WO2001080462A2

WO2001080462A2 - System and method for adjusting transmission power of respective light beams in a wavelength-division multiplexed network

Info

Publication number: WO2001080462A2
Application number: PCT/US2001/011934
Authority: WO
Inventors: Richard A. Barry; Naimish Patel; Eric Swanson; Joseph S. Zahavi
Original assignee: Sycamore Networks, Inc.
Priority date: 2000-04-13
Filing date: 2001-04-12
Publication date: 2001-10-25
Also published as: AU2001251576A1; WO2001080462A3

Abstract

A laser beam power adjustment system adjusts the power of each of the laser beams transmitted by source nodes to respective destination nodes in a network in which information is transmitted using wavelength-division multiplexing techniques. Initially, the laser beam power adjustment system adjusts the power so that, for each source node, all of the light beams used to transfer information are of sufficient power to ensure that, when the light beams are received at the respective destination nodes, the light beams will be received with at least a power level that is sufficient to accomodate the sensitivity requirements of the receiving transducer at the destination node, thereby to provide satisfactory communications between the source and destination nodes. Thereafter, for each light beam transmitted from a source node to a respective destination node, the laser beam power adjustment system adjusts the power to accomodate likely coherent and incoherent crosstalk of other light beams with the respective light beam.

Description

SYSTEM AND METHOD FOR ADJUSTING TRANSMISSION POWER OF

RESPECTIVE LIGHT BEAMS IN A WAVELENGTH-DIVISION

MULTIPLEXED NETWORK

FIELD OF THE INVENTION

The invention relates generally to the field of digital data networks and more particularly to networks in which digital data is transferred using wavelength division multiplexing technigues. The invention particularly provides an arrangement for controlling power of information transfer channels in such wavelength division multiplexing ("WDM") networks.

BACKGROUND OF THE INVENTION

Digital data networks are used to transfer data and other information among a plurality of devices, such as computers for processing, mass storage subsystems for storage and retrieval, connections to other networks such as the public telephony system and the like. As networks become larger, and with ever increasing numbers of devices being connected therein for transmitting and receiving information, the demand for information transfer bandwidth increases correspondingly. Since light beams have a higher information carrying capacity than electronic signals, optical information transfer techniques, in which information is transferred using beams of light through optical fibers are being increasingly used. In addition, since optical fibers can typically carry light beams of many different wavelengths concurrently, wave-division multiplexing ("WDM") techniques, in which information is transferred through a single optical fiber using a plurality of beams of light concurrently, are being employed to further increase the bandwidth.

Several problems arise, however, in connection with use of wavelength-division multiplexing in connection with transmission of information through a network. Typically, in a WDM network, each light beam is generated by a laser, which receives an electrical signal from electronic circuitry that processes or otherwise generates information for transmission. Each light beam is transmitted through an optical fiber to a transducer, which receives the respective light beam and converts it to electrical form for provision to electronic circuitry that will use the information that is transmitted. The light beam as provided to the receiving transducer needs to be of at least some minimum power level in order for it

(that is, the transducer) to be able to transduce the light beam to electrical form, with at least some maximum bit error rate. Generally, if the power level of the light beam as provided to the transducer increases above the minimum power level, the bit error rate can be reduced somewhat. On the other hand, if the power of the light beam as provided to the transducer increases far enough, the transducer will saturate, which can also lead to reception errors. Accordingly, the power of light beam as provided to the receiving transducer preferably will be below a maximum power level to ensure that it is not saturated. Thus, the power with which each light beam is transmitted needs to be adjusted to ensure that the power as provided to the respective transducer is within the maximum and minimum power at the transducer, so that the light beam can be adequately transduced and the transducer will not be saturated. In the adjustment process, issues such as amplification which may be provided in the optical path between the transmitter and transducer and attenuation of the light beam in the optical fiber over the optical path need to be taken into account when determining the power with which a light beam is to be transmitted.

Although adjusting the power for a single light beam, that is, a light beam comprising a single wavelength, in a network may be relatively uncomplicated, significant complications arise in a WDM network, in which multiple light beams are transmitted through a single optical fiber, and further in which light beams of the same wavelength may be used in different parts of the network. In such a network, care must be taken to ensure that, for amplifiers which are typically provided in the network, the power for each respective light beam at the inputs of the respective amplifiers is not high enough that it (that is, the respective light beam) will be allocated an undue or unnecessary share of power boost by the respective amplifiers. In addition, if multiple light beams are to have the same wavelength, care must be taken that the power of one of the light beams as used in one part of the network is not so great that it will leak into the other part of the network in which a light beam of the same wavelength is to be used, thereby to avoid crosstalk between the first light beam and the second light beam of the same wavelength.

SUMMARY OF THE INVENTION The invention provides a new and improved system and method facilitating adjustment of power with which each respective light beam is transmitted in a WDM network. In brief summary, the invention provides a laser beam power adjustment system that adjusts the power of each of the laser beams transmitted by source nodes to respective destination nodes in a network in which information is transmitted using wavelength-division multiplexing techniques. Initially, the laser beam power adjustment system adjusts the power so that, for each source node, all of the light beams used to transfer information are of sufficient power to ensure that, when the light beams are received at the respective destination nodes, the light beams will be received with at least a power level that is sufficient to accommodate the sensitivity requirements of the receiving transducer at the destination node, thereby to provide satisfactory communications between the source and destination nodes. Thereafter, for each light beam transmitted from a source node to a respective destination node, the laser beam power adjustment system adjusts the power to accommodate likely coherent and incoherent crosstalk of other light beams with the respective light beam.

BRIEF DESCRIPTION OF THE DRAWINGS

This invention is pointed out with particularity in the appended claims. The above and further advantages of this invention may be better understood by referring to the following description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a functional block diagram of a WDM network providing a system facilitating adjustment of power with which respective light beams are transmitted, constructed in accordance with the invention; FIG. 2 is a functional block diagram of portions of the network depicted in FIG.l, which is helpful in understanding the power adjustment system; and

FIG. 3 is a flowchart depicting operations of the power adjustment system used in the network depicted in FIGS. 1 and 2.

DETAILED DESCRIPTION OF AN ILLUSTRATIVE EMBODIMENT

FIG. 1 is a functional block diagram of a network 10 in which digital data is transferred using wavelength- division multiplexing (WDM) techniques, and providing a system facilitating adjustment of power with which respective light beams are transmitted, constructed in accordance with the invention. With reference to FIG. 1, network 10 includes a plurality of nodes 11(1) through 11 (N) generally identified by reference numeral 111 (n) ) , each comprising information source/destination 12 (n) and an optical network node 13 (n) to communications media 14 (n) which interconnect the nodes 11 (n) to form the network 10. The topology of the network 10 is in the form of a ring, and thus each optical network node 13 (n) can receive information from a preceding node ll(n-l) in the network and transmit information to a successive node ll(n+l) in the network, thereby to facilitate transfer of information in a clockwise direction, as shown in FIG. 1, through the network 10. In one embodiment, there are actually two communications media provided to interconnect each node 11 (n) with the successive node ll(n+l) in the network, and two communications media to interconnect each node 11 (n) with the preceding node ll(n-l) in the network. In that embodiment, one set of communications media interconnecting the respective node 11 (n) and successive and preceding nodes ll(n+l) and ll(n-l) is used to transfer information in a clockwise direction in the network 10, and the other set of communications media interconnecting the respective node 11 (n) and successive and preceding nodes ll(n+l) and ll(n-l) is used to transfer information in a counter-clockwise direction in the network 10. Since the operations in connection with transfer of information in the clockwise and counterclockwise directions are symmetric, and components in the respective nodes 11 (n) used to transfer information are also symmetric as between transfer in the clockwise and counter-clockwise directions, in the following, operations in connection with transfer in one direction and components therefor will be discussed, with the understanding that such operations and components can be used in connection with transfer of information in either the clockwise or the counter-clockwise direction.

With continued reference to FIG. 1, the information source/destination 12 (n) of each node 11 (n) may be in the form of any equipment which can generate digital data for transfer over a network and/or which may need to receive and store or use digital data. Such equipment may include, for example, an individual digital computer or mass storage subsystems, local area networks ("LANs") including digital computers and/or mass storage subsystems together with other components such as network printers useful in LANs, wide area networks ("WANs") including, for example, a plurality of LANs, the public switched telephony network ("PSTN"), and so forth. The optical network node 13 (n) of the respective node 11 (n) can receive information from the information source/destination 12 (n) of the respective node 11 (n) to be transferred over the network to another node 11 (n' ) (n'≠n) as the destination node, and transmit the information over the communications medium 14 (n) connected thereto. Information may be transmitted using any convenient information transfer protocol, including continuous transmission, in message packets or any other convenient form. The information transmitted by the optical network node 13 (n) will be transferred through each successive optical network node 13 (n' ' ) , 13 (n' ' ' ) , ... (n' ' , n'''≠n, n' ' ) as necessary until it (that is, the information) reaches the destination node 11 (n' ) . When the information reaches the destination node 11 (n' ) , the optical network node 13 (n' ) will retrieve the information from the communications medium 14(n'-l) connected thereto for provision to its respective information source/destination 12 (n' ) . Generally, each node 11 (n) in the network 10 can operate both as a source node, to generate and transmit information over the communications media 14 (n) to another node, and as a destination node, to receive and use information transmitted thereto from a source node, and so the optical network node 13 (n) in the network 10 can operate to both transmit information over, and receive information from communications media 14 (n) and 14 (n-1) , connected thereto. However, it will be appreciated that some nodes may operate only as source nodes or as destination nodes, and for such nodes the optical network node may not need to be able to receive or transmit information, respectively, over the network, except as will be described below in connection with the control channel. As noted above, information is transferred through the network 10 using WDM techniques. The communications media 14 (n) are in the form of optical fibers which transfer information in the form of light beams. The communications media 14 (n) effectively transfer information between each pair of source and destination nodes 11 (nS), 11 (nD) , with the optical network node 13 (nS) of the source node 11 (nS) injecting a light beam carrying the information into the communications medium 14 (nS) connected thereto, and the optical network node 13 (nD) of the destination node 11 (nD) ejecting the light beam carrying the information from the communications medium 14(nD-l) connected thereto. The optical network node 13 (nD) preferably ejects the light beam from the communication path provided by the communication medium so as to minimize, or at least significantly reduce, the amount of energy of the light beam which might be transferred over the succeeding communications medium 14 (nD) .

Generally, information between each pair of source and destination nodes 11 (n) , 11 (n' ) will be transferred using a light beam of a particular wavelength. Light beams of different wavelengths can be used to transfer information between different pairs of source and destination nodes 11 (nS), 11 (nD) ; 11 (nS' ) , 11 (nD' ) ; ll(nS''), 11 (nD' ' ) , and so forth, thereby providing wavelength-division multiplexing of the communications between respective pairs of such nodes. In addition, if the volume of information transferred between a pair of source and destination nodes 11 (nS) , 11 (nD) , is sufficiently large, or if the rate even for a relatively brief time is sufficiently high, it may be convenient to allow the pair of source and destination nodes to use multiple wavelengths to transfer the information. Essentially, each wavelength provides a channel over which the respective pairs of source and destination nodes 11 (nS) and 11 (nD) can transfer information.

The wavelengths of the light beams are chosen so that communications between one pair of source and destination nodes 11 (nS), 11 (nD) will not interfere with communications between another pair of source and destination nodes 11 (nS'), 11 (nD' ) , and, accordingly, preferably the same wavelength will not be used at least if a portion of the path over the communications media for the light beam between the one pair of source and destination nodes 11 (nS) , 11 (nD) , does not overlap with the path between the other pair of source and destination nodes 11 (nS' ) , 11 (nD' ) . On the other hand, if a destination node 11 (nD) , when ejecting the light beam can provide that, even if some energy from the ejected light beam seeps into the succeeding communications medium, the amount of energy is sufficiently low that it would not interfere with communications between another pair of source and destination nodes 11 (nS' ) , 11 (nD' ) the path between which does not overlap the path between the first pair of source and destination nodes 11 (nS) and 11 (nD) , the light of the same wavelength may be used in connection with communications between the other pair of source and destination nodes 11 (nS'), 11 (nD' ) . The invention provides an arrangement for adjusting the power with which each source node 11 (nS) transmits each light beam is transmitted thereby, so as to provide that

(i) the light beam as received at the destination node 11 (nD) is within a range in which, at the low end of the range, the light beam will be of sufficient minimum power, when it arrives at the transducer of the destination node 11 (nD) that converts the light beam to electrical form, to meet the sensitivity and noise requirements of the transducer, yet not be of so high a power as to saturate the transducer; (ii) if an amplifier is provided in the optical path between each source node 11 (nS) and respective destination node 11 (nD) , the amplifier is maintained in its linear range so as to avoid generation of undesirable signal artifacts, (iii) for each destination node 11 (nD) for a light beam of one wavelength λi, incoherent crosstalk with light beams of other wavelengths λ₂, λ₃ ... is not so great as to interfere with communications over the light beam of wavelength λι_; and (iv) coherent crosstalk as between light beams of the same wavelength λi, used in multiple portions of the network is not so great as to interfere with communications in each of the respective portions.

These will be further described in connection with FIG. 2.

FIG. 2 schematically depicts a portion of the network 10 depicted in FIG. 1, in somewhat greater detail. FIG. 2 particularly depicts details of an illustrative optical network node, namely optical network node 13(1) of node 11(1), in network 10. Other optical network nodes 13 (n) (n≠l) are generally similar to optical network node 13(1), and will not be described in detail. With reference to FIG. 2, the optical network node 13(1) of node 11(1) essentially comprises two portions, including a transmitter portion 13(1) (T) and a receiver portion 13(1) (R) . The transmitter portion 13(1) (T) receives information in electrical form from information generating circuitry comprising, for example, the information source/destination 12(1) of the node 11(1), converts the information to one or more light beams for transmission to respective destination nodes 11 (nD) , and transmits the light beams over the communications medium 14(1). The receiver portion 13(1) (R) receives light beams over the communications medium 14 (N) from node 11 (N) and, for those light beams of the wavelengths for which the node 11(1) is the destination node, diverts the respective light beams and transduces them to electrical form for provision to information utilization circuitry comprising the information source/destination 12(1). For light beams of wavelengths for which the node 11(1) is not the destination node, the receiver portion 13(1) (R) allows those light beams to pass therethrough undiverted onto the communications medium 14(1).

As noted above, the transmitter portion 13(1) (T) receives information in electrical form from information generating circuitry comprising, for example, the information source/destination 12(1) of the node 11(1), converts the information to one or more light beams for transmission to respective destination nodes 11 (nD) , and transmits the light beams over the communications medium 14(1). The transmitter portion 13(1) (T) includes one or more lasers 20(1) through 20 (M) (generally identified by reference numeral 20 (m) ) , an associated variable optical attenuator 21 (m), an optical multiplexer 22, a booster amplifier 23 and an optical add multiplexer 24. The optical add multiplexer 24 actually forms, with an optical drop multiplexer described below in connection with the receiver portion 13(1) (R) , a conventional optical add/drop multiplexer ("OADM"). Each laser 20 (m) receives an electrical signal representing information to be transmitted to another node 11(1) as a destination node 11 (nD) and generates in response thereto a modulated light beam of a predetermined wavelength and power. The respective variable optical attenuator 21 (m) receives the light beam generated by the associated laser 20 (m) and attenuates the light beam to provide a light beam at a power level that is selected to generally optimally satisfy items (i) through (iv) above. The optical multiplexer 22 receives the attenuated light beams and couples all of them onto a single optical fiber for provision to the booster amplifier 23. The booster amplifier 23, in turn, amplifies the light beams provided thereto by the optical multiplexer 22 to provide a predetermined amplification, or "gain," with all of the light beams receiving the same gain. The optical add multiplexer 24 then couples the amplified light beams as- received from the booster amplifier onto the communications medium 14(1), thereby to transfer them (that is, the light beams) to the node 11(2) along with light beams of wavelengths for which the node 11(1) is not the destination node.

The receiver portion 13(1) (R) of the optical network node 13(1) of node 11(1) includes an optical drop multiplexer 30, a pre-amplifier 31, an optical demultiplexer 32 and one or more light beam-to-electrical signal transducers 33(1) through 33 (T) (generally identified by reference numeral 33 (t) ) . As noted above, the optical drop multiplexer 30 forms, with the optical add multiplexer 24 in the transmitter portion 13(1) (T) , a conventional optical add/drop multiplexer. The optical drop multiplexer 30 diverts light beams that it receives of respective wavelengths for which the node 11(1) is the destination node and couples them to the pre-amplifier 31, which amplifies the light beams of all of the wavelengths provided thereto by the optical drop multiplexer 30 by a predetermined gain. The pre-amplifier 31, in turn, provides the amplified light beams to the optical demultiplexer 31, which disambiguates the respective light beams and couples them to respective transducers 33 (t). Each transducer 33 (t), in turn, generates a respective electrical signal in response thereto for provision to the information utilization circuitry.

As noted above, each of the other nodes 11 (n) (n≠l) which can operate as both source and destination nodes 11 (nS) and 11 (nD) will have optical network node components similar to those described above in connection with node 11(1). It will be appreciated, however, if a particular node 11 (n) operates only as a source node, and not as a destination node, its optical network node 13 (n) need not have a receiver portion 13 (n) (R) . Similarly, if a particular node 11 (n' ) operates only as a destination node, and not as a source node, its optical network node 13 (n') need not have a transmitter portion 13 (n') (T) . In addition, if a node 11 (n' ' ) operates as a source node using only a single light beam, its transmitter portion 13 (n' ' ) (T) need only have a single laser and variable optical attenuator and no multiplexer 22. Similarly, if a node ll(n''') operates as a destination node using only a single light beam, its receiver portion 13 (n' ' ' ) (R) need only have a single transducer 33 (t), and no demultiplexer 32. In addition, variable optical attenuators and amplifiers may be provided at additional locations in the transmitter portion 13(1) (T) as well as in the receiver portion 13(1) (R) , and one or more of the lasers 20 (m) may generate respective light beams of selectable output power, in which case respective variable optical attenuators 21 (m) may be omitted. Furthermore, depending on a number of factors, including, for example, the size of the network, the lengths of the communications media 14 (n) , the power of the light beams as coupled through the multiplexer 22 and by the optical add multiplexer 24 onto the respective communications medium, and the power requirements of the respective transducers 33 (t), booster amplifiers 23 and/or pre-amplifiers 31 may not be needed in transmitter and/or receiver portions 13 (n) (T) , 13 (n) (R) of one or more of the nodes 11(1) . On the other hand, depending on some or all of these factors, one or more in- line amplifiers, such as in-line amplifier 40 in communications medium 14 (3) , may be provided, which will serve to amplify all of the light beams which are transferred through the communications medium 14(3) from node 11(3) to node 11(4). The transmitter portion 13(1) (T) of the optical network node 13(1) of node 11(1) will include one laser 20 (m) and associated variable optical attenuator 21 (m) for each destination node 11 (nD) for which node 11(1) is a source node. Thus, if the node 11(1) is to be able to communicate, as a source node, with all of the other nodes 11(2) through 11 (N) in the network 10, it will be appreciated that its transmitter portion 13(1) (T) will include N-l lasers 20 (m) and associated variable optical attenuators 21 (m) (in that case, M=N-1). In addition, it will be appreciated that the other nodes 11(2) through 11 (N) will have, in their respective receiver portions 13(2) (R) through 13 (N) (R) , optical drop multiplexers 30 which can divert the light beams of the respective wavelengths, and transducers 33 (t) which can transduce the respective diverted light beams to generate an electrical signal for use by the respective information utilization circuitry as described above in connection with receiver portion 13(1) (R) . In that case, the lasers 20 (m) will generate light beams of different wavelengths λ(l) (2) through λ(l) (N) (where the first index "(1)" identifies the source node and the second index " (2)" through " (N)" identifies the destination node) respectively, which are transmitted to nodes 11(2) through 11 (N) respectively, so that the optical drop multiplexers 30 of the different nodes 11(2) through 11 (N) will be able to disambiguate the light beams of the different wavelengths that they receive and divert the light beam of the appropriate wavelength. It will be appreciated that, if the node 11(1) does not need to communicate, as a source node, with certain ones of the other nodes 11 (n) in the network 10 as destination nodes, its (that is, node ll(l)'s) transmitter portion 13(1) (T) can be provided with a corresponding fewer number of lasers 20 (m) and associated variable optical attenuators 21 (m) .

Similarly, the transmitter portions 13 (n) (T) (n≠l) of the optical network nodes 13 (n) of the other nodes 11 (n) are constructed generally similarly. Thus, if each of the nodes 11 (n) is to operate as a source node for transmission to each of the other nodes as destination nodes, there may be required light beams of Nx(N-l) wavelengths λ(l) (2) through λ(N) (N-l) to accommodate that. On the other hand, depending on several factors, including, for example, the amount, if any, of leakage of a light beam that is to be diverted by the optical drop multiplexer 30 at a node 11 (n) into the next communications medium 1 (n) , light beams of the same wavelengths can be used in diverse non-overlapping regions of the network 10. For example, if the transmitter portion 13(1) (T) of node 11(1) uses a light beam of wavelength λ(l) (2) to communicate with node 11(2), the same wavelength may be used for the light beam of wavelength λ(3) (4) used for communications between node 11(3) and 11(4), that is, λ ( 1) (2) =λ (3) (4 ) . This can serve to reduce the number of unique wavelengths that may be required for communications among nodes 11 (n) in the network 10. It will be appreciated that, in order to be able to use a light beam for communications between node 11(3) and 11(4) of the same wavelength as is used for communications between node 11(1) and 11(2), the optical drop multiplexer 30 of node 11(2) will need to divert at least a portion of the light beam of wavelength λ(l) (2) sufficient to ensure that any portion of the light beam which reaches node 11(4) will be sufficiently attenuated, by losses in the communications media 11(2) and 11(3) and optical network node 13(3), that it is below the power required by the receiver for the wavelength in receiver portion 13(4) (R) of node 11(4). In that case, only the light beam of the same wavelength λ(3) (4) as transmitted by node 11(3) will be received by the node 11(4), and so there will be no coherent crosstalk between the two light beams. It will also be appreciated that the same wavelength will need to be used in non-overlapping regions, since if, for example, λ(l) (5)=λ(2) (3) , the receiver portion 13(3) (R) of node 11(3) will divert not only the light beam of wavelength λ(2) (3) transmitted by node 11(2), but also the light beam λ(l) (5) transmitted by node 11(1) as source node for which node 11(5) is the intended destination node.

In accordance with the invention, the network 10 is also provided with a laser beam power adjustment system, which will be identified by reference numeral 50, for adjusting the power with which each of the light beams is transmitted by the transmitter portions 13 (n) (T) of each of the nodes 11 (n) . In particular, in network 10, the laser beam power adjustment system 50 controls the attenuation provided by each of the variable optical attenuators 21 (m) in each of the transmitter portions 13 (n) (T) to ensure that all of the requirements (i) through (iv) above are satisfied. The laser beam power adjustment system 50 may form part of one of the nodes 11 (n) and communicate with the other nodes over either separate connections, or over a control channel (not separately shown) similar to that described in U.S. Patent Application Serial No. 09/251,814, filed February 17, 1999, in the name of Richard A. Barry, et al . , and entitled System And Method Providing Control Channel For Wavelength-Division Multiplexed Network, assigned to the assignee of the present invention and incorporated herein by reference. Alternatively, the laser beam power adjustment system may comprise a separate component that communicates with all of the nodes either over separate connections or over a control channel.

In any case, in controlling the attenuation of a variable optical attenuator 21 (m) in the transmitter portion 13 (n) (T) of a source 11 (nS), the laser beam power adjustment system 50 enables the associated laser 20 (m) to generate a light beam for transmission to the node 11 (n' ) which is to be the destination node, and determines whether the transducer 33 (t) that is to receive the light beam is adequately receiving the light beam. Generally, the laser beam power adjustment system 50, at the beginning of adjusting the power for a laser 20 (m) , begins by adjusting the associated variable optical attenuator 21 (m) to provide a relatively large attenuation, so that the light beam will have a relatively low power at the transducer 33 (t) for the light beam at the receiver portion 13 (n') (R) of the destination node 11 (nD) . Thereafter, the laser beam power adjustment system can reduce the attenuation provided by the variable optical attenuator 21 (m) in steps, in each step enabling the power of the light beam as provided to transducer 33 (t) to increase, until it determines that the power of the light beam at the transducer 33 (t) is sufficient for the sensitivity requirements of both the transducer 33 (t) and the pre-amplifier 31. In one embodiment, the laser beam power adjustment system 50 can determine whether the sensitivity requirements of the pre-amplifier 31 have been satisfied by determining the power level of the light beam as received at the transducer 33 (t), and, based on the gain provided by the pre-amplifier 31, determining the amount of power of the light beam as received by the preamplifier 31 that would be required to provide the power at the transducer 33 (t). Other arrangements, such as measurement of the input power of the light beam at the pre-amplifier 31, will also be apparent to those skilled in the art .

In any case, the amount of reduction of attenuation in each respective step is selected to ensure that, in the last iteration, that is, the iteration in which the attenuation is reduced to the level at which power of the light beam at the transducer 33 (t) is sufficiently high as to satisfy the transducer's sensitivity requirements, the power will not be so high as to saturate the transducer (reference item (i) above) . In addition, it will be appreciated that the process ensures that the light beam be allocated a not undue amount of power by a booster amplifier, such as booster amplifier 23 or inline booster amplifier 40, since the amount of attenuation provided by variable optical attenuator 21 (m) will be such as to ensure that the light beam will not be allocated an undue amount of amplification thereby (reference item (ii) above) , other than that which may be necessary to provide the required power to transducer 33 (t). In addition, if the receiver portion 13 (n' ) (R) forms part of a node 11 (n) that is the destination node for multiple light beams, and if power had previously been adjusted for one or more other light beams, as the attenuation is reduced for the light beam whose power is currently being adjusted, thus increasing its power level at the input to the pre- amplifier 31, the laser beam power adjustment system 50 can determine whether the total input power of all of the light beams input to the pre-amplifier, including the power for that light beam, would saturate the preamplifier 31. If the laser beam power adjustment system 50 determines that these criteria cannot be contemporaneously met, that is, if it determines, for example, that a power level of the light beam that is required to provide sufficient power at the transducer 33 (t) or at the input to the pre-amplifier 31 would increase the power level for all light beams input to the pre-amplifier 31 to above its saturation level, the laser beam power adjustment system may indicate an error condition. After the laser beam power adjustment system 50 has conditioned the variable optical attenuator 21 (m) to provide the appropriate attenuation for the light beam generated by laser 20 (m), and if it (that is, the laser beam power adjustment system 50) determines that the power level of the light beam is satisfactory at the inputs of both the transducer 33 (t) and the pre-amplifier 31 without the total power of all light beams to the pre-amplifier 31 saturating the pre-amplifier, it can further adjust the attenuation as necessary to accommodate levels of incoherent and coherent crosstalk (reference items (iii) and (iv) above). In those operations, the laser beam power adjustment system 50 can attempt to determine whether crosstalk exists at a level which may require compensation by energizing other light beams which may give rise to crosstalk, such as (in the case of incoherent crosstalk) light beams of other wavelengths which are directed to or through the destination node 11 (nD) or (in the case of coherent crosstalk) light beams of the same wavelength elsewhere in the network, and adjust the attenuation of the variable optical attenuator 21 (m) as necessary to accommodate any interference. Alternatively, the laser beam power adjustment system 50 can measure the level of coherent and incoherent crosstalk of the light beam as received at the transducer 31, estimate the level based on, for example, mathematical models representing characteristics of the various components in the communication path, or perhaps generate a guess as to the likely levels of incoherent and coherent crosstalk, and adjust the attenuation accordingly. Generally, if the laser beam power adjustment system 50 finds that crosstalk will need to be accommodated, it will need to reduce the attenuation provided by variable optical attenuator 21 (m) , thereby to increase the power of the light beam as provided to the transducer 33 (t). After adjusting the attenuation, the laser beam power adjustment system 50 will also determine whether the power level is high enough to saturate the transducer 33 (t) or the pre-amplifier 31, and, if so, it can determine that the light beam of the particular wavelength cannot be used as described above.

In the operations as described above, the laser beam power adjustment system 50 performs the operations for each light beam, preferably starting from the light beam with the longest path towards the shortest path, since light beams with longer paths generally will be more subject to attenuation by the communication media and other components along the path from the source node 11 (nS) to the destination node 11 (nD) . In addition, by establishing the power required for the light beams for the longer paths first, the power level for those beams at each intermediate nodes 11 (n) along the respective paths can also be determined. In that case, the level of coherent crosstalk at the intermediate nodes 11 (n) along the path, and incoherent crosstalk at other nodes 11 (n) in the network can also be determined and the power level for the light beams for the shorter beams adjusted accordingly.

With this background, the operations performed by the laser beam power adjustment system 50 will be described in connection with the flow chart in FIG. 3. With reference to FIG. 3, the laser beam power adjustment system 50 initially ranks the communications paths for each source node/destination node pair according to their respective lengths (step 100) and selects the source node and destination node associated with the longest unprocessed path (step 101) . The laser beam power adjustment system 50 can be provided with the lengths by, for example, a system administrator. On the other hand, the laser beam power adjustment system 50 can determine the relative lengths of the communication paths between the respective pairs of source and destination nodes by loss measurements as described in the aforementioned Barry application. It will be appreciated that initially the laser beam power adjustment system 50 will select the source node and destination node associated with the longest path. If a plurality of communications paths have the same length, or if the path lengths are unknown, the laser beam power adjustment system can select one of the paths accordingly to any convenient selection mechanism.

After the laser beam power adjustment system 50 has selected a source node and destination node in step 101,. it performs a series of steps to determine an initial attenuation setting for the variable optical attenuator 21 (m) power level for the variable optical attenuator 21 (m) associated with the laser 20 (m) that will be used to transmit the light beam therebetween. Initially in those operations, the laser beam power adjustment system 50 selects an attenuation setting for the variable optical attenuator 21 (m) associated with the laser 20 (m) that will be used to transmit the light beam therebetween (step 102), enables the laser 20 (m) to transmit a light beam with the variable optical attenuator 21 (m) at the selected attenuation setting (step 103), and determines whether the light beam as received at the transducer 33 (t) that is to receive the light beam is at an acceptable power level (step 104). Preferably the initial attenuation setting selected in step 102 will be relatively high, so that when the laser 20 (m) begins transmitting in step 103 the power level of the light beam transmitted thereby will be relatively low, in particular preferably below the minimum power level required for the transducer 33 (t) which receives the light beam.

If the laser beam power adjustment system 50 makes a negative determination in step 104, that is, if it determines that the light beam is at an unacceptable power level, it will determine whether the variable optical attenuator 21 (m) is currently at its minimum attenuation setting (step 105). If the laser beam power adjustment system 50 makes a positive determination in step 105, it will not be able to further reduce the attenuation to increase the power level of the light beam as provided to the transducer 33 (t) . In that case, the laser beam power adjustment system 50 sequences to step 110 to signal an error to indicate that the communication path cannot be used at the power and wavelength provided with the particular laser 20 (m) . Thereafter, the laser beam power adjustment system can exit (not shown) , or it can proceed to step 111 to determine whether there are any additional source node/destination node pairs whose light beam' s power level is to be established, and, if so, return to step 101 to process the next source node/destination node pair.

Returning to step 105, if the laser beam power adjustment system makes a negative determination in that step, then it will be able to further reduce the attenuation provided by the variable optical attenuator, to further increase the power of the light beam as provided to the transducer 33 (t) . Accordingly, if the laser beam power adjustment system makes a negative determination in step 105, it sequences to step 106 to reduce the level of attenuation provided by the variable optical attenuator 21 (m) (step 105) and return to step 104 to again determine whether the light beam as received at the transducer 33 (t) that is to receive the light beam is at an acceptable power level. It will be appreciated that, since the laser beam power adjustment system 50 started with a relatively high attenuation so that the light beam as received at the transducer 33 (t) will be at a relatively low power, it (that is, the laser beam power adjustment system 50) will reduce the attenuation in step 106, thereby to increase the power of the laser beam as received at the transducer 33 (t). It will be appreciated that the laser 20 (m) will remain energized, generating the light beam, during steps 104 through 106.

The laser beam power adjustment system 50 performs steps 104 through 106 through one or more iterations, decreasing the attenuation of variable optical attenuator 21 (m) , and thereby increasing the power of the light beam as provided to transducer 33 (t), in each iteration, until it (that is, the laser beam power adjustment system 50) determines in step 104 that the power level of the light beam as received at the transducer is acceptable. It will be appreciated that, since in these operations the power level of the light beam as provided to the transducer 33 (t) increases from a relatively low level, when the laser beam power adjustment system 50 determines that the power level is acceptable, it (that is, the power level) will be near the low end of the range of acceptable power levels for the transducer 33 (t). After the laser beam power adjustment system 50 determines in step 104 that the power level of the laser beam as received at the transducer 33 (t) is acceptable, it can proceed to perform a number of steps to further adjust the attenuation provided by the variable optical attenuator 21 (m) in view of possible coherent and incoherent crosstalk that might arise in connection with light beams for which power levels have been previously established. It will be appreciated that, for the first pair of source and destination nodes for which the power level is set, there will be no light beams whose power levels have been previously established, in which case the laser beam power adjustment system 50 can skip these steps. In any case, initially, the laser beam power adjustment system 50 determines whether the power levels of any of the light beams of other wavelengths than the one whose power level was established in steps 102 through 105 which are directed to or through the selected destination node 11 (nD) is sufficiently high as may cause coherent or incoherent crosstalk (step 107) . If the laser beam power adjustment system 50 makes a positive determination in step 107, it sequences to step 108 to adjust the attenuation level of the variable optical attenuator 21 (m) to facilitate an increase in the power of the light beam as received by the transducer 33 (t) to accommodate the possible crosstalk.

After increasing the power of the light beam to accommodate possible crosstalk from other light beams, the laser beam power adjustment system 50 determines whether the new power level of the laser beam at the transducer 33 (t) is outside of the acceptable power level range (step 109) . It will be appreciated that, if the power level is outside of the acceptable range, it will generally be too high, that is, at or above the saturation level for the transducer 33 (t). If the laser beam power adjustment system 50 makes a negative determination in step 109, which will occur if the power level is within the acceptable range, it will sequence to step 111 to determine whether there are any additional source node/destination node pairs whose light beam' s power level is to be established. If the laser beam power adjustment system makes a positive determination, it will return to step 101 to process the next source node/destination node pair.

Returning to step 109, if the laser beam power adjustment system 50 makes a positive determination in that step, it may be unable to establish an acceptable power level for the light beam, particularly at the wavelength selected for the light beam, and so it (that is, the laser beam power adjustment system 50) signals an error (step 110) . Thereafter, the laser beam power adjustment system can exit (not shown) , or it can proceed to step 111 to determine whether there are any additional source node/destination node pairs whose light beam's power level is to be established, and, if so, return to step 101 to process the next source node/destination node pair.

Returning to step 107, if the laser beam power adjustment system 50 makes a negative determination in that step, which will be the case if it determines that the power levels of none of the light beams of other wavelengths than the one whose power level was established in steps 102 through 105 which are directed to or through the selected destination node 11 (nD) is sufficiently high as may cause coherent or incoherent crosstalk, it will proceed to step 111 to determine whether there are any additional source node/destination node pairs whose light beam's power level is to be established, and, if so, return to step 101 to process the next source node/destination node pair.

The laser beam power adjustment system 50 will perform steps 101 through 111 through a number of iterations, determined by the number of source node/destination node pairs for which power levels for light beams need to be established. After the laser beam power adjustment system 50 has processed all of the source node/destination node pairs it will make a positive determination in step 111, and exit (step 112) . Alternatively, instead of exiting at this point, the laser beam power adjustment system 50 can repeat the crosstalk adjustment steps (steps 107 through 111) for each of the other source node/destination node pairs again, in the same order, to determine, for each source node/destination node pair, whether a light beam whose power level was adjusted and established after that for the respective source node/destination node pair would be likely to cause coherent or incoherent crosstalk with the light beam for the respective source node/destination node and, if so, adjust the attenuation of the variable optical attenuator 21 (m) to accommodate the crosstalk. If necessary, the laser beam power adjustment system 50 can repeat this until the crosstalk has been accommodated for light beams associated with all of the source node/destination node pairs, or until it has performed a selected maximum number of iterations. The invention provides a number of advantages. In particular, it provides an arrangement by which power of the light beams used for transferring information in a WDM network can be readily adjusted to ensure that communications can take place, while providing a fair allocation of attenuation in the various amplifiers used in the respective nodes, and further accommodating coherent and incoherent crosstalk in communications as between various pairs of source and destination nodes which communicate using light beams of the same wavelength.

It will be appreciated that numerous modifications may be made in connection with the network 10 and laser beam power adjustment system 50 described above in connection with FIGS. 1 through 3. For example, although the invention has been described as making use of the power level of the light beam as received at the transducer 33 (t), it will be appreciated that other measures can be used in addition or instead, including, for example, the bit error rate for information transferred over the light beam.

In addition, for each node operating as a destination node, the laser beam power adjustment system 50 can also adjust the attenuation settings of the variable optical attenuators 21 (m) (or the power output from the respective lasers 20 (m) ) for the source nodes to ensure that input noise requirements of the pre-amplifier 31 are satisfied and that the pre-amplifier is not saturated if all of the source nodes transmit light beams thereto concurrently. In addition, although the optical network node 13(1) has been described as having the structure described above in connection with FIG. 2, as mentioned above additional components may also be provided, including, for example, additional amplifiers or variable optical attenuators along any of the light paths. Furthermore, although the laser beam power adjustment system 50 has been described as adjusting the attenuation provided by variable optical attenuators 21 (m) , it will be appreciated that, if a laser 20 (m) has a variable power output, it (that is, the laser beam power adjustment system 50) can instead adjust the power output of the laser 20 (m) (in which case a variable optical attenuator 21 (m) may not need to be provided therefor) , or in addition if a variable optical attenuator 21 (m) is provided therefor.

Although the laser beam power adjustment system 50 has been described as initializing and adjusting power levels for light beams used for communications between all source node/destination node pairs in the network 10, it will be appreciated that it may instead or also be used to initialize and adjust power levels for light beams used for communications between a single node 11 (nX) that is, for example, added to the network 10 after the power levels for the light beams used in connection with communications between other nodes have been established. In that case, the laser beam power adjustment system 50 may perform the initial adjustment (steps 101 through 105) of the power levels of only the light beams for which the single node 11 (nX) is a source or destination node, and thereafter perform the crosstalk accommodation steps (steps 108 through 112) as necessary in connection with all light beams. It will be appreciated that, if the laser beam power adjustment system 50 is unable to accommodate crosstalk as between a previously-established light beam and a new light beam for which the node 11 (nX) is a source or destination node, preferably the laser beam power adjustment system 50 will ensure that communications using the previously-established light beam continue.

The foregoing description has been limited to a specific embodiment of this invention. It will be apparent, however, that various variations and modifications may be made to the invention, with the attainment of some or all of the advantages of the invention. It is the object of the appended claims to cover these and such other variations and modifications as come within the true spirit and scope of the invention.

What is claimed as new and desired to be secured by Letters Patent of the United States is:

Claims

1. A system for adjusting transmission power of light beams in a wavelength-division multiplexed network, the network comprising a plurality of nodes having respective variable power light beams generators and transceivers, in relation _ to a predetermined power criterion, the system comprising:

A. a power criterion confirmation module associate with each of said transceivers for determining when the power criterion has been satisfied for the respective transceiver; and

B. a power adjustment control configured to control the power generated by each of the respective light beam generators in relation to the determination by the power criterion confirmation module that the power criterion has been satisfied for the respective transceiver, the power adjustment control for each light beam generator being configured to enable the respective light beam generator to increase the power for its respective light beam until the power criterion confirmation module determines that the power criterion for that light beam has been satisfied.

2. A system as defined in claim 1 in which the power criterion is a power level for the respective light beam.

3. A system as defined in claim 1 in which the power criterion is an error rate for digital data transferred over the respective light beam.

4. A system as defined in claim 1 in which the light beam generator includes a variable power light source, the power adjustment control being configured to* adjust the power of the variable power light source.

5. A system as defined in claim 4 in which the variable power light source is a laser.

6. A system as defined in claim 1 in which the light beam generator includes a light source configured to generate a light beam, and a variable optical attenuator configured to provide an attenuation for the light beam, the power adjustment control being configured to adjust the attenuation provided by the variable optical attenuator.

7. A system as defined in claim 6 in which the light source is a laser.

8. A system as defined in claim 1 in which each light beam is transferred through the network along a light path, each light path having a length from the respective light beam generator to the respective transceiver, the power adjustment control being configured to control the power generated by each of the light beam generators in an order relating to the length of the light path for the light beam generated thereby.

9. A system as defined in claim 8 in which the power adjustment control controls the power of each light beam from those light beams having longer paths to those light beams having shorter paths.

10. A system as defined in claim 1 in which the power criterion is a power level for the respective light beam to reduce crosstalk in connection with at least one other light beam.

11. A system as defined in claim 10 in which each light beam is transferred through the network along a light path through respective nodes, and in which the power adjustment control is configured to

A. control the power for respective light beams seriatim, and B. for each light beam whose path terminates at a respective node, to determine an amount of crosstalk therefor and adjust the power level to accommodate the power of the respective light beam in relation to the amount of crosstalk determined.

12. A system as defined in claim 11 in which the power adjustment control is configured to increase the power of a respective light beam in relation to the amount of crosstalk determined.

13. A system as defined in claim 12 in which the path of at least one light beam includes a pre-amplifier, the pre-amplifier having a saturation power level, the power adjustment control further determining whether the pre- amplifier is saturated by the increase in power of a respective light beam therethrough.