Center Wavelength Shift Research Articles

Temperature-insensitive fiber Bragg grating strain sensor

A fiber Bragg grating strain sensor, whose reflection bandwidth is insensitive to temperature, is presented. The cross-sectional area is designed to change linearly. Under axial stress, there is a linear relationship between stress and average strain. Experimental results show that when temperature increases, reflection center wavelength shifts to longer wavelength, and there is a good linear relationship between center wavelength and temperature. When stress increases, reflection center wavelength shifts to longer wavelength, and reflection bandwidth increases. There are good linear relationships between reflection center wavelength and stress as well as reflection bandwidth and stress.

Origins of Spectral Changes in Fiber Bragg Gratings Due to Macrobending

The effect of macrobending on fiber Bragg gratings (FBGs) in standard and specialist use single-mode fibers has been investigated for bend diameters down to 6 mm. The bend dependence of various spectral properties of FBGs, including center wavelength, were found to depend upon fiber type. A maximum centre wavelength shift of 0.39 nm was measured for a FBG written in low bend loss fiber bent to a 6 mm diameter. To investigate the cause of the observed wavelength shift, and the dependence on fiber design, a number of theoretical models have been applied to the problem. The results of modeling suggest that bend-induced changes in mode structure may explain the observed differences between fiber types, while wavefront distortion might be the cause of the general shift to shorter FBG center wavelengths when the bend diameter decreases.

Greatly etched fiber Bragg grating hydrogen sensor with Pd/Ni composite film as sensing material

Pd/Ni composite film was deposited on the side-face of chemically etched fiber Bragg grating as sensing element by magnetron sputtering process. The composition, structure and micro morphology of Pd/Ni composite film were investigated by energy dispersive X-ray analysis (EDAX), X-ray diffraction (XRD) and scanning electron microscopy (SEM). The atomic ratio of Pd:Ni in Pd/Ni composite film is controlled at about 91:9. XRD result proves that the Pd/Ni composite film has good structural stability during hydrogen absorption and desorption process. During the hydrogen response experiment, central wavelength shift of the etched FBG increases linearly with the increase of hydrogen concentration. When the hydrogen concentration is 4% in volume ratio, wavelength shift of the etched FBG is about 60 pm, and the hydrogen sensor is not sensitive to methane. The experimental results show the sensor's hydrogen response is reversible, and the hydrogen sensor may have great potential for hydrogen's distributed measurement.

Wide Range Temperature Sensors Based on One-Dimensional Photonic Crystal with a Single Defect

Transmission characteristics of one-dimensional photonic crystal structure with a defect have been studied. Transfer matrix method has been employed to find the transmission spectra of the proposed structure. We consider a Si/air multilayer system and refractive index of Si layer has been taken as temperature dependent. As the refractive index of Si layer is a function of temperature of medium, so the central wavelength of the defect mode is a function of temperature. Variation in temperature causes the shifting of defect modes. It is found that the average change or shift in central wavelength of defect modes is 0.064 nm/K. This property can be exploited in the design of a temperature sensor.

Diffractive properties of imaginary-part photonic crystal slab

The diffraction spectra of imaginary-part photonic crystal (IPPC) slabs are analyzed using the scattering-matrix method. By investigating the thickness dependence of the diffraction, we find a remarkable red shift of central wavelength of the diffraction spectrum, which obviously distinguishes from the phenomenon of spectral hole. We observe that diffraction efficiency can be enhanced more than 20-fold by optimizing the geometry parameters. These imply that the diffraction spectra of the IPPC slab can be controlled at will and used to guide the design to achieve useful nanoscale devices.

Analysis of Wavelength and Intensity Interrogation Methods in Cascaded Double-Ring Sensors

Optical waveguide sensors based on cascaded double-ring resonators are analyzed theoretically and experimentally, for both wavelength interrogation and intensity interrogation methods. It is shown that for the wavelength interrogation, the sensitivity can be increased by over three orders of magnitude by combining the intensity ratio detection of two central peaks, compared with the simple digital operation mode of the central peak wavelength shift detection of the transmission spectrum. While the detection limit of the wavelength interrogation method is several times smaller than that of intensity interrogation method, the latter has advantages of simplicity and low cost which can outweigh the sensitivity reduction for many practical applications. Experimental results showed good agreements with the theoretical analysis.

Bragg reflector formed on oxidised porous silicon

The manufacturing of a porous silicon Bragg reflector on P+ silicon substrate is investigated and the effects of oxidation on the Bragg reflector parameters is studied, notably the shift of the central wavelength after oxidation. This Letter shows how to anticipate the changes due to oxidation rather than to endure them. First, the variations in the refractive index and thickness after oxidation were studied. Then, from these measurements, an oxidised porous silicon Bragg reflector centred at 1550 nm was formed by anticipating the shift of central wavelength during oxidation. There was a good correlation between the experimental and theoretical spectra of the Bragg reflector.

Artifact reduction by wavelength calibration for hyperspectral imagers

Hyperspectral imaging data collected by airborne and satellite platforms have been used in a variety of fields, such as geology, agriculture and forestry. In order to make such quantitative and precise applications, accurate spectral calibrations of hyperspectral imaging data are necessary to be achieved. The center wavelength for each band obtained in a laboratory needs to be adjusted, since it changes easily by vibration and variation of observation condition. The shift of center wavelength triggers an artifact in atmospheric correction necessary for quantitative estimation of ground surface target. The quantitative estimation of the artifact appeared around absorption bands is important for accurate spectral calibration. This paper describes a new estimation method for the artifact, and shows that the artifact reduction is achieved by calibration of center wavelength with our proposed methods. Furthermore, we evaluate the artifact influence on practical remote sensing application, such as tree classification. Our estimation method accurately evaluates the artifact and makes wavelength corrected hyperspectral image without artifact. We clarify that the spectral change of artifact having a tendency in cross-track that is more influential in classification than spectral change of tree species. This method has been effectively applied to a variety of land use applications using data acquired with airborne hyperspectral imagers.

Measurement of Surface Plasmon Polariton Enhanced Goos–Hanchen Shift Based on Grating and Liquid Crystal Technologies

We measured the surface plasmon polariton (SPP) enhanced Goos-Hanchen (GH) shift based on liquid crystal (LC) and grating technologies. An optical setup is used to convert the spatial displacement to incidence angle variation to a Littrow mounted diffraction grating. As a consequence, the GH shift information could be obtained from the back-reflected center wavelength that fulfills the Littrow condition. An LC cell is used to adjust the polarization state of the incident light without mechanical movement. About 10- μm GH shift difference between transverse-electric (TE) and transverse-magnetic (TM) mode lights were measured associated with the SPP excitation. The corresponding center wavelength shift of the returned beam is 404 pm. The relationship between energy conversion and GH shift is also investigated.

Direct observation of two-color pulse dynamics in passively synchronized Er and Yb mode-locked fiber lasers

We report direct experimental observation of interesting pulse synchronization dynamics in a cavity-combined Er and Yb mode-locked fiber lasers by measuring the relative position between the two-color pulses in the shared fiber section. The influence of the 1.03 μm pulse on the 1.56 μm single pulse as well as bound soliton pairs can be clearly identified as an effective phase modulation through the XPM effect with the walk-off effect taken into account. For the 1.56 μm single pulse under synchronization, the dependence of the relative position variation and the center wavelength shift on the cavity mismatch detuning is found analogous to the typical characteristics of FM mode-locked lasers with modulation frequency detuning. Moreover, depending on the cavity mismatch, the passively synchronized 1.56 μm bound soliton pairs are found to exhibit two different dynamical behaviors, i.e., phase-locked (in-phase) as well as non-phase-locked. The physical origins for these two kinds of bound soliton pairs are investigated experimentally by disclosing their locations with respective to the copropagating 1.03 μm pulse.

Research on the fiber Bragg grating sensor for the shock stress measurement

A fiber Bragg grating (FBG) sensor with an unbalanced Mach-Zehnder fiber interferometer for the shock stress measurement is proposed and demonstrated. An analysis relationship between the shock stress and the central reflection wavelength shift of the FBG is firstly derived. In this sensor, the optical path difference of the unbalanced Mach-Zehnder fiber interferometer is ~3.1 mm and the length of the FBG is 2 mm. An arctangent function reduction method, which can avoid sine function's insensitive zone where the shock stress measurement has a reduced accuracy, is presented. A shock stress measurement of water driven by one stage gun (up to 1.4 GPa), with good theoretical accuracy (~10%), is launched.

Experimental Study on the FBG Sensing Character under Three-Dimensional Uniform Stress Field

FBG sensing character and model under three-dimensional stress are studyed in this paper. FBG center wavelength shifts of the two polarization directions are different and the shifts are determined by the changes of refractive index and the axial stain. The experiment indicates that the resonance peak wavelength shift amounts of the two directions are proportional to the transverse load.

All-optical wavelength tuning in Ti:PPLN Šolc filter via the photorefractive effect

We demonstrate fine-wavelength tuning of a periodically poled Ti:LiNbO 3 (Ti:PPLN) Šolc filter by use of the photorefractive (PR) effect. The center wavelength of a Ti:PPLN Šolc filter is controlled by the PR effect, which is induced by second-harmonic generation (SHG). The refractive index change caused by the PR effect is calculated from the measured shift in center wavelength from the Ti:PPLN Šolc filter. The experimental results show that all-optical fine-wavelength tuning in a Ti:PPLN Šolc filter is possible by the use of a second-order nonlinear effect and that a Ti:PPLN waveguide has much higher resistance to PR damage than a Ti:LiNbO 3 waveguide. The measured wavelength tuning rate as a function of the pump beam power is about − 0.033 nm/mW.

Simultaneous Force and Temperature Sensor Using Small-Diameter FBGs

In order to decrease the influence on mechanical performance of smart composite, small diameter optical fibers are usually preferred as strain sensors. Therefore, fiber Bragg gratings in small diameter were manufactured. In applications, the FBG measures both strain (or force, pressure, displacement) and thermal expansion. In order to overcome this inconvenience and to measure both parameters at the same time and location, a simple temperature insensitive force sensor was proposed. A pair of small-diameter FBG was attached on a uniform strength beam. One of the FBG was affixed on the upper surface of the cantilever, and the other one on the bottom. The central wavelength shifts of the two FBGs are proportional to load value F and –F respectively. Therefore, coefficient matrix is well conditioned. The force and temperature in sensing point could be discriminated. Experiments were carried out to study the force and temperature cross sensitivity. Experimental results match the theoretical ones well. The temperature and force were calculated and the errors were 2% and 7% respectively.

Efficient distributed moisture-ingress sensing using diamond-like carbon-nanocoated long-period gratings

We have demonstrated the possibility of efficient distributed water ingress sensing by making use of LPGs nanocoated with diamond-like carbon (DLC). Two long-period gratings (LPGs) with different coating thicknesses were tested. A portion of each LPG was soaked and the responses were measured by tracking both the center-wavelength shift and the LPG resonance minimum depth and by simulating a two-detector sensing arrangement in combination with a diffraction grating. The responses exhibit large linear sections and thus allow for a simple distributed sensing over the grating length.

UV광 측정용 아조벤젠 코팅된 FBG의 열적 효과 제거 및 파장 의존성에 대한 연구

In the paper, we have demonstrated an azobenzene-coated fiber Bragg grating (FBG) for monitoring ultraviolet light (UV) intensity in remote measurement. The elasticity of the coated azobenzene polymer is changed by the UV light, which induces a center wavelength change corresponding to the change of the FBG’s grating period. The wavelength shift resulting from both UV light and other light with the wavelength out of the UV range was about 0.18 nm. In order to improve the accuracy of the measurement, the center wavelength shift caused by radiant heat of the light source was sufficiently removed by using a thermal filter. The amount of the center wavelength shift was consequently reduced to 0.06 nm, compared to the result without the thermal filter. Also, the FBGs coated by using azobenzene polymer were produced by two different methods; thermal casting and UV curing. Considering temperature dependence, UV curing is more suitable than thermal casting in UV sensor application of the azobenzene-coated FBG. In addition, we have confirmed the wavelength dependence of the optical sensor by means of four different band pass filters. Thus, we found out that the center wavelength shift per unit intensity is 0.029 [arb. unit] as a maximum value at 370 nm wavelength region and that the absorption spectrum of the azobenzene polymer was very consistent with the wavelength dependence of the azobenzene-coated FBG.

Enhanced Contrast of Wavelength-Selective Mid-Infrared Detectors Stable against Incident Angle and Temperature Changes

A thermal infrared detector with enhanced contrast of its wavelength-selective function has been fabricated. A 2 ×2-mm2-size detector with p++-Si–Pt thermopiles on a Si substrate is the base device. A polymer material is deposited on the hot junctions of the thermopiles, working as a wavelength-selective absorber to integrate spectroscopic function into the thermopile detector. The polymer used is polyacrylonitrile (PAN), with a sharp isolated absorption peak at a wavelength of 4455 nm; the detector shows +35% output increase from the baseline at this wavelength. Very little dependence of the center wavelength (CWL) on the incident angle has been confirmed. Temperature dependencies were measured over the range from 20 to 80 °C. The detector shows a smaller CWL shift, which is ∼1/20 of that of the conventional detector using an optical bandpass filter with a dielectric multilayer on a Si substrate. These features are suitable for miniaturizing the sensor module including the optical setups.

Simultaneous sensing of displacement and temperature with a single FBG

A novel fiber Bragg grating (FBG) sensor with simultaneous sensing of displacement and temperature is presented. The FBG is affixed on the cantilever inclinedly. The midpoint of FBG exactly coincides with the zero strain layer of a rectangular beam. The vertical displacement can be measured by the broadened bandwidth of FBG as the bandwidth is insensitive to temperature, while the temperature can be measured by the center wavelength shift as the wavelength shift is insensitive to vertical displacement. With 0.1 nm spectral resolution of the analyzer, sensitivities of bandwidth-displacement and center wavelength-temperature are 0.48 nm/mm and 0.05 nm/°C, resolutions are 0.2 mm and 2.0 °C, and sensing ranges of displacement and temperature are up to 8.5 mm and 45°C respectively. Experimental results match theoretical analyses very well.

Highly sensitive liquid-level sensor based on weak uniform fiber Bragg grating with narrow-bandwidth

A highly sensitive liquid-level sensor based on weak uniform fiber Bragg grating (FBG) with narrow-bandwidth is proposed and demonstrated. The resonance wavelength of FBG varies as a result of the change of axial strain. According to this characteristic, a precise approach utilizing fiber grating as sensing element to measure height of liquid is achieved. The grating is directly bonded to a float which can transfer buoyancy into axial strain on the fiber. Measurement of liquid level can be realized by monitoring the central wavelength shift of the sensing FBG. The experimental results show that for a liquid-level variation of 25 mm, a high sensitivity of 5.9×10−6 m/pm with a good linearity of 0.991 is achieved. Within bidirectional change of liquid level, crossing sensitivity and long term stability taken into account, the system is of good reliability and stability.

Thermal Wavelength Stabilization of Fiber Bragg Gratings Using Bi-Metal Structure

A stabilized laser is essential for optical fiber communication network. One of the passive technique for stabilization of central wavelength of laser is based on the application of fiber Bragg gratings. Due to the positive coefficient of thermal expansion of optical fiber, the Bragg gratings within the fiber written by excimer laser gives about 0.01nm/oC shift on the central wavelength respect to the ambient temperature which leads serious problem in the communication network. Since both the temperature and tension force are linearly proportional to the central wavelength of fiber Bragg gratings. A feasible approach to derive the wavelength stabilization is to decrease the tension force of fiber Bragg gratings respect to the increase of ambient temperature. In this paper, a bi-metal structure with similarly negative coefficient of thermal expansion is used to decrease the tension force while the environmental temperature increases. Results show that the theory provides a fundamental solution of the physical data of the temperature compensated fixture for near zero shift of central wavelength. The practical compensation of the bimetal structure is non-linear due to the thermal expansion of the arm of the fixture, while the compensation is linear respect to the ambient temperature by neglecting the thermal expansion of the arm. However, this package is feasible for mass production and can be used for athermalization of the fiber Bragg gratings in optical communication system.