GB2368479A

GB2368479A - Dispersion compensator

Info

Publication number: GB2368479A
Application number: GB0025984A
Authority: GB
Inventors: Nicholas John Doran
Original assignee: Marconi Communications Ltd; Marconi Co Ltd
Current assignee: Marconi Communications Ltd; BAE Systems Electronics Ltd
Priority date: 2000-10-24
Filing date: 2000-10-24
Publication date: 2002-05-01
Also published as: AU2001294012A1; GB0025984D0; WO2002035271A1

Abstract

The invention provides a dispersion compensator (10) for compensating dispersion arising in one or more optical radiation waveguides coupled to the compensator, the compensator (10) including: <SL> <LI>(a) a circulator (20) for receiving input radiation to be dispersion corrected, for directing the input radiation to a dispersion correcting path (30, 40, 50, 60), and for receiving radiation from the dispersion correcting path (30, 40, 50, 60) for output from the compensator (10); and <LI>(b) the correcting path (30, 40, 50, 60) comprising an optical fibre waveguide (30) whose effective length is dependent upon wavelength of components of the input radiation propagating along the path (30), the effective length being arranged to change with wavelength of the components such as to provide an enhanced degree of dispersion correction. </SL> A plurality of wavelength selective fibre gratings (40, 50, 60) cause this wavelength dependence of the effective length.

Description

DISPERSION COMPENSATOR The present invention relates to a dispersion compensator for compensating dispersion in optical fibre waveguides.

Optical radiation is defined as radiation whose free-space wavelength lies within a range of 100

nm to 10 u. m.

Convention optical fibre waveguides employed in contemporary optical communication systems exhibit radiation dependent dispersion, often referred to as chromatic dispersion, which results in pulse broadening in communication traffic bearing radiation propagating therealong. Thus, the dispersion is a bandwidth limiting phenomenon. Such dispersion is conventionally, at least to a first order of magnitude, corrected by arranging for conventional optical fibre waveguides to be connected in series with respective compensating fibres having a complementary dispersion characteristic. It is known that one optical communication system manufacturer can manufacturer compensating optical fibre waveguides providing in excess of 90% dispersion compensation. Other communication system manufacturers have obtained dispersion compensation substantially in a range of 60% to 80%. However, for certain categories of optical fibre waveguides, for example NZDSF optical fibre waveguides manufactured by Timewave and LEAF, it has not presently been possible to devise a form of compensating fibre which can provide compensation by providing a corresponding dispersion matching characteristic.

Moreover, dispersion compensation over a relatively wide range of radiation wavelengths using dispersion compensation fibre waveguides has presently been virtually impossible to achieve.

The inventor has appreciated that, instead of employing solely a compensation fibre waveguide for providing dispersion compensation, it is beneficial in practice to employ a dispersion compensator including a dispersion correcting fibre waveguide incorporating one or more fibre gratings formed along the waveguide.

Thus, according to the present invention, there is provided a dispersion compensator for compensating dispersion arising in one or more optical radiation waveguides coupled to the compensator, the compensator including:

(a) radiation directing means for receiving input radiation to be dispersion corrected, for directing the input radiation to dispersion correcting means, and for receiving radiation from the dispersion correcting means for output from the compensator; and (b) the correcting means comprising a dispersion correcting path whose effective length is dependent upon wavelength of components of the input radiation propagating along the path, the effective length being arranged to change with wavelength of the components such as to provide an enhanced degree of dispersion correction.

The invention provides the advantage that it is capable of providing a more accurate degree of dispersion compensation in comparison to that provided solely by using one or more dispersion compensating optical fibre waveguides.

Preferably, the path comprises one or more wavelength selective reflecting structures operable to provide the path with its effective length dependent upon radiation component wavelength. The one or more wavelength selective structures enable different effective lengths of the path to be selected for providing dispersion compensation at specific radiation wavelengths.

Conveniently, for ease of fabrication and for reducing manufacturing cost, the one or more structures comprise at least one fibre grating. Moreover, it is desirable that the path is provided by an optical fibre waveguide into which the at least one fibre grating is formed, thereby providing a unitary structure. A unitary structure is capable of reducing manufacturing cost and providing enhanced reliability.

Preferably, the path is provided by a dispersion compensating optical fibre waveguide. Such compensating fibre is commercially available and is to be contrasted with standard optical fibre which is most often used in optical communication systems.

In order to provide a suitable range of effective path lengths, it is preferably that the at least one grating is spatially separated from its neighbouring grating by a path distance in a range of 30 m to 150 m. Indeed, it is further desirable for practical operation that a first grating of the at least one grating is spatially separated from the radiation directing means by a path length in a range of 2 km to 10 km.

Preferably, in order to provide improved radiation wavelength-gain characteristics to the compensator, it is desirable that the one or more reflecting structures are arranged to exhibit progressively greater reflectivity in their series away from the radiation directing means.

When implementing the compensator in practice using standard optical components, it is preferable that the radiation directing means is an optical circulator. Conveniently, so that the compensator can be connected in in-line configuration into a optical communication system, it is preferable that the circulator is a three-port circulator.

Embodiments of the invention will now be described, by way of example only, with reference to the following diagrams in which: Figure 1 is a schematic illustration of a first embodiment of a dispersion compensator according to the invention; and Figure 2 is a diagram of dispersion compensation characteristics addressed by the present invention.

Referring now to Figure 1, there is shown a dispersion compensator according to the invention.

The compensator is indicated generally by 10 and comprises an optical circulator 20 and a continuous length of dispersion compensating optical fibre waveguide 30. The circulator 20 includes a first optical input port A for receiving input radiation, a second optical output port B for outputting radiation which has been compensated for dispersion within the compensator 10, and a third optical port C connected to the fibre waveguide 30. The waveguide 30 incorporates a first section of length Lo and a second section LG including k fibre gratings; k is an integer having a value of unity or greater. The first section is connected at its first end to the port C, and at its second end runs continuously into the second section. The second section includes the k fibre gratings cascaded in series starting with a first grating 40 nearest the first section and a klb grating 50 most remote from the first section.

Each grating along the waveguide 30 is operable to reflect radiation over a corresponding range of radiation wavelengths. For example, the first waveguide 40 is operable to reflect radiation

over a waveband AR1, a second grating 60 is operable to reflect radiation over a waveband A ; L2, and so on to the klb grating 50 which is operable to reflect radiation over a waveband A ; Lk.

Moreover, a length of the waveguide 30 of length Li separates the first grating 40 from the second grating 60, and so on until a length Lk-I which separates the (k-l) th grating from the kth grating 50. The lengths Li to Lek-l are each in the order of 100 m long, for example in a range of 30 to 150 m long. Moreover, the first section is in the order of 3 to 4 km in length, for example in a range of 2 km to 10 km.

The gratings along the second section, for example the gratings 40,50, 60, are formed directly by exposing the fibre waveguide 30 to an ultraviolet interference fringe pattern which causes permanent grating-like refractive index perturbations in the waveguide 30.

Each grating reflects over its associated waveband range, for example each grating reflects over a 3.2 nm wavelength range encompassing channel wavebands corresponding to eight neighbouring wavelength division multiplexed (WDM) channels at a channel spacing of 50 GHz.

By spatially distributing the gratings, dispersion correction provided by the waveguide 30 can be increased or decreased as required over specific preferred wavebands. Thus, near-perfect dispersion correction can be achieved on a step-wise linear basis over a relatively wide range of radiation wavelengths. In practice, the gratings are formed onto the waveguide 30 to a positional error tolerance in the order of a 1 m.

Operation of the compensator 10 will now be described with reference to Figure 1.

Communication traffic bearing radiation propagates along a waveguide to the input port A. The circulator 20 directs the radiation towards the port C whereat the radiation propagates out along the first section. The radiation experiences a degree of dispersion correction as it propagates to the gratings. Each grating selectively reflects back components of the radiation corresponding to its associated grating bandwidth, the components propagating back along the waveguide 30 through the first section and finally to the port C. The circulator 20 directs reflected radiation received thereat from the port C to the output port B as dispersion corrected radiation which propagates from the port B away from the compensator 10.

Thus, dispersion degraded radiation input at the input port A can be dispersion corrected in the compensator 10 and output as corresponding corrected radiation at the output port C.

The value of k can be selected depending upon the form and accuracy of dispersion compensation required. For example, k can beneficially be in a range of 3 to 20 in practice. The compensator 10 can be employed in a number of potential application. A first example application is to provide dispersion compensation of LEAF-type optical fibre waveguides which has hitherto not been practicable. A second example application is to provide dispersion compensation of standard monomode optical fibre waveguides where the waveguide 30 itself can be based on other types of standard optical fibre waveguide. The compensator 10 can also be used to provide slope compensation in a communication link comprising a plurality of serially disposed repeater nodes where most of dispersion correcting fibre waveguide employed along the link provides imperfect compensation, the compensator 10 being used on some of the nodes, for example every 5"'node, to provide residual dispersion correction resulting in close to 100%, for example 98%, overall dispersion correction along the link.

It should be noted that because the fibre waveguide 30 employs reflection therealong in combination with the circulator 20, the waveguide 30 is equivalent to twice its length of conventional non-reflecting dispersion correcting fibre waveguide.

In the compensator 10, radiation components in certain wavebands will experience mutually differing degrees of loss depending upon where their corresponding gratings are incorporated along the fibre waveguide 30. As a consequence, some gain management facility is beneficially included within the compensator 10. For example, the gratings in the second section can be arranged to provide progressively greater reflectivities away from the first section, for example the second grating 60 has a reflectivity greater than the first grating 40, and so on.

Chromatic dispersion in optical fibre waveguides arises as a consequence of the waveguides exhibiting a refractive index which is dependent upon the wavelength of radiation propagating therealong. WDM radiation employed in contemporary optical communication systems comprises a relatively broad spectrum of wavelengths, for example 120 channels at a channel spacing of 50 GHz results in an overall frequency radiation frequency range of 0.6 THz. As a consequence of such a relatively broad spectrum, radiation components of relatively shorter wavelength in the WDM radiation propagate with a different propagation velocities along the waveguides in comparison to radiation components of relatively longer wavelength. Such differing propagation velocities result in pulse broadening in the systems which is a bandwidth

limiting factor. The compensator 10 is capable of providing a deconvolution of such pulse broadening.

In Figure 2, there is shown a graph indicated by 100 including a horizontal axis 110 along which radiation wavelength is represented and a vertical axis 120 along which relative refractive index exhibited by the compensator 10 is represented. For different radiation wavelengths, differing degrees of refractive index change with wavelength are required to obtain near-perfect compensation of dispersion in the compensator 10. Thus, slopes of compensation curves 130, 140 relating to radiation wavelengths of nominally 1.3 nm and 1.5 nm respectively are mutually different. Incorporation of the gratings in the compensator 10 allows different lengths of dispersion compensating fibre to be selected depending upon the wavelength of radiation components present.

It will be appreciated by one ordinarily skilled in the art of dispersion compensator design that modifications can be made to the compensator 10 without departing from the scope of the invention. For example, the number of gratings k can be unity or greater. Moreover, a variety of different types of optical waveguide can be employed for the fibre waveguide 30. Moreover, other types of optical device can be employed instead of the circulator 20 provided that they are capable of performing a similar function to the circulator 20, namely directing radiation to be compensated through the fibre waveguide 30.

In use, the compensator according to the invention can be included to provide pre-compensation prior to outputting radiation into a fibre waveguide susceptible to dispersion. Alternatively, it can be included to provide post-compensation to radiation which has suffered dispersion. Moreover, it can also be included to provide compensation at an intermediate position along a fibre waveguide susceptible to dispersion effects.

Claims

CLAIMS 1. A dispersion compensator (10) for compensating dispersion arising in one or more optical radiation waveguides coupled to the compensator, the compensator (10) including: (a) radiation directing means (20) for receiving input radiation to be dispersion corrected, for directing the input radiation to dispersion correcting means (30,40, 50,60), and for receiving radiation from the dispersion correcting means (30,40, 50,60) for output from the compensator (10); and (b) the correcting means (30,40, 50,60) comprising a dispersion correcting path (30) whose effective length is dependent upon wavelength of components of the input radiation propagating along the path (30), the effective length being arranged to change with wavelength of the components such as to provide an enhanced degree of dispersion correction.
2. A compensator (10) according to Claim 1 wherein the path comprises one or more wavelength selective reflecting structures (40,50, 60) operable to provide the path (30) with its effective length dependent upon radiation component wavelength.
3. A compensator according to Claim 2 wherein the one or more structures comprise at least one fibre grating (40,50, 60).
4. A compensator according to Claim 3 wherein the path (30) is provided by an optical fibre waveguide into which the at least one fibre grating (40,50, 60) is formed.
5. A compensator according to Claim 3 or 4 wherein the path is provided by a dispersion compensating optical fibre waveguide (30).
6. A compensator according to Claim 3,4 or 5 wherein the at least one grating (40,50, 60) is spatially separated from its neighbouring grating by a path distance in a range of 30 m to 150 m.
7. A compensator according to Claim 3,4, 5 or 6 wherein a first grating (40) of the at least one grating is spatially separated from the radiation directing means (20) by a path length in a range of 2 km to 10 km.
8. A compensator according to any one of Claims 2 to 7 wherein the one or more reflecting structures (40,50, 60) are arranged to exhibit progressively greater reflectivity in their series away from the radiation directing means (20).
9. A compensator according to any preceding claim wherein the radiation directing means (20) is an optical circulator.
10. A compensator according to Claim 9 wherein the circulator (20) is a three-port circulator.
11. A dispersion compensator substantially as hereinbefore described with reference to one or more of Figures 1 and 2.