Abstract:
Methods for utilizing optical systems in order to introduce digitally tunable amounts of temporal dispersion into optical signals and methods and systems for providing angular dispersion compensated output from optical switching/routing systems. The method for introducing controlled amounts of temporal dispersion into a signal includes the steps of (a) selectively directing an electromagnetic radiation beam to a predetermined optical path, and (b) subsequently selectively directing the electromagnetic radiation beam to another predetermined optical path. The angular dispersion compensated optical system includes a switching/routing optical system and a beam deflection element optically disposed on an output side of the switching/routing optical system. During operation of the angular dispersion compensated optical system, the beam deflection element selectively deflects an output electromagnetic radiation beam originating from the switching/routing optical system in order to render, after selective deflection, a direction of propagation of the electromagnetic radiation output beam parallel to the direction of propagation of an input beam of the switching/routing optical system.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       [0001]     This invention was made partially with U.S. Government support from the U.S. Air Force under Contract F30602-98-C-0079. The U.S. Government has certain rights in the invention. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     This invention relates generally to optical communications systems, and more particularly to optical switch technology.  
         [0003]     With the advent of substantial new uses for electro/optic systems, there exists a greater need to effectively control the multiplexing, demultiplexing and routing of signals that are Wavelength Division Multiplexed (WDM), and Dense Wavelength Division Multiplexed (DWDM), on optical fibers. Among the operations required in managing such WDM and DWDM systems are addition of signals into empty channels (Add) and selection of signals from populated channels (Drop), split, or route signals of different channels (interleaving and channelizing), and cross connecting from m channels to n channels (m×n cross-connect).  
         [0004]     U.S. Pat. No. 5,771,320 discloses a free space optical switching and routing system utilizing a switchable grating based approach together with novel noise suppression techniques. This family of devices provides for an optical switching and routing system that is useful for interconnecting any of an input array&#39;s optical channels to any of an output array&#39;s optical channels.  
         [0005]     In most typical optical communications systems, the optical channels are carried through the system mostly by means of optical fibers. The optical fibers provide, in most applications, the input channels for the optical switching and routing system and also, receive the output channels of the optical switching and routing system. There is a need for decreased fiber coupling losses.  
         [0006]     In optical fiber communications systems, it is sometimes desirable to introduce controllable amounts of temporal dispersion. There is a need for methods of introducing tunable amounts of temporal dispersion into optical signals.  
         [0007]     There is also a need for reduced crosstalk between channels in optical switching and routing Systems.  
         [0008]     It is therefore an object of this invention to provide methods for utilizing optical systems in order to introduce digitally tunable amounts of temporal dispersion into optical signals.  
         [0009]     It is a further object of this invention to provide methods and systems for providing angular dispersion compensated output which lead to decreased fiber-coupling losses.  
       BRIEF SUMMARY OF THE INVENTION  
       [0010]     The objects set forth above as well as further and other objects and advantages of the present invention are achieved by the embodiments of the invention described hereinbelow.  
         [0011]     The present invention provides methods for utilizing optical systems in order to introduce digitally tunable amounts of temporal dispersion into optical signals and methods and systems for providing angular dispersion compensated output from optical switching/routing systems. The method of this invention for introducing controlled amounts of temporal dispersion into a signal includes the steps of (a) selectively directing an electromagnetic radiation beam to a predetermined optical path, and (b) subsequently selectively directing the electromagnetic radiation beam to another predetermined optical path. The method can also include the step of redirecting the subsequently selectively directed electromagnetic radiation beam to a predetermined direction. Redirecting the subsequently selectively directed electromagnetic radiation beam to a predetermined direction can result in a longer propagation distance and a larger amount of temporal dispersion.  
         [0012]     The angular dispersion compensated optical system or sub-system of this invention includes a switching/routing optical sub-system and a beam deflection element optically disposed on the multiple beam output side of the switching/routing optical sub-system. During operation of the angular dispersion compensated optical system or sub-system, the beam deflection element selectively deflects output electromagnetic radiation beams originating from the switching/routing optical sub-system in order to render, after selective deflection, a direction of propagation of the electromagnetic radiation output beams parallel to the direction of propagation of an input beam of the switching/routing optical system.  
         [0013]     Utilization of a pixellated switched grating as the beam deflection element enables decreasing the detected crosstalk from all the non-selected beam paths.  
         [0014]     For a better understanding of the present invention, together with other and further objects thereof, reference is made to the accompanying drawings and detailed description and its scope will be pointed out in the appended claims. 
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING  
       [0015]      FIG. 1   a  is a graphical schematic representation of a prior art grating based switching/routing optical system;  
         [0016]      FIG. 1   b  is a graphical schematic representation of a section of the prior art grating based switching/routing optical system of  FIG. 4   a;    
         [0017]      FIG. 2  is a graphical schematic representation of another prior art grating based switching/routing optical system;  
         [0018]      FIGS. 3   a  and  3   b  are graphical schematic representation of embodiments of a grating based switching/routing optical system utilized for introducing controlled amounts of temporal dispersion;  
         [0019]      FIG. 4  is a graphical schematic representation of an embodiment of an angular dispersion compensated grating based switching/routing optical system;  
         [0020]      FIG. 5  is a graphical schematic representation of another embodiment of an angular dispersion compensated grating based switching/routing optical system;  
         [0021]      FIG. 6  is a graphical schematic representation of an embodiment of an angular dispersion compensated grating based switching/routing optical system utilizing a compensating switched steering grating;  
         [0022]      FIG. 7  is a graphical schematic representation of an embodiment of an angular dispersion compensated grating based switching/routing optical system utilizing a compensating pixellated switched steering grating; and  
         [0023]      FIG. 8  is a graphical schematic representation of another embodiment of an angular dispersion compensated grating based switching/routing optical system utilizing a compensating pixellated switched steering grating. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0024]     In order to better understand the present invention described below, it should be noted that certain terms used in the description of the invention have interchangeable usage. For example, the term “optical” refers not only to optical components, but also to electro-optical components and the terms “optical beam” and “electromagnetic radiation beam” may be used interchangeably.  
         [0025]     Furthermore, terms such as “beam paths” and “channels” may also be interchanged, in certain instances, based upon their usage as recognized in the art.  
         [0026]     In addition, identical components may be referred to with identical reference numerals within the specification and drawings for simplifying an understanding of the various components of this invention.  
         [0027]     Methods for utilizing optical systems in order to introduce digitally tunable amounts of temporal dispersion into optical signals and methods and systems for providing angular dispersion compensated output are disclosed herein below.  
         [0028]      FIGS. 1   a  and  1   b  illustrate a prior art grating based switching/routing system  10  including four switchable gratings  412 ,  414 ,  416 ,  418 . The operation of the switching/routing system  10  is described in U.S. Pat. No. 5,982,515, hereby incorporated by reference. Due to the diffractive nature of the grating based switching/routing system, it suffers from a lateral chromatic shift that varies with each output channel.  FIG. 1   b  depicts the diffraction of an optical beam  428  by a switchable grating  418  and illustrates the lateral chromatic shift. The origin of the temporal dispersion increments is due to the variation in optical path length with wavelength, which can result from propagation in an angularly dispersed state and can also result from propagation with the lateral chromatic shift shown in  FIG. 1   b . For each switched path, there are differing amounts of distance propagated while the spectral components of the input are angularly separated. As a result, the longer wavelength spectral components of a given input beam are delayed with respect to the shorter wavelength components of that input.  
         [0029]     In one embodiment, shown in  FIG. 2 , a cascade  30  of 2n switched gratings  32 ,  34 ,  36 ,  38 ,  42 ,  44 ,  46 ,  48  are used to introduce one of  2   n  equal increments of temporal dispersion on an input optical signal  40 . These increments of temporal dispersion are also accompanied by time shifts. Prior art transmission-grating-based optical time delay systems, such as the system described in U.S. Pat. No. 5,982,515, can be used to provide tunable temporal dispersion.  
         [0030]     Referring again to  FIG. 2 , when all the switched gratings  32 ,  34 ,  36 ,  38 ,  42 ,  44 ,  46 ,  48  are transparent, an input optical signal (“electromagnetic radiation beam”)  40  is transmitted with a baseline amount of temporal dispersion due to propagation through the system  30 . (The baseline amount is substantially smaller than the grating induced temporal dispersion.) As the gratings  32 ,  34 ,  36 ,  38 ,  42 ,  44 ,  46 ,  48  are switched providing for varying displacements down a central plane  41 , increasing quantities of dispersive delay between the short and long wavelength components of the optical signal  40  are introduced. Switching each of the gratings  32 ,  34 ,  36 ,  38 ,  42 ,  44 ,  46 ,  48  directs the input optical signal  40  to a predetermined optical path. When the beam propagates in the diffracted direction other than the incident direction of the input optical signal  40 , increments of temporal dispersion are introduced into the optical signal. When the optical beam  40  propagates substantially parallel to the direction of the input beam due to experiencing equal amounts of clockwise and counterclockwise deviation (or when the optical beam  40  experiences substantially equal amounts of clockwise and counterclockwise deviation), there is no angular dispersion and, therefore, no grating induced temporal dispersion accumulates. (Herinafter, rendering the direction of propagation of the optical beam substantially parallel to the direction of the input beam refers to rendering the direction of propagation of the optical beam substantially parallel to the direction of the input beam due to experiencing equal amounts of clockwise and counterclockwise deviation.)  
         [0031]     In the embodiment shown in  FIG. 2 , the subsequent gratings are switched until a direction of propagation of the output electromagnetic radiation beam  50  is the same as a direction of propagation of the input signal  40 .  
         [0032]     Larger amounts of temporal dispersion may be introduced in other embodiments, such as that shown in  FIGS. 3   a  and  3   b , where the diffraction angles are increased. Referring to  FIG. 3   a , the system  60  includes gratings  62 ,  64 ,  66 ,  78 ,  82 ,  84  that are capable of directing an input optical signal  70  to a path at a larger diffraction angle than that shown in  FIG. 1 . Also referring to  FIG. 3   a , the system  60  includes redirecting elements  68 ,  72 ,  74 ,  76  (gratings or mirror arrays). The redirecting elements  68 ,  72 ,  74 ,  76  direct the optical signal  70  (“electromagnetic radiation beam”) to a predetermined direction and allow a longer propagation distance and a larger amount of temporal dispersion. In the embodiment shown in  FIG. 3   a , the redirecting elements  68 ,  72 ,  74 ,  76  and the switching of subsequent gratings  78 ,  82 ,  84  result in a direction of propagation of the output beam  80  that is parallel but opposite (anti-parallel) to a direction of propagation of the input signal  70 .  
         [0033]     Referring to  FIG. 3   b , the system  90  includes gratings  88 ,  92 ,  94 ,  105 ,  106 ,  108  that are capable of directing an input optical signal  70  to a path at a larger diffraction angle than that shown in  FIG. 1 . Also referring to  FIG. 3   b , the system  90  includes redirecting elements  96 ,  98 ,  102 ,  104  (gratings or mirror arrays). Redirecting elements  96  and  104  can be, in one embodiment, pixellated or switchable redirecting gratings. Redirecting elements  98  and  102  can be, in one embodiment, fixed gratings or mirrors. The redirecting elements  96 ,  98 ,  102 ,  104  direct the optical signal  70  (“electromagnetic radiation beam”) to a predetermined direction and allow a longer propagation distance and a larger amount of temporal dispersion than in the configuration of  FIG. 2 . In the embodiment shown in  FIG. 3   b , the redirecting elements  96 ,  98 ,  102 ,  104  and the switching of subsequent gratings  105 ,  106 ,  108  result in a direction of propagation of the output beam  109  that is parallel to a direction of propagation of the input signal  70 .  
         [0034]     Both the embodiments of  FIGS. 2, 3   a  and  3   b  include a cross talk suppressing element  39  and  75 , such as that described in U.S. Pat. No. 5,982,515. In the embodiments of  FIG. 2 , the cross talk suppressing element  39  is located at central plane  41 . In the embodiments of  FIG. 3   a , the cross talk suppressing element  75  between the switchable grating cascades as shown. In this location all the possible optical paths may be spatially separated and, therefore, deviating elements (such as, but not limited to, spatial light modulators (SLMs) and pixilated switchable gratings) can be used to selectively increase loss in the non-selected channels, thereby reducing crosstalk.  
         [0035]     In the embodiment shown in  FIG. 2  the direction of propagation of the output beam  50  can be rendered parallel to the direction of propagation of the input signal  40 . In an uncompensated system, such as the embodiment of the switching/routing sub-system  120  shown in  FIG. 4 , half of the selectable beam paths (channels)  112 ,  116  exit the last switched grating parallel to the input beam  110  (heading upward) and half the selectable beam paths (channels)  114 ,  118  exit the last switched grating parallel to the other diffracted order  115  (heading downward). For efficient coupling of the output to optical fibers, it is advantageous to choose the common output direction parallel to the input (or equivalently, ensure that each path has an equal number of clockwise and counterclockwise diffractions of equal angle) so that the angular dispersion will be substantially eliminated.  
         [0036]     In the embodiment of an angular dispersion compensated optical switching/routing system  100  shown in  FIG. 4 , a beam deflection element  126  (a pixellated non-switchable steering grating in the embodiment of  FIG. 4 ) is used to provide the compensating counterclockwise diffraction to the channels  114 ,  118  that have an unbalanced clockwise diffraction. One channel  112  experiences no diffractions and is thus not angularly dispersed. Another channel  116  experiences symmetric clockwise and counterclockwise diffraction in the switching/routing sub-system  120 , is not angularly dispersed and, therefore, does not require diffraction by the steering grating  126 . The pixellated non-switchable steering grating  126  is patterned to selectively deflect the channels  114 ,  118  that have experienced an unbalanced clockwise diffraction from the optical switching and routing sub-system  120 . The pixellated non-switchable steering grating  126  imparts a compensating clockwise diffraction on channels  114  and  118 . Non-Diffracting (transparent, also referred to as clear) pixels in the pixellated steering grating  126  are used to transmit beams  112  and  116  without diffraction.  
         [0037]     In another embodiment  130  of an angular dispersion compensated system of this invention, shown in  FIG. 5 , the input electromagnetic radiation (“optical”) beam  132  is normally (perpendicularly) incident on the input face of the optical switching and routing system  120 . The embodiment  130  of the angular dispersion compensated system of this invention includes a beam steering deflection element  140  (a steering grating in the embodiment shown in  FIG. 5 ). The steering grating  140  has half the frequency of the switched gratings  122 ,  124 . In order to eliminate angular dispersion in the output channels  146 ,  148 ,  152 ,  154 , it is necessary to assure that the beam  132  is subjected to an equal number of clockwise and counter clockwise diffractive deviations, i.e., that the output is diffracted parallel to the input. Referring to  FIG. 5 , a non-switchable pixellated grating  150  is used to provide the compensating diffraction to the channels  146 ,  148 ,  152 ,  154 . It is desirable to use volume holographic gratings for high efficiency gratings, and for those elements the pixellation is necessary to provide for the proper slant angle for the modulation fringes (to satisfy the Bragg condition). Alternatively, blazed surface relief gratings and other types of gratings can be used.  
         [0038]     The pixellated non-switchable grating  126  shown in  FIG. 4  can be replaced by a single non-pixellated switched grating  155 , as shown in  FIG. 6 , for the case of a single desired output location at a time. Referring to  FIG. 6 , in one embodiment, the entire switchable steering grating is switched “off” (non-diffracting) for the case when the channels  112 ,  116  that have no diffraction or a balanced diffraction being selected; and the entire steering grating is switched to “on” when the channels  114 ,  118  that have an unbalanced diffraction are selected to provide compensating diffraction. An added advantage of the embodiment shown in  FIG. 6  is a decrease in the crosstalk in roughly half the channels.  
         [0039]     The switched grating  155  in  FIG. 6  can be replaced with a pixellated switched grating  160 , as shown in  FIG. 7 , in which case it can also be used to decrease the crosstalk in all the non-selected channels. The grating pixels are set to steer the desired channel as shown, and to steer crosstalk signals present in the non-selected channels out of those channels and hence out of the ensuing fiber couplers (not shown). For example, if the second channel  114  is desired, pixel  166  will be set in the deflecting state. Accordingly, crosstalk will be reduced if the top  168  (first) and third  164  switched grating pixels are set diffractive to deflect the crosstalk present in those exit beams out of the output channels; and, the bottom  162  (fourth) pixel would be set non-diffractive, in order to deflect crosstalk present in the bottom channel  162  away from the fiber coupler (not shown) for that channel. The switching of selected grating pixels enables the selective deflection of crosstalk signals out of the system, reducing the detected crosstalk.  
         [0040]     Similarly, the switched grating  150  in  FIG. 5  can be replaced with a pixellated switchable grating  170 , as shown in  FIG. 8 , in which case it can also be used to decrease the crosstalk in all the non-selected channels. The grating pixels are set to steer the desired channel as shown, and to steer all the non-selected channels away from the path shown and hence out of the output channels. Referring to  FIG. 8 , the pixels in the pixellated switchable grating  170  that correspond to selected channels diffract the beam  132  while the pixels in the pixellated switchable grating  170  that correspond to the non-selected channels are set to (i.e. switched “off”) not diffract the beam  132 .  
         [0041]     One embodiment of the switchable diffraction grating utilized in this invention is the switchable volume diffraction element (grating) described in U.S. Pat. No. 5,771,320, herein incorporated by reference. The embodiments of the optical switching and routing systems described in U.S. Pat. No. 5,771,320 utilize volume phase diffraction (holographic) gratings that permit switching of the incident energy between two or more orders. The primary mechanisms considered which permit this diffracted-order switching are electrical switching, optical switching, and polarization switching. The switched gratings can be optically switched, electrically switched, polarization switched, or switched based on other mechanisms. In the electrical switching embodiment, a voltage across the switched grating component switches the grating among diffracting and transparent states. Switching controls (such as logic and a voltage source) effect the transition between diffracting and transparent states (for example, controls C 1 , C 2 , C 3  and C 4  in  FIG. 2 ). It should be noted that although switching controls are not shown, or separately labeled, in  FIGS. 3 through 8 , switching controls are present and needed in all switchable or switchable pixellated gratings.  
         [0042]     It should be noted that if the order of grating plane spacings increases rather than decreases (the latter is shown in the  FIGS. 4, 5 ,  6 ,  7 ,  8 ), the same principles hold but the pitch of the pixellation is altered, i.e., typically fewer and larger semi-field pixels are used in place of rapidly alternating small channel-to-channel pixels.  
         [0043]     It should also be noted that, although the above described embodiments refer to two dimensional switching/routing systems, three dimensional systems, such as crossconnect switch configurations, are also within the scope of this invention.  
         [0044]     Although the invention has been described with respect to various embodiments, it should be realized this invention is also capable of a wide variety of further and other embodiments within the spirit and scope of the appended claims.