Abstract:
Disclosed is an MEMS variable optical attenuator comprising a substrate having a planar surface, optical fibers having an optical signal transmitting end and an optical signal receiving end, respectively, coaxially arranged on the substrate, a micro-electric actuator arranged on the substrate for providing a driving stroke along a direction perpendicular to an optical axis of the optical beam, at least one lever structure arranged on the substrate for receiving the driving stroke of the micro-electric actuator at a first end thereof and transferring an amplified displacement distance to an optical shutter through a second end thereof, an optical shutter arranged on the substrate and connected to the second end of the lever structure so as to be moved by the amplified displacement distance, thereby being displaced to an attenuation position of the optical beam.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an optical attenuator that uses an element of a micro-electro-mechanical system (MEMS) device, and more particularly to an MEMS variable optical attenuator capable of amplifying a displacement distance of an optical shutter so that the displacement distance is compatible with a large mode field diameter (MFD) of an optical signal transmitting end or an optical signal receiving end of an optical fiber. 
   2. Description of the Related Art 
   An optical attenuator is an optical component for use in optical telecommunication networks. The optical attenuator includes a pair of optical waveguides having an optical signal transmitting end and an optical signal receiving end, respectively, and attenuates an optical power of an optical beam passing out the transmitting end of the optical waveguide and entering the receiving end of the optical waveguide by causing insertion loss of the optical beam. 
   Generally, optical power levels are regulated over wide ranges based on a configuration of optical telecommunication systems. For example, the optical power levels are determined by an optical transmission loss typically varied based on a length of an optical transmission line, the number of connection points of optical fibers, and the number and performance of optical components such as optical couplers coupled to the optical transmission line. An optical attenuator is needed in optical telecommunication networks to reduce an optical power when an optical signal with a excessive power level greater than an allowed power level is received by an optical signal receiver. The optical attenuator further may be used in evaluating, adjusting and correcting telecommunication equipments and optical measurement equipments. 
   Such optical attenuators are classified into two types, a fixed optical attenuator for reducing an optical power by a fixed amount of attenuation and a variable optical attenuator capable of attenuating an optical power of incident light beams by a varied amount of attenuation based on user&#39;s requirements. Such optical attenuators are required to be produced at low cost with high reliability and small size. 
   To satisfy such requirements, an optical attenuator that uses an element of an MEMS device has been suggested. Such MEMS optical attenuator is realized by forming a microstructure acting as an actuator on a substrate such as silicon by using a thin film processing technology. Generally, an MEMS actuator is driven to move by a driving force caused by thermal expansion or an electrostatic force. As the MEMS actuator moves, an optical shutter coupled to the MEMS actuator is displaced so as to be inserted into a gap between two optical waveguides, thereby partially intercepting optical beams traveling from an optical signal transmitting end (or an exit end) of the optical waveguide such as an optical fiber to an optical signal receiving end (or an incident end) of the optical waveguide. 
     FIGS. 1A and 1B  illustrate a perspective view and a plan view, respectively, of a conventional variable optical attenuator using an actuator driven by an electrostatic force. 
   Referring to  FIGS. 1A and 1B , an MEMS variable optical attenuator includes a substrate having a pair of optical waveguides  19   a ,  19   b  provided thereon, wherein one waveguide has an optical signal transmitting end and the other has an optical signal receiving end, an electrostatic actuator comprised of driving electrodes  12   a ,  12   b , a ground electrode  14 , a spring  15  and a movable mass  16 , and an optical shutter  17  connected to the movable mass  16  of the electrostatic actuator. 
   The driving electrodes  12   a ,  12   b  and the ground electrode  14  are supported by an oxide layer called an “anchor” and formed on the substrate  11 , and thereby fixed to the substrate  11 . The movable mass  16  is connected to the ground electrode  14  via the spring  15  and has a comb shape. The driving electrodes  12   a ,  12   b  have respective extended portions  13   a ,  13   b , each with a comb shape. The comb of each of the extended portions  13   a ,  13   b  is interdigitated with the comb of the movable mass  16 . 
   When driving signals are applied to the driving electrodes  12   a ,  12   b  so as to generate a potential difference between the driving electrodes  12   a ,  12   b  and the ground electrode  14 , an electrostatic force arises between the interdigitated combs of movable mass  16  and extended portions  13   a ,  13   b , thereby causing the movable mass  16  to move. As the movable mass  16  moves, the optical shutter  17  is inserted into a gap defined by the optical signal transmitting end  19   a  and the optical signal receiving end  19   b  so as to partially intercept optical beams incident onto the optical shutter  17 . 
   Advantageously, optical waveguides are optical fibers. To improve optical performance of the optical fibers, an optical collimator can be used. The optical collimator enlarges a mode field diameter of the optical fiber, thereby reducing alignment loss of optical beams, amount of variation of wavelength dependence loss (WDL) and polarization dependence loss (PDL) of light beams, reflection loss and initial insertion loss of light beams. As a result, it is possible to achieve a superior optical performance of the optical fiber. 
   However, even though the optical collimator has such advantages as described above, it cannot be adopted in a conventional MEMS variable optical attenuator due to its large mode field diameter (MFD). The conventional MEMS variable optical attenuator is provided with an actuator having a driving stroke of about 10 μm which is compatible with a MFD of a typical optical fiber. However, in the case of using an optical collimator, a MFD of the optical fiber increases to 100 μm, or to 200-300 μm under certain circumstances, so that it is difficult to achieve an adequate attenuation level of the incident light beams by using the conventional MEMS actuator having a short driving stroke. 
   To solve the above problem, it is necessary to lengthen the actuator&#39;s driving stroke so that a displacement distance of an optical shutter increases, but there is a limit to lengthening a driving stroke of an actuator because an MEMS variable optical attenuator is implemented in a very small sized chip. In a conventional MEMS variable optical attenuator, a driving stroke of an actuator is limited by a gap “d” defined by two facing combs, a comb of the movable mass  16  and a comb of the extended portions  13   a ,  13   b  of the driving electrodes  12   a ,  12   b . Accordingly, if the driving stroke of the actuator is lengthened to be compatible with the MFD of the optical collimator only by using the gap “d”, it cannot satisfy the need for a small sized MEMS optical variable attenuator. 
   Accordingly, to realize an MEMS variable optical attenuator having an excellent optical performance and a small size, it is necessary to modify a structure of an MEMS actuator so that a driving stroke of the MEMS actuator can be amplified to be compatible with a large MFD of an optical collimator. 
   SUMMARY OF THE INVENTION 
   Therefore, the present invention has been made in view of the above problems, and it is an object of the present invention to provide an MEMS variable optical attenuator provided with an actuator capable of providing an optical shutter with a large displacement distance greater than a driving stroke of the actuator by using a lever so that the displacement distance of the optical shutter may be compatible with a large mode field diameter of a collimator, thereby achieving a desired amount of attenuation of a optical power. 
   In accordance with the present invention, the above and other objects can be accomplished by the provision of an MEMS variable optical attenuator comprising a substrate having a planar surface, optical fibers having an optical signal transmitting end and an optical signal receiving end, respectively, coaxially aligned with each other on the substrate, a micro-electric actuator arranged on the substrate for providing a driving stroke in a direction perpendicular to an optical axis of an optical beam, at least one lever structure arranged on the substrate for receiving the driving stroke of the micro-electric actuator at a first end thereof and providing an optical shutter with a displacement distance which is greater than the driving stroke through a second end thereof, an optical shutter arranged on the substrate and connected to the second end of the lever structure so as to be moved by the amplified displacement distance, thereby being displaced to an attenuation position of the optical signal. 
   In accordance with one aspect of the present invention, there is provided an MEMS variable optical attenuator comprising a substrate having a planar surface, optical fibers having an optical signal transmitting end and an optical signal receiving end, respectively, coaxially arranged on the substrate, an electrostatic electrode section fixed on the substrate and generating an electrostatic force in response to an electronic input signal and, a movable mass arranged on the substrate and moving by the electrostatic force in a direction perpendicular to an optical axis, a ground electrode section fixed on the substrate and connected to the movable mass by a first elastic structure, a lever structure arranged in perpendicular to a moving direction of the movable mass and having a first end connected to the movable mass via a second elastic structure and a second end opposite to the first end, a supporting structure arranged on an opposite side of the movable mass with respect to the lever structure and connected to a portion of the lever structure by a third elastic structure, the portion being near the first end of the lever structure, and an optical shutter arranged on the substrate and connected to the second end of the lever structure. 
   In accordance with another aspect of the present invention, there is provided with an MEMS variable optical attenuator comprising a substrate having a planar surface, optical fibers having an optical signal transmitting end and an optical signal receiving end, respectively, coaxially aligned with each other on the substrate, an electrostatic electrode section fixed on the substrate and generating an electrostatic force in response to the electronic input signal, a movable mass arranged on the substrate and moving by the electrostatic force, two ground electrodes arranged at both sides of the movable mass and connected to the movable mass by respective first elastic structures, a first and second lever structures, each with a first end and a second end, which are arranged in perpendicular to a moving direction of the movable mass, the first ends of the first and second lever structures being connected to the movable mass by respective second elastic structures, a supporting structure arranged on an opposite side of the movable mass with respect to the first and second lever structures and connected to respective portions of the first and second lever structures by respective third elastic structures, the respective portions being near the respective first ends of the first and second lever structures, and an optical shutter arranged on the substrate and connected to the second ends of the first and second lever structures. 
   In accordance with still another aspect of the present invention, there is provided an MEMS variable optical attenuator comprising a substrate having a planar surface, optical fibers having an optical signal transmitting end and an optical signal receiving end, respectively, coaxially aligned with each other on the substrate, an electrostatic electrode section fixed on the substrate and generating an electrostatic force in response to the electronic input signal, a movable mass arranged on the substrate and being moved by the electrostatic force, two ground electrodes fixed on the substrate, arranged at both sides of the movable mass, and connected to the movable mass by respective first elastic structures, a first and second lever structures, each with a first end and a second end, arranged in perpendicular to a moving direction of the movable mass, the first ends of the first and second lever structures being connected to the movable mass by respective second elastic structures, two supporting structures arranged on an opposite side of the movable mass with respect to the first and second lever structures and connected to respective portions of the first and second lever structures by respective third elastic structures, the respective portions being near the respective first ends of the first and second lever structures, and an optical shutter arranged on the substrate and connected to the second ends of the first and second lever structures. 
   Preferably, the movable mass includes an extended structure which is arranged in parallel with the first and second lever structures and has a length which is almost equal to the total lengths of the first and second lever structures, and the first ends of the first and second lever structures are connected to the movable mass by the third elastic structures. 
   Preferably, each of the movable mass and the electrostatic electrode section has a comb shape and the combs are interdigitated with each other. 
   The MEMS variable optical attenuator in accordance with the present invention is capable of attenuating the optical power of the optical beam by a desired amount of attenuation even in the case that an optical collimator is provided to the optical signal transmitting end or the optical signal receiving end of the optical fiber. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which: 
       FIGS. 1A and 1B  illustrate a perspective view and a plan view of a conventional MEMS variable optical attenuator, respectively; 
       FIG. 2  illustrates a plan view of an MEMS variable optical attenuator in accordance with a first embodiment of the present invention; 
       FIG. 3  illustrates a perspective view of an MEMS variable optical attenuator in accordance with a second embodiment of the present invention; 
       FIGS. 4A and 4B  illustrate plan views showing the operation of the MEMS variable optical attenuators of the first and second embodiments of the present invention, respectively; 
       FIG. 5  illustrates a plan view of an MEMS variable optical attenuator having an improved movable mass and driving electrodes in accordance with the present invention; 
       FIG. 6  illustrates a plan view of an MEMS variable optical attenuator in accordance with a third embodiment of the present invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   A detailed description of an MEMS variable optical attenuator in accordance with preferred embodiments of the present invention will be given below with reference to the accompanying drawings. 
     FIG. 2  illustrates a plan view of an MEMS variable optical attenuator in accordance with a first embodiment of the present invention. 
   Referring to  FIG. 2 , an MEMS variable optical attenuator in accordance with a first embodiment of the present invention includes a substrate  21  having a pair of optical fibers with an optical signal transmitting end  20  and an optical signal receiving end  30 , respectively, an electrostatic actuator comprised of a driving electrode  22 , ground electrodes  24   a ,  24   b  and a movable mass  12 , a lever structure  35  for amplifying a driving stroke of the actuator so as for an optical shutter to be displaced by a displacement distance greater than the driving stroke of the actuator, and an optical shutter  27  coupled to the lever structure  35 . 
   The driving electrodes  12   a ,  12   b  and the ground electrode  14  are structures (hatched portion) formed over the substrate  11  and supported by an oxide layer (not shown). The movable mass  26  is connected to the ground electrodes  24   a ,  24   b  located at both sides thereof via respective first elastic structures  31   a ,  31   b  and suspended over the substrate  21 . The movable mass  26  and the driving electrode  22  preferably have interdigitated comb structures to effectively generate an electrostatic force. 
   The first elastic structures  31   a ,  31   b  act as linear springs and allow the movable mass  26  to move along a predetermined path by a driving stroke. 
   In accordance with this embodiment of the present invention as described above, arranging the first elastic structures  31   a ,  31   b  at both sides of the movable  26  is advantageous in that the movable mass  26  is able to perform a precise straight line motion. However, locations and the numbers of the first elastic structures and the ground electrodes are not limited to the arrangement shown in FIG.  2 . The first elastic structure can be arranged in a different position from that of  FIG. 2  so as for the movable mass  26  to be returned to its original position after being displaced. 
   The lever structure  35  is almost perpendicular to a moving direction of the movable mass  26 . The lever structure  35  has a first end connected to the movable mass  26  via a second elastic structure  32  and a second end perpendicularly coupled to the optical shutter  27 . 
   The MEMS variable optical attenuator in accordance with the first embodiment of the present invention further includes a supporting structure  25  fixed on the substrate  21  and connected to a portion of the lever structure  35  by a third elastic structure, the portion being near to the first end of the lever structure  35  and acting as a fulcrum of a lever. The supporting structure  25  is coated with a metal which is the same material as the ground electrodes  24   a ,  24   b , so that the supporting structure  25  may serve as an additional ground electrode. The second and third elastic structures  32 ,  33  function to help the lever structure  35  to operate smoothly. 
   In the MEMS variable optical attenuator shown in  FIG. 2 , if a desired electrical signal is applied to the driving electrode  22  and an elastic force arises between the driving electrodes and the ground electrodes, the movable mass is displaced toward the driving electrode  22 . After the elastic force is removed or decreases, the movable mass  26  is returned to the initial position due to a restoring force of the first elastic structures  31   a ,  31   b . The displacement distance of the movable mass is determined by a gap D 1  defined by a tip of the movable mass  26  and a body of the driving electrode  22 . 
   The driving stroke corresponding to the size of the gap D 1  is transferred to the first end of the lever structure  35  through the second elastic structure  32 , and thus the first end of the lever structure  35  moves by the same distance as the gap D 1 . As the first end of the lever structure  35  moves by the gap D 1 , the second end connected to the optical shutter moves by a displacement distance greater than the gap D 1  because the gap D 1  is amplified to the displacement distance by the lever structure  35  and the fulcrum. 
   As described above, as the second end of the lever structure  35  moves by the amplified displacement distance, the optical shutter  27  perpendicularly coupled to the second end of the lever structure  35  is driven to be inserted into a gap between the optical signal transmitting end and the optical signal receiving end of the optical fibers. 
   The amount of amplification of the displacement distance is determined by a position of the fulcrum. That is, the amount of the amplification is determined by a leverage ratio. The leverage ration is defined by a ratio of a first length l 1  (from the first end to the fulcrum) of the lever structure  35  to a second length L 1  (from the second end to the fulcrum) of the lever structure  35 . 
   For example, in the case that the second length L 1  is 10 times greater than the first length l 1 , the displacement distance of the optical shutter coupled to the second end of the lever structure  35  is amplified to 10 times the driving stroke of the movable structure  26 . That is, assuming that the actuator has a driving stroke of 10-30 μm, the displacement distance of the optical shutter can be amplified to 100-300 μm. Accordingly, the displacement distance of the optical shutter can be compatible with the MFD of the optical collimator used in the MEMS variable optical attenuator. 
   As described above, to achieve a great amplification of the displacement distance of the optical shutter, it is desirable that the fulcrum of the lever is formed to be near the first end of the lever structure. 
     FIG. 3  illustrates a perspective view of an MEMS variable optical attenuator according to a second embodiment of the present invention. The MEMS variable optical attenuator in accordance with the second embodiment of the present invention includes two lever structures bilaterally symmetrically arranged. 
   Referring to  FIG. 3 , an MEMS variable optical attenuator of the second embodiment of the present invention includes a substrate having optical fibers with a transmitting end  129   a  and a receiving end  129   b , respectively, thereon, an electrostatic actuator comprised of a driving electrode  122 , ground electrodes  124   a ,  124   b  and a movable mass  126 , two lever structures  135   a ,  135   b  which are bilaterally symmetrically arranged, and an optical shutter  127  coupled to the lever structures  135   a ,  135   b.    
   The driving electrode  122  and the ground electrodes  124   a ,  124   b  are supported by an oxide layer  128  and fixed on the substrate  121  in similar manner to the MEMS variable optical attenuator shown in FIG.  2 . The movable mass  126  is connected to the ground electrodes  124   a ,  124   b  arranged at both sides thereof by first elastic structures  131   a ,  131   b , respectively, and suspended over the substrate  121 . The first elastic structures  131   a ,  131   b  act as linear springs, thereby enabling the movable mass  126  to move along a predetermined path by a driving stroke. 
   The first and second lever structures  135   a ,  135   b  are arranged in perpendicular to a moving direction of the movable mass  126 , and first ends thereof are aligned on the same straight line and adjacent to the other. The first ends of the first and second lever structures  135   a ,  135   b  are connected to movable mass  126  by second elastic structures  132   a ,  132   b , respectively. 
   The first lever structure  135   a  has a fulcrum at a portion close to the first end thereof. The second lever structure  135   b  has a fulcrum at a portion close to the first end thereof. The portions near the first and second lever structures  135   a ,  135   b  are connected to a supporting structure  125  fixed on the substrate  121  by third elastic structures  133   a ,  133   b , respectively. The supporting structure  125  is coated with a metal which is the same material as the ground electrodes  124   a ,  125   b , thereby serving as a ground electrode. 
   Second ends of the first and second lever structures  135   a ,  135   b  are connected to the optical shutter  127  by third elastic structures  133   a ,  133   b , respectively. Further, the lever structures  135   a ,  135   b  are bilaterally symmetrically arranged at both sides of a virtual line X-X′ connecting the optical shutter  127  and the center of the movable mass  126 . 
     FIGS. 4A and 4B  are plan views showing the operation of the MEMS variable optical attenuator shown in FIG.  3 . 
     FIG. 4A  illustrates the MEMS variable optical attenuator in which electrical signals corresponding to the amount of attenuation of optical beams are not applied to the driving electrode  122 . As explained with reference to  FIG. 3 , the movable mass  126  is connected to the ground electrodes  124   a ,  124   b  by the respective first elastic structures and moves along a straight path perpendicular to an optical axis of optical fibers. The first ends of the first and second lever structures  135   a ,  135   b  are connected to the movable mass  126  by the second elastic structures  132   a ,  132   b , respectively, and the second ends of the first and second lever structures  135   a ,  135   b  are connected to the optical shutter  125  by fourth elastic structures  134   a ,  134   b , respectively. The portions of the first and second lever structures  135   a ,  135   b , which are close to the first ends of the first and second lever structures are connected to the supporting structure  125  by third elastic structures  133   a ,  133   b , respectively, thereby serving as fulcrums. When the electronic signal corresponding to the amount of the attenuation of optical beams is applied to the driving electrode, an elastic force arises between the driving electrode and the ground electrodes, so that the movable mass  126  moves toward the driving electrode  122  as shown in FIG.  4 B. Along the moving direction of the movable mass  126 , the first ends of the first and second lever structures  135   a ,  135   b  move by a distance that is the same as a driving stroke of the movable mass  126 . As soon as the first ends of the first and second lever structures  135   a ,  135   b  move, the second ends of the first and second lever structures  135   a ,  135   b  move in an opposite direction to the moving direction of the first ends. That is, the second ends of the first and second lever structures move toward the optical axis of an optical beam. The displacement distances of the second ends of the first and second lever structures  135   a ,  135   b  are increased by a leverage force exerted by the lever structures  135   a ,  135   b  and the fulcrums by an amount obtained by multiplying the driving stroke of the movable mass by a leverage ratio, wherein the leverage ratio is defined as a ratio of a first length l2, from the first ends of the lever structures  135   a ,  135   b  to the fulcrums, to the a second length L2, from the second ends of the lever structures  135   a ,  135   b  to the fulcrums. 
   Accordingly, the optical shutter  127  moves by a displacement distance greater than that of the movable mass  126 , in which the displacement distance of the movable mass  126  is defined by a gap between the movable mass  126  and the driving electrode  122 . 
   Particularly, by connecting the optical shutter  127  to the second ends of the two lever structures  135   a ,  135   b , the optical shutter  127  moves in a direction perpendicular to the optical axis of the optical signal transmitting end  129   a  and the optical signal receiving end  129   b.    
   In accordance with the first and second embodiment of the present invention, the movable mass and the driving electrode have a comb shape so as to increase an elastic force generating area. In the case that the movable mass and the driving electrodes have the comb shape, the elastic force generating area is larger than when the driving electrode and the movable mass have a flat panel shape. However, the MEMS variable optical attenuator in accordance with the present invention will be modified in various shapes. That is, the movable mass and the driving electrode can be formed to have shapes other than a comb shape. 
   Another example shape of the movable mass and the driving electrode is disclosed in FIG.  5 . 
   Referring to  FIG. 5 , other elements except for the movable mass and the driving electrode are the same as the MEMS variable optical attenuator shown in  FIG. 3. A  movable mass  146  is connected to ground electrodes  144   a ,  144   b  by first elastic structures  151   a ,  151   b , respectively. First ends of a first and a second lever structures  155   a ,  155   b  are connected to the movable mass  146  by second elastic structures  152   a ,  152   b , and second ends of the first and second lever structures  155   a ,  155   b  are connected to an optical shutter  147  by fourth elastic structures  154   a ,  154   b , respectively. 
   Further, a portion of the first lever structure  155   a  is connected to a supporting structure  145  by a third elastic structure  153   a  and a portion of the second lever structure  155   b  is connected to the supporting structure  145  by a third elastic structure  153   b . The portions connected to the supporting structure  145  act as fulcrums of a lever, wherein the portions are near the first ends of the lever structures  135   a ,  135   b.    
   In this embodiment, the movable mass  146  has two extended portions  146 ′,  146 ″ arranged in parallel with a body of the movable mass  146 , which extend toward the driving electrode  142 . The driving electrode  142  has extended portions  142 ′,  142 ″ arranged in parallel with a body of the driving electrode  142 , which extend toward the movable mass  146 . The extended portions  142 ′,  142 ″ of the driving electrode  142  do not overlap with the extended portions  146 ′,  146 ″ of the movable mass  146 , but are positioned between the body of the movable mass  146  and the extended portions  146 ′,  146 ″ of the movable mass  146 . 
   Such shapes of the movable mass  146  and the driving electrode  142  provide an increased elastic force generating area, thereby improving movement efficiency of the movable mass which is driven to move by an electrostatic force. 
     FIG. 6  illustrates a plan view of an MEMS variable optical attenuator in accordance with a third embodiment of the present invention. This embodiment provides an MEMS variable optical attenuator different from the MEMS variable optical attenuator in accordance with the second embodiment of the present invention in connection of the lever structures. 
   The MEMS variable optical attenuator in accordance with the third embodiment of the present invention includes a substrate  161  having a pair of optical fibers with an optical signal transmitting end  169   a  and an optical signal receiving end  169   b , respectively, an elastic actuator comprised of a driving electrode  162 , ground electrodes  164   a ,  164   b  and a movable mass  166 , two lever structures  175   a ,  175   b  bilaterally symmetrically arranged, and an optical shutter connected to the lever structures  175   a ,  175   b . The driving electrode  162  is fixed on the substrate  161  and supported by an oxide layer  168  formed on the substrate  161 . The movable mass  166  are connected to the ground electrodes  164   a ,  164   b  arranged at both sides thereof by first elastic structures  171   a ,  171   b  and suspended over the substrate  161 . Here, the first elastic structures  171   a ,  171   b  act as a linear spring defining a driving stroke of the movable mass, thereby enabling the movable mass  166  to move linearly by the driving stroke. 
   The first and second lever structures  175   a ,  175   b  are arranged in a straight line perpendicular to a moving direction of the movable mass  176 . Second ends of the first and second lever structures  175   a ,  175   b  are adjacent to each other. In this embodiment, the first ends of the first and second lever structures  175   a ,  175   b , which outwardly extend from the substrate  161 , are connected to the movable mass  166  by second elastic structures  172   a ,  172   b , respectively. The first and second lever structures  165   a ,  165   b  have respective fulcrums connected to supporting structures  165   a ,  165   b  fixed on the substrate  161  by third elastic structures  173   a ,  173   b , respectively. The fulcrums are formed to be near the first ends of the lever structures. Because the fulcrums on the two lever structures  175   a  and  175   b  are distanced from each other, two supporting structures  165   a  and  165   b  are needed. The supporting structures  165   a ,  165   b  are coated with a metal which is the same material as the ground electrodes  164   a ,  164   b , thereby being able to serve as a ground electrode. 
   The second ends of the first and second lever structures  175   a ,  175   b , which are the opposite ends of the first ends are connected to the optical shutter  167  by third elastic structures  173   a ,  173   b , respectively. 
   Here, the movable mass  166  having a width limited by the ground electrodes  164   a ,  164   b  arranged at both sides of the movable mass  166  should be connected to the second ends of the first and second lever structures  175   a ,  175   b . However, there is a difficulty in connecting the second ends of the first and second lever structures  175   a ,  175   b  to the movable mass  166  by second elastic structures  172   a ,  172   b , respectively, because the movable mass  166  has a narrow width. 
   To solve this problem, with reference to  FIG. 6 , there is provided with an extended structure  166   a  which is arranged in parallel with the first and second lever structures  175   a ,  175   b  and has the same length as the total of the lengths of the first and second lever structures  175   a ,  175   b . Both outward ends of the extended structure  166   a  are connected to the first ends of the first and second lever structures  175   a ,  175   b , respectively. 
   It is preferable that the first and second lever structures  175   a ,  175   b  are arranged bilaterally symmetrically on a virtual line connecting the optical shutter  167  and the center of the movable mass  166 . 
   As described above, the MEMS variable optical attenuator in accordance with the present invention satisfies a need of small size as well as a need of a large displacement distance of the optical shutter, which is compatible with a large MFD of the optical collimator installed at the optical signal transmitting end or the optical signal receiving end of the optical fiber. Accordingly, the MEMS variable optical attenuator of the present invention may precisely attenuate an optical power of the optical beam by the desired amount even in the case that the optical signal transmitting end or the optical signal receiving end of the optical fiber has an optical collimator. 
   Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.