Abstract:
A method for calibrating a preferred disposition for a moveable first mirror of an optical switch core of a photonic crossconnect device relative to a second mirror thereof, the method comprising the steps of determining approximate geometric coordinates of the first mirror relative to the second mirror, effecting a laser light crossconnection between the first and second mirrors to produce data from which to provide first order corrections to refine the geometric coordinates, and effecting a further laser light crossconnection between the first mirror and a third mirror, to produce data from which to provide second order corrections to further refine the geometric coordinates, whereby to calibrate the first mirror such that upon initiation of a laser light crossconnection involving the first mirror, a switching element detects deviation of the first mirror from the preferred disposition thereof and effects corrective changes.

Description:
REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of now abandoned prior U.S. Provisional Patent Application Ser. No. 60/368,276, filed Mar. 27, 2002 by Babu Narayanan et al. for CALIBRATION OF PHOTONIC CROSSCONNECTS, which patent application is hereby incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to automatic precise alignment of laser light transmitting and receiving mirrors, and is directed more particularly to alignment of mirrors in an optical switch core of a photonic crossconnect device. 
     BACKGROUND OF THE INVENTION 
     Photonic crossconnect devices (PXCs) are generally known and comprise an optical crossconnect with no optical-electrical conversions performed in the data path in the device. The typical crossconnect device includes a plurality of input ports, a plurality of output ports, and an optical switch core which provides relative connections between input and output ports. The optical switch core itself typically comprises, among other things, electrostatically controlled microelectromechanical (MEMs) mirrors which are tilted on command so as to direct optical signals between various ports. However, manufacturing variations and changes on operating environments require that photonic crossconnect devices be calibrated so as to ensure the reliable operation of the devices. 
     There is a need for a method for calibration of the optical switch core (or cores) of the PXC so that crossconnections can be made quickly and reliably. 
     SUMMARY OF THE INVENTION 
     An object of the invention is, therefore, to provide a method for calibrating the switching elements of a photonic crossconnect device such that crossconnections can be effected quickly and reliably. 
     With the above and other objects in view, a feature of the present invention is the provision of a method for calibrating a most efficient disposition for a first mirror of a first switching element of an optical switch core of a photonic crossconnect device relative to a second mirror of a second switching element of the optical switch core, the first element first mirror being moveably mounted relative to the optical switch core, and first switching element sensors being mounted on the first switching element for sensing (i) geometrical coordinates of the first mirror in space, (ii) offset values for the first switching element sensors, and (iii) gain values for the first switching element sensors. The method comprises the steps of setting offset and gain values for the first switching element sensors of the first switching element to default values, and determining approximate first order geometric coordinates of the first mirror of the first switching element relative to the second mirror of the second switching element by at least one of (i) heuristics and (ii) mathematics. The method further comprises the steps of effecting a laser light crossconnection between the switching elements&#39; first and second mirrors. Data is provided therefrom that is used to refine the first order geometrical coordinates of the first mirror of the first switching element, and effecting at least one further laser light crossconnection between the first switching element, first mirror and a third switching element third mirror. Data is produced therefrom that is used to correct the offset and gain values for the first switching element sensors, whereby to calibrate the first mirror such that upon initiation of a laser light crossconnection involving the first mirror, the switching element sensors for the first switching element are adapted to detect deviation of the first mirror from the most efficient disposition thereof and enable the first switching element to effect corrective changes. 
     The above and other features of the invention, including various novel details of construction and combinations of parts and steps, will now be more particularly described with reference to the accompanying drawings and pointed out the claims. It will be understood that the particular method embodying the invention is shown by way of illustration only and not as a limitation 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 
       Reference is made to the accompanying drawings in which is shown an illustrative embodiment of the invention, from which its novel features and advantages will be apparent. 
       In the drawings: 
         FIG. 1  is a diagrammatic illustration of an optical switch core for a photonic crossconnect device; 
         FIG. 2  is a diagrammatic illustration of a photonic crossconnect device having optical switch cores, each having an array of switching elements; and 
         FIG. 3  is a diagrammatic illustration of a photonic crossconnect device in combination with detector modules and a calibration controller. 
         FIG. 4  is a diagrammatic illustration of a photonic crossconnect device in combination with detector modules and a calibration controller showing a second crossconnection between a first mirror and a third mirror. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     As noted above, photonic crossconnect devices (PXC) are generally known and comprise an optical crossconnect with no optical-electrical conversions performed in the data path of the device. The PXC  10  ( FIGS. 2 and 3 ) is provided with input ports  12  and output ports  14 . External fibers  16  are connected to the PXC ports  12 ,  14  to transport the optical signals that are to be switched in the PXC. Switching in the PXC  10  occurs within an optical switch core  18  ( FIGS. 1–3 ). It is assumed that the PXC  10  is non-blocking, that is, any port  12 ,  14  can be cross-connected to any other port in the system. An optical switch core  18  is diagrammatically illustrated in  FIGS. 1–3 . 
     For purposes of this description, it is assumed that the PXC  10  has duplex ports. PXC duplex ports include an input port and an output port and are interconnected to other equipment via the external fibers  16 . 
     A PXC may be configured as a redundant system. An example of a redundant configuration is shown in FIGS.  2  and  3 , wherein the system is provided with two independent optical switch cores  18 A and  18 B and two-way switches  20  within the ports  12 ,  14 . In an alternative configuration, there is one optical switch core, but two-way switches within the ports to achieve redundancy. Various combinations of splitters  22  and/or switches  20  may be used to achieve redundancy. An example of a configuration with splitters/switches, dual cores and internal alignment lasers  30  (from optical detector modules  32 ) is shown in  FIG. 3 . 
     The optical switch core  18  includes a number of switching elements  24 . For each input to the optical switch core  18 , there is one switching element  24 A, and for each output there is one switching element  24 B. The switching element  24  may comprise a collimator (not shown) and a microelectromechanical (MEMs) mirror  26  ( FIG. 3 ) that is electrostatically controlled. It is known to tilt an input mirror  26 A and an output mirror  26 B in three dimensional space with respect to two orthogonal axes. Switching elements  24  work so that the two mirrors in question,  26 A,  26 B, point to each other so as to carry an optical signal from the input mirror  26 A to the output mirror  26 B. 
     The optical switch core  18  may be programmed using a set of parameters for each switching element  24 . For example, a switching element that is based on a MEMs mirror with hinge sensors may have the following parameters associated with it: (1) geometrical coordinates for positioning the mirror in three dimensional space relative to a reference; (2) offset values for the hinge sensors; and (3) gain values for the hinge sensors. The geometrical coordinates can be recorded using hinge sensor readings. For instance, for each input switching element there may be recorded the desired sensor values for all the sensors in use so that the input switching element  24 A is pointed towards a selected output switching element  24 B. The same may be done for every output switching element pointing towards an input switching element. Such sets of initialization values are referred to as “Parameters of Zeroth order”. Hinge sensor values may change over time as a result of drift in the offset or gain of the hinges, or as a result of a correlation between the different hinges of the same mirror. For each switching element, parameters of first and second orders may be used to capture such variations. For example, first order parameters may be related to drift in sensor offset values, while second order parameters may be related to drift in sensor gain values. 
     In the case of electrostatically driven mirrors with two hinge sensors, X and Y sensors, an angle of rotation of the mirror may be related to an applied voltage as follows:
     V x0 : The sensor offset for the X axis when the angle of rotation is 0 degrees.   V y0 : The sensor offset for the Y axis when the angle of rotation is 0 degrees.   α: The sensor gain for the X axis   β: The sensor gain for the Y axis   

     Where V x  is the X axis sensor voltage and V y  the Y axis sensor voltage, the following equations apply:
 
 V   x   =V   x0 +αθ  (1)
 
 V   y   =V   yo +βφ  (2)
 
     Here, θ and φ are the angles of rotation with respect to the X and Y axes of the mirror, respectively. The equations (1) and (2) assume that a response is linear with respect to the angle of rotation, and there is no dependence on the angle of rotation with respect to the other axis. The V yo  and V x0  can be considered offset terms, and α and β can be considered gain terms. 
     The equations (1) and (2) are easily modified to incorporate dependence between X and Y axes of rotation, as well as lower order terms, as follows:
 
 V   x   =V   x0   +αθ+y θ 2   +f (θ,φ)  (3)
 
 V   y   =V   y0 +βφ+δφ 2   +g (θ,φ)  (4)
 
     In operation, it is desired that an input mirror  26 A align with an output mirror  26 B. In order to accomplish this, the mirror  26 A has to be turned with respect to the X axis and Y axis. Given a desired angle, a servo control for the mirrors uses the equations above to relate the sensor voltages to the desired angle. 
     From equation (1), it is clear that to solve for the unknowns V xo  and α, it is necessary to solve a system of two equations with two unknowns. Hence, it is necessary to make at least two distinct connections involving a mirror to obtain two equations. It may be desirable to use more than two connections to get more accurate values, especially for the gain parameter α. In case of equation (4), more than two connections are necessary. The same holds true for the unknowns V yo  and β relative to the Y axis. 
     Calibration of the PXC  10  may be carried out in two phases—fine calibration and coarse calibration. Fine calibration, in turn, involves first and second orders of correction, or refinement. Coarse calibration is used to generate a rough set of values so that connections can be made reasonably quickly. For example, coarse calibration may be based on heuristics and/or mathematical formulas to obtain the geometric coordinates of the mirrors in a MEMs based PXC, with offset and gain values set at default values. A simple method is to make all possible connections and record the configurations of the input and output switching elements after successfully making every connection. For example, in a MEMs based PXC with hinge sensors, record the hinge sensor readings after each successful connection. 
     Coarse calibration is essentially determining initial estimates. Once this is done, the subsequent phases of calibration refine the coarse values and determine increasingly accurate values. 
     First order correction of calibration data can be achieved by refining the coarse calibration parameters associated with the switching elements  24 . In the case of a MEMs based PXC, this involves correcting the calibration data for drift in sensor offsets. First order correction of the data for a switching element may be achieved by having the switching element participate in one connection. This is sufficient inasmuch as equation (1) or (2) is used in this case. 
     If the ports of the PXC are bi-directional ports, it is convenient to perform loopback connections (i.e., crossconnect the input of a port to the output of the same port) to generate data for the first order correction. A laser source is necessary to make a crossconnection. The laser source may be an external laser source (external to the PXC system) or an internal laser source that is internal to the PXC stem. In both cases, it is desirable to ensure that the light used to update calibration data does not leave the PXC system. 
     In the case of the redundant PXC system, as shown in  FIG. 3 , the two-way switches  20  at the output ports  14  of the PXC  10  are used to ensure that the light does not leave the PXC system. The following steps are preferred: 
     (1) ensure that the crossconnection is not active in core  18 B; 
     (2) set the switch  20  at the output port  14  to select from core  18 B; 
     (3) make a loopback connection in core  18 A and record data for the first order corrections; 
     (4) tear down the loopback connection in core  18 A; 
     (5) set switch  20  to select from core  18 A; and 
     (6) make a loopback connection in core  18 B and record data for the first order corrections. First order correction may be carried out continuously for ports that are involved in crossconnections. 
     Second order correction of calibration data involves refining the first order parameters. In the case of a MEMs based PXC, this involves correcting offset and gain values for the switching elements  24 . As mote than one parameter is being corrected, at least one additional crossconnection is required per switching element being calibrated. The additional crossconnection may be created, for example as shown in  FIG. 4 , by causing a crossconnection to be created between the first mirror  26 A and a third mirror  26 B′. Using the data from the two crossconnections, second order correction is made. It is useful to make several connections, more than two, and apply a technique such as linear regression to obtain robust values. 
     Ports  12 ,  14  in the PXC  10  may be reserved for calibration to ensure availability of free ports for the connections necessary to make second order corrections. As in the previous correction phase, one may use internal or external light for the crossconnections. One should ensure that the light does not leave the PXC system by making proper use of the switches  20  at the PXC ports  12 ,  14 . 
     The above methods for updates of calibration data can be performed with the PXC in operation. It may also be performed with a partially equipped system where only some ports are deployed and some are not. The above methods require only deployed ports to participate in crossconnections. The method may be employed periodically on ports that are idle in an operational PXC system or may be employed upon user demand. The calibration data may be recorded persistently in a database within the system. The data may be copied periodically to an off-line location  28  ( FIG. 3 ) where trends in calibration data can be studied to monitor system performance. Trend analysis can also be performed within the system controller to trigger alarms (not shown) on some optical switching elements  24  or ports  12 ,  14  of the system if calibration data is seen to be changing beyond expected limits. 
     There is thus provided a method for calibrating the switching elements of a photonic crossconnect device, thereby facilitating quick and reliable crossconnections. 
     It will be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described and illustrated in order to explain the nature of the invention, may be made by those skilled in the art within the principles and scope of the invention as expressed in the appended claims.