Showing papers on "Photonic-crystal fiber published in 1985"

Color Centers in Glass Optical Fiber Waveguides

Abstract: Color centers formed in the core and cladding of optical fiber waveguides by exposure to nuclear radiation can greatly increase the attenuation in the infrared spectral region of interest for optical communications. The radiation-induced paramagnetic defect centers in pure silica, silica doped with Ge, P, or B, and heavy metal fluoride glasses have been identified and thoroughly characterized by electron spin resonance (ESR) techniques. This paper will review the results of recent studies of color centers in optical fiber waveguide materials and their Identification via correlations of the radiation-induced optical absorptions and defect centers elucidated by ESR.

Overview Of Radiation Effects In Fiber Optics

Abstract: The optical properties of fiber waveguides can be degraded by exposure to nuclear radiation, primarily through the generation of color centers in the fiber core. This paper will review recent studies of the radiation-induced absorption in state-of-the-art fiber optics. Particular emphasis is placed on the development of more radiation resistant pure and doped silica core waveguides and on the understanding of the damage processes in these materials. The results of studies of radiation damage in high birefringent, polarization-maintaining fibers and in heavy metal fluoride glasses and fibers will also be reviewed.

Multimode fiber-lens optical coupler

Abstract: A novel expanded beam coupling arrangement for use in association with single mode fibers is disclosed. An appropriate length of multimode fiber is fused to the endface of an input single mode fiber, where the length of the multimode fiber is chosen to provide the desired lensing conditions of the input beam. The multimode fiber is thus used as a lens, but provides many advantages over prior art optical connectors which use conventional quarter-pitch GRIN lenses epoxied to the fiber endfaces. In particular, the misalignment associated with the epoxied arrangement is reduced since the multimode fiber-lens connector of the present invention may be chosen to comprise the same outer diameter as the single mode fiber. Additionally, the use of a section of optical fiber as a lens allows for a fused connection to be used instead of an epoxied connection, which results in a more stable and rugged interface between the fiber and the lens.

Determination of single-mode fiber coupler design parameters from loss measurements

Abstract: A method is presented for determining the minimum spacing between the fiber core centers in a single-mode fiber coupler which consists of two substrate blocks each provided with a fiber fixed in a curved groove. Immersion liquids with known refractive indices are placed on the top surface of one coupler block and the introduced power loss of the light guided within the fiber is measured. By fitting these results to calculated curves the minimum spacing may be determined with an error of less than 0.2 μm.

Method of making a polarization-insensitive, evanescent-wave, fused coupler with minimal environmental sensitivity

Abstract: An optical coupler and method of making same is described. The coupling ratio of the coupler is polarization-insensitive. The optical coupler described herein is made from single-mode optical fibers. Each optical fiber has a length of nearly exposed core which is fused to the exposed core of the other optical fiber while the fibers are maintained in parallel juxtaposition with one another without twisting. By creating a fused core coupler from single-mode optical fibers in which the cores are in parallel juxtaposition with one another, the coupling ratio of the subject invention does not change with changes in polarization of light passing through each single-mode fiber and, thus, is polarization insensitive.

How modal noise in multimode fibers depends on source spectrum and fiber dispersion

Abstract: The speckle contrast for multimode fibers, and thus the modal noise, is essentially given by the impulse response of the fiber and the power spectrum of the source. Theoretical and experimental results show that laser sources with a large spectrum of narrow longitudinal modes may cause high speckle contrast and important modal noise over more than 1-km length in graded-index multimode fibers.

Amplified spontaneous Raman scattering and gain in fiber Raman amplifiers

Abstract: The spectrum of amplified spontaneous Raman scattering and gain in a fiber Raman amplifer has been calculated analytically as a function of distance and pump power. The model used makes no assumptions on the magnitude of the gain and considers the pump nondepletion region. From the results, the on/off ratio has been calculated and is found to have a variation with length that depends on frequency detuning. An enhancement in on/off ratio is also found for small fiber lengths but at limited gain. The use of a narrow-bandpass optical fiber leads to quite different behavior of the on/off ratio than for the unfiltered case.

Low-loss laser-to-fiber coupling with negligible optical feedback

Abstract: A novel lens design for the coupling of single-frequency lasers to monomode fibers achieves both high coupling efficiency (> 40 percent, best value 70 percent) and extremely low optical power feedback ( \sim 10^{-7} of the emitted power). The lens is formed out of a ∼ 1 mm length of a silica thread fused to the monomode fiber, with the open end of the silica thread modeled as an aspherical lens.

High-birefringence polarizing fiber with flat cladding

Abstract: This paper presents the theoretical and experimental investigations on polarization characteristics of PANDA fiber with flat cladding. It is theoretically shown that the modal birefringence in the flat-clad fiber is almost the same as that in the circular-clad fiber. The flat-clad PANDA fiber has been fabricated by grinding off the preform cladding on the opposite faces and then drawing with a low furnace temperature. The modal birefringence of the fiber is B = 5.9 \times 10^{-4} and the separation of the bending loss edges for the two polarization modes is \Delta\upsilon = 0.51 in the normalized frequency. The polarizing region can be tuned from 1.3 to 1.56 μm by varying the fiber-bending diameter from 3.5 to 4.5 cm. A 1.5-m length of polarizing fiber exhibits extinction ratios of 44.9 and 44.4 dB with insertion losses for the guided mode of 0.25 and 0.41 dB at 1.3 and 1.56 μm, respectively.

Optical contact evanescent wave fiber optic coupler

Abstract: A single mode, evanescent wave, fiber optic coupler. The coupler comprises a pair of substrates, each substrate having an optically worked fiber embedded therein and wherein the substrates are secured together such that the optical fibers are in optical contact and are in effective fusion with each other.

Transmission properties of a multimode optical-fiber taper

Abstract: A simple and useful formula governing the skew rays propagating through a multimode fiber taper has been derived by using geometrical optics. Using this formula, the transmission properties of the fiber taper are studied. The total light transmitted and the effective numerical aperture for both the meridional rays and the skew rays are compared with those of a uniform fiber.

Similarities and Differences Between Fiber Acoustics and Fiber Optics

Abstract: An introductory comparison of guided wave properties and sensor applications between weakly guiding acoustic and optical fibers is given. The mathematical expressions of dispersion relations, cutoff c onditions, etc for the flexural, t orsional and radial-axial acoustic modes in a weakly guiding acoustic fiber a re shown to be identical to those for the hybrid, transverse electric and transverse magnetic optic modes in a weakly guiding optical fiber. But no analogy is found between the longitudinal acoustic mode and any optical mode. Cladded glass f iber waveguides using pure and doped fused silica as core or cladding materials can be used for e ither f iber acoustic or fiber optical waveguides or both simultaneously . For both types of waveguides, dispersion properties, loss mechanisms, coupling considerations and industrial applications are discussed.

Modal-insensitive biconical taper couplers

Abstract: The modal-insensitive coupler consists of a number N of optical multimode fiber lengths, each fiber length having a core of initial predetermined cross-sectional area. The fiber lengths are fused together in a coupling section such that the cores of the fiber lengths have a reduced cross-sectional area in the coupling section which supports a minimum number of modes of optical energy at a predetermined wavelength, and such that the fiber lengths have different core cross-sectional areas to produce preselected coupling ratios.

Fiber laser sensor

Abstract: A fiber laser sensor is comprised of two fiber lasers which are each optically coupled to a third fiber so that energy is exchanged between each of the laser fibers and the third fiber. The coupling is accomplished by arranging the laser fibers parallel to and on either side of a third fiber and properly spacing the three fibers to produce optical coupling along the length of the fibers. The central fiber ends are non-reflective so that light generated by one of the lasers and coupled to the central fiber is lost except for a small fraction of the energy transferred to the other laser. Accordingly, the structure operates with the least loss when the two lasers oscillate at a common frequency but 180° out-of-phase. The resonant cavities of the two fiber lasers are made non-identical by making the path length one cavity slightly longer than the other to produce Vernier tuning of the laser resonant frequency orders relative to each other with only one common resonant frequency. The device can be used as a sensor because the common laser output frequency is sensitive to changes in the optical path length of either or both of the laser cavities.

Optical coupling system with intermediate coupling medium

Abstract: An optical coupling system between a semiconductor laser and an optical fiber is disclosed. The system is characterized in that a medium having a refractive index greater than 1 but smaller than the refractive index of the semiconductor laser material is packed between the end surface of a distributed feedback type or distributed Bragg reflector type semiconductor laser and the end surface of an optical fiber.

A new confocal combination lens method for a laser-diode module using a single-mode fiber

Abstract: A new confocal combination lens method is proposed, which can overcome severe misalignment tolerances for an InGaAsP laser diode to single-mode fiber coupling. Lens 2 in the confocal two-lens method is divided into two lenses, that is, lenses 21 and 22. The optimal distance between lenses 1 and 21 is found to be shorter than that for a confocal condition. Lens 22 is attached to the input endface of the single-mode fiber, thus forming a virtual fiber. Using 0.06- and 0.18-pitch GRIN rod lenses as lenses 21 and 22, the lateral and axial misalignment tolerances for the virtual fiber are improved by three and nine times, respectively, in comparison with those for the single-mode fiber. High-coupling efficiency of -3.0 dB of the laser diode to the single-mode fiber can be easily obtained including Fresnel reflections.

Wavelength-selective coupling of two-core optical fiber: application and design

Abstract: Wavelength-selective coupling characteristics of evanescent fields between nonidentical single-mode fibers are investigated theoretically. We study the application of two-core fiber, which has two adjacent cores embedded in a cladding, especially for fiber-optical amplifiers, using three-wave mixing or stimulated Raman scattering. The theory shows how to minimize the power-transfer efficiency between two cores either at the signal or at the pump wavelength, while maintaining 100% power transfer at the other wavelength. As a result, owing to the wavelength-selective coupling, a periodic interaction of the signal and pump lights is expected only in one of the two cores. This permits an efficient amplification of signal light. The attractive feature of the use of the two-core fiber is that various optics, such as lenses, beam splitters, and grating filters, can be eliminated from the setup.

Optical fiber coupler

Abstract: A fiber optic coupler comprises a multimode fiber bus, an input branch fiber, and an output branch fiber. Coupling between the fibers is accomplished by means of a reflector, disposed in the core of the multimode optical fiber. The reflector is quite small, such that only a small fraction of the light propagating in the multimode fiber is incident thereon. The incident light is reflected towards the output branch fiber and coupled thereto through a lens. Light propagating in the input branch fiber towards the multimode fiber is focused on the reflector by means of a lens. The reflector is oriented to reflect the focused light in a generally cone-shaped beam directed longitudinally down the axis of a multimode fiber. Preferably, the cone-shaped beam diverges so that substantially all the modes within the acceptance cone of the multimode fiber are excited. Such excitation of the modes is preferably equalized by selecting the numerical aperture of the multimode fiber such that only the portion of the reflected beam which has a relatively uniform intensity is within the acceptance cone of the fiber. The invention is particularly advantageous for local area networks and distributed sensor systems.

Accurate analysis of single-mode graded-index fiber directional couplers

Abstract: We present an accurate coupled-mode analysis to evaluate the coupling characteristics of identical single-mode graded-index fiber directional couplers. We also give an empirical relation for calculating the coupling length of a fiber coupler formed by fibers having profile parameter q , normalized frequency V , and normalized separation d/a ; d is the distance between the fiber centers and a , the radius of the fiber core.

Method and means of removing claddings from optical fibers

Abstract: An optical fiber is processed by ablating cladding material with an intense electromagnetic energy source. The intensity of another beam directed along the propagation axis of the fiber is monitored to detect near exposure of the fiber core. The ablating electromagnetic energy source is preferably circularly polarized. Apparatus for performing this process is disclosed. Fibers produced by this process are used to form optical couplers or, when a metallic layer is provided over the nearly exposed core region, an optical polarizer.

Process for producing plastic optical fibers

Abstract: A process for producing a plastic optical fiber which consists of a core made of a homo- or co-polymer of alkyl methacrylate and a cladding made of a fluorine-containing polymer, which process comprising forming the cladding around the core and irradiating the optical fiber with an electron beam, the optical fiber produced by the process being used at a temperature higher than 80° C. without significant loss of light transmission.

Form polarizing fibers and method of fabrication

Abstract: A polarizing optical fiber has a core formed of a plurality of layers of dielectrics having different refractive indices. The dielectric layers form a composite structure having different refractive indices for light of different polarizations. The differing refractive indices cause the polarization states of light guided by the fiber to be non-degenerate so that energy ordinarily will not couple from one polarization to the other. The fiber includes a cladding that may have a refractive index either less than both core indices to provide a polarization maintaining fiber or greater than or equal to one of the core indices to provide a polarizing fiber. The method of fabrication of the layered core form birefringent optical fiber includes forming a structure of a plurality of layers of the dielectrics, heating the structure to form a monolith, stretching the monolith to form the fiber core, and adding the cladding to the core.

Stable fiber optic polarizer

Abstract: This invention relates to fiber optic apparatus and methods for polarizing light for use in fiber optic devices, such as rotation sensors A phase grating (52) formed of alternating layers of dielectrics (54,56) having different indices of refraction is placed adjacent a half coupler (12) that includes a fiber (20,22) having a cladding thickness which forms an interaction region The evanescent field of light of a selected polarization interacts with the phase grating (52) to couple out of the fiber (20,22) while light of a second selected polarization remains in the fiber (20,22)

Apparatus for measuring the cut-off wavelength of a single-mode optical fiber

Abstract: An apparatus for measuring the cut-off wavelength of a single-mode optical fiber includes two fiber bending devices one of which provides the fiber, during test, with a large radius bend and the other of which provides the fiber, during test, with a small radius bend. The outputs from the fiber during the large and small radius bending are compared in order to precisely measure the cut-off wavelength.

Optical wavelength demultiplexer

Abstract: An optical wavelength demultiplexer includes a retainer means which curvedly retains an optical fiber. A mirror-polished surface is formed on the convex side of a curved portion of the optical fiber which is retained by the retainer means in such a manner that the mirror-polished surface is in close proximity to the core of the optical fiber. Further, a diffraction grating is disposed such as to oppose and contact the mirror-polished surface. The pitch of the diffraction grating is set to be λ/2n f , where λ represents the wavelength of light to be separated, and n f the effective refractive index of the optical fiber. The direction of the pitch is selected to be coincident with the extending direction of the optical fiber.