Abstract:
Small mechanical devices fabricated from semiconductor substrates called micro-electromechanical systems (MEMS) are used in the telecommunications industry for various purposes, such as switching and attenuation. Typically a small mirror or arm is moved into or out of the optical path of a beam of light to redirect of attenuate the signal. The present invention relates to a drive circuit for a MEMS device that converts a low voltage source into a higher more useful voltage pulse. The present invention also relates to an optically powered MEMS device utilizing the aforementioned drive circuit. Moreover, the drive circuit according to the present invention can be used to generate a single exponentially decaying pulse that can both generate an actuation force and a braking force for the MEMS device.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   The present application claims priority from U.S. patent application Ser. No. 60/380,279 filed May 15, 2002. 

   TECHNICAL FIELD 
   The present invention relates to a drive circuit for a micro-electro-mechanical systems (MEMS) device, and in particular to a low voltage drive circuit that can be powered by light traveling in an optical network. 
   BACKGROUND OF THE INVENTION 
   Switches and other devices utilizing MEMS technology have recently become popular in the fiber optic communication industry due to their versatility, reproducibility, and durability. Conventional MEMS switches include a tiny reciprocating or pivoting mirror that requires a separate electrical power source to generate the voltage, e.g. 30V to 60V, required to move the mirror from a rest position to an active position. One such switch is disclosed in U.S. Pat. No. 6,303,885 issued Oct. 16, 2001 to Bryant Hichwa et al, which is incorporated herein by reference. The switch disclosed in the aforementioned reference is a bi-stable or latching MEMS switch that includes a mirror mounted on a reciprocating beam, which is supported by spring arms. Latching MEMS actuators generally only require a short duration, high voltage pulse to generate an electrostatic actuating force. However, due to the elastic nature of the arms supporting the beam with the mirror, the device tends to oscillate or “ring” before coming to a complete stop. To eliminate this ringing, breaking systems have been developed such as the one disclosed in U.S. patent application Ser. No. 09/810,825 filed Mar. 16, 2001 naming Mao et al as inventors, which is incorporated herein by reference. Unfortunately, these devices still require a steady high voltage source. Other devices using MEMS in the fiber optic communications industry include attenuators and dynamic gain equalizers. 
   The concept of using optical energy to actuate a switch is disclosed in U.S. Pat. No. 6,310,339 issued Oct. 30, 2001 in the name of Hsu et al; U.S. Pat. No. 5,714,773 issued Feb. 3, 1998 to Burrows et al; U.S. Pat. No. 5,859,719 issued Jan. 12, 1999 to Dentai et al; and U.S. Pat. No. 6,075,239 issued Jun. 13, 2000 to Aksyuk et al. Unfortunately, the Hsu et al device still requires a separate power source to provide a base voltage, to which the optical power is added to raise the total voltage over a threshold voltage, which actuates the switch. The remaining three references disclose devices that only generate very low voltages, and in reality would not be able to generate the energy required to power a conventional MEMS device, only simple specially designed devices. 
   An object of the present invention is to overcome the shortcomings of the prior art by providing a drive circuit for a conventional MEMS device that utilizes a low voltage source to generate a high voltage pulse. 
   Another object of the present invention is to provide an actuator for a MEMS switch that both drives and brakes the system with very little if any ringing. 
   Another object of the present invention is to provide a MEMS device that is powered by optical power only. 
   Another object of the present invention is to provide an efficient drive circuit that draws almost no current except during the short period of an optical switching cycle. 
   Another object of the present invention is to provide a drive circuit that does not need continuous optical power, only needing optical power when a switching cycle is anticipated. 
   SUMMARY OF THE INVENTION 
   Accordingly, the present invention relates to a micro-electro-mechanical device comprising: 
   a power source for generating a relatively-low substantially-constant voltage; 
   an electrical circuit for converting the constant voltage into a relatively higher voltage pulse; and 
   a MEMS device including an actuator for moving a body from a first position to a second position after receiving the voltage pulse. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be described in greater detail with reference to the accompanying drawings which represent preferred embodiments thereof, wherein: 
       FIG. 1  is a schematic illustration of a conventional 2×2 MEMS switch; 
       FIGS. 2   a  and  2   b  are schematic illustrations of a comb drive actuator for the switch of  FIG. 1 ; 
       FIG. 3   a  is a circuit diagram of the drive circuit for a MEMS device according to the present invention; 
       FIG. 3   b  is one example for a power source for the drive circuit of  FIG. 3   a ; and 
       FIG. 4  is another example of a power source for the drive circuit of  FIG. 3   a  utilizing optical power. 
   

   DETAILED DESCRIPTION 
   An example of a conventional MEMS device is illustrated in  FIG. 1 , in which a bi-stable 2×2 switch  20  includes a reciprocating center body  22  pivotally mounted to fixed supports  24  and  26  via spring arms  28  and  30 , respectively. An actuator  34 , as will be hereinafter described with reference to  FIG. 2 , is used to move the center body  22  between two stable positions. The center body  22  includes a hollow portion  36  defined by side walls  40  and  42 . The hollow portion  36  enables the side walls  40  and  42  to slightly deform during actuation of the center body  22 , thereby providing an energy barrier, i.e. a latch, that must be overcome before returning to the other position. Typically the end  95  of the center body  22  includes a reflective surface on each side thereof for reflecting optical signals between waveguides, as is well known in the art. 
   The actuator  34  can take several forms such as magnetic, electrical and electrostatic.  FIG. 2   a  is a simplified top view of a portion of an electrostatic comb drive  60  according to a preferred embodiment of the present invention in a first switch position or state. The comb drive  60  includes fixed fingers  62 ,  64  and movable fingers  66 , only one of which is shown for simplicity of illustration. Comb drives typically have dozens, if not hundreds, of inter-digitized fingers, and are often formed in thin films of silicon using photolithography and anisotropic etching techniques. In this embodiment, each finger has a wide section  68 ,  70  and a narrow section  72 ,  74  attaching the wide sections  68 ,  70  to the base  76  or movable element  78 , respectively. 
   The wide sections  68 ,  70  increase the inter-finger capacitance when they are aligned, thus decreasing the electrostatic potential. In a particular embodiment the narrow sections are about 3 microns wide and the wide sections were at least 7 microns wide. The gap between the fingers when the wide sections are aligned is about 1-2 microns. In another embodiment, the narrow sections were about 3 microns wide and the wide sections were about 13 microns wide. It is generally desirable that the wide sections be at least three times wider than the narrow section to facilitate bi-directional operation of the electrostatic comb drive. When a voltage is applied between the fixed and moving electrodes, the moving part experiences an attractive force to pull it toward the fixed part so that the thick portions are aligned and the gap between the fingers is the least. 
   Although the wider sections are illustrated as rectangular blocks, other shapes may be fabricated to achieve desired electrostatic drive performance. For example, the wider sections  68  and  70  could be wider near the tip to facilitate more rapid initial acceleration of the movable portion, tapering to a narrower width near the narrow section  72  and  74  to reduce the total electrostatic force-time product. Similarly, it is not necessary that the wider sections  68  on the fixed fingers  62  and  64  be the same or even similar to the wider sections  70  on the movable fingers  66 . 
     FIG. 2   b  is a simplified top view of the comb drive shown in  FIG. 2   a  in a second position. The movable fingers  66  have been attracted to the fixed fingers  62 ,  64  by applying a voltage pulse between the two halves of the comb drive. The voltage pulse was maintained long enough to accelerate the movable element  78  of the drive to a sufficient energy to reach the second position. The voltage pulse can be maintained after the wide portions  68  and  70  of the two sets of fingers pass each other to slow the movable element  78  before it reaches the second position. An inflection point occurs between the two stable positions, at which point the forces are balanced, and after which the actuator  34  begins to apply a force in the opposite direction for braking the switch. Spring arms  28  and  30  or other motive elements can contribute to the movement of the movable element  78 . A latching technique, such as the one described with reference to  FIG. 1 , holds the movable element  78  in the second position. Another feature is that once the movable element  78  reaches the target position, voltage of the same polarity can be used to switch the movable element  78  back to the initial position, also with deceleration. Thus the same or very similar electric pulse can be used to toggle the switch between states. 
   A preferred voltage pulse for actuating and braking the MEMS device  20  is provided by the electric circuit  100  illustrated in FIG.  3 . In general, an inductor L 1  is used to send a high voltage spike to charge a drive capacitor C 1 , which discharges slowly creating an exponentially decaying voltage pulse used to drive the aforementioned actuator  34 . 
   In detail, an on/off switch S 1  closes the circuit  100  and activates the input voltage V cc  across a first resistor R 3 , turning on a transistor Q 1 , which in turn applies V cc  across an inductor L 1 . The input voltage V cc  is much less than is normally required to power a MEMS device, e.g. 2 to 10 volts, typically 3 volts. A second resistor R 2  establishes a current (I 1 ) and therefore the energy in the inductor L 1  according to the equation E inductor= ½(V cc /R 2 ) 2 L 1 . Typically the current (I t ) in inductor L 1  ranges between 100 μA and 10 mA, depending on the inductance of the inductor L 1 . Inductors with smaller inductances require larger currents, i.e. smaller resistances R 2 , to store the required energy and generate the required voltage spike to power the actuator  34 , e.g. comb drive  60 . The transistor Q 1  turns on and saturates when the switch S 1  is activated, thereby switching the transistor Q 1  into a conducting mode with about a 10Ω resistance between the source and the drain. When the switch S 1  is released the transistor Q 1  switches to the off state. The switching speed of the transistor Q 1  establishes the amplitude of the voltage spike according to the equation V(t)=−L 1  dl 1 (t)/dt. Due to the negative sign in the inductor relationship, a positive voltage appears at the lower node of the inductor L 1  when the switch S 1  is turned off, i.e. this is a falling edge triggered device. This positive voltage at the lower node causes a diode D 4  to conduct, and the drive capacitor to charge up to the required voltage. The voltage spike only lasts for about 1 μs, but that is enough time to charge the capacitor to the required voltage, which for the preferred embodiment is between 60V and 90V. The diode D 4  prevents any charge from flowing backwards therethrough, thereby ensuring that the stored voltage drains from the drive capacitor C 2  and storage capacitor C 1  through a bleed resistor R 1  with effective resistance R B . Zener diode D 2  may be used to set the maximum voltage of the high voltage pulse or to protect the MEMS device from excessive voltages. 
   If we assume ideal electrical components, all of the energy in the inductor L 1  will be transferred to the capacitor C 1 . The energy in the capacitors is also given by the equation E capacitor= ½V s   2 C T , wherein V s  is the peak amplitude of the voltage spike at the lower node of the inductor L 1 , and C T  is the total capacitance, equal to C 1  plus C 2  in this example circuit, since the two capacitors are in parallel. Since the voltage across the capacitors C 1  and C 2  decreases exponentially with time constant t=R B C T , the effective width of the pulse can be controlled by changing the resistance R B  and/or changing the effective capacitance of the capacitor C T , e.g. changing the specific capacitance of the capacitor C 1  or adding additional capacitors in series or parallel. As an example: if R B  is 10 MΩ and C T  is 50 pF, then t=R B C T =10 MΩ×50 pF=500 μs, i.e. the amplitude of the voltage has decreased by one half after 500 μs. In other words, a 3V source has been converted into a 60 V and 90 V spike lasting 1 μs, which has been converted into a pulse with a peak amplitude between 60 V and 90 V decaying exponentially with a time constant of 500 μs. 
   One method of ensuring that the amplitude of the voltage spike will be constant regardless of a variation in the supply voltage amplitude is to ensure that the inductor is saturated for all supply voltage amplitudes above the lowest expected value. Accordingly, the inductor L 1  will store the same maximum amount of energy for all supply voltage amplitudes, since increasing the current through a saturated inductor will not store additional energy. 
   Tests have shown that only minimum ringing results for a large range in voltage amplitudes as long as the correct time constant is used for the exponential decay. The higher peak amplitude imparts a higher velocity to the center body  22 , i.e. the center body  22  reaches the force inflection point sooner, thereby still receiving enough braking force. Lower amplitude pulses impart a slower velocity to the center body  22  resulting in the center body  22  reaching the inflection point later, but with less energy that is still removed without ringing. Ideally, the system is designed so that the slope of the voltage decay corresponds to the speed of the actuator  34  to correctly balance the driving and braking forces. 
     FIGS. 3   b  and  4  represent two examples of power sources that can be used to provide the voltage V cc . The first and simplest example ( FIG. 3   b ) is a 3V lithium battery  201 . The second example ( FIG. 4 ) includes a plurality of photodiodes  202  (or other device for converting light into energy) connected in series. With this arrangement light can be used to power the device. A separate switching signal can be sent to the switch if the corresponding information signal is to be switched or preferably the light beams that are to be switched can be used to power the switch. In a specific example, a wavelength division multiplexed (WDM) signal traveling along an optical fiber  203  may include a special wavelength channel, which determines whether the signal should be switched. When the signal approaches the switch  20 , the signal channel is demultiplexed from the remainder of the signal by a WDM filter  204  to power the switch  20  and redirect the signal along an alternate path  207 . The mere existence of the signal channel at a predetermined wavelength may activate the switch  20  or information stored in the signal channel may result in the signal continuing on along path  206  or being redirected along path  207 . The number of photodiodes  202  required is obviously dependent upon the efficiency thereof. Typically to generate the required 3 Volts, five to twelve photodiodes generating from 0.25 V to 0.6 V each are required. Alternatively, the WDM filter  204  can be replaced by any form of beam splitting device that taps a portion of the original signal to power the switch  20 .