Optical: systems and elements – Optical amplifier – Correction of deleterious effects
Reexamination Certificate
1999-10-04
2001-10-23
Hellner, Mark (Department: 3662)
Optical: systems and elements
Optical amplifier
Correction of deleterious effects
C359S341500, C359S349000
Reexamination Certificate
active
06307668
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to optical amplifiers, and more particularly relates to an ultra-wide bandwidth fiber based optical amplifier which divides the erbium wavelength band (1520 nm-1610 nm) into three separate bands, separately amplifies each of the three bands in parallel configuration, and then recombines the bands to provide uniform gain flatness over the entire bandwidth.
The design of wavelength division multiplexed (WDM) systems in the 1.5 &mgr;m range is currently constrained by the limited bandwidth available from conventional erbium doped fiber amplifiers. The presently available bandwidth is limited to about 20 nm because of the highly structured gain spectrum of conventional erbium doped fibers. The use of gain equalization filters can extend the usable bandwidth up to about 40 nm (about 1525 nm to about 1565 nm). This 40 nm gain spectrum allows the use of more channels in a WDM system. However, proposed 10 Gb/s systems will require the use of the entire 80-90 nm bandwidth with very small channel spacings.
One possible solution to provide greater bandwidth would be to provide an erbium doped fiber that has a gain spectrum over a greater bandwidth. This would allow a single fiber amplifier to provide a gain spectrum over a greater bandwidth. Erbium doped fluoride fibers have shown gain spectrums of 25 nm without gain equalization filters, and newer, tellurite erbium doped fibers have gain spectrums in different ranges, but the gains are highly non-uniform. To date, it has been impossible to provide a single erbium doped fiber which has a uniform gain spectrum over more than a 25 nm bandwidth.
Another proposed solution is to divide the erbium bandwidth into two bands and separately amplify the separated bands in parallel configuration. This concept allows the use of two different amplifiers which can be optimized for a flat gain region within a specific band. This solution was proposed in the Apr. 10, 1997 publication of Electronics Letters (Vol. 33. No. 8). The article describes a broadband amplifier which divides the available bandwidth into two bands a 1520 nm-1570 nm band (1554 nm band) and a 1570 nm-1610 nm band (1.58 &mgr;m band). The configuration of each band is based on a cascade configuration with a 980 nm pumped EDFA and a 1480 nm pumped EDFA using a combination of silica and fluoride fibers to optimize gain flatness. The EDFA unit for the 1.55 &mgr;m band showed a relatively flat gain spectrum from 1530 nm-1560 nm, and the EDFA unit for the 1.58 &mgr;m band showed a relatively flat gain spectrum from 1576 nm-1600 nm. The result is a wide bandwidth amplifier having a 54 nm flat gain spectrum. Although demonstrating an improved gain bandwidth of 14 nm over the prior single amplifier systems, this parallel configuration still loses significant bandwidth between the optimum gain spectrums, i.e. between 1560 nm and 1576 nm.
An 80 nm gain flattened amplifier using only silica erbium doped fibers was described in the Nov. 6, 1997 publication of Electronic Letters (Vol. 33 No. 23). Expansion of the gain flattened bandwidth from 54 nm to 80 nm was achieved by using two separate EDFA sections. The entire bandwidth is amplified in a first common section. After the first section, the optical channels are split into two bands, a C-band with a range of 1520 nm-1570 nm and an L-band with a range of 1570 nm-1620 nm. The C-band branch has a single stage amplifier, while the L-band branch has a two stage amplifier. The gain bandwidth in the C-band was shown to be 36.9 nm while the gain bandwidth in the L-band was shown to be 43.4 nm giving a total gain bandwidth of 80.3 nm. While the system demonstrates an even greater gain spectrum, the gain spectrum in both the C-band and L-band are non-uniform which makes real-life utilization of the entire gain spectrum difficult. The author's solution to improve gain spectrum flatness in the L-band is to change the inversion level, however, this comes at the expense of bandwidth. Accordingly, the entire 80 nm bandwidth would not be usable in an actual commercial device.
Furthermore a significant concern which prevents practical implementation of these proposed parallel designs is the problem of multipath interference (MPI) which is a phenomenon which naturally occurs when recombining two or more wavelength bands into a single fiber. Neither article discusses the problem or mentions an), possible solution to the problem.
Accordingly, while there have been attempts to provide a wide bandwidth amplifier having a greater gain spectrum, none of the present solutions solves the ultimate challenge of providing uniform gain flatness over the entire 1.5 &mgr;m bandwidth.
SUMMARY OF THE INVENTION
The present invention seeks to solve the prior art shortcomings by dividing the erbium wavelength band into three separate bandwidths 1520 nm-1541 nm (C
1
band), 1541-1565 (C
2
band) and 1565-1610 (L band) and separately amplifying each bandwidth with a specially designed amplifier block optimized to provide a flat gain spectrum within the limited bandwidth. The amplifier then recombines the separately amplified band to provide an ultra-wide bandwidth amplifier with a flat gain spectrum over the entire 90 nm bandwidth.
The concept of splitting the C band into two separate bands may seem controversial at first since it clearly adds complexity where none would seem to be needed. However, it will be shown herein that there are significant advantages to be found in this approach. The underlying physics of each of the three bands is significantly different, and these differences have many subtle effects on gain, noise figures, output power, saturation/inversion conditions, and required pumping power. By separating the conventional C band into two separate C
1
and C
2
bands, the lower limit of the C
1
band can be broadened to include 1520 nm with the proper choice of glass host, thus gaining up to 5 nm of bandwidth and compensating for channels lost at the intersection of the C
1
and C
2
bands. In the C
1
band, there is an inherent gain peak at 1530 nm. Eliminating this peak becomes much easier with a total C
1
bandwidth of 20 nm versus 35 nm for the conventional (1525-1565 nm) C bandwidth. This will allow for implementation of a gain equalization filter that optimizes performance in this band without imposing tradeoffs in other adjacent bands. In the C
2
band there will be no gain equalization filtering needed due to the inherent gain flatness of this band. Optimally designed Erbium doped fiber will be used in this band to further improve gain flatness. The splitting of the conventional C band will also reduce the effects of spectral hole burning between the 1530 nm peak and the 1550 nm peak. In addition, dispersion compensation becomes easier to implement due to the reduced bandwidth of each separate band.
The present invention also addresses the challenges of separating and then efficiently recombining multiple wavelength bands, which typically causes a dip in gain at the intersection of the two bands and also causes multi-path interference (MPI). The problem is resolved by constructing all three amplifier blocks with the same optical transmission length. The C
1
and C
2
band amplifier blocks, which include shorter erbium doped fibers than the L band amplifier block, are physically lengthened using lengths of single mode fiber so that the total length of the optical transmission path of each amplifier block is generally equal. Fiber lengths are controlled to within 500 microns. Selected amplifier blocks further include delay control devices which selectively delay signals passing through the respective amplifier block to provide further fine adjustment to signal recombination.
More specifically, the wide bandwidth optical amplifier of the present invention includes first, second and third amplifier blocks. A demultiplexer device splits the 1.5 &mgr;m wavelength band into first (C
1
), second (C
2
) and third (L) bandwidths, and outputs the respective bandwidths to the input ends of the first, second an
Bastien Steven P.
Jiang Shijun
Krishnan Mala
Manzur Tariq
Barlow Josephs & Holmes
Hellner Mark
Optigain, Inc.
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