnifications to the gas pressure and temperature. The spectral curve could be demodulated using a digital bandpass filter to extract the interference spectrum according to the spatial frequencies of resonance cavities. The findings indicate that the material and structural properties of the resonance cavities have an impact on the respective temperature sensitivity and pressure sensitivity. The measured pressure sensitivity and temperature sensitivity of the proposed sensor are 114 nm/MPa and 176 pm/°C, respectively. Therefore, the proposed sensor combines ease of fabrication and high sensitivity, making it great potential for practical sensing measurements.

Polarization response of planarized optical waveguides to determine the anisotropic complex refractive index of graphene oxide thin films.

The polarization response of graphene oxide (GO)-coated planarized optical waveguides is used to determine the complex refractive index of GO film. GO films with thicknesses between 0.10 and 0.71 µm were coated on planarized optical waveguides. GO-coated waveguides exhibit large polarization dependent losses-and the polarization response depends strongly on the GO coating thickness. The response was used, together with finite element analysis, to determine the complex refractive index of the GO film. The complex refractive indices of GO films for both TE- and TM-polarized light at a wavelength of 1550 nm were found to be 1.71+0.09i and 1.58+0.05i, respectively. The uncertainties of nGO and kGO for TE-polarized light are ±0.02 and ±0.03, respectively, whereas the uncertainties of nGO and kGO for TM-polarized light are ±0.05 and ±0.02, respectively.

Fabrication of a carbon nanotube/tungsten disulfide visible spectrum photodetector.

Two-dimensional-material-based photodetectors are gaining prominence in optoelectronic applications, but there are certain factors to consider with bulk material usage. The demand for a highly responsive and highly efficient device with an inexpensive fabrication method is always of paramount importance. Carbon nanotubes (CNT) are well known, owing to their upheld vigorous structural and optoelectronic characteristics, but to fabricate them at a large scale involves multifarious processes. A visible range photodetector device structure developed using a simple and inexpensive drop-casting technique is reported here. The optoelectronic characteristics of the device are studied with IV measurements under the light and dark conditions by incorporating a thin CNT layer on top of tungsten-disulfide-based heterojunction photodetector to enhance the overall characteristics such as detectivity, responsivity, photocurrent, rise time, and fall time in the visible range of the light spectrum with a violet light source at 441 nm. In the DC bias voltage range of -20 to 20 V, IV measurements are carried out under dark and illumination conditions with different incident power densities. The threshold voltage is recognized at 2.0 V. Photocurrent is found to be highly dependent on the state of the incident light. For 0.3074mW/cm2 illuminated power, the highest responsivity and detectivity are determined to be 0.57 A/W and 2.89×1011 Jones. These findings encourage an alternative fabrication method at a large scale to grow CNTs for the enhancement of optoelectronic properties of present two-dimensional-material-based optoelectronic and photonics applications.

Cascaded Fabry-Perot interferometer-regenerated fiber Bragg grating structure for temperature-strain measurement under extreme temperature conditions.

We demonstrated an optical fiber sensor based on a cascaded fiber Fabry-Perot interferometer (FPI)-regenerated fiber Bragg grating (RFBG) for simultaneous measurement of temperature and strain under high temperature environments. The FPI is manufactured from a ∼74 µm long hollow core silica tube (HCST) sandwiched between two single mode fibers (SMFs). The RFBG is inscribed in one of the SMF arms which is embedded inside an alundum tube, making it insensitive to the applied strain on the entire fiber sensor, just in case the temperature and strain recovery process are described using the strain-free RFBG instead of a characteristic due-parameter matrix. This feature is intended for thermal compensation for the FPI structure that is sensitive to both temperature and strain. In the characterization tests, the proposed device has exhibited a temperature sensitivity ∼ 18.01 pm/°C in the range of 100 °C - 1000 °C and excellent linear response to strain in the range of 300 °C - 1000 °C. The measured strain sensitivity is as high as ∼ 2.17 pm/µÉ› for a detection range from 0 µÉ› to 450 µÉ› at 800 °C, which is ∼ 1.5 times that of a FPI-RFBG without the alundum tube.

Regenerated grating produced in a multimaterial glass-based photosensitive fiber with an ultrahigh thermal regeneration ratio.

This work demonstrates thermal regeneration of gratings inscribed in a new type of multi-material glass-based photosensitive fiber. And isothermal annealing procedure has been carried out on a type-I seed grating (SG) imprinted in erbium-doped zirconia-yttria-alumina-germanium (Er-ZYAG) silica glass-based fiber, which is initiated from room temperature of 25°C up to 900°C. The findings show that the created regenerated grating (RG) has an ultrahigh thermal regeneration ratio with a value of 0.72.

Tungsten-disulphide-based heterojunction photodetector.

Two-dimensional (2D) materials have realized significant new applications in photonics, electronics, and optoelectronics. Among these materials is tungsten disulphide (WS2), which is a 2D material that shows excellent optoelectronic properties, tunable/sizable bandgap in the visible range, and good absorption. A polycrystalline WS2 thin film is successfully deposited on a substrate using radio frequency magnetron sputtering at room temperature. The x-ray diffraction pattern reveals two hexagonal structured peaks along the (100) and (110) planes. Energy-dispersive x-ray spectroscopy reveals a non-stoichiometric WS2 film with 1.25 ratio of S/W for a 156.3 nm thick film, while Raman shifts are observed at the E2g1 and A1g phonon modes located at 350.70 cm-1 and 415.60 cm-1, respectively. A sandwiched heterojunction photodetector device is successfully fabricated and illuminated within the violet range at 441 nm and 10 V of bias voltage. The maximum photocurrent values are calculated as 0.95 µA, while the responsivity is observed at 169.3 mA W-1 and detectivity 1.48×108 Jones at illuminated power of 0.6124 µm. These results highlight the adaptability of the present technique for large-scale applications as well as the flexibility to promote development of advanced optoelectronic devices.

Nanosecond pulse laser generation at 1.55 and 2 µm regions by integrating a piece of newly developed chromium-doped fiber-based saturable absorber.

This paper demonstrated the nanosecond pulse laser operation at 1.55 and 2 µm wavelength regions using a newly develop chromium-doped fiber (CrDF) as a saturable absorber (SA) to convert efficiently continuous-wave laser operation to nanosecond pulse laser operation. The laser uses an erbium-doped fiber (EDF) and thulium-doped fiber as the gain medium. A piece of 10 cm long CrDF was integrated into both laser cavities to generate nanosecond pulse laser operation. In 1.55 region generation, an additional single-mode fiber (SMF) 100 m long was added into the EDF laser cavity. Stable pulse generation occurred at a repetition rate of 1 MHz with a pulse width of 432 ns and a signal-to-noise ratio (SNR) of 66 dB. The highest peak power of 24 mW was obtained at 142 mW pump power. In 2 µm region generation, the obtained repetition rate was 10 MHz with a pulse width and SNR of 59 ns and 41 dB, respectively. The highest peak power was only 8.3 mW. By looking into the findings, the newly developed CrDF SA has a potential to be further enhanced toward better generation of ultrashort pulse fiber lasers.

1.3 µm fiber grating in a thin-core fiber for LP01-LP11 mode converters and sensing ability.

We demonstrate an all-fiber structure that can realize LP01-LP11 mode conversion and twist measurement. It is a thin-core fiber (TCF) grating at a wavelength of 1310 nm cascaded to a short segment of a TCF of a different core size. It is found that the different core size of the TCF between the fiber and the grating has an impact on the excitation of a higher-order mode and mode conversion efficiency. The fiber structure exhibits a good linear response to twisting, strain, and temperature. Depending on the associated mode, the mode intensity and the wavelength for exciting the peaks of the grating have different sensitivities to twisting angle, applied strain, and temperature. These properties can be exploited for simultaneous measurement.

Effect of two annealing processes on the thermal regeneration of fiber Bragg gratings in hydrogenated standard optical fibers.

In this work, we demonstrate the thermal regeneration of fiber Bragg gratings written in the hydrogenated standard communication optical fibers by two annealing processes. The first annealing process is done at an intermediate temperature (500°C, 700°C, and 900°C) for a specific period of time before cooling down to room temperature. The second annealing is at 1000°C in which the thermal regeneration is attained. The experimental results show that the regenerated gratings that are preannealed at 700°C have charted a reflectivity larger than 65%. They have higher thermal stability compared to that of the standard annealing process. Meanwhile the difference in temperature sensitivity is very small. The temperature sensitivities of regenerated gratings, which have undergone only two annealing processes, are 16.1 pm/°C and 15.8 pm/°C, respectively.

Temperature-independent hygrometry using micromachined photonic crystal fiber.

An in-fiber Mach-Zehnder interferometer (MZI) is proposed and experimentally demonstrated for relative humidity (RH) and temperature measurements. The MZI is formed by a grapefruit-shaped photonic crystal fiber (G-PCF) cascaded with a short section of multimode fiber that serves as a mode coupler. To enhance sensitivity to humidity, femtosecond laser micromachining was performed to remove a portion of cladding of the G-PCF to expose its core to the ambient medium. The output interference spectrum is fast Fourier transformed to produce a spatial frequency spectrum that describes the intensity composition of the cladding modes in the MZI. In our investigation, it was observed that the interference dip intensity has a sensitivity of -0.077 dB/% RH to the change of RH in the range of 25%-80% RH, whereas the dip wavelength has a temperature sensitivity of ∼3.3 pm/°C in the range of 25°C-70°C. In addition, the dip intensity was insensitive to temperature. These characteristics have provided convenience in eliminating temperature cross talk and achieving accurate humidity measurement.

Mach-Zehnder interferometric magnetic field sensor based on a photonic crystal fiber and magnetic fluid.

A Mach-Zehnder interferometric magnetic field sensor based on a photonic crystal fiber (PCF) and magnetic fluid (MF) was designed and experimentally demonstrated. The sensing probe consists of a single-mode-(SM)-multimode-PCF-SM fiber structure through arc fusion splicing. It was then laser engrave notched with the femtosecond laser so that the PCF cladding was selectively infilled MF. A well-defined interference pattern was obtained on account of the tunable refractive index of the MF infilled PCF cladding. The transmission spectra of the proposed sensor under different magnetic field intensities have been measured and theoretically analyzed. The results show that the sensitivity of the proposed sensor can reach -0.13 dB/mT and 0.07334 nm/mT in the magnetic field intensity from 1 mT to 20 mT and 2 mT to 20 mT, respectively.

A Recent Progress of Steel Bar Corrosion Diagnostic Techniques in RC Structures.

Corrosion of steel bar is one of key factors undermining reinforced concrete (RC) structures in a harsh environment. This paper attempts to review the non-destructive procedures from the aspect of the corrosion measurement techniques, especially their advantages and limitations. Systematical classification of diagnostic methods is carried out to determine any probable corrosion issues before the structures become severe, and helps choose the suitable method according to different construction features. Furthermore, the three electrochemical factors method is introduced to inspire researchers to combine various techniques to improve corrosion evaluation accuracy. The recommendations for future work are summarized, in conclusion.

Temperature sensing using CdSe quantum dot doped poly(methyl methacrylate) microfiber.

This work describes noncontact temperature measurements using wavelength shifts of CdSe quantum dot (QD) doped poly(methyl methacrylate) microfiber. The sensor is fabricated using a drawing method by bridging two tapered single mode fibers with a polymer microfiber (PMF) approximately 3 µm in diameter. A set of a PMF section with and without the doping of the CdSe-ZnS core-shell QD was applied as sensing probes and used to measure temperatures over the range of 25°C-48°C. The experimental results show that the doped PMF is able to achieve a higher performance with a reasonably good sensitivity of 58.5 pm/°C based on the wavelength shifting, which is about 18 times that of the undoped PMF temperature sensitivity. The proposed sensor showed a linear temperature sensing range that matches well with the physiologically relevant temperatures. Moreover, these results open the way for long-term and high-stability realization of temperature sensing optical fibers.

Strain measurement at temperatures up to 800°C using regenerated gratings produced in the highGe-doped and B/Ge co-doped fibers.

In this work, we have proposed a sensor for strain measurement in high-temperature environments up to 800°C by employing two regenerated fiber Bragg gratings. Two seed gratings (SGs) are inscribed in high Ge-doped and B/Ge-codoped fibers, respectively, which possess different temperature sensitivities. To achieve two gratings with different strain sensitivities, one of the gratings is chemically etched to reduce the fiber diameter for strain sensitivity enhancement. A thermal annealing process is carried out to activate the grating regeneration in the SGs. The temperature and strain calibration experiments indicate that the proposed structure has uncertainty values of 23.42 µÎµ and 5.83°C over the ranges of 0-1000 µÎµ and 20°C-800°C, respectively.

All-fiber magnetic field sensor based on tapered thin-core fiber and magnetic fluid.

A method for the measurement of a magnetic field by combining a tapered thin-core fiber (TTCF) and magnetic fluid is proposed and experimentally demonstrated. The modal interference effect is caused by the core mode and excited eigenmodes in the TTCF cladding. The transmission spectra of the proposed sensor are measured and theoretically analyzed at different magnetic field strengths. The results field show that the magnetic sensitivity reaches up to -0.1039 dB/Oe in the range of 40-1600 e. The proposed method possesses high sensitivity and low cost compared with other expensive methods.

Potassium permanganate (KMnO4) sensing based on microfiber sensors.

Two straight microfiber sensors are proposed and demonstrated for the detection of various concentrations of a potassium permanganate (KMnO4) solution. Two types of straight microfibers, namely, silica microfiber and poly(methyl methacrylate) microfiber, have been fabricated by using the flame brushing technique and the direct drawing technique, respectively. Based on the varied KMnO4 concentrations of the solution from 1% to 6%, the measurement of the peak voltage of the transmission power was made. The results show that the sensitivity of the silica microfiber sensor and the polymer microfiber is obtained at 184.5 µW/% and 32.57 µW/% with a resolution of 0.0035% and 0.0064%, respectively. Hence, the silica microfiber is more sensitive than the polymer microfiber for KMnO4 concentration measurements.

Curvature and Temperature Measurement Based on a Few-Mode PCF Formed M-Z-I and an Embedded FBG.

We have experimentally demonstrated an optical fiber Mach-Zehnder interferometer (MZI) structure formed by a few-mode photonic crystal fiber (PCF) for curvature measurement and inscribed a fiber Bragg grating (FBG) in the PCF for the purpose of simultaneously measuring temperature. The structure consists of a PCF sandwiched between two multi-mode fibers (MMFs). Bending experimental results show that the proposed sensor has a sensitivity of -1.03 nm/m-1 at a curvature range from 10 m-1 to 22.4 m-1, and the curvature sensitivity of the embedded FBG was -0.003 nm/m-1. Temperature response experimental results showed that the MZI's wavelength, λa, has a sensitivity of 60.3 pm/°C, and the FBG's Bragg wavelength, λb, has sensitivity of 9.2 pm/°C in the temperature range of 8 to 100 °C. As such, it can be used for simultaneous measurement of curvature and temperature over ranges of 10 m-1 to 22.4 m-1 and 8 °C to 100 °C, respectively. The results show that the embedded FBG can be a good indicator to compensate the varying ambient temperature during a curvature measurement.

D-Shaped Polarization Maintaining Fiber Sensor for Strain and Temperature Monitoring.

A D-shaped polarization-maintaining fiber (PMF) as fiber optic sensor for the simultaneous monitoring of strain and the surrounding temperature is presented. A mechanical end and edge polishing system with aluminum oxide polishing film is utilized to perform sequential polishing on one side (lengthwise) of the PMF in order to fabricate a D-shaped cross-section. Experimental results show that the proposed sensor has high sensitivity of 46 pm/µÎµ and 130 pm/°C for strain and temperature, respectively, which is significantly higher than other recently reported work (mainly from 2013) related to fiber optic sensors. The easy fabrication method, high sensitivity, and good linearity make this sensing device applicable in various applications such as health monitoring and spatial analysis of engineering structures.

Thulium-doped fiber laser utilizing a photonic crystal fiber-based optical low-pass filter with application in 1.7 µm and 1.8 µm band.

This paper describes a low pass filter based on photonics crystal fiber (PCF) partial ASE suppression, and its application within a 1.7 µm to 1.8 µm band thulium-doped fiber amplifier (TDFA) and a thulium-doped fiber laser (TDFL). The enlargement of air holes around the doped core region of the PCF resulted in a low-pass filter device that was able to attenuate wavelengths above the conventional long cut-off wavelength. These ensuing long cut-off wavelengths were 1.85 µm and 1.75 µm, and enabled a transmission mechanism that possessed a number of desirable characteristics. The proposed optical low-pass filter was applied within a TDFA and TDFL system. Peak spectrum was observed at around 1.9 µm for conventional TDF lasers, while the proposed TDF laser with PCF setup had fiber laser peak wavelengths measured at downshifted values of 1.74 µm and 1.81 µm.