EP1506441A2

EP1506441A2 - Gires-tournois etalons and dispersion compensation

Info

Publication number: EP1506441A2
Application number: EP03722862A
Authority: EP
Inventors: Xuewen Indigo Photonics Limited SHU; Kevin Christopher Byron; Lee Ann Indigo Photonics Ltd KENNEDY; Catherine Anne Indigo Photonics LTD Sugden
Original assignee: Aston University; Aston Photonic Technologies Ltd
Current assignee: Aston University
Priority date: 2002-05-13
Filing date: 2003-05-13
Publication date: 2005-02-16
Also published as: AU2003230022A8; WO2003096082A2; AU2003230022A1; WO2003096082A3

Abstract

A first embodiment of the invention provides a Gires-Tournois etalon (DGTE) (20) comprising a cladding mode suppressed optical fibre (22), including an etalon section (24) in which a weakly reflective optical waveguide grating (26) and a strongly reflective optical waveguide grating (28) are provided. The two gratings (26, 28) are chirped fibre Bragg gratings (FBGs). The chirped FBGs (26, 28) are arranged to together define an etalon cavity. Another aspect of the invention provides a dispersion compensator (60) comprising two DGTEs (62, 64) according to the first embodiment of the invention. The DGTEs (62, 64) have a linearly varying dispersion over a selected optical bandwidth. The first DGTE (62) has a positive dispersion slope and the second DGTE (64) has a negative dispersion slope. The magnitudes of the dispersion slopes of the two DGTEs (62, 64) are substantially equal, so that the dispersion compensator (60) has a constant dispersion across the selected bandwidth. The DGTEs (62, 64) are optically coupled to one another via a four-port optical circulator (66).

Description

Gires-Toumois etalons and Dispersion Compensation

The invention relates to Gires-Toumois etalons and to dispersion compensators incorporating them.

Dispersion is a major problem for optical transmission systems. As pulses of light travel down an optical fibre transmission line they broaden due to chromatic dispersion. In addition, due to dispersion slope, the dispersion experienced by each wavelength component of an optical pulse can be different. The dispersion acquired by an optical pulse limits the length of transmission line across which a data signal (made up of a series of optical pulses) can be transmitted and successfully decoded at the receiver.

Tuneable dispersion compensators are becoming key devices in high speed optical transmission systems due to dispersion tolerances rapidly being reduced with increasing bit rate, especially when the bit rate reaches 40Gbit/s or beyond. Dynamic control of dispersion is also demanded during reconfiguration of transmission networks to compensate for effects such as the change of the link lengths or introduction of wavelength dependent routing.

A number of solutions have been proposed to address the problem of dispersion. The majority of these perform dispersion compensation on the pulses. This involves reforming the optical pulses to their original shape so that the data signal can then either be decoded or retransmitted across another transmission line.

One known dispersion compensation method utilises negative dispersion optical fibre (DCF) to reverse the effects of dispersion in normal transmission fibre. Although this works well as a bulk dispersion compensator (all channels), the amount of compensation applied cannot be adjusted and it does not compensate for higher-order dispersion terms (e.g. dispersion slope). Another solution uses chirped fibre Bragg gratings (CFBG), the different wavelengths of light being reflected from different positions along the grating, thereby applying a negative dispersion to the pulse. CFBG based devices are long (of the order of 10cm), making them difficult and expensive to fabricate and package, and they can suffer from high delay ripple effects

(10-20ps). A third approach is based on complex fibre Bragg gratings having a grating structure which includes multiple phase shifts and complex amplitude functions. However, these devices are primarily used only for dispersion slope compensation, and due to their fabrication complexity they suffer from low yields.

Various dispersion tuneable devices based on tuneable chirped fibre Bragg gratings, virtually imaged phased arrays, and integrated optical filters have been demonstrated. Although chirped fibre Bragg grating based devices are low loss and fully compatibility with optical fibre, they can generally only be used to provide compensation on a single optical channel.

An alternative arrangement to optical fibre based dispersion compensation devices is based on thin-film Gires-Toumois etalons (GTE). These bulk optic devices rely on reflections from a thin film etalon cavity to induce a negative dispersion effect on an optical pulse. However, they can only apply identical dispersion across their whole operating bandwidth. In addition, bulk optic GTE based devices suffer from high insertion losses due to the optical signal having to exit and renter the transmission fibre.

According to a first aspect of the present invention there is provided a Gires- Tournois etalon comprising: an optical waveguide including a grating section in which an optical waveguide grating is provided; and an optical reflector optically coupled to the optical waveguide, the optical waveguide grating and the optical reflector being arranged to together define an etalon cavity.

The optical reflector preferably comprises a second optical waveguide grating provided within a second grating section of the optical waveguide, at least part of the spectral profile of each grating including the same wavelength range. The optical waveguide grating is preferably a weakly reflective optical waveguide grating and the second optical waveguide grating is preferably a strongly reflective optical waveguide grating. The Gires-Toumois etalon may further comprise one or more additional optical waveguide gratings of intermediate reflective strength, giving a total of n optical waveguide gratings, at least part of the spectral profile of each additional grating including the same wavelength range as the spectral profiles of the weakly reflective grating and the strongly reflective grating, and the gratings being arranged to together define n-1 etalon cavities.

The or each optical waveguide grating is preferably a Bragg grating.

The optical waveguide gratings may be arranged in a spaced relationship with one another or may be arranged abutting one another. The optical waveguide gratings may be uniform period Bragg gratings, chirped Bragg gratings, phase-shifted Bragg gratings or sampled Bragg gratings.

Alternatively, the optical waveguide gratings may be arranged such that they at least in part overlap one another. The optical waveguide gratings may be chirped Bragg gratings, phase-shifted Bragg gratings or sampled Bragg gratings.

The chirped Bragg gratings may have a linear chirp or a non-linear chirp, such as a quadratic chirp.

The optical waveguide gratings may each be the same type of Bragg grating, or may be different types of Bragg grating. Where two or more gratings are chirped Bragg gratings they may be chirped in the same direction or in different directions, and they may have the same rate of chirp or different rates of chirp.

Where the Gires-Toumois etalon comprises three or more chirped Bragg gratings, the lengths of the cavities defined by adjacent pairs of gratings may be unequal.

The optical reflector may alternatively comprise a bulk-optic mirror. The optical reflector may further alternatively comprise a cleaved end face of the optical waveguide. The optical reflector may further alternatively comprise a photo-induced reflection point provided within the optical waveguide.

The Gires-Tournois etalon may further comprise one or more tuning means to which one or more of the optical waveguide gratings are coupled, the tuning means being operable to alter the periodicity of a grating, thereby changing the dispersion characteristics of the etalon. The tuning means may comprise apparatus for applying an axial force to a grating. The axial force may be an axial strain or an axial compression. The apparatus is preferably operable to apply a linear or non-linear strain gradient along the length of the grating. The tuning means may alternatively comprise heating apparatus operable to heat a grating. The heating apparatus is preferably operable to apply a linear or non-linear temperature gradient along the length of the grating. The tuning means may further alternatively comprise apparatus operable to change the spectral profile of a grating by changing the effective refractive index of a material, surrounding at least part of the etalon section of the optical waveguide containing the grating, utilising the electro-optic effect or the magneto-optic effect.

The optical waveguide may be an optical fibre or may be a planar optical waveguide.

According to a second aspect of the present invention there is provided a Gires-Tournois etalon comprising: an optical waveguide including an etalon section in which an optical waveguide grating structure is provided, the optical waveguide grating structure having a periodic refractive index variation equivalent to two or more at least partially overlapped complex Bragg gratings, a first one of the complex Bragg gratings being weakly reflective and a second one of the complex Bragg gratings being strongly reflective.

The complex Bragg gratings may be chirped Bragg gratings, phase-shifted Bragg gratings or sampled Bragg gratings. The chirped Bragg gratings may have a linear chirp or a non-linear chirp, such as a quadratic chirp. The sampled Bragg gratings may be phase sampled, amplitude sampled or phase and amplitude sampled. The complex Bragg gratings may each be the same type of Bragg grating, or may be different types of Bragg grating. Where two or more of the complex Bragg gratings are chirped Bragg gratings they may be chirped in the same direction or in different directions, and they may have the same rate of chirp or different rates of chirp.

The Gires-Tournois etalon may further comprise tuning means to which at least part of the optical waveguide grating structure is coupled, the tuning means being operable to alter the periodicity of the grating structure, thereby changing the dispersion characteristics of the etalon. The tuning means may comprise apparatus for applying an axial force to the grating structure. The axial force may be an axial strain or an axial compression. The apparatus is preferably operable to apply a linear or non-linear strain gradient along the length of the grating. The tuning means may alternatively comprise heating apparatus operable to heat the grating structure. The heating apparatus is preferably operable to apply a linear or non-linear temperature gradient along the length of the grating. The tuning means may further alternatively comprise apparatus operable to change the spectral profile of the grating structure by changing the effective refractive index of a material, surrounding at least part of the etalon section of the optical waveguide, utilising the electro-optic effect or the magneto-optic effect.

According to a third aspect of the present invention there is provided a dispersion controller comprising: a first Gires-Tournois etalon according to the first or second aspect of the present invention, wherein an optical pulse propagating through the waveguide interacts with the etalon, the distance each wavelength component of the pulse travels before exiting the etalon varying with wavelength, such that a wavelength dependent delay is applied to each wavelength component, thereby applying dispersion to the pulse.

The dispersion compensator may further comprise a second Gires-Tournois etalon according to the first or second aspect of the invention optically coupled to the first Gires-Tournois etalon. The first and second etalons may be optically coupled together via an optical waveguide coupler or an optical waveguide circulator.

The first and second Gires-Toumois etalons may apply the same or different dispersions to an optical pulse propagating through the dispersion compensator. The first and second etalons preferably apply dispersion slopes of substantially equal magnitude and opposite sign to the pulse, the first and second etalons thereby together applying a non-zero dispersion and a substantially zero dispersion slope to the pulse.

The dispersion compensator may further comprise one or more additional Gires-Tournois etalons according to the first or second aspects of the invention, the etalons being optically coupled together via one or more optical waveguide couplers or optical waveguide circulators.

According to a fourth aspect of the present invention there is provided a dispersion slope compensator comprising: a Gires-Tournois etalon according to the first aspect of the present invention, the reflectivity of the weak optical waveguide grating varying across the operational bandwidth of the Gires-Tournois etalon, thereby giving the etalon a dispersion profile across which the dispersion slope varies as function of wavelength.

The reflectivity of the weak optical waveguide grating may vary linearly or non- linearly as a function of wavelength.

According to a fifth aspect of the present invention there is provided a dispersion slope compensator comprising: a first Gires-Tournois etalon, according to the first or second aspect of the present invention, having a first group delay amplitude and a first free spectral range; and a second Gires-Tournois etalon, according to the first or second aspect of the present invention, having a second, smaller group delay amplitude and a second free spectral range, the first and second free spectral ranges being different.

The first free spectral range may be larger than the second free spectral range, the dispersion slope compensator thereby having a negative dispersion slope. Alternatively, the first free spectral range may be smaller than the second free spectral range, the dispersion slope compensator thereby having a positive dispersion slope.

The dispersion slope compensator may further comprise one or more tuning means to which one or both of the Gires-Toumois etalons are coupled, the tuning means being operable to alter the relative wavelength difference between the Gires-Tournois etalons. The tuning means may comprise apparatus for applying an axial force to one or both etalons. The axial force may be an axial strain or an axial compression. The tuning means may alternatively comprise heating apparatus operable to heat one or both etalons.

According to a sixth aspect of the present invention there is provided a dispersion compensator comprising: a first optical waveguide including a first etalon section in which first and second optical waveguide gratings are provided, at least part of the spectral profile of each grating including the same wavelength range, and the gratings being arranged to together define a first etalon, wherein an optical pulse propagating through the waveguide interacts with the etalon, the distance each wavelength component of the pulse travels before exiting the etalon varying with wavelength, such that a wavelength dependent delay is applied to each wavelength component, thereby applying dispersion to the pulse. The optical waveguide gratings are preferably Bragg gratings. The Bragg gratings may be uniform period Bragg gratings. Alternatively, the gratings may be complex Bragg gratings. The gratings are preferably substantially identical, having substantially identical spectral profiles and reflectivities.

Further alternatively, the first grating may be a uniform period Bragg grating and the second grating may be a complex Bragg grating. The or each complex Bragg grating may be a chirped Bragg grating, a phase-shifted Bragg grating or a sampled Bragg grating. The sampled Bragg grating may be amplitude sampled, phase sampled or both amplitude and phase sampled.

A chirped Bragg grating may have a linear chirp or may have a quadratic chirp. Where both complex Bragg gratings are chirped Bragg gratings, the gratings may both have a linear chirp, and may have the same rate of chirp or different rates of chirp. Alternatively, both chirped Bragg gratings may have a non-linear chirp, such as a quadratic chirp. Further alternatively, one of the chirped Bragg gratings may have a linear chirp and the other a non-linear chirp, such as a quadratic chirp. The chirped Bragg gratings may be chirped in the same direction or in opposite directions.

The optical waveguide gratings are preferably arranged in a spaced relationship with one another, and are most preferably closely spaced. The gratings may alternatively be located adjacent to one another. The gratings may further alternatively be arranged such that they at least in part overlap one another.

The dispersion compensator may further comprise an additional optical waveguide including an additional etalon section in which first and second additional optical waveguide gratings are provided, at least part of the spectral profile of each grating including the same wavelength range, and the gratings being arranged to together define an additional etalon, wherein the optical pulse propagating through the waveguide interacts with the additional etalon, the distance each wavelength component of the pulse travels before exiting the additional etalon varying with wavelength, such that a wavelength dependent delay is applied to each wavelength component, thereby applying dispersion to the pulse.

The first etalon and the additional etalon may apply the same or different dispersions to the pulse. The first etalon and the additional etalon preferably apply dispersion slopes of substantially equal magnitude and opposite sign to the pulse, the etalons thereby together applying a non-zero dispersion and a substantially zero dispersion slope to the pulse.

The dispersion compensator may comprise a plurality of additional etalons, the etalons preferably being arranged in optical series with one another.

The dispersion compensator may further comprise an optical isolator provided between a or each pair of etalons. The dispersion compensator may further comprise an optical amplifier, such as an Erbium doped fibre amplifier.

The dispersion compensator may alternatively or additionally comprise an optical reflector provided after the etalon or etalons, thereby providing for double pass operation of the etalon or etalons. The optical reflector may comprise a bulk optic optical reflector, such as a bulk optic mirror, or may alternatively comprise an optical waveguide based optical reflector, such as an optical waveguide grating or a Gires-Tournois etalon according to the first or second aspect of the present invention.

The dispersion compensator may alternatively or additionally comprise one or more Gires-Tournois etalons according to the first or second aspect of the present invention.

The dispersion compensator may alternatively or additionally further comprise a bulk-optic Gires-Tournois etalon optically coupled to the etalon or etalons. The dispersion compensator may further alternatively or additionally further comprise a further optical waveguide Bragg grating, which is preferably a chirped Bragg grating. The dispersion compensator preferably further comprises tuning means to which one or both gratings of the or each etalon are coupled, the tuning means being operable to alter the periodicity of a grating, thereby changing the dispersion which may be applied to the pulse by the respective etalon. The tuning means may comprise apparatus for applying an axial force to the grating. The axial force may be an axial strain or an axial compression. The apparatus is preferably operable to apply a linear or non-linear strain gradient along the length of the grating. The tuning means may alternatively comprise heating apparatus operable to heat the grating. The heating apparatus is preferably operable to apply a linear or non-linear temperature gradient along the length of the grating. The tuning means may further alternatively comprise apparatus for change the spectral profile of the grating by changing the effective refractive index of a material surrounding the part of the etalon section of the optical waveguide containing the grating, utilising the electro- optic effect or the magneto-optic effect.

The dispersion compensator may additionally comprise an optical waveguide circulator or an optical waveguide coupler, the circulator or coupler preceding, as seen by an optical pulse, the or each etalon, to thereby enable the compensator to be used in reflection.

The or each optical waveguide is preferably an optical fibre, or may alternatively be a planar optical waveguide.

Specific embodiments of the invention will now be described in detail, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is a diagrammatic representation of a prior art bulk-optic Gires- Tournois etalon;

Figure 2 is a diagrammatic representation of a Gires-Tournois etalon according to a first embodiment of the invention;

Figure 3 shows the experimentally recorded spectral profile in reflection (R) of the Gires-Tournois etalon of Fig. 2; Figure 4 shows the experimentally recorded group delay (Δτ) profile of the

Gires-Toumois etalon of Fig. 2; Figure 5 shows the experimentally recorded dispersion (D) profile of the

Gires-Tournois etalon of Fig. 2;

Figure 6 is a diagrammatic representation of a Gires-Tournois etalon according to a second embodiment of the invention; Figure 7 shows the experimentally recorded spectral profile in reflection (R) of the Gires-Tournois etalon of Fig. 6;

Figure 8 shows the experimentally recorded group delay (Δτ) profile of the

Gires-Tournois etalon of Fig. 6;

Figure 9 shows the experimentally recorded dispersion (D) profile of the Gires-Tournois etalon of Fig. 6;

Figure 10 is a diagrammatic representation of a Gires-Tournois etalon according to a third embodiment of the invention;

Figure 11 shows the theoretically modelled spectral profile in reflection (R) of the Gires-Tournois etalon of Fig. 10; Figure 12 shows the theoretically modelled group delay (Δτ) profile of the

Gires-Tournois etalon of Fig. 10;

Figure 13 shows the theoretically modelled dispersion (D) profile of the Gires- Tournois etalon of Fig. 10;

Figure 14 shows the apodization function used to fabricated a Gires-Tournois etalon comprising a single grating structure according to a fourth embodiment of the invention;

Figure 15 shows the theoretically modelled group delay (Δτ) profile of the

Gires-Tournois etalon of Fig. 14;

Figure 16 shows the theoretically modelled dispersion (D) profile of the Gires- Toumois etalon of Fig. 14;

Figure 17 is a diagrammatic representation of a dispersion compensator according to a fifth embodiment of the invention;

Figure 18 shows group delay (Δτ) profiles of the dispersion compensator of

Fig. 17 for three different dispersion settings ((a) approx. -200ps/nm, (b) approx. Ops/nm, and (c) approx. +200ps/nm) across four channels;

Figure 19 shows the variation in dispersion (D) as a function of applied strain

(με) for the dispersion compensator of Fig. 17, for each of the four channels shown in Fig. 18; Figure 20 shows the group delay (Δτ) profiles, over a single channel, for four different dispersion settings ((a) approx. -210ps/nm, (b) approx. -160ps/nm,

(c) approx. Ops/nm, and (d) approx. +190ps/nm) of an improved dispersion compensator according to the fifth embodiment of the invention; Figure 21 shows the group delay ripples for each of the dispersion settings of

Fig. 20;

Figure 22 is a diagrammatic representation of a dispersion compensator according to a sixth embodiment of the invention;

Figure 23 shows the combined time delay profile of the dispersion compensator (DC) of Fig. 22 and the time delay (Δτ) profiles of each of its four constituent Gires-Tournois etalons;

Figure 24 shows theoretically modelled spectral profiles in reflection of a weak grating (W) and a strong grating (S) within a dispersion slope compensating

Gires-Tournois etalon according to a seventh embodiment of the invention; Figure 25 shows the theoretically modelled group delay (Δτ) profile of a dispersion slope compensator according to an eighth embodiment of the invention;

Figure 26 shows the theoretically modelled dispersion (D) profile of the dispersion slope compensator according to the eighth embodiment of the invention;

Figure 27 is a schematic representation of an optical dispersion compensator according to a ninth embodiment of the present invention;

Figure 28 is a schematic representation of an optical dispersion compensator, including an optical coupler and tuning means, according to a tenth embodiment of the present invention;

Figure 29 is a schematic representation of an optical dispersion compensator, including an optical circulator, according to an eleventh embodiment of the present invention;

Figure 30 is a schematic representation of an optical dispersion compensator, having two grating etalons, one being coupled to a tuning means, according to a twelfth embodiment of the present invention;

Figure 31 shows the theoretical dispersion (D) profile of the first etalon of the dispersion compensator of Fig. 30; Figure 32 shows the theoretical dispersion (D) profile of the second etalon of the dispersion compensator of Fig. 30;

Figure 33 shows three different theoretical resultant group delay (ΔT) profiles, for three different wavelength offsets (A, B, C) between the spectral profiles of the first and second etalons, of the dispersion compensator of Fig. 30;

Figure 34 shows three corresponding dispersion (D) profiles (A, B, C) calculated from the three group delay profiles of Fig. 33;

Figure 35 is a diagrammatic representation of an optical dispersion compensator, including a Gires-Tournois etalon and a grating etalon, according to a thirteenth embodiment of the present invention;

Figure 36 is a diagrammatic representation of an optical dispersion compensator, including a chirped fibre Bragg grating, according to a fourteenth embodiment of the present invention;

Figure 37 is a diagrammatic representation of an optical dispersion compensator according to a fifteenth embodiment of the present invention;

Figure 38 is a diagrammatic representation of an optical transmission system including an optical compensator according to the present invention being used to apply post-transmission compensation to an optical signal;

Figure 39 is a diagrammatic representation of an optical transmission system including an optical compensator according to the present invention being used to apply pre-transmission compensation to an optical signal;

Figure 40 shows the dispersion (D) profiles of a three cavity Gires-Tournois etalon according to a fifth embodiment of the invention when the cavity lengths are equal and unequal; Figure 41 shows the dispersion (D) profile of the Gires-Tournois etalon

(unequal cavity lengths) according to the fifth embodiment of the invention and the dispersion profile of a Gires-Tournois etalon having two unequal length cavities;

Figure 42 shows the group delay (Δτ) profiles of the Gires-Tournois etalons of Fig.41 ;

Figure 43 shows the dispersion (D) profiles of two-cavity and three cavity

Gires-Tournois according to a sixth embodiment of the invention; Figure 44 shows the group delay (Δτ) profiles of the Gires-Tournois etalons according to the sixth embodiment of the invention;

Figure 45 shows group delay (Δτ) profiles of a dispersion compensator according to a tenth embodiment of the invention, for three different dispersion values: (a) D=-620ps/nm; (b) D=0ps/nm; and (c)

D=+620ps/nm;

Figure 46 shows the group delay (Δτ) profiles of a dispersion compensator according to an eleventh embodiment of the invention, for three different dispersion values: (a) D=-750ps/nm; (b) D=0ps/nm; and (c) D=+750ps/nm;

Figure 47 illustrates how a dispersion slope compensator having a negative dispersion slope is constructed from a first DGTE (DGTE1 ) having a large GDA and a second DGTE (DGTE2) having a small GDA, the FSR of the first (large GDA) DGTE being greater than that of the second (small GDA) DGTE; Figure 48 illustrates how a dispersion slope compensator having a negative dispersion slope is constructed from a first DGTE (DGTE1) having a large GDA and a second DGTE (DGTE2) having a small GDA, the FSR of the large-GDA DGTE (DGTE1) being smaller FSR than that of the small-GDA DGTE (DGTE2); Figure 49 shows the measured group delay profile of a dispersion slope compensator according to the thirteenth embodiment of the invention; Figure 50 shows the dispersion values for 20 channels of the dispersion slope compensator of the thirteenth embodiment; Figure 51 shows the measured group delay profile of a dispersion slope compensator according to a fourteenth embodiment of the invention;

Figure 52 shows the theoretically calculated spectral profile (a), group delay profile (b) and dispersion profile (c) of a dispersion compensator according to a twenty-second embodiment of the invention; Figure 53 is a diagrammatic representation of a dispersion compensator according to a twenty-third embodiment of the invention;

Figure 54 shows the dispersion (D) profiles of the two FP etalons of the dispersion compensator of Fig.53; Figure 55 shows the dispersion profile (a), group delay profile (b) and spectral profile in transmission (T) of the dispersion compensator of Fig. 53 when the dispersion value is set at Ops/nm;

Figure 56 is a diagrammatic representation of a dispersion compensator according to a twenty-fourth embodiment of the invention;

Figure 57 is a diagrammatic representation of a dispersion compensator according to a twenty-fifth embodiment of the invention;

Figure 58 is a diagrammatic representation of a dispersion compensator according to a twenty-sixth embodiment of the invention; Figure 59 is a diagrammatic representation of a dispersion compensator according to a twenty-seventh embodiment of the invention;

Figure 60 is a diagrammatic representation of a dispersion compensator according to a twenty-eighth embodiment of the invention;

Figure 61 is a diagrammatic representation of a dispersion compensator according to a twenty-ninth embodiment of the invention;

Figure 62 is a diagrammatic representation of a dispersion compensator according to a thirtieth embodiment of the invention; and

Figure 63 is a diagrammatic representation of a dispersion compensator according to a thirty-first embodiment of the invention.

Referring to Fig. 1 , the traditional bulk-optic Gires-Tournois etalon 10 is a special configuration of a Fabry-Perot etalon, and consists of a partially reflective first mirror 12 and a 100% reflective second mirror 14 which together define the etalon cavity 16. The second mirror 14 reflects all light incident on it, to yield the all-pass transfer function of the Gires-Toumois etalon 10. The spectral characteristics of the first mirror 12 can be altered to control the dispersion properties of the Gires-Toumois etalon 10.

Fig.2 shows a Gires-Tournois etalon (GTE) 20 according to a first embodiment of the invention. The GTEs described hereinafter are referred to as distributed GTEs (DGTE) in order to emphasise the physical property of these GTEs that the resonance location within the etalon cavity is wavelength dependent. The DGTE 20 in this example comprises an optical waveguide, in the form of cladding mode suppressed optical fibre 22, including an etalon section 24 in which a weakly reflective optical waveguide grating 26 and a strongly reflective optical waveguide grating 28 are provided. In this example the weakly reflective optical waveguide grating 26 is a partially reflective

(-11%) chirped fibre Bragg grating (FBG) and the strongly reflective grating 28 is a strong (-99%) chirped FBG. The weak grating 26 has a length of

6mm, a bandwidth of ~10nm and a central wavelength of 1550nm. The strong grating 28 has a length of 10mm, a bandwidth of ~18nm and a central wavelength of 1550nm. The two chirped FBGs 26, 28 have the same chirp rates. The two chirped FBGs 26, 28 were written into the optical fibre 22 using the phase-mask fabrication process. A frequency doubled argon ion laser was used to write the chirped FBGs 26, 28 and they were apodised with a super-Gaussian profile during the writing process. This method of fabricating gratings will be well know to the person skilled in the art so it will not be described in detail here.

The two chirped FBGs 26, 28 are arranged to together define an etalon cavity. In this example the FBGs 26, 28 overlap one another, with their centres being off-set from one another by approximately 2mm. This separation gives the DGTE 20 a channel spacing (free spectral range) of 50 GHz. The spectral profile in reflection (R) of the DGTE 20 is shown in Fig. 3. As can be seen, the reflectivity of the DGTE 20 is high and the fluctuation in the reflectivity is less than 1dB.

The group delay (Δτ) profile of the DGTE 20 is shown in Fig. 4, from which the periodic response of the DGTE 20 is clearly evident. In fact the group delay is periodically quasi-quadratic. In addition, the group delay can be clearly seen to have a slope. This is caused by the dispersive nature of the chirped FBGs 26, 28. The dispersion (D) profile of the DGTE 20 is shown in Fig.5. The dispersion of the DGTE 20 can be seen to be periodically quasi-linear within a given bandwidth i.e. for each channel. Good linearity in the dispersion can be achieved by controlling the reflectivity of the weak gratingl 6: essentially, lower reflectivity results in better dispersion linearity.

As can be seen from Figs. 4 and 5, the usable bandwidth of the DGTE 20 is about 8nm. This is mainly determined by the spectral characteristics of the weak grating 26. The bandwidth of the DGTE 20 can be easily expanded to cover the whole telecoms C-band, or even wider, by using longer gratings or by giving the gratings a larger chirp rate.

Despite the differences in its spectral characteristics, namely the limited bandwidth and the slope present on the group delay profile, the DGTE 20 according to the present invention clearly exhibits similar periodic characteristics to traditional bulk-optic GTEs.

A DGTE 30 according to a second embodiment of the invention is shown in Fig. 6. In this example the DGTE 30 comprises an optical waveguide, in the form of cladding mode suppressed optical fibre 32, including an etalon section 34 in which a weakly reflective (-4%) optical waveguide grating 36 and a strongly reflective (-97%) optical waveguide grating 38 are provided. In this example, both the weak grating 36 and the strong grating 38 are uniform period FBGs. The weak grating 36 has length of ~0.2mm and the strong grating has a length of -0.8mm. Both gratings 36, 38 have a central wavelength of -1546.1 nm and a bandwidth of -1.9nm. The gratings 36, 38 are arranged in a spaced relationship with one another, with a separation distance between the grating centres of -4.1 mm.

The spectral profile in reflection (R) of the DGTE 30 is shown in Fig.7. The group delay (Δτ) profile and the dispersion profile of the DGTE 30 are shown in Figs. 8 and 9 respectively.

The usable bandwidth of a DGTE and the amount of dispersion for each channel which it can apply to an optical signal can both be increased by increasing the number of etalon cavities within the DGTE. Fig. 10 shows a DGTE 40 according to a fourth embodiment of the invention. In this embodiment, the DGTE 40 comprises a planar optical waveguide 42, including an etalon section 44 in which three chirped FBGs are provided: a weak grating 46 having a reflectivity of 0.8%; an intermediate strength grating 48 having a reflectivity of 23.6%; and a strong grating 50 having a reflectivity of 50%. Each grating has a length of 8mm and a bandwidth of ~13.6nm. The gratings are arranged to overlap one another, with the centres of adjacent gratings being offset by -1 mm. The intermediate grating 48 is shown offset for clarity, but it will be appreciated that the three FBGs are in fact co-linear with one another. The three gratings 46, 48, 50 together define two etalon cavities, each cavity having a length of ~1 mm.

Figs. 11 to 13 show the spectral profile in reflection (R), the group delay (Δτ) profile, and the dispersion (D) profile respectively of the DGTE 40.

A fifth embodiment of the invention provides a DGTE comprising an optical fibre, including an etalon section in which four chirped FBGs are provided, giving the DGTE three etalon cavities. Each grating has a length of 16mm and a chirp rate of -32nm/cm. The gratings are arranged overlapping one another. The centres of the first two gratings are offset from one another by 2mm - 0.074μm, the centres of the second and third gratings are offset by 2mm, and the centres of the third and fourth gratings are offset by 2mm - 0.77μm. The DGTE therefore has a cavity length of ~2mm, giving the etalon an FSR of 50GHz. Making the lengths of the three cavities non-identical in this way improves the linearity of the dispersion (D) profile 280 of the etalon, as compared to the dispersion profile 282 of a DGTE constructed from the same gratings but having equal cavity lengths, as shown in Fig. 40.

Figures 41 and 42 respectively compare the dispersion (D) profile 280 and the group delay profile 284 of the DGTE according to the fifth embodiment with the dispersion profile 286 and the group delay profile 288 of a DGTE having two cavities of unequal length. The two-cavity DGTE comprises two chirped Bragg gratings each having a length of 16mm and a chirp rate of -32nm/cm. The gratings are again arranged to overlap one another, with the centres of the first two gratings being offset by 2mm and the centres of the second and third gratings being offset by 2mm - 0.119μm. Figs. 41 and 42 clearly illustrate how increasing the number of cavities within a DGTE increases the bandwidth and dispersion available for each optical channel.

A sixth embodiment of the invention provides a two-cavity DGTE comprising an optical fibre including an etalon section in which three chirped FBGS are provided, and a three-cavity DGTE comprising an optical fibre including an etalon section in which four chirped FBGs are provided. The gratings each have a length of 16mm and are arranged to overlap one another, such that the centres of adjacent gratings are offset by 2mm, giving both DGTEs FSRs of 50GHz.

In contrast with the fifth embodiment, the etalon cavities in the DGTEs according to the sixth embodiment are all of equal length. An equivalent effect to having unequal cavity lengths is achieved by giving one grating within each DGTE a different chirp rate. In the two-cavity DGTE the first and second gratings have a chirp rate of -32nm/cm and the third grating has a chirp rate of -32.071 nm/cm. This is equivalent to an offset between the first and second gratings of 2mm and an offset between the second and third gratings of 2mm - 0.65μm. In the three-cavity DGTE the first, second and fourth gratings have a chirp rate of -32nm/cm and the third grating has a chirp rate of -

32.103nm/cm. This is equivalent to the centres of the first and second gratings being offset by 2mm - 0.05μm, the second and third grating centres being offset by 2mm, and the centres of the third and fourth gratings being offset by 2mm - 0.7μm.

Fig. 43 shows the dispersion (D) profile 290 of the two-cavity DGTE and the dispersion profile 292 of the three-cavity DGTE. Fig. 44 shows the group delay (Δτ) profile 294 of the two-cavity DGTE and the group delay (Δτ) profile 296 of the three-cavity DGTE.

A seventh embodiment of the invention provides a DGTE which comprises a single optical waveguide grating structure fabricated within a planar optical waveguide. The grating structure has a periodic refractive index variation which, in this example, is equivalent to two overlapped chirped Bragg gratings: one weak grating and one strong grating. The equivalent chirped Bragg gratings each have a length of 8mm and a bandwidth of 13.6nm.

The grating structure is fabricated using the phase mask fabrication technique. This method of fabricating gratings will be well known to the skilled person and will therefore not be described in detail here. A chirped phase mask is used here, with a frequency doubled argon ion laser. The grating structure is fabricated using a single scan of the laser beam across the phase mask. During the scan a complex apodization function is applied to the amplitude (A) of the laser beam as a function of position (z) along the phase mask. In this example, the apodization function takes the form of a sine wave modulated super Gaussian function, as shown in Fig. 14. Figs. 15 and 16 show the time delay (Δτ) response and the dispersion (D) response of the

DGTE respectively.

The grating structure may alternatively be fabricated using a phase mask in which all the phase information of the DGTE grating structure is encoded within the phase mask. The skilled person will realise that a super Gaussian apodization function may be applied to the grating structure in order to perform standard apodization of the grating structure.

A eighth embodiment of the present invention provides a dispersion compensator 60 as shown in Figs. 17 to 21. The dispersion compensator 60 comprises two DGTEs 62, 64 according to the first embodiment of the invention. However, the skilled person will appreciate that one or both of the DGTEs 62, 64 may alternatively take the form of any DGTE according to the first or second aspects of the present invention, and in particular any of the above described embodiments of the invention.

The DGTEs 62, 64 have a linearly (or quasi-linearly (which is a simple method of increasing dispersion tuning ranges)) varying dispersion (D) over a selected optical bandwidth, as can be seen in inset Figs. 17a and 17b. The first DGTE 62 is designed to have a positive dispersion slope and the second DGTE 64 is designed to have a negative dispersion slope over the selected bandwidth. The magnitudes of the dispersion slopes of the two DGTEs 62, 64 are substantially equal, so that the dispersion compensator 60 has a constant dispersion across the selected bandwidth.

The DGTEs 62, 64 are optically coupled to one another via a four-port optical circulator 66. The first port 66a forms the input to the dispersion compensator 60, the second port 66b is coupled to the first DGTE 62, the third port 66c is coupled to the second DGTE 64, and the fourth port 66d forms the output of the dispersion compensator.

Group delay (Δτ) profiles of the dispersion compensator 60 for three different dispersion settings ((a) approx. -200ps/nm, (b) approx. Ops/nm, and (c) approx. +200ps/nm) are shown in Fig. 18. Each group delay profile displays a channel spacing of 50GHz for four channels (ch1-4).

The dispersion value is determined by the relative spectral shift between the two DGTEs 62, 64. The spectral shift between the two DGTEs 62, 64 can be controlled by tuning the wavelength of one or both of the chirped FBGs 26, 28 in one or both of the DGTEs 62, 64. As discussed above, the wavelength of an FBG may be tuned either by applying an axial strain to the grating or by heating the grating. In this example the magnitude of the wavelength shift between the two DGTEs 62, 64 is controlled by the application of axial strain. Fig.19 shows the variation in dispersion (D) as a function of applied strain (με) for each of the four channels (ch1 -4).

It is evident from Figs. 18 and 19 that the dispersion values achieved for the four channels are almost identical. The linearity of the group delay curves and of the strain tuning curves are not ideal. This is because the dispersion profile of the first DGTE 62, which has a positive dispersion slope, is quasi-linear (like Fig.5). The linearity of the group delay profile and the strain tuning curve can be improved by using a positive-dispersion-slope DGTE which has a truly linear dispersion profile.

Fig. 20 shows the group delay (Δτ) profiles, over a single channel, for four different dispersion settings ((a) approx. -21 Ops/nm, (b) approx. -160ps/nm, (c) approx. Ops/nm, and (d) approx. +190ps/nm) of an improved dispersion compensator 60 in which the first DGTE 62 is replace with a DGTE having a truly linear dispersion profile. As before, the dispersion compensator has a 50GHz-channel-spacing. The improvement in the linearity of the group delay is clearly visible for each of the dispersion settings. Fig. 21 shows the group delay ripples for each of the dispersion settings. The group delay ripple is better than +/-3ps over the full dispersion tuning and wavelength ranges.

A dispersion compensator 70 according to a ninth embodiment of the invention is shown in Fig. 22. In this embodiment the dispersion compensator 70 comprises four DGTEs 72, 74, 76, 78 according to the first embodiment of the invention. In this example, the four DGTEs 72, 74, 76, 78 are coupled together via two four-port optical circulators 80, 82. A first port 80a of the first circulator forms the input to the dispersion compensator 70. The second port 80b and the third port 80c are connected to the first 72 and second 74 DGTEs respectively. The fourth port 80d is connected to the first port 82a of the second circulator. The second port 82b and the third port 82c of the second circulator are connected to the third 76 and fourth 78 DGTEs respectively. The remaining port 80d of the second circulator forms the output of the dispersion compensator 70. The skilled person will appreciate that the four DGTEs could be coupled together using an alternative arrangement, such as a six-port optical circulator or a combination of optical couplers.

The time delay (Δτ) profiles of each of the four DGTEs 72, 74, 76, 78, and the combined time delay profile of the dispersion compensator (DC) 70, are shown in Fig. 23. Only four channels are shown for clarity, but the skilled person will appreciate that such a device can provide dispersion compensation for many more channels than that. The dispersion compensator 70 can apply a dispersion of -3400ρs/nm to each optical channel, and can therefore provide the necessary dispersion compensation for a 200km standard single mode fibre optical link within a 10Gbit/s transmission system operating at a channel spacing of 25GHz.

A dispersion compensator according to a tenth embodiment of the invention comprises two DGTEs, being the three-cavity DGTE of the fifth embodiment and a two-cavity DGTE comprising two chirped Bragg gratings each having a length of 16mm and a chirp rate of -32nm/cm. The gratings are arranged to overlap one another, with the centres of the first two gratings being offset by 2mm and the centres of the second and third gratings being offset by 2mm - 0.119μm. The dispersion (D) profiles and the group delay (Δτ) profiles of the two- and three-cavity DGTEs are shown in Figs. 41 and 42 respectively.

The group delay (Δτ) profiles of the dispersion compensator for three different dispersion values are shown in Fig. 45: (a) D=-620ps/nm; (b) D=0ps/nm; and (c) D=+620ps/nm. The dispersion compensator has a usable bandwidth of 25GHz.

A dispersion compensator according to an eleventh embodiment of the invention comprises three DGTEs, which in this example comprise a two- cavity DGTE according to the sixth embodiment of the invention and two three-cavity DGTEs according to the sixth embodiment of the invention. The group delay (Δτ) profiles of the dispersion compensator for three different dispersion values are shown in Fig. 46: (a) D=-750ps/nm; (b) D=0ps/nm; and (c) D=+750ps/nm. The dispersion compensator has a usable bandwidth of 25GHz.

Dispersion compensators can be constructed from a combination of 2, 3, 4 or more DGTEs to provide fixed dispersion compensators or tuneable dispersion compensators. The greater the number of DGTEs combined to form a dispersion compensator, the larger the available dispersion range. In addition, a dispersion compensator can be constructed from a single DGTE. However, a single DGTE dispersion compensator will not have zero dispersion slope.

The fact that a single DGTE does not have zero dispersion slope can be an advantage in certain applications, since this characteristic allows a single DGTE to act as a dispersion slope compensator. A dispersion slope compensating DGTE according to a eleventh embodiment of the invention is substantially the same as the DGTEs 20, 30, 40 described above, with the following modification. The reflectivity of the first (weak) grating is not constant across the operating bandwidth of the DGTE. For example, as shown in Fig. 24, for a two grating DGTE, the reflectivity (R) of the weak grating (W) increases linearly with wavelength (λ) across the operating bandwidth of the DGTE, while the reflectivity of the strong grating (S) remains substantially constant. An twelfth embodiment of the invention provides a dispersion slope compensator based on two DGTEs. In this example, similarly to the dispersion slope compensator according to the previous embodiment, the weak grating in each DGTE has a linearly varying reflectivity and the strong gratings have a substantially constant reflectivity. As shown in Figs. 25 and 26 respectively, the time delay (Δτ) has a different gradient across each channel (five channels are shown) and each channel experiences a different dispersion (D). This type of dispersion slope compensator can therefore be used to compensate for dispersion slope across the different channels within a transmission system.

Thirteenth and fourteenth embodiments of the invention provide dispersion slope compensators comprising two DGTEs of different group delay amplitude (GDA) and different free spectral range (FSR). The thirteenth embodiment provides a dispersion slope compensator having a positive dispersion slope i.e. for compensating for a negative dispersion slope, and the fourteenth embodiment provides a dispersion slope compensator having a negative dispersion slope.

Fig.47 illustrates how a dispersion slope compensator having a negative dispersion slope is constructed from a first DGTE (DGTE1) having a large GDA and a second DGTE (DGTE2) having a small GDA, the FSR of the first (large GDA) DGTE being greater than that of the second (small GDA) DGTE. The wavelength separation (Δλ) of corresponding peaks in the group delay profiles of the first and second DGTEs increases (Δλι<Δλ₂<Δλ₃ etc) with channel number, due to the mismatch in the FSRs of the two DGTEs. The combined group delay slope (dispersion) of the compensator depends on the wavelength separation between corresponding peaks of the two DGTEs. As can be seen in Fig.47, the slope of the group delay profile i.e. the dispersion) of the dispersion compensator decreases with increasing wavelength separation between corresponding peaks in the group delay profiles of the first and second DGTEs. That is to say, the dispersion compensator has a negative dispersion slope. However, referring to Fig.48, if the large-GDA DGTE (DGTE1) has a smaller

FSR than that of the small-GDA DGTE (DGTE2), a dispersion slope compensator having a positive dispersion slope is produced. The group delay slope (i.e dispersion) increases as the wavelength separation between corresponding peaks in the group delay profiles of the two DGTEs decreases

(Δλι>Δλ₂>Δλ₃ etc).

When two DGTEs with different FSRs are combined, they will generate a beat frequency, which limits the maximum number of channels of different dispersion values. Assuming the difference in the FSRs of the two DGTEs (Δf =f 2-f ι , where fi and f₂ are FSR of DGTE1 and DGTE2, respectively) is very small, i.e. f-ι=f₂«FSR, the beat frequency can be written as fιf₂/|Δfl «FSR²/|Δfl, and the maximum number of channels is given by FSR/)Δfl. The dispersion slope (DS) of the dispersion compensator can be estimated using the relationship

1.4 -Af - (A +fl₂) ESR² where Di and D₂ are the amplitudes of the dispersion for the first DGTΕ

(DGTΕ1) and the second DGTE (DGTE2) respectively.

The dispersion slope compensator according to the thirteen embodiement comprises two DGTEs (of different GDA and FSR) optically coupled via a four-port optical circulator. Each DGTE comprises two chirped fibre Bragg gratings fabricated in hydrogen-loaded cladding-mode-suppressed optical fibre. The gratings were fabricated using the phase mask fabrication technique, utilising a frequency doubled argon ion laser (244nm) and a chirped phase mask. The gratings in each DGTE are separated by -4.1 mm, giving each DGTE an FSR of ~25GHz. Specifically, the first DGTE has an FSR of 25GHz and the second DGTE has an FSR of 25.7GHz. The GDA (and thus the dispersion amplitude) of each DGTE is controlled by the reflectivity of the first (weak) grating, the second grating having a reflectivity of >95%. In this example, the first DGTE has a dispersion amplitude of 770ps/nm and the second DGTE has a dispersion amplitude of 260ps/nm. Fig.49 shows the measured group delay profile of the dispersion slope compensator of the thirteenth embodiment. It can be clearly seen that the dispersion of the channels having wavelengths of between 1544.5nm and

1549.0nm increases from negative to positive. The dispersion values 300 for 20 channels of the dispersion slope compensator are shown in Fig.50. From these dispersion values the dispersion slope of the dispersion slope compensator can be calculated (using the best fitting line) to be

+208.1 ps/nm², which is very close to the theoretical estimation of

+206.1 ps/nm².

The group delay ripples are less than ±4ps and the ripples in transmission amplitudes are <0.8dB across a usable bandwidth of about 14GHz for all channels.

The dispersion slope compensator according to the fourteenth embodiment of the invention again comprises two DGTEs (of different GDA and FSR) optically coupled via a four-port optical circulator. In this embodiment the first DGTE (DGTE1) has a dispersion amplitude of 260ps/nm and an FSR of 25.7GHz, and the second DGTE has a dispersion amplitude of 640ps/nm and an FSR of 26GHz.

Fig.51 shows the measured group delay profile of the dispersion slope compensator according to the fourteenth embodiment. It can be clearly seen that the dispersion of the channels having wavelengths of between 1544.5nm and 1549.0nm increase from negative to positive. The dispersion values 302 for 20 channels of the dispersion slope compensator are shown in Fig.50. From these dispersion values the dispersion slope of the dispersion slope compensator can be calculated (using the best fitting line) to be -94.9ps/nm², which is very close to the theoretical estimation of 94.0ps/nm².

The group delay ripples are less than ±4ps and the ripples in transmission amplitudes are <0.8dB across a usable bandwidth of about 14GHz for all channels. Fig. 27 shows a schematic representation of a ninth embodiment of the invention. The fifteenth embodiment provides a dispersion compensator 110 comprising an optical waveguide, in the form of an optical fibre 112, in which two optical waveguide gratings, chirped fibre Bragg gratings (FBGs) 114, 116 in this example, are provided in an etalon section 118 of the fibre 112, and are arranged to together define a Fabry-Perot (F-P) etalon 120.

The gratings 114, 116 may be fabricated using any suitable fabrication technique, such as the two-beam holographic fabrication technique or the phase mask fabrication technique. These grating fabrication techniques will be well known to the skilled person and so they will not be described in detail here. The spectral profiles of the gratings 114, 116 in this example are substantially the same, covering substantially the same wavelength range. The gratings 114, 116 have a length of between 1 mm and 20mm, both gratings 114, 116 being of substantially the same length. However, if a more complex dispersion profile is required for an etalon longer gratings may be used.

In this embodiment the gratings 114, 116 are arranged adjacent to one another (the dashed line in Fig. 27 indicates the boundary between the two gratings 114, 116).

The dispersion compensator 110 of this embodiment operates in transmission, as indicated by the arrows in Fig. 27.

In operation, an optical pulse propagating through the dispersion compensator 110 is transmitted through the input section 112a of the optical fibre 112 to the etalon 118. As the skilled person will know, an optical pulse comprises a number of different wavelength components. During the interaction of the pulse with the etalon 118, the distance each wavelength component travels before exiting the etalon 118 varies according to its wavelength. A wavelength dependent delay is thereby applied to each wavelength component of the optical pulse. The interaction of the optical pulse with the etalon therefore applies dispersion to the optical pulse. On leaving the etalon 118, the optical pulse is transmitted through the output section of the optical fibre 112.

A dispersion compensator 130 according to a sixteenth embodiment of the invention is shown in Fig. 28. In this embodiment the dispersion compensator 130 similarly comprises an optical fibre 132 in which two FBGs 134, 136 are provided in an etalon section 138 of the fibre 132, and are arranged to together in a spaced relationship to define an F-P etalon 140.

In this example one FBG 134 is a uniform period FBG and the other FBG 136 is a chirped FBG, which may have a linear chirp or a quadratic chirp. The chirped FBG 136 is coupled to tuning means 142 operable to alter the periodicity of the FBG 136. In this example the tuning means 142 takes the form of tuning apparatus operable to apply an axial strain to the FBG 136. By changing the periodicity of the FBG 136 the amount of dispersion which may be applied to a pulse by the etalon 140 can be changed.

To enable the dispersion compensator 130 to operate in reflection the compensator 130 further comprises an optical fibre coupler 144. A first port 144a on one side of the coupler 144 forms the input to the dispersion compensator 130 and the second port 144b on that side of the coupler 144 forms the output to the dispersion compensator 130 (as indicated by the arrows in Fig. 2). One port 144c on the other side of the coupler 144 is optically coupled to the fibre 132. The fourth port 144d is not used and the fibre end should be angled or index matched to prevent unwanted reflections.

A seventeenth embodiment of the invention, shown in Fig. 29., provides a dispersion compensator 150 which comprises an optical fibre 152 in which two chirped FBGs 154, 156 are provided in an etalon section 158 of the fibre 152, and are arranged together in a closely spaced relationship to define an F-P etalon 160. The chirped FBGs may both be linearly chirped, or may both be quadratically chirped or one may be linearly chirped and the other quadratically chirped. The dispersion compensator 150 also comprises a three port optical circulator

162 to enable the compensator 150 to operate in reflection. The first port

162a forms the input to the dispersion compensator 150, the second port

162b is optically coupled to the fibre 152, and the third port 162c forms the output of the dispersion compensator 150. As indicated by the arrow within the optical circulator 162 in Fig. 29, an optical pulse entering the circulator 162 through the first port 162a is transmitted to the second port 162b from where it propagates to and interacts with the etalon 160. Exiting the etalon 160, the reflected optical pulse re-enters the circulator 162 through the second port 162b and is transmitted to the third port 162c, from which it exits the dispersion compensator 150.

The dispersion compensators 110, 130, 150 of the ninth, tenth and eleventh embodiments each have a single F-P etalon structure 120, 140, 60. Due to their nature, the dispersion applied to an optical pulse by these F-P etalons 120, 140, 160 is not the same for each wavelength component of an optical pulse. That is to say, a dispersion compensator 110, 130, 150 based on a single F-P etalon 120, 140, 160 has a dispersion slope characteristic in addition to its dispersion characteristic. This is not a problem in optical transmission systems which operate at low (less than 110Gbit/s) optical pulse bit rates, since these transmission systems can tolerate the existence of dispersion slope across the optical bandwidth of the pulse. The average dispersion required can be set by a single F-P etalon dispersion compensator 110, 130, 150 because at low bit rates it doesn't matter if dispersion deviates a little across the spectral bandwidth of the compensator 110, 130, 150, and thus the pulse.

However, systems operating at higher bit rates require a dispersion compensator to display little or no dispersion slope across the optical bandwidth of a pulse. This can be achieved using the dispersion compensator 180 according to a eighteenth embodiment of the invention, as shown in Fig. 30.

The dispersion compensator 180 according to this embodiment comprises two etalons 190, 200 coupled together via a four port optical circulator 162, and is constructed as follows. In a first optical fibre 182 two FBGs, which in this example are chirped FBGs 184, 186, are provided within a first etalon section

188 and are arranged in a spaced relationship to together define a first F-P etalon 190. In a second optical fibre 192 two further chirped FBGs 194, 196 are provided within a second etalon section 198 and are arranged in a closely spaced relationship to together define a second F-P etalon 200.

The theoretical dispersion (D) profile of the first etalon 190 is shown in Fig. 31 and the theoretical dispersion (D) profile of the second etalon 200 is shown in Fig. 32. As can be seen from Figs. 31 and 32 the dispersion profiles of the first and second etalons 190, 200 can be different.

The FBGs 184, 186, 194, 196 of the first and second etalons 190, 200 may each have the same spectral profile, giving each etalon 190, 200 the same spectral profile. Alternatively, the FBGs 184, 186 in the first etalon 190 may have the same spectral profiles and the FBGs 194, 196 in the second etalon 200 may have the same spectral profiles as each other, but different spectral profiles to the FBGs 184, 186 in the first etalon 190, thereby giving the two etalons 190, 200 different spectral profiles. At least part of the spectral profile of each etalon 190, 200 must cover the same wavelength range, including the spectral bandwidth of an optical pulse to be compensated.

The total dispersion which may be applied to an optical pulse by the dispersion compensator 180 is equal to the sum of the dispersions applied to the pulse by each etalon 190, 200. Fig. 33 shows three different theoretical resultant group delay (ΔT) profiles, for three different wavelength offsets (A, B, C) between the spectral profiles of the first 190 and second 200 etalons. Fig. 34 shows three corresponding dispersion (D) profiles (A, B, C) calculated from the three group delay profiles of Fig. 33.

In order to give the dispersion compensator 180 zero dispersion slope, the first etalon 190 must have a dispersion slope characteristic of equal magnitude but opposite sense to the dispersion slope characteristic of the second etalon. The two dispersion slopes additively cancel each other out, therefore the dispersion compensator 180 applies the same amount of dispersion to each wavelength component of an optical pulse. By concatenating two etalons in this manner the total dispersion of the dispersion compensator 180 can be increased or modified.

The dispersion compensator 180 further comprises tuning means 164 to which the FBGs 184, 186 of the first etalon 190 are coupled. The tuning means comprises tuning apparatus operable to apply an axial compression to the FBGs 184, 186, to thereby change the periodicity of the FBGs 184, 186 and thus alter the amount of dispersion which may be applied to an optical pulse by the first etalon 190. The tuneability of the first etalon 190 allows the total dispersion which may be applied to an optical pulse to be altered.

The first port 162a of the circulator 162 forms the input to the dispersion compensator 180. The second port 162b of the circulator 162 is optically coupled to the first fibre 182. The third port 162c of the circulator 162 is optically coupled to the second fibre 192. The fourth port 162d of the circulator 162 forms the output of the dispersion compensator 180. As indicated by the arrow shown within the circulator 162 in Fig. 30, an optical pulse entering the dispersion compensator 180 through the first port 162a of the circulator 162 will be routed to the second port 162b and on to the first etalon 190. The reflected optical pulse re-enters the circulator through the second port 162b and is routed to the third port 162c, and on to the second etalon 200. The reflected optical pulse again re-enters the circulator through the third port 162c and is routed to the fourth port 162d through which it exits the dispersion compensator.

A dispersion compensator 210 according to a nineteenth embodiment of the invention is shown in Fig. 35. In this embodiment the dispersion compensator 210 comprises an FBG etalon 220 and a bulk-optic Gires-Tournois (G-T) etalon 226 optically coupled together via a four port optical circulator 228 to enable the dispersion compensator 210 to operate in reflection. The dispersion compensator 210 is constructed as follows.

In a first optical fibre 212 two chirped FBGs 214, 216 are provided within an etalon section 218. The FBGs 214, 216 are arranged in a closely spaced relationship to together define an F-P etalon 220. The two FBGs 214, 216 are coupled to tuning means 222, which in this example takes the form of a Peltier heating device operable to alter the temperature, and thus the periodicity, of the FBGs 214, 216. The first optical fibre 212 is optically coupled to the third port 228c of the circulator 228. The GT etalon 226 is optically coupled to the second port 228b of the circulator 228 via a second optical fibre 224 by means of one or more optical lenses (not shown). The first port 228a of the circulator forms the input to the dispersion compensator 210 and the fourth port 228d of the circulator forms the output of the dispersion compensator 210.

Because the GT etalon 226 is a bulk optic device it suffers from relatively high losses due to having to couple the optical signal out of the connecting fibre 224 to launch the optical signal into the GT etalon 226, and then having to couple the reflected optical signal back into the fibre 226. However, the presence of the GT etalon 226 enables particular tuning of the profile of the dispersion compensator 210 and of the amount of dispersion which can be applied to an optical pulse. In this way the dispersion profile of the dispersion compensator 210 can be optimised, and the amount of loss kept to a minimum.

As indicated by the arrow shown within the circulator 228 in Fig. 28, an optical pulse entering the dispersion compensator 110 through the first port 228a of the circulator 228 will be routed to the second port 228b and on to the GT etalon 226. The reflected optical pulse re-enters the circulator through the second port 228b and is routed to the third port 228c, and on to the F-P etalon 220. The reflected optical pulse again re-enters the circulator through the third port 228c and is routed to the fourth port 228d through which it exits the dispersion compensator 210.

Figure 36 shows a dispersion compensator 230 according to a twentieth embodiment of the present invention. The dispersion compensator 230 according to this embodiment is substantially the same as the dispersion compensator 210 according to the thirteenth embodiment, with the following modification. The G-T etalon 226 of the thirteenth embodiment is here replaced by a chirped FBG 232 which acts as fixed dispersion element. The dispersion which may be applied to an optical pulse by the dispersion compensator 230 is variable by means of the F-P etalon 220.

A twenty-first embodiment of the invention, shown in Fig. 37, provides a dispersion compensator 240 comprising an optical waveguide, in the form of a first planar optical waveguide 242 formed within a planar substrate 244, in which two optical waveguide gratings, chirped Bragg gratings 246, 248 in this example, are provided in an etalon section 250 of the waveguide 242, and are arranged to together define an F-P etalon 252. The F-P etalon 252 is formed by the two gratings 246, 248 and the first waveguide 242.

A second planar waveguide 254 forms the input to the dispersion compensator 240 and is optically coupled to the first waveguide 242 in a coupling region 256. A third planar waveguide 258 forms the output to the dispersion compensator 240. An optical pulse to be compensated propagates through the input waveguide 254 and is coupled into a first part 242a of the first waveguide 242 as it propagates through the coupling region 256. The optical pulse is then transmitted to and reflected by (as indicated by the arrows) the first grating 246. The reflected optical pulse propagates back to the coupling region 256 where it is coupled into the second part 242b of the first waveguide 242. The pulse is then transmitted to and reflected by (as indicated by the arrows) the second grating 248. The reflected pulse propagates back to the coupling region 256 where it is coupled into the output waveguide 258.

The dispersion compensators according to the described embodiments of the invention may be used within an optical transmission system to provide post- transmission dispersion compensation or pre-transmission dispersion compensation. Figure 38 (which does not form part of the invention) shows a transmission system 260 for providing post-transmission dispersion compensation. The system 260 comprises a transmission (TX) station 262, an optical fibre transmission link 264, a dispersion compensator 150 according to the eleventh aspect of the invention and a receive (RX) station 266. The dispersion compensator 150 imposes a dispersion on the optical pulses before they arrive at the receive station 266 to compensate for existing dispersion. Figure 39 (which does not form part of the invention) shows a transmission system 270 for providing pre-transmission dispersion compensation. The system 270 comprises a transmission (TX) station 272, an optical fibre transmission link 274, a dispersion compensator 180 according to the eigtheenth embodiment of the invention and a receive (RX) station 276. The dispersion compensator 180 imposes a dispersion on the optical pulses before they are transmitted across the fibre link 274 to impose a pre-chirp which is substantially equivalent to the amount of dispersion expected to be acquired by the pulses as they propagate through the fibre link 274.

A twenty-second embodiment of the invention provides a dispersion compensator comprising an optical fibre in which two identical chirped FBGs are provided within an etalon section of the fibre. In this example the FBGs each have a length of 8mm, a coupling coefficient of 0.54 and a chirp rate of 4nm/cm. The FBGs are arranged overlapping one another, with the centers of the gratings being offset by ~2mm, in order to form a Fabry-Perot (FP) etalon having a 50GHz channel spacing.

The theoretically calculated spectral profile, group delay profile and dispersion profile of the dispersion compensator are shown in Fig. 52 (a, b and c respectively). Similarly to a traditional bulk-optic FP etalon, both the group delay profile and the dispersion profile have periodic characteristics over the stop band of the etalon, with more than ten channels available. For each channel, the group delay is quadratic, thereby resulting in a linear variation in dispersion.

A dispersion compensator having more channels can be produced by using gratings of larger chirp rate and/or having a longer grating length. A dispersion compensator having an increased usable bandwidth and dispersion tuning range can be constructed by combining two or more FP etalons.

One such device is provided by the twenty-third embodiment of the invention, shown in Fig. 53. In this example the dispersion compensator 310 comprises first 312 and second 314 FP etalons, each according to one of the fifteenth, sixteenth, seventeenth or twenty-second embodiments of the invention, arranged optically in series. An optical isolator 316 provided between the FP etalons 312, 314 in order to prevent resonances being generated between the two FP etalons.

The first and second FP etalons 312, 314 are designed to have a positive dispersion slope and a negative dispersion slope respectively. The magnitudes of the dispersion slopes are equal (or near-equal) to each other so that there is a wavelength region within which the dispersion is constant. The dispersion value of the dispersion compensator 310 is determined by the relative spectral shift between the FP etalons 312, 314. This can be altered by either thermal or strain tuning one or both of the FP etalons 312, 314.

The dispersion (D) profiles (318, 320 respectively) of the first FP etalon 312 and the second FP etalon 314 are shown in Fig. 54. Fig. 55 shows the dispersion profile (a), group delay profile (b) and spectral profile in transmission (T) of the dispersion compensator 310 when the dispersion value of the dispersion compensator is set at Ops/nm.

Fig. 56 shows a dispersion compensator 330 according to a twenty-fourth embodiment of the invention. The dispersion compensator comprises first 332, second 334 and third 336 FBG FP etalons, each according to one of the fifteenth, sixteenth, seventeenth or twenty-second embodiments of the invention, arranged optically in series. The dispersion compensator 330 further comprises two optical isolators 338, 340 provided between the first and second FP etalons and the second and third FP etalons respectively.

A dispersion compensator 350 according to a twenty-fifth embodiment of the invention is shown in Fig. 57. The dispersion compensator 350 is substantially the same as the dispersion compensator 310 according to the twenty-third embodiment of the invention, with the following modification. The same reference numerals are retained for corresponding features.

In this embodiment the dispersion compensator 350 further comprises an optical amplifier, in the form of an Erbium doped fibre amplifier (EDFA) 352 provided after the second FP etalon 314. The EDFA 352 optically amplifies an optical signal transmitted by the two FP etalons 312, 314 and may be used to compensate for any insertion losses within the dispersion compensator

350.

Referring to Fig. 58, a twenty-sixth embodiment of the invention provides a dispersion compensator 360 comprising first 362 and second 364 FP etalons, each according to one of the fifteenth, sixteenth, seventeenth or twenty- second embodiments of the invention, and a DGTE 366 according to any one of the first to seventh embodiments of the invention. The FP etalons 362, 364 and the DGTE 366 are coupled together via a three port optical circulator 368. The first FP etalon is coupled to the first port of the circulator 368, the DGTE 366 is coupled to the second port of the circulator 368 and the second FP etalon is coupled to the third port of the circulator 368.

In use, an optical signal is transmitted by the first FP etalon 362, routed from the first to the second port of the circulator 368, reflected by the DGTE 366, routed from the second to the third port of the circulator 368 and transmitted by the second FP etalon 364.

A twenty-seventh embodiment of the invention provides a dispersion compensator 370 as shown in Fig. 59. The dispersion compensator 370 is substantially the same as the dispersion compensator 360 according to the previous embodiment, with the following modification. The same reference numerals are retained for corresponding figures.

The dispersion compensator 370 according to this embodiment further comprises an optical amplifier, in the form of an EDFA 372, provided after the second FP etalon 364.

A dispersion compensator 380 according to a twenty-eighth embodiment of the invention is shown in Fig.60. This dispersion compensator 380 is substantially the same as the dispersion compensator 360 according to the twenty-sixth embodiment, with the following modifications. The same reference numerals are retained for corresponding features. In this embodiment the dispersion compensator 380 further comprises a second DGTE 382. The three-port circulator 368 of Fig.58 is consequently replaced by a four-port optical circulator 384.

Fig.61 shows a dispersion compensator 390 according to a twenty-ninth embodiment of the invention. The dispersion compensator 390 comprises first 392 and second 394 FP etalons, each according to one of the fifteenth, sixteenth, seventeenth or twenty-second embodiments of the invention, and an optical reflector, which in this example takes the form of a bulk optic dielectric film mirror 396. The mirror 396 is provided after the FP etalons 392, 394, thereby providing for double pass operation of the FP etalons. The dispersion compensator 390 further comprises a three-port optical circulator 398 for routing input and output optical signals.

A dispersion compensator 400 according to a thirtieth embodiment of the invention is shown in Fig.62. This dispersion compensator is substantially the same as that of the previous embodiment, with the following modifications. In this embodiment the functions of the second FP etalon 394 and the mirror 396 of Fig.61 are performed by a DGTE 402.

A dispersion compensator 410 according to a thirty-first embodiment of the invention is shown in Fig.63. This dispersion compensator is substantially the same as that of the previous embodiment, with the following modifications. In this embodiment the dispersion compensator comprises a second FP etalon 412 and a second DGTE 414, with a consequent change from a three-port optical circulator to a four-port circulator 416

The described distributed Gires-Tournois etalons and dispersion compensators provide various advantages. Since the described DGTEs are all-in-fibre devices, they are fully compatible with optical fibre based systems, such as telecommunications systems, and their insertion loss is much lower than bulk-optic GTE devices. In addition, the insertion loss can potentially be reduced to be less than that of many previously reported dispersion compensation devices. It is anticipated that the insertion loss can be controlled to be 3dB or less by optimizing the designs of the described dispersion compensators.

Due to their all-fibre nature, both the DGTEs and the dispersion compensators described herein can be made to be very compact. The shortness of the gratings used also contributes to their compactness. The DGTEs described are therefore a highly manufacturable and low cost alternative to bulk-optic GTEs and to many known dispersion compensation devices. The shortness of the gratings within the DGTEs also gives the advantage that the DGTEs display low delay ripple. The devices can be manufactured very simply, with the gratings being fabricated at the same time or sequentially. Because the gratings are short they can be fabricated with a high degree of control. The DGTE based dispersion compensators described avoid many of the manufacturing problems which plague devices based on long grating structures, such as phase errors within the structure of the gratings and difficulties in packaging the gratings. The DGTEs and the dispersion compensators are also easily tuneable and have excellent optical characteristics.

The use of chirped gratings allows the reflection profile of a dispersion compensator to be broadened, thereby extending the number of optical channels which can be compensated for. As the chirp is increased, the total bandwidth of the dispersion compensator is correspondingly increased. The bandwidth of each optical channel is defined by the free spectral range of the DGTE(s) within the dispersion compensator, which is determined by the separation distance between the gratings forming each DGTE. The bandwidth of an optical channel can therefore be changed by suitably altering the grating separation.

The described dispersion compensators may be used to compensate a single optical channel or multiple optical channels simultaneously, since their dispersion profiles repeat periodically (the period defining the channel spacing). The described dispersion compensators may be used to compensate one channel, for example, just before the receiver within a transmission system, or they can be used to compensate multiple channels, for example to apply a pre-chirp or to provide dispersion compensation at an intermediate location within a transmission system. In the case of multiple channels, if the gratings are uniformly chirped, then all of the channels experience the same dispersion. By offsetting some of the channels, by changing the DGTE's cavity length for those channels, a dispersion compensator may also have a dispersion slope, resulting in each channel being located at a different position on the dispersion compensator's dispersion curve. The dispersion compensators can therefore perform dispersion compensation and dispersion slope compensation simultaneously. The dispersion compensators may be operated as per-channel dispersion compensators or as band dispersion compensators.

The skilled person will appreciate that the described dispersion compensators may also be used as dispersion controllers, for example to perform optical pulse compression.

The dispersion compensators described provide the advantage that a single type of dispersion compensator can be used for more than one purpose, at different locations within an optical transmission system, such as telecommunications systems. This means that the number of different components in a system, and thus the number of different component spares which must be stocked, can be kept to a minimum.

The dispersion compensators may include tuning means, providing the advantage that if more dispersion is required, for example because the transmission system properties have changed, the wavelength of the dispersion compensator can be changed to move up or down its dispersion profile. The tuneability of the dispersion compensators may also prove advantageous within metro telecommunications systems where the dispersion compensators may be used as 'set and leave' dispersion elements. Various modifications may be made without departing from the scope of the present invention. As the skilled person will appreciate, each of the optical fibre based embodiments may alternatively be constructed in planar optical waveguides, and vice versa. In addition, a different type of optical fibre may be used to that described.

Where the use of chirped gratings is described, the skilled person will appreciate that other types of complex gratings, such as phase-shifted gratings or sampled gratings, may be used in their place. This includes the use of different types of complex gratings within a single DGTE.

Referring to the DGTE formed using a single complex grating structure, it will be understood that the complex grating structure may be equivalent to a different number, type and/or arrangement of gratings. For example, the grating structure may be equivalent to 3, 4 or more overlapped gratings, the number of gratings being dependent upon the complexity of the grating structure. Indeed, any of the described DGTEs may alternatively be fabricated in this way as a single complex grating structure, using an appropriate phase mask design.

Regarding the dispersion slope compensating DGTE, the first (weakly reflective) grating may have a different reflectivity profile to that described.

In the dispersion compensators which utilise two or more DGTEs, the DGTEs may be provided in a different arrangement to that described and may be coupled together using different optical connections to those described. The skilled person will understand that the dispersion slope profile of a dispersion compensator comprises the combined dispersion slope profiles of its component DGTEs. The achievement of zero dispersion slope for a dispersion compensator comprising two DGTEs is described above. Where the dispersion compensator comprises three or more DGTEs a desired dispersion slope profile can be produced from a wider variety of component DGTE dispersion slope profiles. For example, a three DGTE dispersion compensator requiring zero dispersion slope may be constructed from a first DGTE having a positive dispersion slope and two further DGTEs each having negative dispersion slopes, the combination of the negative dispersion slopes being equal and opposite to the positive dispersion slope of the first DGTE.

The gratings within the F-P etalons may be chirped or uniform-period depending on the optical bandwidth across which the dispersion compensator is required to work. The combination of gratings within an F-P etalon may be different to those described. The dispersion profiles of the etalons may different to those described. When an F-P etalon comprises two chirped gratings, the gratings may have the same or different rates of chirp and may be chirped in the same or opposite directions. As the skilled person will appreciate, where an optical coupler is shown this may be replaced by an optical circulator, and vice versa.

The tuning apparatus described may be replaced by an electro-optic or magneto-optic based tuning means operable to alter the effective refractive index of the material surrounding a grating.

The length, strength, chirp rate and wavelength off-set of the gratings within the DGTEs and FP etalons can all be varied to control the dispersion properties, bandwidth and free spectral range of the etalons and the dispersion compensators constructed from them.

The described FP etalon based dispersion compensators can be fabricated as all-in-fibre devices, and are therefore an attractive alternative for use as dispersion compensation and control elements in optical fibre communication systems. Tunable dispersion compensators based on such devices have highly desirable features including multi-channel operation, very low group delay ripple (compared with conventional chirped FBGs), low insertion loss (compared with conventional bulk-optic FP etalons) and low cost. The number of optical channels can be changed by altering the chirp and/or length of the constituent gratings.

Claims

1. A Gires-Tournois etalon comprising: an optical waveguide including a grating section in which an optical waveguide grating is provided; and an optical reflector optically coupled to the optical waveguide, the optical waveguide grating and the optical reflector being arranged to together define an etalon cavity.

2. An etalon as claimed in claim 1 , wherein the optical reflector comprises a second optical waveguide grating provided within a second grating section of the optical waveguide, at least part of the spectral profile of each grating including the same wavelength range, the optical waveguide grating being a weakly reflective optical waveguide grating and the second optical waveguide grating being a strongly reflective optical waveguide grating.

3. An etalon as claimed in claim 2, wherein the Gires-Tournois etalon further comprises one or more additional optical waveguide gratings of intermediate reflective strength, giving a total of n optical waveguide gratings, at least part of the spectral profile of each additional grating including the same wavelength range as the spectral profiles of the weakly reflective grating and the strongly reflective grating, and the gratings being arranged to together define n-1 etalon cavities.

4. An etalon as claimed in claims 2 or 3, wherein the optical waveguide gratings are arranged in a spaced relationship with one another or are arranged abutting one another, the optical waveguide gratings being uniform period Bragg gratings, chirped Bragg gratings, phase-shifted Bragg gratings or sampled Bragg gratings.

5. An etalon as claimed in claims 2 or 3, wherein the optical waveguide gratings are arranged such that they at least in part overlap one another, the optical waveguide gratings being chirped Bragg gratings, phase-shifted Bragg gratings or sampled Bragg gratings.

6. An etalon as claimed in any of claims 2 to 5, wherein the optical waveguide gratings are the same type of Bragg grating or are different types of Bragg grating.

7. An etalon as claimed in any of claims 2 to 6, wherein where two or more gratings are chirped Bragg gratings the gratings may be chirped in the same direction or in different directions and may have the same rate of chirp or different rates of chirp, the chirp being a linear chirp or a non-linear chirp, such as a quadratic chirp.

8. An etalon as claimed in claim 1 , wherein the optical reflector comprises a bulk-optic mirror, a cleaved end face of the optical waveguide, or a photo- induced reflection point provided within the optical waveguide and the optical waveguide grating is a Bragg grating.

9. An etalon as claimed in any preceding claim, wherein the Gires- Tournois etalon further comprises one or more tuning means to which one or more of the optical waveguide gratings are coupled, the tuning means being operable to alter the periodicity of a grating, thereby changing the dispersion characteristics of the etalon.

10. An etalon as claimed in any preceding claim, wherein the optical waveguide is an optical fibre or a planar optical waveguide.

11. A Gires-Tournois etalon comprising: an optical waveguide including an etalon section in which an optical waveguide grating structure is provided, the optical waveguide grating structure having a periodic refractive index variation equivalent to two or more at least partially overlapped complex Bragg gratings, a first one of the complex Bragg gratings being weakly reflective and a second one of the complex Bragg gratings being strongly reflective.

12. An etalon as claimed in claim 11 , wherein the complex Bragg gratings may each be the same type of Bragg grating, or may be different types of Bragg grating, such as chirped Bragg gratings, phase-shifted Bragg gratings or sampled Bragg gratings.

13. An etalon as claimed in claims 11 or 12, wherein where two or more of the complex Bragg gratings are chirped Bragg gratings they may be chirped in the same direction or in different directions, and they may have the same rate of chirp or different rates of chirp.

14. An etalon as claimed in any of claims 11 to 13, wherein the optical waveguide may be an optical fibre or may be a planar optical waveguide.

15. A dispersion compensator comprising: a first Gires-Tournois etalon as claimed in any of the preceding claims, wherein an optical pulse propagating through the waveguide interacts with the etalon, the distance each wavelength component of the pulse travels before exiting the etalon varying with wavelength, such that a wavelength dependent delay is applied to each wavelength component, thereby applying dispersion to the pulse.

16. A dispersion compensator as claimed in claim 15, wherein the dispersion compensator further comprises a second Gires-Tournois etalon as claimed in any of claims 1 to 14 optically coupled to the first Gires-Tournois etalon, the first and second etalons being optically coupled together via an optical waveguide coupler or an optical waveguide circulator.

17. A dispersion compensator as claimed in claim 16, wherein the first and second Gires-Tournois etalons apply the same or different dispersions, and apply dispersion slopes of substantially equal magnitude and opposite sign, to an optical pulse propagating through the dispersion compensator, the first and second etalons thereby together applying a non-zero dispersion and a substantially zero dispersion slope to the pulse.

18. A dispersion compensator as claimed in any of claims 15 to 17, wherein the dispersion compensator further comprises one or more additional Gires-Tournois etalons as claimed in claims 1 to 10 or 11 to 14, the etalons being optically coupled together via one or more optical waveguide couplers or optical waveguide circulators.

19. A dispersion slope compensator comprising: a Gires-Tournois etalon as claimed in claims 1 to 10 the reflectivity of the weak optical waveguide grating varying across the operational bandwidth of the Gires-Tournois etalon, thereby giving the etalon a dispersion profile across which the dispersion slope varies as function of wavelength.

20. A dispersion slope compensator as claimed in claim 19, wherein the reflectivity of the weak optical waveguide grating varies linearly or non-linearly as a function of wavelength.

21. A dispersion compensator comprising: a first optical waveguide including a first etalon section in which first and second optical waveguide gratings are provided, at least part of the spectral profile of each grating including the same wavelength range, and the gratings being arranged to together define a first etalon, wherein an optical pulse propagating through the waveguide interacts with the etalon, the distance each wavelength component of the pulse travels before exiting the etalon varying with wavelength, such that a wavelength dependent delay is applied to each wavelength component, thereby applying dispersion to the pulse.

22. A dispersion compensator as claimed in claim 21 , wherein the dispersion compensator further comprises an additional optical waveguide including an additional etalon section in which first and second additional optical waveguide gratings are provided, at least part of the spectral profile of each grating including the same wavelength range, and the gratings being arranged to together define an additional etalon, wherein the optical pulse propagating through the waveguide interacts with the additional etalon, the distance each wavelength component of the pulse travels before exiting the additional etalon varying with wavelength, such that a wavelength dependent delay is applied to each wavelength component, thereby applying dispersion to the pulse.

23. A dispersion compensator as claimed in claim 22, wherein the first etalon and the additional etalon apply dispersion slopes of substantially equal magnitude and opposite sign to the pulse, the etalons thereby together applying a non-zero dispersion and a substantially zero dispersion slope to the pulse.

24. A dispersion compensator as claimed in claims 22 or 23, wherein the dispersion compensator comprises a plurality of additional etalons, the etalons being arranged in optical series with one another.

25. A dispersion compensator as claimed in any of claims 21 to 24, wherein the dispersion compensator comprises an optical reflector provided after the etalon or etalons, thereby providing for double pass operation of the etalon or etalons.

26. A dispersion compensator as claimed in claim 25, wherein the optical reflector comprises a bulk optic optical reflector, such as a bulk optic mirror, or an optical waveguide based optical reflector, such as an optical waveguide grating or a Gires-Tournois etalon as claimed in any of claims 1 to 14.

27. A dispersion compensator as claimed in any of claims 21 to 26, wherein the dispersion compensator comprises one or more Gires-Tournois etalons as claimed in any of claims 1 to 14.

28. A dispersion slope compensator comprising: a first Gires-Tournois etalon, as claimed in any of claims 1 to 14, having a first group delay amplitude and a first free spectral range; and a second Gires-Tournois etalon, as claimed in any of claims 1 to 14, having a second, smaller group delay amplitude and a second free spectral range, the first and second free spectral ranges being different.

29. A dispersion slope compensator as claimed in claim 28, wherein the first free spectral range is larger than the second free spectral range, the dispersion slope compensator thereby having a negative dispersion slope.

30. A dispersion slope compensator as claimed in claim 28, wherein the first free spectral range is smaller than the second free spectral range, the dispersion slope compensator thereby having a positive dispersion slope.