Showing papers on "Optical fiber published in 1990"

Dense wavelength division multiplexing networks: principles and applications

Abstract: The very broad bandwidth of low-loss optical transmission in a single-mode fiber and the recent improvements in single-frequency tunable lasers have stimulated significant advances in dense wavelength division multiplexed optical networks This technology, including wavelength-sensitive optical switching and routing elements and passive optical elements, has made it possible to consider the use of wavelength as another dimension, in addition to time and space, in network and switch design The independence of optical signals at different wavelengths makes this a natural choice for multiple-access networks, for applications which benefit from shared transmission media, and for networks in which very large throughputs are required Recent progress in multiwavelength networks are reviewed, some of the limitations which affect the performance of such networks are discussed, and examples of several network and switch proposals based on these ideas are presented Discussed also are critical technologies that are essential to progress in this field >

Limitations on lightwave communications imposed by optical-fiber nonlinearities

Abstract: Optical nonlinearities in the context of lightwave systems limitations are described. The nature and severity of system degradation due to stimulated Raman scattering, carrier-induced phase noise, stimulated Brillouin scattering, and four-photon mixing are discussed. The system power limitations are plotted as a function of a number of wavelength-multiplexed channels. Methods for scaling these results with changes in system parameters are presented. >

Phase noise in photonic communications systems using linear amplifiers

Abstract: Spontaneous emission noise limits the capacity and range of photonic communications systems that use linear optical amplifiers. We consider here the question of phase detection in such systems. Amplitude-to-phase-noise conversion occurs owing to the nonlinear Kerr effect in the transmission fiber, resulting in optimal phase noise performance when the nonlinear phase shift of the system is approximately 1 rad. Error-free state-of-the-art systems that use phase detection at multigigabit rates are thereby limited to a range of a few thousand kilometers.

Silica waveguides on silicon and their application to integrated-optic components

Abstract: A marriage of optical fibre fabrication technology and LSI microfabrication technology gave birth to fibre-matched silica waveguides on silicon: thick glass layers of high-silica-content glass are deposited on silicon by flame hydrolysis, a method originally developed for fibre preform fabrication. Silica channel waveguides are then formed by photolithographic pattern definition processes followed by reactive ion etching. This ‘high silica (HiS) technology’ offers the possibility of integrating a number of passive functions on a single silicon chip, as well as the possibility of the hybrid integration of both active and passive devices on silicon. This paper reviews the NTT HiS technology and its application to integrated-optic components such as optical beam splitters, optical switches, wavelength-division multi/demultiplexers and optical frequency-division multi/demultiplexers. The clear and simple waveguide structures produced by the HiS technology make it possible to design and fabricate these components with high precision and excellent reproducibility.

Power transmission line monitoring system

Abstract: Electric power line phase conductor (line) monitoring system in which a single measuring station can measure all the power parameters of voltage and current and phase angle plus ambient and line temperatures. The measuring station fastens to the line, typically draws power from the line, is small, light, and manually installed without the need for a line power outage. The measured data is transmitted from the measuring station up on the power line phase conductor to a ground receiving and data processing station by either radio or a fiber optic cable or an optical-through-air data link. Each type of data link, fiber optic, radio or optical-through-air, can be built to carry either analog (linear) or digital data. Line true RMS voltage and true RMS current values, line phase angle, along with ambient and line temperature values are digitized and transmitted in digital form through a digital data link. Real time analog voltage and current sinusoidal waveforms are transmitted in analog form through a linear data link with only group delay which for a 50-foot long fiber optic cable would be roughly only 50 nanoseconds. Voltage data is obtained by an elegantly simple voltage divider technique which takes advantage of the highly dielectric character of plastic and glass materials in general and fiber optic cables in particular to form a dual purpose unibody cable assembly containing both the optical fibers of the data link and the large resistance resistive link of the voltage divider. Resistive link materials are described which have high precision resistance tolerance and low temperature coefficients of resistance, and thus improve the accuracy of the voltage sensor by establishing precisely the total resistance of the resistive link and by minimizing changes in the magnitude of that resistance due to changes in ambient temperature.

Nonlinear amplifying loop mirror.

Abstract: A novel device arrangement for all-optical switching that permits efficient exploitation of waveguide nonlinearities is discussed. It is based on a long optical fiber loop mirror with an integral short asymmetrically located optical amplifier. The device performance is demonstrated by using a Nd3+-doped fiber amplifier. Switching is obtained for peak signal powers of less than 1 W and an amplifier pump power of 10 mW.

Distributed-temperature sensing using stimulated Brillouin scattering in optical silica fibers

Abstract: Temperature distribution in silica single-mode optical fibers, from -30 to +60 degrees C, is successfully measured by using Brillouin optical-fiber time-domain analysis. A temperature measurement accuracy of 3 degrees C with a spatial resolution of 100 m is attained over a fiber length of 1.2 km.

Electrical signal processing techniques in long-haul fiber-optic systems

Abstract: The potential for electrical signal processing to mitigate the effect of intersymbol interference in long-haul fiber-optic systems is discussed. Intersymbol interference can severely degrade performance and consequently limit both the maximum distance and data rate of the system. Several techniques for reducing intersymbol interference in single-mode fiber systems with single-frequency lasers are presented, and those techniques which are appropriate at high data rates in direct coherent detection systems are identified. The performances of linear equalization (tapped delay lines), nonlinear cancellation (variable threshold detection), maximum-likelihood detection, coding, and multilevel signaling are analyzed. The results for a simulated binary 8-Gb/s system show that simple techniques can be used to reduce intersymbol interference substantially, thereby increasing the system margin by several decibels. A six-tap linear equalizer increases the dispersion-limited distance (due to chromatic or polarization dispersion) by 20% (or reduces the optical power penalty by as much as a factor of two) in direct detection systems, even when the distortion is nonlinear. A nonlinear cancellation technique (adjusting the decision threshold in the detector based on previously detected bits) can more than double the dispersion-limited distance and/or data rate. >

Guiding-center soliton in optical fibers.

Abstract: For a relatively long optical pulse in a fiber with a dispersion distance z(0) much larger than the loss distance, a soliton cannot exist in an ideal sense. However, with a proper choice of the initial amplitude and amplifier distance z(a), a nonlinear pulse (a guiding-center soliton) propagates like a soliton over a distance much larger than the dispersion distance when it is periodically amplified at distances much shorter than the dispersion distance. The guidingcenter soliton is shown to satisfy the nonlinear Schrodinger equation with a correction of order (z(a)/z(0))(2). Numerical examples supported by analytical results are presented for distortionless propagation of the guiding-center solitons with a pulse width of 40 psec in a dispersion-shifted fiber of D = 1 psec/(nm-km).

Photoinduced refractive-index changes in germanosilicate fibers.

Abstract: Photoinduced guided index changes approaching 10−4 in the range 488–784 nm, measured using a simple interferometric technique, are reported in germanosilicate single-mode optical fibers exposed to the 488-nm line of an Ar+ laser running multifrequency. The wavelength dependence and dynamics of the writing process are characterized, and the material dispersion of the induced Δn(λ) is shown to be weak. The effect is placed in the context of related research on color centers in these fibers, and two different mechanisms are proposed that lead to quantitative estimates in rough agreement with the measured Δn values.

Efficient mode conversion in telecommunication fibre using externally written gratings

Abstract: A novel approach to grating writing in optical fibres is described. This method involves writing the grating point by point through the side of the fibre using a UV laser source. Efficient mode conversion between forward propagating modes is demonstrated. The wavelength at which conversion occurs is determined by the periodicity of the grating. The grating can be designed to couple light at any wavelength of interest.

Optical-fiber temperature sensor based on upconversion-excited fluorescence.

Abstract: The codoping of a heavy-metal fluoride glass by two rare-earth ions, erbium and ytterbium, permits the semiconductor laser infrared excitation of two visible neighboring fluorescence lines of erbium by upconversion, the intensity ratio of which is a single variable function of temperature.

A technique to measure distributed strain in optical fibers

Abstract: The technique is based on Brillouin optical-fiber time-domain analysis (BOTDA), which uses Brillouin interaction between counterpropagating pump and probe light waves. Experimental results for fibers wound on drums at various tensions are presented. A strain measurement accuracy of better than 2*10/sup -5/ and a spatial resolution of 100 m are achieved. >

Semiconductor lasers for coherent optical fiber communications

Abstract: The current status of semiconductor lasers used in coherent optical fiber communications is reviewed for nonexperts in the field. The issues of spectral purity, tuning, modulation, and advanced fabrication methods for photonic integration are discussed, with examples drawn from current experimental devices. >

Fibre optic single mode rotary joint

Abstract: The invention is a rotary joint for singlemode optical fibers, having a fixed and a rotating part to permit the transmission of optical signals across a rotational interface (such as a winch or turret) with minimal insertion loss and, in particular, low reflections (good return loss). There is no need of conversion to electrical signals; the device is passive. It may be use an oil of refractive index matched to that of the glass fibers and to that of fiber tapers or lenses used to expand the beam emitted from one fiber and contract it for transmission into the other fiber. The device is bidirectional. By use foil, through precision techniques for building and mounting the optical and mechanical components, and by use of advanced bearings, both the insertion loss and unwanted reflections (return loss) can be minimized, thereby making it suitable for use with singlemode fiber. Insertion loss can be further reduced in conjunction with index-matching fluid by using optical elements (lenses, tapers, fibers) having angled or curved facets rather than perpendicular facets. Oil filling has the further advantage of pressure compensation allowing the device to operate at any ambient pressure. Lenses having curved surfaces can be accommodated by the use of fluid having a refractive index different from that of the lens material.

Incorporated bragg filter temperature compensated optical waveguide device.

Abstract: A temperature compensated embedded grating optical waveguide light filtering device includes an optical fiber having an elongated core in which light is guided toward a core portion of a predetermined limited length that has embedded therein a multitude of grating elements extending with a substantially equal longitudinal spacing substantially normal to the longitudinal axis and collectively constituting a grating that reflects, of the light propagating in the path toward and reaching the grating elements, that of a wavelength within a narrow range about a central wavelength determined by the spacing of the grating elements and by the index of refraction of the material of the core as influenced by the temperature of and longitudinal strains applied to the grating, back into the path for longitudinal propagation therein opposite to the original propagation direction. Each end of the fiber portion is attached to a different one of two compensating members made of materials with such coefficients of thermal expansion relative to one another and to that of the fiber material as to apply to the fiber longitudinal strains the magnitude of which varies with temperature in such a manner that the changes in the central wavelength that are attributable to the changes in the longitudinal strains substantially compensate for those attributable to the changes in the temperature of the grating.

Ultrashort-pulse propagation in optical fibers

Abstract: A more exact model is suggested for the description of nonlinear light propagation in fibers. In addition to the previously discussed self-phase modulation, parametric, dispersion, self-steepening, and Raman self-scattering effects, this model also takes into account the Stokes losses associated with the material excitation, the dependence of nonlinear effects on the light frequency, and the frequency dependence of the fiber mode area. The self-steepening effect is taken into account more correctly in comparison with previous models. The effects influence considerably the femtosecond soliton propagation. The model is generalized for the case of various fiber dispersion properties along the fiber length. The possibility of obtaining high-quality pulses of less than 15-fsec duration by compression of fundamental solitons with approximately 100-fsec duration in fibers with slowly decreasing dispersion is shown.

Beam expanding fiber using thermal diffusion of the dopant

Abstract: A beam-expanding fiber (BEF) for embedding optical devices has been fabricated by utilizing thermally induced Ge diffusion in silica single-mode fibers (SMFs). The preparation of fiber samples and their heat treatment is described, and the effect of heat on Ge dopant distribution and diffusion is discussed. Modal-intensity distributions were studied and found to confirm the broadening of the modal field distribution after heat treatment of the fiber. Localized heat treatment to obtain BEFs is considered, and device characteristics are discussed. The BEF can arbitrarily change the spot size of a propagating mode without changing the normalized frequency. >

Statistical treatment of the evolution of the principal states of polarization in single-mode fibers

Abstract: A simple relationship is found for the evolution of the principal states of polarization (PSPs) and their differential group delay in fiber links. A simple expression is found, using the relationship, for the probability of the differential group delay (DGD), considering the evolution of the PSPs as a Brownian motion. The theory has been verified experimentally on an optical cable composed of 12 single-mode, shifted-dispersion fibers 2.2-km long. The results show that the DGD grows as the square root of the length when the length of the fiber is far larger than the correlation length of the perturbation. The measured value of DGD can vary substantially in two fibers belonging to the same ensemble, and in the same fiber, considering two frequencies differing by more than 5 nm. >

Evanescent wave absorption spectroscopy using multimode fibers

Abstract: Evanescent waveabsorption in an aqueous dye solution has been performed using multimode fused silica fiber which was unclad at the sensing region. Evanescent absorbance values for methylene blue in a concentration range 3×10−8 to 5×10−6 M are reported. In order to produce modes close to cutoff in the sensing region, tunneling modes were launched into the clad fiber. Spatial filtering was used to restrict the light launched to those modes which have substantial power in their evanescent field in the unclad region. The measured evanescent absorbance of the dye solution was found to vary linearly with the exposed core length and to exhibit a square root dependence on concentration. The former effect is predicted from standard theory while the latter is attributed to adsorption on the core surface which obeys a Debye–Huckel‐type concentration dependence. In addition, a concentration enhancement of two orders of magnitude was observed due to this surface adsorption. While this effect limits the use of the technique for a reversible sensor it may be exploited in disposable probes.

Fiber optical probe connector for physiologic measurement devices

Abstract: A biological fiber optic probe chemical indicator device is disclosed. Structurally, the probe comprises at least one strand of optical fiber terminating in a coupling arrangement with a disposable sleeve assembly including a chemical indicator system for physiological substances. The disposable sleeve sensor element comprises a tubular housing with annular wall portions defining a female coupling channel means and a casing for a chemical reagent indicator.

Thermal effects on the Brillouin frequency shift in jacketed optical silica fibers

Abstract: Brillouin frequency shift in two kinds of jacketed optical single-mode silica fiber, with an ultraviolet curable resin coat and a nylon coat, has been measured at temperatures ranging from -30 to +60 degrees C. It has been found that there are two reasons for the Brillouin frequency shift change in jacketed optical fibers against temperature change. One is the Brillouin frequency shift change for bare fibers. The other is the thermal stress due to the differences in thermal expansion coefficients in bare fiber and its coating material.

Laser fusion of biological materials

Abstract: Apparatus and methods for laser fusion of biological structures are disclosed employing a laser for delivery of a beam of laser radiation to an anastomotic site, together with a reflectance sensor for measuring light reflected from the site and a controller for monitoring changes in the reflectance of the light of the site and controlling the laser in response to the reflectance changes. In one embodiment, the laser radiation is delivered through a hand-held instrument via an optical fiber. The instrument can also include one or more additional fibers for the delivery of illumination light (which can be broadband or white light or radiation from a laser diode) which is reflected and monitored by the reflectance sensor. Reflectance changes during the course of the fusion operation at one or more wavelengths can be monitored (or compared) to provide an indication of the degree of tissue crosslinking and determine when an optimal state of fusion has occurred.

Erbium-doped fiber amplifier

Abstract: This invention is a rare earth doped optical amplifier with increased gain and lowered pump thresholds. The amplifying scheme is based on a 3 level lasing system rather than the more prevalent 4 level lasing system. Additionally, the transmission mode of the optical fiber at the pump wavelength has a radius which is substantially equal to or greater than the radius of the distribution profile of the rare earth ions in the fiber amplifier core. With the inventive amplifier, a gain of 37 dB and a saturation power of 11.3 dBm has been obtained with only 54 mW of launch power at λ=1.49 μm.

Statistical properties of Rayleigh backscattering in single-mode fibers

Abstract: An analytical model that takes into account the influence of the laser linewidth on Rayleigh backscattering is given. For an unmodulated source the power spectral density is found to be identical to the delayed self-homodyne spectrum of the laser. The backscattered signal was measured for different laser linewidths, and the results are compared with the theory. The effects of Rayleigh backscattering noise on fiber gyroscopes and bidirectional optical systems are discussed. >

Simultaneous measurement of temperature and strain: cross-sensitivity considerations

Abstract: Interferometric sensors using optical fibers as a transduction medium have been shown to be sensitive to a variety of physical measurands. A result of this is that the resolution of a system designed to sense strain, for example, may be compromised by fluctuations in the temperature of the environment. The possibility of simultaneously determining the strain and temperature applied to the same piece of highly birefringent fiber is discussed. Second-order effects are shown to be important for long sensing lengths or in the presence of high strains or temperature changes. The results of experiments carried out to verify the theoretical predictions are also described. >

Three-parameter optical fiber sensor and system

Abstract: A single sensor is provided as part of a fiberoptic probe to measure up to three parameters, namely pressure (or force or displacement), temperature, and heat flow or fluid velocity. A solid elastomeric optical element is formed at the end of optical fiber transmission medium, and adjacent light reflective and temperature dependent materials are formed on the resulting convex surface of the optical element. The amount of light reflected is proportional to the force or pressure against the element. The temperature dependent material is preferably a luminescent material. Over the luminescent material is formed a layer of material that is absorptive of infrared radiation, thereby allowing a determination of characteristics of heat or fluid flow by measuring the rate at which heat is carried away from the infrared heated layer. The sensor can be formed at the end of a single optical fiber, thereby having extensive applications where a very small sensor is required. One such application is a medical or clinical one, where the sensor is mounted in a catheter for providing pressure, flow and temperature of the blood in a blood vessel.

The effect of four-wave mixing in fibers on optical frequency-division multiplexed systems

Abstract: The limiting effects of four-wave mixing on optical frequency-division multiplexing (OFDM) systems are described. The optical nonlinearity in a single-mode fiber imposes a fundamental limitation on the capacity of optical frequency-division multiplexed systems. In particular, four-wave mixing (FWM) crosstalk may severely degrade the system performance when the fiber input powers are large and/or the channel spacing is too small. Theoretical and experimental results of the effects of FWM in OFDM systems are presented. The theoretical results demonstrate the dependence of FWM on various system parameters. An analysis of FWM in both undirectional and bidirectional transmission systems is included. The receiver sensitivity degradation from FWM crosstalk is measured in a 16-channel coherent system. >

Fiber-Optic Hydrophone

Abstract: The design and preliminary performance results of a fiber-optic (FO) Michelson interferometric hydrophone are described.

Distributed multiplexed optical fiber bragg grating sensor arrangement

Abstract: An arrangement for determining the magnitudes of selected physical quantities at a multitude of spaced locations, comprising an elongated optical fiber having a waveguiding core and including a multitude of separate longitudinally spaced Bragg sensing gratings of substantially identical initial periodicity for all of the sensing gratings, each of the sensing gratings being situated at a different one of the locations; means for applying to the optical fiber longitudinal stresses the magnitude of which at each of the locations is dependent on the extent of deviation of the magnitude of the respective physical quantity being monitored at that particular location from its initial value, with attendant change in the periodicity; means for launching light pulses into the core for propagation longitudinally thereof and for return from any of the sensing gratings of a detectable light echo at a central wavelength determined by instantaneous value of the periodicity of the respective sensing grating when the wavelength of the light reaching the respective sensing grating at least momentarily coincides with the central wavelength thereof; means for so controlling the wavelength at which the launching means issues light as to coincide, at different times of a detection cycle, with at least all of the central wavelengths at which the sensing gratings return the echoes within the range of longitudinal stresses expected to be applied to the gratings by the applying means; means for conducting the light echoes to a detecting location spaced from the optical fiber; means for detecting the light echoes as they arrive at the detecting location and for issuing a series of detection signals in response thereto; and means for allocating the detection signals to the respective sensing gratings on the basis of time intervals elapsed between the issuance of the respective light pulses and the arrival of the echoes responsive thereto at the detecting means, and to the respective longitudinal stresses applied by the applying means to the respective sensing gratings on the basis of the respective wavelengths of the light pulses in response to which the respective gratings return the echoes.