Showing papers on "Single-mode optical fiber published in 1975"

Mode Coupling Effects in Graded-index Optical Fibers

Abstract: Mode coupling theory is applied to the study of multimode optical fibers with graded-index cores. For coupling caused by random bends in the waveguide axis, the results predict the dependence of the induced losses on the fiber's characteristics. The impulse response is determined for fibers with random bends having several different power spectra. The results are used to predict the transmitted power, the delay time, and the rms pulse width in fibers with graded-index cores.

Mode conversion coefficients in optical fibers.

Abstract: A simple method has been devised for the experimental determination of mode conversion coefficients in multimode fibers and involves only the observation of the far-field output as the angle of incidence of a collimated input beam is changed. The normalized mode coupling coefficient in a liquid-core fiber is D = 3 x 10-6 rad2 / m and increases by as much as a factor of 10 when transverse pressure is applied. Values some 2 orders of magnitude larger are found in glass-core fibers. There is good agreement between the theory presented and experiment.

Threshold characteristics of multimode laser oscillators

Abstract: The oscillation characteristics of a multimode laser are considered in detail, and for the first time saturation by all of the cavity modes is included. Analytical expressions are obtained for the mode amplitudes, linewidths, and over‐all spectral characteristics of lasers operating above and below threshold. It is found that with increasing excitation the spontaneous emission spectrum is gradually resolved into a set of narrow discrete modes. Slightly above threshold the number of modes reduces in a homogeneoulsy broadened laser until only a single mode is oscillating strongly. The theoretical results are in agreement with experimental data that have been reported using semiconductor lasers. With inhomogeneous broadening the spectral width goes through a minimum near threshold.

Polarization optics of index-gradient optical waveguide fibers.

Abstract: An investigation of the polarization optical properties of index-gradient glass fibers confirmed the applicability of the linear birefringent model to fibers of this type. The phase retardation (delta(s) = 58 degrees /cm) is homogeneously distributed along the whole length of the fiber. The depolarization occurring when the incident polarization angle coincides with the fast (or slow) axis of the fiber is negligibly small. The measuring technique employed allows the phase retardation between the wave components in the privileged directions within the fiber to be determined as a function of the temperature (Deltadelta(s)/DeltaT = 0.09 degrees /K cm). As the fiber is twisted the polarization vector rotates synchronously with the twisted endface of the fiber.

Surface-wave radiation loss from curved dielectric slabs and fibers

Abstract: A newly developed systematic procedure to calculate the real and imaginary parts of the change in propagation constant of a surface-wave mode on a curved open waveguide of general cross section is used here to determine these quantifies for the TE modes of an asymmetric slab waveguide and for all the modes of an optical fiber. Comparison of these results with the existing literature points up the care which must be taken in making approximations in this problem, since errors of significant factors in the radiation loss have been made in previous work.

Mode coupling in a multimode optical fiber with microbends

Abstract: Experimental observations of the radiation pattern of a short multimode fiber with different induced deformations are reported here. Theoretical calculations are also made, based on the assumption of strong coupling between two particular modes of the fiber, and are in good agreement with experiment. We obtain numerical evaluations of the coupling coefficients and excess loss as a function of the amplitude of the deformation, and indications on the requirements to be met in cabling the fibers are derived from the experimental results.

Light scattering from optical fibers with arbitrary refractive-index distributions

Abstract: A scattering theory is presented, which is capable of predicting the light-distribution pattern that arises when a clad optical fiber of arbitrary core refractive-index distribution is illuminated by a laser beam perpendicular to its axis. Theoretical predictions are compared to experimental results in the backscattering direction, with excellent agreement for fibers of practical interest having core and cladding diameters on the order to 50 and 150 μm, respectively. Considerable sensitivity to parameter variations has been observed and implications for diameter and refractive-index distribution determinations are discussed.

Excitation of parabolic-index fibers with incoherent sources

Abstract: We study the excitation of a parabolic-index fiber by an incoherent source. The theory is based on approximating the fiber modes by Laguerregaussian functions. The dependence of the total light power injected into the fiber core on the separation between source and fiber, and on the transverse displacement of the source, is shown in graphic form. Also shown are far-field radiation patterns, which indicate the distribution of power versus mode number, for several launching conditions and plots of power versus azimuthal mode number for given values of the compound mode number. The study of launching efficiency versus source radius leads to prescriptions for optimizing the ratio of source to fiber core radius without a matching lens. Use of a lens for matching the image of a small source to the size of the fiber core increases the launching efficiency relative to the power consumption of the light-emitting-source diode.

Excitation of the fundamental and low-order modes of optical fiber waveguides with gaussian beams. 2: offset beams.

Abstract: The excitation efficiencies of the dominant and low-order modes in an optical fiber are theoretically determined for an incident Gaussian beam that is offset with respect to the fiber axis. The excitation efficiencies are shown to be strongly influenced by both the numerical aperture of the fiber and the wavefront curvature of the incident beam.

Angle selective coupler for optical fibers

Abstract: There is disclosed an angle selective coupler for coupling optical energy into and/or out of optical fibers. The coupler consists of a section of the optical fiber modified in such a way as to allow optical excitation of a plurality of higher order modes of optical transmission, each mode being defined by a given angle of propagation relative to the fiber axis. In one embodiment the coupling section comprises a single strand of glass fiber waveguide which is tapered along its length. The existence of the tapered section allows coupling of radiation from an external source into a given propagation angle in the fiber. In another embodiment no taper is used, but the fiber cladding is etched down to a thin layer and higher index material is deposited over the thin layer. This allows coherent coupling of radiation into one or more higher order modes in the optical fiber as determined by the angle of the incident light beam with respect to the cladding of the fiber. In either embodiment the device uses total internal reflection to function as an angle selective input or output connector for efficient coupling of optical energy into a given propagation angle within the fiber waveguide. The existence of a plurality of such angles permits the coupler to be used in a modal multiplexing duplexer.

Low dispersion optical fiber wave guiding structures with periodically deformed waveguide axis

Abstract: The disclosure is directed to optical fiber waveguiding structures, and techniques for fabricating the same, which are designed to have reduced modal dispersion as a result of the deliberate enhancement of mode coupling in the fibers. The mode coupling is achieved in each of the structures by means of an enveloping outer jacket, typically of plastic, which is selectively deformed so as to modify the straightness of the fiber axes in a spatially periodic manner. Several alternative techniques are presented for properly deforming the outer jacket either before or after it is combined with the fibers. Since, with these structures and techniques, the outer jacket provides the desired fiber perturbations, the fibers themselves can be fabricated using standard techniques without the need for any special procedures for modifying their parameters.

Single mode operation of cw dye lasers

Abstract: This paper describes an investigation of the special problems of single mode operation in cw dye lasers. Single mode operation in homogeneously broadened lasers can be obtained, if the gain due to spatial hole burning is compensated by dispersive elements, as prisms, Fabry-Perot etalons and single dielectric interfaces within the resonator. The gain and the influence of the dispersive elements were calculated for two commonly used types of resonators. The calculations make it possible to design simple continuously tunable single-mode systems.

Modes in fiber optical waveguides with ring index profiles

Abstract: Calculations have been made of mode properties of optical fibers with ring index profiles. Three idealized profiles with regions of depressed index on axis were chosen for study. The quantities of interest are the propagation constant for various modes, the group delay, and the near and far field mode patterns. Near and far field patterns are compared with Gaussian TEM laser modes. The results are particularly relevant to studies of nonlinear interactions in fiber waveguides.

Optical waveguide transmitting light wave energy in single mode

Abstract: An optical waveguide for optical communication transmitting light wave energy in a single mode composed of a core of a first transparent dielectric material and successive layers of a second and a third transparent dielectric material coaxially covering the core, the refractive index of the first dielectric material being higher than that of the third dielectric material and the refractive index of the second dielectric material being lower than that of the third dielectric material.

Mode coupling in an optical fiber with core distortions

Abstract: The variations in the geometry of a step-index optical fiber are determined as functions of position along the axis of the fiber by an analysis of the backscattered light produced when a beam from a cw laser is incident perpendicular to the fiber axis. The power spectrum computed from this distortion function is then utilized with coupled-mode theory to predict the mode coupling, the reduction in pulse dispersion, and the accompanying increased radiation loss of the fiber. The theoretical calculations support experimental observations and account for a partial reduction in the multimode pulse dispersion.

Higher-order mode fiber optics t-coupler

Abstract: A higher order mode, simplex, fiber optics T-coupler is based on using only the higher order lossy modes of single fiber optics whether the cable is single fiber or a fiber bundle. This allows for T-couplers that discriminate on modes.

Length dependence of pulse dispersion in a long multimode optical fiber.

Abstract: Shuttle pulse measurements performed on a 1280-m long multimode optical fiber were used to determine pulse dispersion along a 6400-m extrapolated length. The fiber's steady-state mode coupling length was determined without breaking the fiber. The impulse responses for one, three, and five trips through the fiber were recorded and Fourier transformed to yield the corresponding baseband frequency responses. It was concluded that the coupling length was approximately 840 m, and that the impulse response became increasingly more symmetrical for fiber lengths beyond this coupling length. This agrees with theory.

Radiation losses of the HE 11 mode of a fiber with sinusoidally perturbed core boundary

Abstract: The radiation losses of the HE11 mode of a fiber with sinusoidal radius fluctuations are calculated in the weak guidance approximation. The loss formula is derived by means of the volume current method and also by using coupled mode theory. The result presented here extends earlier results to the case of radiation escaping nearly perpendicular to the fiber axis.

Skew rays in optical fibers.

Abstract: In order to justify the use of meridional analysis, the authors solve the Eikonal equation without approximationin two cases of index profile. Limit conditions are studied and attention is given to skew rays with inclination over the fiber axis greater than the critical angle of the meridional case. It is shown that skew rays introduce a queue in the impulse response of a fiber. In the case of solid state laser sources, they do not carry significant energy, and then can be neglected.

Measurement of very short optical delays in multimode fibers

Abstract: We describe a new method for measuring group‐delay differences between the modes of an optical fiber of the order of 1 psec. This is accomplished by Fourier analyzing the intensity output of the fiber, which is fed with a quasimonochromatic signal whose frequency is linearly modulated in time. We present the results of an experiment, in which time delays of the order of 10 psec have been measured. This confirms the potential of the method for measuring dispersion in optical fibers.

Dispersion-Optimized Optical Single-Mode Glass Fiber Waveguides

Abstract: The optical dispersion in singly clad dielectric monomode waveguides is calculated by taking into account the frequency dependence of the refractive indices of the core and the cladding materials. The total dispersion is composed of the material dispersion and the dispersion of the waveguide structure. Fibers can be optimized in such a way that these two portions have opposite signs. The portion due to material dispersion, however, is about 2 orders of magnitude greater than that due to waveguide structure dispersion (considering typical glasses at wavelengths of 0.9 μm and 1.06 μm). Thus, complete compensation of these portions is impossible, and the fibers cannot be made dispersion-free at these wavelengths. The total dispersion at 1.06 μm, however, is smaller than at 0.9 μm by a factor of ⅔.

Optical fiber transmission compensator

Abstract: In an optical fiber, light energy is transmitted in a number of modes having characteristic group velocities, and it is frequently necessary to limit transmission to modes whose group velocities are not excessively disparate. The invention avoids this limitation by providing a compensator (preferably) midway between two stations. The compensator pairs a fast mode in a fiber leading to one of the stations with a slow mode in a fiber leading to the other station, so that the group velocities for all pairs are approximately equal. The compensator comprises first, second and third focusing systems which are aligned with the fiber ends. The first system focuses energy received from one fiber into concentric rings whose radii are characteristic of the modes used in the fiber. The second system transposes, to a ring of maximum radius, energy focused at a ring of minimum radius, and vice versa. The third system delivers transposed energy to the other fiber in modes determined by the radii of the rings to which the energy has been transposed.

Advances in Communication through Light Fibers

Abstract: Publisher Summary This chapter focuses on the advances in communication through light fibers. Recent breakthrough in the purification of glass has made it possible to manufacture very low-loss optical fibers. The fibers provide almost all the attractive features of an ideal optical transmission line, such as, low cost, small size, lightweight, flexible, larger bandwidth, high tensile strength, no short circuiting issues, resistant to nuclear radiation, and heat resistance. These optical fibers are used as the communication media for transmitting optical signals. The bandwidth of a single-mode fiber is limited by the dispersion of the normal mode and by the material dispersion effects. One of the unique characteristics of a dielectric fiber waveguide is that below a certain frequency called the cut-off frequency of a particular mode, that mode simply no longer exists. A fiber of a given cross section will support a finite number of guided modes and a continuous spectrum of radiation modes. Any irregularity in the fiber because of diameter variation, bending, interface irregularities, or the presence of scattering centers will produce coupling of the energy of one mode to the others. Mode coupling within an optical fiber may produce the desirable effect of reducing the delay distortion that result from uncoupled multimode operation or may produce the undesirable effect of signal contamination.

Multiple mode magnetometers

Abstract: A magnetometer in which at least two modes of drive are employed in exciting sensor and/or sample so that the sensor provides an output indication of magnetic moment which is a function of both drive modes. The drive modes may be periodic at specific driving frequencies, aperiodic or may comprise a unidirectional portion of a mode. The output of the sensor contains components representative of the frequency or frequencies of the first mode modulated at the frequency or frequencies of the second mode. This sensor output is then analyzed to provide an indication of sample magnetic moment as a function of the frequency components of both modes of drive in order to eliminate contributions to the indicated magnetic moment from background signals or other perturbations which may be present under single mode operation. The multiple mode magnetometer is useful in several embodiments for production line applications, investigations of magnetic properties of inhomogeneous magnetic materials, as well as for general magnetic moment measurements. The concept of a multimode magnetometer also may be extended to include a multiplicity of modes.

Propagation mode converting optical coupler

Abstract: PURPOSE:To oscillatably couple only the light beam of foundamental mode of an optical fiber among the output optical beam from a semiconductor laser to said optical fiber.