Showing papers on "Dispersion-shifted fiber published in 1990"

Radiations by solitons at the zero group-dispersion wavelength of single-mode optical fibers

Abstract: Nonlinear pulse propagation at the zero group-dispersion wavelength is studied analytically. It is discovered that, after the characteristic initial frequency splitting, evolution of the pulse envelope that is shifted down to the anomalous regime is described by the nonlinear Schr\"odinger equation with higher-order dispersion as a perturbation. The effect of the perturbation on the pulse is to excite radiation at a frequency inversely proportional to the small parameter \ensuremath{\beta}. The amplitude of the radiation is exponentially small (\ensuremath{\propto}exp(-1/\ensuremath{\beta})] and can be calculated only by the perturbation method that goes beyond all orders.

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.

Bi-domain two-mode single crystal fiber devices

Abstract: A mode converter comprises an a-axis LiNbO3 optical fiber exhibiting a ferroelectric bi-domain structure. The fiber is subject to an electrical field that induces a +π/2 phase retardation in one domain of the fiber and a -π/2 phase retardation in the other domain. A light signal launched in the fundamental mode of the fiber is converted into a light signal propagating in the second order mode. When the electrical field is selected so that the phase retardations are not multiples of π/2, the mode conversion is partial and the LiNbO3 fiber can operate as an optical switch or as an amplitude modulator. The mode converter can also be operated as a second harmonic generator. The fiber is heated to a phase matching temperature so that a signal launched in the fundamental mode of the fiber and at a frequency ω is converted to the second order mode at a frequency 2ω. The LiNbO3 fiber can also simultaneously operate as an optical switch and as a second harmonic generator. Other non-linear interactions are possible such as sum or difference frequency generation or parametric generation. The various embodiments of the present invention are reciprocal.

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.

A 100-channel optical FDM transmission/distribution at 622 Mb/s over 50 km

Abstract: A 100-channel optical frequency-division multiplexing (FDM) transmission/distribution experiment at 622 Mb/s is demonstrated for a fiber length of 50 km. The feasibility of a polarization-insensitive waveguide frequency selection switch for 10-GHz intervals and a frequency-shift-keying (FSK) direct-detection scheme employing a Mach-Zehnder filter is verified. The demodulation circuit employs a Mach-Zehnder filter and a balanced receiver, which utilizes optical power more efficiently than the Fabry-Perot filter. No receiver sensitivity degradation is observed due to interchannel crosstalk of the 128-channel tunable waveguide frequency selection switch (FS-SW) or fiber four-wave mixing for transmissions over a 50-km-long nondispersion-shifted (NDS) fiber and a 26-km-long dispersion-shifted (DS) fiber. >

Experimental study of soliton transmission over more than 10,000 km in dispersion-shifted fiber

Abstract: Transmission of 50-psec solitons in a 75-km recirculating loop of dispersion-shifted fiber (D = 1.38 psec/nm/km at λs), using low-gain erbium amplifiers spaced 25 km apart, displays jitter in pulse arrival times consistent with low error rates for transmission over 9000 km and for bit rates ≲4 Gbits/sec. Furthermore, a study of soliton pair propagation in the same loop shows no significant interaction over 9000 km for pair spacings ≥5τ.

Efficient power coupling from a 980-nm, broad-area laser to a single-mode fiber using a wedge-shaped fiber endface

Abstract: A theoretical and experimental study of an efficient technique for coupling a broad-area laser to a single-mode fiber without the use of bulk optical components was carried out. The technique uses a wedge-shaped fiber endface to approximate a cylindrical lens action to correct for the phase mismatch between the curved laser beam wavefront and the planar fiber beam. A theoretical formula for the coupling efficiency in the absence of both angular and transverse misalignments is derived. By optimizing the wedge angle and the axial separation between the laser facet and the fiber endface, a maximum coupling efficiency of 47%, compared to 20% for the square endface, achieved, resulting in 15.2 mW of power in the single-mode fiber. The fabrication process consists of a simple polishing procedure requiring a wedge-shaped polishing tool and is highly reproducible. >

Comparison of some fiber optic configurations for measurement of luminescence and Raman scattering.

Abstract: Measurements are made for a number of dual fiber optic configurations to determine their relative sensitivity using bare fibers and graded-refractive-index lenses. An analysis of the background fiber emission for a typical silica-on-silica fiber (Diaguide, 200-microm core) is presented, and the origin (core or cladding) for several prominent Raman peaks is determined. Also, a forward-scattering fiber geometry is introduced, and the dependence of sensitivity on the type of optical termination and fiber separation is determined.

Distributed amplification for lightwave transmission system

Abstract: Bidirectional lightwave transmission is restored and uniform amplification of lightwave signals over long spans of optical fiber is achieved by employing distributed amplification over the spans. Distributed amplification is achieved with an amplifying optical fiber which includes a long length of optical fiber having a dilute rare-earth dopant concentration substantially in the fiber core region, and a corresponding pump signal generator at at least one end of the doped fiber having the appropriate wavelength and power to cause amplification of optical signals by both Raman effects and stimulated emission from the rare-earth dopants. Dilute concentrations are understood as the range of concentrations substantially satisfying the condition that the gain from the rare-earth dopant, when near saturation, is substantially equal to the fiber loss.

Fiber optic gyroscope

Abstract: In a fiber optic gyroscope comprising a light source, a fiber optic coupler, an optical integrated circuit having a function of a polarizer and a branching optical waveguide, and a fiber optic coil, and detecting an angular velocity applied to the fiber optic coil about the axis thereof, a polarization maintaining optical fiber having its length L is connected to an input/output end of the optical integrated circuit nearer the light source in an optical system of optical fiber from the light source to the optical integrated circuit, the polarization maintaining optical fiber of its length L resulting in a difference in group delay time between the orthogonal two polarization modes and the difference at least exceeding a coherence length of light from the light source. An optical system of optical fiber from the light source to the optical integrated circuit except for the polarization maintaining optical fiber of its length L is formed by a single mode optical fiber, and the fiber optic coupler is formed by a single mode optical fiber.

Amplification of ultrashort solitons in erbium-doped fiber amplifiers

Abstract: Pulse amplification in erbium-doped fiber amplifiers is studied by considering a general model that includes both gain saturation and gain dispersion. The effects of gain dispersion are studied numerically for the case in which a fundamental soliton is launched at the amplifier input. The results show that fiber amplifiers may be useful for simultaneous amplification and compression of weak optical pulses. Under high-gain conditions the input pulse is found to split into a train of amplified subpulses whose width and repetition rate are governed by the gain bandwidth. The numerical results are in qualitative agreement with recent experiments. >

Concentration effect on optical amplification characteristics of Er-doped silica single-mode fibers

Abstract: Optical amplification characteristics for Er-doped silica core single-mode fiber amplifiers with different Er concentrations pumped by 1.48- mu m-wavelength laser diodes are studied. Optical gain drastically depends on Er concentration, even when the Er concentration is less than 1000 p.p.m. In the case of the 40-mW incident pump power, the maximum net gain for the fiber containing 77 p.p.m. Er is higher than that for the fiber containing 970 p.p.m. Er by 11.4 dB. >

Wavelength dependence of bending loss in monomode optical fibers: effect of the fiber buffer coating

Abstract: The bend-induced optical loss in a monomode fiber has been studied as a continuous function of wavelength in the range 1.2-1.6 microm. The loss was observed to be an oscillatory function of wavelength. These observations were explained by using a model based on interference between the guided core mode in the fiber and a "whispering gallery" mode ejected from the core by the bend. The wavelengths of the minima in the oscillatory function were accurately predicted by the model when the propagation of optical power in the fiber buffer coating was taken into account.

Transmission limitations due to fiber nonlinearities in optical FDM systems

Abstract: Transmission limitations due to stimulated Brillouin scattering and four-wave mixing processes are investigated for optical frequency division multiplexing (FDM) systems. The applicability of the dispersion-shifted (DS) and nondispersion-shifted (NDS) fibers is discussed, taking account of channel frequency separation, total channel numbers, input signal power, transmission length, and receiver sensitivity degradation. Experimental results on Brillouin gain spectra and the wave generation efficiency in four-wave mixing processes are also presented to discuss the applicability of the two types of single-node fiber. It was found that NDS fibers operated at a wavelength of 1550 nm can be widely deployed in multichannel systems both for the long-haul and information distribution transmissions, if the signal waveform distortion due to fiber chromatic dispersion is precluded. The delay equalizer will be useful for a high-speed system employing bit rates over 10 Gb/s and repeaterless spans over 300 km. For such an application, DS fiber is preferable. Concerning information distribution network applications, the NDS fiber should be more attractive as a transmission medium for FDM system applications. >

Optical fiber sensor with localized sensing regions

Abstract: A coupling mechanism for a non-identical dual core optical fiber. The fiber is sensitized along selected portions of its length by coating the fiber in a pattern of bands of specified spacing with material that is sensitive to the presence of a predetermined parameter or environmental field. By virtue of inequality of propagation properties and mismatch of optical phase of the fields of the two cores, evanescent wave coupling is largely or wholly suppressed except in those portions of length coated in the specified spatially periodic manner. Optical power launched into one of the two cores remains propagating within that core alone over an extended length of the fiber, except for that period of time when the predetermined parameter or environmental field is sensed. The specific spatial periodicity of the pattern of coating bands on the outside of the fiber acts in concert with the nonidentical propagation parameters of the two cores to effect intercore coupling of optical energy. This coupling, representing sensitivity to any environmental field which causes strain in the coating, is effective only on those portions of the dual core fiber length which bear a specific coating pattern.

Ultrafast, low power, and highly stable all-optical switching in an all polarization maintaining fiber Sagnac interferometer

Abstract: An ultrafast, low-power, and highly stable all-optical switch in a nonlinear Sagnac interferometer is reported. To achieve low-power, highly stable, and walkoff free switching, use is made of a small-core dispersion-shifted polarization-maintaining fiber loop (200 m in length) which has a small group delay difference between the wavelengths of the input signal and the control pulse. To achieve complete polarization stability, a wavelength-sensitive polarization-maintaining fiber coupler is employed. Highly stable and walkoff free all-optical switching is demonstrated at 5 Gb/s. >

Optical communication system comprising a fiber amplifier

Abstract: Disclosed is an optical communication system comprising at least two optical fibers of dissimilar core sizes, joined by a fusion splice. In one embodiment, the larger-core fiber is a communication fiber, and the smaller-core fiber is an erbium-doped amplifier fiber. A taper region is included adjacent the splice. The diameter of the smaller-core fiber increases within the taper region as the splice is approached along the smaller-core fiber. The taper region is substantially free of constrictions. As a consequence of the taper region, the optical losses associated with the splice are relatively low, even when there is relatively high mismatch between the mode field diameters (at a signal wavelength) in the respective fibers.

Determination of homogeneous linewidth by spectral gain hole-burning in an erbium-doped fiber amplifier with GeO/sub 2/:SiO/sub 2/ core

Abstract: Spectral gain hole-burning was observed at low temperatures in an erbium-doped fiber amplifier with GeO/sub 2/:SiO/sub 2/ core. At the peak wavelength lambda =1.535 mu m, homogeneous linewidths determined from the observed hole widths have a power-law dependence on temperature. At room temperature, the extrapolated homogeneous linewidth is 4 nm and the inhomogeneous linewidth is 8 nm. >

Enhanced-nonlinearity single-mode lead silicate optical fiber

Abstract: A single-mode lead silicate optical fiber has been fabricated to permit lower-power all-optical switching. The core glass has a nonlinear index of refraction eight times that of silica. The loss of the fiber is less than 2 dB/m. Over 17π of phase shift was obtained, as measured by self-phase modulation, in a 29-cm length of fiber. At the 1-kW peak power level required to produce this large phase shift, two-photon absorption and stimulated Raman scattering did not significantly degrade the desired nonlinear behavior.

Tunable diode laser frequency modulation spectroscopy through an optical fiber: High‐sensitivity detection of water vapor

Abstract: Frequency modulation spectroscopy through a single‐mode optical fiber has been demonstrated for monitoring H2O vapor at 7665 cm−1. Using a 2.5 mW distributed feedback laser and 100 m of optical fiber, a minimum detectable absorption of 5×10−7 was achieved. To obtain this sensitivity limit, very large spurious signals and technical noise were suppressed using a dual‐channel detection scheme.

Optical equalization of fiber chromatic dispersion in a 5-Gb/s transmission system

Abstract: A 5-Gb/s lightwave transmission experiment demonstrates the effectiveness of an all-pass-type fiber Fabry-Perot optical equalizer for eliminating an error floor caused by laser chirp and chromatic dispersion. >

10 Gbit/s optical PSK homodyne transmission experiments using external cavity DFB LDs

Abstract: 10 Gbit/s optical PSK homodyne transmission experiments using a modified decision-driven optical phase locked loop with 1.55 μm external cavity laser diodes were conducted. A receiver sensitivity of −34.2 dBm or 297 photon/bit was experimentally achieved. There was no degradation in receiver sensitivity after signal transmission through 151 km of dispersion shifted fibre.

Dynamic gain equalization in two-stage fiber amplifiers

Abstract: Relative gain differences among channels in WDM-amplified lightwave systems can be corrected in two-stage fiber amplifiers having complementary gain spectra in each stage. With two channels spaced by 2.5 nm, relative gain corrections of 1 dB were demonstrated. Simulations show that this method can be used to dynamically equalize the channel gain in long-distance transmission systems. >

Detection of high-frequency ultrasound with a polarization-maintaining fiber

Abstract: The effect of a high-frequency ultrasound wave incident normally upon a polarization-maintaining optical fiber is investigated theoretically and experimentally. Two different mechanisms accounting for the birefringence change in the fiber resulting from radial strain produced by the ultrasonic wave are identified and studied. A polarization-maintaining fiber can respond to lower frequencies than a single-mode fiber in the polarimetric mode. Moreover, it does so with better sensitivity and requires no real-time control of the polarization state of the input light. The frequency separating the ranges of significance of the two mechanisms is experimentally found to be about 1 MHz (in water). The principle and the realization of a polarimetric ultrasonic sensor using a polarization-maintaining fiber are also described. >

Fiber-optic sensors using high-resolution optical time domain instrumentation systems

Abstract: Applications of high-resolution optical time domain instrumentation systems for the sensing of strain and temperature using optical fibers are discussed. It is demonstrated that optical fiber segmentation using partially reflective splices allows measurement of localized fiber parameters. In particular, localized fiber strain is measured using specially designed in-line reflectors spaced less than 1 m apart. It is shown that the time domain response to temperature variations can be improved by using silicone fluid-filled fibers with thermooptic coefficients as high as -4.2*10/sup -4// degrees C. The applications of fiber-optic reentrant loops to further improve on system-imposed measurement resolution are discussed. It is shown that recirculation of the probing optical pulse through the sensing fiber yields an effective increase in fiber interaction length. >

Double-core active fiber optical amplifier having a wide-band signal wavelength

Abstract: The invention relates to an optical amplifier, in particular for optical fiber telecommunication lines (1), operating with a transmission signal in a predetermined wavelength range, which amplifier comprises a fluorescent active optical fiber (6) doped with erbium, having two cores (11 and 12, 101 and 102), one (11, 101) of which is connected to a fiber (4) in which a transmission signal to be amplified and a luminous pumping energy are multiplexed, and to an outgoing fiber adapted to transmit the amplified signal, whereas the second core (12, 102) is optically coupled to the first core and is capable of absorbing the spontaneous erbium emission which would constitute a noise source, allowing a signal to be amplified in a wavelength range substantially corresponding to the tolerance range of the commercially available signal laser emitters.

Spectral characteristics of high-power 1.5 mu m broad-band superluminescent fiber sources

Abstract: The spectral variation of spontaneous emission from erbium-doped single-mode fibers is studied with the aim of producing high-power (more than 5 mW), broadband (in excess of 10 nm) amplified spontaneous emission sources for fiber gyroscope applications. In particular, the evolution of spectral shape and center wavelength with fiber length and output power in the previously unstudied high-power regime where saturation effects dominate is demonstrated. A visibility curve for a potential twin-peaked nonresonant erbium-doped fiber gyroscope source with a short (210 mu m) coherence length is also presented. >

Gain characteristics of an Er/sup 3+/-doped multicomponent glass single-mode optical fiber

Abstract: Experimental results on gain characteristics of an Er/sup 3+/-doped multicomponent glass single-mode optical-fiber amplifier are reported. This amplifier shows a gain spectrum with twin gain peaks of 1.535 and 1.543 mu m, providing a broadened gain bandwidth. The apparent 6-dB gain bandwidth is 12 nm. Furthermore, the signal gain of 17 dB and 15-mW pump power is realized at a signal wavelength of 1.536 mu m, and a signal gain coefficient of 1.4 dB/mW is achieved. >

Pressure-induced birefringence in a coated highly birefringent optical fiber

Abstract: An attempt is made to explain the pressure-induced birefringence in a birefringent fiber based on a consideration of the effect due to the difference between the elastic properties of the stress-applying regions and the cladding of the fiber. The elastic properties and the thickness of a single fiber coating are also incorporated into the analysis. Numerical results for some typical coating materials are presented and compared with existing experimental data. The implications of the results with regard to sensor applications are also discussed. A polarimetric sensor using a suitably coated highly birefringent fiber should be useful for the detection of static and low-frequency acoustic pressures. >

Apparatus and method for producing an in-line optical fiber attenuator

Abstract: Signal strength in an optical fiber transmission path is attenuated in one direction to a desired lower level by incorporating a selected length of coreless, undoped fiber of the same diameter into the signal-carrying fiber. On encountering the insert, the beam diameter expands to a pre-selected value at the far end of the insert. The ratio of the beam diameter at this point to the diameter of the adjacent fiber end establishes the attenuation factor. Steps are taught for achieving high consistency in both structural strength of the attenuator as well as the attenuation factor.