Showing papers on "Polarization-maintaining optical fiber published in 1985"

Polarization-state control schemes for heterodyne or homodyne optical fiber communications

Abstract: In heterodyne or homodyne optical fiber communications, the fluctuation of the polarization state in the fiber may deteriorate the receiver sensitivity. This paper reviews the principles and features of six polarization-state control schemes so far proposed, including two new attempts, one using Faraday rotators and the other using rotatable fiber cranks (RFC's), both of which have been proposed very recently. Experiments simulating actual optical heterodyne receivers are also described.

Theoretical analysis of optical fiber laser amplifiers and oscillators

Abstract: Using the formalism of mode overlap, a theoretical analysis of optically pumped fiber laser amplifiers and oscillators is developed. The concept of normalized overlap coefficients is introduced to account for the effects of the transverse structure of the interacting signal and pump modes on the device characteristics. Simple and accurate closed-form expressions are derived for the gain of fiber amplifiers and the threshold and energy conversion efficiency of fiber laser oscillators in terms of the fiber and laser material parameters and the pump and signal modes. When applied to step-index Nd:YAG fiber lasers, this study predicts optimum fundamental mode oscillation in fibers with a V number of 5-25 with submilliwatt thresholds and nearly quantum-limited conversion efficiencies.

Pulse compression in optical fiber filters

Abstract: A method is described for compressing optical pulses at any wavelength in an optical fiber. It uses the negative dispersion property of permanent phase gratings created within the fiber. No external grating pair is required.

Distributed, spatially resolving optical fiber strain gauge.

Abstract: A distributed, spatially resolving optical fiber strain gauge in which the core of the optical fiber is written with periodic grating patterns effective for transmitting and reflecting light injected into the core. Spectral shifts in the transmitted and reflected light indicate the intensity of strain or temperature variations at positions of the grating corresponding to the associated wavelengths of injected light.

Self‐induced polarization changes in birefringent optical fibers

Abstract: Exact solutions are presented for the intensity‐dependent polarization state of a light wave in a birefringent optical fiber. The theory takes into account both the linear polarization evolution and the nonlinear ellipse rotation. It is shown that, contrary to current belief, self‐induced polarization changes are possible even with equal excitation of the fiber’s principal axes. The theory is applicable to the operation of recently demonstrated fiber‐optic logic gates, pulse shapers, and intensity discriminators.

Generation and detection of squeezed states of light by nondegenerate four-wave mixing in an optical fiber.

Abstract: Nondegenerate four-wave mixing in a single-mode optical fiber is proposed as a method of squeezed state generation. An analysis of the near-degenerate mixing process for forward propagation in realistic fibers is presented along with the theory of an experimentally feasible detection strategy. The effects of the quantum nature of the optical nonlinearity and absorption are modeled by treating the fiber medium as a collection of anharmonic oscillators. Methods of suppressing undesired effects such as stimulated Brillouin scattering are presented as is a technique for providing the phase-shifted local oscillator wave necessary for the detection of squeezing. Preliminary experiments on fiber characterization and the detection of four-wave parametric fluorescence are described.

Optical fiber coupler with tunable coupling ratio and method of making

Abstract: An optical coupler for single mode optical signals having a tunable (variable) coupling ratio, and a method of fabricating the coupler. A pair of virtually identical optical fibers are fused together at a narrowed waist region, each fiber being formed of a core and cladding, each being tapered toward the waist in such a manner as to ensure adiabatic propagation of light in the structure. An optical signal carried by one fiber first passes through a decreasing taper region and then passes through an increasing taper region of one or the other fiber (or both). The decreasing taper rate of the input portion of one fiber is such that the optical signal radiates out of the core (where V=1 locally) and into the cladding, as it approaches the waist. The increasing taper rates of the output portions of the two fibers are such that a predetermined coupling ratio is obtained. The coupler is bent in the region of the waist whereby a coupling ratio can be selected between the incoming fiber portions having the decreasing taper and the outgoing fibers having increasing tapers.

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.

Apparatus and method for performing laser material processing through a fiber optic

Abstract: Pulse laser energy in the near infrared and visible spectrum is passed through a single fiber optic at power levels required for material and metal processing. A neodymium-YAG laser used in pulsed mode is coupled to the end of a quartz fiber optic which transmits peak powers in the kilowatt range. In order to transmit higher amounts of average power, a prepared fiber end allows beam coupling through core-air and core-cladding zones. The beam at the output of the fiber optic is focused to achieve power densities capable of cutting, drilling, and welding of metals etc. The main advantage is greater flexibility of laser beam manipulation.

Theory of noise accumulation in linear optical-amplifier chains

Abstract: The noise properties of the two main kinds of linear optical amplifier, phase-insensitive and phase-sensitive, are reviewed, with particular emphasis on their minimum-noise limits. An optical communications link is considered, in which identical sections of attenuating fiber alternate with identical linear amplifiers. The limits on the overall signal-to-noise ratios of such chains are derived. More attention is paid to the signal-to-noise ratio appropriate to phase-sensitive detection, but some results for direct detection are also given.

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.

Review of long wavelength single-mode optical fiber reflectometry techniques

Abstract: This paper reviews the worldwide activity on long-wavelength ( >1.1-\mu m) single-mode optical fiber reflectometry and compares and contrasts the many different approaches being taken to equipment design.

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.

Fiber Optic Sensors: An Overview

Abstract: Fiber optic sensor technology is explained in terms of theory and applications. Various components common to all fiber optic sensors are compared. Two classes of sensing devices are emphasized: amplitude-modu-lated sensors and phase-modulated sensors. Specific examples of amplitude-modulated devices-one a pressure sensor using an optical reflection technique and another an accelerometer using a microbend technique-are described. Four types of fiber optic interferometers used in phase-modulated sensors are discussed. The effect of the optical fiber jacket on the sensitivity of phase-modulated sensors is considered. A miniature pressure sensor and a highly sensitive fiber optic accelerometer, both employing phase modula-tion, are described.

Precision measurement of modal birefringence of highly birefringent fibers by periodic lateral force.

Abstract: Modal birefringence of highly birefringent fibers can be measured nondestructively by the elastooptic modulation method. Based on this modulation method, a new method for precisely measuring the wavelength dependence of modal birefringence in highly birefringent fibers is presented using an incoherent light source such as a fiber Raman laser.

Fiber‐optic logic gate

Abstract: Operation of a fiber‐optic logic gate is demonstrated for the first time. The principle of operation is based upon the intensity‐dependent polarization rotation in birefringent fibers. Using a birefringent fiber combined with polarizers in the crossed state, the ON/OFF state of an AND operation is clearly observed with an extinction ratio of approximately 10 dB. The fiber‐optic logic gate would be a promising device for optical information processing in the future because of the feasibility of compact configuration and very fast‐speed operation.

Super radiant light source

Abstract: 57 A super-radiant light source having an output with a low temporal coherence well suited, for example, to optical inertial sensor applications, includes a single mode optical waveguide such as an optical fiber having its core doped with an active laser material, such as neodymium. Pump light is coupled into the optical fiber at an intensity sufficient to produce a significant amplification of the spontaneous emission. The reflectivity at the exit end of the fiber is low enough so that even with a high gain for the light in one traverse through the fiber, the fiber is operated well below threshold for laser oscillation. The resulting emission is of high intensity, but has a low temporal coherence. In one form, the pump light is coupled into one end of the fiber and a dichroic filter is interposed in the light path between the pump light and the optical fiber to pass pump light into the core and reflect the spontaneous emission of the active material back into the core and toward the output.

Proprietes de polarisation des reflecteurs de Bragg induits par photosensibilite dans les fibres optiques monomodes.

Abstract: Reflecting Bragg filters having some polarizing properties can be photoinduced in monomode optical fibers exposed to a linearly polarized laser beam. Their reflectivity does indeed reach a maximum when the polarization direction of a reading beam is parallel to the polarization direction of the writing beam. This behavior is independent of the relative angle between the polarization direction of the writing beam and the birefringence axes of the fiber. These polarizing properties disappear for long exposure times of the fiber to the writing beam. From these results we conclude that the photosensitization process is not uniform over a cross section of the fiber; and periodic modification of the refractive index is anisotropic and therefore a small photoinduced birefringence exists. We have verified that effect; this result gives more weight to the hypothesis that the photoinduced filters are essentially phase gratings.

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.

Optical fiber magnetometer

Abstract: An optical fiber magnetometer having omnidirectional capability is disclo herein for measuring a total magnetic field independent of its physical orientation or the direction of the field or fields. A relatively long optical fiber defining a sensing arm for exposure to a magnetic field is wound in the form of a spheroid (like rubber bands on a golf ball or yarn threads on a baseball) to provide optical path lengths of substantially the same total length in every direction through the spheroid winding. The plane of polarization of light transmitted through the optical fiber winding is caused to rotate (Faraday effect) when the fiber or components thereof is exposed parallel to a magnetic field. The extent of plane rotation is determined, inter alia, by the total magnetic field passing through the spheroid winding.

Single mode, single polarization optical fiber with accessible guiding region and method of forming directional coupler using same

Abstract: An optical fiber comprising a core and cladding having different refractive indices and forming a single-mode guiding region, the core having a non-circular cross-section defining two transverse orthogonal axes which, in combination with the different refractive indices, permit the de-coupling of waves polarized along the axes. The guiding region is located sufficiently close to the surface of the fiber along a selected length of the fiber, to allow coupling to a guided wave. The outer surface of the fiber has a non-circular cross-section with a predetermined geometric relationship to the guiding region and the orthogonal transverse axes so that the location of the guiding region and the orientation of the axes can be ascertained from the geometry of the outer surface.

Single-mode fiber optic saturable absorber

Abstract: A fiber optic saturable absorber for processing optical signals is characterized by an optical fiber (20) from which a portion (40) of the cladding (36) is removed to form a facing surface (28). A light-absorbing substance (30) having non- linear light-absorbing characteristics is applied to the facing surface (28) such that a portion of the optical signal energy is transferred from the fiber (20) to the substance (30) where it is absorbed. The device selectively attenuates the optical signal and noise, and can be used to reduce pulse waveform distortion caused by pulse broadening and by amplification of system noise.

Temperature independent fiber optic sensor

Abstract: Disclosed is an improved fiber optic sensor of the type in which a fiber optic waveguide component of the sensor is configured to be responsive to an external parameter such that curvature of the fiber optic waveguide is altered in response to forces induced by changes in the external parameter being sensed. The alteration of the curvature of the fiber optic waveguide causes variations in the intensity of light passing therethrough, these variations being indicative of the state of the external parameter. The improvement comprises coating material covering the exterior portion of the fiber optic waveguide, the coating material having an expansion coefficient and thickness such that distortion of the fiber optic waveguide caused by thermally induced stresses between the coating material and the glass fiber is substantially eliminated. Also disclosed is a support member for supporting the curved fiber optic waveguide, the support member and fiber optic waveguide being configured and arranged to minimize the effects of thermal stress tending to separate the waveguide from the support member.

Analysis of a fiber-optic passive loop-resonator gyroscope: Dependence on resonator parameters and light-source coherence

Abstract: Analysis has been made of a fiber-optic passive loop resonator, the heart of the gyroscope The performance characteristics are heavily dependent on the resonator parameters, such as coupling intensity loss of the input light, coupling coefficient between two fiber lines in an evanescent-field directional coupler, and losses of the light traveling in the fiber loop, as well as on the temporal coherence of the input light A two input and two output system is considered for a passive loop-resonator gyroscope Differential output operation of the two output signals is described

In-line single-mode fiber polarization controllers at 1.55, 1.30, and 0.63 Mum

Abstract: Single-mode fiber-optic quarterwave plates utilizing linear birefringence induced by coiling the fiber in a loop are described. Optimal quarterwave plates are obtained by one turn of 1.0-cm radius for 0.6328-μm light for a single-mode fiber with an outer diameter of 81 μm. For single-mode fiber with a diameter of 120 μm, optimal quarterwave plates consist of one turn of 0.95-cm radius for 1.30-μm wavelength or two turns of 1.3-cm radius for 1.55-μm wavelength. Devices were made for cabled and uncabled fiber which provide complete polarization control by adjusting the angles of two such coils.

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.

Self-aligning optical fiber directional coupler and fiber-ring optical rotation sensor using same

Abstract: A continuously drawn optical fiber comprising a core and cladding having different refractive indices and forming a single-mode guiding region, and the outer surface of the fiber having a cross-section forming a pair of orthogonal exterior flat surfaces so that the location of the guiding region can be ascertained from the exterior geometry of the fiber, the guiding region being offset from the center of gravity of the transverse cross-section of the fiber and located sufficiently close to at least one of the flat surfaces to allow coupling to a guided wave through that surface by exposure or expansion of the field of the guiding region.

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.