Fiber Bragg grating has embraced the area of fiber optics since the early days of its discovery, and most fiber optic sensor systems today make use of fiber Bragg grating technology Researchers have gained enormous attention in the field of fiber Bragg grating (FBG)-based sensing due to its inherent advantages, such as small size, fast response, distributed sensing, and immunity to the electromagnetic field Fiber Bragg grating technology is popularly used in measurements of various physical parameters, such as pressure, temperature, and strain for civil engineering, industrial engineering, military, maritime, and aerospace applications Nowadays, strong emphasis is given to structure health monitoring of various engineering and civil structures, which can be easily achieved with FBG-based sensors Depending on the type of grating, FBG can be uniform, long, chirped, tilted or phase shifted having periodic perturbation of refractive index inside core of the optical fiber Basic fundamentals of FBG and recent progress of fiber Bragg grating-based sensors used in various applications for temperature, pressure, liquid level, strain, and refractive index sensing have been reviewed A major problem of temperature cross sensitivity that occurs in FBG-based sensing requires temperature compensation technique that has also been discussed in this paper

https://www.spiedigitallibrary.org/journals/optical-engineering/volume-59/issue-6/060901/Fiber-Bragg-grating-sensors-for-monitoring-of-physical-parameters/10.1117/1.OE.59.6.060901.pdf

Fiber Bragg grating sensors for monitoring of physical parameters: a comprehensive review

The harsh environment associated with the next generation of nuclear reactors is a great challenge facing all new sensing technologies to be deployed for on-line monitoring purposes and for the implantation of SHM methods. Sensors able to resist sustained periods at very high temperatures continuously as is the case within sodium-cooled fast reactors require specific developments and evaluations. Among the diversity of optical fiber sensing technologies, temperature resistant fiber Bragg gratings are increasingly being considered for the instrumentation of future nuclear power plants, especially for components exposed to high temperature and high radiation levels. Research programs are supporting the developments of optical fiber sensors under mixed high temperature and radiative environments leading to significant increase in term of maturity. This paper details the development of temperature-resistant wavelength-multiplexed fiber Bragg gratings for temperature and strain measurements and their characterization for on-line monitoring into the liquid sodium used as a coolant for the next generation of fast reactors.

Temperature Resistant Fiber Bragg Gratings for On-Line and Structural Health Monitoring of the Next-Generation of Nuclear Reactors.

Fiber tip photodegradation through OH diffusion currently limits the long term operation of high-power fiber lasers and amplifiers operating near 3 µm. To address this issue, we investigate the resistance to OH diffusion of fluoride and oxide endcaps manufactured out of ZrF$_4$4, AlF$_3$3, GeO$_2$2, SiO$_2$2 and Al$_2$2O$_3$3 fibers. To this extent, the endcaps are spliced at the output of a 20 W continuous-wave fiber laser operating at 2.8 µm and their degradation over a 100 h time period is monitored. While the fluoride-based endcaps underwent failure during the first 10 h, their oxide counterparts survived the experiment, although showcasing degradation which was reflected as an increase of the endface temperature over time. To overcome this issue, we propose a novel method to completely suppress OH diffusion which consists in sputtering a nanoscopic diffusion barrier film made of silicon nitride (Si$_3$3N$_4$4) on the output face of the endcap. The effectiveness of the approach is validated on Al$_2$2O$_3$3, ZrF$_4$4 and AlF$_3$3 endcaps which show no sign of degradation after being used for more than a 100 h at the output of a 3 µm high-power fiber laser.

Endcapping of high-power 3 µm fiber lasers.

Type II π-phase-shifted Bragg gratings stable up to ~1000°C are written inside a standard single mode silica optical fiber (SMF-28) with infrared femtosecond pulses and a special phase mask Inscription through the protective polyimide fiber coating is also demonstrated The birefringence of the Bragg gratings and, as a result, the polarization dependence of their spectra are strongly affected by the femtosecond laser polarization Using optimized writing conditions, the full width at half maximum of the π-phase-shifted passband feature can be ~30 pm in transmission, while the polarization-dependent shift of its central wavelength can be less than 8 pm, for a 7 mm long grating structure This makes such gratings a unique tool for high-resolution measurements of temperature, load and vibration in extreme temperature environments

High-temperature stable π-phase-shifted fiber Bragg gratings inscribed using infrared femtosecond pulses and a phase mask.

这份报纸在调查可靠性的工作的一个试验性的程序上报导，耐久性，并且为是的申请纤维布拉格栅栏(FBG ) 包装在结构的疲劳测试军事平台期间散布了紧张传感器。传感器可靠性上的 FBG 制造过程的影响被调查。另外，为宽广区域的包装的方法论并且 FBG 察觉到到防卫平台的数组被开发并且测试装表面。

High-Strain Fiber Bragg Gratings for Structural Fatigue Testing of Military Aircraft

Type I fiber Bragg gratings (FBG) written through the coating of various off-the-shelf silica fibers with a femtosecond laser and the phase-mask technique are reported. Inscription through most of the common coating compositions (acrylate, silicone and polyimide) is reported as well as writing through the polyimide coating of various fiber cladding diameters, down to 50 µm. The long term annealing behavior of type I gratings written in a pure silica core fiber is also reported as well as a comparison of the mechanical resistance of type I and II FBG. The high mechanical resistance of the resulting type I FBG is shown to be useful for the fabrication of various distributed FBG arrays written using a single period phase-mask. The strain sensing response of such distributed arrays is also presented.

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Femtosecond FBG Written through the Coating for Sensing Applications

We report on a splice-free erbium-doped all-fiber laser emitting over 20 W at a wavelength of 1610 nm, with a slope efficiency of 19.6 % and an overall efficiency of 18.3% with respect to the launched pump power at 976 nm. The simple cavity design takes advantage of fiber Bragg gratings written directly in the gain fiber through the polymer coating and clad-pumping from a single commercial pump diode to largely simplify the assembling process, making this cavity ideal for large-scale commercial deployment. Two single-mode and singly erbium-doped silica fibers were fabricated in-house: the first to assess the effects of a high erbium concentration (0.36 mol.% Er2O3), yielding a low efficiency of 2.5 % with respect to launched pump power, and the second to achieve the improved result mentioned above (0.03 mol.% Er2O3). Numerical simulations show the link between the performance of each cavity and ion pair-induced quenching.

20 W splice-free erbium-doped all-fiber laser operating at 1610 nm.

This paper reports on an experimental investigation into the differences between directly and remotely bonded optical fiber Bragg grating (FBG) sensors when measuring Lamb waves in an aluminum plate. A sensor array, comprising 16 individual short gauge-length FBG sensing elements, is used to decompose the wave into its constituent modes for the directly bonded case, where the array is attached directly to the plate and for the remotely bonded case where the same array is located in a suspended fiber 250 mm downstream from the fiber/plate contact region. The experimental results were consistent with model predictions and show that while the directly bonded fiber array can resolve the full multimodal Lamb wave spectrum in the plate, the remotely bonded sensor measures only a single nondispersive longitudinal acoustic mode predicted by the analytical solution for a cylindrical waveguide. The inability of remote sensing to distinguish between different modes of Lamb wave propagation represents a significant limitation with respect to the diagnostic utility of this sensing approach.

Remote Sensing of Lamb Waves Using Optical Fibres—An Investigation of Modal Composition

When a laser is mode-locked, it emits a train of ultra-short pulses at a repetition rate determined by the laser cavity length. This article outlines a new and inexpensive procedure to force mode locking in a pre-adjusted nonlinear polarization rotation fiber laser. This procedure is based on the detection of a sudden change in the output polarization state when mode locking occurs. This change is used to command the alignment of the intra-cavity polarization controller in order to find mode-locking conditions. More specifically, the value of the first Stokes parameter varies when the angle of the polarization controller is swept and, moreover, it undergoes an abrupt variation when the laser enters the mode-locked state. Monitoring this abrupt variation provides a practical easy-to-detect signal that can be used to command the alignment of the polarization controller and drive the laser towards mode locking. This monitoring is achieved by feeding a small portion of the signal to a polarization analyzer measuring the first Stokes parameter. A sudden change in the read out of this parameter from the analyzer will occur when the laser enters the mode-locked state. At this moment, the required angle of the polarization controller is kept fixed. The alignment is completed. This procedure provides an alternate way to existing automating procedures that use equipment such as an optical spectrum analyzer, an RF spectrum analyzer, a photodiode connected to an electronic pulse-counter or a nonlinear detecting scheme based on two-photon absorption or second harmonic generation. It is suitable for lasers mode locked by nonlinear polarization rotation. It is relatively easy to implement, it requires inexpensive means, especially at a wavelength of 1550 nm, and it lowers the production and operation costs incurred in comparison to the above-mentioned techniques.

/pdf/automation-of-mode-locking-in-a-nonlinear-polarization-3l2fo88vlu.pdf

Automation of Mode Locking in a Nonlinear Polarization Rotation Fiber Laser through Output Polarization Measurements.

An array of ten distributed femtosecond fibers Bragg gratings (FBG) was written through the polyimide coating of a standard silica fiber using a single period uniform phase-mask. All FBG wavelengths are spectrally distributed over 40nm.

Joé Habel

Papers

Femtosecond FBG Written through the Coating for Sensing Applications

20 W splice-free erbium-doped all-fiber laser operating at 1610 nm.

Remote Sensing of Lamb Waves Using Optical Fibres—An Investigation of Modal Composition

Automation of Mode Locking in a Nonlinear Polarization Rotation Fiber Laser through Output Polarization Measurements.

Flexible phase-mask writing technique of robust femtosecond FBG for distributed sensing