Abstract:
The present invention provides a device for performing wavelength add/drop multiplexing utilizing micromachined free-rotating switch mirrors. The free-space nature of the switch mirrors allow use of the front and back sides of the mirrors for reflecting signals. According to one embodiment of the present invention a WADM is provided in which micromachined switch mirrors are arranged in a polygonal (e.g., hexagonal) geometry, which allows full connectivity. 
     According to one embodiment a WADM is provided for deployment in a unidirectional two-fiber optical network including service and protection fiber routes. According to this embodiment the WADM includes a first input port for receiving a WDM signal from the service fiber route and a second input port for receiving a WDM signal from the protection fiber route. The WADM also includes a first output port for transmitting a WDM signal to the service fiber route, a second output port for transmitting a WDM signal to the protection fiber route, a third input port for receiving locals signals from a local access port and a third output port for dropping signals to a local access port. 
     The WADM further includes a reconfigurable switching matrix comprising a plurality of free-space micromirrors, for performing routing of signals from the various input ports to the various output ports. 
     According to an alternative embodiment a WADM is provided for deployment in a bidirectional two-fiber optical network including two service/protection routes.

Description:
PRIOR PROVISIONAL PATENT APPLICATION 
     The present application claims the benefit under 35 U.S.C. § 119(e) of U.S. Provisional Application No. 60/112,112 filed Dec. 14, 1998. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to optical networks. In particular, the present invention relates to a device for performing add/drop multiplexing in two-fiber ring networks. 
     BACKGROUND INFORMATION 
     With the growing capacity demand for optical fiber communications, wavelength add/drop multiplexers (“WADM”) are essential components in any optical network. In particular WADMs are critical components in wavelength division-multiplexed (“WDM”) regional-access ring or bus networks to provide access to local customers. 
     Current technology utilizes configurable wavelength 2×2 switches inserted in wavelength paths. FIG. 1, which is prior art, depicts an example of a conventional WADM architecture. The conventional WADM includes input port  140 , demultiplexer  110 , multiplexer  120 , output port  130  and a plurality of 2×2 switches  105 ( 1 )- 105 (M). A WDM signal including a plurality of multiplexed signals λ 1 -λ M  is received at input port  140  and transmitted to demultiplexer  110 . Wavelengths λ 1 -λ M  received via local access ports (not shown) may be added via respective switches  105 ( 1 )- 105 (M). Conversely, wavelengths λ 1 -λ M  from the demultiplexed signal may be dropped via switches  105 ( 1 )- 105 (M) to local access ports (not shown). A particular wavelength λ is dropped to and added from the local port if the respective 2×2 switch ( 105 ) is in a cross-state, while it is sent directly to output port  130  when the switch is in a through state. 2×2 switches  105  may be of a discrete or integrated form. 
     Ring networks have become very popular in the carrier world as well as in enterprise networks. A ring is the simplest topology that is two-connected, i.e., provides two separate paths between any pair of nodes. This allows a ring network to be resilient to failures. These rings are called self-healing because they incorporate protection mechanisms that detect failures and reroute traffic away from failed links and nodes onto other routes rapidly. A unidirectional ring carries working traffic only in one direction of the ring (e.g., clockwise). 
     FIG. 2 a , which is prior art, depicts the topology of a unidirectional ring network. A unidirectional ring network carries working traffic in only one direction of the ring (e.g., clockwise), along service fiber  230 . WADMs  210   a - 210   d  provide functionality for dropping and adding wavelengths via local access ports  220   a - 220   d  respectively. For example, working traffic from WADM  210   a  to  210   b  is carried clockwise along the ring and working traffic from WADM  210   b  to  210   a  is also carried clockwise on a different set of links in the ring. Protection fiber  240  provides a backup route in the case of a fiber cut or equipment malfunction in the working fiber  230 . Traffic from WADM  210   a  to WADM  210   b  is sent simultaneously on working fiber  230  in the clockwise direction and protection fiber  240  in the counter-clockwise direction. 
     FIG. 2 b , which is prior art, depicts the topology of a bi-directional two-fiber ring network. Note that both fiber routes  230   a  and  230   b  in FIG. 2 b  carry a non-overlapping sub-set of wavelengths (e.g., even and odd number wavelengths). Thus, both fiber routes  230   a  and  230   b  are working/protection fiber since one direction can function as the protection route for the other direction (because the wavelengths are non-overlapping). For example, in an even/odd arrangement, signals in the protection routes would be even number wavelengths in odd number wavelength fiber routes and odd number wavelengths in even number wavelength fiber routes. 
     Typically, WADMs require additional functionality to enable loop-back for maintenance or to switch the signal to a restoration path in the case of a fiber cut or equipment malfunction. FIG. 3, which is prior art, depicts typical connectivity requirements for a WADM in a uni-directional ring network. WADM  210  must be able to switch signals from WS IN  (west service input)  230   a  to WP OUT    240   b  (west protection output) for loop-back maintenance. Also, if a failure or fiber cut occurs on the east side of WADM  210 , wavelengths from local access ports  220  must be switched to WP OUT    240   b  for restoring the network traffic. Likewise WADM  210  must switch signals arriving from WS IN    230   a  originally destined for ES OUT    230   b  to WP OUT    240   b.    
     Although the functions required as shown in FIG. 3 may be achieved by a 3×3 cross-bar matrix or three 1×3 switches for each wavelength path, the utilization of switch points is inefficient. This results in an increase of the complexity of the electronic controls, size and cost of the WADM device. 
     SUMMARY OF THE INVENTION 
     The present invention provides a device for performing wavelength add/drop multiplexing utilizing micromachined free-rotating switch mirrors. The free-space nature of the switch mirrors allow use of the front and back sides of the mirrors for reflecting signals. According to one embodiment of the present invention a WADM is provided in which micromachined switch mirrors are arranged in a polygonal (e.g., hexagonal) geometry, which allows full connectivity. 
     According to one embodiment a WADM is provided for deployment in a unidirectional two-fiber optical network including service and protection fiber routes. According to this embodiment the WADM includes a first input port for receiving a WDM signal from the service fiber route and a second input port for receiving a WDM signal from the protection fiber route. The WADM also includes a first output port for transmitting a WDM signal to the service fiber route, a second output port for transmitting a WDM signal to the protection fiber route, a third input port for receiving locals signals from a local access port and a third output port for dropping signals to a local access port. 
     The WADM further includes a reconfigurable switching matrix comprising a plurality of free-space micromirrors, for performing routing of signals from the various input ports to the various output ports. 
     According to an alternative embodiment a WADM is provided for deployment in a bidirectional two-fiber optical network including two service/protection routes. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1, which is prior art, depicts an example of a conventional WADM architecture. 
     FIG. 2 a , which is prior art, depicts the topology of a unidirectional ring network. 
     FIG. 2 b , which is prior art, depicts the topology of a bi-directional two-fiber ring network. 
     FIG. 3 depicts typical connectivity requirements for a WADM in a unidirectional ring network. 
     FIG. 4 is a block diagram of a WADM utilizing micromachined free-space mirrors for deployment in a unidirectional ring network according to one embodiment of the present invention. 
     FIG. 5 depicts a microactuated switch mirror according to one embodiment of the present invention. 
     FIG. 6 is a block diagram of a demultiplexer utilizing OCA microplasma technology according to one embodiment of the present invention. 
     FIG. 7 depicts a unidirectional two-fiber ring network, assuming a fiber cut occurs according to one embodiment of the present invention. 
     FIG. 8 a  depicts a WADM node with unidirectional traffic under the situation of normal service according to one embodiment of the present invention. 
     FIG. 8 b  depicts a WADM node with unidirectional traffic under the situation of failure on east side service and protection routes according to one embodiment of the present invention. 
     FIG. 8 c  depicts a WADM node with unidirectional traffic under the situation of failure on west side service and protection routes according to one embodiment of the present invention. 
     FIG. 8 d  depicts a WADM node with unidirectional traffic under the situation of loop-back according to one embodiment of the present invention. 
     FIG. 9 a  depicts the configuration of a switching matrix of micromirrors in a WADM in a unidirectional two-fiber optical network under normal service conditions according to one embodiment of the present invention. 
     FIG. 9 b  depicts the configuration of a switching matrix of micromirrors in a WADM in a unidirectional two-fiber optical network under service failure of east side service and protection fiber routes according to one embodiment of the present invention. 
     FIG. 9 c  depicts the configuration of a switching matrix of micromirrors in a WADM in a unidirectional two-fiber optical network under service failure of west side service and protection fiber routes according to one embodiment of the present invention. 
     FIG. 9 d  depicts the configuration of a switching matrix of micromirrors in a WADM in a unidirectional two-fiber optical network under loop-back conditions according to one embodiment of the present invention. 
     FIG. 10 depicts a WADM with a signal access port in a unidirectional two-fiber network according to one embodiment of the present invention. 
     FIG. 11 depicts a bidirectional two-fiber ring network with a fiber failure on the east route of a WADM B according to one embodiment of the present invention. 
     FIG. 12 depicts a WADM with multiple access ports in a bidirectional two-fiber network according to one embodiment of the present invention. 
     FIG. 13 a  depicts a WADM node with bidirectional traffic under the situation of normal service according to one embodiment of the present invention. 
     FIG. 13 b  depicts a WADM node with bidirectional traffic under the situation of failure on east side service and protection routes according to one embodiment of the present invention. 
     FIG. 13 c  depicts a WADM node with bidirectional traffic under the situation of failure on west side service and protection routes according to one embodiment of the present invention. 
     FIG. 13 d  depicts a WADM node with bidirectional traffic under the situation of loop-back according to one embodiment of the present invention. 
     FIG. 14 a  depicts the configuration of a switching matrix of micromirrors in a WADM in a bidirectional two-fiber optical network under normal service conditions according to one embodiment of the present invention. 
     FIG. 14 b  depicts the configuration of a switching matrix of micromirrors in a WADM in a bidirectional two-fiber optical network under service failure of east side service and protection fiber routes according to one embodiment of the present invention. 
     FIG. 14 c  depicts the configuration of a switching matrix of micromirrors in a WADM in a bidirectional two-fiber optical network under service failure of west side service and protection fiber routes according to one embodiment of the present invention. 
     FIG. 14 d  depicts the configuration of a switching matrix of micromirrors in a WADM in a bidirectional two-fiber optical network under loop-back conditions according to one embodiment of the present invention. 
     FIG. 15 depicts a WADM with a signal access port in a unidirectional two-fiber network according to one embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 4 is a block diagram of a WADM utilizing micromachined free-space mirrors for deployment in a unidirectional ring network according to one embodiment of the present invention. WADM  405  includes a first demultiplexer  110   a  for demultiplexing a WDM signal arriving from a west service fiber  230   a  (“WS IN ”), a second demultiplexer  110   b  for demultiplexing a WDM signal arriving from an east protection fiber  240   b  (“EP IN ”), a first multiplexer  120   a  for performing multiplexing of signals for transmission onto a west protection fiber  240   a  (“WP OUT ”) and a second multiplexer  120   b  for performing multiplexing of signals onto an east service fiber  230   b  (“ES OUT ”). WADM  405  also includes add port  225  for receiving signals from a local access port (not shown), drop port  227  (for transmitting signals to a local drop port (not shown)) and switch fabric  415 . Switch fabric  415  includes a plurality of free-space micromachined mirrors  420   a - 420   i . Although FIG. 4 does not depict a particular method for coupling of the various fibers to the multiplexers and demultiplexers, it is assumed that this would be understood by a practitioner skilled in the art. In particular, although not depicted in FIG. 4, WADM  405  includes a first input port coupled between west service fiber  230   a  and first demultiplexer  110   a , a second input port coupled between east protection fiber  240   b  and second demultiplexer  110   b , a third input port coupled between add port  225  and a local access port (not shown) and a third output port coupled between drop port  227  and a local access port (not shown). 
     WADM  405  performs adding (to add port  225 ) and dropping (to drop port  227 ) of a maximum number of N wavelengths from local customers. WADM transmits a remaining number of M−N wavelengths through the node. For example, WADM  405  shown in FIG. 4 performs adding and dropping of two wavelengths λ 1  and λ 2  and transmits wavelengths λ N+1 , λ N+2 , . . . , λ M  through the node. In general, the number of mirrors  420  and layout in switch fabric  415  will depend upon the number of wavelengths added/dropped from the node. The number of wavelengths added/dropped at a particular WADM is reconfigurable up to a maximum capacity depending on the physical structure of the WADM. 
     WADM controller  455  controls the actuation of mirrors  420  in switch fabric  415 . Each micromachined mirror  420  may assume an actuated or non-actuated state, which determines the routing of wavelengths. For example, if mirror  420   c  is deployed, wavelength λ 1  received from WS IN    230   a  is dropped to drop port  227 . Or, for example, if mirror  420   h  is actuated, λ 1  from add port  225  is reflected to ES OUT    230   b . Various example configurations of micromachined mirrors  420  in switch fabric  415  are described in detail below. The actuation of a particular micromachined mirror is described in more detail below. 
     FIG. 5 depicts a microactuated switch mirror according to one embodiment of the present invention. FIG. 5 shows mirror  503 , which includes reflecting surface  560 . Mirror  503  is coupled to translation plate  540  via pushrod  510  and hinge joint  525 . Microactuated mirror  420  also includes spring  530 . Translation plate  540  includes scratch drive actuator  550 . Mirror  503  is pivoted on a substrate (not shown) via hinge joint  525 . Pushrod  510  couples switch mirror  420  with translation plate  540  through hinge joints  525  and convert plate translation into mirror rotation efficiently. Translation plate  540  is integrated with high-precision scratch drive actuators  550 . Translation plate  540  translation distance and therefore switch mirror  420  rotation angle is determined by the number of bias pulses applied to scratch drive actuator  550 . Drive actuators  550  are controlled by mirror actuation control unit  560 , based upon switching decisions determined by WADM controller  455 . In particular, upon the receipt of a signal to actuate a particular mirror  420 , mirror actuation control unit  560  applies a bias voltage via drive actuators  550 , which causes that particular mirror to actuate. Conversely, to de-actuate a mirror  420 , mirror actuation control unit  560  couples drive actuators  550  to ground. 
     FIG. 6 is a block diagram of a demultiplexer utilizing OCA microplasma technology according to one embodiment of the present invention. Demultiplexer  110  receives a WDM input signal  615  and generates N+M output signals  620   a - 620   d . Rather than employing conventional OCA demultiplexer technology where filters for different wavelengths are distributed on both sides of a glass plate, a high reflection coating  610  is employed on one of side of the glass plate so that all outputs  620   a  are on the other side. The advantage of this approach is that the free-space outputs can incident directly into the micro-mirror switches  120  with proper alignment. By reversing the light propagation, this technology may be used to perform Muxing (not shown here). Thus, multiplexers  120  in WADM utilize a similar approach. However, the present invention is not limited to the use of the single-sided high reflection coating approach depicted in FIG.  6 . With adequate packaging and fiber-interconnection, other MUX/DEMUX technologies may be used with the free-rotating micro-mirrors  120  as a part of an overall WADM architecture. 
     FIG. 7 depicts a unidirectional two-fiber ring network, assuming a fiber cut occurs according to one embodiment of the present invention. In particular, FIG. 7 depicts WADMs  405   a - 405   d  coupled via service fiber  230  and protection fiber  240 . FIG. 7 also shows a hypothetical fiber cut  610  in the service fiber  230  output of node  405   b  and protection fiber  240  input of node  405   b.    
     FIGS. 8 a - 8   d  depict routing operations performed at various WADMs depending upon a single fiber failure that occur in a unidirectional fiber ring network according to one embodiment of the present invention. In particular, FIG. 8 a  depicts the resulting configuration of WADM  405   a  in the case of a fiber cut shown in FIG.  7 . Note that WADM  405   a  assumes normal functioning in that wavelengths λ 1  and λ 2  are added/dropped while wavelengths λ N+1  . . . λ M  are transmitted through WADM  405   a . Thus, some signals are routed back to the protection fiber  240  and express through WADM  405   a . This is indicated by the thicker dashed line in FIG. 8 a.    
     FIG. 8 b  depicts the resulting configuration of WADM  405   b  in the case of a fiber cut as shown in FIG.  7 . In particular, WADM  405   b  “observes” a fiber-failure on the ES OUT    230   b  and EP IN    240   b  fiber routes. All of the through wavelengths λ N+1  . . . λ M  are routed to WP OUT    240   a . In additional, all the wavelengths from the local add port  225  are also switched to the WP OUT  fiber route  240   a . FIG. 8 c  depicts the configuration of WADM  405   c  in the case of a fiber failure as depicted in FIG.  7 . In particular, WADM  405   c  observes a failure on fiber routes WS IN    230   a  and WP OUT    240   a . Signals used by WADM  405   c  from protection fiber  240  EP IN    240   b  are routed to drop port  227 , while signals from add port  225  are switched to ES OUT  fiber  230   b . In addition, the unused signals from EP IN    240   b  are routed to ES OUT    230   b.    
     FIG. 8 d  depicts a loopback configuration, which is necessary for all nodes (e.g.,  405   a - 405   d ). As shown in FIG. 5 d , signals from WS IN    230   a  that are not dropped to drop port  227  are switched back to WP OUT    240   a . Similar operation holds for signals arriving from EP IN    240   b.    
     FIGS. 9 a - 9   d  depict exemplary configurations for a WADM to achieve the switching functionality as illustrated in FIGS. 8 a - 8   d  respectively. However, unlike FIGS. 8 a - 8   d , in FIGS. 9 a - 9   d  it is assumed that wavelength λ 1  is used by the respective WADM  405 , while wavelength λ 2  is not used. It is assumed for this example that N=2. However, the present invention is compatible and may be implemented for any arbitrary number of wavelengths N. FIG. 9 a  depicts normal operations (i.e., WADM  405   a ), in which mirrors  420   c ,  420   f  and  420   g  are all actuated (on position, reflection state). Mirror  420   c  serves to reflect wavelength λ 1  arriving from fiber WS IN    230   a  to drop port  227 . Mirror  420   g  serves to reflect signals of wavelength λ 1  from add port  225  to fiber ES OUT    230   b . Mirror  420   f  serves to reflect wavelengths λ N+1  . . . λ M  arriving from EP IN    240   b  to WP OUT    240   a.    
     FIG. 9 b  depicts a configuration at WADM  405   b  assuming a fiber failure as shown in FIG.  7 . Corresponding to the functionality depicted in FIG. 5 b , mirrors  420   b ,  420   h  and  420   i  are actuated. Mirror  420   b  serves to reflect wavelength λ 1  arriving from fiber WS IN    230   a  to drop port  227 . Mirror  420   h  serves to reflect signals of wavelength from add port  225  to fiber ES OUT    230   b . Mirror  420   i  is actuated to reflect wavelengths λ N+1  . . . λ M  arriving from WS IN    230   a  to WP OUT    240   a  (loopback operation). 
     FIG. 9 c  depicts a configuration at WADM  405   c  assuming a fiber failure as shown in FIG.  7 . Mirrors  420   a ,  420   b  and  420   g  are actuated. Note that wavelength λ 1  arriving from EP IN    240   b  is dropped because mirrors  420   c  and  420   d  are off. Wavelength λ 1  from local add port  225  is reflected via mirror  420   g  to ES OUT    230   b . Wavelengths λ N+1  . . . λ M  arriving from EP IN    240   b  are reflected to ES OUT    230   b  via mirror  420   a.    
     FIG. 9 d  depicts a configuration at a WADM  405   d  to achieve a loopback operation. Mirrors  420   g ,  420   h  and  420   i  are actuated to switch signals arriving from WS IN    230   a  to WP OUT    240   a . Mirrors  420   a - 420   c  can then be actuated (with the remaining mirrors deactuated) to switch signals from EP IN    240   b  to ES OUT    230   b.    
     FIG. 10 depicts a WADM with a signal access port in a unidirectional two-fiber network according to one embodiment of the present invention. The architecture depicted in FIG. 10 is similar to that shown in FIG.  3 . However, WADM  405  includes additional demultiplexer  110   c  and multiplexer  120   c . Thus, demultiplexer  110   c  and multiplexer  120   c  are combined in the access port  220  to combine the signals. This results in multiwavelength single-fiber access to the customer and therefore cost savings in fiber installation. Utilizing this approach WADM functions can be accomplished via the same mirror arrangement as depicted in FIGS. 9 a - 9   d.    
     FIG. 11 depicts a bidirectional two-fiber ring network with a fiber failure on the east route of WADM  405   b  according to one embodiment of the present invention. Note that this situation is similar to the fiber failure scenario depicted in FIG. 7 pertaining to the unidirectional fiber ring network. 
     FIG. 12 depicts a WADM with multiple access ports in a bidirectional two-fiber network according to one embodiment of the present invention. Note that the connectivity requirements differ from the unidirectional case as shown in FIG.  3 . In particular, FIG. 12 shows demultiplexer  110   a  receiving odd wavelengths signals from W IN  fiber  230   a . Odd number wavelengths may be added/dropped at WADM  405 , with the resulting signal multiplexed via multiplexer  120   a  and transmitted to ES OUT  fiber  230   b . Even number wavelengths are also transmitted via demultiplexer  110   a  through WADM  405  to multiplexer  120   a , forming a protection route (shown in thick dashed lines). 
     Demultiplexer  110   b  receives even number wavelengths from E IN  fiber  230   c . Even number wavelengths may be added/dropped at WADM  405 , with the resulting signal multiplexed via multiplexer  120   b  and transmitted to W OUT  fiber  230   d . Odd number wavelengths are also transmitted via demultiplexer  110   b  through WADM  405  to multiplexer  120   b , forming a protection route (shown in thick dashed lines). 
     FIGS. 13 a - 13   d  depict traffic flow for various WADM nodes in a bidirectional fiber network assuming a failure in the fiber as shown in FIG.  11 . In particular, FIG. 13 a  depicts a WADM node operating in normal service. FIG. 13 b  depicts a WADM node operating with a failure on its east routes. FIG. 13 c  depicts a WADM node operating with a failure on its west routes. FIG. 13 d  depicts loop-back operation of a WADM node. 
     FIG. 13 a  depicts normal operation of a WADM node  405 , in particular WADM  405   a  shown in FIG.  11 . Note that odd wavelength signals arriving from W IN  fiber route  230   a  may be added/dropped, while even wavelength signals arriving from W IN    230   a  fiber (shown in thick dashed lines) are passed through WADM node  405 . Similarly, even wavelength signals arriving from E IN  fiber route  230   c  may be added/dropped at WADM node  405 , while odd wavelength (shown in thick dashed lines) signals are passed through the node. 
     FIG. 13 b  depicts the situation at WADM node  405   b  shown in FIG. 11 in which there is a failure on the E OUT    230   b  and E IN    230   c  fiber routes. In particular, signals from W IN    230   a  are dropped or switched to W OUT    230   d  and signals from add ports  225  are switched to W OUT    230   d  instead of E OUT    230   b . Thus, as shown in FIG. 13 b  odd signals transmitted onto fiber route W OUT    230   b  form a protection route. 
     FIG. 13 c  depicts the situation at WADM node  405   c  shown in FIG. 11, in which there is a failure on fiber routes W OUT    230   d  and W IN    230   a . In this case, signals from E IN    230   c  are dropped or switched to E OUT    230   b  and signals from add ports  225  are switched to E OUT    230   b  instead of W OUT    230   d . Thus, as shown in FIG. 13 c , even signals transmitted onto fiber route E OUT    230   b  form a protection route. 
     FIG. 13 d  depicts a loopback configuration for a WADM node  405  according to one embodiment of the present invention. In this case, odd signals arriving from W IN    230   a  are passed to W OUT    230   d , while even signals arriving from E IN    230   c  are passed to E OUT    230   b . Moreover, odd signals arriving from W IN    230   a  are added/dropped and passed through to E OUT    230   b  while even signals arriving from E IN    230   c  are added/dropped and passed through to W OUT    230   d.    
     FIGS. 14 a - 14   d  depict an exemplary free space mirror configurations at a WADM in a bidirectional two-fiber network in various configurations according to one embodiment of the present invention. It is assumed for these examples that WADM  405  can switch wavelengths λ 1 -λ 4 . Also, it is assumed that wavelengths λ 1  and λ 2  are used by the local access ports. In FIG. 14 a  (corresponding to FIG. 14 d  (normal service at node  405   a )) mirror  420   c  is on to reflect λ 1  from W IN    230   a  to drop port  227  and mirror  420   g  is on to reflect λ 1  from add port  225  to E OUT    230   b . Similarly, mirrors  420   b ,  420   h  and  420   e  are off to transmit λ 2  from E IN    230   c  to drop port  227  and λ 2  from add  225  to W OUT    230   d . The remaining through wavelengths from W IN    230   a  are transmitted to E OUT    230   b , while the through wavelengths from E IN    230   c  are reflected to W OUT    230   d  by mirrors  420   f  and  420   k.    
     FIG. 14 b  depicts a mirror configuration for a WADM  405  in a bidirectional two-fiber network with failure on east fiber routes (WADM  405   b  in FIG.  11 ). Instead of adding λ 1  to E OUT    230   b , mirror  420   g  is now off to transmit λ 1  to the protection W OUT    230   d . λ 2  arriving from W IN    230   a  is reflected to drop port  227  by mirror  420   b . λ 2  from add  225  is transmitted to W OUT    230   d . The unused wavelengths from W IN    230   a  are reflected to W OUT    230   d  by mirrors  420   i  and  420   l.    
     FIG. 14 c  depicts a mirror configuration for a WADM  405  in a bidirectional two-fiber network with failure on west fiber routes (WADM  405   c  in FIG.  11 ). FIG. 14 d  depicts a mirror configuration for a WADM  405  in a bidirectional two-fiber network with failure in a loopback configuration. All odd wavelengths from W IN    230   a  are switched to W OUT    230   d  and all even wavelengths from E IN    230   c  are switched to E OUT    230   b.    
     FIG. 15 depicts a WADM with a signal access port in a bidirectional two-fiber network according to one embodiment of the present invention. The architecture depicted in FIG. 15 is similar to that shown in FIG.  12 . However, WADM  405  includes additional demultiplexer  110   c  and multiplexer  120   c . Thus, demultiplexer  110   c  and multiplexer  120   c  are combined in the access port  220  to combine the signals. This results in multiwavelength single-fiber access to the customer and therefore cost savings in fiber installation. Utilizing this approach WADM functions can be accomplished via the same mirror arrangement as depicted in FIGS. 14 a - 14   d.