Lucent Technologies Inc.

Alcatel Lucent

Atul Kumar Srivastava

Yan Sun

James W Sulhoff

John L Zyskind

Sonali Banerjee

Gengxian Luo

Robert William Tkach

Andrew Roman Chraplyvy

Paul Francis Wysocki

Thomas Huntington Wood

Robert Meachem Jopson

Liyan Zhang

Jianhui Zhou

Virendra S Shah

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

The effects of optical impairments on optical signal transmission are substantially reduced in a lightwave transmission system by positioning optical amplifiers and network elements in respective upstream-downstream combinations. By placing an optical amplifier at a position upstream from its corresponding network element, sufficient amplification can be provided by the optical amplifier to compensate for losses introduced by its corresponding network element. Advantageously, the corresponding downstream network element provides sufficient attenuation of the forward travelling lightwave signals so that power-dependent nonlinear effects in the optical fiber do not significantly distort the lightwave signals. Moreover, because of the downstream location of the network element in relation to its corresponding network element, the network element substantially suppresses backward travelling optical signal components such as those caused by Rayleigh backscattering, Stimulated Brillioun Scattering (SBS), and the like. As such, the network element prevents unwanted back reflections and back scattered signals from affecting operation of its corresponding optical amplifier.

System and method for reducing effects of optical impairments in optically amplified lightwave communication systems

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

A two-wavelength WDM analog CATV transmission system which utilizes wavelength channel spacing of about 2.2 nm and laser-dithering. The system includes a pair of optical transmitters (DBF lasers), a pair of modulators, a dithering device, a polarization controller, a 3-dB combiner, an optical amplifier (EDFA), an attenuator, a length of optical fiber, a filter and a CATV receiver. The two lasers have wavelengths of .lambda.1 and .lambda.2, where the difference between .lambda.1 and .lambda.2 is approximately 2.2 nm. The modulators are driven with analog signals comprising 77 NTSC channels between 55.25 and 535.25 MHz A 2-GHz dithering tone is applied to the .lambda.1 laser and the output of the .lambda.2 laser is passed through the polarization controller to align the polarization of the two transmitters. The two modulated and aligned outputs are combined by the 3-dB combiner and launched into the EDFA The output of the EDFA is divided by the splitter four ways and one of the four outputs is passed through the attenuator and launched into the transmitting end of the optical fiber. The receiving end of the fiber is connected to a filter, which provides input into the CATV receiver. By utilizing a wavelength channel spacing of about 2.2 nm and dithering one of the lasers, worst-channel Composite Triple Beat distortion is reduced to -60 dBc, an acceptable level for deployment of WDM analog CATV systems.

Two-wavelength WDM Analog CATV transmission with low crosstalk

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.

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Eatontown, NJ, United States of America

Atul Kumar Srivastava