WO2004019503A2

WO2004019503A2 - Method and apparatus for providing staggered groupwise dispersion compensation in a wdm optical communication system

Info

Publication number: WO2004019503A2
Application number: PCT/US2003/026109
Authority: WO
Inventors: Stephen G. Evangelides, Jr.
Original assignee: Red Sky Systems, Inc.
Priority date: 2002-08-20
Filing date: 2003-08-20
Publication date: 2004-03-04
Also published as: WO2004019503A9; WO2004019503A3; AU2003265538A8; AU2003265538A1

Abstract

A method and apparatus (Fig 1) is provided for compensating for dispersion in a WDM optical communication system that includes a transmitter, receiver, and an optical fiber transmission path coupling the transmitter to the receiver. The method begins by imparting dispersion precompensation to a WDM optical signal having a prescribed bandwidth before the WDM optical signal traverses the optical fiber transmission path (140). The dispersion compensation is provided by first dividing the prescribed bandwidth of the WDM optical signal into a first plurality of sub-bands, and, within each of the first plurality of sub-bands, compensating each wavelength in the sub-band for dispersion by imparting a first fraction of a delay that would be needed to compensate for dispersion at a prescribed wavelength. The prescribed wavelength is equal to an average of the wavelengths located in the sub-band. The first plurality of sub-bands are then recombined to reconstitute the WDM optical signal and the reconstituted WDM optical signal is directed onto the optical fiber transmission path. The method continues by imparting dispersion postcompensation to the WDM optical signal after the WDM optical signal traverses the optical fiber transmission path by dividing the prescribed bandwidth of the WDM optical signal into a second plurality of sub-bands, and, within each of the second plurality of sub-bands, compensating each wavelength in the sub-band for dispersion by imparting a second fraction of a delay that would be needed to compensate for dispersion at a prescribed wavelength (340). The prescribed wavelength is equal to an average of the wavelengths located in the sub-band. The wavelengths in the sub-bands of the first plurality of sub-bands do not form a one-to-one correspondence to the wavelengths in the sub-bands of the second plurality of sub-bands.

Description

METHOD AND APPARATUS FOR PROVIDING STAGGERED GROUPWISE DISPERSION COMPENSATION IN A WDM OPTICAL COMMUNICATION

SYSTEM

Statement of Related Application

[0001] This application claims the benefit of priority to U.S. Provisional Patent Application Serial No. 60/404,585, filed August 20, 2002, entitled "Staggered Groupwise Dispersion Compensation."

Field of the Invention

[0002] The present invention relates generally to WDM optical transmission systems, and more particularly to a method and apparatus for providing dispersion compensation in a WDM optical transmission system.

Background of the Invention

[0003] In recent years, Wavelength Division Multiplexed (WDM) and Dense Wavelength Division Multiplexed (DWDM) optical transmission systems have been increasingly deployed in optical networks. Although DWDM optical transmission systems have increased the speed ana capacity of optical networks, the performance of such systems, especially those providing bit rates of 10 Gb/s or more, has traditionally been limited by various factors such as chromatic dispersion and the non-linearity in an optical fiber's refractive index, which can cause spectral broadening of optical pulses and degrade the transmission of high speed optical signals. Because such optical signal degradation tends to accumulate along transmission paths, chromatic dispersion and non- linearity can significantly limit the transmission distance of high speed optical signals. [0004] Chromatic dispersion refers to the fact that different wavelengths of light pass through an optical fiber at different speeds, thereby causing a pulse of light propagating through the optical fiber to broaden. Chromatic dispersion is often characterized as first, second and third order dispersion. First order dispersion is the rate of change of the refractive index with respect to wavelength in the fiber. First order dispersion is also referred to as group velocity. Second order dispersion is the rate of change of the first order dispersion with respect to wavelength. Second order dispersion produces the pulse broadening. Third order dispersion is the rate of change of broadening with respect to a change in wavelength. This is often referred to as the dispersion slope. [0005] For wide bandwidth DWDM systems the effects of both second and third order dispersion must be mitigated. By proper choice of constituent fibers a dispersion map may be constructed such that the path average dispersion is zero for one wavelength in the useable bandwidth. Due to third order dispersion the path average dispersion for all the other wavelengths in the useable bandwidth will not be zero. For long systems with wide bandwidths the accumulated delay of the wavelengths far from the zero dispersion wavelength will be significant. For example, consider a transmission line of length 2000 km constructed such that the zero dispersion wavelength is in the center of the 28 nm bandwidth. The constituent fiber has a dispersion slope of 0.07 ps/nm²-km (this is typical most fiber). The channels at wavelengths +14 nm and -14 nm from the band center will then see an average dispersion of ±14 nm x 0.07 ps/nm²-km = ±0.98 ps/nm-km. At the end of the transmission line this results in an accumulated delay of ±1960 ps/nm. This is large enough to cause an eye closure penalty of more than 2 dB. The magnitude of the penalty will depend on the amount of nonlinear phase the signal has accumulated. In the interest of optimum performance each channel of a DWDM system will require additional dispersion compensation. Depending on system length and performance margin allowance the allowable tolerance can be as great as ± 300 ps/nm for shorter systems to ± 50 sp/nm for very long systems. Where tolerances allow many channels can be compensated at once. This is frequently referred to as bandwise slope compensation and is much less expensive to implement than providing dispersion slope compensation for each channel individually. As system lengths increase the tolerance to incomplete compensation decreases; hence the number of channels that can be compensated together (i.e., bandwise) decreases.

[0006] Several solutions have been proposed to mitigate the effects of dispersion and dispersion slope in transmission fibers. The most common technique involves the use of a compensating optical fiber having an appropriate length and which has a dispersion that is opposite to the dispersion of the transmission fiber at the desired wavelength. As a result, the dispersion at the desired wavelength in the fiber is substantially canceled by the dispersion in the compensating fiber. Since dispersion is wavelength dependent, the amount of dispersion compensation that is needed differs from wavelength to wavelength. Where tolerances allow many adjacent channels can be compensated at once. This is frequently referred to as group wise slope compensation. It is much less expensive to implement than doing slope compensation for each channel individually. However as system lengths increase the tolerance to incomplete compensation decreases; hence the number of channels that can be compensated together decreases and at some distance it becomes necessary to compensate channels individually. Where performance margins allow it is always more economical to do slope compensation on groups of wavelengths. [0007] Accordingly, it would be desirable to provide a method and apparatus for providing dispersion compensation in a WDM or DWDM optical transmission system, which is less difficult and expensive to implement than channel-by-channel dispersion compensation while giving rise to errors that are less than those that arise with current groupwise dispersion compensation schemes.

Summary of the Invention

[0008] In accordance with the present invention, a method and apparatus is provided for compensating for dispersion in a WDM optical communication system that includes a transmitter, receiver, and an optical fiber transmission path coupling the transmitter to the receiver. The method begins by imparting dispersion precompensation to a WDM optical signal having a prescribed bandwidth before the WDM optical signal traverses the optical fiber transmission path. The dispersion compensation is provided by first dividing the prescribed bandwidth of the WDM optical signal into a first plurality of sub-bands, and, within each of the first plurality of sub-bands, compensating each wavelength in the sub- band for dispersion by imparting a first fraction of a delay that would be needed to compensate for dispersion at a prescribed wavelength. The prescribed wavelength is equal to an average of the wavelengths located in the sub-band. The first plurality of sub-bands are then recombined to reconstitute the WDM optical signal and the reconstituted WDM optical signal is directed onto the optical fiber transmission path. The method continues by imparting dispersion postcompensation to the WDM optical signal after the WDM optical signal traverses the optical fiber transmission path by dividing the prescribed bandwidth of the WDM optical signal into a second plurality of sub-bands, and, within each of the second plurality of sub-bands, compensating each wavelength in the sub-band for dispersion by imparting a second fraction of a delay that would be needed to compensate for dispersion at a prescribed wavelength. The prescribed wavelength is equal to an average of the wavelengths located in the sub-band. The wavelengths in the sub- bands of the first plurality of sub-bands do not form a one-to-one correspondence to the wavelengths in the sub-bands of the second plurality of sub-bands. [0009] In accordance with one aspect of the invention, the wavelengths in the sub- bands of the second plurality of sub-bands are staggered with respect to the wavelengths in the sub-bands of the first plurality of sub-bands.

In accordance with another aspect of the invention, the first fraction of the delay is about equal to about one-half.

[0010] In accordance with another aspect of the invention, the second fraction of the delay is about one-half.

[0011] In accordance with yet another aspect of the invention, the steps of performing dispersion precompensation and postcompensation are performed by dispersion compensating optical fibers.

[0012] In accordance with another aspect of the invention, the dispersion compensating optical fibers are single mode optical fibers.

[0013] In accordance with another aspect of the invention, the steps of performing dispersion precompensation and postcompensation are performed by fiber diffraction gratings.

[0014] In accordance with another aspect of the invention, the first plurality of sub- bands and the second plurality of sub-bands are equal in number. [0015] In accordance with another aspect of the invention, the first plurality of sub- bands and the second plurality of sub-bands are unequal in number. [0016] In accordance with another aspect of the invention, the method further comprises the step of recombining the second plurality of sub-bands to reconstitute the WDM optical signal.

[0017] In accordance with another aspect of the invention, a WDM optical transmission system is provided for transmitting a WDM optical signal having a prescribed bandwidth. The transmission system includes a transmitter unit, a receiver unit, and an optical transmission path interconnecting the transmitter and receiver units that is traversed by the WDM optical signal. A first dispersion compensator is associated with the transmitter unit for imparting dispersion precompensation to the WDM optical signal. The first dispersion compensator is configured to divide the prescribed bandwidth of the WDM optical signal into a first plurality of sub-bands, and, within each of the first plurality of sub-bands, compensate each wavelength in the sub-band for dispersion by imparting a first fraction of a delay that would be needed to compensate for dispersion at a prescribed wavelength. The prescribed wavelength is equal to an average of the wavelengths located in the sub-band. The first dispersion compensator is also configured to recombine the first plurality of sub-bands to reconstitute the WDM optical signal and direct the reconstituted WDM optical signal onto the optical transmission path. A second dispersion compensator is associated with the receiver unit for imparting dispersion postcompensation to the WDM optical signal. The second dispersion compensator is configured to divide the prescribed bandwidth of the WDM optical signal into a second plurality of sub-bands, and within each of the second plurality of sub-bands, compensate each wavelength in the sub-band for dispersion by imparting a second fraction of a delay that would be needed to compensate for dispersion at a prescribed wavelength. The prescribed wavelength is equal to an average of the wavelengths located in the sub-band. The wavelengths in the sub-bands of the first plurality of sub-bands do not form a one-to- one correspondence to the wavelengths in the sub-bands of the second plurality of sub- bands.

Brief Description of the Invention

[0018] FIG. 1 shows a simplified block diagram of an exemplary wavelength division multiplexed (WDM) transmission system in accordance with the present invention.

[0019] FIG. 2 shows one example of a chromatic dispersion compensator that may serve for both dispersion pre-compensation and post-compensation.

[0020] FIG. 3 shows a functional illustration of the pre- and post-dispersion compensation imparted to the wavelength components by the chromatic dispersion compensator depicted in FIG. 2.

[0021] FIG. 4 shows another example of a chromatic dispersion compensator that may serve for both dispersion pre-compensation and post-compensation.

[0022] FIG. 5 shows a functional illustration of the pre- and post-dispersion compensation imparted to the wavelength components on a group by group basis by the dispersion compensator depicted in FIG. 4.

[0023] FIGs. 6-7 show functional illustrations of the pre- and post-dispersion compensation imparted to the wavelength components in accordance to the staggered groupwise dispersion compensation technique of the present invention.

Detailed Description of the Invention

[0024] FIG. 1 shows a simplified block diagram of an exemplary wavelength division multiplexed (WDM) transmission system in accordance with the present invention. The transmission system serves to transmit a plurality of optical channels over a single path from a transmitting terminal to a remotely located receiving tenninal. While FIG. 1 depicts a unidirectional transmission system, it should be noted that if a bidirectional communication system is to be employed, two distinct transmission paths are used to carry the bi-directional communication. The optical transmission system may be an undersea transmission system in which the terminals are located on shore and one or more repeaters may be located underwater

[0025] Transmitter terminal 100 is connected to an optical transmission medium 200, which is connected, in turn, to receiver terminal 300. Transmitter terminal 100 includes a series of encoders 110 and digital t^ransmitters 120 connected to a wavelength division multiplexer 130. For each WDM channel, an encoder 110 is connected to a digital transmitter 120, which, in turn, is connected to the wavelength division multiplexer 130. In other words, wavelength division multiplexer 130 receives signals associated with multiple WDM channels, each of which has an associated digital transmitter 120 and encoder 110. Transmitter terminal 100 also includes a chromatic dispersion compensator 140 that precompensates for dispersion arising in transmission medium 200. [0026] Digital transmitter 120 can be any type of system component that converts electrical signals to optical signals. For example, digital transmitter 120 can include an optical source such as a semiconductor laser or a light-emitting diode, which can be modulated directly by, for example, varying the injection current. WDM multiplexer 130 can be any type of device that combines signals from multiple WDM channels. For example, WDM multiplexer 130 can be a star coupler, a fiber Fabry-Perot filter, an in- line Bragg grating, a diffraction grating, cascaded filters and a wavelength grating router, among others.

[0027] Receiver terminal 300 includes a series of decoders 310, digital receivers 320 and a wavelength division demultiplexer 330. WDM demultiplexer 330 can be any type of device that separates signals from multiple WDM channels. For example, WDM demultiplexer 330 can be a star coupler, a fiber Fabry-Perot filter, an in-line Bragg grating, a diffraction grating, cascaded filters and a wavelength grating router, among others. Receiver terminal 300 also includes a chromatic dispersion compensator 340 that provides post-compensation for dispersion arising in transmission medium 200. [0028] Optical transmission medium 200 includes rare-earth doped optical amplifiers 210ι-210_n interconnected by transmission spans 240j-240_n+ι of optical fiber, for example. If a bi-directional communication system is to be employed, rare-earth doped optical amplifiers are provided in each transmission path. Moreover, in a bi-directional system each of the terminals 100 and 300 include a transmitter and a receiver. In a bi-directional undersea communication system a pair of rare-earth doped optical amplifiers supporting opposite-traveling signals is often housed in a single unit known as a repeater. While only four rare-earth optical amplifiers are depicted in FIG. 1 for clarity of discussion, it should be understood by those skilled in the art that the present invention finds application in transmission paths of all lengths having many additional (or fewer) sets of such amplifiers.

[0029] FIG. 2 shows one example of a chromatic dispersion compensator that may serve for both dispersion pre-compensator 140 and dispersion post-compensator 340. In operation, the dispersion compensator first demultiplexes the optical signal into its individual wavelength components λj, λ₂, λ₃, ... λ_n, equalizes the dispersion of each wavelength component individually, and finally recombines the wavelength components onto a common path for continued transmission. In FIG. 2, the signal enters a l n demultiplexer 303, which demultiplexes the signal so that wavelength components λ_ls λ₂, λ₃, ... λ_n are directed onto output paths 309j, 309₂, 309₃, . . . 309_n, respectively. The wavelength components propagating along the n output paths respectively enter dispersion compensating fibers 304j, 304₂, 304₃, ... 304_n and possibly loss elements 307j, 307₂, 307₃, ... 307_n. The signals are subsequently recombined by multiplexer 305 before exiting the dispersion compensator on fiber 306. For both the dispersion pre-compensator and the dispersion post-compensator, the dispersion in each of the plurality of compensating fibers 304j, 304₂, 304₃, ... 304_n is selected so that one half of the accumulated delay over the transmission path for wavelength components λj, λ₂, λ₃, ... λ_n, respectively is compensated by the fiber at the transmit end of the line and the other half of the accumulated delay is compensated by the fibers at the receive end. As a result, the total dispersion compensation imparted to each wavelength component after traversing both the dispersion pre-compensator and the dispersion pre-compensator returns the delay to zero or near zero. While the dispersion compensating elements employed in FIG. 2 are depicted as dispersion compensating fibers, those of ordinary skill in the art will recognize that many other optical devices may be employed to provide the necessary dispersion compensation. For example, a fiber diffraction grating may be used instead of fiber.

[0030] FIG. 3 shows a functional illustration of the pre- and post-dispersion compensation imparted to the wavelength components by the chromatic dispersion compensator depicted in FIG. 2. The delay imparted to channel ^ is denoted D(^ ). As shown, precompensation and postcompensation each impart to each and every channel one-half the delay required to achieve a net delay of about zero. That is, dispersion compensation is provided on a channel-by-channel basis (i.e. a different delay is imparted to each channel), but for each channel, the delay imparted at the transmitter is the same as that imparted at the receiver.

[0031] As previously mentioned, compensating for dispersion on a channel by channel basis, as in FIGs. 2 and 3, is relatively expensive to perform. Accordingly, a groupwise dispersion compensation technique is sometimes employed in which channels are aggregated into groups or bands that each receives dispersion compensation. That is, the channels within a given group or band all receive the same dispersion compensation. An example of such a groupwise dispersion compensation technique is shown in FIG. 4. [0032] FIG. 4 shows another example of a chromatic dispersion compensator that may serve for both dispersion pre-compensator 140 and dispersion post-compensator 340. In operation, the dispersion compensator first splits the bandwidth of the optical signals into a series of bands, equalizes the dispersion of each band individually, and finally recombines the signals onto a common path for continued transmission. In FIG. 4, the signals reach the compensator on fiber path 201 and enter a l N optical splitter 203, which divides the power of the optical signal onto output paths 209], 209₂, 209₃, . . . 209_N. The signals propagating along the N output paths respectively enter optical bandpass filters 204j, 204₂, 204₃, . . . 204_N which have a center wavelength of λ_ls λ₂, λ₃, ... λn, respectively. The optical bandpass filters 204 separate the usable bandwidth into N distinct bands. The signals emerging from bandpass filters 204] , 204₂, 204₃, . . . 204_N each enter a respective dispersion equalizing fiber 205 \, 205₂, 205₃, . . . 205_N and possibly loss elements 208j, 208₂, 208₃, . . . 208 j. The signals are subsequently recombined in coupler 206 before exiting the dispersion compensator on fiber 207. The amount of compensation or delay provided to each channel within a given group is the average of the individual delays that would be required for each channel within the group. The total delay imparted to each group may be provided in part at both the transmitter and the receiver. It should be noted that the dispersion compensator shown in FIG. 2 can also be used to perform groupwise dispersion compensation. In this case the demultiplexer 303 divides the signal into groups of wavelengths rather than individual wavelength components.

[0033] Similar to FIG. 3, FIG. 5 shows a functional illustration of the pre- and post- dispersion compensation imparted to the wavelength components on a group by group basis by the dispersion compensator depicted in FIG. 4. As used in FIG. 5, m is the number of groups into which the channels are divided, j is the number of channels in each group, gmupi is the average wavelength of group i, and D( govpi ) is the dispersion compensation imparted to each wavelength in group i. As shown, precompensation and postcompensation each impart to every channel within a group one-half of the delay required to achieve a net delay of zero or near zero for the average wavelength within that group.

[0034] As previously mentioned, the groupwise dispersion compensation technique depicted in FIGs. 4 and 5 gives rise to errors that increase with the length of the transmission path, the dispersion slope, and the channel spacing. For group wise slope compensation of a transmission line of length L, having a dispersion slope D', and a group of channels spaced Δλ apart the worst-case compensation error can be calculated as follows: J-l

E ^Λ~"rr 'o"r' max x Aλ x D' x L

[0035] To reduce the error arising from groupwise dispersion compensation, the present invention provides an amount of delay at the transmitter and receiver that are based on different groups of wavelengths. That is, each wavelength receives different amounts of compensation at the transmitter and at the receiver. For example, in the functional illustration of FIG. 6, at the transmitter, the wavelengths are grouped into m bands denoted Tj-T_m. As shown, group Ti includes channels λι-λ₄, group T₂ includes channels λ₅-λ₈, and, more generally, group T_m-1 includes channels λ_mj₊ι to channels λ_mj₊₄. The dispersion compensation provided at the transmitter to each channel in a given group is equal to the delay that would be imparted to the average wavelength within that group. At the receiver, however, the wavelengths are grouped into a different combination of bands. For example, in the particular embodiment of the invention depicted in FIG. 6, the groups at the receiver are staggered with respect to the groups at the transmitter. That is, group R) includes channels λι-λ₂, group R₂ includes channels λ₃-λ₆, and, more generally, group R_m includes channels λ_mj+₃ to λ_mj+6- Like the dispersion compensation imparted at the transmitter, the dispersion compensation provided at the receiver to each channel in a given group is equal to the delay that would be imparted to the average wavelength within that group. As a result of this grouping, the delay imparted to wavelengths λj and λ₂ at the

transmitter, for example, corresponds to and the delay imparted to

wavelengths λi and λ₂ at the receiver is

Likewise, the delay imparted to

wavelengths λ₃ and λ at the transmitter corresponds to

) and the delay

imparted to wavelengths λ₃and λ at the receiver is

Dispersion compensation is provided to the remaining groups of wavelengths in a similar fashion. [0036] It should be noted that the particular groupings of wavelengths shown in FIG. 6 is applicable when the total number of wavelengths is not divisible by the number of wavelengths in each group. FIG. 7, on the other hand, shows an alternative grouping of wavelengths that is applicable when the total number of wavelengths is divisible by the number of wavelengths in each group. More generally, the present invention encompasses any groupings of wavelengths in which the grouping of wavelengths at the transmitter do not form a one-to-one correspondence with the grouping of wavelengths at the receiver. FIGS. 6 and 7 depict one example of such an arrangement in which the groupings of wavelengths at the transmitter are staggered with respect to the grouping of wavelengths at the receiver.

[0037] The inventive staggered groupwise dispersion compensation technique depicted in FIGs. 6 and 7 may be performed with any dispersion compensators available to those of ordinary skill in the art, such as the dispersion compensators shown in FIGs. 2 and 4, for example.

[0038] The present invention advantageously reduces the worst-case compensation error relative to that arising with the conventional groupwise dispersion compensation technique, which was calculated above. The worst-case error for groups of 4 channels can is determined to be.

Error

[0039] A comparison of the errors arising with the conventional groupwise dispersion compensation technique and the inventive staggered groupwise dispersion compensation for groups of 4 channels (i.e., j =4) shows that the present invention gives rise to a maximum error only 1/3 as large. Likewise, if groups of 8 channels are employed, the present invention gives rise to a maximum error of 1.5D^' ΔλL_{sys em5} in comparison to a maximum error of 3.5D ΔλL_Sy_Stem, for the conventional technique. In general it can be shown that for an arbitrary number of channels in a group the maximum error in the staggered compensation scheme is less than or equal to '/_ the maximum error in the conventional group wise compensation technique.

Claims

Claims:

1. A method for compensating for dispersion in a WDM optical communication system that includes a transmitter, receiver, and an optical fiber transmission path coupling said transmitter to said receiver, said method comprising the steps of:

A. imparting dispersion precompensation to a WDM optical signal having a prescribed bandwidth before the WDM optical signal traverses the optical fiber transmission path by; dividing the prescribed bandwidth of the WDM optical signal into a first plurality of sub-bands; within each of the first plurality of sub-bands, compensating each wavelength in the sub-band for dispersion by imparting a first fraction of a delay that would be needed to compensate for dispersion at a prescribed wavelength, said prescribed wavelength being equal to an average of the wavelengths located in the sub-band; recombining the first plurality of sub-bands to reconstitute the WDM optical signal and directing the reconstituted WDM optical signal onto the optical fiber transmission path;

B. imparting dispersion postcompensation to the WDM optical signal after the WDM optical signal traverses the optical fiber transmission path by; dividing the prescribed bandwidth of the WDM optical signal into a second plurality of sub-bands; within each of the second plurality of sub-bands, compensating each wavelength in the sub-band for dispersion by imparting a second fraction of a delay that would be needed to compensate for dispersion at a prescribed wavelength, said prescribed wavelength being equal to an average of the wavelengths located in the sub-band; and wherein the wavelengths in the sub-bands of the first plurality of sub- bands do not form a one-to-one correspondence to the wavelengths in the sub- bands of the second plurality of sub-bands.

2. The method of claim 1 wherein the wavelengths in the sub-bands of the second plurality of sub-bands are staggered with respect to the wavelengths in the sub- bands of the first plurality of sub-bands.

3. The method of claim 1 wherein said first fraction of the delay is about equal to one-half.

4. The method of claim 1 wherein said second fraction of the delay is about one-half.

5. The method of claim 3 wherein said second fraction of the delay is about one-half.

6. The method of claim 1 wherein the steps of performing dispersion precompensation and postcompensation are performed by dispersion compensating optical fibers.

7. The method of claim 6 wherein said dispersion compensating optical fibers are single mode optical fibers.

8. The method of claim 1 wherein the steps of performing dispersion precompensation and postcompensation are performed by fiber diffraction gratings.

9. The method of claim 1 wherein the first plurality of sub-bands and the second plurality of sub-bands are equal in number.

10. The method of claim 1 wherein the first plurality of sub-bands and the second plurality of sub-bands are unequal in number.

11. The method of claim 1 further comprising the step of recombining the second plurality of sub-bands to reconstitute the WDM optical signal.

12. A WDM optical transmission system for transmitting a WDM optical signal having a prescribed bandwidth, comprising: a transmitter unit; a receiver unit; an optical transmission path interconnecting the transmitter and receiver units and being traversed by the WDM optical signal; a first dispersion compensator associated with the transmitter unit for imparting dispersion precompensation to the WDM optical signal, said first dispersion compensator being configured to i. divide the prescribed bandwidth of the WDM optical signal into a first plurality of sub-bands; ii. within each of the first plurality of sub-bands, compensate each wavelength in the sub-band for dispersion by imparting a first fraction of a delay that would be needed to compensate for dispersion at a prescribed wavelength, said prescribed wavelength being equal to an average of the wavelengths located in the sub-band; iii. recombine the first plurality of sub-bands to reconstitute the WDM optical signal and direct the reconstituted WDM optical signal onto the optical transmission path; a second dispersion compensator associated with the receiver unit for imparting dispersion postcompensation to the WDM optical signal, said second dispersion compensator being configured to i. divide the prescribed bandwidth of the WDM optical signal into a second plurality of sub-bands; ii. within each of the second plurality of sub-bands, compensate each wavelength in the sub-band for dispersion by imparting a second fraction of a delay that would be needed to compensate for dispersion at a prescribed wavelength, said prescribed wavelength being equal to an average of the wavelengths located in the sub-band; and wherein the wavelengths in the sub-bands of the first plurality of sub- bands do not form a one-to-one correspondence to the wavelengths in the sub- bands of the second plurality of sub-bands.

13. The WDM optical transmission system of claim 12 wherein said second dispersion compensator is further configured to recombine the second plurality of sub- bands.

14. The WDM optical transmission system of claim 1 wherein the wavelengths in the sub-bands of the second plurality of sub-bands are staggered with respect to the wavelengths in the sub-bands of the first plurality of sub-bands.

15. The WDM optical transmission system of claim 12 wherein said first fraction of the delay is about equal to one-half.

16. The WDM optical transmission system of claim 12 wherein said second fraction of the delay is about one-half.

17. The WDM optical transmission system of claim 15 wherein said second fraction of the delay is about one-half.

18. The WDM optical transmission system of claim 12 wherein said first and second dispersion compensators include dispersion compensating optical fibers

19. The WDM optical transmission system of claim 18 wherein said dispersion compensating optical fibers are single mode optical fibers.

20. The WDM optical transmission system of claim 12 wherein said first and second dispersion compensators include fiber diffraction gratings.

21. The WDM optical transmission system of claim 12 wherein the first plurality of sub-bands and the second plurality of sub-bands are equal in number.

22. The WDM optical transmission system of claim 12 wherein the first plurality of sub-bands and the second plurality of sub-bands are unequal in number.