Characterization Of Fiber Bragg Gratings Research Articles

Characterization of Fiber Bragg Grating for Sensor Applications

Characterization of Fiber Bragg Grating for Sensor Applications

Characterization of Fiber Bragg Grating (FBG) for Weight Measurement System

Currently, FBG sensor has been developed in many applications due to its wavelength is sensitive to pressure change. This study aims to analyze the FBG wavelength shifting due to an applied load. In this research, the FBG was placed transversely right in the middle of the specimen that was made by using silicon rubber with thicknesses of 7 mm and 14 mm. The FBG wavelength in the two specimens was investigated by an interrogator when the load is put on to the specimens in three variations: without load, 5 kg, and 10 kg. The result show that the shift of FBG wavelength in specimen with the same thickness is proportional to load increasing. Furthermore, the test specimen with a thickness of 7 mm is more sensitive than 14 mm. Accordingly, the specimen with a thickness of 7 mm is a good option to be used as a weight measurement system.

Measurement and Discrimination of Asymmetric Non-uniform Strain Distribution Based on Spectrum Characterization of FBG Sensors

The asymmetric deformation of glass-to-metal seals is an important defect that would lead to the failure of the pressure boundary in nuclear reactors. In this research, an efficient approach to measure the asymmetric deformation and prevent the potential failure was proposed based on the spectrum characterization of fiber Bragg grating (FBG) sensors. The asymmetric deformation was induced from small-size large-gradient nonuniform strain in sealing materials; as a result, it could be monitored through the spectra of embedded FBGs affected by strain variations. The theoretical analysis of the FBG spectrum was carried out using the transfer matrix model (TMM) to validate the measuring feasibility and reliability. Based on the theoretical results, the asymmetric deformation was measured by the distributed embedded FBG experimentally. By combining the reconstructed spectrum and the experimental results, the asymmetric deformation of glass-to-metal was proved to be monitored, and the defect was able to be prevented during the manufacturing process effectively via the proposed method.

Fiber Bragg Grating (FBG) Sensors in a High-Scattering Optical Fiber Doped with MgO Nanoparticles for Polarization-Dependent Temperature Sensing

The characterization of Fiber Bragg Grating (FBG) sensors on a high-scattering fiber, having the core doped with MgO nanoparticles for polarization-dependent temperature sensing is reported. The fiber has a scattering level 37.2 dB higher than a single-mode fiber. FBGs have been inscribed by mean of a near-infrared femtosecond laser and a phase mask, with Bragg wavelength around 1552 nm. The characterization shows a thermal sensitivity of 11.45 pm/°C. A polarization-selective thermal behavior has been obtained, with sensitivity of 11.53 pm/°C for the perpendicular polarization (S) and 11.08 pm/°C for the parallel polarization (P), thus having 4.0% different sensitivity between the two polarizations. The results show the inscription of high-reflectivity FBGs onto a fiber core doped with nanoparticles, with the possibility of having reflectors into a fiber with tailored Rayleigh scattering properties.

Characterization of Fiber Bragg Grating for applications in Nuclear Research Reactors

The work presented in this paper characterize the fiber Bragg grating (FBG) for temperature measurement for the safe operation in material testing reactors (MTR). Fibre Bragg Grating is the key technology used to covers many applications in nuclear reactors in order to sense different physical parameters such as temperature, strain, etc. This paper investigates the modelling of Bragg grating based on MATLAB, through solved coupled mod equations in order to achieve maximum reflectivity. In addition, modelling of fiber Bragg grating as a temperature sensor for nuclear applications using opti-grating software is presented. Furthermore, Apodization techniques for signal processing also used to provide accurate measurements which required in nuclear applications.

Characterization of femtosecond-inscribed fiber Bragg gratings in the visible spectral region.

The femtosecond-laser pulse inscription and characterization of fiber Bragg gratings for operation at visible wavelengths was performed using several types of optical fibers, including single-mode and graded-index fibers designed for near-infrared wavelengths. The obtained bandwidths are very narrow (∼0.12-0.36 nm) for the used exposure conditions, even in graded-index fibers. Thermal and strain characterization was performed, with results about half of those found for C-band gratings. The wavelength dependence of the sensitivity is compared with a Sellmeier model.

Photoluminescence mapping of oxygen-defect emission for nanoscale spatial characterization of fiber Bragg gratings

Confocal photoluminescence (PL) microscopy is used to gain insight into the inner structure of Ge-doped Fiber Bragg Gratings (FBGs). These measurements pinpoint room temperature PL emission from oxygen-related defects in the visible range, whose spatial distribution exhibits a periodicity associated with the spatial modulation of the refractive index printed inside the fiber core of the FBG. The period measured by PL mapping performed at submicrometric resolution matches the period of the refractive index variation determined from the optical transmission wavelength using the Bragg condition. Since the PL emission of oxygen-related defects can be used to probe local chemical changes inside fused silica, this novel and non-destructive experimental approach can be implemented for the direct characterization of FBGs, to study the effects of gas conditioning, ageing, and degradation under various environments.

High-T_g TOPAS microstructured polymer optical fiber for fiber Bragg grating strain sensing at 110 degrees

We present the fabrication and characterization of fiber Bragg gratings (FBGs) in an endlessly single-mode microstructured polymer optical fiber (mPOF) made of humidity-insensitive high-Tg TOPAS cyclic olefin copolymer. The mPOF is the first made from grade 5013 TOPAS with a glass transition temperature of Tg = 135°C and we experimentally demonstrate high strain operation (2.5%) of the FBG at 98°C and stable operation up to a record high temperature of 110°C. The Bragg wavelengths of the FBGs are around 860 nm, where the propagation loss is 5.1 dB/m, close to the fiber loss minimum of 3.67 dB/m at 787 nm.

Growth characterization of fiber Bragg gratings inscribed in different rare-earth-doped fibers by UV and VIS femtosecond laser pulses

Besides well-known applications of fiber Bragg gratings (FBGs) in sensing and telecommunication, current research focuses also on an integration of gratings in actively doped fibers in order to realize high-power monolithic fiber lasers. The inscription of Bragg gratings directly into the active laser material has been achieved successfully, but the influence of the different dopants in rare-earth-doped fibers on the inscription process has usually failed to be considered. In this paper we report on a systematic investigation of Bragg grating growth behavior during inscription with UV and VIS fs pulses in various actively doped fibers. Furthermore we present an analysis of the initial grating growth in active fibers with different rare-earth dopant concentration under different femtosecond laser exposure conditions. A positive influence of cerium doping on grating growth and photosensitivity has been found. Ytterbium doping has been found to decelerate grating growth, meaning decreased photosensitivity to femtosecond pulses. The specific doping type seems to play an important role in the fabrication of high-reflective Bragg gratings in rare-earth-doped fibers.

Complete Fiber Bragg Grating Characterization Using an Alternative Method Based on Spectral Interferometry and Minimum- Phase Reconstruction Algorithms

We present an alternative method for the complete characterization of linear passive optical devices using the measurement of the spectral power density of an intermediate interferometric transfer function. Important advantages of this method compared to other published methods are the simplicity of the interferometric setup and the fact that iterative algorithms are not required. This method makes use of spectral interferometry to implement an intermediate transfer function which is mathematically related to the optical device transfer function and whose phase response is proven to be minimum. This last fact permits the unique retrieval of the phase response from the magnitude response using the Hilbert transform (in particular, minimum-phase reconstruction algorithms). Therefore, the whole intermediate transfer function (amplitude and phase responses) can be obtained using only power measurements. Once the intermediate interferometric function has been fully established, the complete optical device transfer function is reconstructed by inverting the mathematical relation between them. We have applied the proposed method to the characterization of fiber Bragg gratings, obtaining results that are in close agreement with the original transfer functions.

Improved thermal and strain performance of annealed polymer optical fiber Bragg gratings

We report on a detailed study of the inscription and characterization of fiber Bragg gratings (FBGs) in commercial step index polymer optical fibers (POFs). Through the growth dynamics of the gratings, we identify the effect of UV-induced heating during the grating inscription. We found that FBGs in annealed commercial POFs can offer more stable short-term performance at both higher temperature and larger strain. Furthermore, the FBGs' operational temperature and strain range without hysteresis was extended by the annealing process. We identified long-term stability problem of even the annealed POF FBGs.

Local and Spectral Characterization of Optical Fibers and Fiber Bragg Gratings with Low Coherence Interferometry

In this article, we describe local and spectral characterization of fibers and integrated optical components using low coherence interferometry. Chromatic dispersion and group birefringence measurements of erbium-doped fibers or photonic crystal fibers are presented, as well as local characterization of fiber Bragg gratings which allows the determination of strain and profile temperature gradient inside a material.

Characterization of Fiber Bragg Gratings by Phase-Sensitive Optical Low-Coherence Reflectometry

Optical low-coherence reflectometry (OLCR) is a very useful metrology tool to characterize optical fiber components such as fiber Bragg gratings (FBGs). These components find a wide variety of applications in the telecommunication and sensor industry. Hence, their characteristics must be precisely analyzed. A high-resolution OLCR setup is presented as a useful investigation technique and detailed in this paper. The precise characterization of different FBGs is shown

Characterization of fiber Bragg gratings using spectral interferometry based on minimum-phase functions

The focus of this paper is on interferometric systems that utilize spectral interferometry based on minimum-phase functions (MPFs) to fully characterize any fiber-Bragg-grating (FBG) spectra without exceptions. The approach presented involves sending a broadband light source (e.g., a short laser pulse) into the FBG of interest and using an optical spectrum analyzer (OSA) to record the spectrum of the interference between the reflected pulse from the grating and the time-delayed version(s) of the original short pulse. The square root of this measured spectrum, which yields the Fourier-transform (FT) magnitude of the input-pulse-sequence electric-field envelope, is then processed to uniquely recover both the phase and the amplitude of the FBG spectrum. The underlying principle for this unique recovery is that by construction, the input pulse sequence sent to the OSA is close to an MPF; thus, it is possible to recover its FT phase spectrum using only the measurement of its FT magnitude spectrum. This is an important result since, by merely measuring the FT magnitude, the entire complex spectrum of the grating can be recovered. Furthermore, this technique can conveniently be used to simultaneously characterize more than one FBG, using a single FT magnitude measurement. This technique has important advantages over existing techniques: a higher resolution and the ability to use longer duration laser pulses.

Polarization-resolved spatial characterization of birefringent Fiber Bragg Gratings

A method that enables polarization-resolved spatial characterization of fiber Bragg gratings is presented. The polarization-resolved reflection spectrum of the grating is measured using optical-frequency domain reflectometry. A polarization-resolved layer-peeling algorithm is used to compute the spatial profile, including the local birefringence and the local polarization-dependent index modulation. A strain-tuned distributed feedback fiber laser is used as source. With closed-loop control of the laser sweep, 0.14% maximum deviation from constant sweep rate is achieved, which is much better than commercial available tunable lasers. The polarization of the source is modulated synchronous with the laser sweep by passing the light through a three-armed Mach-Zehnder-type interferometer having different retardation. The method is used to investigate the polarization-dependence of the index modulation amplitude of a fiber Bragg grating.

Spatial characterization of fiber Bragg grating structures using transversal pressure

A technique for low cost and high precision characterization of fiber Bragg gratings based structures, using transversal pressure, is presented. It is based on the analysis of the amplitude and wavelength changes of the structure reflection spectrum, while being precisely pressed by an electromechanically actuated arm. The theory and the experimental results, which are in good agreement with the expected ones, are also presented.

Fabrication and characterization of fibre Bragg gratings for near 2 µmoperation

Reflective fibre Bragg gratings for operation at 2 µmhave been fabricated with the use of a lithographic phase mask. Bragg gratingsoperating at a wavelength of 2025 nm with reflectivities of up to 93.3% wereproduced with typical bandwidths of 0.4 nm (FWHM). The gratings were writtenusing a pulsed 266 nm UV laser source and were monitored in real timewith the use of a scanning monochromator and a 2 µmTm fibre fluorescence source. The reflection of the fibre Bragg gratings could bemade with particular specifications for use as internal fibre mirrors in fibre lasers or in WDM applications.

Accurate characterization of fiber Bragg grating index modulation by side-diffraction technique

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.

Limitations of the interferometric side diffraction technique for fibre Bragg grating characterization

We make a quantitative investigation of the interferometric side diffraction technique for characterization of fibre Bragg gratings. As opposed to more commonly used non-interferometric alternatives, this method also allows for determination of the grating's distributed phase. It is shown that for weak gratings the detected power and phase are directly proportional to the amplitude and phase of the refractive index modulation in the fibre. While the phase measurement is only limited by the amount of noise present, the relation between modulation amplitude and detected power becomes non-linear when using small probe beam wavelengths and gratings with a modulation depth Δn≈10 −3 and above. This behavior is inherent to the side diffraction and thus care has to be taken when performing apodization measurements on very strong gratings. The sensitivity to noise is briefly discussed and using a crude estimate for the noise level, the theory indicates that this method is suitable down to a grating modulation depth of at least Δn≈10 −5.

Characterization of wavelength-selective fiber-optic devices using a modified phase-shift method

We present a novel measurement setup that can be used for the complete characterization of fiber Bragg gratings and wavelength selective fiber-optic devices. Our setup is based on the phase-shift method (PSM), which we have modified to include the measurement of polarization-induced effects such as polarization-dependant loss (PDL) and polarization-mode dispersion (PMD). We measure the spectral response of devices used in transmission and in reflection, the wavelength dependency of the group delays due to chromatic and polarization-mode dispersion, and the wavelength dependency of the polarization-dependent loss. Experimental results are presented and sources of error are discussed. Comparisons with the Jones matrix eigenanalysis method for the measurement of PDL and differential group delay due to PMD have been carried out.