Numerical modeling of many optical-fiber-based devices' operation requires an exact knowledge of fiber's parameters like core diameter and dopant, usually GeO2, concentration. While diameters are typically specified in the fiber's data sheet, material composition, including dopant in the core, is rarely available. We present a procedure utilizing a reverse engineering approach to find GeO2 concentration in single-mode step-index optical fiber. Our method consists of several stages. First, we measured the numerical aperture NA for several commercially available fibers employing a onedimensional far-field scan. The far-filed mode intensity was acquired by a Ge detector placed on a rotation stage with a stepper motor for fiber end-face positioned on the motor's axis of rotation. We calculated NA for the angular detector position when the light intensity reached 13.5%, 5%, and 1% of its maximum value. Then, taking the corresponding values of core and cladding diameters and using the Sellmeier formula for pure (cladding) and GeO2 doped (core) silica glass, we found the concentration of GeO2 numerically matching calculated NA to the experimental data. We found that dopant concentration equals 9.0, 18.0, 34.0, 34.5, and 39.8 mol% for the fibers, respectively, 980-HP, UHNA1, UHNA3, UHNA4 and UHNA7 produced by Coherent. To verify the correctness of our method, we performed this procedure for several fibers with a known level of GeO2 concentration in the core fabricated by the Laboratory of Optical Fiber Technology, Maria Curie-Sklodowska University. The results of this simulation coincide with expectations with great accuracy.
Controllable excitation of spatial and polarization modes is of high importance in numerous applications, such as nonlinear optics, mode division multiplexing, interferometric measurements or sensing. We propose an effective method for selective excitation of different combinations of modes from LP01 and LP11 groups in a birefringent fiber. In the proposed method, the mode selection is realized with only a Wollaston prism, a rotatable polarizer and a half-wave plate, which ensures the possibility of high-power operation, low wavelength dependence, and tunability. Our approach makes it possible to excite almost all possible combinations of the LP01 and LP11 polarization modes and to continuously tune the relative coupling efficiencies of different modes by transverse shifting of the Wollaston prism. We demonstrate experimentally that the suppression rate of the unwanted modes with respect to the targeted mode exceeds 20 dB, and discuss the system configurations ensuring the highest possible coupling efficiencies for specific modes combinations. As example applications we show direct soliton and supercontinuum generation in the LP11 mode, broadband conversion of a supercontinuum from the LP01 to LP11 mode, broadband generation of vortex beams, gain tunability of intermodal four-wave mixing and cross-polarization four-wave mixing.
We experimentally demonstrate the possibility of adiabatic conversion of LP11 modes to vortex modes in a twisted highly birefringent fiber with a gradually increasing twist rate. Based on the values of effective indices, the LP11 modes are selectively converted to right-/left-handed circularly polarized vortex modes HE21 with a total angular momentum of ±2 and to quasi-TE01/TM01 modes with a total angular momentum of 0. The proposed conversion method has a purely topological origin, therefore, it is broadband in nature, in contrast to the methods based on resonant effects, and can be applied as an all-fiber broadband source of vortex beams.
Funding: Narodowe Centrum Nauki (DEC-2016/22/A/ST7/00089, Maestro 8).
Abstract: We report on experimental studies of polarimetric sensitivity to torsion and temperature in a series of spun highly birefringent side-hole fibers with spin pitches ranging from 5 to 200 mm. The polarimetric sensitivities to torsion and temperature were measured by monitoring a displacement of the spectral interference fringes arising in the output signal because of interference of ellipticaly polarized modes. The experimental results show that the sensitivity to torsion normalized to the fringe width in the spun highly birefringent fibers increases asymptotically with the twist rate to the value of 1/π rad-1 . In contrast to the torsional sensitivity, the temperature sensitivity decays asymptotically to zero with increasing fiber twist rate. Therefore, the spun fibers with short spin pitches are especially well suited for torsion measurements because the torsional sensitivity is enhanced in such fibers while at the same time the cross-sensitivity to temperature is reduced.
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