Modal analysis of the self-imaging phenomenon in optical fibers with an annular core

We investigate the occurrence of self-images, or Talbot images, in a spatially multimode field that propagates along an optical fiber whose core has an annular-shaped cross section. By use of full-vectorial modal analysis, we study the effect of the transverse fiber dimensions on the self-imaging properties. According to our analysis, good self-images can be expected when the fiber core is thin and the modes are far from their cutoffs. However, as the core diameter is made larger to increase the number of modes available in the imaging, the general self-imaging properties tend to deteriorate.     

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.     

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.     

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.     

Investigation of optical fibers for gas-phase, ultraviolet laser-induced-fluorescence (UV-LIF) spectroscopy

We investigate the feasibility of transmitting high-power, ultraviolet (UV) laser pulses through long optical fibers for laser-induced-fluorescence (LIF) spectroscopy of the hydroxyl radical (OH) and nitric oxide (NO) in reacting and non-reacting flows. The fundamental transmission characteristics of nanosecond (ns)-duration laser pulses are studied at wavelengths of 283 nm (OH excitation) and 226 nm (NO excitation) for state-of-the-art, commercial UV-grade fibers. It is verified experimentally that selected fibers are capable of transmitting sufficient UV pulse energy for single-laser-shot LIF measurements. The homogeneous output-beam profile resulting from propagation through a long multimode fiber is ideal for two-dimensional planar-LIF (PLIF) imaging. A fiber-coupled UV-LIF system employing a 6 m long launch fiber is developed for probing OH and NO. Single-laser-shot OH- and NO-PLIF images are obtained in a premixed flame and in a room-temperature NO-seeded N(2) jet, respectively. Effects on LIF excitation lineshapes resulting from delivering intense UV laser pulses through long fibers are also investigated. Proof-of-concept measurements demonstrated in the current work show significant promise for fiber-coupled UV-LIF spectroscopy in harsh diagnostic environments such as gas-turbine test beds.     

Effect of metal coating on the optical losses in heated optical fibers

The effect of heating in air on the optical losses in metal-coated fibers has been studied. Two fibers were drawn from the same silica preform and coated by different metals (copper and aluminum). Dependences of a change in the optical losses on the temperature were measured in a 20–400°C range at a 50°C step. The optical losses of metal-coated fibers heated to temperatures below 300°C change mostly due to the microbending contribution. At temperatures above 300°C, the main contribution to increasing optical losses is due to the absorption on OH groups. It is established for the first time that the contribution to optical losses due to the OH groups is much more pronounced in Al-coated fibers than in Cu-coated ones. In addition, the Al-coated fibers exhibit growth in the optical losses above 300°C due the absorption on molecular hydrogen.     

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.     

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.     

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.     

Mode conversion in rectangular-core optical fibers

Mode conversion from the fundamental to a higher-order mode in a rectangular-core optical fiber is accomplished by applying pressure with the edge of a flat plate. Modal analysis of the near and far field images of the fiber's transmitted beam determines the purity of the converted mode. Mode conversion reaching 75% of the targeted higher-order mode is achieved using this technique. Conversion from a higher-order mode back to the fundamental mode is also demonstrated with comparable efficiency. Propagation of a higher-order mode in a rectangular-core fiber allows for better thermal management and bend-loss immunity than conventional circular-core fibers, extending the power-handling capabilities of optical fibers.     

Interfacial changes of optical fibers in the cementitious environment

Embedded optical fiber sensors have recently been employed for strain and crack monitoring in concrete structures. The performance of the sensor is strongly affected by the fiber/matrix interface. For strain monitoring, effective stress transfer between fiber and matrix is required. A high interfacial bond is therefore desirable. On the other hand, crack sensing may rely on fiber debonding and bending, which is only possible with a weak interfacial bond. In the cementitious environment, the interfacial properties are known to vary with time, and this may affect the long-term performance of embedded optical sensors. The objective of the present investigation is to study the interfacial changes when specimens containing embedded optical fibers (with different coatings) are subject to different environmental conditions including wet curing, wetting/drying and freezing/thawing. Fibers removed from the matrix are examined under the SEM. Also, fiber pull-out specimens are prepared and tested. The results show that the fiber pull-out test can reveal significant changes in interfacial behaviour that cannot be detected from SEM examination. The pull-out test is therefore demonstrated to be a useful technique for the characterization of time dependent interfacial behavior for embedded optical fiber sensors.     

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.     

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.     

Birefringence and internal stress in polystyrene optical fibers

The radial variations of the birefringence of the core of polystyrene optical fibers have been observed and characterized. Variations are much more important on the edge than along the axis, and they introduce a positive radial gradient of the ordinary refractive index that essentially governs light propagation. To explain the existence of such a birefringence, the conditions of the cooling of fibers during their drawing have been simulated. The radial variations of the temperature between the axis and the edge may increase by 10° and, because of the fast variation of the viscosity of polymers in the neighborhood of their glass transition, an important molecular orientation may result in the regions in which this transition first ocurs during the drawing. Comparison is performed between the behavior of polystyrene and poly(methyl methacrylate) core fibers.     

Observations of and applications for self-imaging in optical fibers

Experimental results of self-imaging in optical fibers are reported along with an analytical model that explains the observations. Some implications for sensor design are discussed.     

Apparatus for the alignment of single-mode optical fibers

An apparatus for the alignment of single-mode optical fibers is described. Design configurations are suggested for the aligning unit and the device used to process the error signal controlling the piezoelectric elements. The apparatus should be of interest to developers of fiber-optic systems as a special-purpose industrial accessory.Translated from Izmeritel'naya Tekhnika, No. 8, pp. 28–29, August, 1995.     

Investigation of the soda aluminosilicate glass system for application to fiber optical waveguides

An extensive investigation of the soda aluminosilicate glass systems has been conducted to determine the suitability of this material for low loss fiber optical waveguides. Based on measurements of scattering loss energy gap, and glass transition temperature we conclude that certain compositions of soda aluminosilicate glass have substantially lower measured scattering loss and less estimated absorption loss than pure fused silica, the best of the present waveguide materials. Scattering losses less than $\text{1}\text{4}$ that of pure fused silica have been observed.     

Analysis of optical fibers by the effective-index method

The effective-index method for determining waveguide dispersion is derived from the scalar wave equation and applied to optical fibers of arbitrary cross-sectional shapes. In the simplest use of the method, the optical fiber is replaced by an equivalent slab waveguide with an index profile derived from the geometrical shape of the fiber. Results from analyzing circular, elliptical, and cusp-shaped fibers are used to illustrate the general features of the method. A procedure is also given for improving the accuracy of the method applied to a class of single-mode fiber.