GB2499648A

GB2499648A - Doped fibre amplifier with a wavelength locking reflector

Info

Publication number: GB2499648A
Application number: GB1203270.2A
Authority: GB
Inventors: Ian Peter Mcclean; Nadhum Kadhum Zayer
Original assignee: Oclaro Technology Ltd
Current assignee: Lumentum Technology UK Ltd
Priority date: 2012-02-24
Filing date: 2012-02-24
Publication date: 2013-08-28
Also published as: GB201203270D0

Abstract

A doped fibre amplifier comprising: at least one light source 402 for emitting pump light having an unlocked wavelength spectrum. At least one doped fibre is optically coupled to the light source and configured to receive an optical signal and the pump light. At least one wavelength locking reflector 410 for example a Bragg grating is configured to wavelength lock the pump light and positioned such that at least a portion of the doped fibre lies between the at least one light source and the at least one wavelength locking reflector 410. The at least one light source may comprise a co-pumping light source for emitting coal-pump light and a counter pumping light source.

Description

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DOPED FIBRE AMPLIFIER

Technical Field

5 The invention relates to doped fibre amplifiers. More specifically, the invention relates to doped fibre amplifiers wherein at least a portion of doped fibre lies between a light source and a wavelength locking reflector.

Background

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In this specification the term "light" will be used in the sense that it is used in optical systems to mean not just visible light, but also electromagnetic radiation having a wavelength outside that of the visible range.

15 Doped fibre amplifiers (DFA) are well known and well used amplifiers in telecommunications, typically used to extend the reach of optical telecommunications systems between Optical-Electrical-Optical (OEO) conversion. DFAs use a light source, typically a pump laser, to inject high power light into a doped amplifier fibre to provide population inversion within the fibre. An optical signal may then be transmitted 20 through the doped fibre and amplified by a process of stimulated emission.

One type of DFA is an erbium doped fibre amplifier (EDFA) in which the amplifier fibre is erbium doped. Pump lasers having a light output wavelength of approximately 980nm or approximately 1480nm are typical.

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As used herein, the term "980 nm" encompasses a band of wavelengths in a region surrounding 980 nm. It will be understood that typical band may span from approximately 970 nm to approximately 997 nm, although the skilled person will appreciate that broader definitions of the 980 nm band may be envisaged. Similarly, as 30 used herein, the terms "1480 nm" and "1550 nm" encompass bands of wavelengths in a region surrounding 1480 nm and 1550 nm respectively.

In one arrangement of an EDFA, the pump laser may be configured to inject pump light at the signal input of the erbium doped fibre (Er fibre), i.e. at the end of the Er fibre into 35 which the optical signal enters the fibre. This is termed co-pumping and is typically

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used to achieve a good optical signal to noise ratio (OSNR) performance. In co-pumping arrangements, it is common to use a 980nm pump laser as they have good OSNR performance. In another arrangement, the pump laser may inject the pump light at the signal output end of the Er fibre. This is termed counter pumping and is typically 5 used to achieve good power conversion efficiency.

More than one pump laser of different output wavelengths may be used if the output power required from an EDFA is high, for example approximately greater than 20dBm for EDFAs using typical two pump lasers. In this case it is known to have a co-10 pumping pump laser at the start of the EDFA to get good OSNR performance and either a co-pumping or counter pumping pump laser as the second pump laser. For example, EDFAs may use a 980 nm co-pumping laser and a 980nm co pumping or 1480 nm counter pumping laser.

15 A 980nm pump source for an EDFA is commonly used with a wavelength selective locking reflector included in a laser output fibre, or "pigtail", between the laser chip and the doped fibre. This is designed to prevent output power perturbations caused by mode hopping within the output spectrum plus acts to minimise wavelength variation and so prevent gain profile variation due to pump wavelength induced gain 20 inhomogeneity. A 1480nm pump commonly does not require the wavelength locker.

The choice of topology used will depend on the type of amplifier and the required output optical power. Pre-amplifiers and line amplifiers tend to be designed to achieve good OSNR and so the second pump is typically co-pumping, an arrangement termed 25 a co-co design. Booster amplifiers are commonly designed to achieve high output optical power and so the second pump is typically counter pumping, an arrangement termed a co-counter design. Higher power light sources typically drive a co-counter pump topology DFA as this is the most efficient.

30 Summary of invention

According to the invention in a first aspect there is provided a doped fibre amplifier comprising: at least one light source for emitting pump light having an unlocked wavelength spectrum; at least one doped fibre optically coupled to the light source and 35 configured to receive an optical signal and the pump light; and at least one wavelength

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locking reflector configured to wavelength lock the pump light and positioned such that at least a portion of the doped fibre lies between the at least one light source and the at least one wavelength locking reflector.

5 Because at least a portion of the doped fibre lies between the light source and the wavelength locking reflector, the reflector may be located within the doped fibre, or in a separate section of optical fibre after the doped fibre. In this way, the wavelength locking reflector does not need to be included in the fibre pigtail of the light source.

10 Optionally, the at least one wavelength locking reflector is configured to lock the pump light to a locked wavelength spectrum within, and narrower than, the unlocked wavelength spectrum.

Optionally, the locked wavelength spectrum has a width in the range from 0.1 nm to 2 15 nm.

Optionally, the unlocked wavelength spectrum has a width of 8 nm or greater.

Optionally, the at least one wavelength locking reflector is integrated into the doped 20 fibre.

Optionally, the at least one wavelength locking reflector comprises a fibre Bragg grating.

25 Optionally, the at least one doped fibre comprises an input end and an output end, and wherein the at least one wavelength locking reflector is located at the output end.

Optionally, the at least one light source comprises a co-pumping light source for emitting co-pump light and a counter pumping light source for emitting counter pump 30 light having the same unlocked wavelength spectrum as the co-pump light, and wherein the at least one wavelength locking reflector is configured to wavelength lock the counter pump light.

Optionally, the at least one wavelength locking reflector is located part-way along the 35 length of the at least one doped fibre.

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Optionally, the at least one wavelength locking reflector is located half-way along the length of the doped fibre.

5 Optionally, the at least one doped fibre comprises first and second optically connected doped fibres, and wherein at least part of the first doped fibre lies between the wavelength locking reflector and the co-pumping light source, and at least part of the second doped fibre lies between the wavelength locking reflector and the counter pumping light source.

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Optionally, the reflectivity of the at least one wavelength locking reflector is in the range from 90% to 100% with respect to the wavelength locked pump light.

Optionally, the reflectivity of the at least one wavelength locking reflector is in the range 15 from 95% to 100% with respect to the wavelength locked pump light.

Optionally, the reflectivity of the at least one wavelength locking reflector is in the range from 40% to 60% with respect to the wavelength locked pump light.

20 Optionally, the reflectivity of the at least one wavelength locking reflector is in the range from 45% to 55% with respect to the wavelength locked pump light.

Optionally, the at least one light source comprises a first co-pumping light source for emitting first co-pump light and a second co-pumping light source for emitting second 25 co-pump light having an unlocked wavelength spectrum, and wherein the at least one doped fibre is configured to receive light from the second light source.

Optionally, the at least one wavelength locking reflector is configured to wavelength lock the second co-pump light.

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Optionally, the at least one doped fibre comprises first and second optically connected doped fibres, wherein the first doped fibre is configured to receive the first co-pump light and the second doped fibre is configured to receive the second co-pump light.

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Optionally, the at least one wavelength locking reflector comprises first and second wavelength locking reflectors, wherein at least a portion of the first doped fibre lies between the first co-pumping light source and the first wavelength locking reflector, and wherein at least a portion of the second doped fibre lies between the second co-5 pumping light source and the second wavelength locking reflector.

Optionally, the pump light emitted by the at least one light source is in the 980 nm range.

10 Optionally, the at least one wavelength locking reflector is transmissive at the wavelength of the optical signal.

Brief description of the drawings

15 Exemplary embodiments of the invention are described herein with reference to the accompanying drawings, in which:

Figure 1 is a schematic representation of a typical co-co EDFA;

20 Figure 2 is a schematic representation of a typical co-counter EDFA;

Figure 3 is a plot showing the gain response of a DFA with different wavelengths of pump light;

25 Figure 4 is a schematic representation of a DFA;

Figure 5 is a schematic representation of a DFA;

Figure 6 is a schematic representation of a DFA; and

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Figure 7 is a schematic representation of a DFA.

Specific description

6

Disclosed herein is a DFA in which at least a portion of a doped fibre lies between a light source and a wavelength locking reflector.

Figure 1 shows a known co-co EDFA design 100. The design of Figure 1 and within 5 the following description details the basic elements of a DFA design to explain the principle of remote locking. It is understood that a DFA normally includes additional optical components such as optical tap couplers, isolators, VOAs, but, for clarity, these are not included in the description. The co-co EDFA 100 comprises first and second co-pumping light sources 102, 104 having first and second output fibres (or pigtails) 10 106, 108. In the exemplary EDFA of Figure 1, the pumping light sources are 980 nm pump lasers and, therefore, the first and second output fibres 106, 108 comprise first and second wavelength locking reflectors 110, 112. A wavelength locking reflector in this instance reflects a specific wavelength profile back along the optical axis of the fibre used in the amplifier. An example of a commonly used wavelength locker in 980 15 nm pumps is a fibre Bragg grating (FBG). The first and second output fibres 106, 108 are optically connected to first and second couplers 114, 116.

As used herein, the term "wavelength locking reflector" encompasses an optical element configured to be reflective in a narrow band of optical wavelengths. For 20 practical considerations, wavelength locking reflectors are commonly described as reflecting light of a single wavelength; however, the skilled person would understand that, in reality, a narrow band of wavelengths must be reflected. Wavelength locking reflectors are commonly used in the manufacture of external cavity lasers and the term would therefore be readily understood by a skilled person. Further, the skilled person 25 would understand the distinction between a wavelength locking reflector and a broadband optical reflector.

An optical signal to be amplified is transmitted via an optical fibre 118, which is connected to an input of the first coupler 114. A first section of doped fibre 120 is 30 optically connected to an output of the first coupler 114 and to an input of the second coupler 116. A second section of doped fibre 122 is optically connected to an output of the second coupler 116.

In operation, the first and second light sources 102, 104, which may be laser pumps, 35 produce light having an unlocked wavelength spectrum. The range of wavelengths

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may be embodied in a plurality of modes of light. The light is emitted into the first and second output fibres 106, 108. The direction of the light emitted by the first and second light sources 102, 104 is shown by the arrows in Figure 1. The first and second wavelength locking reflectors 110, 112 reflect a locked wavelength spectrum, which is 5 smaller range of wavelengths and may comprise a plurality of modes, to lock the output of the first and second light sources 102, 104. The wavelength locking reflectors 110, 104 therefore provide an external cavity for the first and second light sources 102, 104.

The "wavelength locked" light is then emitted into the first and second doped fibres 10 120, 122 via first and second couplers 114, 116. The optical signal enters the DFA via optical fibre 118, is transmitted through the doped fibres 120, 122 from left to right and is amplified by stimulated emission.

Figure 2 shows a known co-counter EDFA design 200. A first light source 202 is co-

15 pumping and is a 980 nm pump laser. A second light source 204 is counter pumping and is a 980 nm pump laser. Light emitted from the first and second light sources 202, 204 is wavelength locked in first and second output fibres 206, 208 by first and second wavelength locking reflectors 210, 212 as described above. The wavelength locked light is input to a doped fibre 220 to provide population inversion within the fibre 220.

20 This allows amplification by stimulated emission of an optical signal entering the doped fibre 220 via optical fibre 218and transmitted through the doped fibre 220 from left to right.

Typical 980nm pump lasers are made from a Fabry-Perot chip, which provides the 25 lasing cavity. The resulting spectral envelope of light output from the lasers is quite wide and is not locked. The inventors have appreciated that, when two 980nm pump lasers of this type are used in a co-counter configuration, such as that in Figure 2, pump instability is common due to light passing from one pump chip into the other and causing beating. This means that practical known co-counter designs are generally 30 designed with a 980 nm co-pumping pump laser and a 1480nm counter pumping pump laser, because the separation of the wavelengths of each pump laser is then large enough to prevent instability. Alternatively a component to prevent 980nm light passing between lasers, such as an optical isolator may be included between the pumps.

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The inventors have appreciated that with the advances of 980 nm pump technology, equivalent optical power can be achieved from a DFA with 980 nm pumps as with 1480 nm pumps, although at a lower total electrical power consumption. It is therefore preferable to use 980nm pump lasers in DFAs as they use less electrical power than 5 1480nm pump lasers for the same optical power output. Therefore use of the 980nm pump laser has advantages above the use of a 1480nm pump to achieve high output power efficiently. However, when designing a co-counter DFA care must be taken that the pump lasers, which are directed at each other along the doped optical fibre, do not disturb each other optically and so cause instabilities. This disturbance may, for 10 example, result in beating of the pump lasers and is more pronounced when using 980 nm pump lasers. A known co-counter design was made stable by coupling two 980nm chips together and feedback was made with a single FBG after the coupler (G. R. Jacobovitz-Veselka OAA 2005 FC4-1). A problem with this is that there is loss of optical power in the coupler, plus the amount of pump power at each end of the Er fibre 15 loop was co-dependent because of the coupling.

Typical 980nm pump lasers have a wavelength locking reflector included in the output fibre to lock the pump laser output wavelength to a fixed wavelength. Known co-counter DFA designs use these 980nm pump lasers and select a wavelength locking 20 reflector for each pump to lock the co-pumping and counter pumping light at wavelengths far enough apart not to cause interference, but still be within the 980nm pump band region. This is described in US5995275. The inventors have appreciated that a problem with this configuration when using the 980nm pump laser band is that the gain ripple of the DFA is very dependent upon the chosen wavelength as shown in 25 Figure 3.

The centre wavelength of light output by a pump laser can vary due to manufacturing variations and/or from operational influences such as drive current and temperature. The gain profile of a FA plotted against wavelength is known to vary with pump 30 wavelength and so to minimise gain profile variation, it is important to restrict the pump laser centre wavelength variation. This can be achieved by creating a cavity between the pump laser chip and an external wavelength locking reflector. However, even with a wavelength locking reflector, there is a small pump wavelength shift which can induce gain profile variation. The longer the wavelength of the wavelength locking reflector 35 (and, therefore, the wavelength of the locked pump light output from the pump laser),

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the more gain profile variation occurs with variation in the pump laser centre wavelength. The efficiency of the pump conversion to signal gain for a 980 nm pump laser is pump laser wavelength dependent. The longer the pump laser wavelength, the more efficient the power conversion. Therefore in known co-counter DFA designs 5 utilising two 980 nm pump lasers, the 980nm pump wavelength is a compromise between the two effects of reduced gain flatness and increased efficiency. Thus, if two wavelengths in the 980 nm band are used, degradation of both efficiency and gain flatness is seen compared to the optimal single wavelength choice.

10 Figure 4 shows a DFA comprising a light source 402, a doped fibre 420 and a wavelength locking reflector 410. Exemplary DFAs may be EDFAs. The light source 402 is configured to emit light having an unlocked wavelength spectrum. The unlocked wavelength spectrum may have a width of 8 nm or greater. The doped fibre 420 is optically coupled to the light source 402 and configured to receive an optical signal 15 from an optical fibre 418 and light from the light source 402. The wavelength locking reflector 410 is configured to lock the light emitted from the light source 402 to a locked wavelength spectrum. As described above, the locked wavelength spectrum is commonly and practically described as a single wavelength by the skilled person. The locked wavelength spectrum is typically a narrow band of optical wavelengths and may 20 comprise a small number of modes. The locked wavelength spectrum may have a width in the range from 0.1 nm to 2.0 nm. Specifically, the locked wavelength spectrum may have a width of 2.0 nm, 1.5 nm, 1.0 nm, 0.5 nm or 0.1 nm.

The input signal is transmitted through the doped fibre 420 from left to right in the figure 25 and the wavelength locking reflector 410 is located at an output end (i.e. at the right hand end in the figure) of the doped fibre 402. Therefore, at least a portion of the doped fibre lies between the light source and the wavelength locking reflector. That is, the wavelength locking reflector is not positioned within the light source output optical fibre.

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The light source 402 is a pump laser having an output wavelength of light in the 980 nm band. The pump laser 402 is optically connected to an output fibre (or pigtail) 406, which is optically connected to an input of a coupler 414. The output fibre 406 and the light source 402 form a light source assembly, which forms a single unit or circuit 35 element. The output fibre 406 has no integrated wavelength locking reflector and so

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the light source assembly emits laser light that is not locked to a specific wavelength, but that has a spectrum comprising a plurality of wavelengths.

The optical fibre 418 is also connected to an input of the coupler 414. An output of the 5 coupler 414 is optically connected to the doped fibre 420. In the exemplary DFA of Figure 4, the doped fibre 420 is a length of silica cored optical fibre doped with trivalent erbium ions. The DFA of Figure 4 is therefore an EDFA. The wavelength locking reflector 410 is incorporated into the doped fibre 402. In alternative DFAs, the wavelength locking reflector 410 may be included in a standard optical fibre optically 10 connected to the output of the doped fibre 402.

In use, the pump laser 402 emits pump light, which is transmitted into the doped fibre 420 via the coupler 414. This creates a population inversion in the doped fibre 420. The optical signal is transmitted through the doped fibre 420 and is amplified by 15 stimulated emission. At least part of the light emitted by the pump laser 402 at a particular wavelength is reflected by the wavelength selective reflector 410. This creates a lasing cavity between the wavelength locking reflector 410 and a rear facet of the pump laser 402. The wavelength of the light reflected is determined by the construction of the wavelength selective reflector 410. This has the effect that the light 20 emitted by the pump laser 402 is locked to the wavelength of light reflected by the wavelength selective reflector 410. In the exemplary DFA of figure 4, the wavelength selective reflector is a fibre Bragg grating (FBG).

FBGs and other wavelength selective reflectors are distributed through a length of 25 optical fibre. Typically, the length of an FBG may be in the range from 1 cm to 30 cm and must be placed a distance away from the laser chip longer than the coherence length of the laser, which may be approximately from 1 m to 2 m. Therefore, the overall length of a pump laser pigtail of the prior art may have a length in the range from 1 m to 2.5 m. If an FBG is incorporated into the output fibre of a light source 30 assembly then this adds considerable length of fibre that must be wound into the DFA module. Also, the additional fibre is often costly optical fibre such as Polarisation Maintaining fibre. By placing the FBG 410 in the doped fibre, or after the doped fibre, which is already several meters long, the light source assembly used in Figure 4 has reduced length of fibre and cost.

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In exemplary DFAs having the configuration of figure 4, the FBG 410 may have a reflectivity at the locked wavelength in the range from 1% to 10%, 1% to 20% or 1% to 30%. In alternative exemplary DFAs having the configuration of Figure 4, the FBG 410 may have a reflectivity in the range from 80% to 100%, 90% to 100% and, in a specific 5 exemplary DFA, a reflectivity of 100%. DFAs comprising FBGs 410 with high reflectivities may be advantageous in particular circumstances as they allow the FBG 410 to reflect a high proportion of the pump light produced by the pump laser 402 back through the doped fibre 420. This creates a "pseudo" co-counter configuration using only a single pump laser 402 and reduces the amount of pump power that may pass 10 straight through the doped fibre 420 without adding to the level of inversion. However, it is noted that a DFA having the configuration of Figure 4 will be operable with an FBG having a reflectivity in any range, dependent on the required DFA application. The ranges of reflectivity mentioned above are exemplary only. The FBG 410 is substantially transmissive (i.e. has substantially 0% reflectivity) at the wavelength of the 15 optical signal, typically in the 1550 nm band.

The FBG 410 is therefore able to act as both a wavelength locking reflector and a pump light reflector. This has the advantage that only a single FBG 410 is required. Further, the pump light travelling in each direction (co and counter) through the doped 20 fibre will be locked to the same wavelength, as only one FBG 410 is in the optical path. Therefore, the pump light reflected back towards the pump laser 402 will not create interference in the laser output power.

If a wavelength locking reflector were included in the light source assembly output fibre 25 and another reflector were used as a pump reflector, then the pump light travelling in each direction (co and counter) may be of similar, but not identical, wavelengths and so interference could occur.

Further exemplary DFAs are described below with reference to the figures. Common 30 features not explained in detail in each exemplary DFA may be considered to have the same configuration and operation as described above.

Figure 5 shows a schematic representation of a co-counter DFA 500. A first light source 502 is a co-pumping light source, and a second light source 504 is a counter 35 pumping light source. The first and second light sources 502, 504 are each pump

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lasers emitting light in the 980 nm band. A wavelength selective reflector 510, which in the exemplary DFA of Figure 5 is an FBG, is located within the doped fibre 520. In particular DFAs, the FBG 510 may be approximately half-way along the length of a doped fibre 520. Therefore, the external cavity between each of the first and second 5 light sources 502, 504 and the FBG 510 is effectively made within the doped fibre 520 and, since there is only one reflector, both pumps are locked to the same wavelength and so do not compete. In other exemplary DFAs, it is possible to position the FBG 510 at different points within the doped fibre 520 so the lengths of doped fibre either side of the FBG 510 are varied. For example, it may generally be desirable to have 10 large signal gain in the first section (i.e. before the FBG 510) of doped fibre 520, and a high power in the second section (i.e. after the FBG 510). The length ratio of the first and second sections may be determined by the operating conditions, specifically the input power of the pump laser and the gain, required for a specific amplifier. In alternatively DFAs, the FBG 510 may be included in a standard optical fibre positioned 15 at either side of the doped fibre 520 rather than within it.

The wavelength locking reflector 510 is an FBG and is configured to lock the wavelength of light emitted by the pump lasers 502, 504 to the same wavelength. The reflectivity at the locked wavelength of the FBG 510 at the locked wavelength may 20 have any value. In an exemplary DFA having the configuration of Figure 5, FBG 510 may have a reflectivity at the locked wavelength in the range from 40% to 60%, 45% to 55%or, more specifically, the reflectivity at the locked wavelength may be 50%.

In all cases the reflectivity of the FBG 510 at the optical signal wavelength, typically in 25 the 1550nm band, should in general be minimal to prevent gain clamping through reflection of 1550 nm light back into the doped fibre 520. Gain clamping is caused by the reflection of a signal back into an amplifier using available pump inversion. If pump power is increased, the power of the reflected light is also increased, which takes more of the available inversion. This prevents the gain increasing linearly with an increase in 30 pump power, thereby clamping the gain.

Light is emitted from the pump lasers 502, 504 and injected into the doped fibre 520 via couplers 514, 516. Population inversion within the doped fibre 520 is thereby provided. The wavelength of the light emitted by the pump lasers 502, 504 is locked to the same 35 wavelength. Therefore, the wavelength of the light emitted by each pump laser 502,

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504 is identical and, as such, does not interfere with the light emitted by the opposing pump laser.

The FBG 510 could be configured to have different reflectivities in different exemplary 5 DFAs dependent on the requirements of the DFA design. For example, if the reflectivity of the FBG 510 at the locked wavelength is low then at least part of the pump light from each pump laser 502, 504 arriving at the FBG 510 would pass through to the section of the doped fibre 520 on the opposite side of the FBG 510. In such arrangements, inversion within the doped fibre 520 would be as a consequence of 10 pump light emitted from both pumps. Not only would this benefit operation, but allows for redundancy in that if one pump was to fail then the amplifier would still be able to operate, although to a lesser performance, unlike if each pump only pumped one of the stages.

15 In the exemplary DFA of Figure 5, the FBG 510 is configured to have a reflectivity of 50%. Therefore, half the pump light is reflected back to the pump laser 502, 504, providing good wavelength locking, and half the pump light passes through the FBG 510 providing more gain in the other section of the doped fibre 520. It is noted that the FBG 510 may also be configured to have a low reflectivity at the locked wavelength, for 20 example in the range from 1% to 10%, 1% to 20% or 1% to 30% and still achieve the operating characteristics mentioned above.

In alternative exemplary DFAs, the FBG 510 may be configured to have a reflectivity at the locked wavelength in the range from 80% to 100%, 90% to 100% or, in a specific 25 DFA 100%. In such DFAs, a high proportion, or even all, of the pump light is reflected back to the pump lasers 502, 504, thus the pump light would have a double pass through the same section of doped fibre.

Figure 6 shows a schematic representation of a co-counter DFA 600. The DFA 600 30 comprises first and second doped fibres 620a, 620b. An FBG 610 is positioned between the first and second doped fibres 620a, 620b. The length of the first and second doped fibres 620a, 620b is equal. However, it will be appreciated that the lengths of the first and second doped fibres 620a, 620b may differ from each other.

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The first and second doped fibres 620a, 620b are optically connected directly to each other. The FBG is integrated into a portion of connecting optical fibre optically connected between the first doped fibre 620a the second doped fibre 620b. In alternative exemplary DFAs, the FBG 610 may be integrated into one of the first doped 5 fibre 620a and the second doped fibre 620b.

The reflectivity of the FBG 610 at the locked wavelength may be configured as described in Figure 5. Further, operation of the DFA 600 is similar to the operation of the DFA in Figure 5 and is therefore not described again here.

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By using first and second doped fibres 620a, 620b the flexibility of the DFA 600 is improved. Each of the first and second doped fibres 620a, 620b may, for example, be replaced to accommodate a different DFA design. Further, in exemplary DFAs comprising the connecting optical fibre, that fibre, and therefore the FBG 610, may be 15 replaced to accommodate a different DFA design.

Figure 7 shows a schematic representation of a co-co DFA 700. The DFA 700 comprises first and second doped fibres 720a, 720b. A first light source, which is a pump laser, 702 is optically connected to an input of a first coupler 714 via a first light 20 source output fibre 706. An optical signal input fibre 718, via which an optical signal is input to the DFA 700, is connected to an input of the first coupler 714. An input end of the first doped fibre 720a is connected to an output of the first coupler 714.

A second light source, which is a pump laser, 704 is optically connected via a light 25 source output fibre 708 to an input of a second coupler 716. An output end of the first doped fibre 720b is connected to an input of the second coupler 716. An input end of the second doped fibre 720b is connected to an output of the second coupler 716.

Each of the first and second pump lasers 702, 704 emits light in the 980 nm band.

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First and second wavelength locking reflectors, which are FBGs, 710, 712are positioned within the first and second doped fibres 720a, 720b respectively. However, in exemplary DFAs, the FBGs may be positioned within the doped fibres 720a, 720b, or at the ends of the doped fibres 720a, 720b. The reflectivity at a locked wavelength of 35 the first FBG 710 may be in the range from 40% to 60%, 45% to 55%, or, more

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specifically, approximately 50%. The reflectivity at the locked wavelength of the second FBG 712 may be as described for the FBG 410 of Figure 4. However, as noted above, FBGs having any reflectivity at the locked wavelength may be suitable for use in the DFA of Figure 7. In this example the two doped fibres 720a, 720b act as separate 5 gain stages and each can take advantage of the remote locking of the pump lasers 702, 704 in a design that suits the specific requirement of the gain stage.

Pump light emitted from the first pump laser 702 is injected into the first doped fibre 720a.The first FBG 710 reflects half of the pump light emitted from the first pump laser 10 702 to provide locking of the pump laser 702 to given wavelength. The second half of the pump light passes through the first FBG 710 to increase the gain in the section of the first doped fibre 720a beyond the first FBG 710. The second half of the pump light, i.e. the pump light that has passed through the FBG 710, is removed from the main signal path by the second coupler 716. In addition, the pump light emitted from the 15 second pump laser 704 is injected into the second doped fibre 720b via the second coupler 716. The pump light emitted from the second pump laser 704 passes through the second doped fibre 720b and is reflected by the second FBG 712. The second FBG therefore locks the second pump laser 704 output to a desired wavelength. Also, the second FBG 712 allows all of the pump light to have a second pass through the 20 second doped fibre 720a thereby increasing the power efficiency. Both the first and second FBGs 710, 712 are substantially transmissive at the optical signal wavelength, which is typically in the 1550 nm band.

In other exemplary DFAs, a gain flattening or amplified stimulated emission (ASE) 25 flattening filter shape may be incorporated into the FBG 510 or in another reflector alongside the FBG 510. This type of arrangement may be utilised in any of the DFAs encompassed within the scope of the appended claims.

The DFAs described herein provide the advantage that the pump reflector can act both 30 as a pump wavelength locking reflector and a pump light reflector. In this way, interference between two pump lasers within a DFA can be reduced. Further, in the DFAs described herein the wavelength locking reflector does not need to be positioned within the pump laser output fibre (pigtail). This reduces the amount of optical fibre required for a pump laser, thereby reducing the size and weight of these units.

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The skilled person will envisage further embodiments of the invention without departing from the scope of the appended claims. For example, in all embodiments, the position of the FBG(s) within the doped fibre may vary dependant upon performance and design requirements. Also, the reflectivity of the FBG(s) may vary dependant upon 5 performance and design requirements.

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Claims

CLAIMS:

1. A doped fibre amplifier comprising:

at least one light source for emitting pump light having an unlocked wavelength 5 spectrum;

at least one doped fibre optically coupled to the light source and configured to receive an optical signal and the pump light; and at least one wavelength locking reflector configured to wavelength lock the pump light and positioned such that at least a portion of the doped fibre lies between 10 the at least one light source and the at least one wavelength locking reflector.

2. A doped fibre amplifier according to claim 1, wherein the at least one wavelength locking reflector is configured to lock the pump light to a locked wavelength spectrum within, and narrower than, the unlocked wavelength spectrum.

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3. A doped fibre amplifier according to claim 2, wherein the locked wavelength spectrum has a width in the range from 0.1 nm to 2 nm.

4. A doped fibre amplifier according to any preceding claim, wherein the unlocked 20 wavelength spectrum has a width of 8 nm or greater.

5. A doped fibre amplifier according to any preceding claim, wherein the at least one wavelength locking reflector is integrated into the doped fibre.

25 6. A doped fibre amplifier according to any preceding claim, wherein the at least one wavelength locking reflector comprises a fibre Bragg grating.

7. A doped fibre amplifier according to any preceding claim, wherein the at least one doped fibre comprises an input end and an output end, and wherein the at least

30 one wavelength locking reflector is located at the output end.

8. A doped fibre amplifier according to any of claims 1 to 7, wherein the at least one light source comprises a co-pumping light source for emitting co-pump light and a counter pumping light source for emitting counter pump light having the same unlocked

35 wavelength spectrum as the co-pump light,

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and wherein the at least one wavelength locking reflector is configured to wavelength lock the counter pump light.

9. A doped fibre amplifier according to claim 8 wherein the at least one 5 wavelength locking reflector is located part-way along the length of the at least one doped fibre.

10. A doped fibre amplifier according to claim 9 wherein the at least one wavelength locking reflector is located half-way along the length of the doped fibre.

10

11. A doped fibre amplifier according to claim 8 to 10, wherein the at least one doped fibre comprises first and second optically connected doped fibres, and wherein at least part of the first doped fibre lies between the wavelength locking reflector and the co-pumping light source,

15 and at least part of the second doped fibre lies between the wavelength locking reflector and the counter pumping light source.

12. A doped fibre amplifier according to any preceding claim, wherein the reflectivity of the at least one wavelength locking reflector is in the range from 90% to 100% with

20 respect to the wavelength locked pump light.

13. A doped fibre amplifier according to claim 9, wherein the reflectivity of the at least one wavelength locking reflector is in the range from 95% to 100% with respect to the wavelength locked pump light.

25

14. A doped fibre amplifier according to any of claims 8 to 11, wherein the reflectivity of the at least one wavelength locking reflector is in the range from 40% to 60% with respect to the wavelength locked pump light.

30 15. A doped fibre amplifier according to claim 14, wherein the reflectivity of the at least one wavelength locking reflector is in the range from 45% to 55% with respect to the wavelength locked pump light.

16. A doped fibre amplifier according to any of claims 1 to 7, wherein the at least 35 one light source comprises a first co-pumping light source for emitting first co-pump

19

light and a second co-pumping light source for emitting second co-pump light having an unlocked wavelength spectrum,

and wherein the at least one doped fibre is configured to receive light from the second light source.

5

17. A doped fibre amplifier according to claim 16, wherein the at least one wavelength locking reflector is configured to wavelength lock the second co-pump light.

18. A doped fibre amplifier according to claim 16 or 17, wherein the at least one 10 doped fibre comprises first and second optically connected doped fibres, wherein the first doped fibre is configured to receive the first co-pump light and the second doped fibre is configured to receive the second co-pump light.

19. A doped fibre amplifier according to claim 18, wherein the at least one 15 wavelength locking reflector comprises first and second wavelength locking reflectors,

wherein at least a portion of the first doped fibre lies between the first co-pumping light source and the first wavelength locking reflector, and wherein at least a portion of the second doped fibre lies between the second co-pumping light source and the second wavelength locking reflector.

20

20. A doped fibre amplifier according to any preceding claim, wherein the pump light emitted by the at least one light source is in the 980 nm range.

21. A doped fibre amplifier according to any preceding claim wherein the at least 25 one wavelength locking reflector is transmissive at the wavelength of the optical signal.

22. A doped fibre amplifier substantially as herein described with reference to the accompanying drawings.