GB2383209A

GB2383209A - Raman optical amplifier with two power control loops

Info

Publication number: GB2383209A
Application number: GB0129785A
Authority: GB
Inventors: Robert Charles Goodfellow
Original assignee: Individual
Current assignee: Individual
Priority date: 2001-12-12
Filing date: 2001-12-12
Publication date: 2003-06-18
Also published as: GB0129785D0

Abstract

A low noise optical amplifier which uses stimulated Raman amplification for dense wavelength division multiplexed optical fibre links and networks, that uses a plurality of pump wavelengths, individually power adjusted, in control loops which use a spectrum monitor as input, to provide a set gain to a particular wavelength channel and gain flattening across the DWDM spectrum. The amplifier has a first power control loop for a given wavelength channel, and a second control loop for maintaining the powers of the remaining channels relative to the given channel.

Description

LOW NOISE OPTICAL AMPLIFIER Background Optical fibres are used to carry information traffic between physical locations which form the nodes of an optical communication network. Between one and a few hundred channels may be carried on each fibre. Each channel is assigned to a different wavelength band. A channel has a narrow spectrum centred on a different optical wavelength. The carrier wavelength of the channel is modulated with the information signal. Dense Wavelength Division Multiplexing (DWDM) uses channel spacings of 50 GHz or 1 OOGHz according to the International Telecommunication Union ITU Standards and closer channel spacings of 25GHz may be used in the future.

The cost of the network is influenced strongly by the distance reached between repeater nodes of the optical communications system because each channel requires its own regenerating optoelectronic and electrical circuits at each repeater node. The total equipment to regenerate all of the signals of a DWDM payload is significant and may be costly. Optical amplification may operate on all channels of a DWDM payload, providing similar gain to each channel with little cross talk between each channel and with a small addition of noise. However, the signals are not regenerated and so signal quality progressively deteriorates following each amplification stage. When reaches 1000km or more are required, it is necessary to position several optical amplifiers along the path, so build up of noise, and build up of gain variation over the DWDM spectral range becomes a limiting design consideration. To preserve signal quality the noise added at each stage can be made minimal. The theoretical minimum noise figure for a fibre amplifier is 3dB and practical amplifiers have 3.5 to 6dB. The amplification provided to each DWDM channel can be engineered to be similar to provide good gain flatness ( < 1dB). Nevertheless, the differences in gain between channels accumulate with each amplifier. With low noise figure amplifiers and management of the power differences between channels, around 100 amplifying stages may be used between repeating nodes.

Conventionally, erbium doped fibre amplifiers are used. The signal is launched into the fibre at a level of around 1 mW and is attenuated by residual loss in the fibre to around 1 to 10 microwatts. The optical fibre amplifier then boosts the power back to around 1 mW. Erbium doped fibre amplifiers introduce noise to the signal due to the amplification of spontaneous emission and beat noise. This addition of noise has been analysed by Yariv1 (1991) and by Haus2 (2001). Haus's analysis shows

that'added noise'is minimised when using backward pumped Raman amplification to compensate the fibre loss. Haus showed that the'added noise'increases if Raman amplification is used to produce positive gain in the fibre.

Chromatic dispersion in the fibre produces signal distortion that develops in approximately in proportion to the fibre length. Conventionally, Dispersion Compensation Modules are distributed along the fibre path. The dispersion compensation module may be a length or lengths of special fibres, or a chirped fibre bragg grating filter used in reflection, or a bulk optic delay line arrangement. All introduce loss. The impact of this loss with regard to noise addition is conventionally reduced by placing the dispersion compensation module between two optical amplifier stages. Using the principles expounded by Haus (reference 2), it is beneficial from noise considerations when using fibre as the compensation technology, to use Raman gain to compensate loss, thereby minimising the'added noise.' Gain Flatness The erbium doped fibre has a gain profile which varies with photon energy because the erbium atoms sit in different environments within the silica amorphous structure. Conventionally this variation of gain with wavelength is reduced by means of a gain flattening filter implemented with interference thin film filters or with tailor made fibre bragg gratings. Residual gain flatness of +/-. 5dB are achievable. Polarisation dependent gain and variations due to changes in the loading, ie number of DWDM channels present, and due to temperature make +/-0. 5dB the practical gain flatness limit.

When multiple amplifiers are cascaded to make longer reach systems, the gain variation accumulates.

The error rate of the receiver varies with input power around an optimum value, so the accumulated gain variation leads to a compromised system performance as the power levels become driven further from the optimum value.

It has been shown in research papers that using two or more Raman pump sources, a tilt may be introduced into the gain variation such that shorter wavelength channels may be amplified more than the longer ones, or vice versa. The variation of power level across the channels of DWDM systems has been controlled by demultiplexing the channels and then using a separate attenuator for each channel to correct the power deviations. This net attenuation applied to all channels means that more optical amplification has to be designed into the system and so more added noise results, and reach is compromised.

Transients. As channels are added or dropped at nodes of the DWDM system, the change in power loading on the chain of amplifiers results in changes in the accumulated gain variation between repeater nodes. Also the degree of gain saturation changes along the chain of optical amplifiers. This produces temporal changes in gain and power along the transmission path The changing power levels alter the non-linear processes in the fibre and the optical receiver performance, resulting in distortions of the signal and this can lead to the generation of errors. The attenuators used for levelling have insufficiently fast response time to compensate for channel loading changes. The input detector 7 receives power from all channels simultaneously so will detect transients due to the addition or dropping of DWDM channels and due to saturation effect in amplifiers earlier in a chain of amplifiers. A fast analogue control loop may be set up between this detector and the Raman pump source to provide feedback to smooth out this transient. The frequency response for this analogue feedback loop may be set to 10-2 to 10-5seconds.

The problem that the invention aims to solve. An amplifying module was required that would (1) contribute very low added noise; (2) will lead to limited accumulated gain variation when several modules are cascaded; (3) will provide adjustable gain so that power per channel at the output of each module is maintained at levels where non-linear propagation effects are small ; (4) the gain may be dynamically controlled so that transient effects may be compensated with fast control loops ; and (5) a module that can incorporate a dispersion compensation module.

BRIEF OUTLINE OF THE INVENTION The invention provides multiple pump laser sources to deliver variable Raman gain and dynamic gain flattening across a DWDM spectrum in the upstream transmission fibre by using a spectrum power monitoring device, with effectively two control loops, one to control the total power of the Raman pump sources and the other to control the relative intensities of the constituent wavelengths of the pump.

This is extended to a two stage arrangement in which a dispersion compensation fibre is operated as an optical amplifier either in a backward Raman pumping arrangement using a plurality of pump wavelengths, or using an Erbium doped dispersion compensation fibre with forward or backward pumping, or using a Erbium fibre amplifier.

DETAILED DESCRIPTION OF'THE INVENTION In Figure 1 is shown a schematic of the DEDM line amplifier. In this example embodiment, we show the transmission fibre 1 which may be 0 to 200km long and have attenuation of 0 to 30dB. The input signal arriving from the transmission fibre 1 essentially passes through the wavelength divisional coupler Into the power splitter 3 where the power is sampled by a photodetector and iow noise amplifier 7. If integrity of the transmission fibre is confirmed by the presence of signal power at 7, then pump power from the Raman laser pump sources 10 is introduced into the manifold 9 and controlled power levels of the pump wavelength are directed into the wavelength coupler 2 by optical path 15.

The Raman pumps for C band-1530-1560nm DWDM signals will be an optical phonon energy away from the signal at around 1430 to 1460nm. The Raman pump power may be in the range 100mi to 1500mW.

Once the Raman pump is activated, amplified signal preferably at a power around-3 to +2dBm will be launched into a dispersion compensation module 4. This may be a selected dispersion compensation fibre or series combination of two types of dispersion compensating fibre, so selected to provide essentially dispersion compensation for the transmission fibre for all of the wavelength of the signal channels. The dispersion compensation fibre or fibres will typically have loss between 3dB and 15dB.

The loss of the dispersion compensation module 4 is compensated by means of a second stage of amplification which may be by means of an erbium doped fibre amplifier or preferably as shown in Figure 1, by means of Raman amplification, for the case where the dispersion compensation is achieved with a fibre. For dispersion compensation modules using fibre bragg gratings or bulk optics solutions the erbium doped amplifier solution is the preferred optical amplifier. The preferred solution of Figure 1 shows Raman optical amplification used to compensate loss in the dispersion compensation module 4. Raman pump energy is introduced by optical path 16 into wavelength selective coupler 5 and"backward pumps"the dispersion compensation fibre 4. This Raman pump energy may be derived from the same pump sources as used for pumping the transmission fibre 1, or may be four separate laser sources. This dual use of the Raman pump sources 10 may be effected by means of the optical manifold 9.

The signal is introduced into power spatter 6 at the output part of the amplifying module. Power splitter 6 provides splitting of the output power of around 1 to 10% and the splitting is preferably, essentially, constant over the DWDM band of wavelengths.

The sampled signal power is introduced into spectral monitor 8. Spectral monitor 8 provides an electrical output signal providing data on the power level of each channel of the DWDM signal. This data representation of the power distribution across the channels of the DWDM signal is passed to the controller 15.

The controller may be a digital signal processor or microprocessor computer. A data signal from the power monitor at the input 7 may be provided to the controller by link 13. A separate data link 14 may be provided to a switch in the manifold to provide a safety shut down for the Raman pump. This is activated on detection of loss of signal or on alternatively on detection of loss of Optical Service Channel signal. For detection of the Optical Service Channel the receiver 7 will have de-multiplexing filtering and a plurality of receivers to allow simultaneous detection of the service channel and input power monitoring.

The controller 15 generates control signals for the Raman pump sources 10 via control path 16 and for the manifold 9 via control path 12. The processor in the controller 15 is programmed to control the output power in each DWDM channel after amplification, to keep the power levels at a level where non-linear effects will be kept to manageable levels. The controller will also keep the relative powers of the DWDM channels to within pre-programmed set limits. The controller will keep the DWDM signal powers to the required limits by regulating the pump power levels introduced into the signal path by WDM couplers 2 and 5.

The sampling sequence at turn on is (1) Sample the power at 7. If the signal power or any optical service channel power is detected as present, then turn on the Raman pump sources and introduce power into WDM couplers 2,5. The detection of presence of signal at 7 is used to start the process and test system integrity. This precaution is to prevent Raman pump laser power being introduced into a broken transmission fibre in which case, under some conditions it could be a safety hazard due to eye safety.

(2) Increment the power of the Raman pump sources sequentially in incremental steps-e. g. of 50mW so that all of the plurality of wavelength Raman sources are activated

(3) Sample the DWDM spectrum at 8. If power exceeds non-linear value on all channels, reduce Raman pump power by the increment on all channels. If the power is too high on some channels at the long wavelength end, then reduce the longer wavelength Raman pumps, in increments until the spectrum is flat within program limits, say 1 dB. If the power is too high on some channels at the short wavelength end, then reduce the shorter wavelength Raman pumps, in increments of 50mW until the spectrum is flat within program limits, say 1 dB. If the power in each channel is lower than the set value, repeat step (2) above.

Repeat the sequence until output spectrum is flat and of the required output power.

The pump powers introduced into the transmission fibre coupler 2 and to the DCM coupler 5 are adjusted dynamically by the controller so that the gain due to Raman amplification essentially compensates the fibre losses. The processor adjusts the power levels of the Raman pump sources to provide, after iteration, a flat gain profile across the DWDM signals as assessed at the monitoring point 8. The Raman pump signals are passed through quarter wave plates to turn the linear polarised laser power into essentially circularly polarised power in order to provide essentially equal gain for signals of different polarisation orientation in the fibre. This may be implemented by putting the device to"render the polarisation circular" in the fibre paths 16 and 17 or to implement this as part of the optical manifold 9. The element 17 is a non-reciprocal device such as an optical isolator or circulator to prevent pump energy from down stream amplifiers affecting the control loops.

The algorithm to preserve gain flatness operates with any number of channels present, and with any mix of channels. The data providing the power distribution over the DWDM channels is processed as follows. The DWDM spectrum is effectively handled in n equal spectral width parts-where n is the number of Raman pump source wavelengths to be used. The Raman pump source wavelengths are selected such that

where XPn is the nth pump source wavelength (in the range 1.4 to 1. 5micron for C band), Xsn is the mid wavelength of the part of the spectrum under consideration, and At. pn is the estimated average optical phonon wavelength (-0. 1 micron) for the nth part of the DWDM spectral band to be involved in the Raman amplification process. (ALpn may be slightly different for fibres with different material compositions but because the Raman gain process is - 30nm wide it may be practical to use the same

Raman wavelengths for different transmission and dispersion compensating fibres. ) The spectrum monitor produces data on the power of the DWDM channels present in the signal. The processor computes the channel powers in each of the n parts. The powers of the pump sources are incremented sequentially upwards until the DWDM channels in each part are approximately equal within set boundaries and power levels of the channels in the fibre are the design values (e. g. OdBm).

If any channel's power, as determined by the spectral monitor, is above the design value then the Raman pumps are adjusted downwards in a sequence that will lead to best flatness of gain over the part DWDM spectrum that the offending channel occupies, whilst maintaining flatness over the n different parts of the spectrum.

Claims

1. A line amplifier equipment comprising a spectrum monitoring means, a plurality of Raman pump sources, a control loop to control the power level of one wavelength channel and a second control loop to maintain the powers of each of the plurality of channels relative to the power level of the first controlled channel.

2. A line amplifier equipment as claimed in claim 1 wherein the Raman pump source is operated so that the gain in the backward pumped amplifier formed in the transmission fibre substantially overcomes the transmission fibre attenuation loss.

3. A line amplifier equipment as claimed in claim 1 wherein a plurality of pump sources provides power to a mid stage module containing dispersion compensation fibre to produce gain to substantially overcome the insertion loss of the dispersion compensating fibre.

4. A line amplifier equipment as claimed in claim 1,2 or 3 wherein the plurality of pump sources are connected to the input transmission line by means of an optical manifold that splits and combines the power.

5. A line amplifier equipment as claimed in claim 4 wherein the optical manifold provides the means to modify separately the power levels of the constituent wavelengths of the group of pump wavelengths.

6. A line amplifier equipment as claimed in claim 5 wherein the constituents of the pump that may be varied include separate orthogonal polarisations of the pump wavelengths or the state of polarisation of the pump wavelengths.

7. A line amplifier equipment as claimed in claim 5 or 6 wherein the manifold is a polymeric, silica or silicon optical waveguide circuit using temperature control to split power between paths within the manifold.

8. A line amplifier equipment as claimed in claim 5 or 6 wherein the manifold is made in an electro-optic material and the power control is achieved using electro-optic devices.

9. A line amplifier equipment as claimed in claim 5 or 6 wherein the manifold is constructed with lenses and mirrors to produce optical beams and the splitting and attenuation are achieved by means of miniature moveable elements such as mirrors, lenses or shutters.

10. A line amplifier equipment as claimed in claim 5 or 6 wherein the manifold is assembled with optical fibre waveguides with optical fibre splitters and with discrete fibre pigtailed variable optical attenuators.

11. A line amplifier equipment as claimed in claim 1 wherein the spectrum monitor has a dispersive component and a photodetector array element.

12. A line amplifier equipment as claimed in claim 1 wherein a calibration process is carried out where parameters are stored in a memory which take into account the dispersive efficiency of the dispersive element in the spectrum monitoring device for each wavelength and the sensitivity of the photoelectric elements for the appropriate wavelengths.

13. A line amplifier equipment as claimed in claim 1 wherein the first control loop may be based on any selected channel of the DWDM spectrum.

14. A line amplifier equipment as claimed in claim 1 wherein the first control loop has a response time faster than one millisecond such that transients due to the introduction or removal of channels may be reduced.

15. A line amplifier equipment as claimed in claim 1 wherein the plurality of sources for the Raman amplification are semiconductor laser diodes.

16. A line amplifier equipment as claimed in claim 1 wherein the plurality of sources for the Raman amplification are fibre lasers using Stoke's-shift wavelength conversion.

17. A line amplifier equipment as claimed in claim 1 wherein the second stage of amplification is implemented with an erbium doped fibre amplifier.

18. A line amplifier equipment as claimed in claim 17 wherein the erbium doped fibre amplifier has a gain flattening filter to substantially reduce the gain variation over the spectrum of the DWDM channels due to the second amplifier stage.

19. A line amplifier equipment as claimed in any or all of the claims above wherein an optical isolator or optical circulator or band stop filter is placed in the fibre signal path to control unwanted reflections of the signal or pump wavelengths, orland to limit pump energy from other optical line amplifiers in a linear series of amplifiers from affecting the control loop.

20. A line amplifier equipment embodying any or all of the previous claims wherein fibre splitters and optical power tapping means are employed to permit signal monitoring or bypassing of system control signals.

21. Optical systems, optical communications links or optical networks which embody line amplifier modules with any or all of the claims herein.

22. A line amplifier equipment as claimed in any or all of the claims above comprising a means to sample the input power with a sensitive receiver to check that signal and/or the optical service channel signal are present at the input so confirming that there are no fibre breaks in the transmission path and a gating means which requires the signal to be present before turning the Raman pump source to power levels higher than eye safety limits.

23. A line amplifier equipment as claimed in claim 22 wherein the input detector is used to detect loss of signal and on detecting this, the Raman pump is shut down to preserve eye safe conditions.

24. A line amplifier equipment as claimed in claim 22 or 23 wherein a filter is placed before the input detector to block the back scattered Raman pump light to ensure the safety shut down is not compromised by the pump radiation

25. Use of a line amplifier equipment as claim 1 in an optical communications link.

26. The application of the line amplifier equipment of claim 1 to a Wavelength Division Multiplexed optical communications link to compensate the accumulated gain tilt resulting from a plurality of upstream optical amplifiers.