We present a comprehensive study of an in-line Mach-Zehnder intermodal interferometer fabricated in a boron-doped two-mode highly birefringent microstructured fiber. We observed different ...interference signals at the output of the interferometer, related to the intermodal interference of the fundamental and the first order modes of the orthogonal polarizations and a beating of the polarimetric signal related to the difference in the group modal birefringence between the fundamental and the first order modes, respectively. The proposed interferometer was tested for measurements of hydrostatic pressure and temperature for different alignments of the input polarizer with no analyzer at the output. The sensitivities to hydrostatic pressure of the intermodal interference signals for
- and
-polarizations had an opposite sign and were equal to 0.229 nm/MPa and -0.179 nm/MPa, respectively, while the temperature sensitivities for both polarizations were similar and equal 0.020 nm/°C and 0.019 nm/°C. In the case of pressure, for the simultaneous excitation of both polarization modes, we observed a displacement of intermodal fringes with a sensitivity depending on the azimuth of the input polarization state, as well as on the displacement of their envelope with a sensitivity of 2.14 nm/MPa, accompanied by a change in the fringes visibility. Such properties of the proposed interferometer allow for convenient adjustments to the pressure sensitivity of the intermodal fringes and possible applications for the simultaneous interrogation of temperature and pressure.
We present a rocking filter in a highly birefringent two-mode fiber that enables resonant coupling between different modes in the LP
group. Our simulations and experimental results prove that such a ...filter allows for resonant coupling between orthogonally polarized LP
modes of the same spatial structure, as well as between modes of the same polarizations and orthogonal spatial distributions. Furthermore, we demonstrate that such rocking filters can be used to generate pure TE
, TM
and HE
beams or their coherent superposition.
We performed an experimental analysis of the effect of phase mask alignment on the Bragg grating reflection spectra around the wavelength of
= 1560 nm fabricated in polymer optical fiber by using a ...multiple order phase mask. We monitored the evolution of the reflection spectra for different values of the angle ϕ by describing the tilt between the phase mask and the fiber. We observed that the peak at
is split into five separate peaks for the nonzero tilt and that separation of the peaks increases linearly with ϕ. Through comparison with theoretical data we were able to identify the five peaks as products of different grating periodicities, which are associated with the interference of different pairs of diffraction orders on the phase mask.
We present fiber Bragg grating (FBG)-based hydrostatic pressure sensing with highly birefringent microstructured optical fibers. Since small deformations of the microstructure can have a large ...influence on the material birefringence and pressure sensitivity of the fiber, we have evaluated two microstructured fibers that were made from comparable fiber preforms, but fabricated using different temperature and pressure conditions. The magnitude and sign of the pressure sensitivity are found to be different for both fibers. We have simulated the corresponding change of the Bragg peak separation with finite-element models and experimentally verified our results. We achieve very high experimental sensitivities of -15 and 33 pm/MPa for both sensors. To our knowledge, these are the highest sensitivities ever reported for birefringent FBG-based hydrostatic pressure sensing.
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 (LP
11
) 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 LP
11
polarization modes of a birefringent microstructured fiber with a specially designed dispersion profile and coupled to the respective LP
11
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 LP
11
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.
We examine experimentally the influence of the fiber inelastic twist on polarimetric sensitivity to hydrostatic pressure and pressure sensitivity in a Rayleigh-scattering-based optical ...frequency-domain reflectometer (OFDR) for highly birefringent side-hole fibers. The fibers were drawn from the same preform with different spin pitches varying from 5 mm to 200 mm. We also demonstrate that the sensitivities of spun fiber can be estimated analytically based on its spin pitch, measured birefringence, and the sensitivity of the corresponding non-twisted fiber, showing good agreement with the experimental results. We proved that polarimetric sensitivity to pressure decreases with the shorter spin pitches, while distributed pressure sensitivity decreases for the one polarization eigenmode and increases for the second polarization eigenmode. Therefore, the spun-fibers can operate well as the polarimetric sensors of other physical parameters inducing circular birefringence even under varying pressure. Moreover, they can be used to obtain the desired difference between sensitivities for both polarization eigenmodes
Helical core fibers (HCFs) suffer from low coupling efficiency and unavoidable excitation of higher order modes below the cutoff wavelength because of a core tilt with respect to the symmetry axis of ...the cladding. We propose an effective way of increasing the coupling efficiency to a HCF by untwisting its beginning section in a hydrogen flame. The proposed solution provides also a control over the excitation of higher order modes in HCFs and can be applied in splicing as well as in a free-space launching configuration. We experimentally demonstrate that by using the proposed method, the coupling efficiency between a SMF-28 and HCF can be increased to the level reachable for straight fibers, which is limited only by a modal fields mismatch. We also present detailed numerical and analytical studies of the coupling efficiency between a HCF and SMF versus the pitch distance in the partially untwisted input section of the HCF, which takes into account fundamental and higher-order modes.
PDF
