Optical: systems and elements – Optical amplifier – Correction of deleterious effects
Reexamination Certificate
2000-03-29
2002-04-23
Hellner, Mark (Department: 3662)
Optical: systems and elements
Optical amplifier
Correction of deleterious effects
C359S341100
Reexamination Certificate
active
06377391
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is in the field of fiber optic communication systems, and, more particularly, is in the field of amplifiers for use in fiber optic communication systems.
2. Description of the Related Art
In order to satisfy the increasing demand for bandwidth, fiber optic communication systems are moving towards wavelength-division multiplexing (WDM), in which many channels at separate wavelengths are carried on the same fiber. Integral to most current fiber communication systems is the incorporation of erbium-doped fiber amplifiers (EDFAs). When EDFAs are integrated into WDM systems, the gain flatness of the amplifiers becomes critical. For example, a fiber communication line
100
comprising a serial chain of loss sections
110
(
i
) and gain sections
120
(
i
) is shown in FIG.
1
A. In
FIG. 1A
, the loss sections
110
(
i
) are represented as lengths of fiber, and the gain sections
120
(
i
) are represented by EDFAs. Each loss section
110
(
i
) and an associated gain section
120
(
i
) is referred to herein as a “loss-gain section.”
In a WDM system, multiple independent channels are input at discrete wavelengths. The gain of EDFAs is a function of the wavelength, with a typical unfiltered gain variation appearing in FIG.
1
B. After propagation through the first loss-gain section
110
(1),
120
(1), the channels around 1532 nanometers have higher powers than the other channels due to the EDFA gain variation. Propagating through multiple loss-gain sections causes this disparity in channel powers to grow, and eventually causes the power of some channels out of the last loss-gain section
110
(
n
),
120
(
n
) to drop to unacceptable levels.
To illustrate the foregoing process, the input power spectrum is plotted in
FIG. 2A
, and the output power spectrum after 5 loss-gain sections (i.e., n=5 in
FIG. 1A
) is plotted in FIG.
2
B. The differences in the two spectra in
FIGS. 2A and 2B
illustrate the large disparity in resulting channel powers caused by EDFA gain variations.
Many prior solutions to this problem have involved adding a filter to the EDFA to produce a gain-filter section which is flatter than the gain of the EDFA alone. However, fabricating a filter with a correct shape, which is independent of EDFA parameters (e.g., signal and pump powers) and which is stable with time and temperature, is not trivial.
SUMMARY OF THE INVENTION
The present invention is directed to gain flattening with nonlinear Sagnac amplifiers. The use of nonlinear Sagnac amplifiers for gain flattening is a novel solution. Instead of using a filter which has a loss which varies as a function of wavelength, the present invention is directed to a filter which has a loss that is a function of power. More particularly, the filter in accordance with the present invention attenuates a specific channel i based on the power of that channel. The filter in accordance with the present invention does not provide a loss that is a conventional broadband power-dependent loss. Rather, the filter in accordance with the present invention provides a narrowband power-dependent loss. In other words, the attenuation of the filter at &lgr;
i
, the wavelength of channel i, is a function of the power around that wavelength (i.e., at &lgr;
i
±&dgr;&lgr;), but is not a function of the power at a separate wavelength &lgr;
i±n
outside of the &lgr;
i
±&dgr;&lgr; window. Such a filter is achieved by replacing a standard linear amplifier, as depicted in
FIG. 1A
, with a nonlinear Sagnac amplifier (NSA), which will be described in detail below.
One aspect of the present invention is an amplification system for reducing power differences in a plurality of output optical signals responsive to a plurality of input optical signals having a plurality of respective optical wavelengths and having a plurality of respective input powers. The amplification system comprises an interferometric loop. A coupler couples the plurality of input optical signals to the loop to cause respective first portions of the input optical signals to propagate in a first direction in the loop and to cause respective second portions of the input optical signals to propagate in a second direction in the loop. The coupler combines the first and second portions after the first and second portions propagate in the loop to produce a plurality of output optical signals. An amplifier is located at an asymmetric location with respect to the center of the loop. The amplifier has a gain spectrum which causes the amplifier to have a plurality of respective gains at the plurality of optical wavelengths. The asymmetric location of the amplifier with respect to the center of the loop causes differences in powers of the first signal portions and the second signal portions of the input optical signals while these portions are traveling through the interferometric loop. The differences in powers of the first and second signal portions cause respective phase shifts in the first and second signal portions to occur in the fiber loop due to the optical Kerr effect. The Kerr-induced phased shifts vary in response to differences in the respective input powers and the respective gains to cause a greater Kerr-induced attenuation of input optical signals having a greater gain-power product. Preferably, the amplifier comprises an erbium-doped fiber amplifier. Certain preferred embodiments further include a wavelength division multiplexed coupler in the loop proximate to the amplifier. A pump source is coupled to the wavelength division multiplexed coupler to provide pump light for the amplifier via the wavelength division multiplexed coupler.
Another aspect of the present invention is an amplification system for reducing output power differences in a plurality of output optical signals responsive to a plurality of input optical signals having a plurality of respective optical wavelengths and having a plurality of respective input powers. The amplification system comprises an interferometric loop which has first and second lengths of optical fiber separated by an optical amplifier. The first length of optical fiber is substantially longer than the second length of optical fiber. A coupler couples the optical signals into the interferometric loop to cause respective first and second portions of the optical signals to counterpropagate in first and second directions in the interferometric loop. The coupler combines the respective first and second portions of the optical signals after propagation through the interferometric loop to produce a plurality of respective output signals at the plurality of optical wavelengths. The plurality of output signals have a plurality of respective output powers. The amplifier has a gain characteristic which causes the amplifier to have a plurality of respective gains at the plurality of optical wavelengths. The first and second portions of the optical signals propagating in the first and second directions experience respective Kerr-induced phase shifts caused by self-phase modulation, by copropagating cross-phase modulation, and by counterpropagating cross-modulation. The location of the amplifier causes light propagating in the first direction to pass through the first length of optical fiber before propagating through the amplifier and the second length of optical fiber. The location of the amplifier also causes light propagating in the second direction to propagate through the second length of optical fiber and the amplifier before propagating through the first length of optical fiber. The location of the amplifier also causes the light propagating in the first direction to experience greater counterpropagating cross-phase modulation than the light propagating in the second direction. The location of the amplifier also causes the light propagating in the second direction to experience greater self-phase modulation and greater copropagating cross-modulation than light propagating in the first direction. The Kerr-induced phase shifts of the plurality of optica
Digonnet Michel J. F.
Vakoc Benjamin J.
Hellner Mark
Knobbe Martens Olson & Bear LLP
The Board of Trustees of the Leland Stanford Junior University
LandOfFree
Gain flattening with nonlinear Sagnac amplifiers does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Gain flattening with nonlinear Sagnac amplifiers, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Gain flattening with nonlinear Sagnac amplifiers will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2858275