We study the theoretical formation of optical vortices using a nanostructured gradient index phase mask. We consider structures composed of spatially distributed thermally matched glass nanorods with ...high and low refractive indices. Influence of effective refractive profile distribution, refractive index contrast of component glasses and charge value on the quality of generation of vortices are discussed. A trade-off between waveguiding and phase modulation effects for various refractive index contrast is presented and analysed.
We demonstrate birefringence enhancement in an optical fiber with artificial core anisotropy. A nanostructured fiber core in the form of a glass slab with layers of germanium-doped silica interleaved ...with the layers of fluorine-doped silica enabled an increase of the birefringence of 0.54×10 -4 without any stress zones. The experimental result confirms the numerical prediction of birefringence enhancement over previously presented fibers by increasing the refractive index contrast between the two core base glasses. Birefringence improvement has been achieved only by core material engineering and not by adding any stress zones or other approaches involving cladding shaping and core geometry design. The modification introduced to core had no impact on the guiding performance of the realized fiber, which remained perfectly matched to the SMF-28 standard in terms of its numerical aperture NA = 0.11 and the effective mode-field diameter MFD = 10.1 μm at the wavelength of 1550 nm.
We demonstrate a polarized all-normal dispersion supercontinuum generated in a birefringent silica microstructured fiber spanning beyond 2.5 µm. To our knowledge, this is the spectra reaching the ...furthest in mid-infrared ever generated in normal dispersion silica fibers. The generation process was studied experimentally and numerically with 70 fs pump pulses operating at different wavelengths on short propagation distances of 48 mm and 122 mm. The all-normal operation was limited by the zero-dispersion wavelength at 2.56 µm and spectral broadening was stopped by OH absorption peak at 2.72 µm. We identified the asymmetry between propagation in both polarization axes and showed that pumping along a slow fiber axis is beneficial for a higher degree of polarization. Numerical simulations of the generation process conducted by solving the generalized nonlinear Schrödinger equation (NLSE) and coupled NLSEs system showed good agreement with experimental spectra.
We present first fiber Bragg grating (FBG) inscribed in polarization maintaining (PM) optical fiber with artificially anisotropic core achieved by an appropriate distribution of discrete ...germanium-doped areas in nanoscale. We proved that this FBG reveals unique properties. Its single-resonance spectral shape, strain and temperature sensitivities are similar to that for grating written in cylindrically symmetrical optical fiber, which indicates that it behaves as the grating fabricated in a non-PM fiber, whereas it is written in the fiber that preserves the orthogonal polarization components of the mode. Moreover, due to the relatively low birefringence of the fiber, the FBG is insensitive to the excited polarization state and twist, and only a negligible shift of the spectrum is noticeable with no changes in its shape. Thus, FBG inscribed in ZEBRA fiber is a kind of hybrid structure that can be used for all-fiber lasers that generate polarized wave, as well as for polarization fiber sensors and components with spectral insensitivity to random or intentional twist.
We present both a theoretical and an experimental study of a novel compact lensed fiber system utilizing a nanostructured GRIN lens. The lens can be integrated with an optical fiber, which ensures a ...unique and efficient focusing in any high index medium, such as a liquid. We use the effective medium approach to design lenses with arbitrary refractive index. To fabricate lenses, we utilize a discrete array of nano-sized rods made of two types of glasses, and apply a standard stack-and-draw fiber drawing technology. The fabricated nanostructured GRIN lenses have a parabolic refractive index profile with a diameter of a standard fiber, very short working distances (55 µm in the air) and a high numerical aperture (NA = 0.16). As a proof-of-concept of the new micro-lensed fiber system, we demonstrate an experiment on optical trapping of micrometer-sized glass beads. We also show that our method is compatible with optical fiber technology and allows for any shape of the refractive index distribution in 2D. Thanks to that a new functionality could be achieved by replacing the GRIN lens with an axicon lens, vortex type elements, micro-lenses arrays or diffraction elements.
We present experimental results on fiber Bragg gratings inscription in nanostructured graded-index (nGRIN) and multi-step index (MSIN) optical fibers, both having non-uniform radial distribution of ...GeO
dopant in the fiber cores. In particular, the positive role of radial shaping the GeO
distribution in the fiber core on grating reflection efficiency is reported. We postulate that an appropriate spatial distribution of the germanium concentration that matches the fundamental mode profile improves grating spectral response due to more efficient grating-mode interaction, as compared with uniformly doped step-index optical fibers with the same overall doping level. Moreover, we show that radially shaped fibers exhibit moderately higher temperature responses than their step-index counterparts.
We report on efficient inscription of fiber Bragg gratings (FBGs) in a new type of single mode fiber with nanostructured core and with an effective parabolic graded index profile, using the standard ...phase mask method and a 248 nm pulsed laser. A nanostructured core allows to obtain high concentration of GeO
in subwavelength glass rods and simultaneously to maintain low average germanium dopant level of silica similarly to standard single mode fibers. We showed that in a nanostructured core fiber, a factor of 3 better efficiency in gratings inscription was achieved, although the fiber has 20% lower average concentration of GeO
with respect to SMF-28. In the nanostructured fiber we obtained a significant improvement in temperature sensitivity, while the strain sensitivity of FBG in nGRIN optical fiber is the same as in case of standard single-mode fiber (SMF-28). We have measured the strain sensitivity of 0.72 × 10
1/με (1.11 pm/με@1.53μm), and the temperature sensitivity is about 30% higher than for FBG in SMF-28 and equals to 10.2 × 10
1/K (15.6 pm/K@1.53μm).
We demonstrate for the first time in optical fibers the "dispersion of the axes" phenomenon, which relies upon spectral dependence of polarization planes of the guided modes. This feature was ...achieved in a specially designed side-hole fiber by combining linear birefringence induced by a pair of holes located in the cladding and the elliptical core tilted with respect to the symmetry plane of the holes. We performed numerical simulations of this design properties. The side-hole fiber with a tilted core was fabricated and experimentally studied. The orientation of the principal polarization planes of the fundamental modes varied by 14.5° in the analyzed spectral range. This value is in a good agreement with the simulation results.
The optical performance of nanostructured gradient index vortex phase masks (VPMs) in various external media was investigated. We utilized a modified stack-and-draw technique to fabricate the ...microscopic-size masks. Its structure contains spatially distributed sub-wavelength rods made of two thermally matched glasses with high and low refractive indices. Measured optical characteristics of the generated vortices with azimuthal phase distribution are presented. The numerical and experimental study shows that such VPMs with the same designed structure but varied thickness can convert fundamental light beam into optical vortices with different charges. Furthermore, we found that the mask performance is unaffected by the refractive index of the surrounding media making it an excellent candidate for microfluidic applications.
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