Strain-induced Wavelength Shift Research Articles

Augmentation of Sensitivity of FBG Sensor Using Microwave Photonics and Nonlinear Effect

A simple and unique scheme of high sensitive Fiber Bragg Grating(FBG) interrogation system based on microwave signal measurement utilizing the concept of microwave photonics and nonlinear Four wave mixing(FWM), is proposed. The concept of FWM process is combined with microwave photonics to enhance the sensitivity of FBG sensor. The strain induced wavelength shift of the sensing FBG is translated to higher wavelength shift of the corresponding higher order FWM signals on either side of FBG sensor wavelength in opposite directions. Thus the wavelength separation between the FWM signals increases with small increase in strain. Utilizing the FWM effect and microwave photonics technique, the microwave phase difference is altered, which ultimately is converted to significant change in RF intensity. A novel mathematical relationship relating FWM, applied strain and the microwave power of the proposed scheme is established. A high strain sensitivity of 36.52 μV/με (26.67 μW/με) is attained using the proposed FWM and microwave photonics technique against initial sensitivity of 4.9 μV/με (0.48 μW/με) using only microwave photonics technique. The proposed scheme automatically compensates the temperature variations and eliminates the requirement of high dispersive component to enhance the sensitivity.

Strain-Insensitive Simultaneous Measurement of Bending and Temperature Using Long-Period Fiber Grating Inscribed on Double-Clad Fiber with CO₂ Laser.

Here we report an optical fiber sensor capable of performing strain-insensitive simultaneous measurement of bending and temperature using a long-period fiber grating (LPFG) inscribed on doubleclad fiber (DCF) with a CO₂ laser at ˜10.6 μm. The LPFG inscribed on DCF, referred to as a DC-LPFG, was fabricated by scanning CO₂ laser pulses on an unjacketed DCF with a specific period. Due to co-directional mode coupling, the fabricated DC-LPFG has discrete attenuation bands widely distributed over hundreds of nanometers. Among these wavelength-dependent loss dips, adjacent two dips with different resonance wavelengths were selected as sensor indicators for the measurement of bending and temperature. For these two indicator dips designated as dips A and B, their bending and temperature responses were investigated in a curvature range of 4.90 to 21.91 m-1 and a temperature range of 30 to 110 °C. With increasing bending applied to the DC-LPFG at room temperature, dips A and B showed different blue shifts. The bending sensitivities of dips A and B were measured to be approximately -0.77 and 0.51 nm/m-1, respectively. Unlike the bending response, they showed red shifts of different amounts with increasing ambient temperature, while the sensor head (i.e., the DC-LPFG) remained straight without any applied bending. The temperature sensitivities of dips A and B were measured to be ˜0.094 and ˜0.078 nm/°C, respectively. Owing to their linear and independent responses to bending and temperature, bending and temperature changes applied to the DC-LPFG could be simultaneously estimated from the measured wavelength shifts of the two indicator dips using their pre-determined bending and temperature sensitivities. Moreover, in a strain range of 0 to 2200 με (step: 200 με), strain-induced spectral variations of dips A and B were also measured, and the strain sensitivities of dips A and B were evaluated as approximately -0.028 and -0.013 pm/με, respectively. These strain-induced wavelength shifts were so small that they had little effect on the measurement results of bending and temperature. Thus, it is concluded that the fabricated DC-LPFG can be employed as a cost-effective sensor head for strain-insensitive separate measurement of bending and temperature.

Reliability of stable fiber Bragg grating sensor system for monitoring temperature and strain individually

In the investigation, the fiber Bragg grating-based sensor system is proposed and demonstrated to monitor temperature and strain individually. In order to obtain a stable and accurate sensing signal, an erbium-doped fiber (EDF) ring laser scheme with a 2 m long unpumped EDF as the saturable absorber is also proposed. Hence, the observed stabilization of output power and sensing wavelength can be less than 0.1 dB and 0.02 nm, and 0.3 dB and 0.02 nm in the sensor system, respectively, in the back-to-back state and 25 km fiber link. Hence, this sensor system can improve the output stability of the sensing signal. In addition, the temperature- and strain-induced wavelength shifts can be operated in the range of 20 °C–70 °C and 0–37 500 µε, respectively.

OFDR with local spectrum matching method for optical fiber shape sensing

Strain measurement is the basis of shape reconstruction by using the optical frequency domain reflectometer, and we propose a local spectrum matching method to determine the strain induced wavelength shift by matching the most similarity portion in the measurement spectrum. Comparison study between conventional cross-correlation and local spectrum matching indicates that the proposed method can effectively eliminate the fake peaks and multi-peaks of the cross-correlation. Shape sensing experiments after the calibration process of the curvature illustrate that the two-dimensional shape reconstruction error can be as low as 1 cm in a sensing fiber length of 1 m.

Single-Photon Emitters in Boron Nitride Nanococoons.

Quantum emitters in two-dimensional hexagonal boron nitride (hBN) are attractive for a variety of quantum and photonic technologies because they combine ultra-bright, room-temperature single-photon emission with an atomically thin crystal. However, the emitter's prominence is hindered by large, strain-induced wavelength shifts. We report the discovery of a visible-wavelength, single-photon emitter (SPE) in a zero-dimensional boron nitride allotrope (the boron nitride nanococoon, BNNC) that retains the excellent optical characteristics of few-layer hBN while possessing an emission line variation that is lower by a factor of 5 than the hBN emitter. We determined the emission source to be the nanometer-size BNNC through the cross-correlation of optical confocal microscopy with high-resolution scanning and transmission electron microscopy. Altogether, this discovery enlivens color centers in BN materials and, because of the BN nanococoon's size, opens new and exciting opportunities in nanophotonics, quantum information, biological imaging, and nanoscale sensing.

Strain-induced red-green-blue wavelength tuning in InGaN quantum wells

Monolithically integrating multi-color pixels from a standard InGaN quantum well active region was demonstrated with a wavelength tuning range of 178 nm. Nanopillar structures were fabricated to enable the wavelength tuning. Strain induced wavelength shift was investigated both experimentally and theoretically. A simple one-dimensional strain relaxation model was shown to accurately predict the wavelength shift as a function of the nanopillar diameter. The strain relaxation was found to depend on the indium composition in the quantum well. No noticeable increase of the defect density was observed after the strain relaxation process.

All Optical Fiber Keyboard Based on 2D Location Sensor With Cascade Gratings

We propose and experimentally demonstrate a compact all-fiber 2D location sensor suitable for 12-key keyboard application. The proposed sensor consists of two groups of fiber bragg gratings (FBGs) one of which is spatially arranged in 4×3 array. Strain-induced Bragg wavelength shifts finely adjust the stopband of incident light by pressing the crossing points of the grating array, leading to corresponding reflections from the reference group due to the one-to-one correspondence between two groups. This sensor has the features of being flexible, low insertion loss, simple structure, and also combines with the advantages of the FBG-based sensors with high sensitivity and accuracy.

A Long-Distance Remote Sensing Technique Using a Multiwavelength Raman Fiber Laser Based on Fiber Bragg Gratings Embedded in a Quartz Tube

A long-distance remote sensing technique using a multiwavelength Raman fiber laser with fiber Bragg gratings (FBGs) embedded in a quartz tube is proposed. A Raman linear cavity for multiwavelength generation was configured by using a long length of single mode fiber and FBGs. To discriminate concurrent sensitivities, such as temperature and strain sensitivities, FBG2 was etched to reduce the cladding diameter. Then, two FBGs with different cladding diameters put in a quartz capillary tube and FBG1 with a large diameter (125 μm) was firmly attached in a quartz capillary tube with UV curable epoxy. Since the two FBGs have identical material composition, the multiwavelength Raman fiber laser outputs have the same temperature sensitivities. However, since the FBG1 with a diameter of 125 μm was substantially fixed in a quartz capillary tube, the strain-induced wavelength shift was effectively suppressed. Since the FBG2 with a small cladding diameter of 60 μm only responds to the external strain change, the multiwavelength Raman fiber laser outputs have different strain sensitivities. Therefore, it is possible to simultaneously measure temperature and strain over a long distance of more than 75 km by using the proposed multiwavelength Raman fiber laser without additional input sources.

Simultaneous measurement of strain and temperature using single tilted fibre Bragg grating

Tilted fibre Bragg grating (TFBG) with grating planes tilted at an angle of 3° corresponding to the fibre axis shows core mode and a large number of cladding-mode resonances in its transmission. Discriminating temperature and strain simultaneously using a single tilted fibre Bragg grating is proposed and demonstrated experimentally. The wavelength shifts of the TFBG in response to temperature and strain are investigated. The results show that the cladding modes and core mode have different strain sensitivities while the temperature sensitivities are equal. So, it is possible to monitor the core-mode resonance and the cladding-mode resonances of the TFBG spectrum, allowing the separation of the temperature and strain induced wavelength shifts.

Full-scale test of a concrete box girder using FBG sensing system

For the structural monitoring of railway bridges, electromagnetic interference (EMI) is a significant problem as modern railway lines are powered by high-voltage electric power feeding systems. Fibre optic sensing systems are free from EMI and have been successfully applied in civil engineering fields. This study presents the application of fibre Bragg grating (FBG)-based sensing systems to precast concrete box railway bridges. A 20 m long full-scale precast concrete box railway girder was fabricated and tested in order to identify its dynamic and static performance. The experimental programme involved two phases. First, the dynamic characteristics of the test were identified using a digitally controlled exciter. An FBG-based accelerometer was installed on the surface of the structure. The second phase involved the measurement of the nonlinear static behaviour until failure. Multiplexed FBG strain sensors were embedded along the length of steel rebar and a strain-induced wavelength shift was measured in order to monitor internal strains. The measured values from the FBG-based sensors are compared with the results using electric signal-based sensors. The results show that the FBG sensing system is promising and can improve the efficiency of structural monitoring for modern railway bridges.

Temperature and strain discrimination using a single tilted fibre Bragg grating

A fibre optic sensor capable of discriminating between temperature and strain, using a single fibre Bragg grating, is presented. The technique exploits the core–cladding mode coupling of a tilted fibre Bragg grating (TFBG). The core and cladding modes exhibit different thermal sensivities, while the strain sensivities are approximately equal. Monitoring the core–core mode coupling resonance and the core–cladding mode coupling resonance of the TFBG spectrum allows the separation of the temperature and strain induced wavelength shifts.

A novel fiber strain sensor based on laser’s transient regime

A novel fiber strain sensor based on the fiber laser's transient regime is reported. A Sagnac fiber loop and a fiber Bragg grating(FBG) that also acts as the sensing element form the linear Fabry-Perot cavity, in which a long-period grating(LPG) is inserted. The original peak wavelength of the FBG, whose bandwidth is 0.23nm, is 1557.98nm and located on the left side of the LPG dip (1557.4nm). The light reflected from the Sagnac loop and the FBG is transmitted through the LPG twice in each round trip, and the transmission loss increases with the strain-induced wavelength shift of the FBG, which in turn modifies the laser build-up time. The strain value can be obtained by measuring the build-up time of the laser. In the present experiment, the sensor's sensitivity and resolution can be adjusted by simply controlling the pumping level. When the pump pulse's high level and low level are 32 and 6mW, respectively, a sensitivity of 7×10-6ε/μs and resolution of 7×10-7ε are achieved.

A novel demodulation scheme for fiber Bragg grating sensor system

A novel fiber Bragg grating (FBG) wavelength demodulation scheme based on a compound interference configuration is introduced. This scheme measures the strain-induced wavelength-shift of the FBG in terms of phase-shift signal. The sensing resolution and sensitivity were demonstrated to be 6.2 n/spl epsiv/ and 1.59/spl deg///spl mu//spl epsiv/, respectively, which match the theoretical value of 1.61/spl deg///spl mu//spl epsiv/ very well.

Temperature-controlled fiber Bragg grating dynamic strain detection system

A fiber Bragg grating (FBG) measurement system for dynamic strain detection has been developed. It exploits a temperature-controlled FBG as a small-band filter to measure the strain-induced wavelength shift of structurally integrated FBG sensors. This interrogation method realizes an optimal dynamic performance of the sensor also under the influence of environmental temperature changes. This has been confirmed relative to a conventional reference sensor under simulated in situ conditions in a laboratory test. The FBG system can be applied for nondestructive vibration testing of geotechnical structures, e.g. bridges and borepiles.

Tangential Load Measurements by a Smart Textile Composite

A cellular textile composite structure integrated with fiber Bragg grating (FBG) sensors is designed for tangential load distribution measurements. The composite consists of many cylindrical shell cells for which a unit cell is fabricated from polyester and glass woven fabrics and epoxy. FBG sensors are bonded on the surface of the cells. The feasibility of using this cellular composite to measure tangential load distribution is demonstrated by experiments. The strain induced wavelength shift in the FBG sensor has a linear relationship with the tangential force within a definite range. Numerical simulations are created by finite element analysis. The influences of the environmental temperature, thickness, and effective moduli of the composite and the influences of the position and direction of the FBG sensors are discussed. The sensitivities of this device are 0.09 nm/N (for PET tension), 0.05 nm/N (for PET compression), 0.04 nm/N (for glass tension), and 0.03 nm/N (for glass compression), and the measurement ranges are 0-±3.43N (PET) and 0-±4.41N (glass). In this measurement range, the system can be used to monitor the tangential load distribution for a large area.

Fiber Bragg grating interrogation and multiplexing with a 3/spl times/3 coupler and a scanning filter

We present a new technique for fiber Bragg grating (FBG) sensor interrogation and multiplexing. The technique combines a scanning bandpass filter used to multiplex by wavelength multiple gratings in a single fiber, and an unbalanced Mach-Zehnder fiber interferometer made with a 3/spl times/3 coupler to detect strain-induced wavelength shifts. A demonstration system interrogates four gratings in a single fiber at a sampling rate up to 20 kHz, with a noise floor measured at less than 10 n/spl epsiv///spl radic/(Hz) above 0.1 Hz.

Matrix dependence of strain-induced wavelength shift in self-assembled InAs quantum-dot heterostructures

We report on the matrix-dependent strain effect in self-assembled InAs quantum-dot heterostructures using photoluminescence measurements. A series of samples were prepared to examine the effect of quantum dot position with respect to the so-called strain-reducing layer (SRL). Since the SRL reduces the residual hydrostatic strain in the quantum dots, long emission wavelength of 1.34 μm is observed for the InAs quantum dots with an In0.16Ga0.84As SRL. The dependence of the emission wavelength on the thickness of the cap layer on SRL also indicates the importance of the role of matrix in the strain relaxation process of the dots. Using In0.16Al0.84As instead of In0.16Ga0.84As as the SRL, a blueshift in wavelength is observed because the elastic stiffness of In0.16Al0.84As is higher than that of In0.16Ga0.84As and less strain is removed from the dots with In0.16Al0.84As SRL.

A Wideband Interferometric Wavelength Shift Demodulator of Fiber Bragg Grating Strain Sensor

The performance of a fiber Bragg grating strain sensor constructed with 3 $\times$ 3 coupler is investigated. A 3 $\times$ 3 coupler Mach-Zehnder (M/Z) interferometer is used as wavelength discriminator, interrogating strain-induced Bragg wavelength shifts. Two quadrature-phase-shifted intensities are synthesized from the as-coupled interferometer outputs, and digital arctangent demodulation and phase unwrapping algorithm are applied to extract the phase information proportional to strain. Due to the linear relation between the input strain and the output of quadrature signal processing, signal-fading problems eliminated. In the experiment, a fiber grating that was surface adhered on an aluminum beam was strained in different ways, and the photodetector signals were transferred and processed in a computer-controlled processing unit. A phase recovery fo 7.8 $\pi$ pk-pk excursion, which corresponds to ~650 $\mu$ strain pk-pk of applied strain, was demonstrated. The sensor system was stable over the environmental intensi쇼 perturbations because of the self-referencing effect in the demodulation process.

Passive, light intensity-independent interferometric method for fibre Bragg grating interrogation

The authors describe a passive technique for sensing the strain-induced wavelength shift of a fibre Bragg grating (FBG) using a Mach-Zehnder interferometer that utilises a 3/spl times/3 coupler. Laboratory data show that a resolution of /spl sim/10 n/spl epsiv///spl radic/(Hz) RMS is achievable with inherent insensitivity to source intensity variations.

Fibre laser sensor with ultrahigh strain resolution using interferometric interrogation

A fibre Bragg grating based laser sensor system with an RMS strain resolution of 5.6 × 10-14/√Hz at 7 kHz is reported. An interferometric readout technique is used to determine strain induced wavelength shifts in the laser with high resolution.