Graded-index polymer optical fiber for high-speed data communication

We successfully obtained a high-bandwidth (1 GHz km) and low-loss (90 dB/km at 0.572 μm of wavelength) graded-index polymer optical fiber by using the interfacial-gel polymerization technique, in which we used an unreactive component to obtain the quadratic refractive-index distribution. This high-bandwidth graded-index polymer optical fiber makes it possible to transmit a high-speed optical signal in a short-range network, which is not possible when we use the step-index type of polymer optical fiber commercially available.     

Direct deflection method for determining refractive-index profiles of polymer optical fiber preforms

We present a method for determining the refractive-index profile of polymer optical fiber preforms through a direct-deflection measurement. The method is simple to use, compact, and has good resolution. The profile is obtained from the deflection data by numerically integrating the differential-ray equation for a radial refractive-index gradient. Corrections for topographical deviations are also discussed. Results for both graded-index and step-index fibers are presented.     

Comparison of the filtered backpropagation and the filtered backprojection algorithms for quantitative tomography

We compare the filtered backpropagation algorithm with the filtered backprojection algorithm for reconstructing the complex refractive-index distribution of semitransparent, cylindrical objects. Before reconstruction, the recorded scattered light is propagated back to the reconstruction area by inverse diffraction. Our comparison is based on computer-simulated data, and experimental optical data obtained from fibers with step-index, graded-index, and uniform-index distributions. The results show that both the filtered backpropagation algorithm and the filtered backprojection algorithm can produce accurate reconstructions of the complex refractive-index distribution as long as the weak-scattering approximation is valid. The good agreement between the results obtained from these two reconstruction algorithms indicates that the errors introduced by the assumption of straight-line propagation inside the object are negligible compared with those introduced by the weak-scattering approximation.     

Scanning near-field optical microscopy as a tool for the characterization of multimode interference devices

We report the scanning near-field optical microscopy (SNOM) characterization of a 4 x 4 multimode interference (MMI) device working at a wavelength of 1.55 microm and designed for astronomical signal recombination. A comprehensive analysis of the mapped propagating field is presented. We compare SNOM measurements with beam-propagation-method simulations and thus are able to determine the MMI structure's refractive-index contrast and show that the measured value is higher than the expected value. Further investigation allows us to demonstrate that good care must be taken with the refractive-index profile used in simulation when one deals with low-index contrast structures. We show evidence that a step-index contrast is not suitable for adequate simulation of our structure and present a model that permits good agreement between measured and simulated propagating fields.     

Optimum refractive-index profile of the graded-index polymer optical fiber, toward gigabit data links

The optimum refractive-index distribution of the high-bandwidth graded-index polymer optical fiber (POP) was clarified for the first time by consideration of both modal and material dispersions. The ultimate bandwidth achieved by the POP is investigated by a quantitative estimation of the material dispersion as well as the modal dispersion. The results indicate that even if the refractive-index distribution is tightly controlled, the bandwidth of the graded-index POP is dominated by the material dispersion when the required bit rate becomes larger than a few gigabits per second. It is also confirmed that the material dispersion strongly depends on the matrix polymer and that the use of a fluorinated polymer whose material dispersion [-0.078 ns/(nm km)] is lower than that of poly(methyl methacrylate) [-0.305 ns/(nm km)] allows for a 10-Gb/s signal transmission.     

Power losses in deformed graded-index polymer optical fibers

We investigate the power losses in bent and elongated graded-index polymer optical fibers (GI POFs). The variations of power losses in deformed GI POFs for various radii of curvature and elongations are measured. A simple tensile test result is used to calculate the average plastic energy density (APED) in a deformed GI POF at various elongations. The results indicate that the APED accumulated in a deformed GI POF can be considered as a key index to study the power loss in POF. Based on the experimental results, a curve-fitted equation is proposed to estimate the power loss using the APED for various radii of curvature. The maximum difference between the proposed equation and the experimental results is less than 3% for the deformed GI POFs.     

Ray-trace aberrations of curved graded-index media

Numerical studies of ray-trace aberrations of a curved graded-index medium as a function of launching conditions and refractive-index profile are reported. The variation of meridional and skew ray aberrations with the launching angle for specific point objects shows that the axis of symmetry of the medium shifts on curving. Further, the aberrations of rays emanating from a point object increase or decrease with curvature, depending on the position of the object point with respect to the new axis of symmetry.     

Evaluation of some GRIN fiber parameters and the associated fraction mode loss due to mechanically induced optical anisotropy

Some of the optical parameters of the bent multimode graded-index (GRIN) optical fiber in terms of indices of refraction, where the bending stresses broke the radial symmetry, are evaluated by use of multiple-beam Fizeau fringes. The variation of the index difference between the cladding index and core index in both the compression and tensile fiber regions is measured. The accuracy of measuring the index is +/- 1 x 10(-4). The spatial resolution of the method is 1.39 microm. Evaluation of the acceptance angle, the numerical aperture, and the V number profiles of the bent fiber from the interference pattern at both sides of the bent fiber are presented. The fraction of the mode number lost has been evaluated. The method was used to study the influence of compression on diminishing the index difference that leads to a dissipation of energy and a considerable mode loss. It is obvious from the experimental data that the change of the index difference due to bending strongly affects the fraction of propagating mode number, especially at the small radii of curvature. Ignoring the variation of the index difference we evaluating the number of propagated modes number leads to an insufficient determination of the mode loss. It subsequently leads to an incorrect determination of the mode dispersion and the interface loss in bent GRIN fibers. The study confirms that the deviation of the guide axis from straightness with the radius of curvature of less than 1 cm could lead to a significant fraction mode loss.     

Graded-index silver chlorobromide fibers for the mid-infrared

We have developed and characterized graded-index optical fibers for the mid-IR spectral range, based on silver chlorobromide (AgClBr) crystals. A preform was fabricated by inserting a cylindrical rod made of AgCl(0.1)Br(0.9) into a tube made of AgCl(0.9)Br(0.1). The preform was heated in an oven, causing diffusion of Cl into the outer layer of the rod, thus reducing its index of refraction. The rod was removed from the tube and was then extruded through a die to form a graded-index fiber. Such a fiber was analyzed, investigated, and compared with a step-index fiber made of AgClBr. The attenuation of a 0.9-mm-diameter graded-index fiber was found to be 2.4 dB/m, and the attenuation of a 1.2-mm-diameter graded-index fiber was 4 dB/m at 10.6 microm.     

Index profiling of anisotropic graded-index planar waveguides from effective indices

We present a method for recovering the refractive-index profile of an anisotropic graded-index waveguide from the effective indices by using a cubic spline interpolation function. The first and second derivatives of cubic splines are continuous to ensure a smooth index profile, which is consistent with practical graded-index distributions. A straightforward iteration with an exact dispersion equation to verify the interpolated profile makes this method easy and reliable for application. This approach is proved by numerical analysis of several typical index distributions and experimental examples showing that the refractive-index profiles in anisotropy can be reconstructed close to the exact profile. Waveguides with both more modes (more than four guiding modes) and fewer modes (two to four) can be universally profiled with good accuracy.     

Influence of refractive-index mismatch in high-resolution three-dimensional confocal microscopy

The effects of the refractive-index mismatch in confocal laser scanning microscopy were extensively studied. The axial aberration induced in the case of fluorescent microspheres was measured. The data were used to take into account the mismatch-induced aberrations and to consider object-size influence. Then we focused on the effect of refractive-index mismatch on the effective system's point-spread function under different mismatch conditions and on depth of focusing. We experimentally verified that the peak of the point-spread function intensity profile decreases and the point-spread function itself progressively broadens as a function of the combined effect of the refractive-index mismatch and of the penetration depth, leading to a worsening of the system's overall performances. We also performed these same measurements by embedding subresolution beads in an oocyte's cytoplasm, which can be considered a turbid medium. We found evidence consistent with the previously developed theoretical model; in particular we found a strong dependence of the intensity peak on the focusing depth.     

Measuring cylindrically symmetric refractive-index profiles: a method

This paper describes a new nondestructive method to measure cylindrically symmetric refractive-index profiles of transparent cylinders. The technique is based on the measurement of the axial displacement of rays that are refracted within the cylinder. Three different types of index profile were experimentally determined. Profile errors of better than one part in 10(3) were achieved using very modest equipment. The effects of certain experimental parameters on the profile accuracy are noted. The technique may be applied to the characterization of optical fiber preforms and graded-index rod lenses.     

In-line optical fiber sensors based on cladded multimode tapered fibers

The use of uniform-waist cladded multimode tapered optical fibers is demonstrated for evanescent wave spectroscopy and sensors. The tapering is a simple, low-loss process and consists of stretching the fiber while it is being heated with an oscillating flame torch. As examples, a refractive-index sensor and a hydrogen sensor are demonstrated by use of a conventional graded-index multimode optical fiber. Also, absorbance spectra are measured while the tapers are immersed in an absorbing liquid. It is found experimentally that the uniform waist is the part of the taper that contributes most to the sensor sensitivity. The taper waist diameter may also be used to adjust the sensor dynamic range.     

A dual modality optical fiber sensor

We propose and demonstrate a fibre optic system based on bi-tapered silica fibre that can simultaneously measure strain and fibre curvature. Both modalities on the signal can be extracted with no measurable crosstalk between them. The experimental signal has a pure phase modulation when strain is applied to the tapered fibre optic section of the sensor and the signal shows only intensity modulation when an un-tapered fibre section is bent. High sensitivity is achieved from the experimental results for strain and bending losses and the estimation of measurement errors is 0.2 and 0.1%, respectively. This system offers low-cost, compactness and it can be adapted for structural health monitoring.     

Analysis of an upside-down taper lens end from a single-mode step-index fiber

We introduce and analyze the upside-down taper lens end drawn from step-index fibers. Also, we model the refractive-index distribution and present the ABCD transformation matrix of this fiber end under paraxial approximation. The analysis can be useful for designing micro-optic image systems and laser diodes to single-mode fiber coupling optics.     

Guided waves in graded-index planar waveguides with nonlinear cover medium

Dispersion relations for TE modes in a planar exponentially graded-index waveguide with self-focusing nonlinear cover material have been solved numerically. It is shown that the threshold power required to pull the field maximum out of the film region into the cover is lower compared with that for the step-index waveguide and agrees well with the experimental results. Empirical relations to calculate the corresponding minimum film thickness and the minimum threshold power are given for the lowest-order mode.     

Surface plasmon resonance sensor using an optical fiber with an inverted graded-index profile

A new optical fiber sensor based on surface plasmon resonance is described. It uses an optical fiber with an inverted graded-index profile. A theoretical analysis of the optical propagation when a point light source was used and a computation of the optical power transmitted by the fiber were performed. Experiments were carried out to measure changes of the transmitted power caused by refractive-index variations of the surrounding dielectric medium. Both the simulation and experiments have shown that the sensor exhibits high sensitivity for changes of the surrounding medium in a refractive index range from 1.33 to 1.39.     

Optical time-domain reflectometer performance improvement using complementary correlated Prometheus orthonormal sequence

The authors deal with the pulse spreading approach using complementary correlated Prometheus orthonormal sequence (CCPONS) to enhance the performance of the optical time-domain reflectometer (OTDR). The approach has the advantage of reduced measurement time when compared with the OTDRs designed using the conventional or Hadamard derived codes. Also, it offers an improved dynamic range when compared with the conventional OTDR. We have demonstrated experimentally a high performance OTDR based on a CCPONS coding scheme and using a low-power Fabry Perot-laser diode. Using a 64-bit CCPONS coding, a one-way signal-to-noise ratio improvement of 3.7 dB was achieved over a conventional OTDR under the same measurement conditions, that is, for the same measurement time (20 000 averages) and same peak power (100 W).     

Specklegram in a grapefruit fiber and its response to external mechanical disturbance in a single-multiple-single mode fiber structure

Specklegram in multimode fiber has successfully been used as a sensor for detecting mechanical disturbance. Speckles in a multimode pure silica grapefruit fiber are observed and compared to that of a step-index multimode fiber, showing different features between them. The sensitivities to external disturbance of two kinds of fiber were measured, based on single-multiple-single mode (SMS) fiber structure. Experimental results show that the grapefruit fiber shows higher sensitivity than does the step-index multimode fiber. The transmission spectrum of the grapefruit fiber was measured as well, showing some oscillation features that are significantly different from that of a step-index multimode fiber. The experiments may provide suggestions to understand the mechanisms of light propagation in grapefruit fibers.     

Broadband parallel-fiber optical link for short-distance interconnection with multimode fibers

In a multimode step-index fiber the propagation angle of a beam is conserved over short distances even if the fiber is bent slightly. This behavior can be exploited for a multiplexed signal transmission by the assignment of different channels to different propagation angles [angle-division multiplexing (ADM)]. Thus parallel transmission can be achieved. Because each channel occupies only a subrange of the fiber's numerical aperture, modal dispersion is reduced compared with single-channel transmission through the same fiber. The transmission properties of an ADM-based transmission line are analyzed for short propagation distances. Passive all-optical setups for multiplexing and demultiplexing operations are proposed. Cross-talk measurements are shown for a transmission with a length of 8 m and 13 multiplexed channels.