Showing papers on "Dispersion-shifted fiber published in 1991"

Fading in lightwave systems due to polarization-mode dispersion

Abstract: System fading caused by polarization-mode dispersion is investigated at 1.7 Gb/s using highly-birefringent, dispersion-shifted fiber at 1.55 mu m. The observed fading, which is manifested by random fluctuations of the bit error rate for a fixed receiver power, is observed to depend on the environmental conditions of the fiber, with the time constant for fading varying from minutes to hours depending on the rate of change of the ambient temperature. The mean dispersion penalty inferred from the observed fluctuations in the bit error rate is consistent with a square-law dependence on the polarization-mode dispersion for small penalties. >

Long-distance soliton propagation using lumped amplifiers and dispersion shifted fiber

Abstract: It is shown both analytically and by numerical simulation, that solitons can traverse great distances through a chain of lumped amplifiers connecting dispersion shifted fiber spans. The fiber spans can also have large fractional variations in D. The resultant pulse distortions and dispersive wave radiation tend to be negligible, as long as the length scale of the variations in energy and dispersion are short relative to the soliton period. >

Dispersion penalty reduction using an optical modulator with adjustable chirp

Abstract: Using a unique Ti:LiNbO/sub 3/ modulator, the value of the modulation chirp parameter that minimizes the transmission power penalty caused by fiber chromatic dispersion was experimentally identified. System experiments at 5 Gb/s using nonreturn-to-zero (NRZ) amplitude-shift-keyed (ASK) transmission with direct detection reception are discussed, and the optimum values of the modulation chirp parameter versus distance for transmission at 1.5 mu m wavelength over fibre having zero dispersion at 1.3 mu m are identified. 5 Gb/s NRZ transmission was achieved through distances of 128, 192, and 256 km of conventional fiber while incurring dispersion penalties of -0.5, 0.1, and 1.1 dB respectively, by operating at the quantum chirp value. >

Effect of fiber nonlinearity on long-distance transmission

Abstract: The effect of the weak refractive-index nonlinearity of optical fibers on pulse shape is investigated using computer simulations of long-distance transmission. Fiber losses are canceled by periodically spaced optical amplifiers whose spontaneous emission noise is, however, not included in the simulations. The analysis is confined to normal pulses and does not consider solitons. Several conclusions are drawn. (1) If wavelength division multiplexing (WDM) of two channels is used in a uniform fiber without dispersion fluctuations, catastrophic buildup of four-wave mixing occurs if one primary channel is located at the zero-dispersion wavelength. (2) If two pulses with different carrier frequencies collide in a uniform fiber with no gain or loss discontinuities, their four-wave mixing products reach a peak during complete pulse overlap, but this spurious power dies away as the pulses separate. (3) Two-channel WDM transmission of light modulated in amplitude-shift keying format appears feasible at 2.5 GB/s over distances of 7500 km. >

Tunable gain equalization using a Mach-Zehnder optical filter in multistage fiber amplifiers

Abstract: Tunable signal gain equalization is demonstrated in three-stage Er/sup 3+/-doped fiber amplifiers using a waveguide type Mach-Zehnder (MZ) optical fiber. A 29-channel multiplexed system is examined where signal wavelengths are positioned from 1.548 to 1.555 mu m. By adjusting the MZ transmittance with the external control current, tunable gain equalization is achieved at the output of each amplifier. >

Single-channel operation in very long nonlinear fibers with optical amplifiers at zero dispersion

Abstract: The author investigates, by means of computer simulations, the performance of a very long, single-channel optical fiber system operating very close to the zero-dispersion wavelength of the fiber. Fiber losses are compensated by optical amplifiers. The optical signal is filtered after each amplifier, is passed through a final optical filter prior to square-law detection, and is finally filtered electrically. It is found that such a system does not work well if the fiber dispersion is strictly constant and if the carrier wavelength of the modulated signal coincides with the zero-dispersion wavelength of the fiber. As a result, the optical signal spectrum spreads to many times its initial width so that power is lost in the optical filters and the signal-to-noise ratio is degraded by the need for admitting a wider band of noise to the receiver. >

A single-mode fiber with chromatic dispersion varying along the length

Abstract: A method for fabrication of a novel type of optical fiber with dispersion varying along the fiber length is described. The method takes into account the calculated dependence of fiber dispersion on fiber core diameter for the measured profile of the preform and the desirable dispersion dependence on the fiber length. The main optical parameters of the drawn fiber are theoretically studied and experimentally measured. The fibers are of great interest for nonlinear fiber optics. Such applications of the fibers, such as high-quality soliton pulse compression, soliton pulsewidth stabilization through compensation of losses, and generation of a high-repetition-rate train of practically uninteracting solitons, are considered. >

Miniature Fabry-Perot interferometers micromachined in silicon for use in optical fiber WDM systems

Abstract: The techniques of silicon micromachining have been used to fabricate a second-generation Fabry-Perot interferometer for use in the near-infrared spectral region. These devices provide a sharp optical transmission peak which can be used as wavelength division demultiplexers in optical fiber communications systems. The wavelength tuning and parallelism control of the mirror elements are achieved electrostatically, by varying the voltage between control electrodes. This second-generation device includes a thin, etch-stopped corrugated diaphragm as the suspension for the movable element. >

Polarization-independent interferometric optical-time-domain reflectometer

Abstract: A polarization-independent interferometric optical-time-domain reflectometer is proposed and demonstrated. The experimental setup is composed of a superluminescent diode and a fiber coupler which combines a polarization-maintaining fiber and a conventional single-mode fiber to achieve a polarization-independent spatial profile of the reflectance in a single-mode fiber or waveguide sample. The polarization independence and the reliability of the measured reflectivity, were confirmed, and the measurement of loss and birefringence in an optical waveguide from its reflectance profile is demonstrated. The influence of dispersion in the fibers on the spatial resolution of the reflectometer is also discussed. >

Erbium-doped fiber amplifier with flattened gain spectrum

Abstract: An optical notch filter was incorporated within the length of all erbium-doped fiber amplifier. Careful choice of the filter characteristics and location made it possible to enhance the amplifier gain at wavelengths around 1550 nm. An amplifier with 27-dB gain and 33-nm bandwidth was demonstrated. The saturation characteristics of the amplifier were uniform across its gain spectrum, making it ideal for wavelength division multiplexed (WDM) application. >

All fiber, low threshold, widely tunable single-frequency, erbium-doped fiber ring laser with a tandem fiber Fabry-Perot filter

Abstract: An all fiber, widely tunable, single‐frequency, erbium‐doped fiber ring laser was constructed with a threshold pump power as low as 10 mW. Tuning over more than 30 nm was obtained by applying 0 to 17 dc V to an intracavity fiber Fabry–Perot filter. Threshold pump power versus wavelength data showed low variation over the tuning range. Mode hopping suppression with a tandem fiber Fabry–Perot filter is proposed and demonstrated. Stable single‐frequency operation was demonstrated with side mode suppression higher than 35 dB.

Nonlinear refractive index in erbium doped optical fiber: Theory and experiment

Abstract: An intensity-dependent refractive index in erbium doped optical fiber has been observed for the first time. The effect has been observed by an interferometric technique using a two-core fiber. The effect is analyzed theoretically, and the important nonlinear parameters for thin material have been determined and are presented. >

Multi-stage optical fiber amplifier

Abstract: In this invention there is disclosed a multi-stage optical fiber amplifier for providing a new functon, that of gain equalization. The disclosed multi-stage optical fiber amplifier comprises at least two stages of amplification where each stage comprises an amplifying fiber having a different gain spectrum. In one embodiment the two stages, which can be pumped separately, have different dopant compositions to provide each stage with a different gain spectrum. The disclosed multi-stage optical fiber amplifier provides overall gain spectrum equalization of the amplified channels of long haul transmission lines. Typically, gain equalization can adjust the gain of one channel while maintaining the gain of a second channel constant. When used in combination with automatic gain control, the amplifier gain can be equalized dynamically to compensate for random variations in the relative optical power of wavelength-multiplexed signals to prevent system impairment caused by low-power channels.

A broad-band single polarization optical fiber

Abstract: The authors report on a single-mode optical fiber that transmits one and only one polarization state of the fundamental mode over a broad wavelength band (13%). The good broadband performance is attributed to the fiber's refractive-index profile, which mathematically allows only one polarization state to cutoff. The authors discuss general design principles and show that the mode field diameter is a qualitative indicator of the fiber's performance. The fiber's guided polarization state is similar to the fundamental mode of a step-index fiber in its low attenuation, 5.5 dB/km, and small mode diameter, 5.8 mu m (both measurements at 840 nm). Most important, the fiber shows a broad single polarization band, 13%, a feature well described by the change in mode field diameter with wavelength of an ideal, circularly symmetry, W-profile fiber. >

Dispersion-induced composite second-order distortion at 1.5 mu m

Abstract: Experimental and theoretical results have shown that the composite second-order (CSO) nonlinearity of 1.5- mu m AM analog laser links is inadvertently affected by the coupling of laser chirp with fiber dispersion in regular single-mode fiber. To counteract the dispersion effect, it is shown that dispersion-shifted fiber can be employed in place of the regular fiber. Other possible ways to reduce distortion are to use lasers with reduced chirp, presumably with a multiquantum-well laser structure, or lasers with well-controlled spatial hole burning. Short fiber spans ( >

High-power Q-switched erbium-doped fiber laser

Abstract: Rare-earth single-mode fibers are of specific interest in the field of optical communication as lasers or in-line optical amplifiers. High-power erbium-doped fiber lasers, operating in the eye safe wavelength region of 1.5 μm, are attractive not only in fiber communication but also for such applications as range finding.

Optical switching using fiber ring reflectors

Abstract: We present the results of studies of fiber ring reflectors that are pulse excited in the negative dispersion regime. An intensity-dependent fiber switch is proposed for improved thresholding and pulse output characteristics over those previously reported. Schemes for fiber switches relying on solitary-wave collision-induced phase shifts are also discussed. A pulse regenerator and logic gates are proposed, each capable of correcting timing drifts.

Long distance soliton lightwave communication system

Abstract: Long distance soliton lightwave communications systems are considered for next generation application in terrestrial and transoceanic environments. These systems employ a chain of lumped fiber amplifiers interconnected by long spans of dispersion shifted optical fiber. In such systems, resultant pulse distortion and dispersive wave radiation are minimized when the soliton period is long relative to the perturbation length which is the longer of either the amplification period defined in terms of the length of the optical fiber span between consecutive amplifiers or the dispersion period defined in terms of the length over which the dispersion exhibits a periodic characteristic. Additional system parameters for optimized soliton transmission include the relationships of both the path-average soliton power to the normal soliton power and the path-average dispersion from one optical fiber span to the next. Single channel and wavelength division multiplexed systems are disclosed.

High gain multi-mode optical amplifier

Abstract: Indiscriminately exciting the modes of a multi-mode optical fiber amplifier is avoided by an optical fiber amplifier design in which the excitation of pump modes in the core of a multi-mode fiber is controlled by controlling the pump light launching. The pump light is directed substantially along the center axis of a multi-mode fiber within a predetermined launch angle. Rather than exciting all modes, only lower order modes are affected.

Prechirp technique for dispersion compensation for a high-speed long-span transmission

Abstract: Dispersion compensation by the prechirp technique is theoretically investigated, and experimental results are discussed. An improvement of more than two times was observed for dispersion-limited transmission lengths in 5-Gb/s, 150-km and 10-Gb/s, 50-km transmission with a semiconductor electroabsorption external modulator and a normal fiber. An allowable transmission fiber dispersion of 10000 ps/nm and 2.5 Gb/s is estimated by computer simulation with this prechirp technique. >

Dispersion in rare-earth-doped fibers.

Abstract: Rare-earth dopants in optical fibers have in general been ignored as a source of dispersion. The host material dispersion has been assumed to be an adequate description of the dispersion properties. Resonant dispersion features owing to absorption by the rare-earth ions were observed that were comparable in magnitude with the host material dispersion. These observations have consequences in the design of fiber lasers, nonlinear fiber transmission systems, and analog information systems.

Distortion due to gain tilt of erbium-doped fiber amplifiers

Abstract: It was found experimentally and theoretically that the second-order intermodulation product, namely composite second-order (CSO) distortion, is caused by coupling of the gain tilt of an erbium-doped fiber amplifier (EDFA) with laser chirp. When the signal light is not at the gain peak of the EDFA, gain tilt exists and EDFA cannot be used in an AM-FDM optical analog video transmission system because of CSO degradation. To counteract the gain tilt induced CSO, the authors demonstrated that codoping Al and setting the laser oscillation wavelength at the gain peak are useful. Er-Al-codoped fiber amplifiers have better linearity than Er-doped fiber amplifiers because the gain tilt of the former is smaller. >

System comprising Er-doped optical fiber

Abstract: The inventive optical fiber communication system comprises Si-based amplifier fiber whose core comprises Ge, Al, and Er. The amplifier fiber has an effective index difference (Δn) greater than 0.03, an effective core diameter a less than 3.5 μm, a maximum Al concentration in the core of at least 6 mole %, a mode field diameter at the pump wavelength that is less than 5 μm, a V-number at the pump wavelength in the range 1.4-2.0, a cut-off wavelength less than 1.4 μm, and an Er distribution whose effective diameter is less than that of the Ge distribution. The fiber has advantageous properties including low amplification threshold and noise. Disclosed is also a method of making optical fiber that can be used to produce fiber having characteristics (e.g., Δn>0.03, high Al concentration) not generally obtainable with prior art methods. In a particular embodiment the method comprises a partial collapse of a tubular preform prior to completion of core material deposition, completion of core material deposition, and final collapse.

Fiber amplifier having modified gain spectrum

Abstract: Disclosed is a fiber amplifier system including a gain fiber having a single-mode core containing dopant ions capable of producing stimulated emission of light at wavelength .lambda.s when pumped with light of wavelength .lambda.p. Absorbing ion filtering means is operatively associated with the gain fiber to alter the gain curve. If the absorbing ions are the same as the gain ions of the gain fiber, the system further includes means for preventing pump light from exciting the gain ions of the filtering means. The excitation prevention means may take the form of means for attenuating pump light. If the absorbing ions are different from the dopant ions of the gain fiber, such absorbing ions can be subjected to light at wavelength .lambda.p, but they will remain unexcited. Such absorbing ions can be used to co-dope the gain fiber, or they can be incorporated into the core of a fiber that is in series with the gain fiber.

Hybrid optical fiber and method of increasing the effective area of optical transmission using same

Abstract: A hybrid fiber which, at the operating wavelength, has a desirable fiber dispersion characteristic and a relatively large effective area is provided. Hybrid fibers are constructed in accordance with the principles of the invention by splicing together a fiber with a large effective area but typically unsatisfactory fiber dispersion characteristics to a fiber having excellent dispersion characteristics but a smaller effective area. The resulting hybrid fiber has an increased effective area and desirable fiber dispersion characteristics at the low-loss operating wavelength.

Optical fiber having enhanced bend resistance

Abstract: An optical fiber (22) of this invention is suitable for use in tethered vehicle. The optical fiber has a W-shaped refractive index configuration and is characterized by a Δ of at least 0.9% and a cutoff wavelength of less than 1500 nm. Further, the refractive index configuration is such that a core (30) is characterized by a relatively high value of Δ + . The value of Δ + is at least 0.6% for a single window of operation, whereas for a dual window it is 0.9%. Also, the refractive index configuration of the optical fiber is characterized by a depressed inner cladding portion (32) which has a Δ - of 0.3% and an outer radius about equal to the product of 2.5 and the radius of the core.

Evolution of the bandwidth of the principal states of polarization in single-mode fibers.

Abstract: The bandwidth in which the first-order approximation of the principal states of polarization of a single-mode fiber can be assumed valid is examined. The principal states of polarization and their bandwidth are found for a fiber with both constant coupling and birefringence, and the relationship with the fiber’s eigenmodes is examined. On the basis of these results, a fiber cascade is analyzed, and a Monte Carlo simulation provides theoretical values of the bandwidth that have been experimentally verified on a 2-km-long concatenation of single-mode dispersion-shifted fibers.

Demonstration of optical switching by means of solitary wave collisions in a fiber ring reflector.

Abstract: We have demonstrated the use of solitary wave collisions in optical pulse switching. Our apparatus consisted of a fiber ring with 11 sections of polarization-maintaining fiber, with successive sections fusion spliced with the axes rotated 90 deg. The configuration yielded enhanced transmission (autocorrelation contrast ratio 2.82:1), in agreement with expectation for this number of sections and the unoptimized fiber coupler that was used. Design criteria for complete switching are presented.

Generation of blue coherent light from a continuous-wave semiconductor laser using an organic crystal-cored fiber

Abstract: Efficient frequency doubling of a continuous‐wave semiconductor laser was attained by using a nonlinear optical fiber with a single crystal core of an organic 3,5‐dimethyl‐1‐ (4‐nitrophenyl)pyrazole. The second‐harmonic power generated with a 15 mm length of the fiber reached 0.16 mW from the incident 16.6‐mW fundamental laser power at 884 nm. Nonlinear optical properties of the crystal and a fabrication technique of the fiber are also reported.

Optical fiber electro-optical module

Abstract: OF THE DISCLOSURE An electro-optical module has at least three ports, with at least one port having an active electro-optical device for connection to electrical circuitry and at least one port having an optical port for connection to an optical fiber link. A passive coupler interfaces the various ports. The passive coupler is formed of optical fibers mounted on glass using an adhesive that exhibits refractive index-matching characteristics. The fiber-on-glass coupler is extremely small, allowing for the module to have minimal size. The coupler may be wavelength selective or wavelength insensitive. If a wavelength-selective coupler is utilized, the end surface of an optical fiber is coated with a dichroic filter to provide wavelength sensitivity. An optical fiber adapted to receive light for connection to a detector is selected to be a multi-mode fiber to increase reception of light. The remaining fibers may be single-mode fibers.