Abstract:
The invention features the drawbridge assembly and its applications in optical switches, optical crossconnects, optical add/drop multiplexers and variable optical attenuators. In optical switches and optical crossconnects, an array of the drawbridge assemblies can be used to redirect the multiple input lights to multiple outputs. In add/drop multiplexers, the drawbridge assemblies can select the light channels to be added and dropped. In the attenuator embodiment, a vertical mirror is inserted into two fibers, the first one as the input and the second one as output. The drawbridge assembly controls the position of the vertical mirror for blocking a certain portion of the light and enabling the attenuation. The continuous change of the mirror position results in variable attenuation. A series of VOA form a multi-channel VOA system on a single substrate.

Description:
RELATED APPLICATIONS  
       [0001]    This application claims the priority of U.S. Provisional Patent Application No. 60/313,766, filed on Aug. 20, 2001, entitled “Optical Crossconnect and Mirror Systems,” to Ai Qun Liu, Xuming Zhang, Vadakke Matham Murukeshan, and Chao Lu, the contents of which are incorporated by reference. 
     
    
     
       TECHNICAL FIELD  
         [0002]    This invention relates to optical mirror systems and their applications in optical switches, optical crossconnects, optical add/drop multiplexers and variable optical attenuator systems.  
         BACKGROUND  
         [0003]    Dense wavelength division multiplexing (DWDM) systems and all-optical networks are in rapid development to provide high-speed signal processing in their native optical form without the need for expensive and time delaying conversions. The require the use of optical switches and optical crossconnect devices to redirect light signals and facilitate switching. Optical add/drop multiplexers are also key components in optical networks to add and drop certain light channels. In addition, single and multi-channel variable optical attenuators (VOAs) have wide applications in fiber-optic communication systems, for example, in limiting and regulating the power in fibers, equalizing power levels of different wavelength channels in dense wavelength division multiplexed (DWDM) systems, flattening the gain of optical amplifiers, and balancing the signals in optical add/drop multiplexers (OADMs).  
         SUMMARY  
         [0004]    According to one aspect of the invention, an optical mirror system includes a substrate; a reflective assembly attached to the substrate; an actuating mechanism attached to the substrate; and a drawbridge assembly mechanically coupled to the reflective assembly where upon activation of the actuating mechanism, the reflective assembly moves between a first position in which the reflective assembly is in a non-reflective state and a second position in which the reflective assembly is in a reflective state.  
           [0005]    One or more of the following features may also be included. The drawbridge assembly includes a holding plate; at least one drawing member having two ends, a first end fixedly attached to the substrate; and a biasing mechanism for mechanically coupling the holding plate to the reflective assembly, wherein the actuating mechanism causes the biasing mechanism to bend the reflective assembly. The holding plate mechanically supports the biasing mechanism and is connected to a second end of the at least one drawing member.  
           [0006]    In certain embodiments, the reflective assembly includes a mounting plate and a vertical mirror mechanically coupled to the mounting plate. Further, the non-reflective state of the reflective assembly forms a non-inclined configuration of the drawbridge assembly maintained by a resting state of the biasing mechanism, and the reflective state of the reflective assembly forms an inclined configuration of the drawbridge assembly caused by the actuating mechanism and a non-resting state of the biasing mechanism.  
           [0007]    As yet another feature, the biasing mechanism includes a flexible structure for shaping the reflective state and the non-reflective state of the optical assembly.  
           [0008]    According to another aspect of the invention, an optical crossconnect system includes a light beam traveling along a path; at least one optical mirror system having a substrate, a reflective assembly attached to the substrate, and an actuating mechanism attached to the substrate. The optical crossconnect also includes a drawbridge assembly mechanically coupled to the reflective assembly where upon activation of the actuating mechanism, the reflective assembly moves between a first position in which the reflective assembly is in a non-reflective state and a second position in which the reflective assembly is in a reflective state.  
           [0009]    The optical crossconnect further includes at least one output fiber for emitting the light beam; at least one input fiber for receiving the light beam, where the light beam passes through the optical system and the path of the light beam is determined by an reflective state and a non-reflective state of the optical mirror system.  
           [0010]    One or more of the following features may also be included. The optical crossconnect system further includes an array of collimating lenses and an array of coupling lenses for signal coupling and collimation of the light beam.  
           [0011]    In certain embodiments, the optical crossconnect system also includes a scalable configuration having a plurality of rows and columns.  
           [0012]    As yet another feature, the drawbridge assembly of the optical mirror system includes a holding plate; at least one drawing member having two ends, a first end fixedly attached to the substrate; and a biasing mechanism for mechanically coupling the holding plate to the reflective assembly where the actuating mechanism causes the biasing mechanism to bend the reflective assembly.  
           [0013]    Embodiments may have one or more of the following advantages.  
           [0014]    In optical fiber crossconnect systems, optical mirrors having a drawbridge assembly provide superior benefits in switching.  
           [0015]    The combination of MEMS and optical technologies utilizes existing miniaturization technologies to fabricate the optical mirror systems. The use of optical mirrors in optical crossconnect systems provides the advantages of compactness, low driving voltage and current, low power consumption, compatibility with existing IC processes, low insertion loss, and a higher switch time. Furthermore, their use eliminates the drawbacks of large size, wobbling, and mechanical instability.  
           [0016]    In particular, the flexible configuration of the reflective and non-reflective states of the system maintains the vertical mirror in an uplifted position thereby minimizing mechanical failure in the reflective and non-reflective switching positions. Important benefits are also achieved by eliminating the movement of the drawbridge assembly. Optical switching is accomplished without movement of the fixed components attached to the substrate. Consequently, the optical mirror system requires no additional actuators, thus reducing the number of elements required.  
           [0017]    Another benefit is the scalability of the optical crossconnect systems. Forming optical crossconnect systems of large arrays can be easily and efficiently achieved, providing low power consumption and lower switch time.  
           [0018]    The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
       
    
    
     DESCRIPTION OF DRAWINGS  
       [0019]    [0019]FIG. 1( a ) is a perspective view of an optical mirror system in a non-reflective state.  
         [0020]    [0020]FIG. 1( b ) is a side view of the optical mirror system of FIG. 1( a ).  
         [0021]    [0021]FIG. 2( a ) is a perspective view of an optical mirror system in a reflective state.  
         [0022]    [0022]FIG. 2( b ) is a side view of the optical mirror system of FIG. 2( a ).  
         [0023]    [0023]FIG. 3 is a perspective view of an optical mirror system in a non-reflective state with an alternate biasing mechanism.  
         [0024]    [0024]FIG. 4 is a perspective view of the optical mirror system of FIG. 3 in a reflective state.  
         [0025]    [0025]FIG. 5 is a diagrammatic view of an optical crossconnect system.  
         [0026]    [0026]FIG. 6( a ) is a perspective view of a single VOA.  
         [0027]    [0027]FIG. 6( b ) is a diagrammatic view of a position relationship of the mirror and the output fiber.  
         [0028]    [0028]FIG. 7 is a diagrammatic view of a multi-channel VOA. 
     
    
     DETAILED DESCRIPTION  
       [0029]    To better understand the invention, it is helpful to clarify the meaning of certain terms. The term “optical mirror system” refers generally to the complete optical system, while the term “vertical mirror” refers to the reflective mirror mounted on the surface of the optical mirror system. Therefore, the optical mirror system includes a vertical mirror.  
         [0030]    The structural components of the optical mirror system and the optical crossconnect system will be described first, with the accompanying FIGS.  1 - 7 .  
         [0031]    Referring to FIG. 1, an optical mirror system  115  includes a substrate  100 , a drawbridge assembly  116 , and a reflective assembly  117 . The drawbridge assembly  116  includes a holding plate  140 , a biasing mechanism  130 , and a drawing member  150 . The reflective assembly  117 , which is mounted on the upper surface of the substrate  100 , includes a vertical mirror  110  and a mounting plate  120 . The optical mirror system  115  further includes a locking mechanism in the form of anti-disintegration interlockers  160  for mechanically connecting the drawbridge assembly  116  to the upper surface of substrate  100 . An electrode  170  serving as the actuating mechanism of the optical mirror system is also shown.  
         [0032]    In one embodiment, the biasing mechanism  130  which defines the configuration of the drawbridge assembly  116 , includes a flexible structure in the form of bending beams  130  or springs  230 .  
         [0033]    In FIG. 1( a ), a perspective view of the optical mirror system  115  in a first position in a non-reflective state is shown for a system  115  having a bending biasing mechanism  130 . In the reflective assembly  117 , the vertical mirror  110  is mechanically coupled to the mounting plate  120  by a microhinge  113  and is fixedly maintained in a vertical position by a head holder  111 . The head holder  110  includes a first free end  111 ( a ) and a second fixed end  111 ( b ). The first free end  111 ( a ) has a groove region for securely holding the vertical mirror  110  in an upright vertical position, and a second fixed end  111 ( b ) is mechanically coupled to the mounting plate  120  by a micro-hinge  114 . The vertical mirror  110  further includes a reflective part  112  which is deposited with metals to increase light reflectance.  
         [0034]    In the embodiment of FIG. 1( a ), the mounting plate  120  is mechanically connected to the bending biasing mechanism  130  of the drawbridge assembly  116 . The biasing mechanism  130  is formed by bending beams which connect the mounting plate  120  to the holding plate  140 . The holding plate  140  is mechanically coupled to the upper surface of the substrate  100  by at least one microhinge  142 . Similarly, the drawing member  150  is also mechanically coupled to the upper surface of substrate  100  by at least one microhinge  152 .  
         [0035]    Specifically, in this embodiment, an elongated drawing beam forms the drawing member  150  having a latching mechanism for mechanically coupling with the upper surface of the substrate  100  and the holding plate  140 . One end of the drawing member  150  has a T-shaped head  151  which mechanically couples with a T-shaped aperture  141  of the holding plate  140 , thereby forming a non-inclining configuration of the optical mirror system  115  in a non-reflective state. As seen in FIG. 1( b ), this non-reflective configuration is formed by the holding plate  140  being held at a non-reflective angle relative to the upper surface of the substrate  100 .  
         [0036]    In this first position in a non-reflective state, the vertical mirror  110  and the mounting plate  120  are positioned sufficiently above the upper surface of the substrate  100  so that a spacing between the mounting plate  120  and the substrate  100  is formed. In this spacing, an input light beam  180  can pass through to form an output light beam  181 .  
         [0037]    In the optical mirror system  115 , the electrode  170  forming the actuating mechanism of the system is mounted on the upper surface of substrate  100  and positioned below the mounting plate  120 . This way, the mounting plate  120  and the vertical mirror  110  are mechanically moved simultaneously in response to the actuating effect of the electrode  170 .  
         [0038]    In this embodiment, the optical mirror system  115  further includes a locking mechanism in the form of an anti-disintegration interlocker  160  for mechanically connecting the drawbridge assembly  116  to the upper surface of substrate  100 . The anti-disintegration interlocker  160  are employed to prevent the drawbridge assembly  116  from disintegrating when the optical mirror system  115  is in dynamic operation. The interlocker  160  includes a latching beam  161  which protrudes from the holding plate  140  and a pressing beam  162  which is securely hinged to the substrate  100 .  
         [0039]    Referring to FIGS.  2 ( a ) and  2 ( b ), FIG. 2( a ) is a perspective view of the optical mirror system  115  in a second position in a reflective state. FIG. 2( b ) shows the optical mirror system  115  on a side view for better appreciation of its reflective state. The reflective assembly  117 , namely the mounting plate  120 , is shown in a downward inclined position relative to its resting state, with the vertical mirror  110  also shown in an inclined configuration. On the other hand, the drawbridge assembly  116  is shown as having a similar configuration as in the first position in a non-reflective state. In particular, the drawbridge assembly  116  is maintained by the holding plate  140  and the drawing member  150 , whereas the bending biasing mechanism  130  causes the reflective assembly  117  to incline downward.  
         [0040]    [0040]FIG. 3 shows another embodiment of the optical mirror system in the first position in a non-reflective state having an alternate spring biasing mechanism  230 . An optical mirror system  216  includes a vertical mirror  210  mechanically coupled to a mounting plate  220  by at least one microhinge  213 . The vertical mirror  210  is maintained in a vertical position by a holder  211 .  
         [0041]    The vertical mirror  210  is mechanically coupled to the mounting plate  220  by a microhinge  213  and is fixedly maintained in a vertical position by a head holder  211 . The head holder  211  includes a first free end  211 ( a ) and a second fixed end  211 ( b ). The first free end  211 ( a ) has a groove region for securely holding the vertical mirror  210  in an upright vertical position, and the second fixed end  211 ( b ) is mechanically coupled to the mounting plate  220  by a microhinge  214 . The vertical mirror  210  further includes a reflective part  212  to maximize light reflectance.  
         [0042]    The mounting plate  220  is mounted on the substrate  200  by extended plates  221 , flexible beams  260 , and anchors  261 . The mounting plate  220  is further connected to the holding plate  240  by a spring biasing mechanism  230 . The holding plate  240  is mechanically coupled to the substrate  200  by a microhinge  242 . The drawing member  250  is also mechanically coupled to the substrate  200  by a microhinge  252 . The microhinge  251  is coupled to the T-shaped aperture  241  in the holding plate  240 . This way, the vertical mirror  210  is lifted by the structure of the drawbridge assembly  216  which includes the spring biasing mechanism  230 , the holding plate  240 , the extended plate  221 , the flexible beam  260 , the anchor  261 , and the drawing member  250 . An electrode  270  serves as the electrostatic actuating mechanism in this embodiment.  
         [0043]    In the non-reflective state of the optical mirror system  215 , when the vertical mirror  210  is lifted, an input light beam  280  passes through the spacing formed between the lifted vertical mirror  210  and mounting plate  220 , and the electrode  270 , to form the output light beam  281 . The mounting plate  220  inclines the drawbridge assembly  216  downward, and any disintegration of the drawbridge assembly  216  is unlikely to occur in dynamic operation if the lengths and widths of the extended plate  221  and the flexible beams  260  are properly selected.  
         [0044]    Referring to FIG. 4, the vertical mirror  210  in FIG. 4 is shown in the second position in a reflective state, with corresponding changes in the configuration of the drawbridge assembly  216  and reflective assembly  217 . In this reflective state, the input light beam  280  transmitted to the optical mirror system  215  is reflected by the reflective part  212  of the vertical mirror  210  and redirected to a different direction in the form of the output light beam  282 .  
         [0045]    [0045]FIG. 5 illustrates a non-blocking N to N, free-space optical crossconnect system  315  using an array of optical mirror systems  310  disposed in columns and rows. In certain embodiments, each of the optical mirror systems has a drawbridge assembly  116  (not shown) and a reflective assembly  117  (not shown). The optical mirror systems  310  are used for redirecting the input optical beams  312 . The optical crossconnect system  315  includes a matrix  300  formed by a plurality of optical mirror systems  310 , a plurality of optical fibers  311  for inputting the light beams, a plurality of optical fibers  321  for outputting the light beams, and a micro-lens array  312  and  322  for collimating and coupling the light beams.  
         [0046]    The collimated light beams passes through the non-reflective optical mirror systems  302  and are redirected by the reflective state optical mirror systems  301  to the output fibers  321 , where the micro-lens array  322  are employed to couple the light signals into the N output fibers  321 . The fibers  331  along with the collimating lenses  332  are used to drop the light beams. For example, one light beam from the input fibers  311  can be directly coupled to one fiber of the fibers  331  if all the optical mirror systems in its light path are in the non-reflective state. Similarly, the fibers  341  and the lenses  342  are employed to add the light beams to the output fibers  321 . This optical crossconnect system  315  provides the functions of both the cross-connection and add/drop multiplexing.  
         [0047]    This configuration of the optical crossconnect system  315  can be easily scaled by using additional rows and columns of optical mirror systems  310  due to their small size. The systems  310  can be of the type shown in FIGS.  1 - 4 . A typical size of an optical mirror system  310  is 0.8 mm×0.8 mm, having a switch time of 0.2 seconds.  
         [0048]    Now the operational features of the optical mirror system  115  and  215  as illustrated in FIGS.  1 - 4  will be described.  
         [0049]    The operation of the optical mirror system  115  and the functional interconnections among the various elements of the system  115  is explained.  
         [0050]    The holding plate  140  and the drawing members  150  operate integrally to drive the drawbridge assembly  116 . The holding plate  140  having the T-shaped aperture  141  is connected to the substrate  100  by the micromachined hinge  142  forming a rotational axis for the vertical displacement and support of the holding plate  140 . When the T-shaped head  151  of the drawing beam  150  are latched into the T-shaped aperture  141 , the holding plate  140  can support the bending biasing mechanism  130  at various angles, forming a drawbridge assembly  116 .  
         [0051]    The angle of the various inclinations of the biasing mechanism  130  and the holding plate  140  is determined by the length of the drawing member  150 , the distance from the T-shaped aperture  141  to the rotational axis of the holding plate  140 , and the separation of the rotational axis of the holding plate  140  and the drawing member  150 . Moreover, the width of the drawing member  150  and the holding plate  140  may also affect the inclination angle of the bending biasing mechanism  130  and the holding plate  140 .  
         [0052]    In this example, the use of the bending biasing mechanism  130  significantly influences the configuration of the resulting drawbridge assembly  116  and the optical mirror system  115 . In this embodiment, a plurality of bending beams are used to mechanically couple the mounting plate  120  to the holding plate  140 . The bending beam has the same thickness as the mounting plate  120  but the width of the bending beam is different than the width of the mounting plate  120 . Due to this configuration, most of the deformation is concentrated on the bending beams while the mounting plate is displaced vertically. The width and length of the bending beams also determine the degree of tension and inflexibility as well as the optical switch time, driving voltage, and driving current. The deformation caused by the vertical mirror  110  and the mounting plate  120  are negligible and the bending beams can support a sufficient amount of deformation caused by the actuating mechanism in addition to the weight of the reflective assembly  117 .  
         [0053]    For the operation of light switching, referring to FIGS.  1 ( a ),  1 ( b ),  2 ( a ) and  2 ( b ), the vertical mirror  110  mounted on the mounting plate  120  is supported by the drawbridge assembly  116  and positioned over the upper surface of the substrate  110  to reflect incoming light beams. A spacing between the upper surface of substrate  100  and the mounting plate  120  allows the reflective assembly  117  to move vertically, enabling the vertical mirror  110  to be displaced in different positions over the substrate  100 . When an electrostatic force such as a driving force is applied between the electrode  170  and the mounting plate  120  by an electrostatic or electromagnetic actuating mechanism, the entire reflective assembly  117  inclines downward, displacing the vertical mirror  110  vertically and causing the optical mirror system  115  to enter a reflective state (FIG. 2).  
         [0054]    Consequently, the vertical mirror  110  enters the path of the input light beam  180  and the reflective assembly  117  redirects the input light beam  180  to the output light beam  182 , thereby achieving optical switching. Optical attenuation is also accomplished as the reflective function of the vertical mirror controls the light energy which is reflected and transmitted to an output light beam. In other words, changing the position of the vertical mirror  110  after partially redirecting the light beam, different light energies of light reflection and transmission are obtained, leading to optical attenuation.  
         [0055]    Subsequently, when the voltage is removed, the reflective assembly  117  returns to its resting state by the recovery force in the bending beams  130 . In this non-reflective state, the non-inclined configuration of the reflective assembly  117  causes incoming light beams to pass through the optical mirror system  115  unchanged in direction, as illustrated in FIG. 1. In short, optical switching and optical attenuation are implemented by adding and removing a driving voltage to the actuating mechanism mounted on the surface of the substrate  100 . This in turn, induces the inclined and non-inclined configurations of the reflective assembly  117  and  217  illustrated in FIGS.  1 - 4 .  
         [0056]    Various actuation mechanisms can be employed to drive the vertical mirror  110  such as electrostatic actuation, electromagnetic actuation, and thermal actuation. In particular, the electrostatic and electromagnetic modes provide the high dynamic response and the low power consumption desirable in generating large array crossconnect systems. Moreover, electrostatic and electromagnetic mechanisms include the advantages of low heating, easy fabrication, compatibility with existing IC process, high tolerability to environmental factors, and particularly, high dynamic response in operation. Although only the electrostatic actuating mechanisms are illustrated in the embodiments, electromagnetic actuating mechanisms can equally be implemented within the optical systems shown.  
         [0057]    The anti-disintegration interlocker  160  is employed to prevent the disintegration of the drawbridge assembly  116  in dynamic operation. In particular, the anti-disintegration interlocker  160  prevents the T-shaped head  151  of the drawing member  150  from collapsing and separating from the T-shaped aperture  141  of the holding plate  140 .  
         [0058]    The operation of the optical mirror system  215  using an alternate biasing mechanism  230  is described. In this embodiment, springs form the biasing mechanism  230  coupling the mounting plate  220  to the holding plate  240  so that the reflective assembly  217  may reflect the optical light beams. Because springs may not maintain a static inclination of the mounting plate  220 , one end of the mounting plate is anchored to the substrate  200  by flexible beams or attached to the substrate  200  by micromachined hinges.  
         [0059]    Generally, the operation of the optical mirror system  215 , as illustrated in FIGS. 3 and 4, is similar to the operation of the optical mirror system  115  shown in FIGS. 1 and 2.  
         [0060]    In the reflective state of the optical mirror system  215 , the reflective assembly  217  is displaced vertically and inclined downward by a driving voltage between the mounting plate  220  and the electrode  270 . In the drawbridge assembly  216 , the spring biasing mechanism  230  is extended with the vertical displacement of the mounting plate  220 . Consequently, as the input light beam  280  is reflected by the vertical mirror  210 , the input light beam  280  is redirected to the output light beam  282 .  
         [0061]    Thereafter, the electrostatic actuating mechanism or voltage is removed and the elastic force in the spring biasing mechanism  230  lifts the mounting plate  220  upward to its resting state, thereby moving the vertical mirror  210  vertically upward. This way, in the non-reflective state of the optical mirror system  215 , the reflective part  212  of the vertical mirror  210  is removed from the path of the input light beam  270 .  
         [0062]    If an actuating mechanism in the form of an electromagnetic actuator is used, a coil is formed on the mounting plate  220  while an external magnetic field is applied using a magnet. The driving current passing through the coil generates a magnetic field which interacts with the external field and drives the mounting plate  220  to displace vertically.  
         [0063]    Referring to FIG. 5, the operation of the free space MEMS-based optical crossconnect system  315  is described. Generally optical crossconnect systems are made in waveguide. However, MEMS-based optical crossconnect systems have advanced rapidly improving the fabrication process of optical crossconnect systems. Compared with its waveguide counterparts, MEMS-based optical crossconnect systems operate in free space and provide high switching contrast, low insertion loss, small crosstalk, tolerance to wavelength and polarization, transparency to data format and speed, compactness and low cost.  
         [0064]    In optical crossconnect systems, the vertical mirrors determine the direction and path of the light beams which pass unmoved and intact without carrying information about data or speed. In contrast, the optical signals operate interferometrically and/or diffractively in waveguide crossconnect systems, thereby strongly relying on the wavelength and polarization. Moreover, nonlinear effects such as Four Wave Mixing (FWM) and Self-Phase Modulating (SPM), influence the transmission quality for different data format and speed.  
         [0065]    The operation of the free space optical crossconnect system  315  begins when light beams from a plurality of input optical fibers  311  are collimated by micro-lens array  312 . The collimated light beams pass through the non-reflective state optical mirror systems  302  and are reflected and redirected by the reflective state optical mirror systems  301  toward a desired light path. Meanwhile, the micro-lens array  322  couple the light signals into a plurality of output optical fibers  321 . The fibers  331  and the collimating lenses  332  are employed to drop the light beams from the input fibers  311 , and the fibers  341  along with the lenses  342  are used to add the light beams to the output fibers  321 .  
         [0066]    Furthermore, if a larger array of optical mirror systems is required, the optical crossconnect system can be easily scaled by forming additional rows and columns of the optical mirror systems, whose small size makes this a concise operation.  
         [0067]    The method of fabricating the optical mirror systems of the present invention is described next. The bulk micromachining and the surface micromachining technologies are the main methods used to fabricate MEMS components, including the vertical mirrors and the actuating mechanisms used in the present invention. In bulk micromachining technology, the surface of the vertical mirror  110  and  210  is directly formed by deep etching of the silicon wafer. The bulk method takes advantage of the property that the etching rate is dependent on the crystalline direction and the doping concentration while etching single crystalline silicon wafers. The surface micromachining method deposits the structural layers sandwiched by the individual sacrificial layers. After the sacrificial layers have been etched (e.g. etching SiO2 by HF), the desired components are released.  
         [0068]    The bulk micromachining is able to fabricate MEMS components with a large thickness (limited by the wafer thickness). However, the component structure in the vertical direction should be simple. In contrast, although the surface micromachining method is capable of fabricating components with complex vertical structures, the thickness of the components is limited to several microns.  
         [0069]    In fact, the vertical mirrors  110  and  210  of FIGS.  1 - 4 , the actuating mechanism, as well as other surface structures of the optical mirror system may be fabricated not only by bulk micromachining technology but also by surface micromachining methods. Regardless of the fabrication and structural defining processes used, the surface of the vertical mirror  110  and  210  should to be coated with a metal layer to increase its reflectivity.  
         [0070]    Referring to FIGS.  6 ( a ) and  6 ( b ), a single VOA system  415  includes a substrate  400 , a drawbridge assembly  420  and two optical fibers  492  and  495 . The drawbridge assembly includes a mirror  412  attached and deposited at the end of a L-shaped plate  410 . The plate  410  is maintained in vertical position by a head holder  411 . A light signal  493  is transmitted in the fiber core  491  of the input fiber  492 . When the light signal  493  enters the free space, the light signal  493  is partially blocked by the mirror  412 . Only a portion of light  496  is coupled into the core  494  of the output fiber  495 . By applying the voltage between the drawbridge assembly  420  and the electrode  470 , the position of the mirror  412  can be finely controlled, resulting in variable attenuation.  
         [0071]    Referring to FIG. 7, a multi-channel VOA system  515  includes a series of VOA system  501  and a substrate  500 . The VOA system  501  includes a drawbridge assembly  502 , an input fiber  503 , and an output fiber  504 .  
         [0072]    A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.