Grating-based Sensor Research Articles

A new subdivision method for grating-based displacement sensor using imaging array

A new subdivision method for grating-based displacement sensor was proposed in the present study, which takes advantages of imaging array's high resolution and pixels’ good uniformity in the space. In this method, the magnified grating image is received by imaging array and grating pitch is directly subdivided by pixels, which is quite different from that moiré fringe is subdivided by complex subsequent circuits in moiré-type displacement sensor. The displacement is statistically calculated by using the whole grating image, which greatly eliminates the errors arising from illumination, electrical signals’ fluctuation, grating defect, and so on. Therefore, the subdivision method is easy to obtain signal with high signal-to-noise ratio, insensitive to some external factors, and able to attain high measurement precision with low cost. In this paper, the principle of subdivision method was illuminated, the systemic resolution was theoretically discussed, the measurement precision was experimentally checked, and the uncertainty of measurement was analyzed. The subdivision system consisted of the grating with 20-μm pitch and the CMOS image sensor with the pixels of 1280×1024 had the resolution of 0.04 μm, and the maximum displacement error was less than 0.4 μm, which has been tested in the Abbe comparator platform.

Long period grating-based humidity sensor for potential structural health monitoring

This paper reports the work on the development of novel humidity sensors using long period grating (LPG) technology in an optical fibre. The sensors are created through coating a thin layer of polyvinyl alcohol (PVA), whose refractive index varies as a function of humidity level, onto an LPG. The LPG written in a single mode optical fibre provides a means to change the coupling between the core and cladding modes thus to induce a wavelength shift in the attenuation bands of the transmission spectrum of the optical sensor device created. In this work different sensor configurations were created by coating various LPGs and then characterizing the resulting sensor performance. The work carried out has shown that, compared to the more familiar fibre Bragg grating (FBG)-based humidity sensors, LPG sensors show higher measurement sensitivity, with more relaxed requirements for the coating thickness and uniformity, with the potential for their development as chemical sensors through optimization of the design of the coating materials chosen. In this research, the sensors which were designed and fabricated were created for future applications in determining moisture ingress in a range of concrete materials when subjected to various environmental conditions.

Hybrid Fiber-Optic/Electrical Measurement System for Characterization of Railway Traffic and Its Effects on a Short Span Bridge

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> The characterization of traffic effects on a short span railway bridge in Northern Portugal with a new hybrid platform that allows the simultaneous assessment of signals generated by a sensing network composed of both electrical and fiber Bragg grating-based sensors was demonstrated. A commercial fiber-optic-based train characterization system using a bridge weight-in-motion algorithm was also developed and tested. By using only three fiber Bragg grating sensors the system allows on-motion determination of train speed and weight distribution. </para>

Interrogation of Fiber Bragg Grating Dynamic Strain Sensors by Self-Mixing Interferometry

An innovative investigation of optical feedback or self-mixing interference within the cavity of a single-longitudinal-mode laser is described as an integral part of a novel interrogation scheme to be employed in a fiber Bragg grating-based sensor for strain measurement. The entire sensor device simply consists of a laser diode with an integrated photodiode which is coupled to a fiber Bragg grating under strain. A small percentage of the injected lightwave resonantly reflected off the grating structure reenters the laser cavity and modifies the emission properties of the laser, resulting in the formation of characteristic sawtooth fringes which contain the embedded strain information. The feasibility of demodulating the induced small wavelength shifts of fiber Bragg gratings under dynamic mechanical loading by self-mixing interferometry is thus presented analytically and experimentally verified. A relatively good corroboration has been achieved.

Surface plasmon resonance sensor based on an array of diffraction gratings for highly parallelized observation of biomolecular interactions

We report a novel surface plasmon resonance (SPR) sensor for highly parallelized biomolecular interaction analysis (BIA). This sensor utilizes an array detection format and angular spectroscopy of surface plasmons on a two-dimensional array of metallic diffraction gratings. Each diffraction grating on a sensor chip serves both as an SPR coupler and a sensing channel in which refractive index changes induced by biomolecular interaction events are measured. We theoretically optimized the diffraction grating-based sensor to yield maximum resolution in the measurement of refractive index changes. Based on this study, we developed a prototype sensor instrument which enables parallel measurements with high-refractive index resolution (5 × 10 −7 RIU) in large number of sensing channels (120). The instrument consists of a two-dimensional array of sensing channels on a sensor chip, a fluidic system for sample delivery, and an SPR optical readout system. By using this instrument and micro spotting to produce a chip array, we evaluated the cross-binding of antibodies against a selected set of endocrine-disrupting compounds (atrazine, 2,4-dichlorophenoxyaceic acid, 4-nonylphenol and benzo[ a]pyrene) to respective protein–analyte conjugates.

Multiplexing of Fiber-Optic Long-Period Grating-Based Interferometric Sensors

Two long-period gratings (LPGs) fabricated in series in an optical fiber form a Mach-Zehnder interferometer, producing a sinusoidal channelled spectrum within the characteristic LPG attenuation bands, which has the appearance of an interference fringe pattern. The phase of the fringes is sensitive to changes in the surrounding environmental parameters, such as refractive index. Fabrication of a number of identical cascaded LPG pairs with different separations produces independent fringe patterns of different frequencies within the attenuation bands. The application of basic Fourier techniques to analyze the transmission spectrum allows the phase of each fringe pattern to be determined independently, facilitating the differentiation of external effects acting on each interferometer

Bragg grating-based fibre optic sensors in structural health monitoring

This work first considers a review of the dominant current methods for fibre Bragg grating wavelength interrogation. These methods include WDM interferometry, tunable filter (both Fabry-Perot and acousto-optic) demultiplexing, CCD/prism technique and a newer hybrid method utilizing Fabry-Perot and interferometric techniques. Two applications using these techniques are described: hull loads monitoring on an all-composite fast patrol boat and bolt pre-load loss monitoring in a composite beam in conjunction with a state-space modelling data analysis technique.

Monitoring Ingress of Moisture in Structural Concrete Using a Novel Optical-Based Sensor Approach

The detection of moisture ingress in concrete is important for structural monitoring and in this work is realised by monitoring the shift in the characteristic wavelength of a fibre Bragg grating-based sensor. The sensor relies upon a moisture-sensitive polymer layer deposited on the fibre Bragg grating (FBG) and the strain induced on it as a result of polymer swelling is monitored. Moisture ingress experiments were carried out using two such optical fibre sensors, placed at varying distances from the edge of the face of standard concrete cubes to the inner part of the concrete sample and subjected to water at a constant temperature. Information on the properties of different types of concrete and thus potentially on the migration of dissolved salts and their effect on reinforcement bars within concrete can be obtained.

Surface Plasmon Resonance Imaging of Biomolecular Interactions on a Grating-Based Sensor Array

A surface plasmon resonance sensor array based upon a grating substrate was developed for the detection of biomolecular interactions. The substrate consisted of a gold grating prepared by wet chemical treatment of a commercial recordable compact disk. A custom-built floating pin microspotter was constructed to deliver solutions containing omega-functionalized linear alkanethiols to the grating surface and produce an array of sensor elements with different exposed functional end groups. This array platform can be used to study biomolecular interactions in a label-free, sensitive, and high-throughput format. To illustrate the performance of this device, a test protein (bovine serum albumin) was exposed to sensor elements containing an array of functionalized alkanethiols possessing either activated carboxylic acid-, amine-, or hydroxyl-terminated regions. Local changes in plasmon resonance were monitored in a fixed-angle imaging configuration. Plasmon images clearly distinguish the degree of protein attachment at the various surfaces. The molecular binding events on the grating were also confirmed by ellipsometry. This grating-based SPR imaging platform represents a simple and robust method for performing label-free, high-sensitivity, and high-throughput detection of biomolecular interactions.

Optimizing integrated optical chips for label-free (bio-)chemical sensing

Label-free sensing is an important method for many (bio-)chemical applications in fields such as biotechnology, medicine, pharma, ecology and food quality control. The broad range of applications includes liquid refractive index sensing, molecule detection, and the detection of particles or cells. Integrated optics based on the use of waveguide modes offers a great potential and flexibility to tailor the sensor properties to these applications. In this paper, the results of a numerical study are presented, showing that this flexibility is founded on the many degrees of freedom that can be used for the integrated optical chip design, in contrast to other technologies such as those based on surface plasmon resonance, for which the materials' properties limit the range of choices. The applications that are explicitly considered and discussed include (1) bulk refractometry, (2) thin-layer sensing, for example biosensors monitoring molecular adsorption processes occurring within some 10 nm of the chip's surface, (3) thick-layer sensing with processes involving molecules or ions to be monitored within a sensing matrix extending to some 100 nm from the chip's surface, for example hydrogel-based layers and chemo-optically sensitive membranes, and (4) particle sensing with particles or, for example, biological cells to be monitored within probe volumes extending to some 1,000 nm from the chip's surface. The peculiarities for the different types of applications will be discussed, and suitable modeling methods presented. Finally, the application-specific design guidelines supplied will enable the optimization of various types of integrated optical sensors, including interferometers and grating-based sensors.

Response of fiber Bragg gratings to longitudinal ultrasonic waves

In the last years, fiber optic sensors have been widely exploited for several sensing applications, including static and dynamic strain measurements up to acoustic detection. Among these, fiber Bragg grating sensors have been indicated as the ideal candidate for practical structural health monitoring in light of their unique advantages over conventional sensing devices. Although this class of sensors has been successfully tested for static and low-frequency measurements, the identification of sensor performances for high-frequency detection, including acoustic emission and ultrasonic investigations, is required. To this aim, the analysis of feasibilty on the use of fiber Bragg grating sensors as ultrasonic detectors has been carried out. In particular, the response of fiber Bragg gratings subjected to the longitudinal ultrasonic (US) field has been theoretically and numerically investigated. Ultrasonic field interaction has been modeled, taking into account the direct deformation of the grating pitch combined with changes in local refractive index due to the elasto-optic effect. Numerical results, obtained for both uniform and Gaussian-apodized fiber Bragg gratings, show that the grating spectrum is strongly influenced by the US field in terms of shape and central wavelength. In particular, a key parameter affecting the grating response is the ratio between the US wavelength and the grating length. Normal operation characterized by changes in wavelength of undistorted Bragg peak is possible only for US wavelengths longer than the grating length. For US wavelengths approaching the grating length, the wavelength change is accompanied by subpeaks formation and main peak amplitude modulation. This effect can be attributed to the nonuniformity of the US perturbation along the grating length. At very high US frequencies, the grating is not sensitive any longer. The results of this analysis provide useful tools for the design of grating-based ultrasound sensors for meeting specific requirements in terms of field intensity and frequencies.

A novel temperature-insensitive fiber Bragg grating sensor for displacement measurement

This paper presents the design and development of an optical fiber Bragg gratingbased displacement sensor. A fiber Bragg grating is glued at a slant orientationonto the lateral side of a specially designed cantilever beam. It is found thatthe bandwidth of the FBG based sensor changes linearly with the variation ofthe displacement at the free end of the beam due to the displacement-inducedstrain gradient. Displacement sensing is realized by measuring the reflected opticalpower of the signal from the grating with a photodetector. A linear response of37.9 mV mm−1 was obtained within a displacement range of 9.0 mm. This sensor is also cost-effective dueto the use of a simple demodulation method and is inherently temperature insensitive,eliminating the need for temperature compensation.

Demonstration of etched cladding fiber Bragg grating-based sensors with hydrogel coating

A novel hydrogel-coated single-mode fiber Bragg grating (FBG) sensor is investigated. The sensor uses a swellable polymer material, hydrogel, as the active sensing component. The sensing mechanism is based on the stress that is induced in the chemically sensitive swellable hydrogel coating. The stress shifts the Bragg wavelength of the FBG. By reducing the cladding diameter of FBG through etching with hydrofluoric acid, the tension tuning force of FBG can be lowered. Therefore, the stress induced in hydrogel can shift the wavelength of FBG more and the sensitivity of the sensor is improved. When the grating fiber is etched 37.5 μm in diameter, the sensitivity of sensor is approximately 10-fold larger than that without fiber etching.

Strain evaluation of epoxy-cured fiber-optic connectors

Optical fiber connectors are passive components used to link two fiber links or a fiber link to a photonic device. One widely used type of fiber connector, a design that uses a thermally cured epoxy adhesive, has been evaluated via Bragg grating-based fiber strain sensors. Strain sensors were used to evaluate the strain incurred by the optical fiber as a result of installation and subsequent environmental testing. Preliminary mechanical modeling and a strain analysis using Bragg grating-based strain sensors are discussed. Since the strain sensors were not exposed to uniaxial loading, mechanical modeling was used to determine the optimum placement of the sensors and the expected response. Also discussed are ongoing studies to evaluate the viscoelastic behavior of the epoxy and its effect on the strain state of the connector assembly.

Distributed dynamic strain measurement using a correlation-based Brillouin sensing system

Conventional fiber Brillouin-based strain sensors are capable of distributed sensing, making them advantageous over fiber Bragg grating-based sensors for structural monitoring applications. However, Brillouin sensors have low spatial resolution and are inappropriate for dynamic strain measurements as they have large measurement times of several minutes. We present a correlation-based continuous-wave technique for high spatial resolution and distributed dynamic strain measurements using stimulated Brillouin scattering. Using our technique, we have successfully measured dynamic strain from a 5-cm vibrating section, at a sampling rate of 8.8 Hz with a strain accuracy of about /spl plusmn/38 /spl mu//spl epsiv/.

Self-aligning, bipolar bend transducer based onlong period grating written ineccentric core fibre

A long period grating-based bend sensor featuring direct bend sensitivity along with linear, bipolar curvature response is demonstrated. The sensor can be aligned for optimum sensitivity using the slight intrinsic curvature of the eccentric core fibre.

Acoustic wave-based sensors using mode conversion in periodic gratings

Using periodic gratings etched into the surface of a piezoelectric plate, surface acoustic waves (SAW) can be converted into bulk waves and vice versa with high efficiency. If parallel grating structures are fabricated on opposite surfaces of a piezoelectric plate, a SAW also can be directed from one surface to the other. Using such structures, acoustic wave-based sensors can be designed that utilize SAW for the detection of chemical analytes on an electrode-free surface, i.e., the back surface. As a result, spurious sensor response and electrode aging that may occur when a chemical analyte comes in contact with the transducers are minimized. The design principles of these grating-based SAW sensors are explained, and the mass sensitivity is investigated using chemical vapor deposited thin polymer films, a type of material used in many practical chemical sensor applications. Experimental results are presented for the detection of nitrogen dioxide (NO(2 )) in sub-ppm concentrations.

Surface plasmon resonance sensors based on diffraction gratings and prism couplers: sensitivity comparison

Theoretical analysis and comparison of the sensitivity of surface plasmon resonance (SPR) sensors using diffraction at gratings and attenuated total reflection (ATR) in prism couplers for two detection methods-resonant angle interrogation and resonant wavelength interrogation is presented. Analytical expressions for sensitivity of these SPR sensors are derived and the influence of the major design parameters of the sensing structures on the sensor sensitivity is discussed. The analysis shows that grating-based SPR sensors using wavelength interrogation are much less sensitive then their prism coupler-based counterparts. In the angular interrogation mode, the sensitivity of SPR sensors using diffraction gratings depends on the diffraction order and does not differ much from that of SPR sensors based on prism couplers.

Novel hybrid interferometer stabilization scheme used in wavelength shift measurement for Bragg grating sensors

A novel hybrid interferometer stabilization scheme used for wavelength shift measurement, such as for in-fiber Bragg grating-based sensors, is described. This scheme combines the advantages of the long term optical imbalance stability offered by the locking system used and a comparative insensitivity to vibration-induced noise at frequencies up to 200 Hz, achieved with the use of a so-called “detection-and-compensation” scheme. By including an additional reference beam and the hybrid interferometer stabilization scheme, the vibration-induced fluctuation in the signal can be reduced by about 20 dB, and the value of the phase variation can be reduced by a factor of 20.

Photoinduced grating-based optical fiber sensors for structural analysis and control

Optical fiber sensor-based instrumentation has been shown to be extremely attractive for the in situ health monitoring, diagnostics and control of civil infrastructure. This paper presents the design, fabrication and implementation of long-period grating (LPG) based optical fiber sensors for the nondestructive evaluation of smart structure systems. The LPG sensors, fabricated in standard SMF-28 Corning fiber, were attached to pieces of rebar and cyclically loaded from no-load through 35 kN loads; sensor performance was monitored using an optical spectrum analyzer. A mechanical extensometer was collocated with the LPG sensor for comparison. Short-period optical fiber Bragg grating (FBG) sensors were also tested under similar conditions. Sensor performance for both the LPG and the FBG was found to be consistent with strain measurements using the extensometer.