Abstract:
In optical communication switching equipment there is a need to set up optical paths through the switch before there is an optical signal present. The present invention provides an optical switching apparatus comprising: an optical switch having a number of optical paths for switching optical channels; a demultiplexer coupled to said switch; a broadband light source coupled to said demultiplexer which distributes said light in said channels to enable correct alignment of said optical paths.

Description:
FIELD OF THE INVENTION 
     The present invention relates to optical switching, and particularly to an apparatus and method for optical switch alignment. 
     BACKGROUND OF THE INVENTION 
     Communications networks are increasingly becoming all optical networks, incorporating optical switching. Optical switches are typically fabricated using micro electro-mechanical systems (MEMS) technology. A recently developed optical switch of this type is described in “Performance of a 576×576 Optical Cross Connect” by H Laor, A Richards, E Fontenot, Proceedings of the National Fibre Optic Engineers Conference Sep. 28, 1999, Chicago, USA”; which is incorporated herein by reference. Such switches may be used to switch wave division multiplex (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 use moveable mirrors to redirect and align optical paths between switching modules within the switch. The switching paths must be correctly aligned to complete an optical signal or channel connection across the switch. 
     There is a need to set up optical paths through the switch before there is an optical signal or channel present. That is the optical path must be set up and verified before transmission of optical channels to prevent misconnection through misalignment of the moveable mirrors in the switch, and hence to prevent the introduction of errors into the optical signal by the switching function of the network. 
     FIG. 1 shows a schematic diagram of a MEMS based optical switch incorporating a prior art optical path alignment arrangement. The optical switch comprises a number of port cards  1  coupled to modules of a MEMS core switch  2  comprising focusing lenses, fixed and moveable mirrors as described below, and in more detail in the above reference for example. Each port card  1  corresponds to a pair of modules in the switch  2  and connects optical fibres (for example  3   a  and  4   a ) to modules of the MEMS core switch  2 , which switches incoming fibres ( 3   a ,  3   b  . . . ) to desired outgoing fibres ( 4   a ,  4   b  . . . ). A typical core network switch has 156 pairs of modules. 
     The optical signal wavelength band is typically determined by standards, for example on or around 1500 nm for core network transmission. The prior art arrangement uses light of a different wavelength injected into the optical path to enable correct alignment of the mirrors in the MEMS core switch  2 . Light from a local light source  5  within each port card  1  is injected into the optical path  3  using a wavelength selective coupler  6 . Another wavelength selective coupler  6  on the other side of the MEMS core  2  of each card  1  is used to filter off the light injected which is then detected by a receiver or detector  7 , and used to indicate that the MEMS mirrors are correctly aligned. A major disadvantage of this arrangement however is the addition of two wavelength selective couplers  6  and a local light source for each port card  1  of the switch. This adds significantly to the per port cost of the switch fabric. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an improved apparatus and method for optical switch alignment. 
     In a first aspect the present invention comprises an optical switching apparatus comprising: 
     an optical switch having a number of optical paths for switching optical channels; 
     a demultiplexer coupled to said switch; 
     a broadband light source coupled to said demultiplexer which distributes said light in said channels to enable correct alignment of said optical paths. 
     Preferably said apparatus further comprises receiver means coupled to said switch and arranged to indicate correct alignment of said optical paths by receiving said light. 
     Preferably said receiver means comprises local receivers for each said channel. 
     Preferably the light source is an E-LED. 
     Alternatively the light source is amplifier spontaneous emissions. 
     Preferably the light source is modulated. 
     Preferably said modulation is amplitude modulation. 
     Preferably the switch is an MEMS switch. 
     In a second aspect the present invention comprises an optical switching apparatus comprising: 
     an optical switch having a number of optical paths for switching optical channels; 
     a demultiplexer coupled to said switch; 
     a modulated broadband light source coupled to said demultiplexer which distributes said light in said channels to enable correct alignment of said optical paths. 
     In a third aspect the present invention comprises an optical switching apparatus for enabling correct alignment of optical paths in an optical switch, said apparatus comprising: 
     a demultiplexer coupled to said switch; 
     an in-band light source coupled to said demultiplexer which distributes said light in demultiplexed optical channels to enable correct alignment of said optical paths. 
     In a further aspect the invention comprises a method of aligning optical paths through an optical switch for switching a number of optical channels, the method comprising: 
     generating a broadband light source in a multiplexed signal comprising said channels; 
     demultiplexing said signal; 
     receiving said light in the demultiplexed channels to indicate correct alignment of said optical paths. 
     In a yet further aspect the present invention comprises a computer program on a machine readable medium, said computer program being capable of performing a method of controlling an optical switching apparatus to align optical paths through an optical switch for switching a number of optical channels, the method comprising: 
     introducing a broadband light source into a multiplexed signal comprising said channels; 
     demultiplexing said signal; 
     receiving said light in the demultiplexed channels to indicate correct alignment of said optical paths. 
    
    
     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 prior art optical switch alignment arrangement; and 
     FIG. 2 shows a typical MEMS switch core optical connection between two modules; 
     FIG. 3 shows an optical switch alignment arrangement according to a preferred embodiment of the present invention; and 
     FIG. 4 shows a switching apparatus according to the invention. 
    
    
     DETAILED DESCRIPTION 
     A micro electro-mechanical systems (MEMS) optical switch is constructed of a number of modules. FIG. 2 shows the optical path which represents a connection between two modules  10  and  20 . Light from a fibre  3   a  passes through a focusing lens  12 . The light which forms the beam is reflected off a fixed mirror  14  to keep device packaging small. It is then reflected off a movable mirror  16  which precisely directs the beam in two axes. If the movable mirror is in position  16 ′, the beam will move along an alternative path shown as  32 . 
     To make an optical connection between modules  10  and  20 , the moveable mirror  16  directs the beam at the movable mirror  26  of a targeted second module  20 . At the same time the moveable mirror  26  of the second module  20  is controlled to deflect the beam  30  towards the fixed mirror  24 , into the lens  22  and hence into the fibre  4   a  completing the connection. It is the co-ordinated control of deflection angles by the two moveable mirrors  16  and  26  that creates the optical connection between the two transmission fibres  3   a  and  4   a.    
     By having two arrays of such modules, optical signals coming in from a first array (corresponding to fibres  3   a ,  3   b  . . . ) may be directed into any of the output fibres  4   a ,  4   b  . . . of the second array. The terms input and output are used for convenience, the optical path through any two modules in a connection being bi-directional. 
     As described above referring to FIGS. 1 and 2, a port card  1  couples a pair of optical fibres (eg  3   a  and  4   a ) to a corresponding pair of modules (eg  10  and  20 ) in a MEMS core switch  2  having a number of such pairs and which is capable of switching between modules on one array (associated with the “input” fibres  3 ) to another array (associated with “output” fibres  4 ). Each fibre  3  or  4  may carry one or a number of optical channels. The term optical channel refers to a wavelength or discrete range of wavelengths which carries a signal and which is distinguished from one or more other optical channels using different wavelengths or ranges of wavelengths as is known in Wave Division Multiplexing (WDM). Typically optical fibres external to the switching apparatus (A and B) are used to carry WDM signals which are then demultiplexed into individual optical channels and switched as individual channels. A demultiplexer  15  separates the optical channels and puts them onto separate fibres  3  ( 3   a ,  3   b  . . .  3   x ) where they are each connected to a port card  1  for switching by the core switch  2 . Alternatively the complete WDM signal or groups of channels may be switched. 
     As described above, there is a requirement to set up a connection between modules before there is signal light in the optical channel being switched to prevent the misconnection of traffic and to avoid the traffic being delayed by the sum of all the optical switch set-up times when first connected to the network. Once the path is correctly set up, the moveable mirrors being correctly aligned in the modules concerned the newly switched optical channel may be used for the transmission of optical signals. Prior art solutions typically use wavelength selective couplers  6  within the port cards to introduce and detect out of band light to optimise mirror alignment in the connection. Two such couplers  6  are used for each channel or port card section  1 . The use of these wavelength selective couplers and a separate light source for each port however significantly increases the cost of port cards, and may also contribute to signal attenuation or other performance deterioration. 
     Light sources for alignment purposes may also be placed around the moveable mirrors. However it can be difficult to ensure that these light sources are aligned to follow a path parallel to that taken by the light from the input fibre. These light sources and detectors also represent additional expense and manufacturing complexity. 
     Referring to FIG. 3, a preferred embodiment switching apparatus and method is there described. The apparatus comprises a number of port cards  51  each coupling fibres (for example  3   a  and  4   a ) to the MEMS core switch  2 . Each port card  51  comprises an injected light receiver or detector  52 . The apparatus also comprises a demultiplexer  55  and optionally a multiplexer  56 , respectively coupled (by fibres  3   a  and  4   a ) to the input and output of the port cards  51 . A light source  53  is available prior to the demultiplexer  55 . Preferably the apparatus further comprises or uses an existing optical amplifier  54  which is coupled to the input of the demultiplexer  55 . 
     The light source  53  is preferably an E-LED (Edge Emitting Light Emitting Diode) which is coupled into the demulitplexer  55  input using the optical amplifier  54  for example. Preferably use is made of an existing amplifier  54  to further reduce the component count of the apparatus. Alternatively amplifier spontaneous emissions from the amplifier  54  may be used as a light source thereby further reducing component count. The demultiplexer  55  divides a combined optical signal in fibre A comprising a number of optical channels into separate optical channels or groups of optical channels corresponding to fibres  3   a ,  3   b  . . . , each individual channel or group of channels being coupled to a port card  51 . The channels are then switched individually and if desired recombined into a common optical signal such as WDM by the multiplexer  56  as is known in the art. The light source  53  is injected into the “incoming” fibres  3  by introducing it into the fibre A before the demulitplexer  55 , into fibre A which distributes it into the demultiplexed optical channels in fibres  3   a ,  3   b ,  3   c  . . . 
     To achieve this the light introduced by the light source  53  is broadband such that the injected light shares the wavelength range of the optical channels in the multiplexed signal in fibre A so that it is distributed across all the optical channels and is present in each optical channel when the demultiplexer  55  divides the multiplexed signal into individual optical channels. The injected light travels along the fibres  3   a  ,  3   b  etc through the core switch  2  and is detected by the injected light receiver  52  of each port card section  51 , which is optimised for detecting the injected light. 
     The light source  53  is preferably modulated according to a predetermined modulation scheme. For example the light source  53  is amplitude modulated to add a tone which can be readily detected by the detectors  52 . This allows the injected light to be introduced while an optical signal is present, the receiver  52  being optimised for detecting this modulated light. It also simplifies the design of the receiver. The receiver or detector  52  is fed a small portion of the signal light from the fibre  4   a  which comprises the injected light. This portion or sample is derived from a splitter  56  for example. 
     The amplifier  54  and light source  53  (eg an E-LED) are located before the demulitplexer divides out the channels. This reduces the component count of the apparatus by requiring only one light source and amplifier to introduce the injected light for correct alignment of the switch. The use of amplifier spontaneous emissions further reduces component count by requiring only the amplifier  54 . The light source is broadband so that it covers all in-band channels incorporated within the combined optical signal fibre A and B. 
     Correct alignment of the mirrors or optical path may be determined by detecting maximum power of the injected light at the receiver  52  for example. 
     While the preferred embodiment has been described with respect to MEMS switches, other types of optical switches could also benefit from the invention, for example those using those using refraction, diffraction or reflection. The invention has application in telecommunications data networking switches for example. 
     For the purposes of this specification, the terms “optical” and “light” should be understood as pertaining not only to the visible part of the electromagnetic spectrum, but also to the infra-red and ultra-violet parts that bound the visible part. 
     While the invention has been described with respect optical fibres components, free space or optical waveguide components could also be used. 
     The light source  53  may comprise two or more broadband sources in order to cover a multiplexed signal having optical channels covering two or more discrete and separate ranges of wavelengths. This approach which avoids having a single broadband source covering all the optical channels as well as wavelengths in-between avoids the introduction of light in between groups of channels which may interfere with switching or other optical control equipment coupled to the switching apparatus. 
     FIG. 4 shows a switching apparatus according to the present invention which comprises an optical switch or switch core  2 , a number of port cards  51   a ,  51   b  . . . , a number of demultiplexers  55  and light sources  53 . The demultiplexer  55  demultiplexes optical signals from incoming external fibres A, C . . . , into a number of optical channels or groups of channels corresponding to fibres  3   a ,  3   b ,  3   c  . . . , which couple the demultiplexers  55  to the port cards  51   a ,  51   b ,  51   c  . . . The port cards couple optical channels to the core switch  2  which switches the incoming ports to outgoing ports, a selection of optical paths within the switch being shown in dashed outline by reference  30 . For example, the optical channel corresponding to fibre  3   a  from the demulitplexer  55  into port card  51   a , is switched to port card  51   c  and from there on into fibre  4   c  where it is multiplexed by multiplexer  56  into external fibre B. 
     Broadband lightsource  53  is introduced into the optical signal coming from external fibre A prior to the demultiplexer  55  such that the broadband lightsource is also demultiplexed, components or different wavelengths of this appearing in each of the demultiplexer outputs corresponding to fibres  3   a-f . In the example given, a detector  52  in port card  51   c  will detect light from light source  53  to indicate that the mirrors within switch  2  are properly aligned between the modules corresponding to port card  51   a  and wavelengths, the detectors  52  in the port cards detect either all the wavelength range of the broadband source  53  or are modified or tuned to detect just the wavelength corresponding to the optical channel they are expecting to receive. In the example given, the detector  52  of port card  51   c  may be arranged to detect light corresponding to the wavelengths of the optical channel associated with fibre  3   a  or any of the wavelengths associated with broadband source  53 . Preferably each broadband light source  53  associated with an incoming external fibre A, C . . . , has a different modulation scheme to distinguish it from other incoming optical signals such that in the switching example shown by optical path Z between port cards  1   f  and  1   h , the receiver or detector  52  of port card  1   h  is optimised to detect the modulation scheme associated with incoming fibre A. 
     The foregoing describes the invention including the preferred form thereof. Alterations and modifications as would be obvious to someone skilled in the art are intended to be incorporated within the scope hereof.