Performance analysis of Strain sensor based on Fiber Bragg Grating

Wavelength independent chemical sensing using etched thermally regenerated FBG

Fiber Bragg gratings written using a phase-mask exhibit a complex refractive index pattern in the fiber core, as observed in differential interference contrast (DIC) microscopy. This results in dual periodicities along the fiber core and gratings at both the Bragg wavelength, twice the Bragg wavelength, and the harmonics of these. Features in grating spectra arise from one or both of these periodicities and spectra for twice the Bragg wavelength (or its odd harmonics) are very sensitive to the details of how the UV complex diffraction field of the phase mask becomes imprinted as refractive index changes in the fiber core.

To remove or reduce the cladding layer thickness of fiber Bragg grating, the refractive index changes in the surrounding environment will affect the fiber Bragg grating effective refractive index of the core, resulting in changes in the Bragg wavelength. Solution concentration changes will cause changes in refractive index of the solution. A new kind of solution concentration sensor is designed with the etched phase shifted fiber Bragg grating, and concentration sensing detection with four kinds of solutions is experimentally demonstrated. With resolution concentration increasing, the phase shifted label wavelength shifts to the red, and the sensor is more sensitive in the case of higher concentration.

Polymer optical fibers (POF) have received increased attention in recent years in the fields of data communication and sensing applications. The lower cost and higher flexibility are the main advantages of POF compared to silica fibers and make them interesting candidates for Fiber Bragg grating (FBG) sensor applications. FBG are convenient measurement devices for strain and temperature measurements, as they can be multiplexed within one fiber yielding a sensor array and the fiber can be embedded in structures. This work investigated the possibility of producing FBG in polymer fibers and their use as sensor units. It could be shown that using excimer laser irradiation at 308 nm, it is possible to write FBG in single-mode POF employing a standard phase mask, side writing technique. Index changes of up to 1.7 × 10-4 and reflectivies of up to 87% could be reached. The induced refractive index change due to pulsed UV irradiation was shown to be negative. The polymer material of the core (Polystyrene (PS) / Polymethyl methacrylate (PMMA)) was not sensitized prior to irradiation. During the grating formation an irradiation induced insertion loss of up to 11 dB/cm was observed. Excimer laser written FBG showed stability of over 9 months for approximately 40% of written FBG. Results of FBG writing using femto second laser irradiation showed FBG with reflectivities of up to 1.2 × 10-4, however these POFBG were not stable. The POFBG were characterized using optical low coherence reflectometry (OLCR) which enables to calculate the FBG location and length as well as the induced amplitude Δnac and mean refractive index change Δndc. Taking into account fiber and insertion losses, good agreement of these calculations and measurements were found. The results show that large variations of the induced index change result from irradiation. Local index change peaks of up to 4 times the mean value were observed, indicating inhomogeneity of the fiber material. Birefringence in the core of the POF (≈ 1.2 × 10-3) is up to a factor 3 higher than in the cladding which is due to the PS content within the core. The birefringence is due to inelastic strain and stress induced in the drawing process. Annealing, uniform irradiation and FBG writing using 308 nm excimer laser light induces a decrease of the absolute birefringence value. The changes upon irradiation are confined to the core of the fiber. Large variations in the initial and final birefringence before and after irradiation support the findings of the OLCR measurements indicating material inhomogeneities in the fiber. POFBG were found to be sensitive to relative humidity, temperature and strain. This is in contrast to glass fiber FBG which do not show humidity sensitivity. POFBG relative humidity sensitivity is non-linear with a change of up to 8 nm for a RH change of ≈ 100%. The non-linearity is introduced by a non-linear water sorption process. The POF grating response to temperature changes under dry conditions (1.5±1 % RH) is –10±0.5 pm/°C. The temperature response of the FBG submerged in water is –36±2 pm/°C due to an increased thermal expansion coefficient and a change in polarizability. Under ambient conditions the grating response to heating is typically ≈ –138 pm/°C, predominantly due to a change in POF swelling, i.e changes in relative humidity and POF water content. The diffusion coefficient of water in this POF at 23.5°C is 6.7 × 10-9cm2/s for sorption and 10 × 10-9cm2/s for desorption. Equilibrium of water content within the fiber and the surrounding air is typically reached after approximately one hour. Calculation showed that a reduction of the fiber diameter can increase this humidity sensitivity response time down to approximately 5 minutes for a fiber diameter of 25 µm.

Numerical analysis of apodized fiber Bragg gratings formation using phase mask with variable diffraction efficiency

A novel method of optimal apodization selection for chirped fiber Bragg gratings

The performance of fiber Bragg grating (FBG) devices is determined by spatial distribution of FBG parameters. For many applications, like chirped FBG used for dispersion compensation, it is sufficient to control these parameters with spatial resolution of around 1 mm. We improve the sensitivity and accuracy of the side-diffraction FBG characterization at the cost of reducing the spatial resolution and demonstrate simultaneous measurement of fiber grating refractive index modulation with accuracy of 2/spl middot/10/sup -6/, grating period variation with accuracy of 5 pm, and spatial resolution of /spl sim/1 mm. The group delay ripple of a chirped FBG calculated from our side-diffraction characterization is in a good agreement with spectroscopic measurements.

Results are presented on the photosensitivity behavior of a phosphate glass optical fibre using 248nm, 500fs laser radiation. Bragg grating exposures performed using a double phase mask interferometer and peak intensities of 0.37TW/cm2, revealed that grating growth becomes non-monotonic in terms of average and modulated refractive index changes, resembling the Type IIA photosensitivity behavior. Average refractive index changes greater than 10−3 were measured after accumulated energy density doses of 6.5KJ/cm2. The Bragg gratings inscribed maintained significant part of their strength up to 377°C. Exposed and side-polished fibre samples were subjected to Knoop micro-hardness measurements for revealing that the irradiated glass undergoes significant volume dilation.

This paper presents a 3D touch trigger probe based on Fiber Bragg Grating (FBG). The sensing principle is Bragg equation λ=2nΛ. Mutative strain and temperature outside alter both the refractive index (n) and grating pitches (Λ) of the fiber core, so the Bragg wavelength λ will change accordingly. The probe adopts FBG sensor system which has four FBGs provided with same parameter (three as sensor FBG and one as match FBG). Laser beam from broadband light source enter sensor FBGs through one coupler, the reflected light is imported to match FBG via another coupler, eventually captured by a high precision optoelectronic detector which monitors energy of the laser reflected by match FBG. The tip ball swings when it contact work pieces, and causes rotation of the plank by rigid connection, the displacement of the tip ball will be transferred to strain exerting on sensor FBGs. Consequently the strain results in Bragg wavelength shift of the reflected laser beam. The displacement of the probe leads to shift of Bragg wavelength of the sensor FBG, therefore, results in energy change of reflected light from the matching FBG. The probe based on FBG sensor brings an untouched branch of the application of Fiber grating sensors. It is also studied on key points of a touch trigger probe such as repeatability, trigger force and resolution.

Due to the limitations of measurement equipment and the influence of factors such as the environment and target, measurement errors may occur during the data acquisition process of airborne LiDAR bathymetry (ALB). The refractive index of water is defined as the propagation ratio of the speed of light waves in a vacuum to that in water; this ratio influences not only the propagation speed of the laser pulse in water but also the propagation direction of the laser pulse entering water. Therefore, the influence of refractive index changes in water on the ALB errors needs to be analyzed. To this end, the principle of ALB is first briefly introduced. Then, the calculation method for the refractive index of water is described with Snell’s law and an empirical formula. Finally, the influence of refractive index changes on ALB errors is analyzed using the derived formula at the water–air interface and in the water column. The experimental results showed that in a constant elevation of 50 m for a bathymetric floor, the refractive index changes in water caused by temperature, salinity, and depth are less than 0.001. The maximum bathymetric error and maximum planimetric error caused by the refractive index changes at the water–air interface are 0.036 m and 0.015 m, respectively. The ALB errors caused by refractive index changes in the water column are relatively low, and the water column does not need to be layered to calculate the ALB errors. The influence of refractive index changes in water on the ALB error is minimal, accounting for only a small proportion of all bathymetric errors. Thus, it is necessary to determine whether the effect of the ALB error due to refractive index changes in water needs to be corrected based on the accuracy requirements of the data acquisition. This study and analysis can provide a reference basis for correcting ALB errors.

Mach–Zehnder interferometer cascaded with FBG for simultaneous measurement of RI and temperature

We present optical fiber sensors to measurement strain and vibration. The sensors are based on fiber Bragg gratings (FBG). We prepared construction of strain sensors with respect to its implementation on the outer surface of concrete structures and with compensation of potential temperature drifts. These sensors are projected with look forward to maximal elongation and strength which can be applied to the sensor. Each sensor contains two optical fibers with FBGs. One FBG is glued into the sensor in points of fixation which are in the line with mounting holes. This FBG is prestressed to half of measurement range, than the stretching and pressing can be measured simultaneously by one FBG. The second FBG is placed inside the sensor without fixation to measure temperature drifts. The sensor can be used to structure health monitoring. The sensors to measurement vibration are based on tilted fiber Bragg grating (TFBG) with fiber taper. The sensor uses the TFBG as a cladding modes reflector and fiber taper as a bend-sensitive recoupling member. The lower cladding modes (ghost), reflected from TFBG, is recoupled back into the fiber core via tapered fiber section. We focused on optimization of TFBG tilt angle to reach maximum reflection of the ghost and taper parameters. In this article we present complete set-up, optical and mechanical parameters of both types of sensors.

We analyze the transmission property of nanostructures made on silver and gold metal for the applications in optical biosensors. Various structures such as slit only, slit groove slit, and multiple slit and groove structures are taken into account to find the effect of various physical parameters such as number of grooves, number of slits and others on the transmission of different wavelength light sources through the structure. A broad wavelength of 400 nm to 900 nm is used to analyze the transmission through the structure. With these structures and broad light source, change in transmission intensity is analyzed with the change in the refractive index. The change in refractive index of the analyte varies transmission intensity and wavelength shift at the output beam which can be used for sensing the amount of analyte such as monitoring glucose amount on blood/saliva, hydrogen peroxide and others. The detection of these analytes can be used to detect the different disease. The analysis of the transmittance through the nanostructure can be used for the detection of several disease such as diabetes and others through the saliva, blood and others non-invasively.

Theoretical and experimental study of refractive index sensors based on etched fiber Bragg gratings

Optical fiber Bragg grating (FBG) sensors have significant potential for use as embedded devices to monitor the structural integrity of engineering materials. The main drawback of the FBG strain sensor is its cross-sensitivity to temperature. This paper reports a simple scheme for multiplexing a FBG and an extrinsic Fabry-Perot interferometric (EFPI) sensor to enable the decoupling of strain from temperature. The EFPI sensor was constructed using a precision bore quartz capillary tube which housed two cleaved optical fibers. The gap between the fiber end- faces served as the Fabry-Perot cavity. Since the coefficients of thermal expansion between the optical fiber and the capillary tube were similar, the EFPI sensor has a very low sensitivity towards temperature. Therefore, when both sensors are placed close together, the EFPI sensor can act as the strain sensor, and temperature can be determined from the FBG wavelength shift after taking out the strain effect. The signal processing for the EFPI sensor was based on a channelled spectrum method using a CCD spectrometer. The same CCD spectrometer was also used to determine the wavelength shift of the FBG. The cross-talk between the EFPI and FBG sensors was evaluated. The feasibility of conducting simultaneous strain and temperature measurements was demonstrated.