Optical communications – Transmitter and receiver system – Including optical waveguide
Reexamination Certificate
1998-12-18
2003-11-18
Pascal, Leslie (Department: 2633)
Optical communications
Transmitter and receiver system
Including optical waveguide
C398S141000, C398S148000, C398S079000, C398S081000, C398S178000
Reexamination Certificate
active
06650842
ABSTRACT:
FIELD OF THE INVENTION
The present invention generally relates to an optical communication network, and more particularly, to a system and method for reducing four-wave mixing along an extended optical fiber link.
BACKGROUND OF THE INVENTION
Optical communication networks serve to transport information at high data rates between a number of physical sites, commonly referred to as nodes. Each of these nodes are interconnected with the various other nodes by information conduits, commonly referred to as links. These links are comprised of at least one, and usually several, optical fibers. Information is usually presented to the optical communication network in the form of time-domain electrical data signals, and may represent any combination of telephony, video, or computer data in a variety of formats.
As depicted in
FIG. 1
, multiple data signals D
1
, D
2
, D
3
and D
4
are typically provided to a site for transport over an optical network
10
. Typically, these data signals are coupled via a digital cross connect switch (not shown) to an optical transmitter. Each optical transmitter includes at least one, and usually several semiconductor lasers. Each semiconductor laser responsively emits light that is intensity-modulated by a corresponding input electrical data signal. This intensity-modulated light is wavelength division multiplexed (WDM) and transmitted over an optical fiber
14
to an optical receiver
16
at a remote site. That is, the transmitter converts the electrical data signals to optical signals each having a carrier frequency in the light spectrum, each input electrical data signal modulating a light carrier having a different light frequency and corresponding wavelength. Thus, as shown, for four electrical input data signals, there are transmitted four optical signals having four different wavelengths across a single optical fiber. In this manner, the data from each of these data signals is wavelength division multiplexed (WDM) over a single optical fiber.
Examples of these electrical data signals can be SONET-compliant STS-48 or STS-192 synchronous data signals each bearing digital data at about 2.5 Gbps or 9.9 Gbps, respectively. Correspondingly, the optical data signals may include SONET OC-48 or OC-192 signals bearing digital data at approximately 2.5 or 9.9 Gbps respectively. Generally, various high data rate electrical data signals are multiplexed over a single optical medium.
Frequently, optical fiber lengths between nodes are so long that several intermediate amplification stages
18
are required along the length of the fiber
14
, as shown in FIG.
1
. In addition to intensity loss along the fiber, the fiber medium can also introduce other impairments, such as chromatic dispersion. Chromatic dispersion causes individual light pulses to become blurred and less discernible along the optical fiber. If chromatic dispersion is not compensated in a distributed fashion, the total dispersion will accumulate along the length of the fiber as shown at
20
in
FIG. 2
, and the optical signal may eventually be unrecoverable at the receiving end.
One common practice for minimizing chromatic dispersion is to incorporate a dispersion compensator (DC) into each line amplifier
26
of a network
24
as shown in FIG.
4
. In this approach, dispersion compensation is accomplished in a distributed fashion, as shown at
28
in FIG.
5
. At each optical amplifier
26
, the dispersion compensator attempts to minimize the chromatic dispersion at that point, as shown at P. The dispersion compensator nearly cancels the chromatic dispersion introduced by the optic fiber
14
up to that point. Furthermore, the dispersion compensator in a WDM system attempts to minimize the chromatic dispersion across a band of wavelengths by exhibiting a dispersion slope characteristic, as a function of wavelength, that is opposite that of the native optic fiber. Conventionally, each dispersion compensator is set to reduce chromatic dispersion to near zero at the amplifier for all wavelengths in a band.
Another potential transmission impairment observed along optical lengths is mixing among optical carriers, as shown at
22
in FIG.
3
and at
29
in FIG.
6
. This mixing is caused by non-linearities in the amplification stages, and in the fiber medium at high carrier power levels. Relatively high launch powers into an optical fiber can cause the optical carriers to mix and create unwanted signal components that interfere with desired carriers. This mixing problem can be circumvented by lowering carrier power, and by steering the optical carrier wavelengths so as to move the unwanted byproducts into a harmless position in the receive spectrum. However, using lower transmit power limits the maximum distance between optical amplifiers. Steering of optical wavelengths complicates the control and selection of the optical carriers, and subverts the desire to use an evenly-spaced comb reference in a dense WDM channel plan.
There is a desire to provide an improved optical communications link that offers improved tolerance to high launch power of an optical carrier into an optical fiber to overcome attenuation through the length of the optical fiber, while minimizing the carrier interactions such as four-wave mixing that usually accompany such high launch power levels.
SUMMARY OF THE INVENTION
The present invention achieves technical advantages as an optical communication link with improved tolerance to high launch power of optical carriers and reduced carrier mixing. In accordance with the present invention, the optical communication link includes an optical fiber, at least one optical amplifier disposed along the optical fiber, and at least one dispersion compensator. In one preferred embodiment, a first dispersion compensator is located along an optical fiber between a transmitter and a first optical amplifier, with first dispersion compensator placed at some substantial distance prior to the first optical amplifier. This allows an optical carrier to be launched at a relatively high power into a fiber under dispersive conditions. A second dispersion compensator may be inserted into the fiber line at a point after the first optical amplifier where the high launch power has been eventually diminished by fiber attenuation and non-linearity of the fiber medium is no longer a problem. The second dispersion compensator brings the dispersion to a manageable value. The loss introduced by the dispersion compensator is tolerated by an ample gain budget. The optical signal then continues towards a second optical amplifier and the dispersion is again allowed to accumulate so that the optical signal is dispersively propagated during and immediately after amplification at the second optical amplifier. In this first preferred embodiment, the amplifiers and dispersion compensators are alternately spaced from one another. Four-wave mixing is reduced by advantageously using the property that chromatic dispersion in the optical fiber tends to disrupt the phase coherence that is required for mixing to occur. The present invention provides manageable chromatic dispersion at the amplifiers which tends to reduce the four-wave mixing that can occur through the amplification stage when multiple wavelengths are amplified.
According to a first method of the present invention, there is provided a method for processing an optical signal along an optical fiber link. The method comprises coupling an optical signal to the link, dispersion compensating the optical signal, allowing the optical signal to become mildly dispersed as it travels through the fiber, and then amplifying the mildly dispersed optical signal using an optical amplifier. The optical amplifier amplifies a moderately dispersed optical carrier, which advantageously reduces four-wave mixing. Consequently, a relatively high launch power of the optical carrier into the optical fiber is allowed. Another dispersion compensator compensates for the accumulated chromatic dispersion at a point past the optical amplifier, after sufficient attenuation has occurred due to
Fee John A.
McKiel, Jr. Frank A.
Pascal Leslie
Phan Hanh
WorldCom, Inc.
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