Abstract:
A reconfigurable multi-add/drop module for optical communications. The system includes a first network interface GRIN lens collimator connected on one end to an optical fiber of a communications network. An output of the GRIN lens is directed to a series of slidable two-section channel filters. Each filter is mechanically movable to a first position that passes all wavelengths. The second position of each filter reflects a particular wavelength to a corresponding add/drop GRIN lens collimator that receives the reflected light and outputs it into an add/drop fiber. The outputs from the add/drop collimators are directed to a single add/drop fiber through use of a power combiner. Light that passes through all of the filters is directed into a second network interface GRIN lens collimator for the purpose of coupling the light onto a second network optical fiber. A carrier wavelength can also be entered/added at the add/drop port.

Description:
This application claims benefit of provisional No. 60/144,909 filed Jul. 21, 1999. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to optical communications systems, and more particularly to a subsystem for routing wavelengths in an optical communications system through use of slidable two-section channel filters allowing direction of a selected wavelength without interrupting express channels. 
     2. Brief Description of the Prior Art 
     Optical communications networks are capable of handling large quantities of data due to their very broad bandwidth. This capability is enhanced through the simultaneous transmission of a plurality of carriers of different wavelengths. A technique known as wavelength division multiplexing is used to place the various carriers on a single optic network and separate the wavelengths at a node for re-routing. A system/module for use in receiving and/or transmitting a selected carrier wavelength at a node in an optical communications system is described in U.S. Pat. No. 5,712,932 by Alexander et al. This system uses circulators and a series of fixed tuned Bragg filters that are switched in or out of the network to receive (drop) or add (transmit) a particular carrier wavelength. A disadvantage of this is that the flow of express channels/wavelengths is disturbed during the switching moments, a problem that can cause a loss of data. A system that avoids the use of switches is described in U.S. Pat. No. 5,706,375 by Mihailov et al. wherein a specific wavelength is selected by tuning a Bragg filter in and out of a corresponding channel. A disadvantage of this system is the complexity and cost of the tuning mechanism. 
     In view of the prior art discussed above, it is apparent that a need exists for an improved optical add/drop module/system that is amenable to low cost and reliable construction. 
     SUMMARY 
     It is therefore an object of the present invention to provide an improved add/drop system for use in an optical communications network. 
     It is a further object of the present invention to provide an add/drop module that does not disturb the flow of express channels during an add/drop procedure. 
     It is another object of the present invention to provide an add/drop module that avoids the use of costly Bragg filters. 
     Briefly, a preferred embodiment of the present invention includes an add/drop module system for use in routing carrier wavelengths through an optical communications network. The system includes a first network interface GRIN lens collimator connected on one end to an optical fiber of a communications network. An output of the GRIN lens is directed to a series of slidable two-section channel filters. Each filter is mechanically movable to a first position that passes all wavelengths. Electrical relays slide the filters from the first position to a second position upon input of a directive signal. The second position of each filter reflects a particular wavelength to a corresponding add/drop GRIN lens collimator that receives the reflected light wavelength and outputs it into an add/drop optical fiber. The outputs from the add/drop collimators are directed to a single add/drop optical fiber through use of a power combiner. Light that passes through all of the filters is directed into a second network interface GRIN lens collimator for the purpose of coupling the light onto a second network optical fiber. Optical carriers arriving on the first network fiber can therefore either be dropped to the add/drop port or passed through for transmission on the second fiber. A carrier wavelength can also be entered added at the add/drop port. In order for an added wavelength to be added to the network, it must correspond to a wavelength of one of the filters, which must be positioned to reflect the signal. With the filter in this position, the added wavelength is reflected and passed to the first GRIN lens collimator which couples the signal onto the first network fiber. 
    
    
     IN THE DRAWING 
     FIG. 1 is a diagram illustrating a module of the present invention having a node receive/transmit port, an add/drop port, and an express channel port; 
     FIG. 2 shows the construction of the two-section filter of the present invention; 
     FIG. 3 a  illustrates an electromechanical device for positioning the two-section filter along a downward diagonal line relative to a wavelength beam; 
     FIG. 3 b  illustrates an electromechanical device for positioning a two-section filter by moving it horizontally, orthogonal to the beam direction; 
     FIG. 3 c  illustrates the use of an electromechanical device for positioning a two-section filter oriented with the filter sections in a horizontal line; 
     FIG. 4 is a schematic of a module with a separate add/drop line for each channel; 
     FIG. 5 is a schematic of a module that requires only one collimator in the add/drop section; 
     FIG. 6 illustrates a module providing bi-directional operation; 
     FIG. 7 illustrates a communications system with access devices; 
     FIG. 8 a  shows a method of connecting a bi-directional module to a network; and 
     FIG. 8 b  shows a method of connecting a module with a separate express channel port to a network. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A preferred embodiment of the present invention is illustrated in FIG. 1 showing a module  10  for use at a node in an optical communications system. The module  10  has a first port  12  for receiving and sending wavelengths from and to a communications network, and a second port  14  for connection to the communications network for passing express channels/wavelengths through the module  10 . An express channel is one that at a particular point in time is not to be directed by the module  10  to an add/drop port  16  for reception at the current node. The module  10  includes a first GRIN lens collimator  18  with an input  20  connected to a fiber  22  in communication with an optical communications network. The actual connection or connectors in the module are not shown, as well as other details that will be readily understood by those skilled in the art. The output  24  of the collimator  18  directs or receives a beam  26  to or from a series of two-section channel filters  28 - 34 . Although four filters are shown, the invention includes any number of filters. Each filter  28 - 34  is slidably set by a corresponding electromechanical apparatus  36 - 42 . The electromechanical construction details of the apparatus  36 - 42  for sliding filters  28 - 34  are not shown because such details will be readily apparent to those skilled in the art. A second GRIN lens collimator  44  is included to receive any wavelengths at input  46  that are not reflected by one of the filters  28 - 4 , i.e. any channels that are not to be received by the system node in which the module  10  is included. These wavelengths will be termed “express” channels. 
     Broadband mirrors  48  and  50  included in the embodiment of FIG. 1 are optional alternative apparatus for conveniently redirecting the beam  26  for a preferred location of components including the input/outputs  12 ,  14  and  16 . Add/drop GRIN lens collimators  52 - 58  are included to transform any wavelengths in the form of beams reflected by filters  28 - 34  from or to fiber cables  60 - 66 . An optical power divider represented by the junction  68  is included to direct any wavelengths on fibers  60 - 66  from filters  28 - 34  to pass through to fiber  16 , i.e. wavelengths to be dropped. In the other direction for adding wavelengths to a network, the power divider  68  provides for wavelength inputs on line  16  to be transferred through lines  60 - 66  and output as beams from collimators  52 - 58  to be reflected off of corresponding filters  28 - 34 . An added wavelength, upon reflection from a filter, travels into the end  24  of collimator  18  and then out to fiber  22 . For example, an incoming wavelength on fiber  22  may enter collimator  18 , be output as a beam  26 , reflected by mirror  48  and by filter  30 , designed and set to reflect the particular incoming wavelength. The beam then enters the collimator  54  and is passed on to fiber  62  and through combiner  68  to fiber  16 , whereupon it can be received by a receiver (not shown). 
     Similarly, a wavelength generated by a transmitter (not shown) may be input on line  16 , passed through combiner  68  to fiber  62 , through the collimator  54 , and output as a beam reflected from filter  30  and passed through filter  28 , reflected by mirror  48 , input to collimator  18 , and output on fiber  12  for transmission on a network (not shown). 
     The design of the novel two-section filter is illustrated in FIG. 2. A filter  70 , similar to filters  28 - 34 , has a channel selective reflecting section  72  and a transmissive section  74 . The filter  70  is constructed on a transparent substrate  76 . The transmissive section  74  preferably has an antireflective coating on each side  78 ,  80  of the transparent substrate to provide minimum transmission loss of all wavelengths passing through section  74 . The reflective section  72  has a wavelength-selective reflecting thin film layer  82  on one side, and an antireflective coating  84  on the opposite side. 
     The main reason for including the transmissive section  74  in the filter, rather than simply sliding a wavelength selective reflecting filter in and out of the beam path as required, is to eliminate the discontinuous interface between the reflective filter edge and air, which would cause a momentary disruption to a wavelength as the edge crossed the wavelength beam path. 
     With the electromechanical apparatus  36 - 42  all directed to slide the corresponding two section filters  28 - 34  so as to place the transmissive sections in the beam path, all the channels are passed into the express output  14 . Activating one of the apparatus  36 - 42  to place a reflective section  72  in the beam path causes the wavelength reflected by the selected filter to be reflected into the corresponding GRIN lens collimator  52 - 58 , and out through the power combiner  68  and add/drop port. Also, as explained above, a wavelength can be injected at port  16 , which is then directed out port  12 . In a similar manner, any number of the filters  28 - 34  can be activated at the same time to add or drop corresponding wavelengths from or to port  16 . 
     The movement of the filter by the electromechanical apparatus is more clearly illustrated in FIGS. 3 a ,  3   b , and  3   c . FIG. 3 a  illustrates an apparatus  86  for sliding a filter  88  in the direction of arrow  90  from a first position indicated by the solid outline for passing a beam  92  through a transparent filter section  94 . In this position, the reflective section  96  is removed from the beam  92  path. The dashed filter outline shows the filter  88  moved in the direction  90  to place the reflective section  96  in the beam path. 
     FIG. 3 b  shows an apparatus  98  for moving a filter  100  in direction  102 . The filter  100  has two sections  104  and  106 . The solid outline shows section  106  in the beam  108  path. The dashed outline  110  shows the filter  100  section  104  in the beam path. 
     FIG. 3 c  shows another arrangement of apparatus  112  for moving a filter  114  with sections  116  and  118  from a first position indicated by the solid lines with section  118  in the beam  120  path to a second position (dashed outline) putting section  116  in the beam  120  path. 
     FIG. 4 illustrates an alternate module embodiment wherein the wavelengths reflected by filters  28 - 34  are not served by the combiner  68  of FIG. 1, but instead each wavelength is added or dropped from or to a separate source or destination through fiber lines  122 - 128 . 
     FIG. 5 shows an alternate embodiment  130  that performs in a similar manner to the module described in reference to FIG.  1 . Instead of the four collimators  52 - 58  of FIG. 1, reflective elements  132 - 138  are used to direct the wavelengths to a single collimator  140 . The first element  132  reflects a wavelength reflected by filter  28  when the reflective section  72  (FIG. 2) is positioned to intersect the beam  142 . Element  132  can be either a mirror, reflection of any wavelength, or a reflective filter that only reflects the wavelength reflected by section  72  of filter  28 . Elements  134 - 138  are single channel reflective filters, each reflecting the same wavelength as its corresponding filter  30 - 34  and passing other wavelengths. For example, filters  32  and  136  must reflect the same wavelength. In further example, filters  136  and  138  must pass the wavelengths reflected by filters  28 ,  30 ,  132  and  134  to allow corresponding incoming wavelengths at port  12  to pass through to the collimator  140  to port  142 . Incoming wavelengths at port  12  that are not reflected by filters  28 - 34  are express channels and pass through to output collimator  144  and to port  146 . Wavelengths entering port  142  from a transmitter (not shown) that correspond to the wavelengths reflected by filters  28 - 34  and  132 - 138  can be directed out port  12  if the required one of filters  28 - 34  is positioned to reflect the beam. 
     An alternate module embodiment  148 , similar to the one shown in FIG. 1 except configured for bi-directional operation, is illustrated in FIG.  6 . The angled mirror  50  and collimator  44  of FIG. 1 have been replaced with a mirror  150  oriented to reflect the beam  152  containing express wavelengths, i.e. those not reflected by filters  28 - 34 , back along the same path through the filters  28 - 34 , and reflected off mirror  48  and through collimator  18  and out port  12 . 
     A typical optical communications network application of the modules described above is illustrated in FIG. 7. A ring architecture  154  includes fiber optic lines  156  interconnecting access devices  158 - 164 . In application of the present invention, each access device could include one or more of the modules described above. Arrows  166  and  168  indicate the capability of the access device to add (transmit) wavelengths and drop (receive) wavelengths to and from the network respectively. According to the above module description, the added and/or dropped wavelengths may be accomplished through a single fiber optic input, such as port  16  in FIGS. 1 and 6, or each wavelength can have a separate line, such as in FIG.  4 . Other combinations variations of the above modules will be apparent to those skilled in the art, and these are included in the spirit of the present invention. For example, a module could be configured with a combination of a number of add/drop channels combined through a combiner such as in FIG.  1  and in addition one or more channels having separate lines such as in FIG.  4 . 
     Various ways of connecting the modules of the present invention to a network will be apparent to those skilled in the art. FIG. 8 a  is an example showing connection of the module of FIG. 6 to the network fiber line  156  through use of an optical circulator  170 . FIG. 8 b  illustrates the use of two circulators  172 ,  174  for use in connecting a module such as module  10  of FIG. 1 to a network fiber  156 . 
     Although the present invention has been described above in terms of specific embodiments, it is anticipated that alterations and modifications thereof will no doubt become apparent to those skilled in the art. It is therefore intended that the following claims be interpreted as covering all such alterations and modifications as fall within the true spirit and scope of the invention.