Optical: systems and elements – Optical amplifier – Correction of deleterious effects
Reexamination Certificate
2001-10-11
2003-09-16
Moskowitz, Nelson (Department: 3663)
Optical: systems and elements
Optical amplifier
Correction of deleterious effects
C359S334000, C359S337100, C359S341410, C372S003000
Reexamination Certificate
active
06621621
ABSTRACT:
FIELD OF THE INVENTION
The invention is directed to optical telecommunications networks, and in particular, to a line amplification system for wavelength switched optical networks.
BACKGROUND OF THE INVENTION
The equipment of an optical network can be generally classified into two categories, namely the switching nodes and the line system. The switching nodes are concerned with switching the channels in the input WDM (wavelength division multiplexing) signal to an output of choice, and with add/dropping the on-ramp/off-ramp user signals into/from the WDM signal. The line system includes the optical components and the fiber between two successive switching nodes, and is concerned with conditioning (line amplification, power control, dispersion control, etc.) the WDM signals to achieve long-haul transmission. Generally, the switching nodes may also include a preamplifier and a postamplifier, which are part of the line system.
Optical network architecture
Current optical networks are based on a WDM physical layer, using point-to-point (pt-pt) connectivity. While ultra-long reach achieved lately provides lower cost networks by substantially reducing the number of line regenerators, regeneration is nonetheless performed for all channels at the switching nodes, as often called ‘hidden regeneration’. This is because point-to-point connectivity implies OEO (optical-to-electrical-to-optical) processing of all channels arriving at a switching node. While optical-to-electrical O/E and E/O conversions are necessary for the off-ramp and on-ramp signals, they are not always necessary for the signals that pass through a switching node. The passthrough traffic, which is unnecessarily OEO processed, accounts for a large percentage (over 50%) of the total traffic at a node. As the number of channels in the WDM signal grows, the cost of the ‘hidden regenerators’ also grows, hindering the profit for the network provider.
The present invention is applicable to a wavelength switched network where each signal travels between a different source and destination node, without unnecessary OEO conversions at all intermediate nodes. The present specification is concerned with the line amplification system of such a network, that is generally described in the co-pending patent applications “Architecture for a Photonic transport Network” (Roorda et al.), Ser. No. 09/876,391, filed on Jun. 8, 2001. The present invention is also concerned with a line control system generally described in the patent application “Method for Engineering connections in a dynamically Reconfigurable Photonic Switched Network” (Zhou et al.), provisional patent application filed Jul. 18, 2001, Ser. No. 60/306,302, formal patent application filed August 2001, Ser. No. 09/930,528. This patent application claims priority from both above-mentioned patent applications. Details about the software architecture and operation of this photonic network are also described, illustrated and claimed in the co-pending provisional patent application “Architecture for an Optical Network Manager” (Emery et al.), Ser. No. 60/298,008, filed on Jun. 13, 2001, which is incorporated herein by reference.
To summarize, the conventional architecture is replaced by a new architecture where repetitive regeneration of all channels in a WDM signal is not necessary, regeneration being performed only for individual channels based on the current network performance. Thus, the challenges in designing a line amplification system for such a network are substantially different from those encountered in conventional transport networks. For example, the number of the channels in a WDM signal on any link of such a network, as well as the bandwidth of the WDM signal, change as channels are arbitrarily added and removed across the network. As well, traditional channel performance optimization methods cannot be applied to end-to-end connections that pass through many nodes without OEO conversion.
Thus, there is a need to provide a line amplification system adaptable to condition a WDM signal with a variable number of channels, variable wavelength-to-channel allocation, and random channel add/drop.
There is also a need to provide a line amplification system that allows for use of OEO regeneration only at the nodes, and only for specific channels that need regeneration, based on the current network connectivity and performance.
There is also a need to provide a line amplification system with a line control system adapted to collect current information on current physical performance parameters of the span and link, to allow for individual channel optimization in the context of dynamic configuration and reconfiguration of the network.
Long reach and ultra-long reach optical transmission
Expansion of long haul optical communication networks has been fueled by the data traffic, and is estimated to be in the order of 70-150%. Particularly, since the popularity of the World Wide Web has enabled business transactions over the Internet, IP (Internet Protocol) and IP-based services have grown and evolved dramatically.
The reach, or the distance traveled by an optical channel along a path between a source node and a destination node, is limited by the combined effect of attenuation and distortion experienced by the signal along the path.
A solution to control attenuation is to place electro-optic repeaters (regenerators) at distances of 40-80 km, for retiming, regenerating and reformatting the optical signal. This solution however implies conversion of the optical signal to an electrical format and re-conversion of the processed electrical signal into an optical format (OEO conversion). With the advent of WDM, the cost of regenerators became prohibitive; this lead to development of optical amplifiers, which amplify an entire transmission band, i.e. a plurality of channels passing through it, without OEO conversion.
There are three types of optical amplifiers: post-amplifiers that connect to a transmitter to boost the output power, line amplifiers that amplify the optical signals along the signal route, and preamplifiers that improve the sensitivity of optical receivers. These different types of amplifiers provide different output power levels, use different input power levels, and generally have different noise figure requirements. The typical distance between two optical amplifiers is 80-100 km.
Although the EDFAs can support very long fiber spans by significantly increasing the optical power of all optical channels passing through them, they exhibit a wavelength-dependent gain profile, noise profile, and saturation characteristics. Hence, each optical channel experiences a different gain along a transmission path. The gain tilt is controlled typically, by selecting the channels of the WDM signal to have a similar gain tilt; however, this is not always possible, especially for networks with a high density of channels. Another solution used lately is to provide the optical amplifiers with dynamic gain flattening means such dynamic gain equalizers (DGE), which flatten-out specific wavelengths and can be tuned as needed.
For transmission speeds over 2.5 Gb/s, signal corruption caused by Chromatic Dispersion (CD) also becomes very important. Chromatic dispersion is the dependence of the speed of light on its frequency (wavelength), measured in ps
m, and is attributable to optical fiber and optical components in general. CD compensation is realized by installing devices with a net CD in the opposite sense. For example, if a network provider wishes to compensate for 1700 ps
m of CD for a particular wavelength or a set of wavelengths, it can use a dispersion compensating module (DCM) that has a negative value of −1700 ps
m of CD in the same wavelength regime. After the compensator, the CD is essentially zero. Sometimes the network provider will compensate the net dispersion to a non-zero value.
Another way to increase the signal reach is to use the Stimulated Raman Scattering effect. Thus, by pumping the fiber using a laser of a certain power(s) and wavelength(s), the signal is furth
Beer Paul Edward
Jones Kevan Peter
Solheim Alan Glen
Wight Mark Stephen
Innovance, Inc.
Moskowitz Nelson
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