Abstract:
The present invention provides an N×M non-blocking optical switch using a novel switching fabric that utilizes micromirrors to switch light signals in a planar waveguide array. The optical switch uses a commercially available micromechanical actuator to actuate each micromirror. The actuator can also be an inexpensive custom made actuator. The actuator and the waveguide substrate are separate units and there are no electrical connections between them. The switch includes a unique design feature whereby the actuator and the micromirror array do not require precision alignment. The optical switch of the present invention is fabricated at low temperatures and assembled using glue.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates generally to optical switches, and particularly to optical switches using micromechanical actuators.  
           [0003]    2. Technical Background  
           [0004]    Currently, network providers are experiencing a large increase in the demand for telecommunications services. Forecasters do not expect that this increase in demand will abate anytime soon. Network designers are attempting to meet the demand by exploiting the multi-terabit bandwidth capacity of single-mode optical fibers in the 1550 nm wavelength region. Early on in the fiber optics era, the networks that were deployed used the optical layer as a transmission medium in point-to-point links. Most of the network functionality took place in the electrical domain. Unfortunately, this approach is very limited. Subsequently, SONET/SDH systems were developed as an all purpose method of carrying voice and data over fiber. However, SONET/SDH systems are also limited by the electronic switching equipment. Typically, the SONET/SDH equipment deployed at access rings operate at the OC-3 or OC-12 transmission rates, which translate to data rates of 155 Mb/s and 622 Mb/s respectively. Interoffice rings usually operate at OC-12, OC-48, or OC-192 transmission rates. The data rates for OC48 and OC-192 are about 2.5 Gb/s and 10 Gb/s respectively. Thus, it would be advantageous to move network functionality such as routing and switching into the optical domain to exploit the full bandwidth capacity of single mode fiber.  
           [0005]    There have been several approaches that have been proposed to realize this goal. One such approach uses a switch array with movable micro-electro-mechanical system (MEMS) mirrors. The input and output optical fibers are set in grooves and are disposed orthogonal to each other. The MEMS mirrors are positioned at the intersection of the input fibers and the output fibers in free space. This method requires fairly large mirrors and collimators. This is due to the inevitable spreading of the light beam as it leaves the waveguide and travels in free-space toward the MEMS mirror. The large mirrors are problematic because of their requirements for angular placement accuracy, flatness, and the difficulty of actuating such a relatively large structure quickly and accurately. These devices typically have an actuation distance of 300 μm to 400 μm, which negatively impacts switching speed. In addition, the individual collimators must be assembled for each input and output fiber, thus increasing fabrication costs.  
           [0006]    In a second approach, a planar waveguide array is used. Trenches are formed at the cross-points of the input waveguides and the output waveguides. Micromirrors are positioned within the trenches in free-space. Each micromirror acts like a shutter and is rotated into a closed position by an electrostatic, magnetic, or some other type of MEMS actuator so that the light signal is reflected from an input waveguide into an output waveguide. When the micromirror is in the open position, the light continues to propagate in the original direction without being switched. This method is also subject to the beam-spreading problem, and it appears that the typical losses from such a switch would be high.  
           [0007]    A third approach uses an index-matching fluid as the switching element. A planar waveguide array is formed on a substrate. Trenches are formed at the cross-points and are filled with a fluid that matches the refractive index of the waveguide core. In order to actuate the switch, the fluid is either physically moved in and out of the cross-point using an actuator, or the fluid is thermally or electrolytically converted into a gas to create a bubble. For this approach to work, the facets cut at the end of the waveguide at the cross-points must be of mirror quality, since they are used to reflect the light into the desired waveguide. Finally, the fluid must be withdrawn cleanly to preserve the desired facet geometry and to prevent scattering losses due to any remaining droplets.  
           [0008]    In yet another approach, a beam or plate is disposed diagonally over a gap in a waveguide. A mirror is suspended from the beam into the gap. An electrode is disposed adjacent to the gap and underneath the beam. When the electrode is addressed, the beam and mirror move into the gap to reflect light propagating in the waveguide. This approach has several disadvantages. This method is also subject to the beam-spreading problem discussed above. Again, it appears that the typical losses from such a switch would be high. Second, the electronics needed to drive the actuator are integrated into the optical substrate.  
           [0009]    All of the approaches discussed above are problematic when considering cost and reproducibility. MEMS mirrors and actuators must be specially made using one of several photolithographic processes. Assembling MEMS mirrors and MEMS actuators in waveguide trenches requires precision alignment. The assembly of MEMS devices often requires high temperatures. Integrating electronics into waveguide/MEMS substrates adds another layer of complexity to the process. Thus, the resultant switch may be expensive to make and difficult to reproduce. These devices may have yield problems as well.  
           [0010]    Thus, a need exists for an optical switch having the advantages of the MEMS design, without the disadvantages of the designs discussed above. A need exists for an optical switch that uses readily available off-the-shelf actuators, or ones that can be sub-contracted to standard low cost machine shops. The switch should be comprised of discrete low cost units that avoid unneeded complexity such as integrating electronics into the optical waveguide substrate. Electrical connections between the substrate and electronic systems should likewise be eliminated. Further, a switch is needed that mitigates the precision alignment heretofore required between the actuator and the micromirror array. Finally, an optical switch is needed that can be fabricated at low temperatures using low cost assembly techniques.  
         SUMMARY OF THE INVENTION  
         [0011]    The present invention provides an optical switch having the advantages of the MEMS design, without the disadvantages of the designs discussed above. The optical switch uses a commercially available actuator, or alternatively, a custom made actuator that is easily and inexpensively made by a standard low cost machining shop. The actuator and the waveguide substrate are separate units. Thus, there is no need to integrate electronics into the optical waveguide substrate. Furthermore, the substrate has no electrical connections. Because of the unique design of the switch, the interface between the actuator and the micromirror array does not require precision alignment. The optical switch of the present invention is fabricated at low temperatures and assembled using glue.  
           [0012]    One aspect of the present invention is an optical device for directing a light signal. The optical device includes a switching membrane having a undeformed first state and a deformed second state, wherein the light signal is not deflected in the first state and the light signal is deflected in the second state.  
           [0013]    In another aspect, the present invention includes an optical device for directing a light signal. The optical device includes a membrane having a first side and a second side, the membrane being actuatable between an undeformed state and a deformed state. At least one micromirror is connected to the second side, wherein the micromirror deflects the light signal in the deformed state.  
           [0014]    In another aspect, the present invention includes an optical device for directing a light signal. The optical device includes at least one optical waveguide for propagating the light signal, the at least one optical waveguide having at least one trench disposed therein. A switching membrane having at least one micromirror is aligned with the at least one trench, the membrane being actuatable between an undeformed state and a deformed state.  
           [0015]    In yet another aspect, the present invention includes an optical switch for directing a plurality of light signals. The optical switch includes an optical circuit for propagating the plurality of light signals, the optical circuit having a plurality of first waveguides intersecting a plurality of second waveguides to thereby form a plurality of cross-points, wherein each cross-point includes a trench. A switch membrane including a first side and a second side, the second side having a plurality of micromirrors is disposed thereon, each of the plurality of micromirrors being in substantial alignment with a corresponding trench. A micromechanical actuator having a plurality of actuating members engages the first side, each actuating member being in substantial alignment with a corresponding micromirror disposed on the second side, whereby the actuating member deforms the switch membrane in a first switch state to thereby position the corresponding micromirror into its corresponding trench.  
           [0016]    In yet another aspect, the present invention includes a method of making an optical device for directing a light signal. The method including the steps of providing a substrate having a first surface and a second surface. At least one mirror is formed in the second surface of the substrate. Excess substrate material is removed from the second surface on either side of the at least one mirror such that the substrate is deformable between a undeformed position and a deformed position.  
           [0017]    Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.  
           [0018]    It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate various embodiments of the invention, and together with the description serve to explain the principles and operation of the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]    [0019]FIG. 1 is a sectional view of the optical switch of the present invention;  
         [0020]    [0020]FIG. 2 is a plan view of the optical waveguide substrate of the present invention;  
         [0021]    [0021]FIG. 3 is a schematic view of the actuator drive mechanism in accordance with a first embodiment of the present invention;  
         [0022]    [0022]FIG. 4 is a detail view of the actuator impact print head of the actuator shown in FIG. 3;  
         [0023]    [0023]FIG. 5 is a schematic view of the actuator drive mechanism in accordance with a second embodiment of the present invention  
         [0024]    [0024]FIG. 6 is a schematic view of the switch membrane in accordance with a third embodiment of the present invention  
         [0025]    [0025]FIG. 7 is a sectional view of the optical switch using the switch membrane shown in FIG. 6;  
         [0026]    FIGS.  8 A- 8 F are diagrams depicting a method for fabricating the switch membranes shown in FIGS.  1 - 6 ;  
         [0027]    [0027]FIG. 9 is a plan view of the switch membrane in accordance with a fourth embodiment of the present invention; and  
         [0028]    FIGS.  10 A- 10 F are diagrams depicting a method for fabricating the switch membrane shown in FIG. 9. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0029]    Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. An exemplary embodiment of the optical switch of the present invention is shown in FIG. 1, and is designated generally throughout by reference numeral  10 .  
         [0030]    In accordance with the invention, the present invention for an optical switch includes a switching membrane having a undeformed first state and a deformed second state. The switching membrane deflects the light signal in the second state, but does not deflect the light signal in the first state. The switching membrane is actuated by a micromechanical actuator that is readily available. It can be implemented using an off the-shelf commercial unit or custom made in a standard machining shop. The membrane, actuator and waveguide substrate are separate units. There is no need to integrate electronics into the optical waveguide substrate and the substrate has no electrical connections. The actuator does not have to be precisely aligned to the waveguide substrate and the switching membrane. The optical switch of the present invention is fabricated at low temperatures and easily assembled, using glue to join the separate units.  
         [0031]    As embodied herein, and depicted in FIG. 1, a sectional view of the optical switch of the present invention is disclosed. Optical switch  10  includes switching membrane  20  which has micromirror  24  disposed on flexible diaphragm  22 . Micromirror  24  may also include stop member  26 . Stop member  26  is wider than trench  44  and prevents micromirror  24  from being damaged by being driven into optical substrate  40 . Switching membrane  20  is connected to optical substrate  40  by a spacer or framing member  14  using assembly glue  16 . Optical substrate  40  includes optical waveguides  42  which propagate the light signals in the optical circuit. Trenches  44  are formed in waveguides  42 . As shown in FIG. 1, micromirrors  24  are in alignment with trenches  42 . Although actuator  30  does not have to be aligned to membrane  20  or waveguide substrate  40 , membrane  20  and mirrors  24  do have to be aligned to trenches  44 . There is +/−1 μm alignment variability between mirror  24  and trench  44 . Switch  10  also includes micromechanical actuator  30 . Actuator  30  includes a plurality of impact pins  32  which are engaged with flexible diaphragm  22 . In one embodiment impact pins  32  are touching, or slightly spaced apart from diaphragm  22 . In another embodiment, impact pins  32  are connected to flexible diaphragm  22 . It is not necessary to precisely align pins  32  with the centerline of stop member  26 . Even if the pins are off-center, they will position mirrors  24  in trenches  44  when they deform diaphragm  22 . Actuator  30  individually controls impact pins  32 . Any micromirror  24  can be positioned within its corresponding trench  44  independently of the other micromirrors. In one embodiment, index-matching fluid  12  is disposed between switching membrane  20  and optical substrate  40 . Index-matching fluid  12  prevents the light signal from spreading and de-collimating as it traverses trench  44 . The refractive index of index-matching fluid  12  is substantially the same as that of waveguides  42 .  
         [0032]    It will be apparent to those of ordinary skill in the pertinent art that modifications and variations can be made to flexible diaphragm  22  of the present invention depending on the type of actuator  30  used, the degree of flexibility and strength required, and/or the need to contain index-matching fluid  12  between substrate  40  and membrane  20 . In the example shown in FIG. 1, flexible diaphragm  22  is comprised of a metallic material. In one embodiment the metallic material is an aluminum foil. In other embodiments, membrane  20  is fabricated by forming an uninterrupted diaphragm or foil using an electro-deposition of nickel, copper, gold, polymer, or a thermal oxide material. In the electro-deposition process, an adhesion under layer may be used, such as silicon. In another embodiment, membrane  20  is fabricated by forming a matrix of beams connecting each base  26  and mirror  24  pair with frame  14 . In yet another embodiment, membrane  20  is a micro-suspension or a micro-spring structure. This embodiment will be discussed more fully below.  
         [0033]    It will also be apparent to those of ordinary skill in the pertinent art that modifications and variations can be made to micromechanical actuator  30  depending on the availability and pricing of suitable commercial actuators. For example, actuator  30  may be comprised of an impact print head, a screw driven linear motor actuator, or a pin matrix device used in Braille displays.  
         [0034]    Those of ordinary skill in the art will also recognize that modifications and variations can be made to optical substrate  40  as well. Cladding layer  46  can be formed using any of the methods and materials commonly known to those of ordinary skill in the art. Semiconductor materials such as silicon can be used. Chemical vapor deposition of silica, fused silica, ceramic materials, metallic materials, or polymeric materials can also be used. A variety of methods and materials can be used to form waveguides  42 . These methods include: sol-gel deposition of silica; amorphous silicon; compound semiconductor materials such as III-V or II-VI materials; doped chemical vapor deposition of silica; organic -inorganic hybrid materials; or polymer materials. In one embodiment, waveguides  42  are formed using photolithographic techniques wherein cladding layer  46  is selectively exposed to radiation using a mask or reticle.  
         [0035]    Excess material is removed, and waveguide material is deposited in a groove etched in the cladding material to form the waveguides  42 . Other techniques such as embossing and micro replication can also be used.  
         [0036]    The optical switch  10  shown in FIG. 1 operates as follows. The light signal propagates through trench  44  without being deflected when switching membrane  20  is not deformed by impact pin  32 . Micromirror  24  is positioned in trench  44  to deflect the light signal when actuator  30  causes impact pin  32  to deform switching membrane  20 . When impact pin  32  is retracted, switching membrane springs back to its original position by an intrinsic restoring force and/or the force produced by index-matching fluid  12 .  
         [0037]    As embodied herein, and depicted in FIG. 2, a plan view of optical switch  10  is depicted showing the placement of micromirrors  24  in optical substrate  40  during switch actuation. Optical substrate  40  includes waveguides  42  intersecting waveguides  48 . Cladding  46  is disposed between waveguides  42  and  48 . In FIG. 2, switching membrane  20  and actuator  30  are not shown for clarity of illustration. Instead, only those micromirrors  24  that are positioned within their respective trenches  44  are shown. Light signals L s1 , L s2 , and L s3  are directed into ports  480 ,  4802 , and  484 , respectively. Light signal L s1  is deflected by mirror  24  at cross-point C P1  and is directed out of switch  10  via port  420 . Light signal L s2  is deflected by mirror  24  at cross-point C P2  and is directed out of switch  10  via port  424 . Light signal L s3  is deflected by mirror  24  at cross-point C P3  and is directed out of switch  10  via port  422 .  
         [0038]    As one of ordinary skill in the art will recognize, a variety of combinations can be utilized to direct light signals L s1 , L s2 , and L s3  through optical switch  10 . Further, the present invention is not limited to the 3×3 example shown in FIG. 2. Micromechanical actuator  30 , switching membrane  20  with micromirrors  24 , and optical substrate  40  with intersecting waveguides  42  and  48 , form an N×M nonblocking cross-bar switch, wherein N is the number of waveguides  42 , M is the number of waveguides  48 , and N×M is the number of micromirrors  24  disposed on switching membrane  20 . N and M are integers.  
         [0039]    As embodied herein, and depicted in FIG. 3, a schematic view of micromechanical actuator  30  in accordance with a first embodiment of the present invention is disclosed. Actuator  30  includes an array of impact pins  32 . Pins  32  are an integrated part of print head  34 . Each impact pins  32  is connected to a coil actuator  36 . Coil actuators  36  are driven by their respective drive circuits  52 . Drive circuits  52  are activated and addressed by control module  50 . In response to a control pulse from control module  50 , drive circuit  52  supplies solenoid  36  with current. As the current flows through solenoid  36 , the armature (not shown) of solenoid  36  is attracted to the core  360  of the solenoid, driving impact pin  32  to deform switching membrane  20 . The actuator and print head arrangement shown in FIG. 3 is capable of a switching speed of approximately 10 ms or better.  
         [0040]    It will be apparent to those of ordinary skill in the pertinent art that modifications and variations can be made to control module  50  and drive circuit  52  of the present invention depending on the nature of the command signals being transmitted and received from the network interface, and the current requirements of solenoid coil  36 . For example, control module  50  may send an active high digital pulse to drive circuit  52 , signaling that its respective micromirror should be positioned within trench  44 . In one embodiment, drive circuit  52  includes an emitter grounded transistor to drive solenoid  36 . The coil of solenoid actuator  36  is connected (not shown) to a power source at one end, and the collector of an emitter-grounded transistor at the other. Drive circuit  52  drives the base of the transistor causing a current to flow through solenoid  36  from the power source. As discussed above, the current flows through the coil, the armature (not shown) of the solenoid  36  is attracted to the core of the solenoid causing impact pin  32  to deform membrane  20 . As one of ordinary skill in the art will recognize any number of circuits can be designed to supply current to solenoid  36  depending on the electrical characteristics of solenoid  36 .  
         [0041]    As embodied herein, and depicted in FIG. 4, a detail view of the actuator impact print head  34  is disclosed. Print head  34  includes plate  38  and impact pins  32 . In one embodiment, plate  38  is made of a Teflon material. One of ordinary skill will recognize that any suitable material will do. Plate  38  includes an array of holes  380  to accommodate impact pins  32 .  
         [0042]    It will be apparent to those of ordinary skill in the pertinent art that modifications and variations can be made to the size and pitch of holes  380  and pins  32  disposed on plate  38 . Obviously, the dimensions and tolerances of the features disposed on plate  38  depend on the size and pitch of trenches  44  disposed in optical substrate  40 . By way of example, the pitch of holes  380  (and pins  32 ) shown in FIG. 4 is about 700 μm. Each pin  32  can be extended approximately 1 mm when deforming membrane  20  (not shown). The diameter of the impact pins  32  is about 500 μm. However, the present invention is not limited to the pitch and pin sizes disclosed in this example. These dimensions can be adjusted to accommodate smaller or larger switch matrices.  
         [0043]    In a second embodiment of the invention, as embodied herein and as shown in FIG. 5, a schematic view of an alternate actuator drive mechanism is disclosed. Actuator  300  includes an array of impact pins  32 . Pins  32  are an integrated part of print head  34 . The same print head used in the first embodiment discussed above can be used in the second embodiment. Each impact pin  32  is connected to linear screw drive unit  302 . Linear screw drive unit  302  is operatively coupled to step motor  304  (only an interfacing screw of the motor is shown). Motors  304  are addressed and driven by control module  50  (not shown for clarity of illustration). Again, it will be apparent to those of ordinary skill in the pertinent art that modifications and variations can be made to control module  50  of the present invention depending on the nature of the command signals being transmitted and received from the network interface, and the drive requirements of motor  304 .  
         [0044]    In a third embodiment of the invention, as embodied herein and as shown in FIG. 6, a schematic view of alternate switch membrane  200  is disclosed. Switching membrane  200  is made by partially etching substrate  208 . Membrane  200  includes plate member  202 , stop member  206 , and micromirror  204 . FIG. 7 is a sectional view of the optical switch using the switching membrane shown in FIG. 6. The descriptions of actuator  30  and optical substrate  40  are identical to the descriptions of those elements described above and will not be repeated. As shown in FIG. 7, pin  32  may break membrane  200  during switch actuation. This will not pose a problem because pins  32  are glued to plate  202 . However, in this design a two-way actuator such as the screw driven linear motor discussed above may be more appropriate because the spring effect inherent in the first embodiment may not return micromirror  24  to a retracted position quickly enough.  
         [0045]    As embodied herein, and depicted in FIGS.  8 A- 8 F, diagrams depicting a method for fabricating the switch membrane shown in FIG. 1 are disclosed. In FIG. 8A, &lt;110&gt; silicon wafer  60  is provided. In another embodiment, wafer  60  is a &lt;110&gt; SOI wafer. Metallic coating  22  is deposited on a first surface of silicon wafer  60 . Metallic coating  22  can be of any suitable material, but there is shown by way of example an aluminum coating. As discussed above, the metal can be nickel, copper, or gold. Non-metallic materials such as silica or polymers may also be used. Masking material  64  such as silicon nitride is then deposited on a second surface of silicon wafer  60 . In FIG. 813, mask  64  is patterned and etched using a photolithographic process. In FIG. 8C, mirror structures  240  are formed in the second surface by anisotropic wet etching. Photoresist coating  66  is applied to the second surface in FIG. 8D. In FIG. 8E, photoresist lithography is performed leaving a portion of the photoresist  66  on either side of mirror structures  240 . In FIG. 8F, wet etching is performed on the exposed portions of the second surface. All of the silicon is removed to expose flexible diaphragm  22 . The photoresist is likewise removed, exposing stop member  26 . Finally, mirror structures  240  are coated with gold layer  242  to form mirrors  24 .  
         [0046]    As embodied herein, and depicted in FIG. 9, a plan view of the switch membrane in accordance with a fourth embodiment of the present invention is disclosed. In FIG. 9, the portion of the membrane  120  exposed to actuator  30  is shown. This embodiment discloses a micro-spring, or a micro-suspension version of membrane  120 . Membrane  120  includes platform  122  which has stop member  26  and mirror  24  (see FIGS.  10 A- 10 F) formed integrally thereon. Platform  122  is connected to membrane  120  by spring member  124  and is suspended over substrate  40  (not shown). Thus, when pins  32  of actuator  30  depress platform  122 , flexible spring member  124  is elongated and mirror  24  is inserted into trench  44  to thereby reflect the light signal.  
         [0047]    As embodied herein and depicted in FIGS.  10 A- 10 F, diagrams depicting a method for fabricating the switch membrane shown in FIG. 9 are disclosed. In FIG. 10A, &lt;110&gt; silicon wafer  60  is provided. Alternatively, wafer  60  can be a &lt;110&gt; SOI wafer. A layer of photoresist  66  is disposed on both sides of wafer  60 . In FIG. 10B, the top surface of wafer  60  is patterned and etched using an RIE etching process to partially form spring member  124 . The underside is also patterned and etched and a silicon nitride mirror mask  64  is form on the underside of wafer  60 . In FIG. 10C, a portion of the underside is covered by photoresist layer  66 . Another portion directly underneath spring member  124  is not covered by material  66 . This portion is etched to completely form spring member  124 . In FIGS. 10E and 10F, mirror  24  and stop member  126  are integrally formed from platform  122 . Subsequently, mirrors  24  are coated with reflective gold coating  142 . One advantage to this approach over the method disclosed in FIGS.  8 A- 8 F is that lithographic processing is not done over mirrors  24 . This avoids the risk of breaking the mirrors. In another embodiment using a SOI wafer, a silica etch stop is employed for a deep etching through wafer  60 .  
         [0048]    It will be apparent to those skilled in the art that various modifications and variations can be made to the present invention without departing from the spirit and scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.