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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]

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]

In this paper, spectrally tailored tapered chirped fiber Bragg gratings (TCFBGs) are considered for use as strain sensors. Both gratings were written in fused tapered optical fiber using linearly chirped fiber Bragg gratings in co-directional and counter-directional chirp configurations. Theoretical and numerical analysis as well as experimental verification of the influence of applied strain on spectral width of both TCFBG structures were carried out. The results show that TCFBGs exhibit monotonic strain response over the wide operating range of the applied force. Compared with standard tapered FBG written using uniform phase mask, in the case of co-directionally written TCFBG with substantially larger grating chirp the monotonic operating range can be easily broadened toward the higher strain values (even above the applied force at which the optical fiber breaks). In turn, the intrinsic chirp of the counter-directionally written TCFBG can be tailored in such a way as to ensure that the monotonicity of its strain response is always satisfied when tensile force is applied. This is due to the spectral broadening of the reflected spectrum when strain increases. [ABSTRACT FROM PUBLISHER]

A new simple and versatile method for inscribing fiber Bragg gratings with an arbitrarily chosen Bragg wavelength is presented. Owing to the optimization of the irradiation process and application of the phase mask with a properly matched chirp, the method offers the ability to adjust the FBG resonance wavelength adjustment within a range of tens of nanometer. However; the resulting grating has a chirp; its spectral properties closely resemble those of the uniform one, which explains why such gratings are called quasi-uniform fiber Bragg gratings (QUFBGs). Optimization of the inscription process involved a two parameter Pareto method. The research presented here was conducted for a 2.5 cm phase mask with a 0.35 nm/mm chirp and allowed for the modelling, optimization and inscription of QUFBGs of FWHM 0.8–0.9 nm, with a transmission minimum lower than –8 dB and Bragg wavelength adjustment range of 11.4 nm. It was estimated that if extended to a commercially available 15 cm phase mask, the method would allow a record-breaking 74.7 nm adjustment range to be obtained. Together with clear consistency of the numerical and experimental results with the design assumptions for  QUFBGs inscription, this record value makes the proposed method competitive with more sophisticated and less reproducible interferometric techniques. [ABSTRACT FROM PUBLISHER]