Abstract:
An active, optical, piezoelectric actuated modulator allows switching between a reflecting state and an anti-reflecting state. The modulator, or switch, is based on the precise controlling of an air gap between a thin film membrane and a substrate. The thin film membrane is deformed by a miniaturized adaptive material, such as electrostrictive or piezoelectric (PZT) material. Maximum optical reflection is realized when the air gap is equal to a multiple of a quarter wavelength of an impinging optical beam, while anti-reflection is achieved when the thickness of the air gap is equal to zero or is different from a multiple of the quarter wavelength of the optical beam.

Description:
PRIORITY CLAIM 
     The present application claims the priority of co-pending provisional application, Ser. No. 60/281,935, filed on Apr. 6, 2001, titled “Active Reflection and Anti-Reflection Optical Switch,” which is assigned to the same assignee as the present application, and which is incorporated herein by reference. 
    
    
     CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application relates to co-pending U.S. pat. application, Ser. No. 09/884,702, filed on Jun. 19, 2001, titled “Piezoelectric Actuated Optical Switch,” which claims the priority of U.S. provisional patent application, Ser. No. 60/246,284, filed on Nov. 6, 2000, both of which applications are assigned to the same assignee as the present application, and are incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The present invention relates generally to optical signal switching, and particularly to a piezoelectric actuated device for switching an optical signal. More specifically, the present invention relates to an active optical modulator that allows switching from a reflecting state to an anti-reflecting state and vice versa. The switch is based on the precise controlling of an air gap between a thin film membrane and a substrate. The thin film membrane is deformed by a miniaturized adaptive material, such as electrostrictive or piezoelectric (PZT) material. Maximum optical reflection is realized when the air gap is equal to a quarter wavelength of the optical beam, while anti-reflection is achieved when the thickness of the air gap is different from the quarter wavelength. 
     BACKGROUND OF THE INVENTION 
     With the increasing popularity of the World Wide Web (“the web”), there is a continual need to increase the available communication bandwidth. The constant traffic on the web requires an infrastructure that is dynamic to accommodate new needs as they emerge. One of the most pressing challenges is the underlying pipeline, that is the bandwidth which accommodates new users and applications. Some of these applications include as video on demand, video conferencing, and so forth. 
     A number of photonic solutions have been proposed to increase the available network bandwidth. These solutions range from point to point connections to wavelength division multiplexed passive optical network systems. The latter solution is effective in principle, however the cost associated with photonic devices in these systems has been an impediment to their acceptance and rapid deployment. 
     Optical data transmission offers many advantages over electrical and broadcast transmission. However, switching optical data from one channel to another has proven to be problematic. Fundamentally, a beam of light is unaffected by passage through an electric or magnetic gradient, thus the usual solid-state methods for switching electric signals are not effective to switch optical signals. Accordingly, various mechanical techniques relying typically on reflection or refraction have been developed to divert optical signals. 
     FIG. 1 is a schematic diagram of a conventional optical switching array  10 . The switching array  10  includes a plurality of input ports, i.e.,  12 ,  16 , and output ports  14  arranged in columns and rows. To switch an optical signal from a first input port  16  to the output port  14 , a diverter  18  located at a point of intersection between the axes of the two ports  16  and  14 , diverts the beam from the input port  16  to the output port  14 . The diverter  18  can be a mirror, a light pipe, a refractive medium, or the like. Most diverters  18  require a form of actuation to move them into or out of the path of a light beam. 
     FIG. 2 shows a cross-section of a MEMS diverter  18 . The diverter  18  is comprised of a base  32  suspended within a frame  34 . The base  32  includes a reflective coating  36 . Between the frame  34  and the bottom of the base  32  is an interdigitated electrostatic actuator  37  comprising interdigitated fingers  38  and  39  of the base  32  and frame  34 , respectively. The interdigitated electrostatic actuator  37  is actuated by applying electric charges to surfaces of fingers  38  and  39  to cause them to attract each other. The electric charges can be applied to specific fingers  38  and  39 , or to sets of fingers  38  and  39 , to modify how much force is applied, and in what direction, to control the induced tilting of base  32 . 
     Conventional MEMS diverters, however, suffer from some drawbacks. In addition to being expensive to produce, they are also sensitive to electrostatic discharges (ESD) and microcontamination. It will be readily appreciated that ESD can destroy the interdigitated electrostatic actuator  37  by melting or fusing fingers  38  and  39 . Similarly, microcontamination in the form of fine particles or surface films, for example, can mechanically jam the interdigitated electrostatic actuator  37  and prevent it from actuating. Microcontamination can also create an electrical short between fingers  38  and  39 , thereby preventing actuation. 
     A low-cost silicon optical modulator based on micro electro mechanical systems principles (MEMS) has been proposed, offering a low-cost, high production volume modulator. This device has been designated MARS, which is an acronym for Moving Anti-Reflection Switch. In one form, this device has a multi-layer film stack of polysilicon/silicon nitride/polysilicon, wherein the polysilicon is doped and forms the electrode material. A precisely controlled air gap between the film stack and the substrate allows switching from a reflecting state to an anti-reflecting state. 
     The operating principle of a conventional MARS device  100  is illustrated in FIGS. 3,  4 , and  5 , and is based upon the change in an air gap  105  between a suspended membrane  110 , e.g., a silicon nitride film, and an underlying substrate  120 . The membrane  110  has a refractive index equal to the square root of the refractive index of the substrate, and a thickness equal to ¼ the wavelength (λ/4) of an incident light beam. 
     If the membrane  110  is suspended above the substrate  120  such that when the air gap  105  equals λ/4, a high reflection state is achieved, otherwise, including when the air gap  105  is close to zero, an anti-reflection state is achieved. These states also hold true for any value of mλ/4, wherein an even number m represents an anti-reflecting state (or mode), and an odd number m represents a reflecting state. An exemplary MARS structure that is referred to as a double-poly MARS device, is described in U.S. Pat. No. 5,654,819. 
     To activate this MARS device, two electrodes are provided and positioned on top of the membrane  110  and the substrate  120 , with a voltage selectively applied therebetween. The applied voltage creates an electrostatic force that pulls the membrane  110  physically closer to the substrate  120 . When thickness (depth) of the air gap  105  between the membrane  110  and the substrate  120  is reduced to substantially λ/2, an anti-reflective device exhibiting substantially zero reflectivity is produced. 
     While this MARS device  100  provides certain advantages over other prior conventional devices, it has a potential catastrophic failure mode due to the lower polysilicon metallization. This failure mode is illustrated in FIG. 5, where in certain adverse conditions, such as large changes in the dielectric properties of the air gap  105 , or with unusual voltage surges (i.e., electrostatic discharge or ESD) in the switching signal the membrane  110  undergoes excessive deflection, and shorts to the substrate  120 , resulting in a device ( 100 ) failure. 
     Accordingly, it would be desirable to have an optical switching device that can redirect a beam of light that is less susceptible to microcontamination and ESD failures, and that is readily fabricated according to developed microfabrication technologies. 
     SUMMARY OF THE INVENTION 
     The present invention addresses and resolves the foregoing concerns that could lead to potential failure of the MEMS-based devices, namely (i) spurious voltage spikes and (ii) large changes in the dielectric properties of the air in the air gap. 
     The active optical switch of the present invention includes a thin film membrane, that is suspended over a substrate, and that is mechanically deformed by a miniaturized motor, to perform the reflection and anti-reflection switching. In a preferred embodiment, the motor is comprised, for example, of an adaptive or electrostrictive material, such as piezoelectric (PZT). The displacing voltage is applied to the motor rather than to the membrane. 
     Consequently, the membrane and the substrate are not electrically charged as are the corresponding components of the conventional MARS device described above in connection with FIGS. 3 through 5. Thus, the switch of the present invention is tolerant of a direct contact between the membrane and the substrate, thereby solving the spurious voltage spikes concern. 
     In addition, in further contrast to the conventional MARS device described above, the switch of the present invention neither uses nor relies on the air properties in the gap between the membrane and the substrate as an electrically conductive medium to activate the motion of the membrane. The movement of the membrane is caused by the contraction or expansion of the motor. 
     This novel design addresses and solves the concern facing the MARS device described above, namely large changes in the dielectric properties of the air in the air gap. Moreover, due to fact that the substrate is no longer required to be electrically charged, it does not have to be made from special material, such as silicon, nor fabricated using special microfabrication techniques, in effect reducing the cost, labor, and material of the optical switch. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features of the present invention and the manner of attaining them, will become apparent and the invention itself will be better understood by reference to the following description and the accompanying drawings, wherein: 
     FIG. 1 is a schematic diagram of a MEMS optical switching array of the prior art. 
     FIG. 2 is a cross-sectional view of a diverter used in the MEMS optical switching array of FIG. 1; 
     FIG. 3 is a schematic representation of a MARS device of the prior art, shown in a resting state; 
     FIG. 4 is a representation of the MARS device of FIG. 3, shown in an anti-reflecting state; 
     FIG. 5 is a schematic view of the MARS device of FIGS. 3 and 4, illustrating a failure mode. 
     FIG. 6 is a cross-sectional view of an optical switch made in accordance with the present invention, taken along line A—A of FIG. 7, and showing the constituent components of the optical switch; 
     FIG. 7 is an enlarged, fragmentary view of a section of the switch of FIG. 6, illustrating the attachment of an active element to a base and a membrane. 
     FIG. 8 is a top plan view of the optical switch of FIG. 6; 
     FIG. 9 is a cross-sectional view of an optical switch according to another embodiment of the present invention, shown prior to activation; 
     FIG. 10 is a cross-sectional view of the optical switch of FIG. 7, shown after activation; and 
     FIG. 11 is an exemplary graph illustrating the relationship between the reflection property and the air gap thickness of the optical switches of FIGS. 6 through 10, as a function of the wavelength of an impinging optical beam. 
    
    
     Similar numerals in the drawings refer to similar elements. It should be understood that the sizes of the different components in the figures might not be in exact proportion, and are shown for visual clarity and for the purpose of explanation. 
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring to FIGS. 6,  7 , and  8 , they illustrate an optical switch  200  made according to a preferred embodiment of the present invention. The optical switch  200  is generally comprised of a base  205  and a membrane  210 . The membrane  210  is movably connected to the base  205  and is suspended thereto by a plurality of active elements  220 ,  222 ,  224 ,  226 . 
     The base  205  is comprised of a substrate  256 , above which the membrane  210  is supported. An air gap  230  is formed between the membrane  210  and the base  205 . 
     The active members  220 ,  222 ,  224 ,  226  are collectively referred to herein as motor  250 . While only four active members  220 ,  222 ,  224 ,  226  are shown in FIG. 8, it should be clear that a different number of active members can be used, without departing from the scope of the invention. 
     The motor  250  is formed of an active material that develops an electric potential (or voltage) in response to mechanical deformation, and that mechanically deforms in response to an applied electric potential. This is commonly known as the piezoelectric effect. Piezoelectric materials are used in a wide variety of applications including transducers, spark generators for butane lighters, and vibration damping. 
     In a preferred embodiment, the active material is piezoelectric (PZT) which is typically either ceramic or polymeric. Common ceramic piezoelectric materials include, for example, quartz, cadmium sulphide, and titanate compounds such as barium titanate, lead titanate, and lead zirconium titanate. Common polymeric piezoelectric materials include, for example, polyvinylidene fluoride (PVDF), copolymers of vinylidene fluoride and trifluoroethylene (VDF/TrFE), copolymers of vinylidene fluoride and tetrafluoroethylene (VDF/TeFE), and copolymers of vinylidene cyanide and vinyl acetate (VDCN/VA). 
     A distinctive feature of the present invention is that the membrane  210  is not directly secured to the base  205 , but is rather linked thereto by means of the motor  250 . Accordingly, the membrane  210  is freely deformable relative to the substrate  256 . 
     In addition, the membrane  210  and the substrate  256  do not need to be electrically charged. As such, the switch  200  is tolerant of even a direct contact between the membrane  210  and the substrate  256 , should this condition materialize. 
     In operation, and as it will be explained later in more detail in connection with FIGS. 8 and 9, a change in the reflection state (i.e., reflective or anti-reflective) of the optical switch  200  is induced by a corresponding change in the depth of the air gap  230 . Such a change in the air gap ( 205 ) depth is induced by the deformation of the membrane  210 . 
     The membrane  210 , which is suspended over the substrate  256 , is mechanically deformed by the miniaturized motor  250 , to perform the reflection and anti-reflection switching. In a preferred embodiment, the motor  250  is comprised, for example, of an adaptive material, such as an electrostrictive or piezoelectric (PZT) material. 
     A displacing potential is applied to the motor  250  rather than to the membrane  210 , by means of a plurality of electrodes. Only two electrodes  271  and  272  are shown in FIG. 8 to stimulate active element  224  of the motor  250  along the radial direction R. It should be clear that each of the other active elements  220 ,  222 ,  226 , may be supplied with similar electrodes, in order to achieve a uniform, desired deformation of the membrane  210 . 
     In addition, in further contrast to the MARS device described above in connection with FIGS. 3-5, the switch  200  neither uses nor relies on the properties of the air within the air gap  230  as an electrically conductive medium to activate the deformation of the membrane  210 , in that the deformation of the membrane  210  is caused by the contraction or expansion of the motor  250 . 
     Having described the general environment and field of the optical switch  200 , and its mode of operation, its constituent components will now be described in greater detail. In the embodiment of FIGS. 6 and 7, the base  205  is comprised of a generally cylindrically shaped leg  252  that contours the air gap  230 , to support the motor  250 . 
     The base  205  is further comprised of the substrate  256 , above which the membrane  210  is supported by the leg  252 . With more specific reference to FIG. 7, the leg  252  includes a stepped edge  300  on which one end  305  of the active element  220  is seated. The active element  220 , is secured to the stepped edge  300  by means of, for example, an adhesive layer  315 . The stepped edge  300  extends integrally in an upper surface  302 . 
     When the active element  220  is secured to the stepped edge  300 , the flat upper surface  325  of leg  252  is flush with the upper surface  330  of the active element  220 . The upper surface  302  extends under the active element  220  and forms an air pocket or gap  303  with the underside  304  (FIG. 7) of the active member  220  (FIG.  6 ). 
     The air pocket or gap  303  is in communication with the air gap  230  to allow the free movement or displacement of the active elements  220 ,  222 ,  224 ,  226 , and to prevent friction between the motor and the base. The remaining active elements  222 ,  224 ,  226  are similarly secured to the base  205 , to result in the optical switch ( 200 ) design shown in FIG.  8 . 
     The substrate  256  extends integrally from the leg  252 , under the membrane  210 . Preferably, the substrate  256  has the same shape as that membrane  210 . In the example shown in FIG. 8, the membrane  210  and the substrate  256  are circularly shaped. It should however be clear that other shapes may alternatively be employed. 
     In the embodiment of FIG. 6, the base  205  further includes a bottom section  270 . The substrate  256  extends above the bottom section  270 , and is separated therefrom by a gap  275 . According to another embodiment, the gap  275  is not included so that the substrate  256  is integral with the bottom section  270 . 
     The base  205 , including the substrate  256  may be formed of a conductive material that is either optically transparent or absorbing over an operating optical bandwidth. Suitable materials for the base  205  include, but are not limited to silicon, gallium arsenide, indium phosphide, germanium, indium tin oxide (ITO) coated glass, wafer or metal, or other suitable material. 
     The active elements  220 ,  222 ,  224 ,  226  of the motor  250  are substantially similar in function and design, and therefore only the active element  220  will now be described in more detail, with reference to FIGS. 6-8. In this embodiment, the upper surface  330  of the active element  220  is generally rectangularly shaped, with the understanding that other shapes may alternatively be employed. 
     The underside of the active element  220  includes two stepped edges  400  and  405 . The stepped edge  400  fits the stepped edge  300  of the leg  252 . The other stepped edge  405  of the active element  220  is shaped to fit a stepped edge  500  of the membrane  210 . The stepped structure  400 ,  405  provides improved structural integrity and stronger adhesive bonding of the motor  250  to the base  205  and the membrane  210 . 
     The active element  220  links the base  205  and the membrane  210  while creating a lever effect, so that the membrane  210  is forced to be deformed along an axial direction D or optical direction (FIG. 6) of an impinging optical beam, such as a laser beam, by the motor  250 . 
     In another embodiment, the active element  220  does not include the stepped edges  400 ,  405 , but a stronger adhesive bonding of the motor  250  to the base  205  and the membrane  210 , might be required. 
     Still with reference to FIGS. 6 through 8, the membrane  210  is comprised of a well  520  that is contoured by a peripheral wall  525 . The peripheral wall  525  is generally circularly shaped (FIG.  8 ), though other shapes could be used. The membrane  210  extends radially outwardly, into one or more stepped edges  500  to support the active elements  220 ,  222 ,  224 ,  226  of the motor  250 , as explained earlier. 
     The well  520  of the membrane  210  is formed of multiple layers of materials, in this example, three layers  563 ,  564 ,  565 . The first layer  563  is composed of polysilicon and extends over the gap  230 . It should be clear that the first layer  563  can be made of any other suitable amorphous silicon. It is also preferred that the first layer  563  be made of a material that is transparent to the optical beam. 
     The thickness of the first layer  563  is preferably one-quarter wavelength of the impinging optical beam being switched. For example, if the metallization is polysilicon, and the wavelength of the optical beam being switched is 1.55 μm, the thickness of first layer  563  is preferably approximately 1100 Angstroms. 
     The second layer  564  is composed of silicon nitride, and is formed over the polysilicon layer  564 . 
     The silicon nitride layer  564  preferably has a refractive index approximately equal to the square root of the substrate  256 , and has a thickness of one-quarter the wavelength of the optical beam. Techniques for tailoring the refractive index of a material are well known and described in, e.g., Smith et al, “Mechanism of SiNx Hy Deposition From N2—SiH4 Plasma”, J. Vac. Sci. Tech. B(8), #3, pp 551-557 (1990). 
     The third layer is preferably similar in composition to the first layer  563 , and can be composed, for example, of polysilicon, or another suitable, optically transparent material. The third layer may alternatively be made of indium tin oxide. 
     The thickness of third layer  565  is preferably one-half the wavelength of the optical beam. An advantage of using polysilicon for layers  563  and  565  is that the index of refraction of these layers essentially matches the index of refraction of substrate  256 . It is also relatively transparent to laser radiation of interest, i.e. 1.3 μm and 1.55 μm. 
     Referring now to FIGS. 9 and 10 they illustrate an alternative optical switch  600  that is generally similar in function to the optical switch  200 . The optical switch  600  is comprised of a base  605 , the motor active elements  220 ,  222 ,  224 ,  226 , and the membrane  210 , and defines a gap  630  between the membrane  210  and the base  605 . 
     The base  605  is generally similar to the base  205  described earlier, but has a simplified design, in that the base  605  does not include the expanded design of leg  252 . In addition, the base  605  includes substrate  656  that is more basic in construction and design than the substrate  256  and the bottom section  270  and gap  275 . 
     FIG. 9 illustrates the optical switch  600  prior to activation, i.e., before the motor  250  changes its physical properties leading to deformation (expansion or contraction). FIG. 10 illustrates the optical switch  600  subsequent to activation, and illustrates the effect generated by an exemplary expansion of the motor  250 . 
     When a potential is selectively applied across the motor  250 , lateral forces “F” to the wall  525  of the membrane  210 . In a preferred embodiment, the lateral forces “F” generated by two oppositely disposed pairs of active members ( 220 ,  222 ) and ( 224 ,  226 ) are generally equal and opposite in direction. According to the present exemplary embodiment, all four forces “F” generated by all four active members  220 ,  222 ,  224 ,  226  are equal in magnitude. 
     As illustrated in FIG. 10, the forces “F” causing a lever effect, force the membrane  210  to deform axially, inwardly, toward the membrane  656 , thus selectively and controllably varying the width of the air gap  630  between the substrate  656  and the membrane  210 . Changing the air gap ( 630 ) thickness would change the reflection characteristics of the optical switch  600  from a non-reflecting state to a reflecting state, or vice versa, enabling it to perform the desired switching function. 
     FIG. 11 is a graph that displays the relationship between the amount of reflection due to the air gap ( 230 ,  630 ) thickness as a function of the wavelength of the incoming optical beam, and the reflection/anti-reflection property of the optical switch ( 200 ,  600 ). The graph shows that the maximum reflection is achieved by this optical switch ( 200 ,  600 ) when the air gap thickness is equal to a factor of ¼ the wavelength of the optical beam. Anti-reflection is reached when the air gap thickness is reduced to zero or is different from a factor of ¼ the wavelength of the optical beam. 
     It should be understood that the geometry, compositions, and dimensions of the elements described herein can be modified within the scope of the invention and are not intended to be the exclusive; rather, they can be modified within the scope of the invention. Other modifications can be made when implementing the invention for a particular environment. As an example, while the various motors have been described herein to be comprised of piezoelectric material, it should be clear that other active materials, such as, electrostrictive material, memory alloy, smart material, and so forth, could alternatively be employed.