Power control in bidirectional WDM optical link

Abstract

A bidirectional WDM optical communications link has WDM signals sent in opposite directions along a shared optical path and using at least one common wavelength. An optical amplifier (20, 21, 22, 70, A.sub.1.sup.D, A.sub.2.sup.U, A.sub.2.sup.D) optically amplifies (144) a first WDM signal separately from a second WDM signal in the other direction. This separated optical amplification is controlled (134) according to indications of transmission quality at the common wavelength, to alter the relative optical powers of the first and second WDM signals to enable crosstalk at the common wavelength to be limited. Cross talk at the common wavelength can be improved by rebalancing relative amounts of cross talk in the different directions, to enable the capacity benefits of using a common wavelength for both directions to be obtained while using greater optical signal power. This is particularly useful where the optical power is asymmetric, such as in WDM PON systems.

Claims

1. A method of operating a bidirectional wavelength division multiplexed (WDM) optical communications link having an optical path shared by first and second WDM signals sent in opposite directions along the shared optical path and using at least one common wavelength, the method comprising: receiving indications of transmission qualities of the at least one common wavelength, of the first and second WDM signals; optically amplifying the first WDM signal while it is separated from the second WDM signal; and controlling the optical amplification according to the indications of transmission quality, to alter the relative optical powers of the first and second WDM signals for mitigating crosstalk at the common wavelength.

2. The method of claim 1, further comprising separating the first and second WDM signals before the optically amplifying the first WDM signal.

3. The method of claim 1 wherein the optically amplifying the first WDM signal occurs before the first WDM signal enters the shared optical path.

4. The method of claim 3, further comprising optically amplifying the first WDM signal after it has left the shared optical path.

5. The method of claim 1, further comprising optically amplifying the second WDM signal separately from the first WDM signal.

6. The method of claim 5, further comprising: determining which direction of transmission of the optical signals has worse transmission quality; and thereafter, performing at least one of: for a worse direction: increasing the optical power level of the respective WDM signal before it enters the shared optical path; and decreasing the optical power level of the respective optical signal after it has left the shared optical path; for a better direction: decreasing the optical power level of the respective WDM signal before it enters the shared optical path; and increasing the optical power level of the respective WDM signal after it has left the shared optical path.

7. The method of claim 1, further comprising, when the method is used in a passive optical network, using a wavelength reuse transmitter to transmit a common wavelength to form part of the second WDM signal.

8. An apparatus for controlling wavelength division multiplexed (WDM) signals in a bidirectional WDM optical communications link having an optical path shared by first and second WDM signals sent in opposite directions along the shared optical path and using at least one common wavelength, the apparatus comprising: an optical amplifier configured to amplify the first WDM signal while it is separated from the second WDM signal; and a processing circuit functioning as a controller configured to: receive indications of transmission qualities of the at least one common wavelength, of the first and second WDM signals; control the optical amplifier according to the indications of transmission quality; alter the relative optical powers of the first and second WDM signals to mitigate crosstalk at the common wavelength.

9. The apparatus of claim 8, further comprising optical circulators configured to: separate the first and second WDM signals from the shared path; and recombine them onto the shared path.

10. The apparatus of claim 8, wherein the optical amplifier comprises a booster amplifier configured to amplify the first WDM signal before it enters the shared optical path.

11. The apparatus of claim 8, wherein the optical amplifier comprises a pre-amplifier element configured to amplify the first WDM signal after it has left the shared optical path.

12. The apparatus of claim 8, wherein the optical amplifier is configured to pass the second WDM signal without amplification.

13. The apparatus of claim 8, wherein the controller is configured: to determine which direction of transmission of the optical signals has worse transmission quality; and to control the cross talk by performing at least one of the following: for that worse direction, increasing the optical power level of the respective WDM signal before it enters the shared optical path, and decreasing the optical power level of the respective optical signal after it has left the shared optical path; for the better direction, decreasing the optical power level of the respective WDM signal before it enters the shared optical path, and increasing the optical power level of the respective WDM signal after it has left the shared optical path.

14. The apparatus of claim 8, wherein the link is asymmetrical and comprises optical transmitters at each end that have different output optical power levels.

15. A bidirectional wavelength division multiplexed (WDM) optical communications link, comprising: a shared optical path shared by first and second WDM signals sent in opposite directions along the shared optical path and using at least one common wavelength; an apparatus for controlling WDM signals in the bidirectional WDM optical communications link, the apparatus comprising: an optical amplifier configured to amplify the first WDM signal while it is separated from the second WDM signal; and a processing circuit functioning as a controller configured to: receive indications of transmission qualities of the at least one common wavelength, of the first and second WDM signals; control the optical amplifier according to the indications of transmission quality; alter the relative optical powers of the first and second WDM signals to mitigate crosstalk at the common wavelength.

16. A computer program product stored in a non-transitory computer readable medium to control a processing circuit for operating a bidirectional wavelength division multiplexed (WDM) optical communications link having an optical path shared by first and second WDM signals sent in opposite directions along the shared optical path and using at least one common wavelength, the computer program product comprising software instructions which, when executed by the processing circuit, causes the processing circuit to: receive indications of transmission qualities of the at least one common wavelength, of the first and second WDM signals; optically amplify the first WDM signal whilst it is separated from the second WDM signal; and control the optical amplification according to the indications of transmission quality, to alter the relative optical powers of the first and second WDM signals for mitigating crosstalk at the common wavelength.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) How the present invention may be put into effect will now be described by way of example with reference to the appended drawings, in which:

(2) FIG. 1 shows a schematic view of parts of a communications link including apparatus for controlling WDM signals in a bidirectional WDM optical communications link,

(3) FIG. 2 shows steps in operating the apparatus of FIG. 1 or other embodiments,

(4) FIG. 3 shows another embodiment having amplification at both sides of the link,

(5) FIG. 4 shows steps in operating the apparatus of FIG. 3 or other embodiments,

(6) FIG. 5 shows steps in control according to a further embodiment with balancing

(7) FIG. 6 shows embodiment having two BIDI-OA nodes at both sides,

(8) FIG. 7 shows a schematic view of parts of the link of FIG. 6, and showing paths taken by signals,

(9) FIGS. 8 and 9 each show two halves of a flow chart of steps in controlling the WDM signals according to another embodiment,

(10) FIG. 10 shows a schematic view of another embodiment having BER as indication of transmission quality,

(11) FIG. 11 shows a schematic view of another embodiment applied to a WDM PON and having BIDI-OAs,

(12) FIG. 12 shows a schematic view of another embodiment applied to a WDM PON and having BIDI-OAs as booster amplifiers with no pre amplifiers,

(13) FIG. 13 shows a schematic view of another embodiment applied to a WDM PON and having BIDI-OAs, without a booster on the downstream signal and

(14) FIG. 14 shows a schematic view of another embodiment applied to a WDM PON and having BIDI-OAs, with only pre-amplifiers, no booster amplifiers.

DETAILED DESCRIPTION

(15) The present invention will be described with respect to particular embodiments and with reference to certain drawings but the invention is not limited thereto but only by the claims. The drawings described are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn to scale for illustrative purposes.

ABBREVIATIONS

(16) AWG Array Waveguide Gratings

(17) BER Bit Error Rate

(18) BIDI-OA Bidirectional Optical Amplifier

(19) EDFA Erbium Doped Fiber Amplifier

(20) Optical Circulator

(21) OLT Optical Line Termination

(22) ONT Optical Network Termination

(23) OSXR Optical Signal to cross-talk Ratio

(24) PON Passive Optical Network

(25) RBS Rayleigh Backscatter

(26) RIN Relative Intensity Noise

(27) RN Remote Node

(28) RX Receiver

(29) TX Transmitter

(30) Xtalk Cross-talk

(31) WDM Wavelength Division Multiplexing

(32) WR Wavelength reuse

DEFINITIONS

(33) Where the term comprising is used in the present description and claims, it does not exclude other elements or steps and should not be interpreted as being restricted to the means listed thereafter. Where an indefinite or definite article is used when referring to a singular noun e.g. a or an, the, this includes a plural of that noun unless something else is specifically stated.

(34) Elements or parts of the described apparatus, nodes or networks may comprise logic encoded in media for performing any kind of information processing. Logic may comprise software encoded in a disk or other computer-readable medium and/or instructions encoded in an application specific integrated circuit (ASIC), field programmable gate array (FPGA), or other processor or hardware.

(35) References to nodes can encompass any kind of switching node, not limited to the types described, not limited to any level of integration, or size or bandwidth or bit rate and so on.

(36) References to software can encompass any type of programs in any language executable directly or indirectly on processing hardware.

(37) References to controllers, processors, hardware, processing hardware or circuitry can encompass any kind of logic or analog circuitry, integrated to any degree, and not limited to general purpose processors, digital signal processors, ASICs, FPGAs, discrete components or logic and so on.

(38) References to a processor are intended to encompass implementations using multiple processors which may be integrated together, or co-located in the same node or distributed at different locations for example.

INTRODUCTION

(39) By way of introduction to the embodiments, how they address some issues with conventional designs will be explained. Current techniques have some problems as follows. As downstream and upstream wavelengths are shared in wavelength reuse systems, any cross-talk source between the two opposite directions must be taken under control. For example, cross-talk can arise from bad isolation in optical components or optical reflections.

(40) This makes optical amplification critical. For instance, if a bidirectional optical amplification scheme were to be placed between an OLT and a feeder fiber in a WDM PON system, with optical amplifiers acting as booster and preamplifier, respectively, any increase the gain in order to increase the power budget can have effects as follows. If, as it actually happens, the isolation of the output optical circulator is not perfect, a significant amount of power generated by the downstream booster amplifier interferes with the upstream signal at the input of the upstream preamplifier, generating cross-talk. The problem is quite serious considering that, in practical systems, downstream output power is much higher than upstream input power at the pre-amplifier.

(41) Accordingly, to address these issues, various performance optimization methods and apparatus in bidirectional optically amplified systems are shown, some passive, some active, some using remodulation. They can be applied for example in Wavelength Division Multiplexing (WDM) access and mobile backhauling networks, based on wavelength reuse techniques to support symmetric bit rate traffic at low cost, to maximize the spectral efficiency and mitigate channel asymmetry effects, e.g. due the different propagation delay in the two directions.

(42) In summary, some examples provide a BIDIrectional Optical Amplification (BIDI-OA) scheme for a wavelength reuse bidirectional system. Some examples exploit BIDI-OA applied at both sides of the trunk fiber to significantly increase the power budget. Some examples provide a power setting method to optimize the Optical Signal to Xtalk Ratio (OSXR) in the upstream and downstream directions. A bidirectional amplification node can be realized by means of two optical circulators and two EDFAs as shown in FIG. 6 and various other figures described below in more detail.

(43) Simplified BIDI-OA architectures where some optical amplifiers are missed are possible (FIGS. 12, 13 and 14). For example, the preamplifiers-only variant is useful in case of very short reach (less than 10 km) but high concentrated losses (e.g. due to re-use of installed PON infrastructure).

(44) FIGS. 1 and 2, a First Embodiment

(45) FIG. 1 shows a schematic view of parts of a communications link including apparatus 10 for controlling WDM signals in a bidirectional WDM optical communications link having a shared optical path shared by first and second WDM signals sent in opposite directions along the shared optical path and using at least one common wavelength. Transmitters 40 are provided to supply wavelength which are wavelength division multiplexed by a mux/demux device 50 and fed onto the bidirectional shared optical path. The WDM mux/demux also separates and demultiplexes the optical signals in the other direction, and is coupled to provides these to receivers 60. An optical amplifier 70 is provided to amplify the first WDM signal separately from the second WDM signal, by separating the upstream and downstream signals using optical circulators 80, 90, at either side of the optical amplifier. In principle the OC 80 may not be needed if the WDM mux demux 50 keeps physically separate paths for the WDM signals in the two directions.

(46) A controller 30 is configured to receive indications of transmission qualities of the at least one common wavelength, of the first and second WDM signals, and to control the optical amplifier according to the indications of transmission quality, to alter the relative optical powers of the first and second WDM signals to enable crosstalk at the common wavelength to be mitigated. The indications of transmission quality can come from the receivers at each end of the optical link as shown and can be based on detecting the optical quality or can be based on a quality of the electrical signal from the receiver, either before or after the received signal is digitised, or before or after some digital processing such as FEC for example. A bit error rate is an example of transmission quality.

(47) By controlling optical amplification in one direction separately from the other, the relative optical powers in the two directions can be controlled, whether the second direction is controlled or not. Thus the cross talk can be limited or balanced in the two directions, which may provide an overall benefit even if it is actually increased in one direction. Thus performance can be improved, so that the benefits of optical amplification can be combined with the benefits of wavelength reuse. The controller can be implemented by a processor of any kind.

(48) FIG. 2 shows steps in operating the apparatus of FIG. 1 or other embodiments. At step 100, traffic is sent in first and second WDM signals in opposite directions over the shared optical path, using at least one common wavelength. At step 110, indications of transmission qualities at least at the common wavelength are obtained for the first and second WDM signals. The controller receives these indications at step 120. At step 134, the controller outputs control signals for the controlling the optical amplifier to control the optical amplification separately in the two directions according to the received transmission qualities. At step 144, the optical amplifier responds to the control signals to change the amplification of the first WDM signal whilst it is separated from the second WDM signal.

(49) Thus cross talk can be controlled, and even where cross talk is not the limiting factor, the better balancing of the optical powers in the two directions can help enable optical power budgeting to be more tightly controlled and enable reduced optical power margins for example, or reduced capital costs or reduced power consumption in operation.

(50) FIGS. 3 and 4, a Further Embodiment

(51) FIG. 3 shows a schematic view of parts of a communications link similar to FIG. 1 but having amplification at both sides of the link. Items corresponding to those shown in FIG. 1 are illustrated with the same reference numerals. In this case there is a booster optical amplifier 21 for amplifying the WDM signal before it enters the shared optical path, in this case a transmission fiber. The optical amplification is carried out whilst the first WDM signal is separated from the second WDM signal in the other direction. An optical pre-amplifier 22 is shown at the far side of the fiber, for amplifying the WDM signal after leaving the fiber transmission fiber, separately from the WDM signal in the other direction. Controllers 30 are provided at each end of the link, for providing control signals to the booster optical amplifier and the optical pre-amplifier respectively. In principle the WDM signals can be passed along more than one link, and can pass along other links or sections which are shared bidirectional or unshared unidirectional, so references to the shared optical path can mean any part or parts of the entire optical path shared by the WDM signals.

(52) FIG. 4 shows steps in operating the apparatus of FIG. 3 or other embodiments. Traffic is sent in first and second WDM signals in opposite directions over the shared optical path, using at least one common wavelength. At step 122, indications of transmission qualities at least at the common wavelength are obtained for the first and second WDM signals and received by the controllers at the two ends. At step 136, the controller at the first end outputs control signals for controlling the optical amplifier for amplifying the first WDM signal whilst it is separated from the second WDM signal sent in the other direction. Again this control is made according to the received transmission qualities, to enable optimization for example by rebalancing of the transmission qualities in the two directions. At step 144 the booster optical amplifier changes boost amplification of the first WDM signal according to the control signals, before the first WDM signal enters the shared optical path.

(53) At step 150 the controller at the second end outputs control signals for controlling the optical amplification of the pre-amplifier located after the transmission fiber (cross talk location) according to the change in amplification at the first end. This involves controlling the amplification separately from the control of the second WDM signal in the other direction. This can enable the changes in power to be compensated so that the receiver is not affected or is less affected by any changes in the amplification at the booster amplifier. At step 154, the optical pre-amplifier at the second end implements the change in amplification indicated in the control signals.

(54) FIG. 5, Steps in Control According to a Further Embodiment with Balancing

(55) FIG. 5 shows steps in operating the apparatus of FIG. 1, or FIG. 3 or other embodiments where there is control of optical amplification in one direction or in both directions separately. At step 125 a controller at either end receives indications of transmission qualities and decides which direction has worse transmission quality. If the first WDM signal is worse, at step 160 control signals are generated to increase the optical amplification for the first WDM signal before it enters the shared optical path where the cross talk occurs, and to decrease the optical amplification after leaving the shared optical path, or: the optical amplification of the second WDM signal is decreased before it enters the shared optical path and increased in amplification after leaving the shared optical path where the cross talk occurs.

(56) If the second WDM signal is worse, at step 170 control signals are generated to increase the optical amplification for the second WDM signal before it enters the shared optical path, and to decrease the optical amplification after it leaves the shared optical path, or: the optical amplification of the first WDM signal is decreased before it enters the shared optical path and increased in amplification after leaving the shared optical path. At step 180 the optical amplifiers at both ends implement the changes in amplification according to the control signals. In each of the cases, the balance of transmission quality between the two directions can be improved, and thus overall the performance can be increased as there is no longer any excess quality margin in either direction. At step 190 the control steps can be repeated if doing so continues to improve the overall transmission quality, or can be ceased otherwise.

(57) FIGS. 6, 7, Embodiment Having Two BIDI-OA Nodes at Both Sides

(58) FIGS. 6 and 7 show schematic views of apparatus having two BIDI-OA nodes at both the sides of a trunk fiber. At one side of the transmission fiber 340, a transceiver has a transmitter TX 380, and receiver RX 370, for each of a number of wavelength channels. An optical circulator 360 is provided for combining or separating the signals of the two directions. The bidirectional path form here is coupled to an AWG 350, for WDM mux and demux. The bidirectional WDM signal path is then coupled to the apparatus for bidirectional optical amplification, involving optical circulators OC for separating and recombining the signals of the two directions, to enable separate control of amplification by amplifiers A.sub.1.sup.D and A.sub.1.sup.U respectively.

(59) At the far side of the transmission fiber 340 there is similar apparatus for bidirectional amplification, an AWG 330, for WDM mux and demux, and a transceiver having a circulator 320, and for each wavelength, a receiver 300, and transmitter 310. The bidirectional amplification has separate control of amplification by amplifiers A.sub.2.sup.D and A.sub.2.sup.U respectively.

(60) A controller (not shown explicitly here) can be used to set output power by means of gains G.sub.1, G.sub.2 respectively of the EDFAs installed in the BIDI-OA nodes. In FIG. 6, superscripts D and U indicate downstream direction and upstream directions, respectively. Superscript DB indicates downstream backscatter. Subscripts 1 or 2 represent near or far ends, and subscripts 4 to 9 as shown represent locations along the optical path as shown in FIG. 7. In the following discussion we will assume that: Amplifiers A.sub.1.sup.D and A.sub.2.sup.U act as boosters for downstream and upstream WDM signals respectively. They work at constant output channel power. The amplifiers A.sub.1.sup.U and A.sub.2.sup.D act as pre-amplifiers for the upstream and downstream WDM signal respectively. They work at constant gain in some examples, or in some examples they have active control of their gain. All the optical circulators have equal insertion loss IL.sub.circ. All the Array Waveguide Gratings (AWG) have equal insertion loss IL.sub.AWG The fiber Rayleigh back scattering ratio BRR is constant for trunk fiber lengths longer than 15 km.

(61) FIG. 7 shows a schematic view of parts of the link of FIG. 6, with additional wavy lines to show the path taken by an upstream signal (right to left in this case), a downstream signal (left to right in this case), crosstalk in the upstream direction caused by backscatter from the downstream signal, and cross talk in the downstream direction caused by backscatter from the upstream signal.

(62) With reference to FIG. 7, optical signal power, optical reflection noise power and the optical signal to cross-talk noise ratio values at the OLT upstream receiver are:
P.sub.1.sup.U [dBm]=P.sub.8.sup.UG.sub.1.sup.UIL.sub.LinkIL.sub.AWG4IL.sub.circ[Eq. 1]
P.sub.1.sup.DB [dBm]=P.sub.8.sup.D+BRR+G.sub.1.sup.UIL.sub.AWG4IL.sub.circ[Eq. 2]
OSXR.sub.OTL [dB]=P.sub.1.sup.UP.sub.1.sup.DB=P.sub.8.sup.UP.sub.5.sup.DIL.sub.Link+BRR[Eq. 3]

(63) Note that:

(64) The input power signal P.sub.1.sup.U ([Eq. 1]) is proportional to both the channel output power provided by A.sub.2.sup.U booster (P.sub.8.sup.U) and the gain of the A.sub.1.sup.U pre-amplifier (G.sub.1.sup.U). The OSXR.sub.OLT ([Eq. 3]) is proportional to the channel output power provided by the A.sub.2.sup.U booster (P.sub.8.sup.U) and inversely proportional to the per-channel output power provided by the A.sub.1.sup.D booster (P.sub.5.sup.D).

(65) The same values at the Optical Network Termination (ONT) downstream receiver are:
P.sub.12.sup.D [dBm]=P.sub.5.sup.U+G.sub.2.sup.DIL.sub.LinkIL.sub.AWG3IL.sub.circIL.sub.splitter[Eq. 4]
P.sub.12.sup.UB [dBm]=P.sub.8.sup.UBRR+G.sub.2.sup.DIL.sub.AWG3IL.sub.circIL.sub.splitter[Eq. 5]
OSXR.sub.ONT [dB]=P.sub.12.sup.DP.sub.12.sup.UB=P.sub.5.sup.DP.sub.8.sup.UIL.sub.Link+BRR[Eq. 6]

(66) Note that:

(67) The input signal power P.sub.12.sup.D ([Eq. 4]) is proportional to both the channel output power provided by A.sub.1.sup.D booster (P.sub.5.sup.D) and the gain of the A.sub.2.sup.D pre-amplifier (G.sub.2.sup.D). The OSXR.sub.ONT ([Eq. 6]) is proportional to the channel output power provided by A.sub.2.sup.U booster (P.sub.5.sup.D) and inversely proportional to the per-channel output power provided by A.sub.1.sup.D booster (P.sub.8.sup.U).

(68) In a wavelength reuse system the upstream signal is more penalized with respect to the downstream signal due residual modulation due the downstream light coming from the central office and the double transmission distance. So, setting the power levels in order to have the same OSXR in downstream and upstream would result into unbalanced BER performance with downstream BER.sub.OLT (measured at the ONT receiver) better than upstream BER.sub.ONT (measured at the OLT receiver). For this reason, we will try to balance BER rather than OSXR values.

(69) FIGS. 8 and 9, Flow Chart

(70) Considering the previous equations regarding received powers, OSXRs and amplifier output powers and gains, the following method is proposed to set output power and gain values for the optical amplifiers.

(71) Set the optical gains of the pre-amplifiers (A.sub.1.sup.U and A.sub.2.sup.D) in order to meet predetermined received powers values (e.g. nominal receiver sensitivities or ONT injection powers).

(72) Gradually change the output powers of booster amplifiers A.sub.1.sup.D and A.sub.2.sup.U, according to the flowchart in FIGS. 8 and 9, in order to jointly minimize the bit error rate (BER) of the worst OLT receiver BER.sub.OLT and the worst ONT receiver BER.sub.ONT.

(73) According to the previous equations, setting the ratio between the output powers we univocally define the OSXR at both the receivers. The method also applies to simplified BIDI-OA variants such those shown in FIGS. 11 to 14.

(74) The working principle of the method, detailed in a flow chart spread across FIGS. 8 and 9, is outlined in the following.

(75) The amplifiers are initialized setting a pre-determined arbitrary output power level on the boosters and setting the preamplifiers to meet ONT and OLT input power specifications.

(76) After T=Tamp+Tmeas seconds, where Tamp is the time to reach the amplifiers steady state and Tmeas is the time window necessary for accurate BER measurement, we read the BER values on both the OLT and ONT receivers and we check if they are different or not.

(77) If the BERs are equal the method stops, otherwise we look at what is the maximum BER between BER.sub.OLT and BER.sub.ONT. This much is shown in FIG. 8. Then, moving to FIG. 9, in the first box of FIG. 9, we test to see if BER.sub.OLT is the maximum one. If yes, we increment by P [dB] the downstream booster output power P.sub.5.sup.D as shown in the right hand path of FIG. 9. In such a way, OSNR.sub.ONT improves and OSNR.sub.OLT impairs.

(78) Then, we decrement by P [dB] the downstream preamplifier gain G.sub.2.sup.D in order to maintain the same optical power at the ONT input, and then wait for the effects to settle, before reading BERs again.

(79) If, instead, BER.sub.ONT is the maximum one, we take the left path in FIG. 9 and increment by P [dB] the upstream booster output power P.sub.8.sup.U. Such a way OSNR.sub.OLT improves and OSNR.sub.ONT impairs. Then we decrement by P [dB] the upstream preamplifier gain G.sub.1.sup.U in order to maintain the same optical power at the OLT input.

(80) After a wait for T seconds to allow for settling, we read the new BER values. If the new maximum value B2 is greater than the previous one B1, then the method hasn't provided the expected benefit so we come back to the previous state and the method stops. Otherwise a further method iteration is run. Then as shown, the method stops, though of course further iterations could be run.

(81) This optical amplification scheme and method enable combining the high spectral efficiency typical of wavelength reuse systems with the long distance reach typical of regular WDM systems where different wavelengths or fibers are used instead for the two propagation directions.

(82) FIG. 10, Embodiment Having BER as Indication of Transmission Quality

(83) FIG. 10 shows a schematic view of apparatus similar to that of FIG. 1, but with a FEC part 62 coupled to the receivers. This outputs bit error indications to a counter 64, which determines a rate of errors and outputs a bit error rate BER indication to the controller 30, as an example of an indication of transmission quality. As before, the controller can determine how to change the amplification based on the BER, and can output control signals to the bidirectional optical amplifier 20 which can amplify each direction of the bidirectional optical path separately.

(84) FIG. 11, WDM PON Example with BIDI-OA

(85) FIG. 11 shows an example similar to that of FIG. 6, but applied to a WDM PON, and corresponding parts are shown with corresponding reference signs. In this case, one end of the link is an OLT in a central office 200. The other end of the link is at a remote node RN 220, having the bidirectional amplifier, and AWG 222. The transmitter is in the form of a wavelength reuse transmitter WRTX 234 in an ONT 230, having a power splitter/combiner 232. Any kind of wavelength reuse transmitter can be used, for example a loop back optical amplifier, or a reflective type, as would be known to those skilled in the art.

(86) FIG. 12: WDM PON with BIDI-OA with Only Booster Amplifiers

(87) FIG. 12 shows an example similar to that of FIG. 11, but with no pre-amplifiers only booster amplifiers. Corresponding parts are shown with corresponding reference signs. This can simplify the control, but reduce the flexibility.

(88) FIG. 13: WDM PON with BIDI-OA without Booster on Downstream Signal

(89) FIG. 13 shows an example similar to that of FIG. 11, but with no booster amplifier on the downstream path. Corresponding parts are shown with corresponding reference signs. This is feasible because the downstream signal typically has a much higher power level anyway. In another example, a controllable optical attenuator could be used at least for the downstream path to provide some control.

(90) FIG. 14: WDM PON with BIDI-OA with Only Pre-Amplifiers

(91) FIG. 14 shows an example similar to that of FIG. 11, but with no booster amplifiers on both the downstream path and the upstream path. Corresponding parts are shown with corresponding reference signs. This can be useful for example for short reach and high power budget systems. Although there is less direct control of any cross talk, the better balancing of the optical powers in the two directions can help enable optical power budgeting to be more tightly controlled and enable reduced optical power margins for example, and reduced capital costs and reduced power consumption in operation.

CONCLUDING REMARKS

(92) A bidirectional WDM optical communications link has WDM signals sent in opposite directions along a shared optical path and using at least one common wavelength. An optical amplifier 20, 21, 22, 70, A.sub.1.sup.D, A.sub.2.sup.U, A.sub.1.sup.U, A.sub.2.sup.D optically amplifies 144 a first WDM signal separately from a second WDM signal in the other direction. This separated optical amplification is controlled 134 according to indications of transmission quality at the common wavelength, to alter the relative optical powers of the first and second WDM signals to enable crosstalk at the common wavelength to be limited. Cross talk at the common wavelength can be improved by rebalancing relative amounts of cross talk in the different directions, to enable the capacity benefits of using a common wavelength for both directions to be obtained while using greater optical signal power. This is particularly useful where the optical power is asymmetric, such as in WDM PON systems.

(93) Other variations and examples can be envisaged within the claims.