Patent Publication Number: US-10313047-B2

Title: Optical wavelength selective switch, an optical network node, an optical network and methods therein

Description:
RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 14/416,761, filed Jan. 23, 2015, which was the National Stage of International Application No. PCT/SE2014/051210, filed Oct. 13, 2014, the disclosures of each of which are incorporated herein by reference in their entirety. 
    
    
     TECHNICAL FIELD 
     Embodiments herein relate to an optical wavelength selective switch, an optical network node comprising the optical wavelength selective switch, an optical network comprising the optical network node and methods therein. In particular, they relate to switching of optical signals. 
     BACKGROUND 
     One of the building blocks of today&#39;s optical networks is the Wavelength Selective Switch (WSS). A WSS comprises a common port and several tributary ports for input and output of optical signals to be switched, e.g. Wavelength Division Multiplexing (WDM) channels. In fiber-optic communications WDM is a technology which multiplexes a number of optical carrier signals onto a single optical fiber by using different wavelengths of laser light. This technique enables bidirectional communications over one strand of fiber, as well as multiplication of capacity. Through an electronic control interface, a WSS may be (re)configured to switch or route any of the incoming WDM channels on the common port to one of the tributary ports, irrespective of how other incoming wavelengths are routed. That is, on a single WSS, wavelengths may be routed between the common and tributary ports. 
     To create a more flexible wavelength routed switch two or more WSS devices may be interconnected in a particular way. Currently, the interconnecting WSS devices is the technology of choice to bring more flexibility to the WDM switching systems for transport networks, e.g. for local interconnection of different access segments in broadband communications networks. A problem with this is the complexity of the architecture and the increased cost due to the use of more than one WSS. That is, the required number of WSS elements and the required number of ports per WSS rapidly increases with the number of directions of an optical interconnect comprising such WSSs. 
     Optical Cross Connects (OXC) and Re-Configurable Optical Add Drop Multiplexers (ROADM) are examples of basic building blocks of transparent optical networks, which utilize switching in the optical domain. An OXC serves as an interconnection node between a number of fibres and/or directions, capable of routing wavelengths between the different directions in a reconfigurable manner. A degree of the OXC refers to the number of directions that are interconnected at the node. The ROADM may provide similar cross-connect functionality as the OXC and in addition also add/drop of wavelengths to/from local ports. 
     One of the problems with today&#39;s OXCs and ROADMs is the high cost associated with the number of required WSSs and WSS ports for introducing a desired flexibility or re-configurability. 
     The proliferation of broadband access to the Internet coupled with the introduction of new bandwidth-hungry applications, like video streaming and online gaming, over the last couple of years, have collectively led to an ever-increasing amount of traffic being exchanged inside communications networks, such as the internet. This trend has turned the design and operation of appropriate, cost-efficient transport networks into a major challenge for network operators. One of the main technologies used to realize transport networks is the optical communications and networking, which—thanks to the WDM concept—may provide larger transport capacities at a lower cost compared to the electronics counterparts. 
     Nonetheless, a problem is that the optical communications and networking technology is somewhat weak in providing flexibility, which is a significant requirement of advanced transport solutions in for example wireless communications networks. 
     SUMMARY 
     It is therefore an object of embodiments herein to provide an improved way of switching of optical signals. 
     According to a first aspect of embodiments herein, the object is achieved by a method in an optical Wavelength Selective Switch, WSS, for multidirectional switching of optical signals. The optical WSS comprises a reflective element, a first tributary port and a second tributary port. The optical WSS switches an optical signal between the first tributary port and the second tributary port with the reflective element. 
     According to a second aspect of embodiments herein, the object is achieved by an optical WSS according to the first aspect above, for multidirectional switching of optical signals. The optical WSS comprises a reflective element, and a first tributary port and a second tributary port. The optical WSS is adapted to switch an optical signal between the first tributary port and the second tributary port with the reflective element. 
     According to a third aspect of embodiments herein, the object is achieved by an optical network node for multidirectional switching of optical signals in a communications network. The optical network node comprises one or more optical WSSs according to the second aspect above. 
     According to a fourth aspect of embodiments herein, the object is achieved by an optical network comprising at least one optical network node, according to the third aspect above. The optical network further comprises a first service node, a second service node, a client node, a first optical access ring and a second optical access ring. The optical network node is connected to the first optical access ring and the second optical access ring. The first service node is connected to the first optical access ring. The first client node is connected to the second optical access ring. The second service node is connected to any other part of the optical network. 
     The optical network node is configured to route a first optical signal on a first wavelength band between the first service node and the first client node. The optical network node is further configured to route a second optical signal on a second wavelength band between the first service node and the second service node. 
     Since the WSS switches the optical signal between the first tributary port and the second tributary port with the reflective element the WSS is capable of multidirectional switching of optical signals. 
     An advantage with embodiments herein is that they enable realizing more complicated network switching functions with fewer numbers of WSSs. This may further reduce signalling in the network as a control module of the switching device will need to keep states for fewer numbers of WSSs and instruct fewer devices. 
     Specifically, embodiments herein allow switching between tributary ports. 
     Further, embodiments herein allow utilizing the same subset of wavelength channels for the following two types of WDM connections at the same time and within a single WSS: a) a connection between a first pair of ports, and b) one or more connections—depending on the size of the optical WSS—between second pairs of ports. This could for example be a) a connection between the common port and a certain tributary port, and b) one or more connections—depending on the size of the optical WSS—between pairs of tributary ports. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Examples of embodiments herein are described in more detail with reference to attached drawings in which: 
         FIG. 1  is a schematic block diagram illustrating a prior art WSS. 
         FIG. 2  is a schematic block diagram depicting an optical WSS according to embodiments herein. 
         FIG. 3  is a flowchart depicting embodiments of a method in an optical WSS according to embodiments herein. 
         FIG. 4 a    is a schematic block diagram depicting an optical WSS according to further embodiments herein. 
         FIG. 4 b    is a schematic block diagram depicting an optical WSS according to further embodiments herein. 
         FIG. 5  is a schematic block diagram depicting an optical WSS according to further embodiments herein. 
         FIG. 6  is a schematic block diagram depicting an optical WSS according to further embodiments herein. 
         FIG. 7 a    is a schematic block diagram depicting embodiments of an optical network node comprising multiple optical WSSs according to embodiments herein. 
         FIG. 7 b    is a schematic block diagram depicting embodiments of an optical network node comprising an optical WSS according to embodiments herein. 
         FIGS. 8 a - e    are schematic block diagrams depicting optical WSSs according to further embodiments herein. 
         FIG. 9 a    is a schematic block diagram depicting embodiments of an optical network node comprising multiple optical WSSs according to embodiments herein. 
         FIG. 9 b    is a schematic block diagram depicting further embodiments of an optical network node comprising multiple optical WSSs according to embodiments herein. 
         FIG. 10 a    is a schematic block diagram depicting further embodiments of an optical network node comprising multiple optical WSSs according to embodiments herein. 
         FIG. 10 b    is a schematic block diagram depicting further embodiments of an optical network node comprising multiple optical WSSs according to embodiments herein. 
         FIG. 11 a    is a schematic block diagram depicting embodiments of a 4×4 OXC comprising multiple optical WSSs according to embodiments herein. 
         FIG. 11 b    is a schematic block diagram depicting a prior art 4×4 OXC comprising optical WSSs according to embodiments herein. 
         FIG. 12 a    is a schematic block diagram depicting embodiments of a 2D ROADM comprising multiple optical WSSs according to embodiments herein. 
         FIG. 12 b    is a schematic block diagram depicting a prior art 2D ROADM comprising multiple WSSs. 
         FIG. 13 a    is a schematic block diagram depicting embodiments of a 6×6 OXC comprising multiple optical WSSs according to embodiments herein. 
         FIG. 13 b    is a further schematic block diagram depicting embodiments of a 6×6 OXC comprising multiple optical WSSs according to embodiments herein. 
         FIG. 13 c    is a further schematic block diagram depicting embodiments of a 6×6 OXC comprising multiple optical WSSs according to embodiments herein. 
         FIG. 14  illustrates connectivity properties for the 6×6 OXC in  FIGS. 13 a   - c.    
         FIG. 15  illustrates a use case for the 6×6 OXC in  FIGS. 13 a   - c.    
         FIG. 16  is a schematic block diagram depicting further embodiments of a 6×6 OXC comprising multiple optical WSSs according to embodiments herein. 
         FIG. 17  illustrates a use case for the 6×6 OXC in  FIG. 16 . 
         FIG. 18  illustrates connectivity properties for the 6×6 OXC in  FIG. 16 . 
         FIG. 19  is a schematic block diagram depicting an optical network. 
         FIG. 20  is a schematic block diagram depicting embodiments of an optical network comprising optical network nodes according to embodiments herein. 
         FIG. 21  is a schematic block diagram depicting further embodiments of an optical network comprising optical network nodes according to embodiments herein. 
         FIG. 22 a    is a schematic block diagram depicting further embodiments of an optical network comprising optical network nodes according to embodiments herein. 
         FIG. 22 b    is a schematic block diagram depicting further embodiments of an optical network comprising optical network nodes according to embodiments herein. 
         FIG. 23  is a schematic block diagram depicting further embodiments of an optical network comprising optical network nodes according to embodiments herein. 
         FIG. 24  is a schematic block diagram depicting further embodiments of an optical network comprising optical network nodes according to embodiments herein. 
     
    
    
     DETAILED DESCRIPTION 
     As part of developing embodiments herein, a problem will first be identified and discussed. 
     In a prior art WSS the various incoming wavelength channels of a common port are dispersed continuously onto a switching element which then directs each of these channels independently to N tributary ports. The switching element may be an array of mirrors, implemented by e.g. Micro Electro Mechanical System (MEMS) mirrors or Liquid Crystal on Silicon (LCoS). The switching element may be configured and reconfigured to direct an optical signal of a particular wavelength or wavelength band injected into the common port into any desired single tributary port.  FIG. 1  depicts a prior art WSS. 
     The mirror configurations may be independently adjusted for different colours of the light, so that different wavelength channels may be independently routed from the common port to any of the tributary ports. Furthermore, the optical WSS architectures are intrinsically bi-directional, so that when a mirror is configured to direct the optical signal of the particular wavelength or wavelength band between the common port and a particular tributary port, the connection on that wavelength may basically be used in both of the directions. 
     As mentioned above, a more flexible wavelength routed switch may be obtained by interconnecting two or more WSS devices in a particular way. A problem with this is the complexity of the architecture and the increased cost due to the use of more than one WSS. That is, the required number of WSS elements and the required number of ports per WSS rapidly increases with the number of directions of an optical interconnect comprising such WSSs. This problem will now be further illustrated with OXC/ROADM network elements. 
     There are different ways of realizing OXC/ROADM network elements. One common avenue is by combining multiple WSSs and power splitters to a structure which has the desired wavelength routing properties. A basic 4-degree ROADM may require 4 WSSs, one per direction, and may provide flexible wavelength routing between the different directions. Add/drop functionality may be directionally dependent and constricted to fixed wavelengths. Reconfiguration of the add/drop wavelengths may require manual intervention. 
     As mentioned above one of the problems with today&#39;s OXC/ROADMs is the high cost associated with the number of required WSSs and WSS ports for introducing the desired flexibility/re-configurability. 
     Additionally, the complexity of control and signalling for ROADM reconfigurations is proportional to the number of employed WSSs. 
     In embodiments herein an optical WSS for multidirectional switching of optical signals and a method for multidirectional switching of optical signals in the optical WSS will be presented. In embodiments herein, multidirectional switching of optical signals comprises switching optical signals between tributary ports of the WSS. The multidirectional switching of optical signals may also comprise switching a first optical signal on a certain wavelength band between a first pair of ports, and switching a second optical signal within the same wavelength band between a second pair of ports. 
     Further embodiments will present an optical network node comprising one or more optical WSS with multidirectional switching capabilities. The network node may for example be an OXC or an ROADM comprising two or more WSSs with multidirectional switching capabilities. 
     Yet further embodiments will present optical networks based on the optical network node. 
       FIG. 2  schematically depicts an optical WSS  200  according to embodiments herein. As mentioned above the optical WSS  200  comprises a reflective element  210 . 
     The optical WSS  200  further comprises two or more tributary ports, such as a first tributary port  231 , and a second tributary port  232 . 
     In some embodiments the optical WSS  200  comprises further ports. For example in addition to the first tributary port  231  and the second tributary port  232  the optical WSS  200  may comprise a third port  233 , such as a third tributary port, and a fourth port  234 , such as a fourth tributary port or a common port. 
     The use of the terms first tributary port, second tributary port, etc. in embodiments herein does not necessarily reflect any positional or functional relation between the ports, but merely indicates that there are two or more ports. Since the number of tributary ports may vary between the embodiments described herein the first tributary port  231 , the second tributary port  232 , etc. should be seen as logical ports and may symbolise different physical ports. 
     According to some embodiments the first tributary port  231  is adjacent to the fourth port  234 , such as the common port, and the second tributary port  232  may be adjacent to the third port  233 , such as the third tributary port. 
     An optical signal from one of the ports may reach the reflective element  210  and be reflected by the reflective element  210 . An incident beam of the optical signal, which is incident on the reflective element  210 , and the normal of the effective reflective plane of the reflective element  210  define a first angle  251 . A reflected beam of the optical signal, which is reflected from the reflective element  210 , and the normal of the effective reflective plane of the reflective element  210  define a second angle  252 . In order to switch the optical signal, the reflective element  210  may be adjusted causing the second angle  252  to equal the first angle  251 . 
     In other words, the switching of the optical signal between the first tributary port  231  and the second tributary port  232  may comprise adjusting the reflective element  210 , causing the first angle  251 , defined by the incident beam of the optical signal, incident on the reflective element  210 , and the normal of the effective reflective plane of the reflective element  210 , to equal the second angle  252 , defined by the normal and the reflected beam of the optical signal, reflected from the reflective element  210 . 
     A third angle  261 ,  262 ,  263  is defined by two adjacent optical beams incident on, or reflected from, the reflective element  210 . 
     The optical WSS  200  may be configured to switch a wavelength channel from the common port to any of the tributary ports by adjusting an effective reflective plane of the reflective element  210 . 
     A control module  270  in the optical WSS  200  may be adapted to perform switching by adjusting the reflective element  210 . The control module  270  may also be adapted to adjust any other adjustable parameter, e.g. angles or distances. 
     The optical WSS  200  may further comprise a memory  290 . 
     The optical WSS  200  may be designed in a way that the ports  231 ,  232 ,  233 ,  234  are mechanically placed next to each other in a row with a very small distance d 1 , d 2 , d 3  from each other. 
     Embodiments of a method in the optical WSS  200  for multidirectional switching of optical signals will now be described with reference to the optical WSS  200  in  FIG. 2  and a flowchart depicted in  FIG. 3 . 
     In short, in embodiments herein the optical WSS  200  utilises additional reflections from the reflective element  210  to introduce switching between tributary ports. 
     When the optical WSS  200  has four or more ports, e.g. the common port and three tributary ports, switching between the first tributary port  231  and the second tributary port  232  may be introduced by configuring the optical WSS  200  to switch between two other ports, e.g. between the common port  234  and the third tributary port  233 . 
     The method comprises the following actions, which actions may be taken in any suitable order. 
     Action  301   
     In some embodiments the optical WSS  200  adjusts the three or more third angles  261 ,  262 ,  263 , causing the three or more third angles  261 ,  262 ,  263  to have the same magnitude. This is done to extend the number of ports available for multidirectional switching as will be explained in more detail below. The adjustment of the three or more third angles  261 ,  262 ,  263  may for example be implemented by adjusting the distance d 1 , d 2  and d 3  between adjacent ports as will be explained in more detail below. 
     The control module  270  may be adapted to perform action  301 . 
     Action  302   
     Since multidirectional switching is not achieved between arbitrary ports of the optical WSS  200 , the optical WSS  200  may select the ports for multidirectional switching. 
     In some embodiments the number of ports available for multidirectional switching is limited due to that the three or more third angles  261 ,  262 ,  263  do not have the same magnitude, e.g. if the distances d 1 , d 2  and d 3  are equal. In those embodiments the optical WSS  200  selects the third port  233  as the tributary port having a largest angle defined by the first angle  251  or the second angle  252 . For example, for a 1:3 WSS the WSS  200  selects the third physical tributary port as the third port  233 . 
     The control module  270  may be adapted to perform action  302 . 
     Action  303   
     As mentioned above, when the optical WSS  200  has four or more ports, e.g. the common port and three tributary ports, switching between the first tributary port  231  and the second tributary port  232  may be introduced by configuring the optical WSS  200  to switch between two other ports, e.g. between the common port  234  and the third tributary port  233 . 
     Thus, before the optical WSS  200  switches the optical signal between the first tributary port  231  and the second tributary port  232 , the optical WSS  200  may configure the optical WSS  200  to switch a first optical signal on a specific wavelength band, between the third port  233  and the fourth port  234 , with the reflective element  210 . 
     In some embodiments the optical WSS  200  configures the optical WSS  200  to switch the first optical signal between the common port  234  and the third tributary port  233 , with the reflective element  210 . 
     The optical WSS  200  may configure the optical WSS  200  to switch the first optical signal between the fourth tributary port  234  and the third tributary port  233 , with the reflective element  210 . 
     The control module  270  may be adapted to perform action  303 . 
     Action  304   
     The optical WSS  200  switches the optical signal between the first tributary port  231  and the second tributary port  232  with the reflective element  210 . 
     In some embodiments the switching of the optical signal between the first tributary port  231  and the second tributary port  232  comprises adjusting the reflective element  210 , causing a first angle, defined by the optical path between the first tributary port  231  and the reflective element  210  and the normal of an effective reflective plane of the reflective element  210 , to equal a second angle, defined by the normal and the optical path between the reflective element and the second tributary port  232 . 
     When the optical WSS  200  has been configured to switch a first optical signal on a specific wavelength band, between the third port  233  and the fourth port  234 , with the reflective element  210 , the optical WSS  200  may switch a second optical signal, within the same specific wavelength band, between the first tributary port  231  and the second tributary port  232 , with the reflective element  210 . 
     The control module  270  may be adapted to perform action  304 . 
     The reuse of the wavelength may be dynamically reconfigured through the normal control procedure of the optical WSS  200 . 
     To perform the method actions for multidirectional switching of optical signals described above in relation to  FIG. 3 , the optical WSS  200  comprises the following arrangement depicted in  FIG. 2 . As mentioned above the optical WSS  200  comprises the reflective element  210 , and the first tributary port  231  and the second tributary port  232 . The optical WSS  200  may further comprise the third port  233  and the fourth port  234 . 
     In some embodiments the third port  233  is the third tributary port, and the fourth port  234  is the common port. 
     In some other embodiments the third port  233  is the third tributary port and the fourth port  234  is the fourth tributary port. 
     The optical WSS  200  is adapted to, e.g. by means of the control module  270  adapted to, switch an optical signal between the first tributary port  231  and the second tributary port  232  with the reflective element  210 . 
     In some embodiments the optical WSS  200  is adapted to, e.g. by means of the control module  270  adapted to, switch the optical signal between the first tributary port  231  and the second tributary port  232  by adjusting the reflective element  210 , causing the first angle  251 , defined by the incident beam of the optical signal, incident on the reflective element  210 , and the normal of an effective reflective plane of the reflective element  210 , to equal the second angle  252 , defined by the normal and a reflected beam of the optical signal, reflected from the reflective element  210 . 
     When the optical WSS  200  further comprises the third port  233  and the fourth port  234 , the optical WSS  200  may be adapted to configure the optical WSS  200  to switch the first optical signal on the specific wavelength band, between the third port  233  and the fourth port  234 , with the reflective element  210 , and switch the second optical signal, within the same specific wavelength band, between the first tributary port  231  and the second tributary port  232 , with the reflective element  210 . 
     The optical WSS  200  may be further adapted to, e.g. by means of the control module  270  adapted to, switch the second optical signal between the tributary port  231  adjacent to the common port  234  and the tributary port  232  adjacent to the third tributary port  233 . 
     The optical WSS  200  may further be adapted to, e.g. by means of the control module  270  adapted to, select the third port  233  as the tributary port having the largest angle defined by the first angle  251  or the second angle  252 . 
     In some embodiments the optical WSS  200  is further adapted to, e.g. by means of the control module  270  adapted to, adjust the three or more third angles  261 ,  262 ,  263  to have the same magnitude. 
     The optical WSS  200  may further be adapted to, e.g. by means of the memory  290  adapted to, store e.g. the angles, the distances etc. The memory  290  comprises one or more memory units. The memory  290  is further adapted to store configurations and applications to perform the methods herein when being executed in the optical WSS  200 . 
     In the following detailed embodiments a first physical tributary port will be referred to with reference numeral  1 . Likewise a second, third and following physical tributary ports will be referred to with reference numerals  2 ,  3 , etc. The common port will be referred to with reference c. In the following embodiments the term physical reflects the positional hierarchy between the tributary ports of the optical WSS. 
       FIG. 4 a    schematically depicts a 1:4 optical WSS  400  according to some first embodiments herein. As mentioned above the 1:4 optical WSS  400  comprises the reflective element  210 . 
     The 1:4 optical WSS  400  further comprises the common port c. 
     The 1:4 optical WSS  400  further comprises a first physical tributary port  1 , a second physical tributary port  2 , a third physical tributary port  3  and a fourth physical tributary port  4 . The term physical reflects the positional hierarchy between the tributary ports of the 1:4 optical WSS  400 . 
     The first embodiments herein utilise so-called unwanted reflections between pairs of the four tributary ports as an additional switching capability. The 1:4 optical WSS  400  in  FIG. 4 a    will be used as an example. When the 1:4 optical WSS  400  is configured to route or switch a given number of wavelength channels between the common port c and the fourth physical tributary port  4 , then the same wavelength channels are reused between a pair of tributary ports, such as the first physical tributary port  1  and the third physical tributary port  3 , for additional connections. 
     Due to the nature of the internal architecture of the optical WSS  200 ,  400  there is no perfect isolation among WSS ports and additional reflections may occur. These reflections are usually considered as “unwanted” and efforts are usually made to minimize them as much as possible. Embodiments herein utilize such “unwanted” reflections in a constructive way, and thereby enhancing the switching functionality of the optical WSS  200 ,  400 . More specifically, the optical WSS  200 ,  400  may be designed in a way that the ports are mechanically placed next to each other in a row with very small distance from each other. This setting contributes to the so-called unwanted reflections. In  FIG. 4 a   , the reflective element  210  is configured to route the wavelength channel from the common port c to the fourth physical tributary port  4 . The side effect of this configuration is, however, that for the same wavelength channel there will be self-reflection for the central tributary port, i.e. the second physical tributary port  2 , and reflection between the first physical tributary port  1  and the third physical tributary port  3 . 
     Note that although the example is based on a 1:4 WSS, the same phenomenon is present in larger and smaller WSSs. For instance, configuring a 1:9 WSS to route a specific wavelength from the common port to port  9 , then on the same wavelength there will be reflections between the pairs of ports ( 1 , 8 ), ( 2 , 7 ), ( 3 , 6 ) and ( 4 , 5 ). Self-reflection occurs for port N/2 when N is an even number. 
     The first embodiments has been further illustrated in  FIG. 4 b   , where it is assumed that all the wavelength channels of the WDM link may be divided into 4 disjoint groups of A, B, C and D. Any of the wavelengths belonging to any of the four groups may be independently routed between the common port c and any physical tributary port  1 ,  2 ,  3 ,  4  of the 1:4 optical WSS  400 . The same wavelength channels that are routed between the common port c and the fourth physical tributary port  4 , i.e., channel group D in this example, is reused to provide a switching capability between the physical second tributary port  2  and the third physical tributary port  3 . With this configuration the switching capacity between the common port c and the first physical tributary port  1  will be equal to that between the second physical tributary port  2  and the third physical tributary port  3 . 
     Some second embodiments will now be described with reference to  FIG. 4 a   . As mentioned above the optical 1:4 optical WSS  400  comprises the reflective element  210 , the first physical tributary port  1 , the second physical tributary port  2 , the third physical tributary port  4  and the fourth physical tributary port  4 . The optical WSS  400  may comprise a common port c. 
     In the second embodiments the 1:4 optical WSS  400  is configured to switch a first optical signal within a specific wavelength band between the first physical tributary port  1  and the fourth physical tributary port  4 , with the reflective element  210 . 
     Then the 1:4 optical WSS  400  may further switch a second optical signal, within the same specific wavelength band, between the second physical tributary port  2  and the third physical tributary port  4 , with the reflective element  210 . 
     This embodiment may be implemented by rotating the reflective element  210  in a direction causing the angle defined by the optical path between the common port c and the reflective element  210 , and the normal of the effective reflective plane to be smaller than the angle needed for switching between the common port c and the tributary port with the largest angle towards the normal, i.e. the fourth physical tributary port  4  in  FIG. 4 a   . In  FIG. 4 a    the reflective element is rotated in a direction causing the normal of the reflective pane to move away from the common port c. 
     Some third embodiments will now be described with reference to  FIGS. 5 and 6 .  FIG. 5  schematically presents a first 1:5 optical WSS  501  with equal distances d between all ports. The first 1:5 optical WSS  501  comprises a fifth physical tributary port  5 . The number of tributary ports that may offer multidirectional switching for the same wavelength band is limited if the distances between the ports are equal. Equal distances results in that adjacent third angles  261 ,  262  are different. When the adjacent third angles are different the first 1:5 optical WSS  501  may perform additional switching on the same wavelength band between the tributary port adjacent to the common port and the tributary port adjacent to the tributary port having the largest angle defined by the optical path between the tributary port and the normal of the effective reflective plane of the reflective element  510 . In other words, for the first 1:5 optical WSS  501  this means that switching on the same wavelength band is possible between the first physical tributary port  1  and the fourth physical tributary port  4  when the first 1:5 optical WSS  401  is configured to switch between the common port c and the fifth physical tributary port  5 . 
       FIG. 6  schematically presents a second 1:5 optical WSS  602  with enhanced multidirectional switching capabilities. In order to perform multidirectional switching between more ports than is possible with the first 1:5 optical WSS  501 , the second 1:5 optical WSS  602  may adjust the three or more third angles  261 ,  262 ,  263 , causing the three or more third angles  261 ,  262 ,  263  to have the same magnitude. The magnitude is indicated in  FIG. 6  as θ. 
     This may for example be implemented by adjusting the distance d 0 , d 1 , d 2  between adjacent ports. 
     Table 1 presents the possible multidirectional switching functions in the second 1:5 optical WSS  602  following the architecture of  FIG. 6 . The right column lists additional multidirectional functions that are possible with a given configuration of the second 1:5 optical WSS  602  in the left column. For example, if the second 1:5 optical WSS  602  is configured to route between the common port c and the fourth physical tributary port  4 , it would be possible to utilize the first physical tributary port  1  and the third physical tributary port  3  for additional connections on the same wavelength. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Configured to switch 
                 Additional Multidirectional Switching 
               
               
                   
                   
               
             
            
               
                   
                 c      5 
                 1      4 and 2      3 
               
               
                   
                 c      4 
                 1      3 
               
               
                   
                 c      3 
                 1      2 
               
               
                   
                   
               
            
           
         
       
     
     Some fourth embodiments comprise one or more combinations of the previous embodiments. For example, an optical WSS with equal third angles and configured to switch two or more optical signals within the same wavelength band between two or more pairs of tributary ports. 
     Embodiments herein are also directed to an optical network node  710  comprising one or more optical WSS  200  as illustrated in  FIG. 7   a.    
     In the following a number of embodiments of the optical network node  710  comprising the one or more optical WSS  200  will be presented. 
       FIG. 7 b    illustrates an example of a 3×3 optical network node comprising a 1:3 multidirectional optical WSS according to embodiments herein. 
     Thanks to the utilization of the connectivity between tributary ports, the 1:3 multidirectional optical WSS may be used to route the traffic from any of the 3 I/O ports  730 ,  731 ,  732  of the structure to any other I/O port when the 1:3 multidirectional optical WSS is used in a bidirectional mode. 
     An optical network node according to embodiments herein do not need more than one WSS to realize this functionality. This means that more flexible optical routers may be built with less number of WSS devices, and therefore at a lower cost. 
       FIG. 8 a  to 8 e    illustrates some properties of the optical WSS according to embodiments herein. 
       FIG. 8 a    illustrates an optical WSS  810  of size 1:3.  FIG. 8 b    illustrates an optical WSS  820  of size 1:4 without support for connectivity between physical tributary ports  1  and  2 .  FIG. 8 c    illustrates a WSS  830  of size 1:4 with support for connectivity between physical tributary ports  1  and  2 .  FIG. 8 d    illustrates a WSS  840  of size 1:5 without support for connectivity between physical tributary ports  2  and  3 .  FIG. 8 e    illustrates a WSS  850  of size 1:5 with support for connectivity between physical tributary ports  2  and  3 . 
     In contrast to the prior art WSS, a tributary port in embodiments herein may additionally exhibit the following properties. 
     Type A: An optical signal may be routed between the considered tributary port, for example the physical tributary port  1  and another tributary port, such as the physical tributary port  2 , through the multidirectional optical WSS  200 . 
     Type B: Routing of a first optical signal between the considered tributary port and another port, such as a common port c or another tributary port, introduces connectivity for a second optical signal within the same wavelength band between two other tributary ports. 
     A WSS tributary port may possess either properties type A or B, both A and B or neither A nor B as illustrated for different types of WSSs in  FIG. 8 a    to  8   e.    
     The switching behaviour described by embodiments herein makes cascading of such WSSs by interconnecting them via the tributary ports interesting. In embodiments below two basic ways of cascading such WSSs along the tributary ports will be described. 
       FIG. 9 a    illustrates the optical network node  910  comprising a first optical WSS  921  and a second optical WSS  922 , each according to embodiments herein, such as the optical WSS  200 . 
     The optical network node  910  may be adapted to switch an optical signal from at least one tributary port, such as from the first physical tributary port  1 , via the second physical tributary port  2 , comprised in the first optical WSS  921  to at least one tributary port, such as via the first physical tributary port  1  to the second physical tributary port  2 , comprised in the second optical WSS  922 . 
     A first scheme of cascading WSSs is through connecting tributary ports of type A as in  FIG. 9 a   .  FIG. 9 a    illustrates three multidirectional 1:3 WSSs with the first tributary port  1  and the second tributary port  2 , each of type A. The third tributary port  3  is of type B. By generalizing to M WSSs, this way of cascading the WSSs creates a chain of M WSSs where optical signals within a wavelength band may be routed through the full WSS chain. Assuming that end points P 4  and P 5  of this chain and common ports P 1 , P 2  and P 3  may be utilized as in/outgoing ports of the structure, the structure constitutes an OXC with M+2 ports based on only M WSSs. As illustrated in  FIG. 9 a    a wavelength band may be routed between any pair of in/outgoing ports of the OXC. For the simple configuration depicted in  FIG. 9 a   , there are however several constraints in terms of internal wavelength blocking. However, additional WSS ports may be used to relieve some of the blocking constraints as depicted in  FIG. 9 b    with a dotted line between two of the third tributary ports  3 . Alternatively, depicted with a dash-dotted line and A/D (P 2 ), some directional add/drop functionality may be introduced. 
     In some embodiments the optical network node  910  is further adapted to switch the optical signal from any one or more of the first tributary port  231 , such as the first physical tributary port  1 , and the second tributary port  232 , such as the second physical tributary port  2 , comprised in a first optical WSS  921  to any one or more of the first tributary port  231 , such as the first physical tributary port  1 , and the second tributary port  232 , such as the second physical tributary port  2 , comprised in the second optical WSS  922 . 
     A second scheme of cascading the WSSs via the tributary ports is by alternatingly connecting ports of Type A with ports that are not of type A. This is illustrated in  FIG. 10 a    for the case of connecting two 1:3 WSSs. The second scheme creates a structure with different connectivity properties compared to the first scheme of cascading the WSSs.  FIG. 10 a    illustrates a structure with full non-blocking connectivity within in/outgoing ports P 1 , P 2 , P 3  as well as within in/outgoing ports P 1 , P 2 , P 4 . This structure may be used to provide a fully non-blocking 2-degree ROADM with 2 add/drop ports. 
     Note that for the second scheme of cascading ideally at least two connections are provided between neighbouring WSSs. For the 1:3 WSS, two connections may be made to one next neighbour. 
     Extending the number of WSS ports as in  FIG. 10 b    may relieve some of the wavelength blocking constraints.  FIG. 10 b    illustrates cascading WSSs of size 1:4 via tributary ports by connecting ports of type A with ports not of type A. Two connections between each neighbouring WSS are provided. 
     In other words, the optical network node  1010  may comprise two connections between the first optical WSS  1021  and the second optical WSS  1022 . The optical network node  1010  may further be adapted to switch the optical signal from the fourth port  234 , such as the common port c, of the first optical WSS  1021 , via the third port  233 , such as the third physical tributary port  3 , of the first optical WSS  1021 , to any one of the first tributary port  231 , such as the first physical tributary port  1 , and the second tributary port  232 , such as the second physical tributary port  2 , of the second optical WSS  1022 . 
     Based on the two basic schemes for cascading WSSs with multidirectional wavelength switching capabilities, several important building blocks for single fiber transmission based transparent optical networks may be built with fewer WSSs or with fewer WSS ports as compared to today&#39;s alternatives. Examples are described below. 
     In  FIG. 11 a    the optical network node  710 ,  1110  is a 4×4 OXC comprising two 1:3 WSSs, namely a first 1:3 WSS  1121  and a second 1:3 WSS  1122  according to embodiments herein. The 4×4 OXC is based on cascading through the first cascading scheme. 
     WSS settings that provide connectivity between different OXC ports are displayed in Table 2. Table 2 is presented in the format (a:b), where ‘a’ represents routing of a considered wavelength between the common port c and port ‘a’ for the first 1:3 WSS  1121  and where ‘b’ represents routing of considered wavelength between the common port c and port ‘b’ for the second 1:3 WSS  1122 . X denotes any port. Note that the only blocking constraint is for routing a wavelength between port pairs P 2 -P 3  and P 1 -P 4  simultaneously, while all other combinations are fully non-blocking.  FIG. 11 b    presents a conventional 4×4 OXC. Compared to the conventional 4×4 OXC, which is fully non-blocking, the number of WSSs is reduced by 50% in embodiments herein. 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                 Port 
                 P1 
                 P2 
                 P3 
                 P4 
               
               
                   
               
             
            
               
                 P1 
                 — 
                 3:3, 2:1 
                 1:x 
                 2:3 
               
               
                 P2 
                 3:3, 2:1 
                 — 
                 3:1 
                 X:3 
               
               
                 P3 
                 1:x 
                 3:1 
                 — 
                 3:3 
               
               
                 P4 
                 2:3 
                 X:3 
                 3:3 
                 — 
               
               
                   
               
            
           
         
       
     
     In other words, when the optical network node  710 ,  1110  is the 4×4 OXC, each WSS  1121 ,  1122  may comprise three tributary ports. The third tributary port  233 , such as the third physical tributary port  3 , comprised in the first optical WSS  1121  may be connected to another third tributary port  233 , such as the third physical tributary port  3 , comprised in the second optical WSS  1122 . The optical network node  1110  further comprises four combined input and output ports: 
     a first input and output port P 1  connected to the common port  234 , c comprised in the first optical WSS  1121 , 
     a second input and output port P 2  connected to the common port  234 , c comprised in the second optical WSS  1122 , 
     a third input and output port P 3  connected to the first tributary port  231 , such as the first physical tributary port  1 , or the second tributary port  232 , such as the second physical tributary port  2 , comprised in the first optical WSS  1121 , and 
     a fourth input and output port P 4  connected to the first tributary port  231 , such as the first physical tributary port  1 , comprised in the second optical WSS  1102 , or the second tributary port  232 , such as the second physical tributary port  2  comprised in the second optical WSS  1102 . 
     In  FIG. 12 a    the optical network node  1210  is a 2D-ROADM comprising two 1:3 WSSs, namely the first 1:3 WSS  1221  and the second 1:3 WSS  1222  according to embodiments herein. The 2D-ROADM is based on cascading through the second cascading scheme. 
     WSS settings that provide connectivity between different ports of the 2D-ROADM are displayed in Table 3. Table 3 is presented in the format a:b, where ‘a’ represents routing of the considered wavelength between the common port c and port ‘a’ for the first 1:3 WSS  1221 . ‘b’ represents routing of the considered wavelength between the common port c and port ‘b’ for the second 1:3 WSS  1222 . 
       FIG. 12 b    presents a conventional 2D-ROADM. Compared to the conventional 2D-ROADM which provides identical functionality, embodiments herein reduce the number of WSSs by two. 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 3 
               
               
                   
               
               
                 Port 
                 P1 
                 P2 
                 P3 (A/D) 
                 P4 (A/D) 
               
               
                   
               
             
            
               
                 P1 
                 — 
                 3:2, 2:3 
                 1:x 
                 3:3 
               
               
                 P2 
                 3:2, 2:3 
                 — 
                 3:3 
                 X:1 
               
               
                 P3 
                 1:x 
                 3:3 
                 — 
                 — 
               
               
                 P4 
                 3:3 
                 X:1 
                 — 
                 — 
               
               
                   
               
            
           
         
       
     
     In other words, when the optical network node  710 ,  1210  is a 2D-ROADM, the optical network node  1210  further comprises four combined input and output ports P 1 , P 2 , P 3 , P 4 : 
     the first input and output port P 1  connected to the common port  234 , c comprised in the first optical WSS  1221 , 
     the second input and output port P 2  connected to the common port  234 , c comprised in the second optical WSS  1222 , 
     the third input and output port P 3  connected to the first tributary port  231 , such as the first physical tributary port  1 , or the second tributary port  232 , such as the second physical tributary port  2 , comprised in the first optical WSS  1221 , and 
     the fourth input and output port P 4  connected to the first tributary port  231 , such as the first physical tributary port  1  or the second tributary port  232 , such as the second physical tributary port  2 , comprised in the second optical WSS  1222 . 
     In  FIG. 13 a    the optical network node  1310  is a 6×6 OXC comprising two cascaded multidirectional 1:5 WSSs  1321 ,  1322 . The 6×6 OXC comprises six input and output ports P 1 -P 6 . 
     Note that for the 1:5 WSS  1321 ,  1322 , when a given wavelength is routed between the common port c and a fifth physical tributary port  5 , the 1:5 WSS  1321 ,  1322  also provides connectivity between the first physical tributary port  1  and the fourth physical tributary port  4  as well as between the second physical tributary port  2  and the third physical tributary port  3 , illustrated with dashed lines in  FIG. 13   a.    
     Also, when the wavelength is routed from the common port c to the third physical tributary port  3  it provides connectivity between the first physical tributary port  1  and the second physical tributary port  2 , indicated by a solid lines in  FIG. 13   b.    
     When the wavelength is routed from the common port c to the fourth physical tributary port  4 , the WSS  1321 ,  1322  may be designed to provide connectivity between the first physical tributary port  1  and the third physical tributary port  3 , indicated by dashed lines in  FIG. 13   c.    
     There are several ways of interconnecting or daisy chaining 1:5 WSSs. Two variants are proposed. 
     A first variant, illustrated by the optical network node  1310 , is based on cascading according to the first scheme twice between type A ports, plus adding a third direct connection between the fifth physical tributary ports  5  of the two WSSs  1321 ,  1322  in  FIG. 13   a.    
     The first variant of the 6×6 OXC provides connectivity between each pair of input and output ports P 1 -P 6 , although there are some blocking constraints. For several combinations of pairs, simultaneous connections may be offered for a given wavelength band. This is illustrated in  FIG. 14  where ‘bold’ connection lines illustrate combinations of simultaneous connections that are possible for a given wavelength band. A target use of this 6×6 OXC is for low cost networks where some flexibility is needed. 
       FIG. 15  illustrates a first interconnecting network node  1510 , comprising the first variant of the 6×6 OXC. The first interconnecting network node  1510  may for example be used in a first configuration to interconnect three rings P 1 -P 2 , P 3 -P 4 , P 5 -P 6 , illustrated with solid lines. 
     The first interconnecting network node  1510  may then re-configure to a second configuration to combine two of the rings to a larger logical ring, illustrated with dashed lines, for specific wavelengths. 
     The first configuration completes the rings whilst the second configuration enables two rings to be interconnected to a single logical ring. 
     A second variant of the 6×6 OXC is illustrated by the optical network node  1610  depicted in  FIG. 16 . The second variant of the 6×6 OXC is based on cascading two WSSs  1621 ,  1622  according to the first scheme and the second scheme simultaneously. The second variant of the 6×6 OXC have slightly different connectivity properties compared with the first variant of the 6×6 OXC. The second variant of the 6×6 OXC may be used as an interconnection point between three optical rings for the case when it may be beneficial for given wavelengths to combine all three optical rings to a single logical optical ring illustrated with dashed lines in  FIG. 17 . The solid and dashed lines in  FIG. 17  illustrate two possible routing configurations for given wavelengths. The solid configuration completes the optical rings while the dashed configuration enables all optical rings to be interconnected to form a single logical ring. 
       FIG. 18  presents connectivity properties for the second variant of the 6×6 OXC. Bold lines illustrate different cases of more than one simultaneous connection for a given wavelength that may be supported by the second variant of the 6×6 OXC. 
     Embodiments herein are also directed towards optical networks, for example employed in radio access networks. Radio access network scenarios with centralized baseband are gaining interest. Centralized baseband means that a base station BaseBand Unit (BBU) or base station digital unit is connected via a fronthaul link to a remote radio head and antenna. The BBU is usually in a centralized location. The fronthaul link may be part of an optical network. Such scenarios require transport solutions able to carry digitalized baseband, e.g. Common Public Radio Interface (CPRI), between Remote Radio Units (RRU) and centralized baseband hotels, such as the BBU. 
     It is expected that some degree of flexibility will be needed in fronthaul transport in order to facilitate service provisioning and to provide a platform for efficient resource usage. 
     For example, with increasing small cell densification one may expect a larger degree of traffic dynamicity in the outer parts of the radio access network as traffic fluctuations may be more pronounced over the coverage area of a small cell compared to a macro. Small cell deployment may also lead to an increasing need for provisioning flexibility in order to adjust to an evolving urban environment where new cells may be added and old cells removed based on changing traffic patterns and evolving competitive landscapes among wireless access providers. By exploiting embodiments herein, an optical network architecture with a desired flexibility may be achieved at low cost and low complexity. 
       FIG. 19  illustrates a general optical network architecture for this use case where small cells are connected via “small cell” optical rings and these optical rings are interconnected at optical access nodes. The optical access nodes are in turn interconnected by an access ring. 
     Embodiments of an optical network  2000  will now be described with reference to  FIG. 20 . 
     The optical network  2000  comprises at least one optical network node  710 ,  2010 , according to embodiments herein. In other words, the optical network node  710 ,  2010  comprises one or more optical WSSs  200  according to embodiments herein. The optical network node  710 ,  2010  may be an optical access network node. 
     The optical network  2000  further comprises a first service node  2021 , and a second service node  2022 . The service nodes  2021 ,  2022  may each be a local BBU, also referred to as a local base band hotel. 
     The optical network  2000  further comprises a client node  2030 . The client node  2030  may be a RRU. 
     The optical network  2000  further comprises a first optical access ring  2041  and a second optical access ring  2042 . The optical access network  2000  may further comprise a third optical access ring  2043 . The first, second and third optical access rings  2041 ,  2042 ,  2043  may collectively be referred to as small cell optical rings  2041 ,  2042 ,  2043 . 
     The optical network node  710 ,  2010  is connected to the first optical access ring  2041  and the second optical access ring  242 . The first service node  2021  is connected to the first optical access ring  2041 . The client node  2030  is connected to the second optical access ring  2042 . The second service node  2022  is connected to any other part of the optical access network  2000 . 
     The optical network node  710 ,  2010  is adapted to route the first optical signal on a first wavelength band between the first service node  2021  and the client node  2030 . The optical network node  710 ,  2010  is further adapted to route the second optical signal on a second wavelength band between the first service node  2021  and the second service node  2022 . 
     In some embodiments the optical network  2000  comprises three or more optical access rings  2041 ,  2042 ,  2043 . Then the optical network node  710 ,  2010  may be adapted to route the first optical signal on the first wavelength band between the first service node  2021  and the client node  2030  via any one or more out of: the first optical access ring  2041 , the second optical access ring  2042 , and the third optical access ring  2043 . 
     The first service node  2021  may perform baseband processing. 
     In some embodiments the first service node  2021  performs packet aggregation. 
     The first service node  2021  may perform channel multiplexing aggregating multiple lower rate optical signals to fewer number of higher rate optical signals. 
     In some embodiments, illustrated in  FIG. 21 , the optical network node  710 ,  2010  may comprise the first optical WSS  2121  and the second optical WSS  2122 , each according to embodiments herein, such as the optical WSS  200 . 
     The optical network node  710 ,  2010  may be adapted to switch or route an optical signal from at least one tributary port, such as from the first physical tributary port  1  via the third physical tributary port  3 , comprised in the first optical WSS  2121  to at least one tributary port, such as the second physical tributary port  2 , comprised in the second optical WSS  2122 . 
     As illustrated in  FIG. 21 , the optical network node  710 ,  2010  may be an optical access node. The first optical WSS  2121  and the second optical WSS  2121  may each be a 1:4 WSS for multidirectional switching according to embodiments herein. The fourth physical tributary port  2134  of the first optical WSS  2121  may be interconnected with the fourth physical tributary port  2144  of the second optical WSS  2122  to form a bypass connection for a fourth optical access ring  2150 . The small cell optical rings  2041 ,  2042 ,  2043  are connected to the other tributary ports of each WSS  2121 ,  2122 . Since the WSSs  2121 ,  2122  internally may provide connectivity between the first physical tributary port  1  and the third physical tributary port  3  the small cell optical rings  2041 ,  2042 ,  2043  are nested in such a manner that any client node on any small cell optical ring  2041 ,  2042 ,  2043  may be connected to any client node on any optical ring  2041 ,  2042 ,  2043 . 
       FIG. 21  also illustrates how a wavelength band, e.g. Lambda group D, may be used to simultaneously provide local connectivity in the small cell rings  2041 ,  2042 ,  2043  and to provide bypass for connectivity between elements of other access nodes. 
     The first service node  2021  may serve all the small cell rings of the access site.  FIG. 22 a    illustrate an example of wavelength allocation where three wavelength channels, Lambda  1 - 3 , are used to serve, i.e. backhaul, three local baseband hotels via the access ring. Simultaneously, these wavelengths, Lambda  1 - 3 , and other wavelengths, Lambda  4 - 11 , are used to serve the fronthaul connectivity needs between the small cells and the baseband hotels.  FIG. 22 b    further illustrates the same example. 
     Embodiments herein provide a scalable transport solution where the total number of RRU clients is not limited by the maximum number of wavelength channels in the access ring. It enables RRU clients to be connected to a local BBU. At the same time it still allows for each client to be accessed via the global access ring. One use of this feature may be to enable low power configurations where the local baseband hotels are put to standby and few selected cells are aggregated to a single hotel as illustrated in  FIG. 23 . 
     An alternative of the presented architecture is depicted in  FIG. 24 , where the local BBU is attached to the small access rings via two tributary ports to provide full protection for connections between each access RRU and the local BBU. 
     In other words, the first service node  2021  may be connected to the first access ring  2041  and the second access ring  2042  near a head or a tail of each access ring  2041 ,  2042 . In this way the connection between the first service node  2021  and the client node  2030  is protected against any single fiber cut on any of the optical rings  2041 ,  2042 ,  2043  connected to the optical network node  710 ,  2010 . 
     In some embodiments the first service node  2021  is connected to the head or the tail of the second access ring  2042  from which there is no direct optical path to any other access ring  2041 ,  2043  through the optical network node  710 ,  2010  without passing through the second access ring  2042 . 
     When using the word “comprise” or “comprising” it shall be interpreted as non-limiting, i.e. meaning “consist at least of”. 
     The embodiments herein are not limited to the above described preferred embodiments. Various alternatives, modifications and equivalents may be used. Therefore, the above embodiments should not be taken as limiting the scope, which is defined by the appending claims.