Multiple function bandwidth management systems

Details Multiple function bandwidth management systems Multiple function bandwidth management systems

2000-03-03

2002-07-16

Hellner, Mark (Department: 3662)

Optical: systems and elements

Optical amplifier

Correction of deleterious effects

C359S337100, C359S337500, C359S349000

Reexamination Certificate

active

06421167

ABSTRACT:

BACKGROUND
1. Field of the Invention
This invention relates to optical networks. More particularly, the invention relates to modules that can be used to construct bandwidth-management systems for optical networks. The modules provide gain, power (or gain) equalization, and dispersion compensation on a band-by-band basis, facilitating flexibility in network design and ease of network expansion.
2. Description of the Problem
Fiber optic communication systems, such as those currently used in telecommunication distribution networks, typically require amplification of the signal light to compensate for optical power losses which occur over long distances. This amplification also serves to compensate for losses due to splitting the signal light between different branches of the network. Modern fiber optic communication systems utilize optical amplification devices based on optical fibers which have a core doped with a rare earth, such as erbium. Such a device is commonly known an erbium-doped fiber amplifier (EDFA). These amplifiers are well known in the art. Typical EDFA systems amplify a signal light by passing the signal light through the doped optical fiber while simultaneously pumping the fiber with a relatively powerful laser having a wavelength approximately equal to the absorption wavelength of the rare earth ions. EDFA amplifiers are common in modern optical networks, and have made possible the operation of extremely long networks covering large geographic areas without resorting to electronic regeneration to compensate for losses.
Despite the above advances in optical amplification, long optical spans still suffer from distortion problems that must be corrected if the network is to operate properly. Distortion may arise from a number of sources. Optical fiber systems inherently exhibit a property called dispersion, a pulse-broadening mechanism that reduces the bandwidth of the system. Dispersion may be caused by modal dispersion, in which different modes of a multimode optical fiber propagate at different group velocities. A second type is chromatic dispersion, which results from a combination of material dispersion in the optical glass and geometric effects of the waveguide. Chromatic dispersion causes different spectral components of a signal to propagate at different velocities, inducing pulse spread in high-bit-rate systems.
Once consecutive pulses have spread out so that they are no longer distinguishable from one another, information is lost. Dispersion may be compensated for on a channel-by-channel basis. Various dispersion compensation mechanisms are known. A common dispersion compensation system consists of a length of dispersion compensating optical fiber connected to the system. This special fiber exhibits dispersion characteristics that cancel the dispersion characteristics of the network. U.S. Pat. No. 5,861,970 issued Jan. 19, 1999 provides a good discussion of dispersion in optical networks and is incorporated herein by reference.
Even with known methods of dispersion compensation, dispersion differences between channels lead to non-ideal compensation over all bands. These differences increase with the number of channels and the system length. Dispersion can exceed several thousand picoseconds per nanometer for long-haul systems.
FIG. 1
illustrates a dispersion map, a plot of dispersion versus system length. The map of
FIG. 1
illustrates approximate dispersion for terrestrial systems using large effective area fiber for 16 channels ranging from 1531 nanometers to 1559 nanometers. In this map, dispersion compensating fibers are used at approximately 80 kilometer intervals. Curve
101
approximates an average dispersion for wavelengths from 1553 to 1559 nanometers, curve
102
approximates an average dispersion for wavelengths of 1540 through 1543 nanometers, and curve
103
approximates dispersion for wavelengths in a range of 1531 to 1533 nanometers. The difference in dispersion compensation, &Dgr;D, for this band of frequencies in this relatively short system is approximately 750 picoseconds per nanometer (ps
m).
FIG. 2
shows a similar dispersion map as shown in
FIG. 1
, but for submarine large effective area fiber. Again the dispersion is compensated for using standard single mode fibers every 80 kilometers. Curves
201
,
202
, and
203
, represent average dispersion for the same wavelength ranges as those shown at
101
,
102
, and
103
in
FIG. 1
, discussed above. In this case, the difference in dispersion across the bandwidth of the system is approximately 900 ps
m. As the spectral bandwidth of deployed systems increases, &Dgr;D will increase, and the required dispersion compensation will vary across the spectrum, making it more and more difficult to design optical networks which provide error-free communication.
Other problems with large optical networks result from inadequate gain equalization. With dense wavelength division multiplexing (DWDM), many channels or transmission signals independent of each other are sent over the same line or optical fiber by multiplexing within the domain of optical frequencies. The transmitted channels are distinguishable from each other because each of them is associated with a specific frequency or wavelength. In an optical network, the different channels must be substantially equivalent to each other in terms of signal level. However, doped fibers, as typically used in optical amplifiers, have an emission spectrum with a peak of limited width; the features vary depending on the glass system into which the dopant is introduced, as well as other factors. The accumulated wide-power variation among channels in DWDM systems can deteriorate the overall system performance significantly.
FIG. 3
shows the impact of gain variation with system length for a submarine system, assuming a gain tilt of −0.5 dB to +0.5 dB from short to long wavelengths. For the channels that experience a larger than nominal gain, shown at
301
, the channel power grows with system length. As the system length increases, the channel power increases above the threshold,
303
, for non-linear interaction of the channels in the fiber. Additionally, the channels that experience a smaller than nominal gain,
302
, will lose power with system length. As the system length increases, the channel power will drop below the detection limit
304
. For adequate signal recovery with an optical amplifier there should be sufficient optical signal at the input. Operating below the detection limit or above the linear behavior limit results in degradation of the signal and the information on that channel is not recoverable.
FIG. 4
shows a similar graph, but this time, for terrestrial optical systems. Curve
402
represents wavelengths decreasing in power and curve
401
represents wavelengths increasing in power. Operational limits are shown at
403
and
404
. Filters are often used to provide gain equalization for optical systems to compensate for the effects described immediately above. Other gain equalization methods exist. Gain equalization is also often referred to as power equalization. U.S. Pat. No. 5,852,510, issued Dec. 22, 1998 provides a good discussion of gain equalization, and is incorporated herein by reference.
The interaction of the various requirements for amplification, dispersion, and gain equalization, as discussed above makes the design and configuration of DWDM optical fiber networks difficult and complex. Accommodating or compensating for one of these factors affects the others, and the effects are not always predictable across the network. The problem is especially acute when network expansion is required. As more modes or channels are added to the network, amplifiers, gain equalizers, and dispersion compensators which previously assured adequate performance, no longer work across the entire spectrum and entire network redesign becomes necessary. What is needed is a network design capability to provide for all of the above needs simultaneously on a channel-by-channel or band-by-band basis. Such a solution should ideally

Affiliated with

Also associated with

Rating

Multiple function bandwidth management systems does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Multiple function bandwidth management systems, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Multiple function bandwidth management systems will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2841054