Abstract:
A method and apparatus suitable for testing the operation of a photonic switch are described. The photonic switch includes switching means arranged to redirect (and hence switch) an incident optical signal. Operation of the switching means can be verified by providing a test optical signal arranged to be incident upon the switching means, the test optical signal path being distinct from the switched optical signal path.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a photonic switch, and in particular to an apparatus and a method suitable for testing the operation of a photonic switch. 
     BACKGROUND OF THE INVENTION 
     Communications networks are increasingly becoming all optical networks, incorporating photonic (optical) switching. Photonic switches are typically fabricated using Micro Electro-Mechanical Structures (MEMS) technology. A recently developed photonic switch of this type is described in “Free-Space Micro Machined Optical Switches for Optical Networking” by LY Lin et al, IEEE Journal of Selected Topics in Quantum Electronics, Vol. 5 No. 1, January/February 1999; which is incorporated herein by reference. Such switches may be used to switch wavelength division multiplexed (WDM) signals as a group, or the WDM signals may be demultiplexed outside the switch and switched individually as channels, or as groups of channels as desired. MEMS switches typically use moveable mirrors to re-direct optical paths within the switch in order to complete an optical signal or channel connection across the switch. 
     FIG. 1 shows a schematic diagram of a typical MEMS photonic switch  100 . The switch  100  is bidirectional, but for simplicity is assumed to comprise  4  inputs in the form of optical fibres  112 ,  114 ,  116  &amp;  118 , and  4  outputs which are also optical fibres  122 ,  124 ,  126  &amp;  128 . Each input and output has an associated lens  104  which collimates the beam from each input and focuses the respective beam at each output. Such a switch is generically referred to as a  4 × 4  switch (number of inputs x number of outputs). 
     The switch  100  is a cross point switch, having a switching device (a mirror,  106 ) located at each of the points at which optical signals emitted from the input fibres would cross with optical signals emitted from the output fibres. The switch  100  thus has a four by four array of mirrors  106  mounted on a surface  102 . 
     In this particular switch, each mirror may be moved between two stable positions. FIGS. 2 a  and  2   b  illustrate these positions. FIG. 2 a  shows the mirror in the inactivated position  106   a , where the mirror is flat i.e. substantially parallel to the surface  102 . FIG. 2 b  shows the mirror having been raised to the activated or upright position  106   b , substantially perpendicular to the surface  102 . This activation may be performed by a variety of means e.g. by micro actuators causing the mirror to be rotated about the hinges  108 . The mirrors are typically formed of materials such as polysilicon, the reflectivity of which is increased by providing a reflective coating  107  such as gold. In the inactivated state, it is typical for the relatively non reflective surface  109  of the mirror to lie adjacent to the surface  102 , so that the reflective coating  107  does not contact the surface  102 . 
     FIG. 1 shows a typical operation of the switch  100 . By raising the appropriate mirrors, an optical signal from each of the inputs  112 ,  114 ,  116  &amp;  118  is directed to a respective output  128 ,  126 ,  122  &amp;  124 . For instance, an optical signal originating from input fibre  112  is formed into a collimated beam  132  by lens  104 . The beam  132  then reflects off the front reflective surface  107  of a raised mirror  106   b  into a further lens  104  which focuses the beam  132  into the output fibre  128 . It will be appreciated that by appropriate control of the array of mirrors  106 , any one of the signals originating from the inputs  112 ,  114 ,  116  &amp;  118  can be switched into any one of the outputs,  122 ,  124 , 126  &amp;  128 . 
     Various solutions have been proposed to test the mirror status or switch connection, in order to verify that the mirrors  106  are functioning correctly and are not, for example, jammed in either the raised  106   b  or flat  106   a  position. 
     One solution is to inject different optical test signals into each input port (i.e.  112 ,  114 ,  116 ,  118 ) to the switch  100  via fibre tap couplers (not shown). Such test signals would be distinct from the normal optical signal being switched e.g. of different wave length and/or modulation characteristics. Each output port (i.e.  122 ,  124 ,  126 ,  128 ) would then be connected to a further tap coupler, in order that the test signals could be extracted, detected and analysed for verification that the desired input to output connections exist. This solution is true connectivity verification. However, due to the number of components required, it would be both bulky and expensive. For instance, in a N×N switch (where N is an integer) the required components would include 2N couplers, N sources, N detectors, as well as numerous splices and fibre interfaces; additionally there would be the assembly cost. 
     An alternative solution is to use electrical parameters (e.g. capacitance, inductance or resistance) to monitor the physical position of the mirrors. However, this would double the number of electrical connections to the switch matrix, and is hence impractical for large arrays of mirrors. 
     Co-pending U.S. application Ser. No. 09/545,545, “Testing Operation of a Photonic Switch”, by the same inventor, describes a method of utilising test optical signals in the plane of the switching mirrors, reflected from the rear of one or more of the mirrors, in order to test whether the mirrors are functioning correctly. This approach has limitations in that it requires the rear of the mirrors to be reflective, and requires an array of mirrors to be in certain predefined configurations for any given mirror to be tested. Simultaneous testing of all mirrors is hence not possible. 
     The present invention aims to address one or more of the problems of the prior art. 
     SUMMARY OF THE INVENTION 
     In a first aspect, the present invention provides a photonic switch having at least one switching means comprising a reflective surface arranged to be moveable between at least a first and a second position, and arranged to switch an incident optical signal by reflectively redirecting the optical path of said signal when in at least one of said positions, the incident and redirected optical paths defining a first plane; the switch further comprising test signal means arranged to provide a test optical signal incident said reflective surface, and measuring means arranged to measure a reflection of the test signal at a predetermined position suitable for determining if said switching means is in said first position from said measurement, the paths of the incident and reflected test signals lying outside of the first plane. 
     In a further aspect, the present invention provides a telecommunications system comprising a photonic switch having at least one switching means comprising a reflective surface arranged to be moveable between at least a first and a second position, and arranged to switch an incident optical signal by reflectively redirecting the optical path of said signal when in at least one of said positions, the incident and redirected optical path defining a first plane; the switch further comprising test signal means arranged to provide a test optical signal incident said reflective surface, and measuring means arranged to measure a reflection of the test signal at a predetermined position suitable for determining if said switching means is in said first position from said measurement, the paths of the incident and reflected test signals lying outside of the first plane. 
     In another aspect, the present invention provides a method of testing the status of a photonic switch, the switch having at least one switching means comprising a reflective surface arranged to be moveable between at least a first and a second position, and arranged to switch an incident optical signal by reflectively redirecting the optical path of said signal when in at least one of said positions, the incident and redirected optical paths defining a first plane; the method comprising the steps of providing a test optical signal incident said reflective surface, the optical path of the test signal not lying within said first plane, and measuring a reflection of the test signal at a predetermined position suitable for determining if said switching means is in said first position from said measurement. 
     Preferably, the method further comprises the step of providing an actuating signal sufficient to move the switching means such that the intensity of the reflected test signal at the predetermined measuring location would be altered, without moving the switching means sufficiently to redirect the incident optical signal. This allows the switching means (normally mirrors) to undergo a small movement to check whether the mirror is stuck, but without the movement being large enough so as to redirect the incident optical signal. If the mirrors move in response to the actuating signal, then this can be detected by the change in reflected test signal incident on the detector. This test allows re-routing to an alternative path if the mirror is fixed in position. 
     In a further aspect, the present invention provides a computer program arranged to perform a method of testing the status of a photonic switch, the switch having at least one switching means comprising a reflective surface arranged to be moveable between at least a first and a second position, and arranged to switch an incident optical signal by reflectively redirecting the optical path of said signal when in at least one of said positions, the incident and redirected optical paths defining a first plane; the method comprising the steps of providing a test optical signal incident said reflective surface, the optical path of the test signal not lying within said first plane, and measuring a reflection of the test signal at a predetermined position suitable for determining if said switching means is in said first position from said measurement. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In order that a greater understanding of the invention can be obtained, embodiments of the invention will now be described with reference to the accompanying drawings, by way of example only and without intending to be limiting, in which: 
     FIG. 1 shows a typical MEMS switch arrangement (PRIOR ART); 
     FIG. 2 a  and FIG. 2 b  show respectively a mirror from the switch of FIG. 1 in the inactivated state and the activated state (PRIOR ART); 
     FIG. 3 shows the testing of a MEMS switch according to a first embodiment of the present invention; and 
     FIG. 4 shows the testing of a MEMS switch according to a second embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 3 shows a first embodiment of the present invention, which largely corresponds to the typical MEMS switch arrangement found in FIG.  1 . By appropriate movement of each of the mirrors  206  between the flat  206   a  and raised  206   b  positions, optical signals from each of the inputs  212 ,  214 ,  216  and  218  can be directed to any of the outputs  222 ,  224 ,  226  and  228 . The apparatus is characterised by having a light source  300  arranged to transmit light so as to be incident upon the array of mirrors  206 . 
     A detector array  310  comprises an array of photo detectors  312 , with a respective photo detector  312  for each of the mirrors  206 . The detector array  310  is arranged such that each photo detector  312  will detect light from the respective mirror  206  when the mirror is in the flat (inactivated) position  206   a.    
     FIG. 3 shows this particular embodiment in use. From the perspective view of the switch  200  shown, it can be seen that two of the mirrors  206  are in the upright position  206   b  with four of the mirrors  206  in the flat position  206   a  i.e. substantially parallel to the switch surface  202 . Optical signals from the inputs ( 212 - 218 ) will thus be switched via the mirrors  206   b  to the outputs ( 222 - 228 ), with the signals lying within the same plane. This plane will be substantially parallel and, in the orientation illustrated, above the plane defined by the surface  202 . 
     The light source is positioned above the plane of the signals, and arranged to provide a beam of light  302  (the outer extremities of the beam  302   a  and  302   b  are shown) that intersects the plane of the switched optical signals. Only light incident upon the flat mirrors  206   a  will be reflected  304  so as to illuminate a photo detector  312 . 
     Analysis of the signals of the photo detectors to determine those that are illuminated ( 312   a ) and those that are not illuminated ( 312   b ) can respectively determine which of the mirrors are in the flat position and which are not. 
     The embodiment thus allows the detection of mirror status (i.e. position and operation) with components positioned outside of the plane of the switched optical signals. Thus the detection system does not interfere in any way with the mirrors, the inputs and outputs or any expansion ports that may exist for control of the mirrors. As this detection technique does not rely on light from the inputs ( 212 - 218 ) or outputs ( 222 - 228 ), no optical couplers are required that would result in an attenuation of the optical signal being switched or added complication or cost. 
     If desired, the detection can be performed at a relatively low speed, thus allowing for large area detectors which minimise the precision of the geometry of the optics required in relation to the light source  300  and the detector array  310 . Only one light source  300  is necessary, although of course it will be appreciated that if desired a separate light source could be utilised for providing a respective signal  302  incident upon each mirror  206 . 
     The light source can be of any desired wavelength, but is preferably of a different wavelength than the optical signals being switched so as to reduce interference. Preferably, the light source is visible radiation, thus permitting the use of relatively inexpensive silicon detector arrays. Finally, as the technique preferably utilises the existing high reflectivity front surface of the mirrors  206 , no extra steps in MEMS processing is required to construct the apparatus of the present invention. 
     If desired, a window transparent to the light from the light source  300  could be used to hermetically seal the upper surface of the switch  200 . Such a window (not shown) could be positioned so as to be substantially parallel to the surface  202 . Suitable materials for such window would include sapphire or glass, which are transparent to the visible wavelengths of light. 
     The invention thus provides a practical and low cost means of continuously monitoring the status of the mirrors in a MEMS optical switch. The network manager in a optical transmission system can utilise this information to identify faults in the switching and mirrors, and to infer the input/output connectivity of that element. If a fault is detected in the switch, then the network manager can appropriately reroute signals around the switch/network so as to minimise the impact of the fault on live traffic. 
     As well as being utilised on-line (i.e. when the system is carrying live optical signals), the method may also be used to test the operation of the mirrors off-line. 
     If desired, the invention can also be utilised to provide a means of testing on-line the operation of the mirrors  206  to ensure that they have not become stuck in, for instance, the flat  206   a  position, prior to actuating the mirrors into the raised  206   b  position. This is achieved by applying a low level of actuating signal to a mirror in the flat position  206   a , sufficient to move the mirror such that the reflected light from the mirror does not fall on the respective detector at the same level of intensity. As the mirror is only moved through a relatively small angle, the optical signals being switched will not be interrupted/affected, whilst a change in detected light intensity will indicate that the mirror  206  is not stuck. 
     Whilst the invention has been described in reference to the particular embodiments shown in FIG. 3, equally it will be understood by a skilled person that various other embodiments will fall within the scope of the present invention. For instance, whilst the detector  310  has been described as a detector array, it will be appreciated that this array could be replaced by a single detector having a surface area sufficient to detect all the optical signals reflected from all of the mirrors in the flat position  206   a . The mirror status could thus be deduced by determining the total optical power incident upon the detector array compared with the optical power which would be reflected by a single mirror in the flat position  206   a.    
     Equally, whilst the invention has been described in relation to monitoring when the mirrors  206  are in the flat position, it will be appreciated that the technique described in principle could be extended to confirm angular positions of the mirrors at any desired angle. This could be achieved by appropriate positioning of the light source  300  and detector  310 . 
     FIG. 4 shows such an alternative embodiment. In this particular embodiment, additional mirrors ( 306   a ) are located adjacent to the switch mirrors  206 . In this instance, the additional mirrors  306   a  are fixed in position so as to lie substantially flat upon the silicon substrate surface  202 . The light source  300 , additional mirrors  306   a  and the detector array  310  are positioned such that an optical signal  302  from the light source  300  will be detected at the respective detector in the array  310 , with the beam  302  being reflected from the fixed mirror  306   a  and the actuated mirror  206  only when in the upright position  206   b . If the respective actuated mirror  206  is in the flat position  206   a , then the reflected light beam  304  will not be incident upon the respective detector within the array  310 . 
     It will be appreciated that in the embodiment shown in FIG. 4, the light source and the detector array positions could be juxtaposed due to the bi-directional nature of light. 
     Equally, if desired the embodiment shown in FIG.  3  and the embodiment shown in FIG. 4 could be combined into a single apparatus, utilising only a single light source  300  but two arrays of detectors  310  respectfully arranged to detect when the mirrors are in the flat position  206   a  or the upright position  206   b.    
     For the purpose of this specification the terms “optical” and “light” should be understood as pertaining not only to the visible part of the electro magnetic spectrum, but also to the infra-red and ultra-violet parts that bound the visible part. 
     Any range as given herein may be extended or altered without loosing the effect sought, as will be apparent to a skilled person from an understanding of the teaching herein.