Generation and conversion of optical vortices in long-period twisted elliptical fibers

We theoretically demonstrate that long-period twisted elliptical fibers have the ability to change in a certain wavelength range the topological charge of the incoming field by two units. We also show that such fibers can generate charge-2 optical vortices from the incoming Gaussian beams.     

Mode coupling in strained and unstrained step-index plastic optical fibers

Using the power-flow equation, we have examined the state of mode coupling in strained and unstrained step-index plastic optical fibers. The strained fibers show much stronger mode coupling than unstrained fibers of the same types. As a result, the coupling lengths where equilibrium mode distribution is achieved and the lengths of fiber required for achieving a steady-state mode distribution for strained fibers are much shorter than the corresponding lengths for unstrained fibers.     

Nonlinear modes in optical fibers

It is shown that the electromagnetic field generated in an optical fiber operating in a nonlinear regime has the form of nonlinear guided modes—cnoidal (periodic) waves which transform into monochromatic fiber modes in the linear operating regime. The cnoidal modes are generated due to nonlinear polarization effects in the dielectric medium. These effects can be described by equations with sinusoidal nonlinearity.     

Generation of optical vortices in spun multihelicoidal optical fibers

We have theoretically studied long-period spun l-helicoidal fibers and their ability to generate singular beams from regular ones. On the basis of perturbation theory in the presence of degeneracy, applied to the scalar waveguide equation, we obtained the structure of coupled modes of such fibers and their spectra. It is shown that the coupled modes consist of the fields, which taken separately bear topological charges that differ by l units. We have numerically studied the process of the passage of a Gaussian beam through such a fiber and demonstrated that long-period l-helicoidal fibers have the ability to change-in a certain wavelength range-the topological charge of the incoming Gaussian beam by l units, generating in this way charge-l optical vortex.     

Bidirectional optical coupler for plastic optical fibers

We have developed a low-loss bidirectional optical coupler for high-speed optical communication with plastic optical fibers (POFs). The coupler, which is fabricated by an injection molding method that uses poly (methyl methacrylate), has an antisymmetric tapered shape. We show that the coupler has low insertion and branching losses. The tapered shape of the receiving branch reduces beam diameter and increases detection efficiency coupling to a photodetector, whose area is smaller than that of the plastic optical fiber. The possibility of more than 15-m bidirectional transmission with a signaling bit rate up to 500 Mbits/s for simplex step-index POFs is demonstrated.     

Thermal-stress-induced birefringence in bow-tie optical fibers

Birefringence induced by thermal stress in bow-tie optical fibers is studied in detail by the use of the finite-element method. Results of computer modeling show that a higher degree of birefringence can be obtained with the use of a larger cladding and larger stress-applying zones in the fiber.     

Radiation scattering in single-layer optical fibers

Translated from Izmeritel'naya Tekhnika, No. 6, pp. 17–18, June, 1991.     

Propagating modes in gain-guided optical fibers

Optical fibers in which gain-guiding effects are significant or even dominant compared with conventional index guiding may become of practical interest for future high-power single-mode fiber lasers. I derive the propagation characteristics of symmetrical slab waveguides and cylindrical optical fibers having arbitrary amounts of mixed gain and index guiding, assuming a single uniform transverse profile for both the gain and the refractive-index steps. Optical fibers of this type are best characterized by using a complex-valued v-squared parameter in place of the real-valued v parameter commonly used to describe conventional index-guided optical fibers.     

Polymer optical fibers and nonlinear optical device principles

Basic principles of the present optoelectronic research activities at Hoechst AG are presented in this contribution. Within the diversified broad band spectrum of optoelectronic materials and applications our interest is focused on new types of polymer optical fibers and nonlinear optical device principles using organic Langmuir–Blodgett films and electrically poled polymers. The basic principles and the limits of the present research and development activities which, have a high market potential are outlined.     

Quantum cryptography using optical fibers

Quantum cryptography permits the transmission of secret information whose security is guaranteed by the uncertainty principle. An experimental system for quantum crytography is implemented based on the linear polarization of single photons transmitted by an optical fiber. Polarization-preserving optical fiber and a feedback loop are employed to maintain the state of polarization. Error rates of less than 0.5% are obtained.     

Cross talk in absorbing optical fibers.

Cross talk between neighboring optical fibers is shown to alter the absorption spectrum of the fibers. There is an enhancement of absorption at shorter wavelengths and a consequent shift of the absorption peak towards the short-wavelength end of the spectrum. Curves are presented to illustrate the effect. Results are given in a dimensionless form, applicable to fibers with arbitrary physical parameters.     

Vortex optical Magnus effect in multimode fibers

A theoretical and experimental analysis is made of the optical Magnus effect in multimode optical fibers excited by a laser beam whose wavefront has a pure screw dislocation and carries the topological charge ±l, where l is the azimuthal quantum number. It is found that the angular rotation of the plane of propagation of a local wave depends on the magnitude and sign of the topological charge and changes qualitatively when the circulation of the polarization is reversed. The phase mechanism is attributed to spin-orbit interaction in the photon ensemble. It is demonstrated experimentally that the optical Magnus effect in a few-mode fiber for the CP₁₁ mode at the beat length is observed as a rotation of the axis of the pure edge dislocation field through an angle proportional to the beat length. Pis’ma Zh. Tekh. Fiz. 23, 76–81 (August 26, 1997)     

The diffusion of water in optical fibers

The diffusion of “water” (OH) into the core of optical fibers can affect the transmission properties. An increase in the water will result in an increased loss of high order modes, a reduction in numerical aperture, and increased microbending losses if there is coupling to these higher order modes. An extra 10ppm OH uniformly distributed through the core would produce 12dB/km loss at 0.95μm and about 1dB/km loss at 0.90μm.Based on the extrapolation of existing diffusion coefficient measurements and solubility data, calculations are made for the diffusion of atmospheric H₂O through the cladding into the core and also for the diffusion of OH from the cladding into the core. Diffusion can occur fairly rapidly during the high temperatures reached when depositing, collapsing, and drawing the fibers, and also slowly during extended period ambient temperature storage and use.     

Tunable four-wave mixing in low-mode-number optical fibers

Partially degenerate four-wave mixing for different pump wavelengths in step-index optical silica fibers with low mode numbers is considered. The frequency shifts that correspond to the phase match and the overlap integrals are calculated for a tunable pump. It is shown that when the interacting waves propagate in some combinations of the modes LP(01) and LP(11), four-wave mixing is possible for tunable pump wavelengths in the region from 0.4 to 0.9 μm, far from the minimum of the material dispersion.     

Thermal focusing of radiation in optical fibers

The influence of external thermal factors on optical fibers is considered. The phenomenon of periodic breakdown observed in gradient-index fibers can be explained by radiation focusing, by which the intensity can grow up to a level sufficient to produce damage of the core material.     

Mode coupling in large-diameter polymer-clad silica fibers

Intermodal diffusion in a large-diameter plastic-clad silica step-index fiber is measured with a near-field projection of the modal pattern onto a CCD camera. The diffusion process is found to obey a 1/θ(2) dependence (where θ is the axis-crossing angle of the mode). The diffusion constant is estimated to be 1.3 × 10(-5) rad(2) m-(1). The contribution of mode mixing to evanescent-wave spectroscopy in such fibers is discussed. When all bound meridional modes are launched, the effect of diffusion is a small reduction- some 10% maximum-in the fraction of evanescent-wave power outside the core for the 0.4 numerical aperture fiber used.     

Modulation transfer functions of optical fibers

Translated from Izmeritel'naya Tekhnika, No. 6, pp. 24–25, June, 1992.     

Dispersion characteristics of segmented optical fibers

A direct scalar two-dimensional routine based on the method of lines is implemented to analyze the dispersion characteristics of segmented fibers. In this kind of structure, dispersion control in a simple profile is achieved by variations in the filling ratio of the coaxiallike structure.