Patent Publication Number: US-2004057656-A1

Title: Double Risley prism pairs for optical beam steering and alignment

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
     [0001] This application claims the benefit of U.S. Provisional Application No. 60/413,282, filed Sep. 24, 2002. 
    
    
     
       BACKGROUND OF THE INVENTION  
       [0002] The present invention generally relates to fiber optic communications and, more particularly, to using double Risley prism pairs for optical beam alignment in a 3-dimensional, all-optical, fiber optical switch and also in free space optical communication.  
       [0003] Fiber optical switches find wide application in communications. Fiber optical switches are increasingly used in the telecommunications industry, where fiber optical switches may be used, for example, in a central office core router of a telecommunications network as cross-connect switches for metro and long haul services.  
       [0004]FIG. 1A shows an optical communication system hierarchy  100  according to the prior art, including long haul and metro telecommunications switching networks, for example, long haul switching network  102  and metro telecommunications switching networks  104  and  106 . Optical communication system hierarchy  100  may include nodes—such as nodes  108 —that communicate using optical communication links—such as links  110 —between the nodes, typically connected in loops. Optical fibers may be used in the links—such as links  110 —as working, protection, add, or drop links, as known in the art, for transmitting signal light beams between nodes—such as nodes  108 . A node may be, for example, a telephone exchange, such as public switched telephone network (PSTN)  112  or cellular network  114 , shown in FIG. 1A connected, for example, by a synchronous optical network (SONET) network  116 . As seen in FIG. 1A, for example, metro telecommunications switching network  104  may be connected via one or more optical links—such as optical links  117 —to residential extended digital subscriber line (x-DSL) network  118 . Also as seen in FIG. 1A, for example, metro telecommunications switching network  106  may be connected via one or more optical links—such as optical links  119 —to internet protocol (IP) router  120 , connecting asynchronous transfer mode (ATM) switch  122  and Ethernet local area network (LAN)  124  for a regional internet service provider (ISP). Also as seen in FIG. 1A, for example, metro telecommunications switching network  106  may be connected via one or more optical links—such as optical links  125 —to a corporate enterprise systems connection (ESCON) network  126 , which may comprise a frame relay ESCON fiber channel network or gigabit Ethernet, as known in the art. Each of PSTN  112 , cellular network  114 , SONET network  116 , residential x-DSL network  118 , IP router  120 , ATM switch  122 , Ethernet LAN  124 , and ESCON network  126  may connected through an optical cross connect switch to a switching network—such as metro telecommunications switching networks  104  and  106 .  
       [0005] Referring now to FIG. 1B, an example of a long haul switching network  130  is illustrated. Long haul switching network  130  may correspond, for example, to long haul switching network  102 , shown in optical communication system hierarchy  100  of FIG. 1A. Long haul switching network  130  may include nodes—such as nodes  108 —that communicate using optical fiber links—such as links  110 —between the nodes. Links—such as links  110 —typically connect the nodes—such as nodes  108 —in loops. For example, nodes  132 ,  134 , and  136  are shown in FIG. 1B connected in a loop by links  131 ,  133 , and  135 . Link  131  connects node  136  with node  132 ; link  133  connects node  132  with node  134 , and link  135  (shown as a broken link) would ordinarily connect node  134  with node  136 . Links—such as links  131 ,  133 , and  135 —may comprise multiple optical fibers that may be used as working, protection, add, or drop fibers, in any combination, as known in the art, for transmitting signal light beams between nodes—such as nodes  132 ,  134 , and  136 . For example, communication between node  136  and node  134  would ordinarily be transmitted over working fibers of link  135 . If link  135  should become disabled, illustrated in FIG. 1B by a break in link  135 , communication can be rerouted for example, over links  131  and  133  between node  136  and node  134  via node  132 , using protection fibers included in links  131  and  133 . Such rerouting can be accomplished, as known in the art, by means of optical cross connect switches or protection switches, which may be optical cross connect switches configured to perform such rerouting.  
       [0006] Referring now to FIG. 1C, examples of several types of connections to a metro telecommunications switching network  140  is illustrated. Metro telecommunications switching network  140  may correspond, for example, to metro telecommunications switching network  104  or metro telecommunications switching network  106 , shown in optical communication system hierarchy  100  of FIG. 1A. Metro telecommunications switching network  140  may include nodes—such as nodes  142 ,  144 ,  146 , and  148 —connected in a loop by links  141 ,  143 ,  145 , and  147 , where link  141  connects node  148  with node  142 ; link  143  connects node  142  with node  144 , and so forth, as shown in FIG. 1C. Links—such as links  141 ,  143 ,  145 , and  147 —may comprise multiple optical fibers that may be used as working, protection, add, or drop fibers, in any combination, as known in the art, for transmitting signal light beams between nodes—such as nodes  142 ,  144 , 146 , and  148 . Each of nodes  142 ,  144 ,  146 , and  148 —as well as nodes  108 , shown in FIGS. 1A, 1B, and  1 C, may comprise one or more optical cross connect switches. Each cross-connect switch may be configured to act as a non-blocking cross-connect switch, protection switch, add/drop module, or mux/demux, as known in the art.  
       [0007] Individual clients, are typically connected into a metro telecommunications switching network—such as metro telecommunications switching network  140 —using an add/drop module. For example, add/drop module  150  may be used, as known in the art and shown in FIG. 1C, to connect LAN  152 , ATM switch  154 , and access router  156  to node  142  of metro telecommunications switching network  140 . Also, for example, mux/demux  158  may be used, as known in the art and shown in FIG. 1C, to connect SONET add/drop multiplexer (ADM)  160 , ESCON node  162 , and enterprise frame relay router  164  to node  109  of metro telecommunications switching network  140 . Also, for example, SONET distributed communication system (DCS)  166  may be connected, as known in the art and shown in FIG. 1C, to node  144  of metro telecommunications switching network  140 . Each node—such as node  144 —of metro telecommunications switching network  140  may appropriately route the signals connected to the node using optical cross-connect switches included in the node and configured—for example, as non-blocking cross-connect switch, protection switch, add/drop module, or mux/demux—to perform the appropriate function. Thus, the cross-connect switch has come to be a fundamental component of telecommunication systems.  
       [0008] An optical cross-connect switch may allow light to be routed between optical fibers in such a way that any optical fiber from one side of the switch can be optically connected to any of the optical fibers on another side of the switch. Metro and long haul services may be provided using dense wavelength division multiplexing (WDM or DWDM). DWDM is a technology that uses multiple lasers and transmits several wavelengths of light simultaneously over a single optical fiber. Each signal travels within its unique color band, which is modulated by the data (text, voice, video, for example). DWDM enables the existing fiber infrastructure of the telephone companies and other carriers to be dramatically increased. DWDM systems exist that can support more than 150 wavelengths. Such systems can provide more than 1,000 giga-bits per second (Gbps or billion bits per second) of data transmission on one optical fiber. Several key components in optical communications networks—including optical add/drop modules (OADM), protection switches, and cross-connect switches—may be implemented using optical switches  
       [0009] Conventional fiber optical switches that connect optical fiber lines are electro-optical. Such conventional switches convert photons from the input side to electrons internally in order to do the signal switching electronically and then convert back to photons on the output side, thus being referred to as optical-electrical-optical (OEO) switches. By way of contrast, an all-optical fiber optical switch, referred to as optical-optical-optical (OOO), is a switching device that maintains the signal as light from input to output. Although some vendors call electro-optical switches “optical switches,” true optical switches, i.e., all-optical switches, support all transmission speeds. Unlike electronic switches, which are tied to specific data rates and protocols, all-optical, or OOO, switches direct the incoming data bit stream to the output port no matter what the line speed or protocol (such as IP, ATM, or SONET) and do not have to be upgraded for any changes to the protocol.  
       [0010] An optical switch is a device that can be used to switch a small and collimated beam of light in free space by either leaving the light path to pass through a location unaffected or changing the light path to a different direction at the location. The switching can be done mechanically, for example, by moving a mirror between two distinct and stable positions—in the path of the light, and out of the path of the light. Switching by changing a light path between two distinct and stable positions may be referred to as digital switching. Digital switching is usually implemented by a switch in which the ends of all of the optical fibers connected to the switch are in the same plane, referred to as being 2-dimensional.  
       [0011] For example, a 2-dimensional optical cross-connect switch can be implemented with a planar array of mirrors that can be moved into and out of the path of the light for switching light beams between optical fibers. Switching can also be done mechanically, for example, by moving an optical element, such as a lens, prism, or mirror, continuously from one position to another in order to redirect a light path from one destination to another, which may be referred to as analog switching. Because the optical element is continuously adjustable in analog switching, the geometrical configuration in which optical fibers are connected to the switch is less constrained. For example, the ends of all of the optical fibers connected to the analog switch need not be in the same plane, so that the analog switch may be referred to as being 3-dimensional.  
       [0012] Recently, attempts have been made to use Risley prism pairs for optical switching. Risley prisms are known from their application to other technologies, as disclosed, for example, in U.S. Pat. No. 6,343,767, issued Feb. 5, 2002 to Sparrold, et al., where the use of Risley prism pairs in a missile seeker is disclosed. U.S. Pat. No. 2001/0,046,345 A1, published Nov. 29, 2001 by Snyder et al. discloses the use of Risley prism pairs for use in 3-dimensional analog optical switches. As disclosed in Snyder et al., optical switching uses one Risley prism pair for beam steering from an input fiber to one of many output fibers. For multiple input, multiple output optical switches, which may be referred to as M×N switches, where M is the number of input fibers and N is the number of output fibers, the beams must not only be steered to “shoot at” a certain location where the output fiber is located but must also be aligned to the output fiber&#39;s incident angle.  
       [0013] When using only one Risley prism pair for the beam steering, the output fiber may be mechanically oriented, for example, to an angle optically aligned to the steered incoming beam. Collimating and focusing lenses may also be used, for example, in the light path of the steered beam to perform the beam alignment not provided by the single Risley prism pair. In multiple input, multiple output switches, however, as the number of output fibers increases, the optical angle of the Risley prism pair, i.e., the range of different angles through which the Risley prism pair can steer a beam of light, must be increased. The increasing optical angle, however, works against the requirement for alignment to the output fiber&#39;s incident angle. In other words, an output fiber adequately aligned to receive light from one Risley prism pair may not be able to receive light from a different Risley prism pair in the switch, thus limiting the number of inputs and outputs that are practical for a given Risley prism pair optical angle. Snyder et al. disclose a Risley prism pair optical angle of a typical value of 1.1 degree of arc. Therefore, the prior art beam steering suffers from serious drawbacks that make it only suitable for 1×N switching rather than the M×N switching needed for cross-connect switch applications.  
       [0014] Furthermore, there are recent developments in free space optics, where optical communication is between two locations, such as buildings, with line of sight being accomplished by laser beam steering using tilting mirrors or mechanical gimbals mechanisms. FIG. 1D shows an example of a free space optical communication system  170 . Optical communication system  170  may comprise, for example, a laser communication (lasercom) terminal  172  (a first node in the above terminology) that communicates over optical communication link  173 , i.e., using a communication light beam, with a second lasercom terminal  174  (a second node). Although, the example illustrates optical communication system  170  used to communicate between two buildings  175  and  177 , lasercom terminals  172  and  174  of optical communication system  170  may comprise a satellite lasercom terminal  176 , or a ground lasercom terminal  178 , as also shown in FIG. 1D, for communication between the earth and a satellite in orbit, for example. For optical beam steering in free space optical communication, a tilting mirror or gimbals mechanism approach is typically used to steer the communication light beams—such as the communication light beam comprising link  173 . Risley prism pairs can also be employed for the same beam steering function achieved presently by tilting mirrors. The optical alignment is less sensitive to mechanical angle variations of the Risley prisms than the tilting mirrors. The same problems of optical beam alignment alluded to above, however, still arise, i.e., the beams must not only be steered to “shoot at” a certain location where an output fiber of a lasercom terminal is located but must also be aligned to the output fiber&#39;s incident angle.  
       [0015] As can be seen, there is a need for optical beam steering and alignment in optical communication systems—such as optical switching networks and free space optical communication systems—that quickly and reliably achieve accurate optical beam alignment. Furthermore, there is a need for optical beam steering and alignment in an analog optical switch that achieves accurate alignment regardless of the number of input and output fibers. Also, there is a need in optical communication systems for optical beam steering and alignment that achieves a larger Risley prism pair optical angle.  
       SUMMARY OF THE INVENTION  
       [0016] In one aspect of the present invention, an optical communication apparatus comprises an input optical fiber that transmits a first light beam; an output optical fiber that transmits a second light beam; a first Risley prism pair disposed in an optical path between the input optical fiber and the output optical fiber; and a second Risley prism pair disposed in the optical path between the first Risley prism pair and the output optical fiber. The first Risley prism pair, which is located on the input optical fiber side, steers the light beam from the input optical fiber and makes it incident on the second Risley prism pair, which is located on the output optical fiber side. The second Risley prism pair steers the incident light beam angle to align it with the incident angle of the output optical fiber, thereby forming an aligned optical path between the input optical fiber and the output optical fiber. The optical path between the two optical fibers remains the same when the input and output are reversed.  
       [0017] In another aspect of the present invention, an optical communication apparatus comprises an input optical fiber that transmits a first light beam; an output optical fiber that transmits a second light beam; a first Risley prism pair disposed in an optical path between the input optical fiber and the output optical fiber; and a second Risley prism pair disposed in the optical path between the first Risley prism pair and the output optical fiber. A first actuator for the first Risley prism pair adjusts a first deflection angle of the first Risley prism pair so that the first Risley prism pair steers the first light beam to be incident on a second outside prism face of the second Risley prism pair. A second actuator for the second Risley prism pair adjusts a second deflection angle of the second Risley prism pair so that second Risley prism pair aligns the second light beam to be incident on a first outside prism face of the first Risley prism pair, thereby forming an aligned optical path between the input optical fiber and the output optical fiber.  
       [0018] In still another aspect of the present invention, a 3-dimensional optical cross-connect switch comprises an input array of input optical fibers including at least one input optical fiber that transmits a first light beam; an output array of output optical fibers including at least one output optical fiber that transmits a second light beam; and a plurality of Risley prism pairs. Each input optical fiber of the input array has a distinct corresponding Risley prism pair in the plurality of Risley prism pairs, and each output optical fiber of the output array has a distinct corresponding Risley prism pair in the plurality of Risley prism pairs. A distinct first Risley prism pair of the plurality of Risley prism pairs, corresponding to the at least one input optical fiber, is disposed in an optical path between the at least one input optical fiber and the at least one output optical fiber. A distinct second Risley prism pair of the plurality of Risley prism pairs, corresponding to the at least one output optical fiber, is disposed in the optical path between the first Risley prism pair and the at least one output optical fiber. A first actuator for the first Risley prism pair adjusts a first deflection angle of the first Risley prism pair. A second actuator for the second Risley prism pair adjusts a second deflection angle of the second Risley prism pair. A controller receives first feedback signals from the first actuator, where the first actuator includes first position sensors that sense the prism angular positions of the first Risley prism pair and provides the first feedback signals to the controller. The controller provides first control signals to the first actuator that adjusts the first deflection angle so that the first Risley prism pair steers the first light beam to be incident on a second outside prism face of the second Risley prism pair. The controller receives second feedback signals from the second actuator, where the second actuator includes second position sensors that sense prism angular positions of the second Risley prism pair and provides the second feedback signals to the controller. Also, the controller provides second control signals to the second actuator that adjusts the second deflection angle so that the second Risley prism pair aligns the second light beam to be incident on a first outside prism face of the first Risley prism pair, thereby forming an aligned optical path between the input optical fiber and the output optical fiber. A signal light beam is transmitted over the aligned optical path between the at least one input optical fiber and the at least one output optical fiber.  
       [0019] In yet another aspect of the present invention, an optical communication system comprises a plurality of nodes with at least one of the plurality of nodes including an optical switch; and a plurality of links with each link of the plurality of links including at least one optical fiber. Each of the plurality of links optically connects two of the plurality of nodes, at least one of the plurality of links includes an input optical fiber connected to the optical switch and transmitting a first light beam, and at least one of the plurality of links includes an output optical fiber connected to the optical switch and transmitting a second light beam. The optical switch comprises a first Risley prism pair disposed in an optical path between the input optical fiber and the output optical fiber; and a second Risley prism pair disposed in the optical path between the first Risley prism pair and the output optical fiber. A first actuator for the first Risley prism pair adjusts a first deflection angle of the first Risley prism pair so that the first Risley prism pair steers the first light beam to be incident on a second outside prism face of the second Risley prism pair. A second actuator for the second Risley prism pair adjusts a second deflection angle of the second Risley prism pair so that second Risley prism pair aligns the second light beam to be incident on a first outside prism face of the first Risley prism pair, thereby forming an aligned optical path between the input optical fiber and the output optical fiber. A signal light beam is transmitted over the aligned optical path between the input optical fiber and the output optical fiber.  
       [0020] In even another aspect of the present invention, an optical communication system comprises a plurality of nodes and at least one link between two nodes in the plurality of nodes. A first node comprises an input optical fiber transmitting a first light beam; a first Risley prism pair disposed in an optical path between the input optical fiber and the link; and a first actuator for the first Risley prism pair, which adjusts a first deflection angle of the first Risley prism pair. A second node comprises an output optical fiber transmitting a second light beam; a second Risley prism pair disposed in the optical path between the first Risley prism pair and the output optical fiber; and a second actuator for the second Risley prism pair. The first actuator adjusts the first deflection angle so that the first Risley prism pair steers the first light beam to be incident on a second outside prism face of the second Risley prism pair. The second actuator adjusts a second deflection angle of the second Risley prism pair so that the second Risley prism pair aligns the second light beam to be incident on a first outside prism face of the first Risley prism pair, thereby forming an aligned optical path between the input optical fiber and the output optical fiber. A signal light beam is transmitted over the aligned optical path so that the first node optically communicates over the link with the second node.  
       [0021] In an additional aspect of the present invention, an actuator for a Risley prism pair comprises an optical path; a first drive motor; and a second drive motor. The first drive motor is connected to a first Risley prism so that a first drive motor rotation is transmitted to a first rotation of the first Risley prism about a central axis without blocking the optical path. The second drive motor is connected to a second Risley prism so that a second drive motor rotation is transmitted to a second rotation of the second Risley prism about the central axis without blocking the optical path. The second rotation and the first rotation are independent. An outside diameter of the actuator with respect to the central axis is less than 2.5 cm.  
       [0022] In a further aspect of the present invention, a method for optical beam alignment comprises steps of: inserting a first light beam into a first Risley prism pair; inserting a second light beam into a second Risley prism pair; adjusting the first Risley prism pair to steer the first light beam to be incident on a second outside prism face of the second Risley prism pair; and forming an aligned optical path by adjusting the second Risley prism pair to deflect the second light beam to be incident on a first outside prism face of the first Risley prism pair.  
       [0023] In a still further aspect of the present invention, a method for optically switching light beams in an optical switch comprises steps of: selecting an input optical fiber and an output optical fiber to be optically connected to each other; inserting a first light beam in the input optical fiber to be transmitted through a first Risley prism pair; inserting a second light beam in the output optical fiber to be transmitted through a second Risley prism pair; adjusting the first Risley prism pair to steer the first light beam to be incident on a second outside prism face of the second Risley prism pair; and adjusting the second Risley prism pair to align the second light beam to be incident on a first outside prism face of the first Risley prism pair; thereby forming an aligned optical path between the input optical fiber and the output optical fiber.  
       [0024] These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0025]FIG. 1A is a diagram showing a hierarchy of optical communication networks in a prior art optical communication system;  
     [0026]FIG. 1B is a diagram for an example of a long-haul network in the hierarchy shown in FIG. 1A for prior art optical communication systems;  
     [0027]FIG. 1C is a diagram for an example of a metropolitan network in the hierarchy shown in FIG. 1A for prior art optical communication systems;  
     [0028]FIG. 1D is a diagram for an example of a building-to-building free space optical communication for prior art optical communication systems.  
     [0029]FIG. 2A is block diagram of a 3-dimensional optical switch, in accordance with an embodiment of the present invention;  
     [0030]FIG. 2B is a schematic diagram illustrating optical beam steering and alignment in a double Risley prism pair for a 3-dimensional optical switch, according to the embodiment illustrated in FIG. 2A;  
     [0031]FIG. 2C is a diagram of an optical path in a double Risley prism pair for a 3-dimensional optical switch, according to the embodiment illustrated in FIG. 2A, and wherein the slant faces of the prisms are oriented in a “face-to-back” fashion commonly known Risley prism pair;  
     [0032]FIG. 2D is a diagram of a 3-dimensional optical switch, such as the one shown in FIG. 2A, in which an optical connection cannot be made for some at least one pair of input and output optical fibers that are too far apart;  
     [0033]FIG. 2E is a variation of the double Risley prism pair, in which the slant faces of the prisms are oriented in a “face-to-face” fashion;  
     [0034]FIG. 2F is another variation of the double Risley prism pair, in which the slant faces of the prisms are oriented in a “back-to-back” fashion;  
     [0035]FIG. 3 is a perspective view of a prism-pair actuator for a 3-dimensional optical switch, according to an embodiment of the present invention;  
     [0036]FIG. 4 is a perspective view of a prism-pair actuator for a 3-dimensional optical switch, according to another embodiment of the present invention;  
     [0037]FIG. 5 is a cross-sectional schematic diagram view of a prism-pair actuator for a 3-dimensional optical switch, according to yet another embodiment of the present invention; and  
     [0038]FIG. 6 is a flow chart illustrating one example of a method for switching optical beams using a 3-dimensional optical switch, in accordance with one embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     [0039] The following detailed description is of the best currently contemplated modes of carrying out the invention. The description is not to be taken in a limiting sense, but is made merely for the purpose of illustrating the general principles of the invention, since the scope of the invention is best defined by the appended claims.  
     [0040] Broadly, the present invention provides optical beam steering and alignment in optical communication systems—such as optical switching networks and free space optical communication systems. In one embodiment, an analog optical switch achieves accurate alignment for a practical number—such as 100 or more—of input and output optical fibers. One embodiment of the present invention may be used in the context of optical communication systems and switching networks, where optical switching may be used to provide components such as optical add/drop modules (OADM), protection switches, and non-blocking cross connect switches.  
     [0041] In one embodiment, the present invention uses a Risley prism pair in conjunction with each input and output optical fiber for optical beam steering and alignment so that each optical path through the switch uses two Risley prism pairs (i.e., a double Risley prism pair). A first Risley prism pair provides beam steering for the input optical fiber by directing a light beam onto the face of a second Risley prism pair. A second Risley prism pair provides beam alignment for the output optical fiber by directing a second light beam back onto the face of the first Risley prism pair. Thus, optical beam alignment for completing the optical path between a first and second optical fiber, for example, or first and second lasercom terminals, can be established without relying on a pre-existing alignment of the second optical fiber or collimating lens, as in prior art beam steering only, using one Risley prism pair. Therefore, in the present invention, an M×N multiple-channel optical switch uses M+N Risley prism pairs, while the prior art single channel optical switch uses only one Risley prism pair.  
     [0042] One embodiment achieves a larger Risley-prism pair optical angle, for example, as much as approximately 20 degrees of arc compared to prior art optical angles of 1.1 degrees of arc, allowing the switch of one embodiment to accommodate a practical number, such as 1,000 or more, of input and output optical fibers while maintaining beam steering accuracy and improving beam alignment.  
     [0043] Another embodiment may be used in the context of free space optics, where optical communication is between two locations, such as buildings, with line of sight currently being accomplished by laser beam steering using tilting mirrors or mechanical gimbals mechanisms. The present invention of double Risley prism pairs can be employed for the same beam steering function achieved by past tilting mirrors. For this application, the double Risley prism pairs provide an improvement over the tilting mirror or gimbals mechanism approach because the optical alignment is less sensitive to mechanical angle variations of the Risley prisms than the tilting mirrors.  
     [0044] Referring now to FIG. 2A, double Risley prism pair (DRPP) optical switch  202 , which may be a 3-dimensional all-optical (OOO) fiber optical switch employing Risley prism pairs, according to one embodiment is illustrated. DRPP optical switch  202  may include an input array  203  of input optical fibers for transmitting light signals and an output array  205  of output optical fibers for transmitting light signals. Because light can propagate in either direction along an optical fiber, the terms “input” and “output” are used for convenience and do not necessarily limit the direction of signal propagation. Input array  203  may include a plurality of input optical fibers such as  208 ,  210 ,  212 , and  214 . Output array  205  may include a plurality of output optical fibers such as  216 ,  218 ,  220 , and  222 . Although exemplary DRPP optical switch  202  is illustrated with an equal number of input and output optical fibers, it is generally the case that the number of input optical fibers need not equal the number of output optical fibers.  
     [0045] Each of input optical fibers  208 ,  210 ,  212 , and  214  of input array  203  and each of output optical fibers  216 ,  218 ,  220 , and  222  of output array  205  may include a collimator  224 . Collimator  224  may include a glass capillary, as known in the art, surrounding the end of the optical fiber and surrounding a collimating lens, which may be a graded index, or grin, lens, with the glass capillary holding the end of the optical fiber in proximity to the collimating lens. Each collimator  224  is disposed to direct light from the optical fiber into a corresponding Risley prism pair  226 . As known in the art, each Risley prism pair may comprise a pair of optical glass prisms rotatable about a common axis  227   a,  and capable of deflecting a beam of light entering the prism pair parallel to the axis  227   a  within a cone  229 , illustrated in FIG. 2C, whose axis of symmetry coincides with the common axis  227   a  of rotation of the Risley prism pair  226 . The apex angle of the cone  229 , i.e., the maximum angle of deflection of the beam of light, may be referred to as the optical angle of the Risley prism pair, for example, optical angle  228  shown in FIGS. 2A, 2C, and  2 D.  
     [0046] Each Risley prism pair  226  may be connected to an actuator  230 , exemplary embodiments of which are illustrated in FIGS. 4, 5, and  6 . The exemplary embodiments illustrated in FIGS. 4, 5, and  6  and more fully described below, in contrast to prior art actuators, have minimized outside diameter  340  in order to maximize the number of input and output optical fibers that may be handled by a single switch, such as switch  202 . Each actuator  230  may be capable of independently rotating each Risley prism of each Risley prism pair, as more clearly seen with reference to FIGS. 4, 5, and  6 , for deflecting each light beam to a selected angle of deflection, such as deflection angle  232  and deflection angle  234 , seen in FIG. 2B.  
     [0047] Each actuator  230  may be connected, as shown in FIG. 2A, to a controller  236 . Controller  236  may be implemented, for example, using one or more microprocessors. Controller  236  may be configured to provide control signals  237  to each actuator  230  and to receive feedback signals  231  from each actuator  230  for controlling deflection angle  232 , for example, to provide optical beam steering, as illustrated in FIG. 2B by the transition from the “First” part of FIG. 2B to the “Second” and for controlling deflection angle  234 , for example, to provide optical beam alignment, as illustrated in FIG. 2B by the transition from the “Second” part of FIG. 2B to the “Third”, for forming a complete optical path  238 , for example, from optical fiber  208  to optical fiber  222 .  
     [0048] The transition from the “First” part of FIG. 2B to the “Second” part of FIG. 2B, for example, illustrates an adjustment to deflection angle  232  so that light beam  233 , shown in the “First” part of FIG. 2B as not lying on any optical path between Risley prism pairs  226 , is directed to lie on optical path  238  between Risley prism pairs  226  as shown in the “Second” part of FIG. 2B. Similarly, the transition from the “Second” part of FIG. 2B to the “Third” part of FIG. 2B, for example, illustrates an adjustment to deflection angle  234  so that light beam  235 , shown in the “Second” part of FIG. 2B as not lying on any optical path between Risley prism pairs  226 , is directed to lie on optical path  238  between Risley prism pairs  226  as shown in the “Third” part of FIG. 2B. After the adjustment to the second deflection angle—in this example, deflection angle  234 —a complete optical path  238 , as described above, from optical fiber  208  to optical fiber  222  may be aligned and established for optical communications.  
     [0049] The controller  236  is a functional element of the system, and it should not be considered as a physical unit since there are numerous controller hardware structures and software architectures in the prior art that can realize the disclosed function of beam steering, aligning, and switching.  
     [0050] Because each optical fiber connected to DRPP optical switch  202 , such as input optical fibers  208 ,  210 ,  212 ,  214  and output optical fibers  216 ,  218 ,  220 , and  222 , has a corresponding Risley prism pair—such as input Risley prism pair  208   a,  shown FIG. 2A, corresponding to input optical fiber  208  and output Risley prism pair  218   a  corresponding to output optical fiber  218 , any input-output fiber pair comprising an input optical fiber and an output optical fiber may be optically connected. For example, pair  208  and  222  may be connected as illustrated by light path  238 . Thus, DRPP optical switch  202  may optically connect any one of input optical fibers—such as  208 ,  210 ,  212 , and  214  of input array  203 —with any one of output optical fibers—such as  216 ,  218 ,  220 , and  222  of output array  205 .  
     [0051] In the example shown in FIG. 2A, with four input optical fibers and four output optical fibers there are 4×4=16 distinct pairs which may be optically connected to function simultaneously. Thus, DRPP optical switch  202  may provide connection for any permutation of input optical fibers—such as  208 ,  210 ,  212 , and  214 —to output optical fibers—such as  216 ,  218 ,  220 , and  222 . With input array  203  used as working, i.e., signal transmitting, inputs and output array  205  used as working outputs, DRPP optical switch  202  may be connected for use as a 4×4 non-blocking cross-connect switch in a switch network, such as metro telecommunications switching network  104  in optical communication system hierarchy  100 , for example, where “non-blocking”, as understood in the art, means that any one of working inputs of input array  203  may be connected to any one of working outputs of output array  205  without blocking the possibility of any other working input from input array  203  from being connected to an unused working output of output array  205 . Thus, as an example, if input optical fiber  208  is connected to output optical fiber  216 , any one of input optical fibers  210 ,  212 , and  214  of input array  203  may still be simultaneously connected with any one of output optical fibers  218 ,  220 , and  222  of output array  205 .  
     [0052] Because DRPP optical switch  202  has four each of working inputs and outputs for illustration, DRPP optical switch  202  is designated as a 4×4 switch. It is contemplated, however, that any same or different number of inputs and outputs could be provided so that, for example, a 1000×2000 switch could be provided and used as a non-blocking cross-connect switch in a similar manner.  
     [0053] In another aspect of the present invention, some input optical fibers of input array  203  can be used as working inputs and some as protection inputs, and some output optical fibers of output array  205  can be used as working outputs and some as protection outputs, where “protection” is used in the sense of providing redundant, or backup, signal paths, as understood in the art. Thereby, DRPP optical switch  202  may be connected for use as a protection switch, as understood in the art. In another aspect of the present invention, some input optical fibers of input array  203  can be used as working inputs and some as add inputs, as known in the art, and some output optical fibers of output array  205  can be used as working outputs and some as drop outputs, as known in the art. Thus, DRPP optical switch  202  may be connected for use as an optical add/drop module, or OADM, as understood in the art.  
     [0054] Referring now to FIGS. 2B and 2C, a double Risley prism pair  200  for forming optical path  238 , for example, in a fiber optical switch—such as DRPP optical switch  202 —or, for example, in a free space optical communication system—such as free space optical communication system  170 —according to one embodiment is illustrated. Optical path  238  from input optical fiber  204  (including collimator  224 ) to output optical fiber  206  (including collimator  223 ) may be formed by placing a first Risley prism pair  226  into optical path  238  between input optical fiber  204  and output optical fiber  206 , and also by placing a second Risley prism pair  225  into optical path  238  between first Risley prism pair  226  and output optical fiber  206 . The prisms of first Risley prism pair  226  may be rotated about axis  227   a  to adjust deflection angle  232  to steer a first light beam  233 , seen in FIG. 2B, along optical path  238  to be incident on an outside prism face  225   a  of second Risley prism pair  225 . Similarly, the prisms of second Risley prism pair  225  may be rotated about axis  227   b  to adjust deflection angle  234  to align a second light beam  235 , seen in FIG. 2B, along optical path  238  to be incident on an outside prism face  226   a  of first Risley prism pair  226  and to coincide with the first light beam  233 . By adjusting deflection angle  234 , as seen in the transition from the “Second” part to the “Third” part of FIG. 2B, until the second light beam  235  coincides with the first light beam  233  along optical path  238 , alignment of optical path  238  may be effected.  
     [0055] Thus, by way of contrast to the prior art, there is no need for precise alignment of axis  227   a  relative to axis  227   b,  nor for any precise alignment of collimators  223  or  224  relative to each other, in the present invention. So long as the placement and alignment of axes  227   a  and  227   b  relative to each other allows formation of optical path  238  within the Risley prism pair optical angle  228  (as shown in FIGS. 2A and 2C) precise alignment of axis  227   a  relative to axis  227   b  and precise alignment of optical fibers  204  and  206  (or collimators  223 ,  224 ) relative to each other is not required because the angular adjustment of the deflection angles  232 ,  234  of the two Risley prism pairs  225 ,  226  (within the optical angle  228  of the Risley prism pairs) compensates for any misalignment between the optical fibers.  
     [0056] More specifically, in the present invention, because the light paths are reversible (i.e., light propagating in the reverse direction along the same path follows that same path), an optical beam alignment for completing the optical path between the input optical fiber and the output optical fiber (i.e., “first light” as known in the art), can be established so long as the optical path from the second light beam (from the output optical fiber) is sufficiently close to the optical path of the first light beam to pass in the reverse direction through the first Risley prism pair and enter the input optical fiber. For example, if the second light beam overlaps the first light beam between the two Risley prism pairs, the two light beams will have a reversible light path in common so that a first light for optical beam alignment can be established.  
     [0057] The drawings (FIGS. 2B and 2C) and the above description refer only to one angle, i.e., the “deflection angle”. However, it should be understood that actually there are two angles adjusted simultaneously by the rotational angles of the two prisms in one Risley prism pair, as known in the art, and disclosed in U.S. patent application Publication No. US20010046345 A1, published Nov. 29, 2001, and incorporated herein by reference. In particular, paragraph [0022] describes the “net deflection” of the Risley prism pair (referred to as “deflection” in the present description) in terms of the “deflection for each prism”.  
     [0058] As seen in the example shown in FIG. 2A, where the Risley prism pair axes  227   a  and  227   b  are parallel to each other, the perpendicular distance  240  between the Risley prism pair axes  227   a  and  227   b  of the most widely separated optical fibers, such as optical fibers  208  and  222 , should fit within the optical angle  228  (i.e., perpendicular distance  240  should be less than the product of the separation distance  242  between input and output Risley prism pairs and the tangent of the optical angle  228 ) in order for any input optical fiber, such as optical fiber  208 , to be optically connected to any output fiber, such as optical fibers  216 ,  218 ,  220 , and  222 . Thus, for a given separation distance  242 , the optical angle  228  limits the perpendicular distance  240  between the most widely separated optical fibers, such as  208  and  222 , between which an optical path  238  can be formed. Therefore, the maximum number of optically connectable input and output optical fibers is limited by the number of Risley prism pairs that can be disposed within the perpendicular distance  240 . Because each optical fiber is surrounded by its associated actuator  230 , if an outside diameter  340  (shown in FIGS. 3, 4, and  5 ) with respect to central axis  327  of each Risley prism pair and its associated actuator  230  can be made smaller, more optical fibers can be fitted within the perpendicular distance  240  in such a way as to allow switching between any pair of input and output optical fibers, such as  208  and  222 . Although other geometrical arrangements for each Risley prism pair&#39;s optical axis orientation and the three-dimensional position may be contemplated and optimized for maximal channel density, it is understood that the maximum number of optical fibers would be limited by the optical angle  228  and outside diameter  340  of the Risley prism pairs and their associated actuators, i.e., the smaller the optical angle  228 , the smaller the maximum number of optical fibers and the larger the outside diameter  340 , the smaller the maximum number of optical fibers.  
     [0059] In other words, as shown in FIG. 2D, if the number of Risley prism pairs  226  placed side-to-side causes perpendicular distance  244  between a pair of optical fibers, or between the associated Risley prism pairs, such as  143   a  and  145   a,  to be too great relative to separation distance  242 , then line  246  connecting face  243   b  of Risley prism pair  243   a  and face  245   b  of Risley prism pair  245   a  will lie outside optical angle  228  so that no optical path can be formed between Risley prism pair  243   a  and Risley prism pair  245   a.  In other words, switching between all pairs of input and output optical fibers is no longer achieved because optical fiber  243   c  cannot be switched to optical fiber  245   c.  Therefore, in order to maximize the number of input and output optical fibers in DRPP optical switch  202  in such a way that an optical path can be formed for every pair of input and output optical fibers within the optical angle of the Risley prism pairs, it is desirable to provide Risley prism pairs with associated actuators having the smallest possible outside diameter  340  (as more clearly shown in FIGS. 3, 4, and  5 ) with respect to central axis  327 .  
     [0060]FIG. 2E illustrates an alternative implementation to the double Risley prism pair shown in FIG. 2C, in which the slant faces  250  of the prisms are oriented in a “face-to-face” fashion so that the outside prism face  226   a  is not a slant face  250 . Similarly, FIG. 2F illustrates an alternative implementation to the double Risley prism pair shown in FIG. 2C, in which the slant faces  250  of the prisms are oriented in a “back-to-back” fashion so that the outside prism face  226   a  is a slant face  250 . One advantage of these two alternative implementations is that the prism&#39;s reachable optical range, as it is projected on a plane, is a closed contour without a hole in it, while the conventional Risley prism pair generates a donut like shape, where a hole in the contour cannot be optically reached.  
     [0061]FIG. 3 shows a perspective view of prism-pair actuator  330   a  for a 3-dimensional analog optical switch, according to an embodiment of the present invention. Actuator  330   a  may include drive motors  342 ,  344  each connected to a prism  346 ,  348  of a Risley prism pair  326 . Drive motors  342 ,  344  may be electrical stepper or DC motors, for example, as known in the art. Drive motors  342 ,  344  may be connected to prisms  346 ,  348  through reduction drives  350 ,  352 . Reduction drives  350 ,  352  may use traction (friction) or gear contact to independently transmit actuator rotation of drive motors  342 ,  344  to rotation of prisms  346 ,  348  about central axis  327  without blocking optical path  328  through collimator  324 . Thus, drive motor  342  may be independently controlled to provide rotation of prism  346  independent of rotation of prism  348 , and drive motor  344  may be independently controlled to provide rotation of prism  348  independent of rotation of prism  346 . Reduction drives  350 ,  352  may be used to provide a reduction or amplification ratio between rotation of drive motors  342 ,  344  and driven prisms  346 ,  348 , depending on the tradeoff between response speed and motor torque. For the situation where fast response speed is required, the reduction gear may not be needed and a direct coupling, for example, between the motor shaft and the prism drive may be used.  
     [0062]FIG. 3 also shows a wiring harness  366  for each of drive motors  342 ,  344 . Each wiring harness  366  may include wires, for example, for supplying drive motors  342 ,  344  with power, providing control signals—such as control signals  237 —to drive motors  342 ,  344 , and providing feedback signals—such as feedback signals  231 —from, for example, position sensors, also referred to as “resolvers”, which may be incorporated within drive motors  342 ,  344  for position sensing. Resolvers may provide a feedback signal  231  containing information about the angle of rotation of each of prisms  346 ,  348  to a controller, such as controller  236 , shown in FIG. 2A. Resolvers may be substituted by, for example, potentiometers, encoders, or other position sensing devices. Alternatively, position-sensing feedback may not be needed, and so not be used, when the mechanical system, for example, stepper motor and harmonic drive, provides sufficient mechanical precision. FIG. 3 also shows couplings  368 , which may be used for coupling drive motors  342 ,  344  to reduction drives  350 ,  352 , and mounting plates  370 , which may provide a mechanically stable attachment, for example, to a case (not shown). In one embodiment, actuator outside diameter  340  with respect to central axis  327  may be reduced to less than 2.5 centimeter (cm), and it is contemplated that diameter  340  may be reduced to less than 1.0 cm.  
     [0063]FIG. 4 shows a perspective view of prism-pair actuator  330   b  for a 3-dimensional optical switch, according to another embodiment of the present invention. Actuator  330   b  may include drive motors  342 ,  344  each connected to a prism  346 ,  348  of a Risley prism pair  326 . Drive motors  342 ,  344  may be electrical stepper or DC servo motors, or piezoelectric rotary motors, for example, as known in the art. Drive motors  342 ,  344  may be connected to prisms  346 ,  348  through cranks  372  and connecting rods  354 ,  356 . Cranks  372  may transfer torque or rotational motion from drive motors  342 ,  344  to connecting rods  354 ,  356 . Connecting rods  354 ,  356  may independently transmit rotation of actuator drive motors  342 ,  344  to rotation of prisms  346 ,  348  about central axis  327  without blocking optical path  328  through collimator  324 . Thus, drive motor  342  may be independently controlled to provide rotation of prism  346  independent of rotation of prism  348 , and drive motor  344  may be independently controlled to provide rotation of prism  348  independent of rotation of prism  346 .  
     [0064] Drive motors  342 ,  344  may be arranged in line with central axis  327 , as shown, to reduce the actuator module cross sectional area by reducing diameter  340 . FIG. 4 also shows prism holders  374 , which may secure prisms  346 ,  348  and transfer rotation from connecting rods  354 ,  356  to prisms  346 ,  348 . FIG. 4 also shows bearing  376 , which may provide rotational support for prisms  346 ,  348 . In one embodiment, actuator outside diameter  340  with respect to central axis  327  may be reduced to less than 2.5 cm, and it is contemplated that diameter  340  may be reduced to less than 1.0 cm.  
     [0065]FIG. 5 shows a cross-sectional schematic diagram view of yet another prism-pair actuator  330   c  for a 3-dimensional optical switch, according to another embodiment of the present invention. Actuator  330   c  may include drive motors  342 ,  344  each connected to a prism  346 ,  348  of a Risley prism pair  326 . Drive motors  342 ,  344  may be hollow-shaft, frameless, electrical stepper, DC servo or piezoelectric motors rotating on bearings  343 ,  345 , for example, as known in the art. Drive motors  342 ,  344  may be connected to prisms  346 ,  348  through hollow shafts  358 ,  360 . Hollow shafts  358 ,  360  may surround optical path  328  and independently transmit actuator rotation of drive motors  342 ,  344  to rotation of prisms  346 ,  348  about central axis  327  without blocking optical path  328  through collimator  324 , which may be supported on a third shaft  361 . For example, hollow shafts  358 ,  360  may be constructed from stainless steel micro tubing. Hollow shaft  358  may have an inside diameter of 2.0 millimeters (mm) and an outside diameter of 2.5 mm. Hollow shaft  360  may have an inside diameter of 3.0 mm and an outside diameter of 3.5 mm.  
     [0066] Thus, drive motor  342  may be independently controlled to provide rotation of prism  346  independent of rotation of prism  348 , and drive motor  344  may be independently controlled to provide rotation of prism  348  independent of rotation of prism  346 . Actuator  330   c  may also include position sensors  362 ,  364  for position sensing and providing feedback signals containing information about the angle of rotation of each of prisms  346 ,  348  to a controller, such as controller  236 , shown in FIG. 2A. Position sensors  362 ,  364  may be substituted by, for example, potentiometers, encoders, or other position sensing devices. Alternatively, position-sensing feedback may not be needed, and so not be used, when the motor, for example, stepper motor can provide sufficient mechanical precision without closed loop control. Drive motors  342 ,  344  may be arranged in line with central axis  327 , as shown, to reduce the actuator module cross sectional area by reducing diameter  340 . In one embodiment, actuator outside diameter  340  with respect to central axis  327  may be reduced to less than 2.5 cm, and it is contemplated that diameter  340  may be reduced to less than 1.0 cm.  
     [0067] Referring now to FIG. 6, a method of optical beam steering and alignment for optically switching light beams is illustrated depicting exemplary method  400  in accordance with one embodiment. Method  400  may include a step  402  in which a Risley prism pair  226  may be provided for a first optical fiber and a Risley prism pair  225  may be provided for a second optical fiber. More generally, a first plurality of Risley prism pairs  226  may be provided in such a way that an input Risley prism pair  226  is provided corresponding to each optical fiber of an input array  203  of input optical fibers  204 . For example, input Risley prism pair  208   a  may be provided corresponding to input optical fiber  208  and input Risley prism pair  210   a  may be provided corresponding to input optical fiber  210 , as shown in FIG. 2A. Step  402  may further include providing a second plurality of Risley prism pairs  226  in such a way that an output Risley prism pair  226  is provided corresponding to each optical fiber of an output array  205  of output optical fibers  206 . For example, output Risley prism pair  216   a  may be provided corresponding to output optical fiber  216  and output Risley prism pair  218   a  may be provided corresponding to output optical fiber  218 , as shown in FIG. 2A. Risley prism pairs  226  may be disposed, as described above, so that it is not required for any deflection angle, such as deflection angle  232  or  234 , for example, to exceed the Risley prism pair optical angle  228  in order to form an optical path between any input-output fiber pair comprising an input optical fiber of input array  203  and an output optical fiber of output array  205 .  
     [0068] Method  400  may include a step  404  in which a light beam is inserted into a first Risley prism pair corresponding to an input optical fiber of input array  203 , for example, Risley prism pair  226  and the light beam is steered to be incident on a second Risley prism pair corresponding to an output optical fiber of output array  205 , for example, Risley prism pair  225  by adjusting the first Risley prism pair.  
     [0069] Method  400  may include a step  406  in which a light beam is inserted into the second Risley prism pair, for example, Risley prism pair  225  and the light beam is aligned to be incident on the first Risley prism pair, for example, Risley prism pair  226  by adjusting the second Risley prism pair  225 .  
     [0070] Method  400  may include a step  408  in which optical alignment of the input optical fiber of input array  203  corresponding to the first Risley prism pair with the output optical fiber of output array  205  corresponding to the second Risley prism pair is performed by adjusting the second Risley prism pair, for example, by rotating the prisms of second Risley prism pair  225  as known in the art, to align the second light beam to coincide with the first light beam to form an optical path, for example, optical path  238 .  
     [0071] Method  400  may include a step  410  in which optical switching is performed by transmitting a signal over the optical path, for example, optical path  238  between the input optical fiber of input array  203  corresponding to the first Risley prism pair and the output optical fiber of output array  205  corresponding to the second Risley prism pair.  
     [0072] It should be understood, of course, that the foregoing relates to preferred embodiments of the invention and that modifications may be made without departing from the spirit and scope of the invention as set forth in the following claims.