Showing papers on "Dispersion-shifted fiber published in 1976"

Curvature loss formula for optical fibers

Abstract: The loss formula for optical fibers with constant radius of curvature of their axes is derived by expressing the field outside of the fiber in terms of a superposition of cylindrical outgoing waves. The expansion coefficients are determined by matching the superposition field to the field of the fiber along a cylindrical surface that is tangential to the outer perimeter of the curved fiber. This method is a direct extension of my derivation of the curvature-loss formula for slab guides.

Fiber waveguides: a novel technique for investigating attenuation characteristics

Abstract: Light from a pulsed GaAs injection laser is coupled into a glass fiber via a taper coupler. The time dependence of the light backscattered within the fiber as the pulse travels down the waveguide is recorded. From these data the total loss may be determined, as well as an estimate of the scattering and mode mixing characteristics of the fiber.

Material and Mode Dispersion in GeO2·B2O3·SiC2 Glasses

Abstract: Material and mode dispersion in fibers made from glasses in the system GeO2-B2O,3-SiO2 have been calculated using refractive index dispersion results for these glasses. Refractive indices were measured on bulk glass specimens using the minimum deviation method at wavelengths from 0.4358 to 1.0830 μm. The resultant data were then fitted to a 3-term Sellmeier dispersion relation for each glass composition. The fitted equations and a recent theory on the relation of the optimum index profile in a graded index optical waveguide to pulse dispersion were used to calculate the exponent α, which characterizes the shape of the profile, for several practical fiber models. The most effective profile shape for reducing pulse dispersion is a function of wavelength, composition, and fictive temperature. The present data were used elsewhere to predict accurately the optimum profile for minimization of pulse dispersion in several fiber systems consisting of germanium borosilicate glasses.

Optical fiber modes using stimulated four photon mixing

Abstract: Stimulated four photon mixing has been used to excite separately all the modes of a ten-mode optical fiber. Photographs were taken and the mode intensities scanned with a moving pinhole. Measured intensities were found to agree well with the intensities calculated for an ideal guide with an abrupt core-cladding interface. In addition a locking effect was observed in which two separate Stokes:anti-Stokes pairs, which normally occur at slightly different frequency shifts from the pump, locked together at the same frequency.

Silica based optical fiber waveguide using phosphorus pentoxide and germanium dioxide

Abstract: An optical fiber waveguide is disclosed wherein germanium dioxide (GeO2) and phosphorus pentoxide (P2 O5) are radially graded in the core of the optical fiber for the purpose of achieving minimum modal dispersion over a broad range of wavelengths. The phosphorus pentoxide has its maximum concentration on the axis of the fiber core and is graded to zero concentration at the core-cladding interface. The germanium dioxide has its maximum concentration at the core-cladding interface, is constant throughout the cladding and is graded to zero concentration on the axis of the fiber core. The ratio of maximum P2 O5 to maximum GeO2 is approximately 11.6 and therefore the numerical aperture is only slightly degraded as a result of the reverse doping of germanium dioxide.

Mode-dependent attenuation of optical fibers: excess loss

Abstract: A theory is presented for calculating the excess loss produced by random perturbations of optical fibers. The theory is applicable to perturbations whose longitudinal spatial frequencies are below the range required for mode coupling. To illustrate the method, losses due to diameter variations are calculated for the case of a step-index optical fiber. The diameter variations are found to produce a strong attenuation of the higher order modes. The total excess loss is approximately wavelength independent.

Speckle-pattern visibility of light transmitted through a multimode optical fiber

Abstract: We show that the reduction in visibility of the speckle pattern obtained from laser light that has been passed through a step-index light guide can be found from a knowledge of the model dispersion of the guide, and the bandwidth of the laser beam.

Method and apparatus for introducing geometrical perturbations in optical fiber waveguides

Abstract: The present invention relates to method and apparatus for fabricating multimode optical fiber waveguides having reduced modal dispersion as a result of deliberate enhancement of mode coupling in the waveguide. The axial alignment and/or the diameter of an optical fiber waveguide is varied by means of one or more modulated heat sources, such as, for example, a microflame jet or CO 2 laser, directed against the fiber waveguide. By controlling the modulation of the heat source, the spatial distribution of the geometrical perturbation pattern can be controlled, thereby producing controlled mode coupling in the fiber waveguide.

Noncircular symmetric optical fiber waveguide having minimum modal dispersion

Abstract: An optical fiber waveguide is disclosed wherein the cladding surrounds a fiber core having a noncircularly symmetric cross section. Equations are presented which dictate the value of the index of refraction that must be provided at each spatial point within the fiber core in order to construct a fiber waveguide having minimum modal dispersion. An especially useful optical fiber waveguide is disclosed with an elliptical fiber core thereby providing a fiber which can be more efficiently coupled to a light emitting source having an elongated source area such as in a heterojunction laser or an edge-emitting light emitting diode.

Graded-index fiber for multimode optical communication

Abstract: Graded-index fibers, which are the most promising for first-generation optical communication links, are improved for multimode transmission by employing two or more glassy dopants in the core mutually graded with respect to the silica host and to each other to provide simultaneously minimized intermode dispersion at a first-selected wavelength and, in addition, a broader wavelength range of minimized intermode dispersion, or minimized intermode dispersion at a second distinct wavelength pertinent to the multimode transmission, or optimization of another dispersion-related property of said fiber.

Reduction of modal and chromatic material dispersion in a multimode optical fiber

Abstract: Modal dispersion and chromatic material dispersion in a multimode optical fiber is reduced by feeding the rays of the light beam into the end of the optical fiber the angle of each ray relative to the axis of the fiber varied in accordance with the wavelength of the ray. The shortest ray is fed at a zero angle and the longest wavelength at angle of φ max. φ max. is defined by the equation ##EQU1## This invention relates to the reduction of both modal and chromatic material dispersion in a multimode optical fibre. Multimode guides are currently used in fibre-bundle optical communications and will also appear in the next generation of individual-fibre systems. Their 50 to 100μm diameter cores are compatible with multimode LED sources and simple connecting and tapping techniques. Unfortunately, their information carrying capacity is limited by the variation of signal group velocity both with mode number and with light wavelength; this presents a serious limitation with large numerical aperture fibres and broadband sources. Prior art in this field has concentrated on gradient-index optical fibres to approximately equalize all ray paths and hence reduce mode dispersion. However, these require fabrication procedures more complex than for step-index guides, are capable of accepting only about half as much light from an LED and neccessitate twice the curvature radius in bends. Source collimation to excite only low order modes can reduce mode dispersion. Differential mode attentuation reducing the role of higher order modes and intermodal coupling tending to average the resultant modal velocity are both effective, but these processes are inherently lossy. All the above methods reduce modal but not chromatic material dispersion. A technique for reducing chromatic material dispersion is disclosed in copending application, Ser. No. 591,498 filed June 30, 1975 in the name of the present assignee, now U.S. Pat. No. 3,988,614. The present invention provides for the suitable alteration of the angular light distribution entering the fibre so as to oppose the modal and chromatic effects, thereby achieving a substantial reduction in the net dispersive pulse broadening.

Angle selective fiber coupler

Abstract: Angle selective input coupling through the side of a slightly tapered section of Corning highly multimode fiber has been experimentally demonstrated for the first time. This coupling technique allows the possibility of fabricating bidirectional (duplex) couplers for systems employing single strands of multimode, low loss fiber.

Graded index fiber waveguides with borosilicate composition: fabrication techniques

Abstract: Fabrication techniques for graded refractive index fibers with SiO2–B2O3 composition are shown to be well controlled. The resulting fibers demonstrate both low loss (less than 3 dB/km) at 900 nm and 1060 nm and low dispersion (260 psec/km including 200-psec/km material dispersion) at 900 nm.

Wavelength dependence of frequency-response measurements in multimode optical fibers

Abstract: A newly developed technique for directly measuring fiber dispersion in the frequency domain as a function of wavelength is described Spectrally filtered white light from a xenon arc lamp is sinusoidally modulated in the range 0 to 1 GHz by an electrooptic modulator and injected into a fiber The procedure is to vary the modulation frequency and measure the corresponding sideband output power with a photomultiplier and spectrum analyzer Ratio measurements between the test fiber and a short reference fiber give the baseband frequency response A number of germanium- and boron-doped fibers have been examined The least dispersive borosilicate graded-index fiber has a 1 dB bandwidth of 1 GHz, after 107 km of propagation at λ = 908 nm The width broadens gradually with increasing wavelengths up to λ = 1100 nm

Mode mixing with reduced losses in parabolic-index fibers

Abstract: We present design criteria for the construction of a modified parabolic-index fiber with intentional mode coupling. Mode coupling serves the purpose of reducing multimode pulse dispersion and is accomplished by introducing carefully designed index fluctuations into the fiber core or by controlled “random” bends of the fiber axis. Radiation losses due to mode coupling can be minimized by terminating the parabolic-index fiber core in an abrupt index discontinuity. The additional modes introduced by this step must be filtered out by periodic mode filters that consist of parabolic-index fiber sections without the refractive-index step.

Measurements of wavelength dependence of group delay in a multimode silica fiber

Abstract: The wavelength dependence of the group delay in a multimode silica fiber was determined from the wavelength‐dependent phase shift of the sinusoidally modulated light transmitted by the fiber. When the lower‐order modes were dominant in the modal power distribution, the measured dispersion at a wavelength of 830 nm was 0.90 ns/10 nm/km. When the launching conditions were changed to excite more of the higher‐order modes, the measured dispersion decreased to 0.60 ns/10 nm/km, indicating that the material dispersion was compensated partially by the waveguide dispersion.

W fiber design considerations

Abstract: The baseband frequency characteristics and the average confinement of light to core properties of a W fiber are analyzed in conjunction with corresponding singly clad (SC) fibers under some ideal assumptions It is found that the bandwidth of a W fiber is almost equal to that of the SC fiber that has no intermediate layer and is much wider than that of the other SC fiber in which the intermediate layer extends to infinity. The average fractional power flow outside the core in a W fiber is drastically reduced in comparison with both SC fibers. Practical design considerations are deduced from a typical example.

Steady-state losses of optical fibers and fiber resonators

Abstract: We study the steady-state loss of a fiber with random, nearest-neighbor coupling and compare it with the mode with the lowest loss of a cavity formed from a section of the same type of fiber. We find that the loss of the cavity is not identical with the loss of the steady-state distribution of the fiber with random coupling. In fact, fiber and fiber resonator behave very differently if the fiber mode of highest order is made very lossy. The loss of the steady-state distribution of the fiber with random, nearest-neighbor coupling approaches a weighted average of the losses of its individual modes plus a contribution from the coupling coefficient that couples the highest-order mode to its neighbors. The cavity loss, on the other hand, becomes independent of the coupling coefficients and of the loss of the highest-order mode, provided this loss becomes much higher than the coupling strength. This behavior leads us to conclude that the loss of the cavity is a weighted average of the losses of all those modes whose coupling strength exceeds their (individual, uncoupled) loss coefficients. Two resonator modes with propagation constants β 1 and β 2 remain uncoupled unless they satisfy the condition β 1 − β 2 = 2πn/L, where n is an integer and L is twice the length of the resonator.

Transmission system for optical fiber

Abstract: PURPOSE:To improve the economy and reliability of the transmission system, by using the optical fiber having low dispersion and less pulse width extension and the stable photo amplifier.

Sweep frequency transfer function measurement applied to optical fiber

Abstract: Up to now, most of the experimental work on propagation through optical fibers was done using pulse measurements techniques. Short-light-pulse sources almost monochromatic and easy to drive to high-peak-power pulses are available on the optoelectronic components market; this is one of the main reasons why fiber manufacturers characterize their fibers by the broadening of the transmitted light pulse. This parameter does not provide enough information on the fiber behavior to the digital transmission system experts, unless one assumes a very-stable pulse shape at the fiber input and uses fast Fourier transform computation on time-domain signals. This paper presents the more classical technique which is being used on transmission cables and devices. The central unit is the network analyzer which can simultaneously provide attenuation versus frequency and phase rotation versus frequency of any optoelectronic device coupled between the reference and the test inputs. Various light sources have been selected for their sinusoidal modulation facilities; most of our measurements were made with fast-light-emitting diodes (GaAlAs 0.82 μm) and some of them with a CW injection laser (GaAlAs 0.82 μm).

Optical fiber producing apparatus

Abstract: PURPOSE:To uniformalize the diameter of an optical fiber by controlling the fiber diameter in accordance with the measured level of the variation in the diameter of a preform.

Proximal scanning systems: improved resolution using inclined optical fibers.

Abstract: The illumination of a plane beneath the radiating aperture of an inclined multimode optical fiber is calculated. Graphical results are presented that show, for a proximal scanning head made up of fiber triplets composed of a central sensing fiber and two illuminating fibers, inclining the illuminating fibers away from the sensing fiber provides improved illumination of the work surface. The improvement in illumination obtained allows closer spacing of the scanning head to the work surface, thus improving resolution. A simple geometrical formula is presented that enables a prediction to be made of the fiber inclination for optimum illumination. This formula holds for large variations in the shape of the fiber polar sensitivity function.

An Ideal Refractive-Index-Distribution and Mode Filter for Band Broadening of Multimode Optical Fiber

Abstract: The focusing fiber whose refractive index distribution has the valley at the periphery of the core, as the ideal index distribution with respect to the bandwidth, is analyzed exactly, by which the group delay spread caused by the core-cladding boundary can be eliminated, and, therefore, the bandwidth can be broadened.