GB2387478A

GB2387478A - Raman optical amplifier

Info

Publication number: GB2387478A
Application number: GB0208487A
Authority: GB
Inventors: Tim Ellingham; Liam Gleeson
Original assignee: Marconi Communications Ltd; Marconi Co Ltd
Current assignee: Marconi Communications Ltd; BAE Systems Electronics Ltd
Priority date: 2002-04-12
Filing date: 2002-04-12
Publication date: 2003-10-15
Also published as: GB0208487D0

Abstract

A Raman optical amplifier 114 has at least one pump laser 113 for outputting a pump spectrum and, optically connected to the pump laser, a non-liner optical fibre 111 for broadening the pump spectrum to increase linearity of gain of the amplifier with respect to wavelength. The increase in linearity means that, where more than one pump laser is used to pump the optical amplifier, fewer pump lasers are required for a given gain linearity. The nonlinear optical fibre may be, for example, a dispersion flattened fibre or a "holey" fibre having longitudinal bores in cladding of the fibre. The amplification may occur in the non-liner optical fibre 111 or in a linear optical transmission fibre 115 to which the non-linear fibre is optically connected.

Description

RADIAN OPTICAL AMPLIFIER

This invention relates to a Raman optical amplifier.

Optical amplifiers are often necessary for optical fibre communication systems. Erbium doped fibre amplifiers (EDFA) are commonly used because they provide efficient optical 5 amplification at wavelengths in the region of 1550 nm, which is within a standard wavelength window for optical fibre communications.

Raman optical amplifiers potentially offer advantages over EDFAs, providing, for example, lower noise, large gain over a wider bandwidth, and simpler construction than EDFAs, including the use of standard silica optical fibre rather than optical fibre doped with 10 erbium. However, although Raman amplifiers provide a broad gain spectrum, the gain is unequal over the wavelength spectrum. Therefore, to use Raman amplifiers in wavelength division multiplex (WDM) optical systems, where amplification is required over all the wavelengths used in the WDM system, several pump lasers operating at different wavelengths are used simultaneously, so that the gain spectra for each pump wavelength overlap to 15 produce a relatively flat gain spectrum. However, the requirement for multiple pump lasers makes the optical amplifier bulky and may make the use of Raman optical amplifiers uneconomic. It is an object of the invention at least to mitigate the foregoing disadvantages.

According to the first aspect of the invention, there is provided a Raman optical 20 amplifier for producing gain in an optical signal, the Raman optical amplifier comprising a pump laser for outputting a pump spectrum and, optically connected to the pump laser, optical fibre means having substantial non-linear optical characteristics for broadening the pump spectrum, such that the broadening of the pump spectrum increases linearity of the gain with respect to wavelength.

25 Conveniently, the optical fibre means having non-linear characteristics is a dispersion flattened optical fibre with an anomalous dispersion of not more than substantially 0. lps/nm/km for promoting four- wave mixing of the pump spectrum therein thereby broadening the pump spectrum.

Alternatively, the optical fibre means having non-linear characteristics is an optical fibre 30 comprising longitudinal bores for confining optical transmission to a solid core thereby broadening the pump spectrum.

Advantageously, a transmission optical fibre having substantially linear optical characteristics is optically connected to the optical fibre means having non-linear characteristics, such that Raman amplification occurs in at least a portion of the transmission optical fibre.

5 Preferably, insertion means are provided for inserting the broadened pump spectrum into the transmission optical fibre for amplifying an optical signal therein to produce an amplified optical signal such that the amplified optical signal may be output from the transmission optical fibre without passing through the optical fibre means having non-linear characteristics. 10 Conveniently, the insertion means is a band splitter located between the optical fibre means having non-linear characteristics and the transmission optical fibre and between the transmission optical fibre and an output of the amplified signal therefrom.

Advantageously, the Raman optical amplifier comprises at least one pump laser for, or each for, outputting a pump spectrum centred on a respective pump wavelength and optical 15 fibre means having non-linear characteristics for broadening the respective pump spectrum or I spectra, such that the broadening of the pump spectrum or spectra decreases the number of a i plurality of pump lasers otherwise required to produce a predetermined linearity of the gain with respect to wavelength.

Preferably, the increase in the linearity of the gain comprises a decrease in gain ripple of 20 at least 50% over a bandwidth spanning from 1 525nm to 1625nm, or part thereof.

According to a second aspect of this invention, there is provided a Raman optical I amplifier for producing gain in an optical signal, the Raman optical amplifier comprising at least one pump laser for outputting a spectrum centred on a pump wavelength, or where there I is more than one pump laser, centred on a respective pump wavelength, and, optically 25 connected to the at least one pump laser, optical fibre means having non-linear optical characteristics for broadening the pump spectrum or spectra, such that the broadening of the i pump spectrum or spectra decreases the number of a plurality of pump lasers otherwise required to produce a predetermined linearity of the gain with respect to wavelength.

Conveniently, the optical fibre means having non-linear characteristics is a zero 30 dispersion optical fibre for promoting four-wave mixing of the pump spectrum therein for broadening the pump spectrum or spectra thereby broadening the pump spectrum.

Alternatively, the optical fibre means having non-linear characteristics is an optical fibre comprising longitudinal bores for confining optical transmission to a solid core thereby broadening the pump spectrum or spectra.

Advantageously, a transmission optical fibre having substantially linear optical 5 characteristics is optically connected to the optical fibre means having non-linear characteristics, such that Raman amplification occurs in at least a portion of the transmission optical fibre.

Preferably, insertion means are provided for inserting the broadened pump spectrum into the transmission optical fibre for amplifying an optical signal therein to produce an 10 amplified optical signal such that the amplified optical signal may be output from the transmission optical fibre without passing through the optical fibre means having non-linear characteristics. Conveniently, the insertion means is a band splitter located between the optical fibre means having non-linear characteristics and the transmission optical fibre and between the 15 transmission optical fibre and an output of the amplified signal therefrom.

Preferably, the predetermined linearity of the gain comprises a gain ripple of not more than 0.25 dB over a bandwidth spanning from 1 525nm to 1 625nm, or part thereof.

According to a third aspect of the invention there is provided a Raman optical amplifier for producing gain in an optical signal, the Raman optical amplifier comprising at least one 20 pump laser for outputting a pump spectrum centred on a pump wavelength, or where there is more than one pump laser, centred on a respective pump wavelength, and, optically connected to the at least one pump laser, optical fibre means having nonlinear optical characteristics for broadening the pump spectrum or spectra, such that the broadening of the pump spectrum or spectra increases linearity of the gain with respect to wavelength and decreases the number of 25 a plurality of pump lasers otherwise required to produce a predetermined linearity of the gain with respect to wavelength.

The invention will now be described by way of example with reference to the accompanying drawings in which: FIG. 1 shows graphically an output spectrum of a pump laser used in a known Raman 30 optical amplifier;

FIG.2 shows graphically a gain spectrum of the Raman optical amplifier of FIG. 1; FIG. 3 shows graphically a gam spectrum of the Raman optical amplifier using the pump of FIG.1 as a first pump laser and using a second pump laser having a pump spectrum offset from that of the first pump; 5 FIG.4 shows a dispersion spectrum of a dispersion flattened fibre suitable for use in the] invention; FIG.5 shows a micrograph of a transverse cross section of an optical fibre suitable for I use in the invention; FIG.6 shows graphically an effective pump laser spectrum of a Raman optical amplifier 10 according to the present invention; FIG. 7 shows graphically a gain spectrum of the Raman optical amplifier using the effective pump laser spectrum of FIG.6; FIG.8 shows graphically a gain spectrum of a Raman optical amplifier using the pump of FIG. 6 as a first pump laser and using a second pump laser having an effective pump 15 spectrum offset from that of the first pump; FIG. 9 shows a schematic diagram of a first embodiment of an optical amplifier according to the invention;] FIG. 10 shows a schematic diagram of a second embodiment of an optical amplifier according to the invention; 20 FIG. 11 shows a schematic diagram of a third embodiment of an optical amplifier according to the invention.

In the figures like reference numerals represent like parts.

It is known that when incident light of sufficient intensity is passed through an optical fibre some energy is converted to optical phonons such that the resultant Raman scattered 25 light has a lower energy and therefore a lower frequency, the Stokes frequency, and thus a longer wavelength, than the incident light. Similarly, Raman scattering also occurs when an incident photon and an optical phonon of the same momentum are simultaneously annihilated to form a scattered photon with a higher energy. The scattered photon is up-shifted in frequency to the antiStokes frequency. The frequency shift is known as the Stokes shift and

- the magnitude of the Stokes shift depends on the frequencies of the optical phonon modes supported by the host matenal.

It is also known that Raman scattering may be used to amplify an optical signal if the signal propagates simultaneously with a high-power pump beam and the signal falls within 5 the Raman gain spectrum for that pump frequency. The pump beam is in-elastically scattered to produce photons at the Stokes frequency that is the frequency of the optical signal to be amplified. Since the signal and the pump beams propagate together through the fibre, the signal stimulates emission of Stokes photons at the same phase and frequency as itself. Silica glass fibres support a wide range of optical phonon frequencies allowing a very wide Raman 10 gain bandwidth.

Referring to FIG. 1, an output spectrum 10 of a pump laser for a known Raman amplifier is shown having an ordinate of power and an abscissa of wavelength, in which the spectrum 10 has a peak power 11 at a wavelength of approximately 1455 nm. The corresponding gain spectrum 20 of the optical amplifier is shown in the graphical 15 representation of FIG. 27 having an ordinate of gain and an abscissa of wavelength, in which it can be seen that even over a region of interest 21 between 1535 nm and 1565 rim the gain is neither constant nor linearly related to wavelength. The peak gain 22 is offset from the peak 11 of the pump spectrum by approximately 100 nm. In order to obtain a flatter gain spectrum it is known to use a number of pump wavelengths so that their individual gains overlap to 20 produce a flatter gain spectrum. Broadband distributed Raman amplification using laser diode pumping at up to 13 different pump wavelengths is known and results in improved gain flatness over wavelength ranges of up to 100 nm. Such a modelled gain spectrum 30, using two pumps, is shown in FIG. 3, again having an ordinate of gain and an abscissa of wavelength. A first pump, centred on a wavelength of 1426 nm, has a first gain spectrum 25 shown by dotted line 32 and a second pump, centred on a wavelength of 1453 nm, has a second gain spectrum shown by broken line 20. Measured combined gain values are shown by the plotted points 33 in FIG. 3, which closely follow the modelled gain spectrum shown by the solid line 30. In order to achieve a required gain spectrum, the first and second pumps are not driven equally, as witnessed by the different gain spectra 20, 32 of the two pumps shown 30 by the respective broken and dotted lines in FIG. 3. The two pumps offset by approximately 27 nm give a minimum combined gain of 10 dB with an optimised gain ripple of approximately 0.75 dB over a 35 rim window between 1529 rim and 1564 nm.

s

It is further known that when light of a first wavelength travels in a similar phase as light of a similar second wavelength for a relatively long time new signals are generated at the same frequency spacing as the original. This is known as four-wave mixing. The effect may also be caused by modulation instability which is a special case of four-wave mixing. The 5 closer the channels, the more power, and the longer the two wavelengths travel in phase, the more mixing occurs. This has the affect of broadening the spectrum of a light beam passed through an optical fibre in which there is little dispersion. Dispersion flattened fibres i. e. timbres having substantially zero dispersion over a significantly large range of wavelengths are known, in which this effect is therefore maximised. A plot 40 of an ordinate of dispersion 10 against an abscissa of wavelength for such a zero dispersion fibre is shown in FIG. 4.

Preferably the fibre is highly non-linear with a dispersion value between O and 0.1 ps.nm/km over at least a 30 rim bandwidth centred, for example, on 1450 nm.

Alternatively, spectrum broadening may be obtained using known "holey" fibres having a pattern of longitudinal bores for confining optical transmission to a solid core through the 15 fibre. As shown in a scanning electron micrograph of FIG. 5, the cladding ofthe fibre consists of a regular lattice of air-filled bores 51 and a core 52 is formed by the omission of a single bore at the centre of the fibre. The highly nonlinear characteristics of such an optical fibre broaden the spectrum of a light beam passed through the fibre, without the requirement for dispersion flattening. Such optical fibres are available from Crystal Fibre Blokken, 84 DK 20 3460, Birkerod, Denmark.

It will be understood that any other suitable optical fibre having substantial non-linear optical characteristics, which also causes sufficient broadening of the pump spectrum to produce a useful improvement in the gain linearity, may be used. By a fibre having substantial non-linear optical characteristics is to be understood an optical fibre having a non 25 linearity nJAeff 2 5 x 10- An, where n2 is a non-linear co-efficient of the fibre and Aeff iS an effective cross- sectional area of the fibre.

Such a broadened pump spectrum 60, produced using four-wave mixing or by passing the pump light through a "holey" optical timbre, is shown in the graph of FIG. 6, having an ordinate of power and an abscissa of wavelength, over the same wavelength range of 1434 nm I 30 to 1476 rim as shown in the un-broadened spectrum 10 in FIG. 1. Using such a broadened pump spectrum a flatter and more linear gain spectrum 70 is obtained, as illustrated in FIG. 7,; than the gain spectrum 20, illustrated in FIG. 2, obtained with the un-broadened pump spectrum 10. With the pump spectra broadening the peak pump power is reduced for a similar

pump input, so more pump power is required to produce the same gain and, at least in the case of four-wave mixing, the higher power leads to further spectrum broadening. This extra power may be obtained by using fibre lasers, which are capable of much higher outputs than semiconductor lasers. Although fibre lasers are more expensive than semiconductor lasers, 5 this cost differential can be offset against a larger number of semiconductor pump lasers required in known optical amplifiers.

The results of combining a first broadened pump specka offset from a second broadened pump spectrum is shown in FIG. 8. The total modelled gain spectrum 80, shown in solid line, is obtained using two pumps, a first pump, centred on a wavelength of 1426 nm, with a first 10 gain spectrum 81, shown in dotted line, and a second pump, centred on a wavelength of 1453 nm, with a second gain spectrum 70, shown in broken line. In order to achieve a required gain spectrum, the first and second pumps are not driven equally, as witnessed by the different gain spectra 70, 81 of the two pumps shown by the respective broken and dotted lines in FIG. 8. The two pumps offset by approximately 27 rim give a minimum combined 15 gain of 10 dB with an optimised gain ripple of approximately 0.15 dB over a 35 nm window between 1529 rim and 1564 nm. This is a fivefold improvement in the gain ripple of the prior art amplifier the gain spectrum of which is shown in FIG. 3.

In use, in a first embodiment of the invention, shown in FIG. 9, a zero dispersion flattened fibre 91 links a known EDFA 92 by means of a band splitter or wavelength division 20 multiplexer 96 to a Raman amplifier 94 and to an output signal line 97. In use, signals are amplified by the EDFA 92 and transmitted through the DFF 91. The pump laser 93 and the DFF 91 together form a dispersed Raman optical amplifier 94, that is, the signal transmitted from the EDFA 92 is amplified by the Raman amplifier 94 along a portion' for example, one half, of the DFF 91 nearest the Raman amplifier pump laser 93. Moreover, the highest 25 amplification occurs nearest to the pump laser 93, where a pump beam from the pump laser 93 is strongest, and where the signal is weakest Preferably a plurality of pump lasers is used, each pump laser having an output spectrum centred on a different wavelength.

As the pump beams pass through the DFF 91, 4-wave mixing results in broadening of 30 the spectra of the pump beams from each of the pump lasers 93. This 4-wave mixing is enhanced by the use of high power pump lasers. As a result of this spectrum broadening, fewer pump laser wavelengths, and hence fewer pump lasers, are required than in the prior art.

It will be understood that in this embodiment, where a DFF is used as the only signal transmission media, a signal wavelength is preferably used which is outside of the zero dispersion region of the DFF, to avoid 4wave mixing of the signal, while retaining 4-wave t mixing of the pump spectra.

5 In a second embodiment of the invention, shown in FIG. 10, an EDFA 102 transmits a signal into a known transmission optical fibre 105, such as a single mode fibre (SMF) or a large effective area fibre (LEAF). The transmission fibre is joined by a splice 109 to a DFF 101 pumped through a first arm of a band splitter or wavelength division multiplexer 106 by one or more Raman optical amplifier pump laser(s) 104. A second arm of the band splitter 10 106 forms an output signal line 107. In use, the DFF 101 and at least a portion of the transmission fibre 105 together form a dispersed Raman optical amplifier 104, in a manner to be described.

Preferably a plurality of pump lasers is used for the Ramon amplifier 104, each pump i laser having an output spectrum centred on a different wavelength.

15 In use, an optical signal amplified by the EDFA 102 is launched into the transmission I optical fibre 105 from which the signal is transmitted into the DFF 101. In the DFF 101, the spectra of pump beams from each of the Raman pump lasers are broadened by 4-wave mixing to provide broadened pump spectra to produce a gain spectrum covering all the signal wavelengths. The signal transmitted from the EDFA 102 is then amplified by the broadened 20 pump spectra in a portion of the transmission fibre 105 nearest the pump lasers 103.

Preferably the DFF 101 is located, for example, in a terminal box together with the pump lasers 103.

In a third embodiment of the invention, shown in FIG. 11, an EDFA 112 is connected to a transmission optical fibre 115 at a transmitter end 1151 of the optical fibre 115. At an 25 opposed, receiver end 1152, the optical fibre 115 is connected to a first side of a passive band splitter 116. On a second side of the band splitter 116, opposed to the first side, a first arm is connected to a second transmission optical fibre 117 leading to an input of a second EDFA 118 and a second arm is connected by a third optical fibre 119 to a DFF span 111 and thence to one or more Raman optical amplifier pump lasers 113. i 30 Preferably a plurality of pump lasers is used, each pump laser having an output spectrum centred on a different wavelength.

In use, the pump lasers 113, the DFF span 111 and at least a portion of the transmission fibre 115 together form a dispersed Raman optical amplifier 114 in a manner to be described.

In order to provide sufficient power for Raman amplification, the pump lasers may be fibre lasers. 5 In use, an optical signal pre-amplified by the first EDFA 112 is transmitted through the transmission optical fibre 115 towards the band splitter 116. Pump beams from the pump lasers 113 are passed through the DFF 111 and spectra of the pump beams are broadened in the DFF 111 by 4-wave mixing, resulting in a broad pump beam from the overlapping spectra passing through the band splitter 116 and into the transmission optical fibre 115 where they 10 cause Raman amplification of the optical signal along a portion of the transmission optical fibre 115 closest to the band splitter 116, to form an amplified optical signal. The amplified optical signal is passed by the band splitter 116 to the second transmission optical fibre 117 and transmitted by that fibre 117 to the second EDFA 118 for further amplification and output. 15 The third embodiment has the advantage that the optical signal is not passed through the DFF 111 and is therefore not subject to distortion in the DFF.

Typically, the pump wavelengths may be in the range 1400-1500 nm and the optical signals in the wavelength range 1500-1600 nm, that is the pump frequencies are separated from the signal frequencies by the Stokes frequency shift of the transmission fibre material.

20 The DFF loop 111 may be replaced by a "holey" optical fibre, as described above, so that the spectra of the pump laser beams are broadened by the "holey" optical fibre rather than by the DFF loop 111.

Although embodiments of the invention have been described in which optical signal transmission is counter-directional to the pump beams, it will be understood that the invention 25 has equal application where the optical signals and pump beams are co-directional.

Claims

1. A Raman optical amplifier for producing gain in an optical signal, the Raman optical amplifier comprising a pump laser for outputting a pump spectrum and, optically connected to the pump laser, optical fibre means having substantial non-linear optical characteristics for 5 broadening the pump spectrum, such that the broadening of the pump spectrum increases linearity of the gain with respect to wavelength.

2. A Raman optical amplifier as claimed in claim 1, wherein the optical fibre means having non-linear characteristics is a dispersion flattened optical fibre with an anomalous dispersion of not more than substantially 0.1 pa/run/km for promoting four-wave mixing of 10 the pump spectrum therein thereby broadening the pump spectrum.

3. A Raman optical amplifier as claimed in claim 1, wherein the optical fibre means having non-linear characteristics is an optical fibre comprising longitudinal bores for confining optical transmission to a solid core thereby broadening the pump spectrum.

4. A Raman optical amplifier as claimed in any of claims 1 to 3, wherein a transmission 15 optical fibre having substantially linear optical characteristics is optically connected to the optical fibre means having non-linear characteristics, such that Raman amplification occurs in at least a portion of the transmission optical fibre.

5. A Raman optical amplifier as claimed in claim 4, wherein insertion means are provided for inserting the broadened pump spectrum into the transmission optical fibre for amplifying 20 an optical signal therein to produce an amplified optical signal such that the amplified optical signal may be output from the transmission optical fibre without passing through the optical fibre means having non-linear characteristics.

6. A Raman optical amplifier as claimed in claim 5, wherein the insertion means is a band splitter located between the optical fibre means having non-linear characteristics and the 25 transmission optical fibre and between the transmission optical fibre and an output of the amplified signal therefrom.

7. A Raman optical amplifier as claimed in any of the preceding claims, comprising at least one pump laser for, or each for, outputting a pump spectrum centred on a respective pump wavelength and optical fibre means having non-linear characteristics for broadening the 30 respective pump spectrum or spectra, such that the broadening of the pump spectrum or

spectra decreases the number of a plurality of pump lasers otherwise required to produce a predetermined linearity of the gain with respect to wavelength.

8. A Raman optical amplifier as claimed in any of the preceding claims, wherein the increase in the linearity of the gain comprises a decrease in gain ripple of at least 50% over a 5 bandwidth sparking from 1525nm to 1625nm, or part thereof.

9. A Raman optical amplifier for producing gain in an optical signal, the Raman optical amplifier comprising at least one pump laser for outputting a spectrum centred on a pump wavelength, or where there is more than one pump laser, centred on a respective pump wavelength, and, optically connected to the at least one pump laser, optical fibre means 10 having non-linear optical characteristics for broadening the pump spectrum or spectra, such that the broadening of the pump spectrum or spectra decreases the number of a plurality of pump lasers otherwise required to produce a predetermined linearity of the gain with respect to wavelength.

10. Raman optical amplifier as claimed in claim 9, wherein the optical fibre means having 15 non-linear characteristics is a zero dispersion optical fibre for promoting four-wave mixing of the pump spectrum therein for broadening the pump spectrum or spectra.

11. A Raman optical amplifier as claimed in claim 9, wherein the optical fibre means having non-linear characteristics is an optical fibre comprising longitudinal bores for confining optical transmission to a solid core thereby broadening the pump spectrum or 20 spectra.

12. A Raman optical amplifier as claimed in any of claims 9 to 11, wherein a transmission optical fibre having substantially linear optical characteristics is optically connected to the optical fibre means having non-linear characteristics, such that Raman amplification occurs in at least a portion of the transmission optical fibre.

25

13. A Raman optical amplifier as claimed in claim 12, wherein insertion means are provided for inserting the broadened pump spectrum into the transmission optical fibre for amplifying an optical signal therein to produce an amplified optical signal such that the amplified optical signal may be output from the transmission optical fibre without passing through the optical fibre means having non-linear characteristics.

30

14. A Raman optical amplifier as claimed in claim 13, wherein the insertion means is a band splitter located between the optical fibre means having non-linear characteristics and the

transmission optical fibre and between the transmission optical fibre and an output of the amplified signal therefrom.

15. A Raman optical amplifier as claimed in any of claims 9 to 14, wherein the predetermined linearity of the gain comprises a gain ripple of not more than 0.25 dB over a 5 bandwidth spanning from 1525nm to 1625nm, or part thereof.

16. A Raman optical amplifier for producing gain in an optical signal, the Raman optical amplifier comprising at least one pump laser for outputting a pump spectrum centred on a pump wavelength, or where there is more than one pump laser, centred on a respective pump wavelength, and, optically connected to the at least one pump laser, optical fibre means 10 having non-linear optical characteristics for broadening the pump spectrum or spectra, such that the broadening of the pump spectrum or spectra increases linearity of the gain with respect to wavelength and decreases the number of a plurality of pump lasers otherwise required to produce a predetermined linearity of the gain with respect to wavelength.

17. A Raman optical amplifier substantially as herein described with reference to and as 15 illustrated in FIGS. 6 to 8 in combination with FIGS. 9, 10 or 11 of the accompanying drawings.

(c oRmPE Application No: GB 0208487.9 r Examiner: Marion Dixon Claims searched: all Date of search: 20 November 2002

Patents Act 1977 Search Report under Section 17 Databases searched: UK Patent Office collections, including GB, EP, WO & US patent specifications, in:

UK C1 (Ed.T): H1C (CA,CBAA,CBAX); H4B (BK16D) Int Cl (Ed.7): HOlS (31094, 3/30); H04B (10/17) Other: Online: EPODOC,WPI,JAPIO,INSPEC,IEL Documents considered to be relevant: Category Identity of document and relevant passage Relevant to claims A WO1999/066607 (Bandwidth Solutions et al) A A EP0911926A (Nippon Telegraph and Telephone) see figs 29-37 A, P US20020044335A (Islam et al) see X JP2002040497A (KDDI Submarine Cable Systems) see figs 1 and 12-14,16 A Electronics Letters, vol 35, no 14, July 1999, pages 1175-1176, "50GHz spaced, 32XlOGbit/s dense WDM transmission in zero-dispersion region over 640km of dispersion-shifted fibre with multiwavelength distributed Raman amplification", Suzuki et al A Conference on Lasers and Electro-Optics Europe, 2000, Digest, page 394, "Broadband and nearly flat parametric gain in single-mode fibers", Provino et al X Document indicating lack of novelty or inventive step A Document indicating technological background and/or state of the art

Y Document indicating lack of inventive step if combined P Documentpublishedonorafterthedeclaredprioritydatebutbeforethe with one or more other documents of sane category. filing date of this invention.

E Patent document published on or after, but with priority date earlier & Member of the same patent family than, the filmy date of this application.

An Executive Agency of the Deparanent of Trade and Industry