WO2003007037A1 - A dispersion compensation device - Google Patents

A dispersion compensation device Download PDF

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Publication number
WO2003007037A1
WO2003007037A1 PCT/AU2002/000946 AU0200946W WO03007037A1 WO 2003007037 A1 WO2003007037 A1 WO 2003007037A1 AU 0200946 W AU0200946 W AU 0200946W WO 03007037 A1 WO03007037 A1 WO 03007037A1
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Prior art keywords
grating
refractive index
gratings
compensation device
dispersion compensation
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PCT/AU2002/000946
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French (fr)
Inventor
Mark Englund
Mark Sceats
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The University Of Sydney
The Commonwealth Of Australia Represented By The Defence Science And Technology Organisation Of The Department Of Defence
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Publication of WO2003007037A1 publication Critical patent/WO2003007037A1/en

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Classifications

    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/24—Coupling light guides
    • G02B6/26—Optical coupling means
    • G02B6/28—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals
    • G02B6/293—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means
    • G02B6/29304—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means operating by diffraction, e.g. grating
    • G02B6/29316—Light guides comprising a diffractive element, e.g. grating in or on the light guide such that diffracted light is confined in the light guide
    • G02B6/29317—Light guides of the optical fibre type
    • G02B6/29322—Diffractive elements of the tunable type
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/24—Coupling light guides
    • G02B6/26—Optical coupling means
    • G02B6/28—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals
    • G02B6/293—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means
    • G02B6/29304—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means operating by diffraction, e.g. grating
    • G02B6/29316—Light guides comprising a diffractive element, e.g. grating in or on the light guide such that diffracted light is confined in the light guide
    • G02B6/29317—Light guides of the optical fibre type
    • G02B6/29319—With a cascade of diffractive elements or of diffraction operations
    • G02B6/2932—With a cascade of diffractive elements or of diffraction operations comprising a directional router, e.g. directional coupler, circulator
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/24—Coupling light guides
    • G02B6/26—Optical coupling means
    • G02B6/28—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals
    • G02B6/293—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means
    • G02B6/29379—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means characterised by the function or use of the complete device
    • G02B6/29392—Controlling dispersion
    • G02B6/29394—Compensating wavelength dispersion
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/25—Arrangements specific to fibre transmission
    • H04B10/2507—Arrangements specific to fibre transmission for the reduction or elimination of distortion or dispersion
    • H04B10/2513—Arrangements specific to fibre transmission for the reduction or elimination of distortion or dispersion due to chromatic dispersion
    • H04B10/2519—Arrangements specific to fibre transmission for the reduction or elimination of distortion or dispersion due to chromatic dispersion using Bragg gratings

Definitions

  • a dispersion compensation device which incorporates a plurality of chirped Bragg gratings which are interconnected and arranged to provide for minimisation of group delay ripples .
  • Chromatic dispersion is a phenomenon which places limits on the rate of photonic signal transmission in optical waveguides and may be defined as the variation of propagation time as a function of wavelength within a waveguide. Chromatic dispersion increases with the bandwidth of a photonic signal and limits the transmission distance, particularly at high data rates.
  • a known method of partially eliminating chromatic dispersion has been to reflect photonic signals using an optical fibre incorporating a chirped Bragg grating.
  • a chirped Bragg grating is a grating that provides an increase or decrease in the period ⁇ of refractive index variations along its length.
  • the Bragg wavelength ⁇ B of light reflected by the grating is given by where n eff is the effective refractive index.
  • group delay When a chromatically-dispersed signal enters a chirped grating, the penetration depth D of the signal into the grating increases with wavelength, thus producing a wavelength-dependent time delay, referred to as "group delay" .
  • group delay ripple a wavelength-dependent time delay
  • the presence of the group delay ripple means that a chirped fibre grating cannot completely compensate for chromatic dispersion.
  • Apodization of the grating refractive index profile can reduce, but not eliminate, reflections at the leading refractive index step of the gratings resulting in a reduced amplitude group delay ripple.
  • the present invention is directed to a dispersion compensation device which provides for minimisation of group delay ripple and to a method of fabricating such device.
  • the invention provides a method of fabricating a dispersion compensation device having first and second Bragg gratings which are chirped to provide substantially the same phase delay ⁇ as a function of wavelength ⁇ and which are interconnected in a manner such that their reflection spectra interfere substantially constructively.
  • the method includes the step of displacing at least one of the refractive index steps of the first grating by a distance ⁇ relative to the or each corresponding refractive index step of the second grating, where ⁇ corresponds to a ⁇ /2 phase shift in reflections attributable to at least one of the refractive index steps following the leading refractive index step of the first grating.
  • the present invention also provides a dispersion compensation device when fabricated by the above-defined method.
  • the invention may be defined still further in terms of a dispersion compensation device comprising first and second Bragg gratings and support means supporting the gratings, both gratings being chirped to provide substantially the same phase delay ⁇ as a function of wavelength ⁇ , the gratings being interconnected in a manner such that their reflection spectra interfere substantially constructively, and the support means being arranged in use to impose a dimensional change on one or both of the gratings in a manner whereby at least one of the refractive index steps of the first grating is displaced by distance ⁇ relative to the or each corresponding refractive index step of the second grating, where ⁇ corresponds to a ⁇ /2 phase shift in reflections attributable to at least one of the refractive index steps following the leading refractive index step of the first grating.
  • the intensity of the group delay ripples resulting from the interconnected first and second gratings is lower than the intensity of the group delay ripples attributable to a single grating. This results from destructive interference of the group delay ripples attributable to the individual gratings.
  • the step of displacing at least one of the refractive index steps of the first grating relative to the or each corresponding refractive index step of the second grating preferably comprises displacing the leading refractive index step of the first grating by distance ⁇ relative to the leading refractive index step of the second grating.
  • the step of displacing at least one of the refractive index steps of the first grating relative to the or each corresponding refractive index step of the second grating may be effected during the process of writing the gratings.
  • this step may be effected by imposing a dimensional change on the grating subsequent to writing the gratings.
  • the dimensional change preferably is effected by support means upon which the gratings are mounted.
  • the support means preferably are arranged (ie., controllable) in use to impose a dimensional change on the first grating in a manner whereby at least one of the refractive index steps of the first grating is displaced by distance ⁇ relative to the or each corresponding refractive index steps of the second grating. More specifically, the support means may be arranged in use to impose a dimensional change on the first grating in a manner whereby the leading refractive index step of the first grating is displaced by distance ⁇ relative to the leading refractive index step of the second grating.
  • the support means may be arranged in use to impose a dimensional change on the first grating in a manner whereby all but the leading refractive index step of the first grating are displaced by distance ⁇ relative to the leading refractive index step of the second grating.
  • the support means may be arranged in use to apply localised heating or cooling to the first grating to effect the dimensional change.
  • the support means may be arranged in use to apply a mechanical force to the first grating to impose the dimensional change.
  • Relative positioning of the first and second Bragg gratings may also be effected by the support means through the application of heating or cooling or through the application of a mechanical force.
  • the first and/or the second grating (s) may be constituted by a plurality of grating structures. Also, any one of the gratings may be apodized.
  • Figure 1 shows a diagrammatic representation of a chirped Bragg grating
  • Figure 2 shows a plot of group delay as function of wavelength
  • Figure 3 shows a plot of the refractive index as a function of position within a chirped grating
  • Figure 4 shows a diagrammatic representation of the preferred embodiment of the dispersion compensation device .
  • Figure 1 shows a grating 10 that is chirped and provides an increase in the period ⁇ of refractive index variations along its length for a penetrating signal I.
  • the period ⁇ increases with penetration depth D into the grating and therefore the Bragg wavelength ⁇ B will also increase with D.
  • Figure 2 shows a plot of group delay as function of wavelength for a typical chirped grating with the plot having a ripple 20 that makes the linear compensation of chromatic dispersion difficult.
  • Non-sinusoidal group delay ripple may arise if the chirped fibre grating has defects such as variations in the chirp profile.
  • Non-sinusoidal group delay ripple may also arise from non-ideal fibre hosts such as non-uniform fibre core diameters.
  • Figure 4 shows an embodiment of the dispersion compensation device in which numeral 40 refers to a waveguide which is provided to guide a photonic signal.
  • An optical circulator 41 is connected to a waveguide incorporating the first Bragg grating 43 by another optical waveguide 42.
  • Numerals 44, 46 refer to waveguide terminators, and numerals 45, 48 refer to Peltier devices upon which the first and second Bragg gratings are mounted.
  • the second Bragg grating, incorporated in a waveguide 47, is connected by an optical waveguide 49 with the optical circulator 41.
  • the output signal is, in use, guided in waveguide 50.
  • the Peltier devices 45 and 48 are employed to exercise localised temperature control over one or other or both of the first and second gratings.
  • the Peltier device 45 is controlled to effect localised cooling of the first grating, so as to induce a dimensional change in the grating whereby the leading refractive index step of the first grating 43 is displaced by distance ⁇ relative to the leading refractive index step of the second grating.
  • This displacement may be effected by either shifting the leading edge refractive index of the first grating itself or by shifting all but the leading edge refractive index steps by distance ⁇ .
  • the distance ⁇ corresponds to a ⁇ /2 phase shift in reflections attributable to at least one (usually the first) of the refractive index steps following the leading refractive index step of the first grating.
  • the ⁇ /2 phase shift and, hence, the distance ⁇ may be computed in cases where all (or most) contributory factors are known. In other cases it will be determined empirically having regard to the following factors:
  • the group delay ripples considered here are a result of a Fabry Perot interference effect between the leading refractive index step and the resonance position within the grating.
  • the cavity length In free space interferometers the cavity length only has to change by ⁇ /2 to see a maxima in amplitude move a complete free spectral range.
  • the cavity length In the grating Fabry Perot interferometric effect which has relevance in the context of the present invention, the cavity length has to change by an amount proportional to ⁇ /2 radians of phase as reflected from the grating.
  • the required change in the length between the two reflection points is then a function of the phase envelope of the grating and is thus grating design specific. This corresponding length can be several orders of magnitude larger than the physical distance required for a conventional Fabry Perot interferometer .
  • the present invention is directed to the concatenation of two chirped gratings with group delay ripple profiles that are substantially in anti-phase across the bandwidth of interest, a variety of • combinations of grating chirp rate and positions of the first refractive index steps will provide the required response, where there is substantially destructive interference between the two individual group delay ripple profiles over the bandwidth of interest.
  • the above described implementation for achieving this is but one of the possible implementations.
  • the required change in length (which is grating design dependent) may exceed the practical limits to optical path length change achieveable by mechanical and/or thermal means . In this instance the only way to implement the path length change will be by grating design change before writing the second grating.
  • the first and the second Bragg gratings are aligned spectrally such that their reflection spectra interfere substantially constructively.
  • This alignment may be effected by application of a mechanical force to one and/or the other of the two gratings or by employing the Peltier devices 45 and 48 in the above described manner.
  • the dispersion compensation device may alternatively be fabricated by writing the gratings such that the leading refractive index step of the first grating is shifted relative to the leading refractive index step of the second grating by the distance ⁇ . In this case support means will not be required to impose displacement of relative refractive index steps within the first and/or second gratings.

Abstract

A dispersion compensation device is disclosed which has first and second Bragg gratings, with both gratings being chirped to provide substantially the same phase delay ζ as a function of wavelength μ and the gratings being interconnected in a manner such that their reflection spectra interfere substantially constructively. Also, at least one of the refractive index steps of the first grating is displaced by distance δ relative to the or each corresponding refractive index steps of the second grating, where δ corresponds to a π/2 phase shift in reflections attributable to refractive index steps following the leading refractive index step of the first grating.

Description

A DISPERSION COMPENSATION DEVICE
FIELD OF THE INVENTION
A dispersion compensation device which incorporates a plurality of chirped Bragg gratings which are interconnected and arranged to provide for minimisation of group delay ripples .
BACKGROUND OF THE INVENTION Chromatic dispersion is a phenomenon which places limits on the rate of photonic signal transmission in optical waveguides and may be defined as the variation of propagation time as a function of wavelength within a waveguide. Chromatic dispersion increases with the bandwidth of a photonic signal and limits the transmission distance, particularly at high data rates.
A known method of partially eliminating chromatic dispersion has been to reflect photonic signals using an optical fibre incorporating a chirped Bragg grating. A chirped Bragg grating is a grating that provides an increase or decrease in the period Λ of refractive index variations along its length. As with any fibre Bragg grating, the Bragg wavelength λB of light reflected by the grating is given by
Figure imgf000003_0001
where neff is the effective refractive index. Thus, if Λ increases with penetration depth D into the grating, the Bragg wavelength λB will also increase with D. When a chromatically-dispersed signal enters a chirped grating, the penetration depth D of the signal into the grating increases with wavelength, thus producing a wavelength- dependent time delay, referred to as "group delay" . Although the group delay increases in a generally-linear manner with wavelength, a non-linearity may be introduced by a group delay ripple . The presence of the group delay ripple means that a chirped fibre grating cannot completely compensate for chromatic dispersion. Apodization of the grating refractive index profile can reduce, but not eliminate, reflections at the leading refractive index step of the gratings resulting in a reduced amplitude group delay ripple.
Also, where large slopes of a group delay versus Bragg wavelength are required it has been found that an increase in slope is accompanied by a corresponding increase in the amplitude of group delay ripple.
SUMMARY OF THE INVENTION The present invention is directed to a dispersion compensation device which provides for minimisation of group delay ripple and to a method of fabricating such device.
Broadly defined, the invention provides a method of fabricating a dispersion compensation device having first and second Bragg gratings which are chirped to provide substantially the same phase delay φ as a function of wavelength λ and which are interconnected in a manner such that their reflection spectra interfere substantially constructively. The method includes the step of displacing at least one of the refractive index steps of the first grating by a distance δ relative to the or each corresponding refractive index step of the second grating, where δ corresponds to a π/2 phase shift in reflections attributable to at least one of the refractive index steps following the leading refractive index step of the first grating. The present invention also provides a dispersion compensation device when fabricated by the above-defined method.
The invention may be defined still further in terms of a dispersion compensation device comprising first and second Bragg gratings and support means supporting the gratings, both gratings being chirped to provide substantially the same phase delay φ as a function of wavelength λ, the gratings being interconnected in a manner such that their reflection spectra interfere substantially constructively, and the support means being arranged in use to impose a dimensional change on one or both of the gratings in a manner whereby at least one of the refractive index steps of the first grating is displaced by distance δ relative to the or each corresponding refractive index step of the second grating, where δ corresponds to a π/2 phase shift in reflections attributable to at least one of the refractive index steps following the leading refractive index step of the first grating.
It has been determined that, with the above defined device and devices fabricated by the above defined method, the intensity of the group delay ripples resulting from the interconnected first and second gratings is lower than the intensity of the group delay ripples attributable to a single grating. This results from destructive interference of the group delay ripples attributable to the individual gratings.
PREFERRED FEATURES OF THE INVENTION
In the above-defined method of fabricating a dispersion compensation device the step of displacing at least one of the refractive index steps of the first grating relative to the or each corresponding refractive index step of the second grating preferably comprises displacing the leading refractive index step of the first grating by distance δ relative to the leading refractive index step of the second grating.
The step of displacing at least one of the refractive index steps of the first grating relative to the or each corresponding refractive index step of the second grating may be effected during the process of writing the gratings. Alternatively, this step may be effected by imposing a dimensional change on the grating subsequent to writing the gratings. In this latter case the dimensional change preferably is effected by support means upon which the gratings are mounted.
The support means preferably are arranged (ie., controllable) in use to impose a dimensional change on the first grating in a manner whereby at least one of the refractive index steps of the first grating is displaced by distance δ relative to the or each corresponding refractive index steps of the second grating. More specifically, the support means may be arranged in use to impose a dimensional change on the first grating in a manner whereby the leading refractive index step of the first grating is displaced by distance δ relative to the leading refractive index step of the second grating. Alternatively, the support means may be arranged in use to impose a dimensional change on the first grating in a manner whereby all but the leading refractive index step of the first grating are displaced by distance δ relative to the leading refractive index step of the second grating. The support means may be arranged in use to apply localised heating or cooling to the first grating to effect the dimensional change. Alternatively, the support means may be arranged in use to apply a mechanical force to the first grating to impose the dimensional change.
Relative positioning of the first and second Bragg gratings may also be effected by the support means through the application of heating or cooling or through the application of a mechanical force. The first and/or the second grating (s) may be constituted by a plurality of grating structures. Also, any one of the gratings may be apodized.
The invention will be more fully understood from the following explanatory information and the description of a preferred embodiment of the invention. The description is provided with reference to the accompanying drawings .
BRIEF DESCRIPTION OF THE DRAWINGS
In the drawings : Figure 1 shows a diagrammatic representation of a chirped Bragg grating,
Figure 2 shows a plot of group delay as function of wavelength,
Figure 3 shows a plot of the refractive index as a function of position within a chirped grating, and
Figure 4 shows a diagrammatic representation of the preferred embodiment of the dispersion compensation device .
DETAILED DESCRIPTION OF THE INVENTION
Before proceeding to describe the device as illustrated in Figure 4, brief explanatory information is provided in the context of the invention with reference to Figures 1 to 3 .
Figure 1 shows a grating 10 that is chirped and provides an increase in the period Λ of refractive index variations along its length for a penetrating signal I. The period Λ increases with penetration depth D into the grating and therefore the Bragg wavelength λB will also increase with D. Figure 2 shows a plot of group delay as function of wavelength for a typical chirped grating with the plot having a ripple 20 that makes the linear compensation of chromatic dispersion difficult. There are many potential sources of group delay ripple in chirped fibre Bragg gratings and they may be grouped into sources that induce sinusoidal or non-sinusoidal group delay ripples. For example a non-sinusoidal group delay ripple may arise if the chirped fibre grating has defects such as variations in the chirp profile. Non-sinusoidal group delay ripple may also arise from non-ideal fibre hosts such as non-uniform fibre core diameters.
The origin of the sinusoidal group delay ripple is described with reference to Figure 3, in which the refractive index profile of the chirped grating 10 of Figure 1 is shown. It is believed that, as an photonic signal I comprising a range of wavelengths λi... λ2, enters the grating (on the left hand side of the profile shown in Figure 3) , there is a partial reflection at the leading refractive index step 30, producing a reflected signal I' . A group delay ripple is generated when the signal I' interferes with the various sub-signals of wavelengths λi reflected at refractive index step n . The preferred embodiment of the dispersion compensation device, which functions to compensate for sinusoidal group delay ripples will now be described. Figure 4 shows an embodiment of the dispersion compensation device in which numeral 40 refers to a waveguide which is provided to guide a photonic signal. An optical circulator 41 is connected to a waveguide incorporating the first Bragg grating 43 by another optical waveguide 42. Numerals 44, 46 refer to waveguide terminators, and numerals 45, 48 refer to Peltier devices upon which the first and second Bragg gratings are mounted. The second Bragg grating, incorporated in a waveguide 47, is connected by an optical waveguide 49 with the optical circulator 41. The output signal is, in use, guided in waveguide 50.
In use, the Peltier devices 45 and 48 are employed to exercise localised temperature control over one or other or both of the first and second gratings. In a preferred case the Peltier device 45 is controlled to effect localised cooling of the first grating, so as to induce a dimensional change in the grating whereby the leading refractive index step of the first grating 43 is displaced by distance δ relative to the leading refractive index step of the second grating. This displacement may be effected by either shifting the leading edge refractive index of the first grating itself or by shifting all but the leading edge refractive index steps by distance δ.
The distance δ corresponds to a π/2 phase shift in reflections attributable to at least one (usually the first) of the refractive index steps following the leading refractive index step of the first grating.
The π/2 phase shift and, hence, the distance δ may be computed in cases where all (or most) contributory factors are known. In other cases it will be determined empirically having regard to the following factors:
The group delay ripples considered here are a result of a Fabry Perot interference effect between the leading refractive index step and the resonance position within the grating. There are two key differences between free space Fabry Perot interferometers and the interference effect considered here. Firstly, the cavity length of the grating Fabry Perot is itself wavelength dependent and hence the chirp observed in group delay ripple generated from this effect. Secondly, the position of maxima and minima in the Airy function describing Fabry Perot interferometer response are orders of magnitude less sensitive in the grating based Fabry Perot than in a conventional free space Fabry Perot interferometer. In free space interferometers the cavity length only has to change by λ/2 to see a maxima in amplitude move a complete free spectral range. In the grating Fabry Perot interferometric effect which has relevance in the context of the present invention, the cavity length has to change by an amount proportional to π/2 radians of phase as reflected from the grating. The required change in the length between the two reflection points is then a function of the phase envelope of the grating and is thus grating design specific. This corresponding length can be several orders of magnitude larger than the physical distance required for a conventional Fabry Perot interferometer . Given that the present invention is directed to the concatenation of two chirped gratings with group delay ripple profiles that are substantially in anti-phase across the bandwidth of interest, a variety of • combinations of grating chirp rate and positions of the first refractive index steps will provide the required response, where there is substantially destructive interference between the two individual group delay ripple profiles over the bandwidth of interest. The above described implementation for achieving this is but one of the possible implementations. However, in some cases the required change in length (which is grating design dependent) may exceed the practical limits to optical path length change achieveable by mechanical and/or thermal means . In this instance the only way to implement the path length change will be by grating design change before writing the second grating.
The first and the second Bragg gratings are aligned spectrally such that their reflection spectra interfere substantially constructively. This alignment may be effected by application of a mechanical force to one and/or the other of the two gratings or by employing the Peltier devices 45 and 48 in the above described manner. Having considered the foregoing disclosure, a person skilled in the art will appreciate that such the dispersion compensation device may alternatively be fabricated by writing the gratings such that the leading refractive index step of the first grating is shifted relative to the leading refractive index step of the second grating by the distance δ. In this case support means will not be required to impose displacement of relative refractive index steps within the first and/or second gratings.

Claims

THE CLAIMS :
1. A method of fabricating a dispersion compensation device having first and second Bragg gratings which are chirped to provide substantially the same phase delay φ as a function of wavelength λ and which are interconnected in a manner such that their reflection spectra interfere substantially constructively; the method including the step of displacing at least one of the refractive index steps of the first grating by distance δ relative to the or each corresponding refractive index step of the second grating, where δ corresponds to a π/2 phase shift in reflections attributable to at least one of the refractive index steps following the leading refractive index step of the first grating.
2. The method as claimed in claim 1 wherein the step of displacing at least one of the refractive index steps of the first grating relative to the or each corresponding refractive index step of the second grating comprises displacing the leading refractive index step of the first grating by distance δ relative to the leading refractive index step of the second grating.
3. The method as claimed in claim 1 or claim 2 wherein the step of displacing at least one of the refractive index steps of the first grating relative to the or each corresponding refractive index step of the second grating is effected during a process of writing the gratings.
4. The method of as claimed in claim 1 or 2 wherein the displacement is effected by imposing a dimensional change on the first grating subsequent to writing the gratings.
5. The method as claimed in claim 4 wherein the gratings are mounted upon support means and the dimensional change is effected by way of the support means .
6. A dispersion compensation device when fabricated by the method as claimed in any one of claims 1 to 5.
7. A dispersion compensation device comprising first and second Bragg gratings and support means supporting the gratings, both gratings being chirped to provide substantially the same phase delay φ as a function of wavelength λ, the gratings being interconnected in a manner such that their reflection spectra interfere substantially constructively, and the support means being arranged in use to impose a dimensional change on one or both of the gratings in a manner whereby at least one of the refractive index steps of the first grating is displaced by distance δ relative to the or each corresponding refractive index step of the second grating, where δ corresponds to a π/2 phase shift in reflections attributable to at least one of the refractive index steps following the leading refractive index step of the first grating.
8. The dispersion compensation device as claimed in claim 7 wherein the support means are arranged in use to impose a dimensional change on the first grating in a manner whereby at least one of the refractive index step of the first grating is displaced by distance δ relative to the or each corresponding refractive index steps of the second grating.
9. The dispersion compensation device as claimed in claim 8 wherein the support means are arranged in use to impose a dimensional change on the first grating in a manner whereby the leading refractive index step of the first grating is displaced by distance δ relative to the leading refractive index step of the second grating.
10. The dispersion compensation device as claimed in claim 8 wherein the support means are arranged in use to impose a dimensional change on the first grating in a manner whereby all but the leading refractive index step of the first grating are displaced by distance δ relative to the leading refractive index step of the second grating.
11. The dispersion compensation device as claimed in any one of claims 8 to 10 wherein the support means are arranged in use to exercise localised temperature control at least one of the first and second gratings and thereby to impose the dimensional change on the first grating.
12. The dispersion compensation device as claimed in any one of claims 8 to 10 wherein the wherein the support means are arranged in use to apply a mechanical force to the first grating and thereby to impose the dimensional change on the first grating.
13. The dispersion compensation device as claimed in any one of claims 8 to 12 wherein the relative positioning of the first and second gratings is effected by the support means through the application of temperature control .
14. The dispersion compensation device as claimed in any one of claims 8 to 12 wherein the relative positioning of the first and second gratings is effected by the support means through application of a mechanical force.
15. The dispersion compensation device as claimed in any one of claims 8 to 14 wherein the first grating is constituted by of plurality of grating structures.
16. The dispersion compensation device as claimed in any one of claims 8 to 15 wherein the second grating is constituted by of plurality of grating structures.
17. The dispersion compensation device as claimed in any one of claims 8 to 16 wherein one or the other of the gratings is apodized.
18. The dispersion compensation device as claimed in any one of claims 7 to 16 wherein both the first and the second gratings are apodized.
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WO2000050944A1 (en) * 1999-02-19 2000-08-31 University Of Southern California Dispersion compensation by using tunable nonlinearly-chirped gratings
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