Abstract:
Dividing a multiplexed optical signal transmitted along a first transmission path into two portions and diverting one of the two portions to a second transmission path and forwarding the other portion of the multiplexed optical signal along that first transmission path, while the energy of the diverted portion comprises the entire spectrum of the multiplexed optical signal. Preferably, the diverted first portion is further divided into at least two parts, each comprising a wavelength range substantially different than the others, and at least one of the two parts is forwarded towards an optical receiver.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to optical communication networks using wave division multiplexing.  
         BACKGROUND OF THE INVENTION  
         [0002]    In the last years many optical networks have been deployed mainly in the long haul and metro areas. Many of these networks are based on Dense Wavelength Division Multiplexing (“DWDM”), a method that enables transmission of several optical channels over a single fiber. Each of these optical channels is transmitted at a different wavelength and the signals each being at a different wavelength, are multiplexed and transmitted over one fiber. At a destination point, the selected wavelengths that are associated with the desired optical channels are detected and separated from the remaining multiplexed signals. In typical networks, the digital signals that are transmitted via these optical channels, typically comply with the well-known SONET/SDH standards. Therefore, the most popular bit rates for each optical channel are 2.5 and 10 Gbit/s. In addition, typical optical networks are constructed to comprise two optical fibers; one for carrying optical signals in one direction, while the other for carrying optical signals in the opposite direction.  
           [0003]    The most simple network topology of an optical network comprises two terminals in a point-to-point configuration. However, when an optical network comprises more than two terminals, several topologies to interconnect these terminals are possible, such as: bus, tree, mesh, ring and the like. In a two fibers ring type network it is possible to implement network protection methods so that when one of the optical fibers is cut at one location along the optical ring, the signal can still be delivered to the required destination along the other fiber of the ring. Typically, in optical networks and especially in a ring type network using DWDM transmission the required wavelength/s is/are dropped at each terminal by using Optical Add/Drop Multiplexers (“OADMs”). A typical such OADM comprises at least one optical filter that enables the separation of one or more of the wavelengths from the others and allows forwarding the transmission at that wavelength/s to the local terminal. Another filter in the OADM is used to allow selectively insertion of one or more wavelengths to the multiplexed optical signal transmitted along the fiber. Therefore a specific optical channel transmitted at a specific wavelength is inserted at its originating location using the “add” function of an OADM and is “dropped” at its destination while using another OADM. In several applications there is a need to “add” and “drop” several optical channels at the same location. In such cases each OADM is equipped with several filters, where each such filter is operative to “add/drop” an optical channel transmitted at a specific wavelength. Typically these OADM devices are preset to “add” and “drop” specific wavelengths and cannot be modified during normal operation so as to be adapted to changes in the operating requirements. Thus, when it is required to add or drop a different wavelength at any location, the OADM must be replaced by another OADM that is preset to allow the drop and/or addition of the appropriate wavelength.  
           [0004]    In optical systems where there is a need to connect several optical networks, e.g. optical rings, the most common solution is to convert the optical signals to their electrical form at the point of transfer to the other ring, rearrange them by using an electrical switching equipment, convert the electrical signals back to their optical form, and then transmit them along the other network. This solution that is called OEO (Optical-Electrical-Optical) is rather expensive, as it requires multiple conversions between the optical and the electrical formats of the transmitted signals. Another possible solution to connect between optical networks is the OOO (Optical-Optical-Optical) solution. In this solution, the optical signals arriving from one network, remain in their optical form, are rearranged by fully photonic switching equipment and sent in their original, optical form to the other network. However, photonic switching is still an immature technology that is very expensive, is unreliable and poses scalability problems. Such a switch is also a sensitive single point of failure that makes the network very much dependent on this centrally located equipment.  
           [0005]    Additionally, when the number of optical channels that are required to be “added/dropped” in one location is greater then the capacity of the existing OADM, it would be required to insert additional optical filters in series with the existing OADM. Such a modification will typically break the signal continuity on the optical ring and might adversely affect the communication between several nodes on this ring. In addition to that, after adding such optical filters, the power budget of the optical ring has to be recalculated and the power of each channel should be adjusted accordingly. Therefore it would be desirable to find an effective solution for an optical network wherein each network node can be modified and expanded without affecting other nodes and without interrupting the communication between other nodes.  
           [0006]    An additional problem arises when such fixed OADMs are used in an optical network comprising tunable optical transmitters and receivers so as to enable transmission and reception of signals at varying wavelengths. Since typical OADMs are not tunable, the capabilities of such a network are severely limited and the tunability advantages cannot be effectively used. The term “tunability” as used herein, is used to denote a feature of a transmitter and/or a receiver whereby this transmitter and/or receiver may be operated at various frequencies/wavelengths. The operating wavelength may be modified over a specific range by e.g. an external control, as may be required for the operation of the communication network. Therefore it would be desirable to find an effective solution for a network comprising tunable optical transmitters and optical receivers.  
         SUMMARY OF THE INVENTION  
         [0007]    It is an object of the present invention to provide a novel, cost effective optical system for the transmission of optical signals.  
           [0008]    It is yet another object of the present invention to provide methods for improving the transmission of optical signals in a network and in a number of connected optical networks.  
           [0009]    It is still another object of the present invention to provide an optical communication apparatus and system that are adapted to overcome the drawbacks of the prior art described above and to allow better flexibility in operating an optical communication network.  
           [0010]    Yet another object of the present invention is to provide a method that enables utilizing tunable transmitters and receivers without being required to carry out substantial changes to already existing networks.  
           [0011]    Other objects of the invention will become apparent as the description of the invention proceeds.  
           [0012]    According to a first aspect of the invention there is provided a optical communication system comprising:  
           [0013]    a first plurality of optical transmitters;  
           [0014]    at least one wavelength division multiplexer, adapted to multiplex optical signals transmitted at different wavelengths to obtain a multiplexed optical signal;  
           [0015]    at least one dropping/adding unit adapted to be provided with said multiplexed optical signal transmitted along a first transmission path and divert a first portion thereof to a second transmission path, wherein the first portion comprises substantially the entire spectrum of the multiplexed optical signal and forwarding a second portion of the multiplexed optical signal along said first transmission path; and  
           [0016]    at least one optical receiver, adapted to receive the first portion of the multiplexed optical signal diverted to the second transmission path.  
           [0017]    Preferably, the system comprising a plurality of optical receivers, each of which is adapted to operate at a wavelength belonging to the wavelength range of the part of the diverted first portion of the multiplexed optical signal.  
           [0018]    In accordance with a preferred embodiment of the invention, the system further comprising at least one filtering means adapted to divide the first portion of the multiplexed optical signal diverted by the at least one dropping/adding unit, into at least two parts, each comprising a wavelength range substantially different than the others, and forwarding at least one of these two parts towards the at least one optical receiver.  
           [0019]    By yet another embodiment of the invention, the at least one dropping/adding unit comprises at least one optical coupler.  
           [0020]    By still another embodiment, the second portion of the multiplexed optical signal is substantially equal to an optical signal having the energy of the original multiplexed optical signal less that of the first portion of the multiplexed optical signal, and a wavelength range of said second portion of the multiplexed optical signal is substantially equal to that of the original multiplexed optical signal.  
           [0021]    According to another aspect of the invention, there is provided an optical communication system for transmitting optical signals in an optical network comprising:  
           [0022]    a plurality of network elements and at least one optical path extending therebetween, wherein at least one of the network elements comprises at least one tunable optical transmitter capable of operating at a first plurality of wavelengths and at least one other of the network elements comprises at least one optical receiver;  
           [0023]    at least one first optical coupler adapted to couple the at least one tunable optical transmitter to the at least one optical path and at least one second coupler adapted to couple the at least one optical receiver to said at least one optical path;  
           [0024]    wherein the at least one first optical coupler is adapted to allow transmission of optical signals from said tunable optical transmitter irrespective of their wavelength and combining them with other optical signals transmitted along said at least one optical path passing through said optical coupler; and  
           [0025]    wherein the optical signals transmitted by the tunable optical transmitter, are transmitted at a wavelength corresponding to a wavelength at which the at least one optical receiver is operative.  
           [0026]    According to yet another aspect of the invention, there is provided an optical communication system adapted for transmitting and receiving of optical signals over a plurality of optical networks, wherein a first of the plurality of optical networks comprises at least two network elements and wherein a second of the plurality of optical networks comprises at least one network element, wherein at least one of the network elements comprises at least one tunable optical transmitter and at least one other of the network elements comprises at least one optical receiver;  
           [0027]    a plurality of optical couplers, adapted to couple the at least one tunable optical transmitter or the at least one optical receiver to at least one optical path;  
           [0028]    wherein the optical coupler associated with the tunable optical transmitter is adapted to allow transmission of optical signals from said tunable optical transmitter irrespective of their wavelength and to combine them with other optical signals transmitted along the least one optical path; and  
           [0029]    wherein optical signals transmitted by the tunable optical transmitter, are transmitted at a wavelength corresponding to a wavelength at which the at least one optical receiver is operative.  
           [0030]    In accordance with yet another embodiment of the invention there is provided an optical communication system for transmitting optical signals over at least one optical path extending between at least two network elements, wherein each of the at least two network elements comprises at least one optical transmitter and at least one optical receiver, and wherein the at least one optical transmitter and the at least one optical receiver are each coupled to the at least one optical path via at least one optical coupler, and wherein each of the at least one optical path is further connected to an optical protection switch adapted to block under normal operating conditions the transmission of optical signals ingressing the optical protection switch, and to allow transmission of optical signals through the optical protection switch in response to a fault occurring along the corresponding at least one optical path.  
           [0031]    Preferably, the at least one optical path is connected in a ring type topology.  
           [0032]    By yet another preferred embodiment of the invention, the optical communication system further comprising means to detect a fault along said at least one optical path and activate said optical protection switch in response to a fault detection.  
           [0033]    According to still another embodiment of the invention, there is provided an optical communication system adapted for transmitting and receiving of optical signals over at least two optical networks, wherein a first of said optical networks comprises a plurality of network elements and wherein a second of said optical networks comprises at least one network element. The network elements comprise at least one optical transmitter and at least one optical receiver, and  
           [0034]    wherein at least one first transmitter associated with the first optical network is adapted to transmit an optical signal destined to another network element belonging to the first optical network at a wavelength selected from among a first plurality of wavelengths, and wherein at least one second transmitter associated with the first optical network is adapted to transmit an optical signal destined to a network element belonging to another optical network at a wavelength selected from among a second plurality of wavelengths from,  
           [0035]    characterized in that no wavelength included in the first plurality of wavelengths is included in the second plurality of wavelengths, and at least the first optical network further comprises at least one optical coupling device operative to allow transfer of an optical signal transmitted at a wavelength selected from among the second plurality of wavelengths, and to block the transfer of an optical signal transmitted at a wavelength selected from among said first plurality of wavelengths from being transmitted to a location that is not included in the first optical network.  
           [0036]    In the alternative or in addition, the at least one first transmitter is capable of transmitting optical signals also at a wavelength selected from among the second plurality of wavelengths, e.g. by using a tunable transmitter.  
           [0037]    Furthermore, the present invention should also be understood to encompass cases where at least one tunable transmitter is used instead of using the at least one first transmitter and the at least one second transmitter, so that when an optical signal should be transmitted to another network element belonging to the first optical network, the signal shall be transmitted by the at least one tunable transmitter at a wavelength selected from among a first plurality of wavelengths, whereas when an optical signal should be transmitted to a network element belonging to another optical network, the signal shall be transmitted by that at least one tunable transmitter at a wavelength selected from among a second plurality of wavelengths.  
           [0038]    According to yet another embodiment of the invention there is provided an optical communication system adapted for transmitting and receiving of optical signals over more than two optical networks, wherein at least a first optical network comprises a plurality of network elements and wherein each of the other optical networks comprises at least one network element. The network elements of the first optical network comprise each at least one optical transmitter and at least one optical receiver and wherein at least one transmitter is adapted to transmit an optical signal at a wavelength selected from among a first plurality of wavelengths when the optical signal is destined to another network element belonging to that first optical network, and wherein at least one transmitter is adapted to transmit an optical signal at a wavelength selected from among a second plurality of wavelengths from such a network element belonging to the first optical network when the optical signal is destined to a network element belonging to another optical network. By this embodiment the system is characterized in that:  
           [0039]    no wavelength included in the first plurality of wavelengths is included in the second plurality of wavelengths;  
           [0040]    at least the first optical network further comprises at least one optical coupling device operative to allow transfer of an optical signal transmitted at a wavelength selected from among the second plurality of wavelengths, and to block the transfer of an optical signal transmitted at a wavelength selected from among the first plurality of wavelengths; and  
           [0041]    wherein at least two of the more than two optical network further comprise OADMs that allow addition and drop of optical signal received thereby which are transmitted at predetermined wavelengths selected from said second plurality of wavelengths.  
           [0042]    In the alternative or in addition, the at least one first transmitter is capable of transmitting optical signals also at a wavelength selected from among the second plurality of wavelengths, e.g. by using a tunable transmitter.  
           [0043]    Preferably, at least one of the first and second pluralities of wavelengths comprises a continuous range of wavelengths.  
           [0044]    In accordance with another aspect of the invention, there is provided an optical communication apparatus comprising a dropping/adding unit connected to a transmission path where a wavelength division multiplexed optical signal is transmitted, and being capable of:  
           [0045]    dropping an optical signal having a first portion of the energy of the wavelength division multiplexed optical signal transmitted on the transmission path wherein the first portion comprises substantially the entire spectrum of the optical signal;  
           [0046]    forwarding the remaining of the wavelength division multiplexed optical signal along the transmission path;  
           [0047]    adding an optical signal transmitted at at least one wavelength to the forwarded optical signal along the transmission path,  
           [0048]    and wherein the optical communication apparatus is adapted to forward the first portion of the wavelength division multiplexed optical signal towards at least one optical receiver.  
           [0049]    Preferably, the apparatus further comprising optical filtering means adapted to divide the first portion of the wavelength division multiplexed optical signal into at least two substantially different portions thereof.  
           [0050]    More preferably, the optical filtering means is adapted to forward at least one of the two portions to the at least one optical receiver.  
           [0051]    According to yet another aspect of the invention, there is provided a method for transmitting an optical signal in an optical communication network comprising a plurality of network elements. The method comprises the steps of:  
           [0052]    a. providing a first network element that comprises at least one tunable optical transmitter capable of operating at a plurality of wavelengths;  
           [0053]    b. providing a second network element that comprises at least one optical receiver operative at a first wavelength included among said plurality of wavelengths;  
           [0054]    c. providing an optical coupler adapted to couple the output of the at least one tunable optical transmitter to an optical transmission path extending between the first and second network elements, adapted to forward transmission of optical signals transmitted by the at least one tunable optical transmitter irrespective of their wavelength and combine them with other optical signals transmitted along the optical transmission path;  
           [0055]    d. tuning the at least one tunable optical transmitter so that a transmission destined to the second network element be transmitted at said first wavelength; and  
           [0056]    e. transmitting an optical signal from the at least one tunable optical transmitter via said optical coupler and along said optical transmission path.  
           [0057]    According to another embodiment of the invention, there is provided a method for transmitting optical signals in a system comprising at least a first and a second communication networks wherein the method comprises the steps of:  
           [0058]    i. providing a first network element associated with the first communication network which comprises at least one optical transmitter;  
           [0059]    ii. transmitting optical signals by the at least one optical transmitter, wherein optical signals are transmitted at a first wavelength selected from a first group of wavelengths consisting of a first plurality of wavelengths if said optical signals are destined to another network element associated with the first communication network, and wherein optical signals are transmitted at a second wavelength selected from a second group of wavelengths consisting of a second plurality of wavelengths when the optical signals are destined to another network element not associated with said first communication network, and no wavelength being a member of the first group of wavelengths is a member of said second group of wavelengths;  
           [0060]    iii. allowing transfer of the optical signals from the first communication network to the second communication network if their wavelength is a wavelength being a member of the second group of wavelengths and preventing the transfer of the optical signals from the first communication network to the second communication network if their wavelength is a wavelength being a member of the second group wavelengths.  
           [0061]    By still another embodiment of the invention, there is provided a method of forwarding an optical signal in an optical communication network which comprises the steps of:  
           [0062]    (i) receiving a multiplexed optical signal comprising a plurality of multiplexed optical signals transmitted at different wavelengths, all of which comprise a wavelength range characterizing said multiplexed optical signal;  
           [0063]    (ii) forwarding a first portion of said multiplexed optical signal towards at least one optical receiver, wherein the wavelength range of the first portion is substantially equivalent to the entire wavelength range of the multiplexed optical signal;  
           [0064]    (iii) forwarding the remaining of the multiplexed optical signal along a transmission path in the optical network.  
           [0065]    Preferably, this method further comprising the steps of:  
           [0066]    a. dividing the first portion of the multiplexed optical signal into a plurality of optical signals, each being at a substantially different wavelength range than the others;  
           [0067]    b. forwarding at least one of the divided optical signals towards at least one optical receiver operative at a wavelength belonging to the wavelength range of the divided optical signal. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0068]    The present invention will be understood and appreciated more fully from the following detailed description, taken in conjunction with the drawings in which:  
         [0069]    [0069]FIG. 1 illustrates schematically a typical, prior art optical network, using WDM technology in a ring type topology;  
         [0070]    [0070]FIG. 2 illustrates schematically a typical structure of a prior art OADM—Optical Add/Drop Multiplexer, used in the optical network shown in FIG. 1;  
         [0071]    [0071]FIG. 3 illustrates schematically an optical network of the present invention that comprises optical coupler arrays;  
         [0072]    [0072]FIG. 4 demonstrates details of an optical coupler array that may be applied in the optical network shown in FIG. 3;  
         [0073]    [0073]FIG. 5 illustrates schematically an optical protection switch that may be applied in the optical network shown in FIG. 3;  
         [0074]    [0074]FIG. 6 presents a typical, prior art optical network, comprising two optical rings interconnected via an electrical switch;  
         [0075]    [0075]FIG. 7 illustrates a typical, prior art optical network, comprising two optical rings interconnected via an optical switch;  
         [0076]    [0076]FIG. 8 illustrates an example of an optical network according to the present invention, comprising two optical rings that are interconnected via an optical coupler array;  
         [0077]    [0077]FIG. 9 demonstrates an optical coupler arrays used to connect the two optical rings in the optical network shown in FIG. 8;  
         [0078]    [0078]FIG. 10 presents a frequency plan used in the optical network of FIG. 11 and  12 ;  
         [0079]    [0079]FIG. 11 illustrates schematically an optical network, comprising two optical rings interconnected via a combination of OADMs and optical coupler array;  
         [0080]    [0080]FIG. 12 illustrates schematically an optical network, comprising three optical rings interconnected via a combination of OADMs and optical coupler arrays; and  
         [0081]    [0081]FIG. 13 demonstrates details of a further improvement of an optical coupler array that may be applied, according to the present invention, in the optical network shown in FIG. 3.  
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0082]    [0082]FIG. 1 describes a prior art optical network of a ring type configuration. The optical network illustrated in FIG. 1 comprises six terminals  10   1 ,  10   2 ,  10   3 ,  10   4 ,  10   5  and  10   6 , each provided with, for example, four optical transmitters ( 20   1-1 - 20   1-4 ,  20   2-1 - 20   2-4 ,  20   3-1 - 20   3-4 ,  20   4-1 - 20   4-4 ,  20   5-1 - 20   5-4  and  20   6-1 - 20   6-4 , each operative to receive a corresponding electrical signal and to transmit it at a different wavelength: λ 1-1 -λ 1,4 , λ 3-1 -λ 3-4 , λ 5-1 -λ 5-4 , λ 2-1 -λ 2-4 , λ 4-1 -λ 4-4 , and λ 6-1 -λ 6-4  respectively) and four optical receivers ( 30   1-1 - 30   1-4 ,  30   2-1 - 30   2-4 ,  30   3-1 - 30   3-4 ,  30   4-1 - 30   4-4 ,  30   5-1 - 30   5-4  and  30   6-1 - 30   6-4 , each operative to receive optical signals at a different wavelength: λ 2-1 -λ 2-4 , λ 4-1 -λ 4-4 , λ 6-1 -λ 6-4 , λ 1-1 -λ 1-4 , λ 3-1 -λ 3-4 , and λ 5-1 -λ 5-4  and to convert it to a corresponding electrical signal respectively). Optical transmitters  20   1-1 - 20   1-4 ,  20   2-1 - 20   2-4 ,  20   3-1 - 20   3-4 ,  20   4-1 - 20   4-4 ,  20   5-1 - 20   5-4  and  20   6-1 - 20   6-4  as well as optical receivers  30   1-1 - 30   1-4 ,  30   2-1 - 30   2-4 ,  30   3-1 - 30   3-4 ,  30   4-1 - 30   4-4 ,  30   5-1 - 30   5-4  and  30   6-1 - 30   6-4  are connected to optical fibers  2  and  4  via OADM arrays  40   1 ,  40   2 ,  40   3 ,  40   4 ,  40   5  and  40   6 , respectively. The operation of OADM arrays  40   1 ,  40   2 ,  40   3 ,  40   4 ,  40   5  and  40   6  will be further described in conjunction with FIG. 2.  
         [0083]    Let us consider an example whereby optical transmitter  20   1-1  is operative to transmit an optical signal at a wavelength of λ 1-1 , to optical receiver  30   4-1  and optical transmitter  20   4-1  is adapted to transmit an optical signal at a wavelength of λ 2-1  to optical receiver  30   1-1 . In a similar manner, each of the other optical transmitters is operative to transmit an optical signal at a specific wavelength to communicate with a specific optical receiver that is operative to receive signals transmitted at that specific wavelength.  
         [0084]    In the example depicted at FIG. 1, each OADM is operative to transmit up to four optical channels (each channel operative at a different wavelength) and to receive up to four optical channels at typically other four specific wavelengths. Therefore the optical transmitters have to be configured to operate at the same wavelengths as the OADM they are connected to and likewise the optical receivers have to be configured to operate at the same wavelengths as the OADM they are connected to.  
         [0085]    The WDM optical signals are transmitted over two optical fibers. In optical fiber  2  the optical signals are conveyed in the CW-Clockwise direction, while in optical fiber  4 , the optical signals are conveyed in the CCW—Counter-Clockwise direction.  
         [0086]    In the example demonstrated, optical signals are transmitted under normal operating conditions, from transmitter  20   1-1  over optical fiber  2 , pass through OADM arrays,  40   1 ,  40   6 ,  40   5  and  40   4  and arrive at optical receiver  30   4-1 . The optical signals transmitted from optical transmitter  20   4-1  are conveyed along optical fiber  2 , pass through OADMs arrays  40   4 ,  40   3 ,  40   2  and  40   1  and arrive at optical receiver  30   1-1 . When a fault is detected in the network (while operating under normal operating conditions) e.g. if there is a fiber cut, for example between OADM arrays  40   2  and  40   3 , there will be no change in the optical signal transmission between optical transmitter  20   1-1  and optical receiver  30   4-1 , but the optical signal from optical transmitter  20   4-1  will arrive to optical receiver  30   1-1  over fiber  4  passing through OADM arrays  40   4 ,  40   5 ,  40   6  and  40   1 . In this manner the ring network architecture comprising two optical fibers  2  and  4 , is protected against a fault occurring at any location of the network. It should be understood that in a similar manner, each of the other optical transmitters communicate with a corresponding optical receiver in the optical network and are adapted to affect the above described network protection method, mutatis mutandis.  
         [0087]    As will be appreciated by those skilled in the art, one of the major disadvantages of the prior art system described, is that each optical transmitter, each optical receiver and each OADM array is preset to operate at a given wavelength. Therefore if any change were to be made to the transmission plan of such a network, it would require manual replacement of equipment in several network nodes. Also, when additional optical channels need to be transmitted between OADMs, the network needs to be modified, typically causing service interruption to several other nodes in the network.  
         [0088]    [0088]FIG. 2 describes optical terminal  10   1  in more details Specifically, FIG. 2 describes the operation of OADM array  40   1  in conjunction with optical transmitters  20   1-1 - 20   1-4  operative at wavelengths λ 1-1 -λ 1-4  and optical receiver  30   1-1 - 30   1-4  operative to receive signals transmitted at wavelengths λ 2-1 -λ 2-4 . An optical signal is transmitted from transmitter  20   1-1  and is splitted by optical splitter  50   1 . One part of the signal is conveyed to the “add” input of OADM  54  while the other is conveyed to the “add” input of OADM  56 . Similarly, optical signals are coupled from optical transmitters  20   1-2 - 20   1-4  to OADMS  54  and  56  via optical splitters  50   2 - 50   4 . OADMS  54  and  56 , each comprise optical filters that enable selective insertion of optical signals at λ 1-1 -λ 1-4  from transmitters  20   1-1 - 20   1-4  into optical fibers  2  and  4  respectively. A multiplexed signal received at terminal  10   1  is conveyed to OADM  54  or OADM  56 , from optical fibers  2  or  4 , respectively. It is then passed through optical filters that separate the signal transmitted at the predetermined wavelengths λ 2-1 -λ 2-4  from the rest of the multiplexed signal. The separated signals from either OADM  54  or  56  are then each conveyed via optical switches  52   1 - 52   4  to the corresponding optical receiver  30   1-1 - 30   1-4 , while the remaining non-separated multiplexed signal is forwarded along that fiber to the proceeding network element. In other words, OADM  54  operates by selectively extracting the optical signals transmitted along optical fiber  2  at the predetermined wavelength λ 2-1 -λ 2-4  and inserting optical signal transmitted by transmitters  20   1-1 - 20   1-4  at the predetermined wavelength  20   1-1 - 20   1-4  along that optical fiber  2 . At the same time, optical signals transmitted at other wavelengths pass through OADM  54  practically unchanged. OADM  56  performs the same function as optical OADM  54  but on optical signals added to or dropped from optical fiber  4 , mutatis mutandis.  
         [0089]    Under normal operating conditions, each of controllers  58   1 - 58   4  controls the corresponding optical switch  52   1 - 52   4  to ensure that optical signals arriving from optical fiber  2  via OADM  54  will be forwarded to the corresponding receiver  30   1-1 - 30   1-4 . When no optical signals are received at receiver  30   1-1 - 30   1-4 , the corresponding optical switch  52   1 - 52   4  is activated by the corresponding controller  58   1 - 58   4  to forward the optical signals arriving at the predetermined wavelength λ 2-1 -λ 2-4  from optical fiber  4  via OADM  56 . In this manner the optical communication is protected in case of a fiber cut upstream along optical fiber  2 . OADM arrays  40   2 ,  40   3 ,  40   4 ,  40   5  and  40   6  are operative in a similar manner, mutatis mutandis.  
         [0090]    Reference is now made to FIG. 3 that presents a non-limiting example of an optical network constructed and operative according to a preferred embodiment of the present invention. The optical network described in FIG. 3 comprises six terminals  100 ,  101 ,  102 ,  103 ,  104  and  105 , each provided with four optical transmitters  122   1 - 122   4 ,  126   1 - 126   4 ,  130   1 - 130   4 ,  134   1 - 134   4 ,  138   1 - 138   4  and  142   1 - 142   4  respectively and four optical receivers  124   1 - 124   4 ,  128   1 - 128   4 ,  132   1 - 132   4 ,  136   1 - 136   4 ,  140   1 - 140   4  and  144   1 - 144   4  respectively. Each of the four pairs comprising an optical transmitter and an optical receiver is coupled to optical fibers  106  and  108  via optical coupler array  110 ,  112 ,  114 ,  116 ,  118  and  120 , respectively. Optical transmitters  122   1 - 122   4 ,  126   1 - 126   4 ,  130   1 - 130   4 ,  134   1 - 134   4 ,  138   1 - 138   4  and  142   1 - 142   4  may be operated to transmit at variable wavelengths λ a1 -λ a4 , λ c1 -λ c4 , λ e1 -λ e4 , λ b1 -λ b4 , λ d1 -λ d4 , λ f1 -λ f4  respectively, while optical receivers  124   1 - 124   4 ,  128   1 - 128   4 ,  132   1 - 132   4 ,  136   1 - 136   4 ,  140   1 - 140   4  and  144   1 - 144   4  are operative to receive optical signals at variable wavelengths λ b1 -λ b4 , λ d1 -λ d4 , λ f1 -λ f4 , λ a1 -λ a4 , λ c1 -λ c4 λ e1 -λ e4  respectively. As should be noted, the nomenclature λ a1 -λ a4 , λ c1 -λ c4 , λ e1 -λ e4 , λ b1 -λ b4 , λ d1 -λ d4 , λ f1 -λ f4  indicates that optical transmitters  122   1 - 122   4 ,  126   1 - 126   4 ,  130   1 - 130   4 ,  134   1 - 134   4 ,  138   1 - 138   4  and  142   1 - 142   4  as well as optical receivers  124   1 - 124   4 ,  128   1 - 128   4 ,  132   1 - 132   4 ,  136   1 - 136   4 ,  140   1 - 140   4  and  144   1 - 144   4  are operative at any desired wavelength chosen out of a plurality of wavelengths, as opposed to the fixed and predetermined wavelength at which optical transmitters  20   1-1 - 20   1-4 ,  20   2-1 - 20   2-4 ,  20   3-1 - 20   3-4 ,  20   4-1 - 20   4-4 ,  20   5-1 - 20   5-4  and  20   6-1 - 20   6-4  and optical receivers  30   1-1 - 30   1-4 ,  30   2-1 - 30   2-4 ,  30   3-1 - 30   3-4 ,  30   4-1 - 30   4-4 ,  30   5-1 - 30   5-4  and  30   6-1 - 30   6-4 , of the prior art network demonstrated in FIG. 1, are operable. This characteristic of optical transmitters and receivers is referred to herein as “tunable” transmitters or receivers, as the case may be. An example of such optical tunable transmitters and receivers was described in our co-pending patent application U.S. Ser. No. 10/266,580. The wavelength control of each of the optical transmitters  122   1 - 122   4 ,  126   1 - 126   4 ,  130   1 - 130   4 ,  134   1 - 134   4 ,  138   1 - 138   4  and  142   1 - 142   4  as well as optical receivers  124   1 - 124   4 ,  128   1 - 128   4 ,  132   1 - 132   4 ,  136   1 - 13   4 ,  140   1 - 140   4  and  144   1 - 144   4  is preferably done by a central management system that sends out commands to every optical transmitter and optical receiver. The transmission of the tuning commands from a central management system may be done by any of the methods known in the art per se, e.g. via another network such as an external IP network or via an in-band communication channel, using a dedicated wavelength for this purpose, and the like.  
         [0091]    In order to utilize efficiently the tunability characteristic of the optical transmitters and receivers, the spectrum comprising plurality of applicable wavelengths should preferably be made available at each network element, as there is no predetermination of the wavelength that will be used in that network element. However, as will be appreciated by those skilled in the art, other combinations can be made to apply different parts of the spectrum at different network elements, and the present invention should be understood to encompass also any such combinations, as well as combinations which include tunable and fixed transmitters/receivers at the same network. Consequently, in this preferred embodiment, the optical network is not based on predefined wavelength specific OADMs as described in FIG. 2, which operating mode is preset to allow a certain transmitting wavelength and a certain receiving wavelength, and prevent transmission of signals being at other wavelengths than the predefined ones. To overcome this drawback, in the system described in this embodiment and illustrated in FIG. 3, each of the optical transmitters  122   1 - 122   4 ,  126   1 - 126   4 ,  130   1 - 130   4 ,  134   1 - 134   4 ,  138   1 - 138   4  and  142   1 - 142   4  as well as optical receivers  124   1 - 124   4 ,  128   1 - 128   4 ,  132   1 - 132   4 ,  136   1 - 136   4 ,  140   1 - 140   4  and  144   1 - 144   4  are connected to/from optical fibers  106  and  108  via optical coupler arrays  110 ,  112 ,  114 ,  116 ,  118  and  120 . For example, optical transmitters  122   1 - 122   4  and optical receivers  124   1 - 124   4  are connected to optical fibers  106  and  108  via optical coupler array  110 . Similarly, optical transmitters  126   1 - 126   4 ,  130   1 - 130   4 ,  134   1 - 134   4 ,  138   1 - 138   4  and  142   1 - 142   4  and optical receivers  128   1 - 128   4 ,  132   1 - 132   4 ,  136   1 - 136   4 ,  140   1 - 140   4  and  144   1 - 144   4  are connected to optical fibers  106  and  108  via optical coupler arrays  112 ,  114 ,  116 ,  118  and  120 , mutatis mutandis. The network shown FIG. 3 is configured in a ring topology. However, it should be appreciated that the present invention also encompasses similar networks configured in other topologies such as bus or tree that can be constructed by implementing similar type of tunable transmitters and receivers coupled to optical fibers in the same manner as described herein.  
         [0092]    [0092]FIG. 4 describes terminal  100  in more details. When a multiplexed optical signal transmitted along optical fiber  106  reaches optical coupler  154 , part of the signal is forwarded via optical splitter  156  towards optical switches  152   1 - 152   4 , while the remaining of the signal is conveyed along fiber  106  to coupler  166 . Similarly, when a multiplexed optical signal transmitted along optical fiber  108  reaches optical coupler  158 , part of the signal is forwarded via optical splitter  160  towards optical switches  152   1 - 152   4 , while the remaining of the signal is conveyed along fiber  108  to coupler  168 . Optical switches  152   1 - 152   4  are each operative to connect to the corresponding receiver  124   1 - 124   4 , an optical signal arriving either from optical fiber  106  or from optical fiber  108 . Each of the signals generated at transmitters  122   1 - 122   4  is transmitted at the appropriate wavelength λ a1 -λ a4 , where each of these wavelengths is selected from among a predetermined plurality of wavelengths. The optical signal transmitted from transmitters  122   1 - 122   4  is splitted at optical splitters  150   1 - 150   4  to produce two similar optical signals preferably but not necessarily, at a substantially equal intensity. One of the two optical signals, from each of the optical splitters  150   1 - 150   4  is forwarded, via optical coupler  162  to coupler  166  to be combined with the signal received from coupler  154  along fiber  106 , while the other is forwarded, via optical coupler  164  to coupler  168  to be combined with the signal received from coupler  158  along fiber  108 . It should be appreciated that optical couplers  154 ,  158 ,  166  and  168  can be designed to provide a large range of coupling/splitting ratio. For example an optical coupler can be designed to provide equal coupling ratio where such a coupler is referred to as a 50/50 coupler, or it can split 90% of the optical signal into one port and the remaining 10% to the other port. In which case it is referred to as a 90/10 coupler. The optimal coupling ratio of each coupler may depend on several parameters, such as: the specific topology of the network, the number of network elements, the transmitter power, the receiver sensitivity etc.  
         [0093]    Under normal operation, controllers  170   1 - 170   4  control the respective optical switches  152   1 - 152   4 , to ensure that optical signals arriving from optical fiber  106  via coupler  154  and optical splitter  156  will be properly forwarded to receivers  124   1 - 124   4 . When no optical signals are received at any of the receivers  124   1 - 124   4 , the corresponding optical switch of optical switches  152   1 - 152   4  is activated by the respective controller  170   1 - 170   4  so as to forward the optical signals arriving from optical fiber  108  via coupler  158  and optical splitter  160 . In this manner the optical communication is protected and can reach its destination even in case of a fiber cut upstream along optical fiber  106 .  
         [0094]    Alternatively, the optical signals may be coupled from optical fibers  106  and  108  to receivers  124   1 - 124   4  via optical couplers that combine the optical signals from both optical fibers instead of optical switches  152   1 - 152   4 . This solution is more cost effective but could insert more attenuation in the signal path to receivers  124   1 - 124   4 . The optical network illustrated in FIG. 3 also comprises a protection switch  146 . This protection switch makes sure that, at any time, each optical signal arrives at any optical receiver only via one of the two optical fibers  106  or  108 . Therefore no problem of an optical interference should arise when optical signals from both optical fibers  106  and  108  are combined at the input to receivers  124   1 - 124   4 . Switch  146 is normally open to prevent undesirable propagation of optical signals along the two fibers ring type of network (optical fibers  106  and  108 ). In the prior art optical network illustrated in FIG. 1, such a switch is not required since each optical signal is blocked by one of OADMs  40   1 ,  40   2 ,  40   3 ,  40   4 ,  40   5  and  40   6 , so that no optical signal is propagating along the ring along optical fibers  2  and  4 . Since no OADMs are implemented in the optical network described in FIG. 3, it would be required to avoid a situation whereby an endless loop is created for each of the optical signals transmitted along optical fibers  106  and  108 . When a fault is detected along either of the optical fibers  106  and  108  or both, protection switch  146  is closed to allow establishing of a protection path for the optical signals. Under normal operating conditions, the optical signal transmitted from optical transmitter  122   1  in the example shown, arrives to optical receiver  136   1  over the CCW optical fiber  108 , passing through optical coupler arrays  110 ,  112 ,  114  and  116 . The optical signals transmitted from optical transmitter  134   1  arrives to optical receiver  124   1  over optical fiber  106  passing through optical coupler arrays  116 ,  114 ,  112  and  110 . However, when a transmission fault is detected, for example a fiber cut occurs between optical coupler arrays  112  and  114 , then the protection switch  146  is closed. Consequently, the optical signals transmitted from optical transmitter  122   1  arrive to optical receiver  136   1  over the CW optical fiber  106 , passing through optical coupler arrays  110 ,  120 , protection switch  146  and optical coupler arrays  118  and  116 . Similarly, optical signals from optical transmitter  134   1  will arrive to optical receiver  124   1  over optical fiber  108  passing through optical coupler arrays  116 ,  118 , protection switch  146  and optical coupler arrays  120  and  110 . In this way, the optical network architecture demonstrated that comprises two optical fibers ( 106  and  108 ) is protected against a fault at any location in the network.  
         [0095]    [0095]FIG. 5 describes in details the protection switch  146  illustrated in FIG. 3. For the sake of clarity, optical transmitters  122   1 - 122   4 ,  126   1 - 126   4 ,  130   1 - 130   4 ,  134   1 - 134   4 ,  138   1 - 138   4  and  142   1 - 142   4  and optical receivers  124   1 - 124   4 ,  128   1 - 128   4 ,  132   1 - 132   4 ,  136   1 - 136   4 ,  140   1 - 140   4  and  144   1 - 144   4 , as well as optical coupler arrays  110 ,  112 ,  114 ,  116 ,  118  and  120  are not shown in FIG. 5. Instead, only optical fibers  106 ,  108  and a detailed illustration of protection switch  146  are shown in FIG. 5. Protection switch  146  comprises two optical switches  174  and  176  that are connected in series with optical fibers  106  and  108  respectively, and are both normally open to prevent the optical signals reaching protection switch  146  from being forwarded.  
         [0096]    When the network&#39;s control identifies a fault in transmission which requires the protection switch  146  to close, an appropriate command is transmitted to the protection switch, resulting in closing switches  174  and  176 , thereby establishing a protection path for forwarding the signals&#39; transmission. Once switch  174  is closed, optical transmitter  178  shall transmit along optical fiber  106  all optical signals received at receiver  180 . Similarly, closure of switch  176  enables the transmission of the optical signals received at receiver  186  to be transmitted along optical fiber  108  by transmitter  184 .  
         [0097]    According to another embodiment of this aspect of the invention, optical transmitter  178  is operative to transmit all optical signals received at receiver  180  while inserting together with the optical signals, an optical test signal being at a different wavelength than the rest of the transmitted optical signals. In this example, the wavelength of the optical test signal is selected so that the signal does not to interfere with any traffic transmitted at any of the wavelengths λ a1 -λ a4 , λ c1 -λ c4 , λ e1 -λ e4 , λ b1 -λ b4 , λ d1 -λ d4 , λ f1 -λ f4 . Under normal operating conditions, the optical test signal is transmitted by transmitter  178  and travels along optical fiber  106 , and then received by optical receiver  180 . As long as optical receiver  180  receives that pre-defined optical test signal, which receipt confirms the integrity of the whole path (along fiber  106 ), controller  182  ensures that optical switch  174  remains open. Similarly, optical transmitter  184  transmits an optical test signal along optical fiber  108  and as long as this test signal is received at optical receiver  186 , controller  188  ensures that optical switch  176  remains open. When one of optical fibers  106  and  108  is cut, the respective optical receiver  180  or  186  will not receive the corresponding optical test signal and consequently, the appropriate controller of  182  and  188  will ensure the closure of the corresponding optical switch  174  or  176 , thereby establishing the protection path.  
         [0098]    In various applications, there arises the need to connect several optical networks, each configured, for example, in a ring topology. FIG. 6 illustrates a connection between two optical networks  191  and  192 , as known in the art. Optical network  191  comprises optical fibers  193  and  194  while the other optical network,  192 , comprises optical fibers  196  and  198 . Both optical networks illustrated in this example further comprise a plurality of network elements and an optical to electric converter (e.g. transceiver)  190   1  and  190   2 , respectively. The optical signals that need to be transferred from network  191  to network  192  are selectively dropped at transceiver  190   1 , converted from the optical domain to their electrical representation and are forwarded to electrical switch  199 . The electrical switch is operative to allow the transfers and possible cross-connect from rings  193  and  194  of network  191  to the optical rings  196  and  198  of network  192  and vice versa, in accordance with the signal&#39;s destination. The electrical signals are then converted back by the electric to optical converter  1902  and transmitted to their destination at network  192 . For the sake of simplicity, the OADMs, the transmitters and receivers of the various network elements are not shown. However, their operation is substantially similar to the corresponding device as described in connection with FIGS. 1 and 2. Nevertheless, as will be appreciated by those skilled in the art, this method is cumbersome and expensive, and requires double conversion from an optical signal to an electrical signal and back to an optical signal. The bandwidth of the traffic that can be transmitted that way is restricted by the bandwidth that can be converted by each of the electro-optical converters  190   1  and  190   2 . Additionally, the method requires an electrical switch that might be expensive and might adversely affect the transparency of the signal path.  
         [0099]    Another somewhat similar prior art method for connecting two optical networks is illustrated in FIG. 7. Again, the first optical network,  200 , comprises two optical rings  202  and  204  and a number of network elements. The second optical network  201 , comprises two optical rings  206  and  208  and a number of network elements. Optical signals transmitted along network  200  are selectively dropped by OADMs  212  and  214 , and transferred to the second optical network  201  via optical switch  210 . The optical signals are re-routed at the optical switch  210  and are forwarded to the second optical network via OADMs  216  and  218 . Similarly, optical signals from optical network  201  are selectively dropped by OADMs  216  and  218 , and transferred to optical network  200  via optical switch  210  and OADMs  212  and  214 . By this method, the optical signals are transferred from one optical network to the other in their optical form, without being converted into their electrical form for the switching stage. Still, in this case there is a need for an optical switch that is based on a photonic technology. Such a photonic switch is expensive and typically introduces a significant attenuation to the optical signals. Additionally, the wavelengths of the optical signals that can be connected between the two optical networks are predetermined, as they are dependent on the wavelengths at which transmissions can be added and/or dropped at OADMs  212 ,  214 ,  216  and  218 .  
         [0100]    The present invention provides another network configuration, as illustrated in FIG. 8, for connecting such two optical networks. Optical network  220  comprises two optical fibers  222  and  223  and a plurality of network elements. The other network  221 , comprises optical fibers  224  and  226  and a number of network elements. The two optical networks further comprise each a protection switch  232  and  234  respectively, and are interconnected via optical coupler array  228 . The transmission of optical signals between network elements that belong to the same optical network is similar to that described in conjunction with FIG. 3. Also, protection switches  232  and  234  are operative as described in conjunction with protection switch  146  in FIG. 5. The optical coupler array  228  shown in FIG. 8 is operative to transmit optical signals from the first optical network to the second one and vice versa. As described in conjunction with FIG. 3, the pairs of optical fibers  222  and  223  of network  220  and fibers  224  and  226  of network  221  are used to provide protected transmission of traffic and to provide an adequate solution in case of a fiber cut in any location in each network. As described above, in a case of a fiber cut any optical signal may appear only on one of the two fibers of an optical ring. Therefore optical coupler array  228  is capable of transferring optical signals from optical fiber  222  to both optical fibers  224  and  226 , and is also capable of transferring optical signals from optical fiber  223  to both optical fibers  224  and  226 . Similarly, the optical coupler array  228  is operative in the other direction mutates mutandis.  
         [0101]    [0101]FIG. 9 further illustrates the optical coupler array  228 . When an optical signal transmitted along optical fiber  223  arrives at optical coupler  236 , it is splitted to produce two similar optical signals, preferably at a substantially equal intensity. A first optical signal is forwarded along fiber  223  towards optical coupler  238  while the other is transferred to optical coupler  240 . Similarly, when an optical signal is transmitted along optical fiber  222 , it would arrive at optical coupler  232 , and be splitted to produce two similar optical signals, preferably at a substantially equal intensity. A first optical signal would be forwarded along fiber  222 , towards optical coupler  234  while the other be transferred to optical coupler  240 . It should be appreciated that under normal operating conditions, protection switch  232  (shown in FIG. 8) is open so that the optical signals arriving from the various network elements that belong to network  220 , arrive at the optical coupler array  228  only via optical fiber  222 . However, if optical fibers  222  and  223  are cut, protection switch  232  is closed and each of the optical signals arriving from the various network elements that belong to network  220 , arrive at optical coupler array  228  via optical fiber  222  or via optical fiber  223 , depending on the location of the fiber cut. Preferably, there should be no situation where the optical signals transmitted in network  220  arrive at optical coupler array  228  simultaneously from both optical fibers  220  and  222 . The combined output of optical coupler  240  is splitted to produce two similar optical signals, preferably at a substantially equal intensity. One of the two optical signals egressing optical coupler  240  is forwarded towards optical fiber  226  of network  221  via optical coupler  246 , while the other optical signal is forwarded to optical fiber  224  via optical coupler  248 .  
         [0102]    Similarly, in the opposite direction, when an optical signal transmitted along optical fiber  224  arrives at optical coupler  250 , it is splitted to produce two similar optical signals, preferably at a substantially equal intensity. A first optical signal is forwarded along fiber  224  towards optical coupler  248  while the other is transferred to optical coupler  242 . Similarly, if an optical signal is transmitted along optical fiber  226 , it would arrive at optical coupler  244 , and be splitted to produce two similar optical signals, preferably at a substantially equal intensity. A first optical signal would be forwarded along fiber  226 , towards optical coupler  246  while the other be transferred to optical coupler  242 . It should be appreciated that under normal operating conditions, protection switch  234  (shown in FIG. 8) is open so that the optical signals arriving from the various network elements that belong to network  221 , arrive at the optical coupler array  228  only via optical fiber  224 . However, when optical fibers  224  and  226  are cut, protection switch  234  is closed and each of the optical signals arriving from the various network elements that belong to network  221 , arrive at the optical coupler array  228  via optical fiber  224  or via optical fiber  226 , depending on the location of the fiber cut. Preferably, there should be no situation where the optical signals transmitted in network  221  arrive at optical coupler array  228  simultaneously from both optical fibers  224  and  226 . The combined output of optical coupler  242  is splitted to produce two similar optical signals, preferably at a substantially equal intensity. One of the two optical signals egressing optical coupler  242  is forwarded towards optical fiber  222  of network  221  via optical coupler  234 , while the other optical signal is forwarded to optical fiber  223  via optical coupler  238 .  
         [0103]    It should be appreciated that optical coupler array  228  is not selective, i.e. it transfers all optical signals, irrespective of their wavelength. Therefore any optical wavelength that is used for communication between the various network elements within the optical network  220  preferably should not be used for communication between the various elements of optical network  221 , nor for communication between elements belonging to the two optical networks.  
         [0104]    In order to enable an easier and more cost-effective method of transmission of optical signals within optical networks and between elements belonging to different networks, it is preferred to divide the available optical wavelengths into at least two groups, each comprising a plurality of wavelengths (e.g. wavelength group  260  and wavelength group  262  as exemplified in FIG. 10). In this embodiment, wavelength group  260  is dedicated for transmission of optical signals to and from network elements belonging to the same optical network (intra-network traffic), while wavelength group  262  is dedicated for transmission of optical signals that are destined to elements that belong to one or more different networks (inter-network traffic). In the example presented in FIG. 10, the wavelengths of group  260  are in the range of 1530-1532 nm and the wavelengths of group  262  are in the range of 1532-1535 nm. This arrangement enables easy differentiation between the two groups by appropriate optical filters and also enables the use of wavelengths that are reserved for the intra-network traffic (wavelength group  260 ) in each of the networks, irrespective of whether such wavelength is used within another network, as no transmission at such a wavelength shall leave that network. It should also be understood that the wavelength range of each of such wavelength groups ( 260  and  262 ) could be selected to match the specific communication requirements (i.e. the number of wavelengths required per each group). As mentioned above, the different groups according to the present invention are not restricted to comprise continuous range of wavelengths, but in the two groups example discussed, wavelength group  260  and/or  262  may comprise a plurality of specific pre-defined wavelengths.  
         [0105]    [0105]FIG. 11 illustrates two optical networks each comprising two optical rings, in a similar configuration to the one illustrated in FIG. 8. However, one important difference between these two configurations is that the connection between the two optical networks in FIG. 11 comprises wavelength selective OADMs  278 ,  280 ,  288  and  290 , as opposed to the optical coupler array  228  of the FIG. 8 network configuration. OADMs  278 ,  280 ,  288  and  290  are operative to selectively add and drop optical wavelength group  262  that is reserved for the inter-network traffic, and to allow transparent and uninterrupted passage therethrough of optical signals that do not belong to wavelength group  262 , including optical wavelength group  260 . Naturally, OADMs  278 ,  280 ,  288  and  290  may be designed to allow uninterrupted passage of optical signals transmitted at a wavelength that belong to group  260 , while performing “add and drop” activities for all optical signals transmitted at a wavelength that does not belong to group  260 . Therefore any wavelength that is used within network  292 , i.e. for communication between the elements that belong to network  292 , can safely be used within network  294 , i.e. for communication between the elements that belong to network  294 .  
         [0106]    When one of the network elements of network  292  is required to transmit an optical signal destined to a network element of network  294  that optical signal shall be transmitted at a wavelength selected from among the plurality of wavelengths comprising group  262 . As previously explained, OADMs  278  and  280  are operative to selectively drop optical signals arriving on optical fibers  272  and  270 , respectively, which are transmitted at a wavelength of group  262 . Any optical signal transmitted at a wavelength that belongs to group  262  is transferred to the “drop” output of each OADM, while other optical signal transmitted at wavelengths that do not belong to that group pass transparently via each of these OADMs. Under normal operating conditions, optical signals transmitted by a network element that belongs to network  292  at a group  262 &#39;s wavelength arrive along optical fiber  272  and pass through OADM  278  to optical coupler  284 . The output of optical coupler  284  is splitted into two optical signals. One is coupled to the “add” input of OADM  288  while the other optical signal is coupled to the “add” input of OADM  290 . The optical signals at the “add” input of OADMs  288  and  290  are coupled to optical fibers  276  and  274 , respectively, ensuring that optical signals transmitted by a network element of network  292  at a wavelength selected among the group  262  wavelengths are transmitted to the appropriate network element of network  294 .  
         [0107]    Each of networks  292  and  294  is provided with a protection switch,  282  and  286 , respectively. The structure and operation of these protection switches are substantially similar to those described in conjunction with protection switch  146  in FIG. 5. Under normal operating conditions, protection switches  282  and  288  are operative not only to block in each of networks  292  and  294  the propagation of signals conveyed at a wavelength that belongs to group  260 , but also to prevent signals transmitted from network  292  (or from network  294 ) to the other network at a wavelength that belongs to group  262 , from returning back to originating network. When optical fiber  272  is cut, protection switch  286  will close to establish an optical protection path along network  294 . Similarly, when optical fiber  274  is cut, protection switch  282  will close to establish an optical protection path along network  292 . Once protection switch  286  is closed, a protection path along optical fiber  270  is established, allowing forwarding of optical signals via OADM  280  to optical coupler  284 . Optical signals that arrive at OADM  278  along optical fiber  272  shall be combined at optical coupler  284  with optical signals transmitted along optical fiber  270  via OADM  280 , thus providing protection in the case of a fiber cut, independent of the location of the cut. In a similar manner optical signals may be transmitted from network  294  (via OADMs  288  and  290  and optical coupler  296 ) to network  292 .  
         [0108]    [0108]FIG. 12 describes an embodiment of the present invention that concerns the interoperability of a plurality of optical networks (three double fiber ring networks in this example). The three networks  300 ,  301  and  302  comprising the optical fiber pairs  303  and  304 ,  305  and  306 , and  308  and  310 , respectively. Similarly to FIG. 11 the network elements that are associated with these networks, are not shown in FIG. 12. Again, similarly to the previous example shown in FIG. 11, the intra-network communication between elements that belong to the same network is done by transmitting optical signals that are selected from a group of a first plurality of wavelengths, group  260 , while inter-network communication between elements associated with different network(s) is done by transmitting optical signals at wavelengths selected from a group of a second plurality of wavelengths, group  262 . Similarly to the network described in connection with FIG. 11, OADMs  312  and  314  are operative to transfer optical signals to and from network  300  that are transmitted at a group  262 &#39;s wavelength, while OADMs  330  and  332  are operative to transfer optical signals to and from network  302  that also transmitted at a group  262 &#39;s wavelength. Protection switches  320  and  322  are operative to provide an optical protection path in networks  300  and  302  respectively, in a similar way to the one described in connection with the network configuration illustrated in FIG. 11. Under normal operating conditions, protection switches  320  and  322  are operative not only to block the propagation of signals conveyed at a wavelength that belongs to group  260  in each of networks  300  and  302 , but also to prevent signals at a wavelength that belongs to group  262  and transmitted to network  300  or to network  302  from returning back to the originating network. Similarly to the architecture shown in FIG. 11, optical couplers  316  and  318  are operative to transfer optical signals to and from network  300  and provide an optical protection path in the case of fiber cut in that network, while optical couplers  338  and  340  are operative to transfer optical signals to and from network  302  and provide an optical protection path in the case of fiber cut in network  302 . Optical coupler array  324  is operative to couple optical signals transmitted to and from optical fibers  305  and  306  (network  301 ) and forward them towards network  300  via optical couplers  316  and  318 . Optical coupler array  342  is operative to couple optical signals transmitted to and from optical fibers  305  and  306  (network  301 ) and forward them towards network  302  via optical couplers  338  and  340 . In this configuration, any optical signal that is transmitted along network is transferred via optical coupler arrays  324  and  342  towards both networks  300  and  302 . When a signal is transmitted from, for example, a network element in network  301 , at a wavelength that belongs to wavelength group  260 (reserved for intra-network traffic) it reaches both networks  300  and  302  but is blocked at the corresponding OADMs which are adapted to allow the transfer of signals at wavelengths which belong to wavelength group  262 . Let us now consider an example where an optical signal transmitted at network  300  at a group  262 &#39;s wavelength and is destined to a network element located in network  302 . Such an optical signal will pass through: OADM  312  or  314 , optical coupler  316  and optical coupler array  324  and arrive to network  301 . After passing along network  301 , this optical signal will pass through optical coupler array  342 , optical coupler  340  and OADM  330  or  332 , and finally reach network  302  where it will be forwarded to its destination.  
         [0109]    As shown in FIG. 12, network  301  also comprises protection switch ( 350 ), which operates in a similar to that by which the protection switches  146 ,  232 ,  234 ,  282  and  286  previously described.  
         [0110]    It should be appreciated that in several optical networks a need to amplify the optical signals as a result of extensive attenuation by the optical fibers and the optical coupling devices may arise. The most popular optical amplifiers that are suitable for this purpose are EDFA—Erbium Doped Fiber Amplifiers. These amplifiers are widely used and can provide a typical gain of more than 25 dB. Since all the optical paths in the networks described in FIGS. 3, 8,  11  and  12  are unidirectional, it is easy to introduce optical amplifiers in any location at the network, as may be required.  
         [0111]    As described above in conjunction with FIGS. 3, 8,  11  and  12 , one of the advantages of the networks described in these Figs. is, that the nodes comprised therein are coupled to the optical fibers via non-selective optical couplers, i.e. couplers that are operative to deliver the signals received irrespective of their wavelength. This feature enables a significant degree of flexibility in the operation of each node in the network. However, the main disadvantage of this configuration is that the optical signal is significantly attenuated on its path from the optical fiber to each optical transmitter and to each optical receiver. The more optical transmitters and optical receivers, the higher the optical signal attenuation is, since the optical signal energy is split to more directions. A solution directed to overcome this problem is described in FIG. 13.  
         [0112]    For the sake of clarity, the terminal configuration is shown in FIG. 13 with respect to only one fiber. However, it should be clearly understood by a skilled man in the art that such a terminal as described below, can be configured to operate with two fibers, as illustrated, for example in FIG. 4. When an optical signal transmitted along fiber  402  arrives at passive optical coupler  416 , part of the optical energy is forwarded to filter array  404  while the rest remains with the optical signal that is conveyed onward along fiber  402 . The function of filter array  404  is to selectively split the optical signal at its input into several sub-ranges, (in this example, four frequency sub-ranges). The width of each sub-range was set in this example to 200 GHz. One output of filter array  404  is coupled via optical coupler/splitter to the input of optical receivers  408   1  through  408   4 . In the other direction, the output of optical transmitters  410   1  through  410   4  is coupled via optical coupler  412  to an input of filter array  414 . Similarly to filter array  404 , filter array  414  is operative to selectively combine several sub-ranges, (in this example, four frequency sub-ranges). The output of filter array is coupled to optical fiber  402  via optical coupler  418 . Optical transmitters/receivers can be tunable over the range of the sub-range of the corresponding port of filter arrays  404  and  414 . When more than 4 optical transmitters/receivers are required in that terminal, additional ports of filter arrays  404  and  414  can be applied. In this example a maximum of 16 optical transmitters/receivers can be implemented in the network element. The main advantage of this network element configuration is a much lower signal attenuation for the case of a large number of optical transmitters and receivers in the network element. For example, for  16  optical transmitters/receivers the signal attenuation from optical coupler  416  to the input of each optical receiver is typically less than 9 dB as opposed to typically 14 dB in the configuration described in FIG. 4. Another advantage of this network element configuration is its modularity and scalability. If we consider an example that a network element was configured initially for 4 optical transmitters/receivers, and an expansion is required, more optical transmitters/receivers may be easily added, without affecting the traffic transmitted along the operating channels, by simply using available ports at filter arrays  404  and  414 .  
         [0113]    The present invention has been described using non-limiting detailed descriptions of embodiments thereof that are provided by way of example and are not intended to limit the scope of the invention. It should be understood that features and/or steps described with respect to one embodiment may be used with other embodiments and that not all embodiments of the invention have all of the features and/or steps shown in a particular figure or described with respect to one of the embodiments. Variations of embodiments described will occur to persons of the art.  
         [0114]    It is noted that some of the above described embodiments describe the best mode contemplated by the inventors and therefore include structure, acts or details of structures and acts that may not be essential to the invention and which are described as examples. Structure and acts described herein are replaceable by equivalents, which perform the same function, even if the structure or acts are different, as known in the art. Therefore, the scope of the invention is limited only by the elements and limitations as used in the claims. When used in the following claims, the terms “comprise”, “include”, “have” and their conjugates mean “including but not limited to”.