Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1998-07-30
2002-06-11
Pascal, Leslie (Department: 2733)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200, C359S199200
Reexamination Certificate
active
06404522
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to optical communication generally, and more particularly to optical communication utilizing wavelength division multiplexing (WDM).
BACKGROUND OF THE INVENTION
Today, the telecommunication industry is experiencing growth in demand for communication services and such demand is further expected to grow in the future. One of the ways to meet such demand is by expanding capacity of information carried over fiber optic cables in optical communication systems. One of the most advanced ways of achieving such expansion today is by using wavelength division multiplexing (WDM) for transmitting simultaneously multiple signals at different wavelengths over the same fiber optic cable.
The concept of WDM is based on the theory that a discrete light frequency can carry its own unique package of information. Thus, for example, two separate frequencies that carry data can be combined and transmitted in a combined form along a fiber optic cable to a receiving end. At the receiving end the two frequencies can be received and separated, and the data carried by each separate frequency can be regenerated. Based on this concept, the first systems that employed WDM multiplexed and demultiplexed signals at wavelengths of 1310 nanometer (nm) and 1550 nm (1.0 nanometer is 1.0*10**−9 meter or 1.0**−3 micrometer).
The advent of wide-band optical amplifiers made WDM of many wavelengths onto the same fiber optic cable practical since a plurality of wavelengths in an operating bandwidth of a wide-band optical amplifier could be amplified simultaneously by the same amplifier. With wide-band optical amplifiers being available, communication over long distances can be implemented.
A common wide-band optical amplifier which is commercially available today is the Erbium Doped Fiber Amplifier (EDFA) which has an operating bandwidth around 1550 nm. Wide-band optical amplifiers at 1310 nm are not yet commercially available today, although development of such optical amplifiers continues.
Other elements of a WDM based system, such as multiplexers/demultiplexers, optical transmitters, optical receivers and tunable optical filters, exist today and are commercially available. However, there are still several problems that have to be resolved in order to provide reliable optical communication systems which utilize WDM.
One of the problems which typically arises in systems utilizing dense wavelength division multiplexing (DWDM) of several tens of channels or more relates to spacing of channel wavelengths. If channel wavelengths are not spaced an adequate distance apart, drifts in wavelength characteristics of optoelectric components over time may cause interference between channel wavelengths. The drifts may be generated, for example, due to a change in temperature. Such drifts are typically of a low frequency type.
Other problems relate to wavelength stability of the optical transmitters, and to degradation and interference effects arising from use of non-ideal fiber optic cables. Such effects include, for example, dispersion, self-phase modulation, and cross-phase modulation.
Descriptions of optical communication systems utilizing WDM and of elements of such communication systems are found in the following publications:
An article titled “Mining the Optical Bandwidth for a Terabit per Second”, by Alan Eli Willner, in IEEE Spectrum, April 1997, pp. 32-41;
An article titled “Record Data Transmission Rate Reported at ECOC 96”, by Paul Mortensen, Laser Focus World, November 1996, pp. 40-42;
An article titled “Multiple Wavelengths Exploit Fiber Capacity”, by Eric J. Lemer, Laser Focus World, July 1997, pp. 119-125;
An article titled “Advances in Dense WDM Push Diode-Laser Design”, by Diana Zankowsky, Laser Focus World, August 1997, pp. 167-172;
An article titled “Multistage Amplifier Provides Gain Across 80 nm”, by Kristin Lewotesky, Laser Focus World, September 1997, pp. 22-24;
The Communications Handbook, CRC Press & EEE Press, 1997, Editor-in-Chief Jerry D. Gibson, Section 65, pp. 883-890; and
An article titled “WDM Local Area Networks”, by Kazovsky et al., IEEE LTS, May 1992, pp. 8-15.
Additionally, in U.S. Pat. No. 5,170,273 to Nishio there is described a cross-talk reducing optical switching system which receives electrical digital signals at its input terminal.
U.S. Pat. No. 5,191,457 to Yamazaki describes a VWDM optical communication network in which optical beams are modulated by channel discrimination signals of different frequencies.
U.S. Pat. No. 5,194,977 to Nishio describes a wavelength division switching system with reduced optical components using optical switches.
U.S. Pat. No. 5,557,439 to Alexander et al. describes wavelength division multiplexed optical communication systems configured for expansion with additional optical signal channels.
U.S. Pat. No. 5,680,490 to Cohen et al. describes a comb splitting system which demultiplexes and/or multiplexes a plurality of optical signal channels at various wavelengths.
U.S. Pat. No. 5,712,932 to Alexander et al. describes reconfigurable wavelength division multiplexed systems which include configurable optical routing systems.
The disclosures of all references mentioned above and throughout the present specification are hereby incorporated herein by reference.
SUMMARY OF THE INVENTION
The present invention seeks to provide a method and system for improving communication performance in optical communication systems utilizing WDM.
In the present invention, an optical communication system utilizing WDM converts data provided over up to N channels to optical signals which are communicated over at least some of N channel wavelengths corresponding to the N channels. The N channel wavelengths are spaced S
1
nm from each other.
Communication performance of the optical communication system may be improved in any of the following cases and in any combination thereof:
(1) not all the N channels are carrying data simultaneously and communication performance of some channels that carry data is low;
(2) time dependent changes, such as temperature changes, cause degradation in communication performance; and
(3) actual capacities of at least some of the N channels are lower than a maximum attainable channel capacity and communication performance of some of the channels that carry data is low.
In the case that the optical communication system is only partially loaded, i.e. only K out of the N channels carry data, the K channels carrying data are detected at a switching unit which forms part of the optical communication system.
After the K channels carrying data are detected at the switching unit, a controller at the switching unit calculates a channel spacing S
2
which is greater than S
1
. Additionally, the controller also computes a number NCW, where NCW characterizes a distribution of channel wavelengths in which NCW channel wavelengths in a sub-group of K channel wavelengths corresponding to the K channels are spaced at least S
2
nm from at least one nearest neighbor channel wavelength in the sub-group of K channel wavelengths.
After computing the values of NCW and S
2
, tile controller uses the values of NCW and S
2
to select a sub-group of K optical transmitters to be used for transmission of the data carried over the K channels. Selection of the sub-group of K optical transmitters is performed by sequentially determining a number of NCW optical transmitters which transmit at channel wavelengths spaced S
2
nm from each other, and then determining the rest of K-NCW optical transmitters which transmit at channel wavelengths that are not spaced S
2
nm from each other.
After determination of the sub-group of K optical transmitters, the controller provides to a router control signals identifying the selected sub-group of K optical transmitters. The router routes the data carried over the K channels to the sub-group of K optical transmitters for transmission thereby.
The selection of the sub-group of K optical transmitters generally enables transmission of the data over channel wavelengths spaced at an incr
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