Abstract:
A tunable optical beamsplitter is disclosed that uses electrowetting techniques to vary the propagation characteristics of one or more light beams. Specifically, electrowetting principles are applied to a region of fluid enclosed within an enclosure to form a plurality of liquid lenses. When a light beam is incident upon the plurality of lenses, the plurality of lenses transforms portions of the light beam in corresponding plurality of output split beams. The region of fluid is controllably moved within the enclosure to modify at least a first optical characteristic of at least a first lens in said plurality of lenses in order to change the propagation characteristics of at least one of the split beams.

Description:
FIELD OF THE INVENTION 
     The present invention relates to optical networking components and, in particular, to optical beamsplitters tunable by electrowetting actuation of fluids. 
     BACKGROUND OF THE INVENTION 
     Optical signals are useful for many applications in modern communications systems. A typical optical communications system comprises a transmitter of optical signals (e.g., a laser-based transmitter that generates a desirable wavelength of light, such as 1550 nm), a length of transmission optical fiber coupled to the source, and a receiver coupled to the fiber for receiving the signals. One or more amplifying systems may be disposed along the fiber for amplifying the transmitted signal. Within the receiver or other components within such systems it is often desirable to split the propagation of a single optical light beam into two or more split light beams propagating in different directions (e.g., to different photodetectors). Optical beamsplitters have traditionally been used to accomplish this beam splitting function. 
     Typical optical beamsplitters are, illustratively, semi-reflective cubes and/or plates placed in the path of a propagating beam at a desired preset incidence angle relative to the beam. When positioned at such an incidence angle, the input beam arrives at the partially reflective surface of the beamsplitter at a certain angle in a way such that a portion of the beam is reflected in one direction while at least one other portion of the beam is permitted to pass through the beamsplitter in another direction. As one skilled in the art will recognize, the performance of these types of beamsplitters typically depends to a large degree on precise positioning of the beamsplitter in relation to the incoming light beam and the destination optical components. 
     SUMMARY OF THE INVENTION 
     While prior tunable optical beamsplitters are acceptable for many uses, they tend to be limited in certain regards. Specifically, prior beamsplitters are not tunable, i.e., once prior beamsplitters were fabricated, they were characterized by certain fixed optical properties such as an optimum incidence angle. Thus, any alteration (tuning) of the direction of travel of the split beams and/or the focal length of the beamsplitter required manual movement of the beamsplitter and or the addition of components (such as lenses) to alter the propagation characteristics of the light beam. The present inventors have recognized that, as optical communications systems become more advanced and complex, there is a growing need for new, cost-effective tunable optical beamsplitters and methods of using those devices for changing the propagation behavior of the resulting split light signals. 
     Therefore, the present inventors have invented a tunable optical beamsplitter that uses electrowetting techniques to form a plurality of lenses in a droplet of liquid disposed in an enclosure. Illustratively, when a light beam is incident upon the plurality of lenses, the plurality of lenses transforms portions of the light beam in corresponding plurality of output split beams. The region of fluid is controllably moved within the enclosure to modify at least a first optical characteristic of at least a first lens in said plurality of lenses in order to change the propagation characteristics of at least one of the split beams. In one embodiment, the first optical characteristic is the radius of curvature of at least one lens in the plurality which, when modified, changes the convergence or divergence of the corresponding output split beam(s). In a second embodiment, the first optical characteristic is the position of at least one of the lenses within the beamsplitter which, when modified, changes the direction of departure of the corresponding output split beam(s). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1  shows an illustrative prior art use of a beamsplitter; 
         FIG. 2  shows a prior art liquid microlens using electrowetting principles to alter the optical properties of the microlens; 
         FIG. 3  shows a prior art liquid droplet enclosed in a channel wherein the droplet is movable using electrowetting principles; 
         FIG. 4A  shows a side view of a beamsplitter in accordance with the principles of the present invention wherein a droplet is enclosed within a channel and is movable using electrowetting techniques; 
         FIG. 4B  shows a cross sectional view of the beamsplitter of  FIG. 4A  representing the view at plane A-A′ in  FIG. 4A ; 
         FIG. 4C  shows the cross-sectional view of the beamsplitter of  FIGS. 4A and 4B  representing the view at plane B-B′ in  FIG. 4B ; 
         FIG. 5  shows how the beamsplitter of  FIGS. 4A ,  4 B and  4 C operates to split an incoming light beam into multiple split light beams; 
         FIG. 6  shows how the beamsplitter of  FIGS. 4A ,  4 B and  4 C can be adjusted to vary the divergence of the split light beams; 
         FIG. 7  shows how the beamsplitter of  FIGS. 4A ,  4 B and  4 C can be adjusted to vary the divergence of individual split light beams independent of the other split beams; and 
         FIG. 8  shows how an array of electrodes can be used in the beamsplitter of  FIGS. 4A ,  4 B and  4 C to vary the direction of travel of a split light beam. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows an illustrative well-known prior art optical system wherein a beamsplitter is used to split a propagating optical beam into two split beams directed in different directions. In that figure, beamsplitter  101 , which is illustratively disposed within an optical networking device, is placed in the path of an incoming propagating light beam  102 . Surface  107  of beamsplitter  101  is partially reflective such that, when light beam  102  becomes incident upon surface  107  of beamsplitter  101 , a portion  103  of light beam  102  is reflected toward device  105  and another portion  104  of light beam  102  passes through beamsplitter  101  toward device  106 . Device  105  and device  106  are, illustratively, photodetectors in an optical receiver. As will be evident to one skilled in the art, beamsplitter  101  must be installed within the optical system of  FIG. 1  in a way such that the beamsplitter is aligned with the incoming beam as well as devices  105  and  106 . If the beamsplitter  101  becomes misaligned, physical realignment of the beamsplitter with the beam and devices is necessary. To date, this realignment was only possible by physically repositioning the beamsplitter device through the use of mechanical actuators and/or manual repositioning. 
     The present inventors have realized that it would be desirable to use optical beamsplitters that are tunable and that do not require physical repositioning of the beamsplitter device. Therefore, the present inventors have invented a tunable optical beamsplitter that uses electrowetting to vary the propagation characteristics (e.g., direction of travel and divergence) of one or more split light beams. The resulting devices consume little power (e.g., &lt;1 milliwatt in some cases), are relatively inexpensive to produce, and are compatible with conventional optical systems. Electrowetting principles (i.e., using electric fields to variably change the properties of a liquid-based device) have previously been used to change the focal length and position of liquid microlenses. Such electrowetting based microlenses are the subject of copending U.S. patent application Ser. No. 10/135,973, entitled “Method and Apparatus for Aligning a Photo-Tunable Microlens” and copending U.S. patent application Ser. No. 10/139,124, entitled “Method and Apparatus for Calibrating a Tunable Microlens,” both of which are hereby incorporated by reference herein. In their simplest form, electrowetting based microlenses use a transparent droplet of liquid to focus incoming light onto a desired focal spot. 
       FIG. 2  shows one prior art embodiment of a simple liquid microlens  201 , described in the &#39;973 and &#39;124 US Patent Applications referenced above, whereby the phenomenon of electrowetting may be used to reversibly change the contact angle θ between a droplet  202  of a conducting liquid (which may or may not be transparent) and a dielectric insulating layer  203  having a thickness “d” and a dielectric constant ε r . The contact angle θ between the droplet and the insulating layer is determined by interfacial surface tensions (also known as interfacial energy) “γ”, generally measured in milli-Newtons per meter (mN/m). As used herein, γ S-V  is the interfacial tension between the insulating layer  203  and the air, gas or other liquid that surrounds the droplet, γ L-V  is the interfacial tension between the droplet  202  and the air, gas or other liquid that surrounds the droplet, and γ S-L  is the interfacial tension between the insulating layer  103  and the droplet  202 . The contact angle θ 1  is determined by the following relationship: 
               cos   ⁢           ⁢     θ   1       =         γ     S   -   V       -     γ     S   -   L           γ     L   -   V                 (   1   )               
An electrode  204 , such as metal electrode is positioned below the dielectric layer  203  and is insulated from the droplet  202  by that layer. The droplet  202  may be, for example, a water droplet, and the dielectric insulating layer  203  may be, for example, a Teflon/Parylene surface.
 
     When no voltage difference is present between the droplet  202  and the electrode  204 , the droplet  202  maintains its shape defined by the volume of the droplet and contact angle θ 1 , where θ 1  is determined by the interfacial tensions γ as explained above. When a voltage V is applied to the electrode  204 , the voltage difference between the electrode  204  and the droplet  202  causes the droplet to spread. The dashed line  205  illustrates that the droplet  202  spreads equally across the layer  203  from its central position relative to the electrode  204 . Specifically, the contact angle θ decreases from θ 1  to θ 2  when the voltage is applied between the electrode  204  and the droplet  202 . The voltage V necessary to achieve this spreading may range from several volts to several hundred volts. The amount of spreading, i.e., as determined by the difference between θ 1  and θ 2 , is a function of the applied voltage V. The contact angle θ 2  can be determined by the following relationship: 
               cos   ⁢           ⁢       θ   2     ⁡     (   V   )         =       cos   ⁢           ⁢       θ   1     ⁡     (     V   =   0     )         +           ɛ   o     ⁢     ɛ   r         2   ⁢   d   ⁢           ⁢     γ     L   -   V           ⁢     V   2                 (   2   )             
         where θ 1  is the contact angle between the insulating layer  203  and the droplet  202  when no voltage is applied between the droplet  202  and electrode  204 ; γ L-V  is the droplet interfacial tension described above; ε r  is the dielectric constant of the insulating layer  203 ; and ε 0  is 8.85×10 −12  F/M—the permittivity of a vacuum.       

       FIG. 3  shows an embodiment of a prior structure  301  that relies on the electrowetting principles described above to move a droplet of conductive fluid  302  through an enclosure  309  that is, for example, a glass tube of circular cross section. Such an embodiment is the subject of copending U.S. patent application Ser. No. 10/231,614, entitled “Optical Waveguide Devices With Electro-Wetting Actuation” which is also hereby incorporated by reference herein in its entirety. In contrast to  FIG. 2 , the embodiment of  FIG. 3  uses a rigid enclosure around a conducting liquid droplet  302  to entirely constrain the movement of the droplet in all directions except for the x-direction. In the embodiment of  FIG. 3  the droplet is constrained, illustratively, by a tube of circular cross-section. Electrowetting principles, such as those described above, are used to reversibly change the contact angle θ between the liquid and the surface of enclosure  309 . The contact angle θ between the droplet and the insulating layer is, once again, determined by interfacial surface tensions and can be calculated by referring to equation 1. When no voltage difference is present between the droplet  302  and the electrode  305 , the droplet  302  maintains its position within the enclosure  309  with contact angle θ 1 =θ 2  where θ 1  is determined by the interfacial tensions γ as explained above. 
     When a voltage V is applied to the electrode  305 , the voltage difference between the electrode  305  and the droplet  302  causes the droplet to attempt to spread, as in the case represented by FIG.  2 . Specifically, the contact angle where boundary  303 A meets the surface of enclosure  309  decreases from θ 2  to θ 1  when the voltage is applied between the electrode  305  and the droplet  302 . The voltage V necessary to achieve this change may range from several volts to several hundred volts. The amount of movement, i.e., as determined by the difference between θ 1  and θ 2 , is a function of the applied voltage V. The contact angle θ 2  can be determined by, once again, referring to equation 2, where θ 1  is the contact angle between the surface of enclosure  309  and the droplet  302  when no voltage is applied between the droplet  302  and electrode  305 ; γ L-V  is the droplet interfacial tension; ε r  is the dielectric constant of the insulating layer  306 ; and ε 0  is 8.85×10 —12  F/M—the permittivity of a vacuum. Since the droplet of  FIG. 3  is constrained in its movement in all directions except the x-direction, a difference in contact angle caused by the applied voltage V leads to a force imbalance between the opposite sides  303 A and  303 B of the fluid droplet. As a result, the fluid droplet moves in direction  310  toward the side of the droplet under higher applied voltage. 
     The present inventors have realized that it would be advantageous to utilize the aforementioned electrowetting techniques to create a tunable beamsplitter. Therefore, in accordance with the principles of the present invention,  FIGS. 4A ,  4 B and  4 C show, respectively, a side cross sectional view, a front cross-sectional view and a top cross-sectional view of a beamsplitter  401  that controls fluid motion via electrowetting principles. In this embodiment, referring to  FIG. 4A , droplet  403  is disposed within an illustrative enclosure  402  having reflective inner surface  408  which is, illustratively, a layer of reflective dielectric material. A lubricating liquid may be disposed within the enclosure  402  to reduce friction between droplet  403  and enclosure  402 . Surface  408  may have, illustratively a transparent coating of CYTOP® disposed on a substrate of well-known Teflon® material. CYTOP® is an amorphous fluorocarbon polymer manufactured by Asahi Glass, Inc. that is characterized, in part, advantageous hydrophobic properties. One skilled in the art will recognize that many materials will be suitable to achieve the characteristics necessary for surfaces in beamsplitters such as that described herein. Enclosure  402  is illustratively of rectangular cross section, however one skilled in the art will similarly recognize that many cross sectional shapes (e.g., ellipsoidal) would be equally advantageous. Electrodes  405  and  406 , located above and below droplet  403 , respectively, are separated by dielectric layer  407  from droplet  403 . Channels  409  in the dielectric layer  407  permit grounding of the liquid droplet by bringing the liquid into contact with ground electrodes  412 . As previously discussed, by changing the relative voltages of electrodes  406  and  405 , one or more portions of droplet  403  can be moved in the x-direction within enclosure  402 . The contact angles θ 2  and θ 3 , which are herein after referred to as the vertical contact angles of, respectively, the leading and trailing edge of the droplet can, once again, be determined by equations 1 and 2. 
       FIG. 4B  shows a cross-section of the beamsplitter of FIG.  4 A. Specifically,  FIG. 4B  represents an illustrative cross section of beamsplitter  401  at plane A-A′ as shown in FIG.  4 A. In  FIG. 4B  it can be seen that, unlike previous embodiments of moving a droplet within an enclosure, electrodes  406 A/C and  406 B/D only extend partially across the width of the droplet  403 . Thus, droplet  403  is divided into portions  403 A and  403 B separated by region  404 , hereinafter referred to as transition region  404 . 
       FIG. 4C  shows a top cross-sectional view of the beamsplitter of  FIG. 4A  used to split illustrative light beam  409  into multiple light beams. Specifically,  FIG. 4C  represents an illustrative cross section of beamsplitter  401  at plane B-B′ as shown in FIG.  4 B. In the illustrative embodiment of  FIG. 4C , when an equal voltage is applied to electrodes  406 C (and/or  406 A as shown in  FIG. 4B ) and  406 D (and/or  406 B as also shown in FIG.  4 B), portions  403 A and  403 B of droplet  403  are caused to move in direction  413 . Thus, for a given voltage applied to electrodes  406 C (and/or  406 A as shown in  FIG. 4B ) and  406 D (and/or  406 B as also shown in FIG.  4 B), it can be said that portions  403 A and  403 B form two lenses  411  and  410 , respectively, having radius of curvature R 2 . As will be apparent to one skilled in the art from the forgoing discussion of electrowetting, the amount of displacement of droplet portions  403 A and  403 B (and hence the radius of curvature of lenses  411  and  410 ) depends directly on the amount of voltage applied to electrodes  406 C (and/or  406 A as shown in  FIG. 4B ) and  406 D (and/or  406 B as also shown in FIG.  4 B). Transition region  404  with radius of curvature R 3  serves to divide the droplet to create two portions  403 A and  403 B of droplet  403 . This transition region results from the lack of voltage applied to the droplet in the transition region  404  between electrodes  406 C and  406 D. The lack of electrodes in the transition region  404  creates a high vertical contact angle θ 3 . As a result, the droplet on either side of region  404  forms two lenses having a semi-circular cross-section with, illustratively, the maximum displacement of the lens being positioned at the midpoint between the boundaries  420  of the transition region  404  and the sidewalls  421 . The radii of curvature R 1 , R 2 , and R 3  depend, respectively, on the vertical contact angles θ 1 , θ 2  and θ 3 . Specifically, the radius of curvature R 3  of the transition area on the leading edge of the droplet depends on the radius of curvature R 1  of the trailing edge of the droplet, the contact angle θ 1  of the trailing edge and the contact angle θ 3  of the transition region. The radius of curvature R 3  of the transition area  404  and can be expressed as: 
             R3   =       h     2   ⁢     (       cos   ⁢           ⁢     θ   1       -     cos   ⁢           ⁢     θ   3         )         -   R1             (   3   )               
where h is the height of the enclosure as shown in FIG.  4 A. Similarly, the radius of curvature R 2  of the lenses  410  and  411  in  FIG. 4C  can be determined by: 
             R2   =     R1   -       h     2   ⁢     (       cos   ⁢           ⁢     θ   1       -     cos   ⁢           ⁢     θ   2         )         .               (   4   )               
Thus, by using the previously described electrowetting techniques to vary the vertical contact angles θ 1 , θ 2  and θ 3 , the radii of curvature R 1 , R 2  and R 3  can be changed, or tuned.
 
     One skilled in the art will recognize that, although two lenses (lenses  411  and  410 ) are shown in this exemplary embodiment, by arranging the electrodes differently (e.g., by adding additional electrodes separated from each other) it will be possible to create any number of lenses separated by transition regions such as region  404 . Referring once again to the illustrative embodiment of  FIG. 4C , electrode  405  is used, by relying once again on electrowetting principles, to form region  403 C of droplet  403  which thus forms a third lens  414  having radius R 1 . The portion of the surface of droplet  403  where lenses  410  and  411  are formed is hereinafter referred to as a first surface of the droplet and the portion of the surface of the droplet  403  where lens  414  is formed is hereinafter referred to as a second surface of the droplet. 
       FIG. 5  shows how the beamsplitter of  FIGS. 4A ,  4 B and  4 C may be used operationally to split incoming beam  409  into illustrative multiple beams  503  and  505 . Specifically, the radii of curvature (R 2  in  FIG. 4C ) of lenses  410  and  411  are selected using the above-discussed electrowetting techniques in a way such that, when light beam  409  is incident upon those lenses they operate to focus a portion of beam  409  onto different focal points,  415  and  416  respectively, within focal plane  501 . The focal length f of lenses  410  and  411  is determined by the equation: 
             f   =       [     R       n   lens     -     n   surround         ]     ·     n   lens               (   5   )               
where R is the radius of the lens, n lens  is the refractive index of the lens, and n surround  is the refractive index of the medium surrounding the lens. The two resulting split beams with focus points  415  and  416  propagate through the liquid until reaching lens  414  which functions to direct split output beams  502  and  504  in desired directions, such as directions  503  and  505 , respectively. Focal plane  501 , hereinafter referred to as the main lens focal plane, is the focal plane of lens  414 . Setting the focal points  415  and  416  of lenses  410  and  411  to be within in focal plane  501  leads to the result that the output beams  502  and  504  are parallel beams (i.e., not converging or diverging beams). Thus, parallel output beams may be achieved by adjusting the radius of curvature of lenses lenses  410  and  411  with the electrowetting techniques discussed herein above.
 
       FIGS. 6A ,  6 B and  6 C illustrates how, by tuning the characteristics of the liquid droplet  403 , such as tuning the radius of curvature as discussed in association with  FIG. 4C , the optical characteristics of the lenses, such as lenses  410  and  411 , and hence the propagating characteristics of the split beams, may be varied. Specifically, as shown in  FIG. 6A  (which is similar to the beamsplitter of FIG.  5 ), when the radius of curvature (R 2  in  FIG. 4C ) of lenses  410  and  411  is such that the focus points  415  and  416  are located at the main lens focal plane  501 , the output beams  605  and  606  are parallel beams. However, as is shown in  FIGS. 6B and 6C , by changing the radius of curvature of lenses  410  and  411  (e.g., by varying the vertical contact angles of the droplet  403 ), the convergence or divergence of the output beams can be changed. Alternatively, the same convergence or divergence may be obtained by changing the radius of curvature of lens  414  in FIG.  4 C. This may be desirable, for example, in achieving alignment between the beamsplitter and other optical components or, alternatively, to increase or decrease the power of the resulting split beams. In the illustrative embodiment of  FIG. 6B , the radius of curvature of the lenses  410  and  411  is increased relative to the radius of curvature in  FIG. 6A and , accordingly, the focal length of the lenses (as determined by equation 5) is also increased. The result is that the focal points  415  and  416  are moved in direction  607  away from the main lens focal plane  501 . The same result (displacement of the focal points  415  and  416  relative to the focal plane  501 ) may be achieved by decreasing the radius of curvature of lens  414 . As a result, as is shown by output beams  601  and  602 , the output beams converge as they propagate to a destination. Conversely, as is shown in  FIG. 6C , when the radius of curvature of lenses  410  and  411  are decreased relative to the radius of curvature in  FIG. 6A , the focal length of the lenses is decreased. Therefore, in this case, the focal points  415  and  416  are moved in direction  608  away from the main lens focal plane  501 . Once again, the same result could be achieved by increasing the radius of curvature of lens  414 . Therefore, as is shown by output beams  603  and  604 , the output beams diverge as they propagate to a destination. 
     In the above discussion, the radius of curvature R 2  of lenses  410  and  411  have been assumed to be equal. However, as is shown in  FIG. 7 , it is possible to use electrowetting techniques to adjust the droplet  403  in a way such that the radius of curvature for each of the two lenses is different. Specifically, in this illustrative embodiment, a higher voltage is applied to electrode  406 C (and electrode  406 A in  FIG. 4B ) than is applied to electrode  406 D (and electrode  406 B in FIG.  4 B). As a result, the radius of curvature of lens  410  is lower than the radius of curvature of lens  411 , thus causing the focus point  415  of lens  410  to be a distance d from the main lens focal plane  501  closer to lens  410 . In this illustrative example, the radius of curvature of lens  411  is such that the focus point  416  is located in the main lens focal plane  501 . As a result, output beam  702  is a parallel beam propagating in direction  703 , while output beam  701  is a diverging beam propagating in direction  704 . As discussed previously, it is desirable to be able to adjust the split beams independent of one another in order to, for example, adjust the power per unit area of an individual split beam. 
     Finally,  FIGS. 8A and 8B  illustrate another method of tuning the propagation characteristics of the output split beams. Specifically, by using an array  804  of electrodes, as opposed to a single electrode (such as  406 D in FIG.  7 ), it is possible to vary the angle of departure and/or the intensity (power) of the output beam. Referring to  FIGS. 8A and 8B , this variation is accomplished by only applying a voltage to a portion of the electrodes in array  804 . As such, a smaller portion of droplet  403  is displaced in direction  809 , thus forming lens  805 . One skilled in the art will recognize that this smaller lens will focus a smaller portion of the light beam entering the beamsplitter from direction  801  as compared to, for example, lens  810 . As a result, the power of output split beam  807  will be correspondingly lower. Also, since the smaller lens  805  is displaced away from transition region  404 , the resulting output beam  807  propagating in direction  802  will have a departure angle of θ D . As is illustrated in  FIG. 8B , when lens  805  is displaced in the opposite direction, closer to the transition region  404 , the departure angle θ D  of output beam  808  propagating in direction  803  decreases when compared to the lens location of FIG.  8 A. Once again, as previously discussed, since the focus point  415  is located in the main lens focal plane  501 , the output beams  807  and  808  are parallel beams. 
     The foregoing merely illustrates the principles of the invention. It will thus be appreciated that those skilled in the art will be able to devise various arrangements which, although not explicitly described or shown herein, embody the principles of the invention and are within its spirit and scope. Furthermore, all examples and conditional language recited herein are intended expressly to be only for pedagogical purposes to aid the reader in understanding the principles of the invention and are to be construed as being without limitation to such specifically recited examples and conditions. Moreover, all statements herein reciting aspects and embodiments of the invention, as well as specific examples thereof, are intended to encompass functional equivalents thereof.