Abstract:
Provided is a Fabry-Perot type wavelength tunable optical filter, comprising a first mirror; a second mirror located over the first mirror; a driving body located over the first mirror, and having both ends fixed to the first mirror through a spacer; a plurality of electrodes, each formed on both ends of the driving body; a rod structure connecting a center of the driving body and the second mirror; a plurality of fixing means, each fixed to the first mirror at both sides of the rod structure through the spacer; and a plurality of elastic bodies connecting the rod structure with the plurality of fixing means and acting as a rotational axis. And the mirror is driven by the rod structure acting as a lever that has an elastic body as a rotational axis, when warping is generated by electro-thermal expansion, electromagnetic force or external force. Thereby the mirror can be driven in the larger wavelength tunable range and the low power consumption.

Description:
BACKGROUND 
   1. Field of the Invention 
   The present invention generally relates to a wavelength tunable optical filter that adjusts a distance between two mirrors to make a desired wavelength of light incident or output and, more specifically, to a Fabry-Perot type wavelength tunable optical filter. 
   2. Discussion of Related Art 
   In general, a driving method of a wavelength tunable optical filter is largely classified into two categories. One is to adjust a distance between mirrors by a force applied to the mirrors and to provide a restoration force by a structure connected to the mirror as in an electrostatic scheme (C. J. Chang-Hasnain,  IEEE J Select. Topics Quantum Electron. , vol. 4, pp. 978–987, 2000), and the other is by a deformation of the driving body that is connected to the mirror as in a thermal expansion scheme (C. K. Kim et al.,  Proceeding of IEEE/LEOS Optical MEMES Conference  2003, pp. 38–39, 2003), an electromagnetic scheme (C. K. Kim, Korean Patent Application No. 10-2003-0029763), and an external mechanical force scheme (N. Yokouchi et al.,  IEEE Photon. Technol. Lett. , vol. 4, pp. 701–703, 1992). 
   A wavelength tuning range of the wavelength tunable optical filter is determined by the maximum displacement of the driving body. For the electrostatic driving scheme to which attraction between two mirrors is applied, when the distance between two mirrors is reduced to less than ⅔ of the initial gap, a sticking phenomenon occurs, thus restricting the driving range to the ⅓ of the initial gap. To overcome this, a structure driven in a direction that the distance between the mirrors becomes farther is introduced. One method for this is that one fixed mirror, one moving mirror combined with an electrode, and a fixed electrode are arranged one after another, and a distance between the moving mirror and the fixed electrode is made larger than the distance between two mirrors, thus enabling a larger wavelength tuning range (D. C. Flanders, U.S. Pat. No. 6,341,039), and another method is that a lever type in which a mirror and an electrode unit are placed to be isolated from each other at both sides of a long rod structure is employed to increase a driving range as well as not to incur the sticking phenomenon (C. F. R. Mateus et al.,  IEEE Photon. Technol. Lett. , vol. 14, pp. 819–821, 2002). 
   The structure in which the rod type driving body is deformed by means of the thermal expansion, the electromagnetic force or the external mechanical force also employs the physical deformation of the driving body only, thus limiting the displacement of the mirror. 
   SUMMARY OF THE INVENTION 
   The present invention is directed to a wavelength tunable optical filter in which a mirror is driven by a rod structure acting as a lever that has an elastic body as a rotational axis, when warping of an actuator is generated by thermal expansion, electromagnetic force or external mechanical force. 
   The present invention is also directed to a Fabry-Perot wavelength tunable optical filter with which power consumption is reduced and a driving in both directions can be enabled in one device. 
   One aspect of the present invention is to provide a wavelength tunable optical filter comprising: a first mirror; a second mirror located over the first mirror to match an optic axis; a driving body located over the first mirror, having both ends fixed to the first mirror through a spacer; a plurality of electrodes, each formed on both ends of the driving body; a rod structure connecting a center of the driving body and the second mirror; a plurality of fixing means, each fixed to the first mirror at both sides of the rod structure through the spacer; and a plurality of elastic bodies connecting the rod structure with the plurality of fixing means and acting as a rotational axis. 
   Another aspect of the present invention is to provide a wavelength tunable optical filter comprising: a first mirror; a second mirror located above the first mirror to match an optic axis; a driving body located over the first mirror, having both ends fixed to the first mirror through a spacer; a plurality of electrodes, each formed on both ends of the driving body; a rod structure passing through a center of the driving body and connected with the second mirror at one end; and a fixing means fixed to the first mirror through the spacer, and fixing the other end of the rod structure to act as a rotational axis. 
   Still another aspect of the present invention is to provide a wavelength tunable optical filter comprising: a first mirror; a second mirror located above the first mirror to match an optic axis; a first and a second driving bodies located over the first mirror in parallel with each other, having both ends fixed to the first mirror through a spacer; a plurality of electrodes, each formed on both ends of the first driving body and the second driving body; and a rod structure passing through a center of the first driving body and the second driving body and connected with the second mirror at one end. 
   Still another aspect of the present invention is to provide a wavelength tunable optical filter further comprising: a third driving body and a fourth driving body located over the first mirror in parallel with each other, having both ends fixed to the first mirror through a spacer; a plurality of electrodes, each formed on both ends of the third driving body and the fourth driving body; and a rod structure passing through a center of the third driving body and the fourth driving body and connected with the second mirror at one end. 
   Still another aspect of the present invention is to provide a wavelength tunable optical filter comprising: a first mirror; a second mirror located over the first mirror whose optic axis is aligned with that of the first mirror; at least two of rod structures located over the first mirror and symmetrically connected to the second mirror; at least two of driving bodies connected to each rod structure, and having both ends fixed to the first mirror; a plurality of fixing means fixed to the first mirror at both sides of each rod structure; and a plurality of elastic bodies connecting the rod structure with the plurality of fixing means and acting as a rotational axis. 
   Still another aspect of the present invention is to provide a wavelength tunable optical filter comprising: a first mirror; a second mirror located over the first mirror whose optic axis is aligned with that of the first mirror; at least two of rod structures located over the first mirror and symmetrically connected to the second mirror; at least two of driving bodies connected to each rod structure, and having both ends fixed to the first mirror; and a fixing means fixed to the first mirror, and fixing the other end of each rod structure to act as a rotational axis. 
   Still another aspect of the present invention is to provide a wavelength tunable optical filter comprising: a first mirror; a second mirror located over the first mirror whose optic axis is aligned with that of the first mirror; at least two of rod structures located over the first mirror and symmetrically connected to the second mirror; at least two of first and second driving bodies connected to each rod structure, and having both ends fixed to the first mirror; and a fixing means fixed to the first mirror, and fixing the other end of each rod structure to act as a rotational axis. 
   A distance between the first mirror and the second mirror is determined by a thickness of the spacer. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other features of the present invention will be described in reference to certain exemplary embodiments thereof with reference to the attached drawings in which: 
       FIG. 1  is a structural diagram of a wavelength tunable optical filter according to a first embodiment of the present invention; 
       FIGS. 2A to 2C  are cross sectional views for illustrating a behavior of a wavelength tunable optical filter according to a first embodiment of the present invention; 
       FIG. 3  is a structural diagram of a wavelength tunable optical filter according to a second embodiment of the present invention; 
       FIGS. 4A to 4C  are cross sectional views for illustrating a behavior of a wavelength tunable optical filter according to a second embodiment of the present invention; 
       FIG. 5  is a structural diagram of a wavelength tunable optical filter according to a third embodiment of the present invention; and 
       FIG. 6  is a structural diagram of a wavelength tunable optical filter according to a fourth embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   The preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings. 
     FIG. 1  is a structural diagram of a wavelength tunable optical filter according to a first embodiment of the present invention, wherein an electro-thermal driving body and a mirror are connected to both ends of a rod structure that acts as a lever, respectively. 
   An upper mirror  10  and a lower mirror  11  are spaced apart from each other by an air layer interposed therebetween. An optic axis of the upper mirror  10  and the lower mirror  11  are aligned. For the upper mirror  10  separated by the predetermined distance from the lower mirror  11 , electro-thermal driving bodies  13   a ,  13   b  fixed to the lower mirror  11  through spacers  12   a ,  12   b  are placed in both sides of the one end of a rod structure  15 , and electrodes  14   a ,  14   b  are formed on the surfaces of outer ends in the electro-thermal driving bodies  13   a ,  13   b , respectively. A center of the electro-thermal driving body  13  and the upper mirror  10  are connected by a rod structure  15 . At both sides of the rod structure  15 , fixing means  16   a ,  16   b  fixed to the lower mirror  11  through the spacers  17   a ,  17   b  are placed, and the fixing means  16   a ,  16   b  are connected to the rod structure  15  through rotational axes  18   a ,  18 b made of an elastic body, respectively. An initial distance between the upper mirror  10  and the lower mirror  11  is determined based on the thickness of the spacer  12   a ,  12   b ,  17   a  and  17   b.    
     FIGS. 2A to 2C  are cross sectional views for illustrating a behavior of a wavelength tunable optical filter according to a first embodiment of the present invention. 
   To provide a better understanding of a driving principle, only the electro-thermal driving bodies  13   a ,  13   b , the rod structure  15 , the rotational axes  18   a ,  18   b  and the upper mirror  10  are schematically illustrated. 
   When a voltage is not applied to the electrodes  14   a ,  14   b , a current does not flow through the electro-thermal driving bodies  13   a ,  13   b . Therefore, as shown in  FIG. 2A , the electro-thermal driving bodies  13   a ,  13   b , the rod structure  15 , the rotational axes  18   a ,  18   b  and the upper mirror  10  remain stable. 
   When the voltage is applied to the electrodes  14   a ,  14   b  to flow the current through the electro-thermal driving bodies  13   a ,  13   b , the electro-thermal driving bodies  13   a ,  13   b  undergo the thermal expansion. Here, outer ends of the electro-thermal driving bodies  13   a ,  13   b  are fixed to the lower mirror  11  through the spacers  12   a ,  12   b , so that the center of electro-thermal driving bodies  13   a ,  13   b , that is, the portion connected to the rod structure  15  are warped upward or downward according as the difference between the thermal expansion coefficients or the internal stresses, and the middle of the rod structure  15  are fixed through the elastic bodies  18   a ,  18   b , so that the upper mirror  10  connected to the rod structure  15  and opposite to the electro-thermal driving bodies  13   a ,  13   b  is driven in a direction opposite to the center of the electro-thermal driving bodies  13   a ,  13   b  through the lever principle.  FIG. 2B  is for a case where the upper mirror  10  is driven downward to become closer to the lower mirror  11  as the center of the electro-thermal driving bodies  13   a ,  13   b  warps upward, and  FIG. 2C  is for a case where the upper mirror  10  is driven upward to become farther to the lower mirror  11  as the center of the electro-thermal driving bodies  13   a ,  13   b  warps downward. 
   Although this embodiment has been illustrated with reference to the thermally expansible driving body, the driving body warped by an electromagnetic force to the current by an external magnetic field, or the driving body warped by applying a physical force to the rod can be applied. 
     FIG. 3  is a structural diagram of a wavelength tunable optical filter according to a second embodiment of the present invention, in which the electro-thermal driving body is connected in the middle of the rod structure. 
   An upper mirror  20  and a lower mirror  21  are spaced apart from each other by an air layer interposed therebetween. An optic axis of the upper mirror  20  and the lower mirror  21  are aligned. For the upper mirror  20  separated by the predetermined distance from the lower mirror  21 , fixing means  23  is placed, which is fixed to the lower mirror  21  through a spacer  22 . The fixing means  23  and the upper mirror  20  are connected by a rod structure  24 . At both sides of the rod structure  24  between the fixing means  23  and the upper mirror  20 , the electro-thermal driving bodies  25   a ,  25   b  are connected respectively, and the other end of each electro-thermal driving bodies  25   a ,  25   b  is fixed to the lower mirror  21  through the spacers  26   a ,  26   b , and electrodes  27   a ,  27   b  are formed respectively on the surfaces of the ends of the electro-thermal driving bodies  25   a ,  25   b . An initial distance between the upper mirror  20  and the lower mirror  21  can be adjusted based on the thickness of the spacer  22 ,  25   a ,  25   b . Preferably, for the electro-thermal driving bodies  25   a ,  25   b , the portion connected to the rod structure  24  is formed narrower than the portion fixed to the spacers  26   a ,  26   b.    
     FIGS. 4A to 4   c  are cross sectional views for illustrating a behavior of a wavelength tunable optical filter according to a second embodiment of the present invention. 
   To provide a better understanding of a driving principle, only the fixing means  23 , the rod structure  24 , the electro-thermal driving bodies  25   a ,  25   b  and the upper mirror  20  are schematically illustrated. 
   When a voltage is not applied to the electrodes  27   a ,  27   b , a current does not flow through the electro-thermal driving bodies  25   a ,  25   b . Therefore, as shown in  FIG. 4A , the fixing means  23 , the rod structure  24 , the electro-thermal driving bodies  25   a ,  25   b  and the upper mirror  20  remain stable. 
   When the voltage is applied to the electrodes  27   a ,  27   b  to flow the current through the electro-thermal driving bodies  25   a ,  25   b , the electro-thermal driving bodies  25   a ,  25   b  undergo the thermal expansion. Each of the outer ends of the electro-thermal driving bodies  25   a ,  25   b  is fixed to the lower mirror  21  through the spacers  26   a ,  26   b , so that the center of electro-thermal driving bodies  25   a ,  25   b , that is, the portion connected to the rod structure  24  are warped upward or downward according as the difference between the thermal expansion coefficients or the internal stresses. Since one end of the rod structure  24  is fixed to the fixing means  23 , the rod structure  24  overall warps in the same direction, and accordingly, the upper mirror  20  connected to the rod structure  24  is also driven in the same direction as the electro-thermal driving bodies  25   a ,  25   b.    
     FIG. 4B  is for a case where the upper mirror  20  is driven upward to become farther to the lower mirror  21  as the center of the electro-thermal driving bodies  25   a ,  25   b  connected to the rod structure  24  warps upward, and  FIG. 4C  is for a case where the upper mirror  20  is driven downward to become closer to the lower mirror  21  as the center of the electro-thermal driving bodies  25   a ,  25   b  connected to the rod structure  24  warps downward. 
   In this embodiment, one end of the rod structure  24  is fixed to the fixing means  23  to serve as a rotational axis, and the upper mirror  20  is driven according to the movement of the electro-thermal driving bodies  25   a ,  25   b  connected to the middle of the rod structure  24 . 
   According to the first and second embodiments, the movement direction of the electro-thermal driving body is determined during the fabrication process, and only one direction between the direction that the mirrors become closer and the direction that the mirrors become farther can be driven. Therefore, according to the present invention, a wavelength tunable optical filter is configured with which both the direction that the mirrors become closer and the direction that the mirrors become farther can be driven. 
     FIG. 5  is a structural diagram of a wavelength tunable optical filter according to a third embodiment of the present invention, in which the mirrors can be driven both in the direction that the mirrors become closer and in the direction that the mirrors become farther. 
   An upper mirror  31  and a lower mirror  32  are spaced apart from each other by an air layer interposed therebetween. An optic axis of the upper mirror  31  and the lower mirror  32  are aligned. For the upper mirror  31  separated by the predetermined distance from the lower mirror  32 , electro-thermal driving bodies  33   a ,  33   b ,  37   a  and  37   b  fixed to the lower mirror  32  through spacers  34   a ,  34   b ,  38   a  and  38   b  are placed in both sides, in parallel with each other, and electrodes  35   a ,  35   b ,  39   a  and  39   b  are formed on the surface of outer ends of the electro-thermal driving bodies  33   a ,  33   b ,  37   a  and  37   b , respectively. Centers of the electro-thermal driving bodies  33   a ,  33   b ,  37   a  and  37   b  and the upper mirror  31  are connected by a rod structure  36 . An initial distance between the upper mirror  31  and the lower mirror  32  is determined based on the thickness of the spacer  34   a ,  34   b ,  38   a  and  38   b.    
   The mirror of the wavelength tunable optical filter fabricated as described above can be driven in both directions. Two electro-thermal driving bodies formed by a single fabrication process on one substrate warp in the same direction due to the current flow because the inherent stress is the same. Therefore, when the electro-thermal driving body is fabricated to warp upward, if the current flows to the electro-thermal driving bodies  33   a ,  33   b  farther to the upper mirror  31 , the electro-thermal driving bodies  37   a ,  37   b  closer to the upper mirror  31  serve as a rotational axis, so that the upper mirror  31  is driven downward by a lever principle, while if the current flows to the electro-thermal driving bodies  37   a ,  37   b  closer to the upper mirror  31 , the electro-thermal driving bodies  33   a ,  33   b  farther to the upper mirror  31  serve as a rotational axis, so that the upper mirror  31  is driven upward. 
   Contrary to this, when the electro-thermal driving body is fabricated to warp downward, if the current flows to the electro-thermal driving bodies  33   a ,  33   b  farther to the upper mirror  31 , the electro-thermal driving bodies  37   a ,  37   b  closer to the mirror  31  serve as a rotational axis, so that the upper mirror  31  is driven upward by a lever principle, while if the current flows to the electro-thermal driving bodies  37   a ,  37   b  closer to the upper mirror  31 , the electro-thermal driving bodies  33   a ,  33   b  farther to the upper mirror  31  serve as a rotational axis, so that the upper mirror  31  is driven downward. 
     FIG. 6  is a structural diagram of a wavelength tunable optical filter according to a fourth embodiment of the present invention. 
   In the first, second, and third embodiments, the rod structure is connected to only one side of the mirror. Therefore, when the rod structure is driven by the lever principle, parallelism between two mirrors can be broken, so that in order to prevent this, the rod structures are symmetrically connected to both sides of the mirror, respectively, as shown in  FIG. 6 , and the electro-thermal driving bodies are connected to the end and the middle of each rod structure as shown in  FIG. 5 . 
   Further, at least two rod structures can be connected to one mirror, and the electro-thermal driving bodies can be connected to each rod structure as described in the first, second and the third embodiments to be driven while keeping the mirrors in parallelism. 
   As described above, according to the present invention, a driving range of the mirror is expanded using a lever principle. When the mirror is driven by thermal expansion, physical expansion or external force without lever structure, it moves as much as the displacement of the center of the driving body, while by the lever principle as described in the present invention, the range of the tunable wavelength can be expanded. The structure using the thermal expansion or the electromagnetic force has a drawback in that it has larger power consumption than the structure driven by the electrostatic force, however, with the lever principle, the driving body requests a little movement to drive the designated wavelength tuning range, thus significantly reducing the power consumption. 
   Further, the wavelength tunable optical filter of the present invention capable of driving upward and downward can expand the wavelength tuning range twice than before, and adjust the current to determine the driving direction. Generally, for the wavelength tunable optical filter using the rotation, the insertion loss and the line width are increased due to the increasing angle between two mirrors, thus degrading an optical characteristic, while in the present invention, at least two driving bodies are symmetrically arranged to keep two mirrors in parallel for every driving range and the optical characteristics can remain the same.