Optical: systems and elements – Optical amplifier – Beam combination or separation
Reexamination Certificate
2000-06-30
2002-12-10
Hellner, Mark (Department: 3663)
Optical: systems and elements
Optical amplifier
Beam combination or separation
C359S337000
Reexamination Certificate
active
06493133
ABSTRACT:
FIELD OF INVENTION
The present invention relates generally to long-haul, fiber optic transmission systems. More particularly, the present invention relates to increasing the capacity of undersea cable systems.
BACKGROUND OF THE INVENTION
An undersea network of fiber-optic telecommunication cables links continents, nations and lands together. Communication traffic on these submarine cables has increased dramatically in recent years and is expected to continue to grow as a result of many factors, including, for example, the globalization of world economies and businesses, the increased demand for international communications capabilities, and the development of multimedia applications and revolutionary resources such as the Internet. These factors necessitate increasing capacity not only by adding cables, but also by optimizing the operation of existing submarine cables. The term “capacity” as used herein refers to the total bit rate of the cable system which represents the sum of the bit rates of each channel on all the fibers within a cable.
Approaches for increasing capacity on existing cables have traditionally been aimed at optimizing bandwidth utilization. Long-haul optical fiber transmission paths, such as those employed by undersea cables, typically extend over 5000 km and operate in the C-band. The C-band spans from about 1525 to about 1565 nm and is optimal for low-absorption losses. Within the C-band, capacity may be increased by increasing the number of channels and the bit rate of the channels. Most long-haul cable systems increase the number of channels through wavelength division multiplexing (WDM). The ultimate capacity of a WDM cable system depends on how closely channels can be packed. Minimum channel spacing is limited, however, by interchannel crosstalk and the degradation of the signal to noise ratio (SNR). It has been found that, to maintain sufficient signal integrity, channel spacing in GHz should exceed, e.g., four times the bit rate in Gb/s. Therefore, for a given bandwidth, the number of channels and bit rate are interrelated and limit the capacity of a cable.
Furthermore, in practice, other factors restrict the use of the entire low-loss bandwidth window of 120 nm near 1550 nm. For example, the number of channels is limited to the bandwidth over which the amplifiers can provide nearly-uniform gain. Other factors that limit the number of channels include nonlinear effects and the tunability of laser transmitters. Therefore, increasing channel bit rate and the number of multiplexed channels is limited by SNR minimums and the current state of the technology.
One approach to increase useable bandwidth involves using the L-band in addition to the C-band. The L-band refers to a bandwidth of about 1570 to about 1610 nm. Combining the L-band with the C-band expands the useable bandwidth from about 1525-1565 (40 nm) to about 1525-1610 nm (80 nm). Use of the L-band has been limited in the past, however, due to several factors, one of the more significant being Raman effects.
In multiple channels systems, the fiber acts as a Raman amplifier such that longer wavelength channels are amplified by shorter wavelength channels when the wavelength difference is within the bandwidth of the Raman gain. The Raman gain of silica fibers is so broad that the amplification can occur for channels spaced as far apart as 200 nm, although the peak amplification occurs between about 110 and about 120 nm from the pump wavelength. The shortest wavelength becomes the most depleted as it can pump many long-wavelength channels simultaneously. It is interesting to note that amplification only occurs when 1 bits are present in both channels simultaneously. This signal-dependent amplification leads to enhanced power fluctuations which add to receiver noise and degrade receiver performance.
The Raman effects between the C- and L-bands are particular problematic compared to the effects within just the C-band, especially in long-haul systems. Given the conventional operating bandwidth within the C-band of less than 40 nm, the Raman effects between channels tend to be insignificant since the peak Raman Stokes shift is about 110 to about 120 nm from the pump wavelength. However, when the C- and L-bands are combined and form a bandwidth of close to 80 nm, the wavelength difference between the shortest and longest channels is quite near the peak Raman Stokes shift.
Aside from Raman effects, combining the C and L-band also is problematic from the standpoint of isolation. More specifically, to isolate bands based on wavelength, a minimum bandgap between them is necessary to maintain the integrity of the channels at the interface of the C- and L-bands. This bandgap tends to be relatively large compared to the spacing between channels and consumes valuable bandwidth thereby reducing capacity.
Therefore, a need exists for increasing capacity on new and existing lines using known technologies while maintaining the integrity of the signals. The present invention fulfills this need among others.
SUMMARY OF INVENTION
The present invention provides an approach for increasing capacity on long-haul cable systems that overcomes the aforementioned problems by employing counter-propagating band signals, preferably, counter-propagating C-band and L-band signals. By using counter-propagating band signals, the Raman effects associated with co-propagating bands are significantly reduced. Applicants suspect that the Raman effects are greatly reduced for counter-propagating signals due to “walk off” and/or “power distribution” effects (described in detail below), although the scope of the present invention is not, in any way, tied to a particular theory. In addition to the benefits of reducing Raman effects, counter-propagating band signals also are more readily isolated. It is more efficient to isolate bands based on their direction of propagation, rather than on their wavelength differences, since a relatively-large bandgap between the bands is not needed to improve the isolation.
In effecting counter-propagating C- and L-band signals on long-haul cable systems, such as undersea cable systems, it has been found that a reduction in backward Rayleigh scattering (BRS) is required. BRS results from random localized variations of the molecular positions in glass that create random inhomogeneities of the reflective index that act as tiny scatters centers. Although BRS is not a problem with counter-propagating band signals over relatively-short transmission paths, such as terrestrial lines of 100-300 km (see, for example, S
uzuki et al
. Bidirectional Ten-Channel 2.5Gbitls WDM Transmission over 250 km Utilizing 76 nm (1531-1607 nm) Gain-band Bidirectional Erbium-doped Fiber Amplifiers, Optical Amplifiers and Their Applications (IEEE/Lasers and Electro-Optics Society, July 1997)), BRS tends to accumulate on long-hauls systems having many cascaded amplifiers. The BRS causes problems in amplification, especially for the L-band which is particularly susceptible to C-band power induced from the BRS of the C-band signals (see, for example, Massicott et al. Low Noise Operation of EY+Doped Silica Fibre Amplifier Around 1-6 &mgr;m (August 1992)). More specifically, as the BRS level rises through accumulation over many amplifiers, it can induce gain and output power fluctuations.
The applicants not only have identified the problems with BRS associated with counter-propagating band signals on long-haul optical fibers, but also have developed a solution. More specifically, it has been found that the cascading effects of BRS can be reduced significantly by isolating and filtering each bandwidth during optical amplification.
The applicants also have developed an approach for isolating and filtering the band signals without incurring high insertion losses and noise figure (NF). More specifically, a novel configuration of bandsplitters is set forth which exploits certain characteristics of bandsplitters. Specifically, a bandsplitter's reflective port has lower insertion loss and NF than its transmission port, while its transm
Chen Chien-Jen
Liang Anhui
Pedersen Bo
Hellner Mark
Tyco Telecommunications (US) Inc.
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