Showing papers on "Single-mode optical fiber published in 1992"

Polarization effect on four-wave mixing efficiency in a single-mode fiber

Abstract: Four-wave mixing in a single-mode fiber is studied for general polarization states in input lights. A theory is developed for fiber four-wave mixing where the polarization states of input light are various and they change randomly when propagating through a fiber line. The results show that intensity beating between the propagating lights plays an important role in four-wave mixing efficiency in a fiber. Experimental results support the theoretical treatment. >

Bending losses of coated single-mode fibers: a simple approach

Abstract: A new approach for the bending losses of coated optical single-mode fibers is developed based on a modified fiber geometry model, and the result is a simple formula. An improved condition for the peak locations is given, that allows for the wavefront curvature of the leaky mode in the bent fiber, since it is derived by wave optics. Simple explicit relations for both the peak spacings and the peak-to-trough ratio are given. Their asymptotic forms for small bend radii are independent of the fundamental-mode parameters. An interpretation of oscillations in the bend-loss curves in terms of a thin-film filter effect of the triple-layer structure on the lateral leaky-mode radiation fully explains the behavior, especially the loss increase in the peaks compared to the case of an infinite cladding. The agreement with numerical and experimental results of other authors is excellent. >

Extrinsic fiber optic displacement sensors and displacement sensing systems

Abstract: An extrinsic Fizeau fiber optic sensor comprises a single-mode fiber, used as an input/output fiber, and a multimode fiber, used purely as a reflector, to form an air gap within a silica tube that acts as a Fizeau cavity. The Fresnel reflection from the glass/air interface at the front of the air gap (reference reflection) and the reflection from the air/glass interface at the far end of the air gap (sensing reflection) interfere in the input/output fiber. The two fibers are allowed to move in the silica tube, and changes in the air gap length cause changes in the phase difference between the reference reflection and the sensing reflection. This phase difference is observed as changes in intensity of the light monitored at the output arm of a fused biconical tapered coupler. The extrinsic Fizeau fiber optic sensor behaves identically whether it is surface mounted or embedded, which is unique to the extrinsic sensor in contrast to intrinsic Fabry-Perot sensors. The sensor may be modified to provide a quadrature phase shift extrinsic Fizeau fiber optic sensor for the detection of both the amplitude and the relative polarity of dynamically varying strain. The quadrature light signals may be generated by either mechanical or optical means. A plurality of the extrinsic sensors may connected in cascade and multiplexed to allow monitoring by a single analyzer.

Dynamics of self-focusing and self-phase modulation in a parabolic index optical fiber.

Abstract: The dynamics that result from the combined effects of spatial diffraction, nonlinearity, and a parabolic graded index for wave propagation in optical fibers are presented. An approximate analytical solution of the nonlinear Schrodinger equation in a graded-index fiber is obtained by using a variational approach. Particular emphasis is put on the variation of both the pulse width and the longitudinal phase delay with the distance of propagation along the fiber.

Interferometric fiber optic displacement sensor

Abstract: A method is presented to produce a change in the optical path length in the gap between two single mode optical fibers proportional to the lateral displacement of either fiber end normal to its axis. This is done with the use of refraction or diffraction at the interface between a guiding and non-guiding media to change the direction of propagation of the light in the gap. A method is also presented for laying a waveguide on a cantilever so that the displacement of the tip of the cantilever produces a proportional path length change in the gap by distancing the waveguide from the neutral axis of the cantilever. The fiber is supported as a cantilever or a waveguide is deposited on a micromachined cantilever and incorporated in an interferometer which is made totally on a silicon substrate with the use of integrated-optic technology. A resonant element in the form of a micro-bridge is incorporated in the ridge waveguide and produces a frequency output which is readily digitizeable and immune to laser frequency noise. Finally, monolithic mechanical means for phase modulation are provided on the same sensor substrate. This is done by vibrating the cantilever or micro-bridge either electrically or optically.

Method of making a fiber having low polarization mode dispersion due to a permanent spin

Abstract: The presence of (typically unintended) birefringence in single mode optical fiber can severely limit the usefulness of the fiber for, e.g., high bit rate or analog optical fiber communication systems, due to the resulting polarization mode dispersion (PMD). It has now been discovered that PMD can be substantially reduced if, during drawing of the fiber, a torque is applied to the fiber such that a "spin" is impressed on the fiber. Desirably the torque is applied such that the spin impressed on the fiber does not have constant spatial frequency, e.g., has alternately clockwise and counterclockwise helicity. At least a portion of optical fiber according to the invention has a spin whose spatial frequency exceeds 4 spins/meter.

Design of a single-mode linear-cavity erbium fiber laser utilizing Bragg reflectors

Abstract: The design constraints required to achieve single-longitudinal-mode operation from a standing-wave, homogeneously broadened, three-level, fiber laser which utilizes intracore Bragg reflectors for cavity feedback are presented. In particular, a closed-form solution to the laser rate equations is found which bounds the region in which only single longitudinal mode operation is supported. The principal resonator geometry to be considered is a two-reflector cavity which utilizes only Bragg reflectors for feedback. Analytical and experimental results are discussed. >

Optical loss property of silica-based single-mode fibers

Abstract: The optical loss property of silica fibers has been investigated theoretically and experimentally based on their Rayleigh scattering and absorption losses. The Rayleigh scattering loss for fibers has been estimated using Rayleigh scattering coefficients and power distribution in the fiber. The Rayleigh scattering coefficients are measured for preforms prepared for fiber fabrication and are discussed for GeO/sub 2/-doped and F-doped glasses. The relationship between the optical loss and fiber parameters is clarified. Moreover, the loss increase due to residual stress which occurs during the drawing process is simulated. The optical loss limitations for GeO/sub 2/-doped and pure silica core fibers are shown. >

Generation of soliton pulse train in optical fibre using two CW singlemode diode lasers

Abstract: A train of fundamental solitons at 1.53 μm has been generated, with pulse durations of 1.5–3.0 ps, at a repetition rate in the range 80–130GHz from the output of two CW single mode DFB lasers amplified in an Er-doped fibre. The transformation of the beat signal into a soliton train resulted from nonlinear propagation of the signal in a special fibre with dispersion decreasing along the length.

Stimulated Raman scattering in a multimode optical fiber: evolution of modes in Stokes waves

Abstract: Stimulated Raman scattering in a 50-μm multimode graded-index optical fiber was studied with a pulsed dye laser as the pump source. With 30 m of the fiber, the scattered waves (the Stokes waves) in the fiber could be made to propagate predominantly in one of the low-order modes of the fiber by careful adjustment of the light-launching conditions. The observed phenomenon is believed to be the result of nonuniform excitation of the modes by the pump. The experimental results are presented and discussed.

Wavelength multiplexed two dimensional image transmission through single mode optical fiber

Abstract: An apparatus and method are provided for transmitting two-dimensional images through a single mode optical fiber. Light from a broadband source is dispersed into component wavelengths, spatially modulated to contain an image, collected for multiplexing in a single mode optical transmission fiber, transmitted through the single mode fiber, and demultiplexed into its component wavelengths to reproduce the transmitted image. The two-dimensional dispersing and collecting elements may be realized using thin film channel waveguides having arrays of optical gratings. Because the dispersive element at the receiving (output) end of the transmission fiber must reproduce the dispersal pattern used at the source, the information transmitting device may be used for data encryption and decryption.

Microlensed optical terminals and optical system equipped therewith, and methods for their manufacture, especially an optical coupling method and optical coupler for use therewith

Abstract: The tip of a first optical fiber 8 is fused integrally to a second optical fiber 7, having a single refractive index and an identical outer diameter, which has a spherical lens 10 with the length and diameter to produce a beam expansion by Gaussian diffusion to at least 80 μm or greater at the exit and a radius of curvature greater than 200 μm. The spherical part of the second fiber is formed by thermal fusion; a spherical lens having the desired diameter at the optical fiber tip is formed by feeding the optical fiber tip into a thermal fusion unit to form the spherical lens. An optical fiber terminal for optical coupling composed of a single mode optical fiber, a non-doped silica fiber beam expansion segment, and a non-doped silica spherical lens, is connected to a polarization-independent optical isolator. A ferrule support having a rotatable spherical segment formed at the tip, in which the second optical fiber and an optical fiber protecting material are inserted and stabilized in a through-hole of the ferrule support, and a coupling unit consisting of a flange and a cap, which contact with the spherical surface of the spherical segment at opposite sides after the ferrule support has been adjusted into a position which produces an optimal beam converging function.

Ion-beam deposited multilayer waveguides and resonators

Abstract: An optical waveguide is formed by depositing multiple layers of thin films using the process of ion beam sputter deposition. The waveguide is made up of alternating layers of a first material and a second material wherein the two materials have differing indices of refraction and low optical loss. These materials are deposited upon a thermally stable substrate creating a multilayer stack which functions as a waveguide. The multilayer waveguide has ultra low propagation losses and good mode discrimination, thus being very efficient for use in single mode applications. The waveguide is easily adaptable to batch processing which reduces costs and increases the performance of the waveguide.

Single-mode selection using coherent imaging within a slab waveguide CO2 laser

Abstract: A technique for selecting a single high‐order lateral mode of a rf excited slab waveguide laser is described. An in‐cavity grid which matches the periodicity of the desired mode, together with slab waveguide dimensions which create coherent imaging by the Talbot effect, have produced a stable output power of 155 W in a high spectral purity, single‐resonator mode.

Chromatic dispersion compensated optical fiber communication system

Abstract: An optical fiber communication system according to the invention comprises, in addition to conventional single mode (SM) optical fiber, dispersion compensating (DC) optical fiber. The DC fiber is selected such that its chromatic dispersion (DDC (λ)) and first derivative of the chromatic dispersion with respect to wavelength (D'DC (λ)) at a wavelength λ=λop have opposite sign from those of the SM fiber. Advantageously, the DC fiber is selected such that LDC ·DDC (λ)+L·D(λ) is approximately zero at λ=λop, and such that D'DC (λ) is approximately equal to --(L/LDC)·D'(λ) at λ=λop, where LDC and L are appropriate lengths of DC and SM fiber, respectively. Typically, L is much greater than LDC, and λop typically is about 1.5 μm, e.g., about 1.56 μm. Preferred embodiments of the invention have low chromatic dispersion over a substantial wavelength region (e.g., dispersion less than 1.5 ps/nm·km over at least 50 nm) that includes λo, facilitating wavelength division multiplexing.

Mode competition using TE03 gyrotron cavities

Abstract: The gyrotron is a powerful source of millimetre wave radiation. Fusion applications require more power per tube than is currently available This in turn means that the resonator must be highly overmoded, with a dense mode spectrum, which leads to mode competition. The influence of external parameters, such as the applied magnetic field, electron beam, and resonator geometry, on mode competition in tubes designed to operate at 150 and 140 GHz in the TE03 mode is investigated theoretically and experimentally. It can be shown that even when the mode spectrum is fairly dense, single mode operation of a gyrotron is possible. The influence of startup conditions on output power is investigated in the 150 GHz experiment. In some cases, modes with an axial index of 2 were excited.

Soliton self-frequency shift in telecommunications fiber

Abstract: When subpicosecond solitons propagate in an optical fiber the soliton self-frequency shift occurs. We have made observations of this effect in standard telecommunication fiber, which show a strong departure from the predicted behavior. This difference is shown to be due to the residual birefringence in the standard fiber. Results are also presented on the power and length dependence and detailed comparisons with numerical simulations are made.

Test apparatus of distributed polarization coupling in fiber gyro coils using white light interferometry

Abstract: An account is given of a novel instrument that employs white-light interferometry to determine the spatial distribution of parasitic polarization coupling along as much as 1 km of high-birefringence optical fiber. This device is directly applicable to test-gyro coils in which localized spurious coupling points are able to affect bias stability. The required measurement time is of the order of 1 min for 1 km of fiber.

Analytical gain equation for erbium-doped fiber amplifiers including mode field profiles and dopant distribution

Abstract: An analytical description of the gain of an erbium-doped fiber amplifier is proposed that takes into account the transverse extension of mode fields and dopant distribution. A single closed-form expression is derived that describes the relation between gain, input powers, and fiber length. The free parameters in this equation are given in their analytical form. A numerical evaluation of the equation is performed for a 980 nm pumped amplifier fiber with different doping radii. A comparison with a calculation using the full numerical integration of differential equations shows that the description is correct for conventional fiber geometries. >

Optical fiber having a core with a repeatedly changing constitutional parameter

Abstract: The present invention is directed to an optical fiber comprising a core and a cladding layer wherein at least one of the constitutional parameters thereof changes along the longitudinal direction of the optical fiber. In order to present a useful optical fiber wherein the occurrence of a Brillouin scattering is prevented, an optical fiber having altered constitutional parameters is found to be effective. The constitutional parameters mean the parameters which determine the constitution of the optical fiber and is capable of influencing the condition of electromagnetic wave transmitting therethrough such as light or acoustic wave. The constitutional parameters include diameter of the core, index of refraction of the core, diameter of the optical fiber, composition of the glass, residual stress of the core. Some examples are disclosed about their manufacturing process and the test results. Much improvement was measured, especially in the use for a single mode optical fiber.

Coaxial optical-fiber frequency shifter

Abstract: A fiber-optic frequency shifter utilizing a traveling torsional acoustic wave to couple the polarization eigenmodes of a highly linearly birefringent optical fiber is demonstrated. The frequency-shifter configuration is such that the acoustic-wave generator and optical fiber are coaxial. The device is thus more rugged than previous free-fiber frequency shifters, which have incorporated a T junction between the acoustic generator and optical fiber. The optical frequency shift was 3.204 MHz, and an optical coupling efficiency of 12% was obtained with 4 W of electrical drive power applied to the acoustic transducer.

Optical fiber coupling device including lenses and magnetooptics

Abstract: An optical device which optically connects a first and a second optical fiber with an optical path provided therebetween and has two or more functions including the function of an optical isolator. A beam of parallel rays obtained by having a beam from an excitation end of a first optical fiber collimated by a lens is passed through a double refraction element, a magnetooptic element, and a double refraction element in order of mention and converged by a lens to be introduced to a second optical fiber through its excitation end, while a beam from the second optical fiber is not coupled to the first optical fiber. Meanwhile, a beam from an excitation port is coupled to the first optical fiber. The optical device is suitable for use in an optical amplification system and a two-way optical transmission system.

Environmentally stable fiber optic polarization maintaining couplers

Abstract: The invention features a fiber optic coupler package including a rigid substrate and at least two side-by-side optical fibers joined in a coupler region. The substrate has a coefficient of thermal expansion substantially matched to that of the fibers. Each fiber extends axially to opposite sides of the coupler region to a primary and secondary regions of fixation, at each side of the coupler region. In the primary region of fixation, each fiber is separately bonded in close proximity to a rigid mounting surface of the substrate using a small amount of adhesive contiguously disposed on opposite sides of the line of tangency formed by the fiber and the mounting surface. The adhesive is distributed to form a pair of substantially identical masses with mirror symmetry relative to a plane defined by the central fiber axis and the line of tangency. In the secondary region of fixation, extended outward from the coupling region beyond the primary regions of fixation, each fiber has a buffer coating and the fiber is bonded through the buffer coating to the substrate. The coupling region may be formed by the biconically tapered laterally fused process or by bonding the fibers together. The fibers of the coupler may be PM fibers, single mode fibers, multimode fibers, or their combination.

Distributed interferometric fiber sensor system.

Abstract: A frequency-modulated continuous-wave technique is used to detect phases of backscattered signals in a single-mode fiber. A distributed interferometric sensor system employing this technique is presented, and interrogation of 28 sensing regions is demonstrated. Spatial resolution 0.7 m and a sensitivity of 3 mradHz are achieved.

Statistical properties of single-mode fiber Rayleigh backscattered intensity and resulting detector current

Abstract: An analytical model for Rayleigh backscattering from a single-mode fiber which takes into account the coherence properties of the optical source, the modulation of the source field and the polarization state changes during propagation along the fiber is presented. It allows a full description of the statistical properties of the backscattered signals. Expressions for the power spectral densities and for the probability density functions of Rayleigh backscattered intensity and of the corresponding detector current are derived from the autocorrelation function of the optical source field. The measurements are shown to correspond to the theoretical results. >

Deterministic dynamics of stimulated scattering phenomena with external feedback

Abstract: A reasonably complete theoretical description of stimulated scattering in the presence of low-reflectivity external feedback is presented, based on an earlier treatment by Lu and Harrison [Europhys. Lett. 16, 655 (1991)] generalized to account for the integral effect of nonlinear refraction. Theoretical results show the Stokes signal to exhibit huge sustained or random bursts of quasiperiodic emission. Detailed comparisons with experimental findings on stimulated Brillouin scattering generated in single-mode optical fiber with weak external feedback by continuous-wave excitation give good agreement regarding both the dynamical features of the Stokes emission and their dependence on the physical control parameters, e.g., pump power, cavity reflectivity, and fiber length. Significantly, our results confirm the essential role of nonlinear refraction to the dynamical behavior and suggest that the noise structure of the initial spontaneous scattering has little influence on this behavior.

Pulse response of a fiber dispersion equalizing scheme based on an optical resonator

Abstract: Results of a theoretical simulation of the dynamic response of a dispersion equalizing scheme using a ring resonator when transmitting chirped Gaussian optical pulses over a long distance fiber are given. It is shown that two pulses which merge into each other due to dispersion are recovered by the resonator. Selection of the optimum resonator parameters in relation to the overall response is described. >

Laser-fabricated low-loss single-mode waveguiding devices in GaAs

Abstract: A maskless laser etching technique was used to fabricate novel waveguides and waveguiding structures directly into the surface of GaAs/AlGaAs heterostructures. The modal and loss properties of these groove-defined structures have been measured as a function of waveguide geometry, and low-loss single-mode waveguides have been produced. The technique was used to fabricate various passive optical devices in a single processing step. Waveguide bend and branch losses were measured and are comparable to those in conventionally fabricated devices. Experimental results are described by simple theoretical models. The technique is attractive as a prototyping tool for developing and testing new integrated optic circuits. >

Analysis of a single-mode fiber with taper lens end

Abstract: The upside-down taper lens optical fiber end is a new type of fiber end that may be useful for micro-optic image systems. We give the index distribution of this kind of fiber end, and the ray refraction formula at a spherical surface and the ray trace equation are used to analyze the ray transformation properties. Under the paraxial approximation, the optical transformation matrix of the upside-down taper lens fiber end is given.