Lucent Technologies Inc.

At&t Bell Laboratories

At+t Corp.

Alcatel Lucent

James W Sulhoff

John L Zyskind

Atul Kumar Srivastava

Yan Sun

David John DiGiovanni

Richard G Smart

Jonathan A Nagel

Julian Stone

Yun C Chung

Gengxian Luo

Robert William Tkach

Andrew Roman Chraplyvy

Paul Francis Wysocki

Neal S Bergano

Robert Meachem Jopson

Jianhui Zhou

Liyan Zhang

Sonali Banerjee

Bruce M Nyman

Virendra S Shah

Robert D Feldman

Richard F Austin

Jennifer Aspell

Joseph D Evankow

John Lehrer Zyskind

Distortion and crosstalk that occurs when operating optical amplifiers in saturation is substantially reduced by passively compensating for gain variations caused by changes in input power to the optical amplifiers. More specifically, in an optical communication system having one or more optical amplifiers, a &ldquo;reservoir&rdquo; optical channel is supplied in addition to the other traffic-carrying optical channels. The wavelength of the reservoir channel is selected such that the power level of the reservoir channel varies in response to changes in power levels of the traffic-carrying channels. Because gain variations are typically highest around the gain peak region in an optical amplifier's gain bandwidth, the reservoir channel in one exemplary embodiment is assigned a wavelength around the gain peak region. As such, the reservoir channel can absorb or otherwise receive most of the distortion and crosstalk while the traffic-carrying channels located away from the gain peak region will experience less distortion and crosstalk. -Importantly, this passive compensation scheme can be used in an amplified system which is operating in the presence of nonlinear distortions caused by gain variations, such as inter-modal distortion and saturation induced crosstalk.

Distortion compensation in optically amplified lightwave communication systems

A high-capacity optical transmission arrangement utilizing a plurality of laser sources and a plurality of wide-band optical amplifiers permit the reliable transmission of 1 Tb/sec rates over significant distances of optical fiber.

High capacity optical transmission arrangement

Substantially error-free communications is achieved in an optical communication system that includes optical amplifiers by detecting bits transmitted in the amplified optical signal using a detection threshold that is derived as a function of a maximum power level associated with a first bit value, e.g., bit &ldquo;0&rdquo;, and a minimum power level associated with a second bit value, e.g., bit &ldquo;1&rdquo;. Importantly, this detection scheme can be used to accurately detect bit patterns in the amplified signal even in the presence of nonlinear distortions caused by gain variations, such as inter-modal distortion and saturation induced crosstalk. In a wavelength division multiplexed (WDM) system comprising semiconductor optical amplifiers, for example, the detection threshold can be set at a level corresponding to P /2N, where P represents the total power in the WDM signal and N represents the number of optical channels in the WDM signal. According to another aspect of the invention, the effect of gain fluctuations becomes smaller as the number of channels increase. In particular, a smoothing effect is realized for the total effective saturation power as gain variations decrease as a function of an increase in the number of channels. As such, the performance of a semiconductor optical amplifier according to the principles of the invention approaches the linear performance of fiber amplifiers as the number of channels increases.

Lightwave communication systems using semiconductor optical amplifiers

A method of modeling erbium doped fiber (&ldquo;EDF&rdquo;) for use in erbium doped fiber amplifiers (&ldquo;EDFA&rdquo;). The EDF has a length and a fractional population density of erbium ions in an excited state. Since the EDF supports N channels, the power propagation along the EDF is characterized by N&plus;1 differential equations as a function of the direction of propagation, z, of the channels along the length l of the EDF and the time t. By applying an average inversion model to a spatially averaged inversion level of the erbium ions in the fiber, the N&plus;1 partial differential equations are reduced to a single ordinary differential equation. This allows an analytical solution at the boundary and initial conditions of the fiber so that an expression for the power of the signal propagating along the fiber can be obtained. With this expression, the single equation can be solved analytically for the inversion level at any point along the EDF.

Methods of modeling erbium doped fiber amplifiers

An Erbium doped fiber amplifier (EDFA) is placed in a laser cavity to clamp the gain of the amplifier. The laser cavity is preferably formed to minimize the effects of relaxation oscillations and of spectral hole burning. In a preferred embodiment, the laser cavity supports a lasing wavelength which makes the saturation energy at the laser wavelength less than that at the signal wavelengths. In an erbium doped fiber implementation, the laser wavelength is shorter than that of the signal wavelengths and is separated from them by no more than approximately 75% of the width of the spectral hole. The compounding of relaxation oscillations may be substantially eliminated by varying the length of the fiber within each laser cavity within a system to change the photon round-trip time within each cavity.

Erbium-doped fiber amplifier with automatic gain control

A method for determining the optimal design of a gain equalization filter (GEF) used in optical fiber amplifiers. The method is suitable for erbium-doped fiber amplifiers (EDFA) that are widely used in wavelength-division-multiplexed (WDM) optical communication systems and networks.

Erbium-doped fiber amplifiers with desired gain-bandwidth characteristics

A wideband optical amplifier employs split-band architecture in which an optical signal passes through a common amplification stage and is then split to pass through parallel gain stages, each of which may be optimized for a particular band being traversed. A broadband grating reflector is used after the input gain section to reflect the signals of one of the bands so that they again will pass through the common input gain section before passing through gain section of the split structure dedicated to their particular wavelength. Meanwhile, the other signals pass through the reflector and move on through the gain section pertinent to their wavelength.

Noise figure in optical amplifiers with a split-band architecture

A wide band optical amplifier employing a split-band architecture in which an optical signal is split into several independent sub-bands which then pass in parallel through separate branches of the optical amplifier. Each branch may be optimized for the sub-band that traverses it. The independent sub-bands are combined before output, resulting in a broad band, high efficiency amplifier. Alternative, hybrid split-band amplifiers are described. As a result of their desirable characteristics, these wide band optical amplifiers may be used in dense WDM communications systems.

Wide band optical amplifier

We have recognized that in a Wavelength Division Multiplex (WDM) ring network with a closed loop, the possibility of lasing at Amplified Spontaneous Emissions (ASE) wavelengths exists. In accordance with the present invention, we provide a technique for protecting against lasing in a WDM ring network comprising a plurality of serially connected node links, each node link including a segment of optical fiber and an access node site for adding or dropping one or more active wavelength channels to a transmission bandwidth of the network. The protection means changes the optical transmission characteristics of the transmission bandwidth of the network to ensure that the network loop gain at any wavelength is always less than the network loop loss.

Stable wavelength division multiplex ring network

An optical amplifier that receives an optical signal at an input of the optical amplifier is arranged so that it amplifies the signal and then adjusts the level of the amplified input signal to a predetermined level. The optical amplifier, which includes a filter to substantially decrease the wavelength dependence of the optical amplifier, then amplifies the adjusted signal and supplies the resulting signal to an output.

Broadband flat gain optical amplifier

In a WDM network employing a plurality of optical amplifiers in at least one optical fiber link, a system and method for dynamically controlling gain in accordance with the collective behavior of the amplifier chain. According to the present invention, the required response time of dynamic gain control is selected substantially inversely relative to the number of amplifiers in the communication path. Illustratively, in a large-scale optical network (with signal channel paths traversing say, a hundred optical amplifiers), the response time of gain control in accordance with the present invention may be on the order of 5 .mu.s or even less.

Method of fast gain control in WDM optical networks

High output power, high gain, and low noise are achieved in a two-stage optical amplifier, suitable for use as a repeater for a long haul lightwave communication system, in accordance with the principles of the invention, by employing a first amplifying stage having a signal gain sufficiently small to prevent self-saturation by amplified stimulated emission (ASE) that uses counter-propagating pump light to cause maximum inversion of the first stage amplifying medium. In an illustrative embodiment of the invention, EDFAs are used in each of two amplifying stages. The length of the EDFA in the first stage is short enough to ensure nearly complete inversion of the EDFA from pump light that counter-propagates with the signal. The counter-propagating pump light allows the invention to advantageously avoid the significant noise figure penalty from the input loss associated with co-propagating pump light. And, noise figure is improved because complete inversion is achieved throughout the EDFA, and, at the input where the noise figure is most sensitive to inversion. The short length also eliminates self-saturation of the EDFA from ASE which degrades the noise figure. However, the length, and hence the gain, of the EDFA in the first stage is long enough to provide sufficient gain so that the noise figure of the two-stage amplifier, as a whole, is determined primarily by that of the first stage. A second EDFA in the second stage of the amplifier may then be configured using co-propagating or counter-propagating pump light for additional signal amplification to provide the required output power and gain for long haul lightwave systems. Other aspects of illustrative embodiments of the invention include the use of passive optical elements including filters, isolators, and attenuators.

High power, high gain, low noise, two-stage optical amplifier

The present invention relates to a method of mitigating gain peaking in a chain of fiber amplifiers by pumping the amplifiers at a predetermined wavelength to produce gain over a specified wavelength range.

Method of mitigating gain peaking using a chain of fiber amplifiers

In an embodiment for obtaining accurate noise figure measurements for any degree of saturation of an optical amplifier, a polarizer is located at the output of the optical amplifier. The amplified spontaneous noise (ASE) produced by an optical amplifier is not polarized, whereas the amplified signal has a well defined state of polarization which is preferably linear. If the amplified signal is not linearly polarized, it can be rendered linearly polarized in one direction by means of a polarization controller located downstream of the polarizer. By setting the polarizer to have its state of polarization orthogonal to that of the linearly polarized amplified signal, the spectral density of the ASE from the polarizer can be measured without associated distortion due to the signal. By sequentially adjusting the polarization controller to minimize and then maximize the signal which it passes, sequential measurements of the ASE spectral density and gain of the optical amplifier can be obtained. Continuous measurements of the ASE can be obtained by placing a splitter such as a 3dB coupler between the polarizer and the optical amplifier and setting the state of polarization of the polarizer to be continuously orthogonal to the state of polarization of the amplified signal from one leg of the splitter. At the same time, a signal from the other leg of the splitter which consists of ASE noise plus the amplified signal can be used to obtain the gain of the optical amplifier. From the measurements of the ASE with and without the amplified signal, the noise figure of the optical amplifier can be calculated.

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Ocean, NJ, United States of America

James W Sulhoff