Abstract:
Apparatus and methods are disclosed for alignment of an input mirror and an output mirror in a photonic cross-connect.

Description:
REFERENCE TO PENDING PRIOR PATENT APPLICATION 
   This patent application claims benefit of pending prior U.S. Provisional Patent Application Ser. No. 60/368,075 filed Mar. 27, 2002 by Humair Raza et al. for ALIGNMENT LASER CONFIGURATION AND APPLICATIONS, which patent application is hereby incorporated herein by reference. 

   FIELD OF THE INVENTION 
   This invention relates to apparatus and methods for aligning optical components, and more particularly to alignment lasers for providing a source of light to align input and output mirrors between ports of an optical cross-connect switch. 
   BACKGROUND OF THE INVENTION 
   Due to the optical nature of a switching fabric in a MEMS-based photonic cross-connect (PXC), some form of light source is needed to align appropriate sets of input and output mirrors in an optical cross-connect switch. Under normal operating conditions, a client signal acts as the light source. However, under other operating conditions, when no client signal is available, and/or when no other external signal is available, cross-connects cannot be established. Accordingly, a source of light for making cross-connects is desired. 
   SUMMARY OF THE INVENTION 
   One object of the present invention is to provide apparatus having an internal source of light for the alignment of input and output mirrors in an optical cross-connect. 
   Another object of the present invention is to provide apparatus having an alignment laser as the internal source of light for the alignment of input and output mirrors in an optical cross-connect where a client signal is not available to effect alignment. 
   Still another object of the present invention is to provide apparatus having an alignment laser as the internal source of light for the alignment of input and output mirrors in an optical cross-connect where client signal is not available at the time of effecting alignment. 
   Yet another object of the present invention is to provide apparatus having an alignment laser as the internal source of light, within an appropriate out-of-band spectrum, for the alignment of input mirrors and output mirrors in an optical cross-connect where external light may be present. 
   Another object of the present invention is to provide a method for aligning an input mirror and an output mirror in an optical cross-connect. 
   With the above and other objects in view, as will hereinafter appear, there is provided an apparatus for alignment of an input mirror and an output mirror in a photonic cross-connect, the apparatus comprising: a first port and a second port selectively connected to one another over a first optical datapath therebetween, the first optical datapath containing the first input mirror and the first output mirror therein; and a first alignment laser optically connected to the first optical datapath so as to selectively inject light along the first optical datapath, wherein the light injected along said first optical datapath is analyzed to provide feedback data for alignment of the first input mirror and the first output mirror. 
   In accordance with a further feature of the present invention, there is provided a method for alignment of a first input mirror and a first output mirror in a photonic cross-connect, the method comprising: injecting light from a first alignment laser into a first optical datapath, wherein the first alignment laser is disposed within the photonic cross-connect, and wherein the first optical datapath passes through the first input mirror and the first output mirror; and analyzing the light from the first alignment laser at a portion of the first optical datapath subsequent to the first input mirror and the first output mirror. 
   The above and other features of the invention, including various novel details of construction and combinations of parts and method steps, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular devices and method steps embodying the invention are shown by way of illustration only and not as limitations of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These foregoing and other objects and features of the present invention will be more fully disclosed by the following detailed description of the preferred embodiments of the invention, which is to be considered together with the accompanying drawings wherein like numbers refer to like parts, and further wherein: 
       FIG. 1  is a schematic view of one embodiment of the present invention wherein alignment laser light is injected through passive couplers; 
       FIG. 2  is a schematic view of another embodiment of the present invention illustrating realignment laser light injection through 2×2 switches at the input of a port module; 
       FIG. 3  is a schematic view of another embodiment of the present invention illustrating an external alignment laser shared by two port modules; 
       FIG. 4  is a schematic view of another embodiment of the present invention illustrating an implementation scheme for forced serialization of alignment laser operation; 
       FIG. 5  is a schematic view of another embodiment of the present invention illustrating simplified operation of an alignment laser based on relaxed requirements; 
       FIG. 6  is a schematic view of another embodiment of the present invention illustrating verification of cross-connects; 
       FIG. 7  is a schematic view of another embodiment of the present invention illustrating quad port module (QPM) and optical data module (ODM) connectivity through an MPT-12; and 
       FIG. 8  is a schematic view of another embodiment of the present invention illustrating insertion loss measurement through alignment lasers. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring first to  FIG. 1 , there is shown a photonic cross-connect and alignment laser configuration  5  comprising first and second port modules  10 ,  15 . A first optical signal module  20  (also referred to hereinbelow as OSM  20 ) is shown between first and second ports  10 ,  15 . First and second optical datapaths  25 ,  30  each extend between first and second port modules  10 ,  15  and through first OSM  20 . An alignment laser  35  is shown in optical connection with first datapath  25  through a first coupler (or tap)  40 . A first optical data module  45  (also referred to hereinbelow as ODM  45 ) is shown in optical connection with first datapath  25  through first coupler  40 . A second optical data module  50  (also referred to hereinbelow as ODM  50 ) is shown in optical connection with first datapath  25  through a second coupler  55 . A third optical data module  60  (also referred to hereinbelow as ODM  60 ) is shown in optical connection with second datapath  30  through a third coupler  65 . A second optical signal module  70  (also referred to hereinbelow as OSM  70 ) is shown in an unconnected state relative to first and second port modules  10 ,  15 . OSM  70  is selectively configured for enabling additional optical datapaths (not shown) between first and second port modules  10 ,  15 . First and second 3-dB splitters  75 ,  80  are shown positioned in first and second ports  10 ,  15 , respectively. First and second 1×2 switches  85 ,  90  are shown positioned in first and second port modules  10 ,  15 , respectively. 
   In this configuration, alignment laser  35  injects laser light into first optical datapath  25  through first coupler (or tap)  40  provided in first port module  10 . This light is detected by ODM  50  so as to verify the integrity of the datapath. The operating wavelength range of alignment laser  35  can be inside or outside the range of wavelengths utilized by the possible input signals that travel through port module  10 . In an embodiment in which the wavelength of alignment laser  35  is within the range of input wavelengths, interaction must be avoided between the input signal from first port module  10  and the alignment laser light from alignment laser  35 . Alternatively, if the selected wavelength of alignment laser  35  is outside the range of the operating wavelengths of the client signals passing through port module  10 , the use of alignment laser  35  is greatly simplified. In this case, the input signal coupling into first optical path  25  may include the same power rating as alignment laser  35 . 
   Referring now to  FIG. 2 , there is shown a photonic cross-connect and alignment laser configuration  105  comprising a port module  110 , and first and second optical signal modules  115 ,  120  (also referred to hereinbelow as OSM  115 ,  120 ). First and second optical datapaths  125 ,  130  each extend between port module  110  and OSM  115 . Third and fourth optical datapaths  135 ,  140  each extend between port module  110  and OSM  120 . An alignment laser  145  is shown optically connected to first optical signal module  110 . A 2×2 switch connects an input signal and alignment laser  145  with first optical datapath  125  and third optical datapath  135 . A first optical data module  155  (also referred to hereinbelow as ODM  155 ) is shown in optical connection with first optical datapath  125  through a first coupler  160 . And a second optical data module  165  (also referred to hereinbelow as ODM  165 ) is shown in optical connection with fourth optical datapath  140  through a second coupler  170 . 
   In an embodiment of the present invention, an input signal or external signal is applied to one of the cores (i.e., OSM  115 ), and alignment laser  145  is used to keep a connection through the backup switch core (i.e., OSM  120 ). Accordingly, this embodiment provides 1:1 equipment protection. 
   External Light Source 
   Referring again to  FIGS. 1 and 2 , a laser light source, i.e., alignment laser  35  ( FIG. 1 ) and/or alignment laser  145  ( FIG. 2 ), is illustrated as part of one of the optical data modules, i.e., ODM  45  and/or ODM  155 , of photonic cross-connect configuration  5  or  105 , respectively. However, similar configurations with these associated properties are also achieved through the use of an external laser light source as discussed hereinbelow. 
   Shared Internal/External Source 
   Referring now to  FIG. 2 , there is shown alignment laser  145  coupled with 2×2 switch  150  so as to provide 1:2 sharing. Alignment laser  145  is used to provide input light for the two cores, which contain OSM  115  and OSM  120 . The sharing of alignment laser  145  can be extended to an m:n ratio, where m represents the number of internal or external laser sources for n port modules. For redundant core architectures, the signal is split 2n ways for a single laser source. Alternatively, each signal is split n ways with an independent light source for each core. 
   Looking now at  FIG. 3 , there is shown a photonic cross-connect and alignment laser configuration  205  comprising first and second port modules  210 ,  215 , and first and second optical signal modules  220 ,  225  (also referred to hereinbelow as OSM  220 ,  225 ). First and second optical datapaths  230 ,  235  each extend between port module  210  and OSM  220 . A first alignment laser  240  (also referred to hereinbelow as a high power laser  240 ) is selectively optically connectable to port modules  210 ,  215  through a first n way coupler  245 . In this embodiment of the present invention, first alignment laser  240  and n way coupler  245  are shown selectively optically connected to correspond to OSM  220  through port module  210 . A second alignment laser  250  (also referred to hereinbelow as a high power laser  250 ) is selectively, optically connectable to port modules  210 ,  215  through a second n way coupler  255 . In this embodiment of the present invention, second alignment laser  250  and second n way coupler  255  are shown in an optically unconnected condition to OSM  225 . However, second alignment laser  250  and second n way coupler  255  are selectively optically connectable to OSM  225 . First and second 3-dB splitters  260 ,  265  are configured in first and second port modules  210 ,  215 , respectively, so as to transmit input signals to first and second OSM&#39;s  220 ,  225 . First and second 1×2 switches  270 ,  275  are configured in first and second port modules  210 ,  215 , respectively, so as to receive output signals from first and second OSM&#39;s  220 ,  225 . 
   Referring still to  FIG. 3 , the coupling of each of first alignment laser  240  and second alignment laser  250  is shown through taps  245 ,  255 , i.e., first n way coupler  245  and second n way coupler  255 , respectively. In an alternative embodiment of the present invention, coupling can also be achieved through 2×2 switches (not shown) as demonstrated in  FIG. 2 . 
   Injecting Laser Light from the Output Port 
   Referring now to  FIGS. 1–3 , photonic cross-connect and alignment laser configurations  5 ,  105 ,  205  may be modified so that the light from alignment laser  35 ,  145 ,  240  or  250  is injected through the switch core in the opposite direction to that of the external signal. In such a construction, couplers or taps are supported at the output port. Light source  35 ,  145 ,  240  or  250  can be an internal or an external photonic cross-connect and laser configuration. In addition, light source  35 ,  145 ,  240  or  250  can be dedicated to each specific port or be a shared resource in a configuration having an m:n ratio. 
   Alignment Laser Applications 
   Alignment lasers may be used to create cross-connects in the absence of a client signal. Such cross-connects may be requested from a variety of applications including, but not limited to, normal cross-connect requests from users, system-initiated maintenance of cross-connects, on-demand verification of cross-connects, self-calibration of mirrors, and shared mesh protection. 
   For a condition when a user-initiated or system-initiated cross-connect request is received and a client signal is not available, the light source provided by the alignment laser can be used to make such a cross-connect. Some applications, such as system-initiated maintenance of cross-connects, are active only when a client signal is not present, and by default will require the use of alignment lasers. In some cases, however, it may also be necessary to use an alignment laser in the presence of an external client signal. 
   In addition to cross-connect related activities, the alignment laser can also be utilized for other system activities such as diagnostics performed at equipment provisioning time and fault isolation within the optical path through the switch core. These additional embodiments are briefly described hereinbelow. 
   The absence of a client signal refers to the absence of an external light source that can be utilized to create a cross-connect. This term should not be confused with the SONET LOS which, depending on the behavior of an attached OEO or client port, may still have an associated light through an AIS signal. 
   User-Initiated Cross-Connect Requests 
   Under normal circumstances, when a cross-connect request is received and some form of external light is available, either through a valid client signal or an AIS, this external light can be used to create the requested cross-connect. In other cases, the alignment laser is utilized to create such cross-connects in the presence or absence of external client signal. 
   In an embodiment of the present invention, a method is provided in which the alignment laser light is not allowed to leave the system. In other cases, however, the alignment laser light can be allowed to leave the system. In the absence of an external signal, the alignment laser light is required to align an appropriate set of mirrors. Alternatively, in the presence of an external client signal, the use of the alignment laser light is driven by the need to isolate the mirror alignment process from the fluctuation of the external client signal. 
   This behavior is applicable to various types of cross-connects such as, but not limited to, equipment and facility protected cross-connects, internal loop-back, and test access, etc. 
   There are various ways to implement the creation of a cross-connect through the use of an alignment laser. Some of these various ways are discussed hereinbelow. 
   Implementation with Forced Serialization of Alignment Laser Operation 
   Referring now to  FIG. 4 , there is shown a forced serialization system  305  for operation of an alignment laser (not shown). A NCM  310 , i.e., a shelf controller  310 , is shown having a configuration to receive a cross-connect request from a component (not shown) external to forced serialization system  305 . NCM  310  is also shown having a configuration to receive internal LOS indications from an optical detector module  315  (also referred to hereinbelow as ODM  315 ) in core A and from an optical detector module  320  (also referred to hereinbelow as ODM  320 ) in core B. A mirror driver module  325  (also referred to herein as MDM  325 ) in core A and a mirror driver module (not shown) in core B control an alignment laser A (not shown) in ODM  315  and an alignment laser B (not shown) in ODM  320 , respectively. A quad port module  330  (also referred to hereinbelow as QPM  330 ) is selectively configurable so as to receive signals from core A or core B. An optical signal module  335  (also referred to hereinbelow as OSM  335 ) and an optical signal module  340  (also referred to hereinbelow as OSM  340 ) are each operably connected to NCM  310  so as to allow the alignment laser (not shown) in core A to be activated by NCM  310  through MDM  325 . 
   Implementation Without Allowing Light to Leave the System 
   Still looking at  FIG. 4 , there is provided a method for controlling the operation of the alignment laser (not shown) using system  305 . System  305  may be configured such that alignment laser light (not shown) is not permitted to leave the system. System  305  may also be configured such that the alignment laser (not shown) is turned off within ODM  315  and ODM  320  in the presence of a client signal (not shown). 
   One method to satisfy each of the above configurations is to serialize the operation of the alignment laser (not shown) for both core A and core B. Hence, the alignment laser (not shown) is required to be on at a given time for only one of core A or core B. The benefit of this approach is that the onset of a client signal is able to be detected in addition to making sure that the light from the alignment laser (not shown) does not leave system  305 . However, this embodiment involves coordination between various modules within the photonic cross-connect (PXC) to operate each alignment laser (not shown). This may not be suitable for real-time control required for some applications such as, for example, shared mesh protection. This method comprises NCM  310  receiving a cross-connect request, NCM  310  receiving internal LOS indications from both ODM  315  and ODM  320 , NCM  310  forcing a 1×2 switch on QPM  330  to select a signal from core A, and NCM  310  instructing MDM  335  to turn on the alignment laser (not shown) residing in ODM  315 . After a cross-connect is set up in core A, the process is repeated for core B. 
   Implementation Scheme Allowing the Light to Leave the System 
   Referring now to  FIG. 5 , there is shown an alignment laser system  345  having relaxed requirements for simplified operation of an alignment laser (not shown). More particularly, under these relaxed requirements, system  345  usually does not permit alignment laser light (not shown) to leave system  345 . However, during cross-connect setup, alignment laser light can leave system  345  for a limited period of time. This period of time at setup is specified through a timer. Also, under these relaxed requirements, system  345  is configured to turn off the alignment laser (not shown) in the presence of client signal. However, during cross-connect setup, system  345  is configured to permit the use of alignment laser light (not shown) in the presence of the client signal over a limited period of time. 
   A system based on these relaxed requirements considerably simplifies the operation of the alignment laser. Based on the local LOS detection made by the input power detector of optical detector module  315  (also referred to herein as ODM  315 ), the decision to turn on the laser can be autonomously made by each MDM  320 ,  325 . This approach has the advantage that (1) NCM  310  does not require coordination therethrough so as to control a switch on QPM  330  or to serialize the operation of the setup of a cross-connect for each of core A and core B, and (2) the interface between NCM  310  and each of MDM  320  and MDM  325  remains the same for the cross-connect setup, with or without alignment lasers. 
   The operation of the alignment laser may include some potential drawbacks. One potential drawback of system  345  includes the creation of a situation in which alignment laser light leaves system  345 . This should not be a significant issue inasmuch as the alignment laser light leaving system  345  will be an unmodulated signal and will also be below −23 dBm. Another potential drawback of system  345  includes the creation of a situation in which weaker client signals may be temporarily undetected when both alignment laser light and client signals are simultaneously present. For most applications, however, this will not be a significant issue inasmuch as the client signal is expected to be around 0dBm and the alignment laser will only be on for a short period of time as controlled by a timer. 
   For completeness, the potential drawbacks are described hereinabove. In practice, however, these potential drawbacks should occur only during setup of the lightpath. The signal is usually selected from a path after confirmation of the quality of the lightpath by an attached Transport Network Element (not shown), i.e., after the setup of the lightpath. In an embodiment of the present invention, confirmation of the quality of the lightpath is conveyed in the form of a defect-clear indication. 
   Maintenance of Cross-Connects 
   Maintenance of cross-connects includes processes to verify the existence and quality of an optical path between specified ports. Such maintenance is required to compensate for drift in the system parameters involved in a given optical connection through the switch core. During this process, some adjustments in the mirror position may be required. It should be noted that for an active cross-connect with the client signal, the cross-connect is maintained as the default condition through a tracking property of the mirror alignment process. However, system-initiated maintenance of active cross-connects requires the use of alignment lasers when the client signal is not present. From an alignment laser point of view, the cross-connect maintenance is analogous to a creation of a cross-connect having different entry conditions. 
   Some of the network level protection systems, such as SONET APS (1:1) and SPMesh (shared path based mesh protection) provide capabilities to exercise equipment and facilities involved in protecting an idle path. The success or failure of such exercise capabilities is based on the observation of a good signal over the entire optical path. On the other hand, a local maintenance system includes operations to individually exercise many, if not all, of the modules required to create an optical path through a switching fabric. Accordingly, the local maintenance system is useful in detecting silent failures within the system at the nodal level and can significantly enhance the coverage of such diagnostics performed at the network level. 
   There are various ways to accomplish the maintenance of cross-connects. For example, maintenance can be scheduled, i.e., after expiry of a specified timer, so as to check if the cross-connect needs to be maintained. If needed, the maintenance is initiated. In another embodiment of the present invention, maintenance can be initiated by an event. Such initiation may occur upon the determination of an automatic condition requiring maintenance of a cross-connect or upon an explicit user request. For an automatic event, the subsequent maintenance due to a sustained outage requires the use of a timer. 
   For scheduled maintenance, such as after the expiration of a configurable timer, a master MDM initiates the realignment process for the connections having an absence of light at an output ODM. A slave MDM associated with each of these connections activates slow servos, which were previously in a frozen state. This realignment process also utilizes the alignment lasers controlled by the slave MDM. After the maintenance activity is finished, the slow servos are once again returned to a frozen state. In other words, after a correction to the alignment is applied, the mirrors will stay at this position until the next scheduled maintenance session. The frozen state of the slow servos is removed upon detection of a valid external signal, whereby the regular tracking property of the slow servo modifies the mirror position based on a maximum loss requirement. 
   It should be noted that maintenance is usually performed on active cross-connects without a client signal and is different than other system-initiated cross-connect activities involving idle ports, such as self-calibration of mirrors. 
   Verification of Cross-Connects 
   One of the main purposes for cross-connect maintenance activity is the verification of cross-connects. As each relates to verification of cross-connects, the existing and planned cross-connect inventory within a network element can be classified into three major categories. These three major categories are explained hereinbelow. It is noted that maintenance of a cross-connect as specified above can be used to perform verification for some of these categories. 
   Active Cross-Connects with Client Signal 
   This is the normal situation, where the cross-connect is in an active state with the client signal present. In this scenario, the verification can be easily reported by polling the state of various components involved in the optical path of the specified cross-connect. For example, the verification logic may proceed through the following sequence: (1) confirmation that the cross-connect exists in the connection map with the appropriate equipment in IS-NR state, and (2) the slow servo is in the “track” state, which implies that the mirrors are appropriately aligned and loss through the core does not exceed a given requirement. 
   Based on the physical and optical properties of the connection, an appropriate user response can be generated. 
   In case of an abnormal situation, some form of fault diagnosis and isolation may be required to generate an appropriate response to the user. 
   Active Cross-Connects with No Client Signal 
   This cross-connect category deals with the situation in which no client signal is available. In such a case, when a user request for verification of a cross-connect is received, it will be reported based on the last scheduled maintenance activity on the specified cross-connect. The underlying assumption is that, in the absence of a client signal, an MDM will autonomously verify active cross-connects at an interval which is enough to compensate for the parameter drift. If there are other issues with an active cross-connect besides parameter drift, then the regular polling of the cross-connect state performed by the NCM is sufficient and the NCM is assumed to reflect the current state of the cross-connect. Hence, even for this category of cross-connects, the last polled state of the cross-connect can be reported back to the user by the NCM. 
   Cross-Connect Does Not Exist 
   Referring next to  FIG. 6 , there is shown a flowchart  350  representing a process for verification of non-existing cross-connects. The method shown in flowchart  350  is implemented when a cross-connect verification request arises with no existing cross-connect. Such a request may be initiated to verify the connectivity between PXC ports without actually creating the cross-connect. This type of verification is useful to detect silent failures within the system and can be either system initiated or user initiated. The system-initiated requests for such idle ports fall under the scope of the feature of self-calibration of mirrors and will be discussed hereinbelow. 
   The requested verification will require that a maintenance activity be performed without an existing cross-connect between the ports in question if no client signal is available. This eventually translates into setting up a cross-connect through the alignment laser and, after the physical and optical path integrity has been verified, tearing down the cross-connect. The user is then notified of the result of the maintenance activity performed on the idle ports in question. Still referring to  FIG. 6 , the flowchart  350  comprises a logical breakdown of steps required to perform such a verification. A first step  355  of flowchart  350  comprises verifying a non-existing cross-connect. A second step  360  comprises creating a cross-connect. A third step  365  comprises determining whether the cross-connect was created or not. A fourth step  370  comprises reporting on the status of the cross-connect. The same behavior is utilized in the presence of a client signal. In this case, with a client signal present, the system may or may not require alignment laser light. 
   Since this type of verification involves the creation and deletion of cross-connects, the NCM will be involved in the initiation of the maintenance activity. The decision to turn on the alignment laser, however, is made locally by the MDM. 
   Some other system features, such as SP Mesh, utilize similar capability to temporarily set up a cross-connect on idle ports using alignment lasers during the negotiation phase of APS protocol. This is discussed hereinbelow in the section entitled “Shared Mesh Protection”. 
   Self-Calibration of Mirrors 
   The self-calibration of mirrors is a system-initiated activity to update the calibration data required to enhance the accuracy of mirror alignment. This feature utilizes the idle ports or mirrors to make test cross-connects at scheduled intervals. These cross-connects are either internal loop-backs or are cross-connects to designated calibration ports. These cross-connects utilize alignment lasers where no client signal is available at an idle port. 
   Shared Mesh Protection 
   Shared mesh protection, which is also referred to hereinbelow as SPMesh, provides a SONET 1:n APS at the optical layer. For SPMesh, the protection path is shared by reserving appropriate resources without active cross-connects. These cross-connects are created only when a protection switching event is accepted by the APS logic of the protocol. When SPMesh is implemented in an opaque application with LOL everywhere behavior, the idle reserved ports along the protection path do not sense any client light and, hence, require alignment lasers so as to create cross-connects. Since, in this case, the cross-connect setup time influences the overall protection switching time, such cross-connect requests require real-time system behavior. 
   Speech is one of the PAC applications that influence the performance requirements for the use of alignment lasers. 
   Equipment Provisioning Diagnostics 
   Alignment lasers can be used to verify the internal optical connectivity of various modules at equipment provisioning time and should be part of provisioning (power up) diagnostics of an ODD and an QPM. Since most of the applications requiring the use of alignment lasers need a robust detection of internal LOS, verifying optical connectivity at the module level eliminates one possible failure mode associated with this detection. 
   QPM and ODM Connectivity Through MPT-12 
   In an embodiment of the present invention, some of the other diagnostics include, for example, measuring the insertion loss within the optical datapath and, alternatively, comparing the measured insertion loss at a time of equipment provisioning with a measured value from the time of equipment manufacture. Additionally, the measured value at the time of manufacture can be stored in an on-board EEPROM. 
   Referring now to  FIG. 7 , there is shown a logical view  380  of a quad port module  385  (also referred to hereinbelow as QPM  385 ) and an optical detector module  390  (also referred to hereinbelow as ODM  390 ) having connectivity through a MTP-12  400 ,  405 . Here, connectivity is indicated in the form of parallel interconnects such as a MTP-8  410 ,  415 . The same connectivity can be achieved through other means including, but not limited to, individual connectors (not shown). One of the diagnostics performed at the time of provisioning is injection of a signal through an alignment laser  417  so as to check if input power can be detected at ODM  390 . It should be noted that this test fails if MTP-12&#39;s  400 ,  405  are not properly connected between ODM  390  and QPM  385 . 
   Fault Isolation Within Optical Path 
   Referring now to  FIG. 8 , there is shown a photonic cross-connect and alignment laser configuration  420  having a first alignment laser  425  and a second alignment laser  430 . Alignment laser  425  and alignment laser  430  are utilized to isolate faults within an optical path through a switch core during operation. The fault isolation includes, but is not limited to, steps such as (1) using a set of linear equations to determine the insertion loss for each of the components along the optical datapath so as to isolate the root cause of a failure detected in the optical datapath, and (2) sequentially creating cross-connects having the same organization with multiple termination points or having multiple origination points, so as to isolate the failures in the optical datapath. 
   Injection of Test Signals 
   Alignment lasers can be used to inject specified test signals into an optical network. Some of the possible uses of these optical test signals include, for example, (1) local auto-discovery of connectivity between PXC and attached TNE&#39;s (Transport Network Elements), (2) remote auto-discovery of connectivity between two adjacent PXC&#39;s (peer-peer connectivity), and (3) verification of idle links between PXC and TNE&#39;s and between two PXC&#39;s.