Two-dimensional displacement (bending) sensor based on cascaded Fabry-Perot interferometers fabricated in a seven-core fiber.

We demonstrated a two-dimensional vector displacement (bending) sensor with high angular resolution based on Vernier effect generated by two cascaded Fabry-Perot interferometers (FPI) in a seven-core fiber (SCF). To form the FPI, plane-shaped refractive index modulations are fabricated as the reflection mirrors in the SCF using slit-beam shaping and femtosecond laser direct writing. Three pairs of cascaded FPIs are fabricated in the center core and the two non-diagonal edge cores of the SCF and applied to the vector displacement measurement. The proposed sensor exhibits high displacement sensitivity with significant direction dependence. The magnitude and direction of the fiber displacement can be obtained via monitoring the wavelength shifts. Moreover, the source fluctuations and the temperature cross-sensitivity can be referenced out by monitoring the bending-insensitive FPI of the center core.

Fiber-end suspended polymer micro-rod inscribed with Bragg grating for highly sensitive ultrasonic detection.

A suspended polymer rod grating is fabricated on a fiber end for highly sensitive ultrasonic detection. Initially, the uniform polymer waveguide is prepared via the interconnection of holey fibers and the photopolymerization of an ultraviolet glue. A femtosecond laser point-by-point technique is then employed to form periodic grating structures inside the customized waveguide. A final uncovered micro-rod is achieved based on different corrosion resistances of the polymer waveguide and the fiber cladding. The polymer rod presents uniform morphology and controllable size with the support of the constructed air-hole microchannel. The self-alignment and the self-adhesion between the polymer waveguide and the fiber core contribute to the stable efficient optical coupling at the fiber-to-polymer joint. When applied to ultrasonic waves, the decreased size and low Young's modulus of the suspended rod provide benefits for the interaction between the polymer grating and the ultrasound strain. This sensor exhibits a noise equivalent pressure of 33 Pa and -10 d B bandwidth of 7.6 MHz. After packing with a waterproof adhesive, the polymer rod shows sufficient robustness for long-term operation. This Letter proposes a new, to the best of our knowledge, strategy for the fabrication of advanced polymer probes in multifunctional sensing.

Sensitivity enhanced vector accelerometer based on FBG-FP inscribed on multicore fiber.

We propose and fabricate a high-sensitivity vector vibration accelerometer with a multicore fiber Bragg grating Fabry-Perot (FBG-FP) structure. The acceleration sensitivities of the FBG and FBG-FP are 0.15 and 1.26 V/g, respectively. After packaging, the acceleration sensitivity of the FBG-FP is further improved to 6.89 V/g, which is 45.9 times higher than that of the FBG. The resonant frequency of the accelerometer increases from 30 to 86 Hz. Both the sensitivity and resonant frequency of the accelerometer are improved. Owing to the asymmetry of the outer core of the multicore fiber, high-sensitivity two-dimensional vector acceleration sensing can be realized.

Magnetic flux leakage detection based on a sensitivity-enhanced fiber Bragg grating magnetic field sensor.

We propose a sensitivity-enhanced fiber Bragg grating (FBG) magnetic field sensor for magnetic flux leakage (MFL) detection. The testing system consists of the FBG, suspended strain concentration structure, and two ceramic tubes bonded on a Terfenol-D base. We show the relation between the MFL and the width and depth of the crack, the lift-off of the sensor away from the surface of the workpiece, and the angle between the orientation of the sensor and the magnetization direction. The experimental results are very consistent with those obtained from finite element analysis simulations. The sensitivity of the sensor is increased to 81.11 pm/mT for increasing magnetic fields and 91.55 pm/mT for decreasing magnetic fields. The MFL test demonstrates that the sensor can identify a crack with a width of 0.5 mm and depth of 2 mm in an 8 mm thick workpiece. To the best of our knowledge, the magnetic field sensor proposed in this work has the highest sensitivity compared with the same types of sensors. Moreover, the application of an FBG-Terfenol-D based magnetic field sensor in the MFL test shows good performance. Compared with traditional electrical MFL testing technologies, the sensitivity-enhanced optical fiber magnetic field sensor has a higher resolution and longer survival time in harsh environments.

Femtosecond laser point-by-point inscription of helical-sampled fiber Bragg gratings.

In this Letter, a helical-sampled fiber Bragg gratings (HSFBGs) fabrication using a femtosecond laser point-by-point (PBP) technique is proposed. The unique helical structure generates sampled gratings owing to its periodicity. A simple, single-step method for inscription of the sampled gratings is described. The effects of geometrical parameters, including length of grating, helical diameter, helical pitch, and off-axis distance on the resonances are studied, and a series of comb-like spectra are obtained.

Femtosecond laser plane-by-plane inscribed ultrahigh-order fiber Bragg grating and its application in multi-wavelength fiber lasers.

We propose an ultrahigh-order fiber Bragg grating (UHO-FBG) containing dense resonants and its application as a novel filtering device in multi-wavelength lasers. The UHO-FBG is fabricated by femtosecond laser plane-by-plane direct inscription. Thanks to the plane-by-plane inscription, high-order Bragg resonances can be formed with multiple reflectance peaks of comparable reflectance in the range of the fiber operating bandwidth and without the transmission depression of long-period gratings in the transmission spectrum. We also experimentally demonstrate the use of UHO-FBG pairs in a distributed Bragg reflector laser, enabling the excitation of multi-wavelength lasers.

Orientation-dependent fiber-optic accelerometer based on eccentric fiber Bragg grating.

A highly localized eccentric fiber Bragg grating (EFBG) accelerometer was proposed, and its orientation-dependent measurement results were demonstrated experimentally. An EFBG was inscribed point-by-point (PbP) in a single-mode fiber (SMF) using a femtosecond laser, and the cladding mode was recoupled to excite the ghost mode through an abrupt taper. Owing to the asymmetry caused by the lateral offset of the EFBG, the ghost mode showed a significant directional response to acceleration. Furthermore, monitoring the fundamental core mode resonance can help calibrate accidental power perturbation or cross-sensitivity.

Vector bending sensor based on an edge-core cladding-type fiber Bragg grating.

A two-dimensional vector bending sensor that is both compact and simple is proposed and demonstrated, based on an edge-core cladding-type fiber Bragg grating (ECLFBG) inscribed in an edge-core. The ECLFBG is written parallel to the edge-core using a femtosecond laser point-by-point technique. The reflection spectrum of this ECLFBG varies significantly depending on the magnitude and direction of the fiber's bend. Combining the trend and sensitivity of the wavelength shift and reflection intensity variations of the ECLFBG, the bending magnitude and direction can be measured simultaneously.

Two-dimensional vector accelerometer based on orthogonal Bragg gratings inscribed in a standard single-mode fiber cladding.

We propose and demonstrate a novel, to the best of our knowledge, two-dimensional vector accelerometer based on orthogonal cladding fiber Bragg gratings (FBGs) inscribed in a standard single-mode fiber (SMF). The cladding FBGs are written by a femtosecond laser point-by-point technique and run parallel with the core. We experimentally demonstrate that the two orthogonal components of acceleration can be directly detected using simplified power-referenced detection. Using this structure, we can simultaneously obtain orientation information and acceleration in a SMF.

Direct femtosecond laser inscription of an IR fluorotellurite fiber Bragg grating.

This study proposes a novel, to the best of our knowledge, development of fluorotellurite glass fiber Bragg gratings (FBGs). Shell-like morphology was achieved using a single femtosecond laser pulse illuminated through the fiber's polymer coating. Different FBG fabrication methods and parameters were systematically studied to optimize performance. The fluorotellurite FBG exhibited a high sensitivity to writing laser power and reflectivity saturation effect in repetitive writing. A low-insertion-loss fluorotellurite FBG with a reflectivity of over 99% and bandwidth of less than 1 nm was successfully inscribed. The flexible inscription methods can write an FBG at any wavelength in the fluorotellurite glass transparent window, and are applicable to infrared fiber lasers or sensors.

Highly elliptical core fiber with stress-induced birefringence for mode multiplexing.

We report the polarization-maintaining properties of a highly elliptical core fiber surrounded by a trench that was designed to optimize the modal effective indices and bending loss for a total of five spatial modes (10 channels). In addition to the asymmetric core structure, the birefringence of the fiber is increased by the thermal stress introduced during the fabrication. The results show a modal birefringence larger than 10-4 for all guided spatial modes. The mode stability to bending is evaluated by selectively exciting/detecting each spatial mode while perturbing the fiber. This few-mode polarization-maintaining fiber is of interest for multiple-input multiple-output (MIMO)-free mode division multiplexing transmission systems.

Membrane-free acoustic sensing based on an optical fiber Mach-Zehnder interferometer.

Traditional optical fiber acoustic sensors are mostly based on mechanical diaphragms and use indirect coupling between the acoustic and optical signals. The detectable frequency range and sound pressure range of such a sensor have limitations because they are influenced by the membrane or a mechanically deformable material. In this paper, a Mach-Zehnder interferometer-based membrane-free acoustic sensing method is developed. The sensing principle relies on direct detection of sound-pressure-induced changes of the refractive index in the open cavity. This enables an inherently flat frequency response over a broad bandwidth. Simulation and experiment were carried out to verify and demonstrate the idea. The results show that the membrane-free acoustic sensor has a flat frequency response from 500 Hz to 20 kHz.

Evaluation of risk levels of trace elements in walnuts from China and their influence factors: Planting area and cultivar.

The potential health risk of trace elements in nut foods has been widely concerned. The accumulations of trace elements by plants in the environment are disturbed by multiple factors. The objective of this work was to investigate the risk levels of trace elements in walnuts and their influence factors (planting area and cultivar). A total of 228 walnut samples were collected from four major walnut producing areas of China. The contents of essential elements for Fe, Cu, Zn, Mo and Se were 35.8, 21.9, 14.8, 0.3 and 0.04 mg/kg, respectively. The contribution of Cu for dietary reference intake was as high as 82.22%. The risk levels of potential toxicological elements and toxic elements within the acceptable limits. Significantly, the hazard quotients (HQs) of Ba and Co were up to 26.14% and 25.31%, respectively. The effect of planting area on trace elements was determined from the aspects of regional distribution and urbanization. Significant differences of essential elements were found between northeast and northwest areas. The urbanization directly influenced toxic elements, which could cause variation up to 85.0% (Pb) and 42.9% (As). Finally, cultivar effect was confirmed that all walnut cultivars were divided into four categories with different trace element characteristics.

Ultrasonic imaging of seismic physical models using a fringe visibility enhanced fiber-optic Fabry-Perot interferometric sensor.

A fringe visibility enhanced fiber-optic Fabry-Perot interferometer based ultrasonic sensor is proposed and experimentally demonstrated for seismic physical model imaging. The sensor consists of a graded index multimode fiber collimator and a PTFE (polytetrafluoroethylene) diaphragm to form a Fabry-Perot interferometer. Owing to the increase of the sensor's spectral sideband slope and the smaller Young's modulus of the PTFE diaphragm, a high response to both continuous and pulsed ultrasound with a high SNR of 42.92 dB in 300 kHz is achieved when the spectral sideband filter technique is used to interrogate the sensor. The ultrasonic reconstructed images can clearly differentiate the shape of models with a high resolution.

Orientation-dependent fiber-optic displacement sensor using a fiber Bragg grating inscribed in a side-hole fiber.

We propose and experimentally demonstrate an orientation-dependent fiber-optic bending sensor. The sensing probe consists of a fiber Bragg grating inscribed in both the fiber core and the surrounding cladding of a section of a side-hole fiber. We utilized a side-illumination technique using a femtosecond laser to achieve the grating structure formation. The transmission intensities of both resonances are highly sensitive bending of the fiber, and the bending response shows orientation dependence. The surrounding temperature fluctuation causes a wavelength shift, but not an intensity variation. Therefore, the proposed sensor can be employed for simultaneous measurement of bending and temperature.

Optical fiber humidity sensor based on the direct response of the polyimide film.

An optical fiber humidity sensor based on an optical Fabry-Perot interferometer is proposed and experimentally demonstrated. The sensor is constructed by a short section of hollow-core fiber coated with a polyimide (PI) film. Taking advantage of the direct response of the PI film, a sensitivity of up to 1.309 nm/%RH can be achieved in the humidity change range from 40% RH to 80% RH. The temperature sensitivity is measured to be 43.57 pm/°C when the temperature changes from 25°C to 55°C. Because of its simple structure, fast response time, convenient production, and good reproducibility, the proposed sensor will be competitive in the field of cultural relic humidity monitoring and pharmaceutical storage.

Miniaturized fiber-taper-based Fabry-Perot interferometer for high-temperature sensing.

A microminiaturized all-fiber Fabry-Perot interferometer (FPI) for high-temperature sensing has been proposed and experimentally demonstrated. The FPI is composed of a micro-air bubble and a taper probe with a tip less than 2 µm in diameter as reflected interfaces. A temperature sensitivity of 14.68 pm/°C near the wavelength of 1550 nm is obtained. The sensor, with its miniature size, can work in an ultra-small space with a large range of temperature variation.

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.

Integrated in-fiber coupler for microsphere whispering-gallery modes resonator excitation.

We present an integrated in-fiber coupler for excitation of whispering-gallery modes of a microsphere resonator. The coupler is simply fabricated by chemical etching away the holey area of a photonic crystal fiber, leaving a freestanding solid core enclosed in a silica housing. Light is coupled into a microsphere through the suspended core with a diameter of 2.1 µm. Since the coupler itself performs as a Fabry-Perot interferometer, asymmetric Fano resonances can be observed in the mixed reflection spectrum. The silica housing of the coupler provides a robust mechanical support to the microsphere resonator. The new Fano resonance coupler shows great potential in biochemical sensing and optical switching applications.

Hybrid optical fiber Fabry-Perot interferometer for simultaneous measurement of gas refractive index and temperature.

We present a hybrid miniature optical fiber Fabry-Perot interferometer for simultaneous measurement of gas refractive index and temperature. The interferometer is fabricated by cascading two short sections of capillary tubes with different inner diameters. One extrinsic interferometer is based on the air gap cavity formed by the capillary tube with large diameter. Another section of capillary tube with small inner diameter performs as an intrinsic interferometer and also provides a channel enabling gas to enter and leave the extrinsic cavity freely. The experiment shows that the different dips or peaks in fringe exhibit different responses to the changes in gas refractive index and temperature. Owing to this feature, simultaneous measurement of the gas refractive index and temperature can be realized.