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We developed a passive silica based large mode area fiber with the nanostructured core with a diameter of 30 μm devoted to fiber Bragg grating inscription for application in an all-fiber laser cavity. The fiber is perfectly matched to the commercial active fiber with NA = 0.06 and core/cladding diameters 30/250 μm. Measured modal characteristics and bending loss of the fiber confirm single-mode operation for bending diameter of 8 cm with bending losses as low as 0.15 dB/m. Although the nanostructured fiber core is low germanium doped, we fabricated high reflectivity (98.5%) UV induced fiber Bragg grating (FBG) and verified its performance in fiber laser setup. The laser cavity formed with highly reflective FBG inscribed in the fiber and Fresnel reflection on the free end of active fiber results in lasing efficiency of 65.5%. The flexibility of the nanostructuring approach gives prospects for the development of free-form large mode area fibers for passive fiber components and matched with any type of active fibers. [ABSTRACT FROM AUTHOR]

Compliant mechanisms have gained an increasing interest in recent years, especially in relation to the possibility of using 3D printers for their production. These mechanisms typically find applications in precise positioning systems of building robotic devices or in sensing where they can be used to characterize displacement. Three-dimensional printing with PLA materials allows fiber optic-based sensors to be incorporated into the structures of properly designed compliant mechanisms. Therefore, in this paper, an innovative technology is described, of a Fiber Bragg Grating (FBG) sensor embedded in a measuring head which was then inserted into a specially designed mechanical transmission element. The shape of this element is based on clippers that allow to freely modify the amplification of displacement amplitude so that the FBG sensor always works in the most optimal regime without any need to modify its external dimensions. Flexural sensitivity of the replaceable measuring head equal to 1.26 (mε/mm) can be adapted to the needs of the flexure design. [ABSTRACT FROM AUTHOR]

A π-phase-shifted fiber Bragg grating (π-FBG) shows high sensitivity to the ultrasonic (US) wave as compared to the conventional FBG due to the strong slow-light phenomenon at the resonance peak. However, its sensitivity is limited by the interrogation schemes. A combination of π-FBG and unbalanced fiber Mach- Zehnder interferometer (F-MZI) are theoretically analyzed and optimized for the highly sensitive acoustic sensor. The coupled-mode theory (CMT) and transfer matrix method (TMM) are used to establish the numerical modelling of π-FBG. For the optimized grating parameters of π-FBG, the proposed sensing system shows the high strain sensitivity of 1.2 × 108=", the highest dynamic strain resolution of 4:1f"= p Hz, and the highest wavelength shift resolution of 4:9 × 10-9 pm. Further, the proposed sensing system strongly supports both time andwavelength division multiplexing techniques. Therefore, the proposed sensing system shows extreme importance in single as well as quasi-distributed US acoustic wave sensing networks. [ABSTRACT FROM AUTHOR]

A novel self-apodization effect that occurs in tapered fiber Bragg gratings (TFBG) is presented. The detailed theoretical analysis and experimental results show that only by feeding the electromagnetic wave into the grating from the taper waist side, the significant reduction of oscillations in a group delay response of TFBG is achieved. Thus, the apodization effect is achieved without any additional technological processes during the grating inscription. Moreover, a counterdirectionally written tapered and chirped fiber Bragg grating that has a negative slope of the group delay response and exhibits self-apodization effect is proposed. [ABSTRACT FROM AUTHOR]

A simple fiber optic inclinometer based on linearly chirped fiber Bragg grating written in both fused taper transitions is presented. Specially designed and inscribed fiber gratings reveal two selective reflection bands, where only one is sensitive to bending and the other is used as a reference. Thus, the presented inclinometer is insensitive to coupling efficiency variations when the sensor head is reconnected to the measurement setup. Furthermore, the inclinometer is characterized by a 0°–70° operating range, 1° resolution and similar sensitivity. In the presented configuration, the inclinometer can operate as a temperature independent strain sensor as well as simultaneously measure tilt and ambient temperature. [ABSTRACT FROM AUTHOR]

Abstract: Scalar diffraction theory is applied to analyze the intensity distribution in the fiber core during fiber Bragg grating (FBG) fabrication using an apodized phase mask. The averaged diffraction field distribution in fiber core was calculated as a function of optical fiber position. The results show that in a case of apodized FBGs fabrication, averaged field intensity profile, and thus refractive index changes in fiber core have complex form. Moreover, it was shown that the influence of optical fiber position behind the phase mask on average intensity distribution in fiber core decreases with increasing of its diameter. [ABSTRACT FROM AUTHOR]

In this paper, the application of a fiber Bragg grating written in a highly birefringent side-hole elliptical core optical fiber for two-axial strain measurement is presented. Hybrid optical fiber structures achieved by combining large side-holes and elliptical core result in a very high birefringence of 1 × 10−3 and thus high initial Bragg peak spectral separation of 1.16 nm, as well as a very high transverse force sensitivity, of up to 650 pm/(N/mm) or even −1150 pm/(N/mm), depending on the fiber orientation with respect to the applied force. Due to the ~22 %m/m GeO2 concentration in the core the fiber being highly photosensitive, which significantly simplifies FBG fabrication by UV illumination without the need for prior hydrogen loading, which worsens thermal stability. Finally, the developed FBGs written in the highly birefringent side-hole elliptical core optical fiber were embedded in the square composite plates and applied for strain measurements. Tests of two-directional four-point bending have shown usability of such FBG for two-axial in-plane strain measurement with a single FBG in iso-thermal conditions. [ABSTRACT FROM AUTHOR]

Polymer–matrix composites degrade under the influence of UV radiation in the range of the 290–400 nm band. The degradation of polymer–matrix composites exposed to UV radiation is characterized by extensive aging of the epoxy matrix, resulting in deterioration of their mechanical properties. Glass fibers/epoxy resin composites were made by an out-of-autoclave method whereas a fiber optic sensor was placed between different layers of laminates. In our work, we used a fiber Bragg grating sensor covered with graphene oxide and embedded in a polymer matrix composite to monitor UV radiation intensity. Measurements of UV radiation may allow monitoring the aging process of individual components of the polymer composite. In order to estimate the number of microcracks of epoxy resin, microstructure observations were carried out using a scanning electron microscope. [ABSTRACT FROM AUTHOR]