We experimentally demonstrate the possibility of quasi-adiabatic conversion of L P 11 modes to vortex modes in a twisted highly birefringent fiber with a gradually increasing twist rate. Based on the value of the effective indices, the L P 11 modes are selectively converted to right- and left-handed circularly polarized vortex modes H E 21 with a total angular momentum of ± 2 and to quasi- T E 01 / T M 01 modes with a total angular momentum of 0. Since the proposed conversion method has a purely topological origin, it is broadband in nature, in contrast to the methods based on resonant effects.

We report on a new phenomenon of light guidance in a fiber core created by an arrangement of holes making a partially open ring. In such structure there is no complete refractive index barrier to confine the light and therefore it cannot guide light if untwisted. However, if the open ring of holes is shifted off the symmetry axis of the twisted fiber then the mode confinement and low loss propagation is possible due to purely geometrical effect related to the increase in the optical path of light following the helical route. Properties of such structures, including confinement loss, modal field distribution, birefringence of fundamental modes were investigated both numerically and experimentally. We also studied the effect of bending, which leads to periodic modulation of the propagation characteristics of the twisted fiber. Furthermore, we demonstrate a possibility of displacement measurements based on bend-induced loss using some of the fabricated fibers, in which the sensitivity to bend is controlled by a twist rate and core structure.

We report on experimental studies of polarimetric sensitivity to torsion in spun highly birefringent fibers. Two classes of spun fibers were examined, namely spun side-hole fibers and birefringent microstructured fibers with different birefringence dispersion, spin pitches, and spin directions. The polarimetric sensitivity to torsion was determined by monitoring a displacement of the spectral interference fringes arising in the output signal because of interference of polarization modes and induced by an additional fiber twist. Both the experimental results and the analytical predictions showed that the sensitivity to torsion normalized to the fringe width in the spun highly birefringent fibers increased asymptotically with the twist rate to the value of 1/ π rad⁻¹. We have also studied the polarimetric response to temperature in the spun side-hole fibers. We have found that, 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 and the range of linear response are both enhanced in such fibers, while at the same time, the cross-sensitivity to temperature is reduced.

We examined the influence of germanium concentration in fiber core on distributed axial strain sensitivity in optical frequency-domain reflectometry based on Rayleigh scattering for single-mode step-index fibers differing in the level of germanium concentration (from 3.6 to 40.6 mol%). We observed a monotonically decreasing dependence of axial strain sensitivity on germanium concentration within a range of 0–3.5 mstrain for uncoated fibers. The lowest axial strain sensitivity of 0.7422 1/strain was obtained for fiber doped with 40.6 mol% GeO2 (UHNA7), compared to 0.7859 1/strain for fiber with 3.6 mol% GeO2 content (SMF-28). We also derived an analytical formula for strain sensitivity in optical frequency-domain reflectometry, explaining the experimental results.

We demonstrate a fiber-based optical vortex beam source operating in broadband or tunable mode in the spectral range of 1100–1400 nm. The vector vortices of the total angular momenta equal to +2, 0, and −2 are obtained by converting the respective linearly polarized (LP₁₁) modes of the two-mode birefringent PANDA fiber with stress-applying elements by gradually twisting its output section. At the input end, the PANDA fiber is powered by broadband supercontinuum or tunable Raman solitons generated in the LP₁₁ polarization modes of a birefringent microstructured fiber with a specially designed dispersion profile and coupled to the respective LP₁₁ modes of the PANDA fiber. Two pulse lasers operating in different regimes (1 ns/1064 nm and 190 fs/1037 nm) were used as the pump to generate supercontinuum or tunable solitons directly in the LP₁₁ modes of the microstructured fiber purely excited with a special Wollaston prism-based method. The high modal and polarization purities of the beams after successive transformations were experimentally confirmed. We also proved the vortex nature of the output beams using shearing interferometry.

This article presents a comprehensive study on the impact of irradiation optical fiber cores with a femtosecond-pulsed laser, operating at a wavelength of 1030 nm, on the signal amplitude in Rayleigh scattering-based optical frequency domain reflectometry (OFDR). The experimental study involves two fibers with significantly different levels of germanium doping: the standard single-mode fiber (SMF-28) and the ultra-high numerical aperture fiber (UHNA7). The research findings reveal distinct characteristics of reflected and scattered light amplitudes as a function of pulse energy. Although different amplitude changes are observed for the examined fibers, both can yield an enhancement of amplitude. The paper further investigates the effect of fiber Bragg grating inscription on the overall amplitude of reflected light. The insights gained from this study could be beneficial for controlling the enhancement of light scattering amplitude in fibers with low or high levels of germanium doping.