Patent Publication Number: US-2015077824-A1

Title: Optical deflecting mirror device having figure &#34;8&#34;-shaped rib and optical deflector

Description:
This application claims the priority benefit under 35 U.S.C. §119 to Japanese Patent Application No. JP2013-191876 filed on Sep. 17, 2013, which disclosure is hereby incorporated in its entirety by reference. 
     BACKGROUND 
     1. Field 
     The presently disclosed subject matter relates to an optical deflector used in an optical scanner for a projector, a headlamp, a bar code reader, a laser printer, a laser head amplifier, a head-up display unit and the like, and, more particularly, to its optical deflecting mirror device. 
     2. Description of the Related Art 
     Recently, optical deflectors used in optical scanners have been micro electro mechanical system (MEMS) devices manufactured by semiconductor manufacturing technology and micro machine technology. 
     In  FIG. 6 , which is a rear-side perspective view illustrating a first prior art optical deflector (see:  FIG. 16  of JP2001-249300), an optical deflecting mirror device  101  constructed by only a single mirror is supported by torsion bars  102 - 1  and  102 - 2  to a support frame (not shown). Also, provided between the support frame and the torsion bars  102 - 1  and  102 - 2  are actuators (not shown) serving as cantilevers. Thus, the optical deflecting mirror device  101  can be rocked around an X-axis by the actuators. 
     In  FIG. 6 , since the thickness of the optical deflecting mirror device  101  is the same as that of the torsion bars  102 - 1  and  102 - 2 , the optical deflecting mirror device  101  is very thin. As a result, the moment of inertia of the optical deflecting mirror device  101  is so small that the resonant frequency “f” of the optical deflecting mirror device  101  is very large. For example, when the thickness and diameter of the optical deflecting mirror device  101  are about 40 μm and about 1.2 mm, respectively, the resonant frequency “f” of the optical deflecting mirror device  101  is 30.0 kHz or more (see:  FIG. 4 ). Thus, the resonant frequency “f” of the optical deflecting mirror device  101  becomes higher than a required resonant frequency f R  (=26.5 kHz) in optical scanners for high definition projectors. That is, f&gt;f R  is satisfied, so that the optical deflecting mirror device  101  can be driven at a higher speed than a required speed. 
     In the first prior art optical deflector of  FIG. 6 , however, since the optical deflecting mirror device  101  is very thin, the rigidity of the optical deflecting mirror device  101  is very small. Therefore, when the rocking angle of the optical deflecting mirror device  101  is large, the optical deflecting mirror device  101  is greatly deformed in a bowl shape, so that the dynamic face-deflection peak-to-valley amount D is very large. For example, when the thickness and diameter of the optical deflecting mirror device  101  are about 40 μm and about 1.2 mm, respectively, the dynamic face-deflection peak-to-valley amount D of the optical deflecting mirror device  101  is about 156 nm (see:  FIG. 4 ). Thus, the dynamic face-deflection peak-to-valley amount D of the optical deflecting mirror device  101  becomes larger than a required dynamic face-deflection peak-to-valley amount D R  (=45 nm) in optical scanners for high definition projectors. That is, D&lt;D R  is not satisfied, so that the optical characteristics of reflected light of the optical deflecting mirror device  101  do not satisfy required optical characteristics in optical scanners for high definition projectors. Note that the required dynamic face-deflection peak-to-valley amount D R  is defined by one-tenth of a wavelength (λ=450 nm) of a laser beam irradiated onto optical deflecting mirror devices. 
     In  FIG. 7 , which is a rear-side perspective view illustrating a second prior art optical deflector (see: JP7-092409), an optical deflecting mirror device  201  constructed by only a single mirror is thicker than torsion bars  202 - 1  and  202 - 2 . Therefore, since the optical deflecting mirror device  201  is very thick, the rigidity of the optical deflecting mirror device  201  is very large. As a result, even when the rocking angle of the optical deflecting mirror device  201  is large, the optical deflecting mirror device  201  is slightly deformed in a bowl shape, so that the dynamic face-deflection peak-to-valley amount D is very small. For example, when the thickness and diameter of the optical deflecting mirror device  201  are about 200 μm and about 1.2 mm, respectively, the dynamic face-deflection peak-to-valley amount D of the optical deflecting mirror device  201  is several nm (see:  FIG. 4 ). Thus, the dynamic face-deflection peak-to-valley amount D of the optical deflecting mirror device  201  becomes smaller than the required dynamic face-deflection peak-to-valley amount D R  (=45 nm) in optical scanners for high definition projectors. That is, D&lt;D R  is satisfied, so that the optical characteristics of reflected light of the optical deflecting mirror device  201  satisfy the required optical characteristics in optical scanners for high definition projectors. 
     In the second prior art optical deflector of  FIG. 7 , however, the moment of inertia of the optical deflecting mirror device  201  is so large that the resonant frequency “f” of the optical deflecting mirror device  201  is very small. For example, when the thickness and diameter of the optical deflecting mirror device  201  are about 200 μm and about 1.2 mm, respectively, the resonant frequency “f” of the optical deflecting mirror device  201  is 15 kHz or less (see:  FIG. 4 ). Thus, the resonant frequency “f” of the optical deflecting mirror device  201  becomes lower than the required resonant frequency f R  (=26.5 kHz) in optical scanners for high definition projectors. That is, f&gt;f R  is not satisfied, so that it is impossible to drive the optical deflecting mirror device  201  at a higher speed than the required speed. 
     In  FIG. 8 , which is a rear-side perspective view illustrating a third prior art optical deflector (see:  FIG. 3  of JP2010-128116), an optical deflecting mirror device  301  coupled to torsion bars  302 - 1  and  302 - 2  has a mirror  301   a  with a reflective front surface and a ring-shaped reinforcement rib  301   b  provided on a back surface of the mirror  301   a.  Therefore, the average thickness of the optical deflecting mirror device  301  is larger than that of the torsion bars  302 - 1  and  302 - 2 . 
     In the third prior art optical deflector of  FIG. 8 , due to the presence of the ring-shaped reinforcement rib  301   b,  the rigidity of the optical deflecting mirror device  301  is larger than that of the optical deflecting mirror device  101  of  FIG. 6 , but is smaller than that of the optical deflecting mirror device  201  of  FIG. 7 . On the other hand, due to the presence of the ring-shaped reinforcement rib  301   b,  the resonant frequency “f” of the optical deflecting mirror device  301  is smaller than that of the optical deflecting mirror device  101  of  FIG. 6 , but is larger than that of the optical deflecting mirror device  201  of  FIG. 7 . For example, when the thickness and diameter of the mirror  301   a  are about 40 μm and about 1.2 mm, respectively, and the width and height of the ring-shaped reinforcement rib  301   b  are about 100 μm and about 150 μm, respectively, the dynamic face-deflection peak-to-valley amount D of the optical deflecting mirror device  301  is about 80 nm and the resonant frequency “f” of the optical deflecting mirror device  301  is about 25.6 kHz (see:  FIG. 4 ). 
     In the third prior art optical deflector of  FIG. 8 , however, since no rib is present at the center of the mirror  301   a,  the rigidity of the optical deflecting mirror device  301  is still small. Therefore, when the rocking angle of the optical deflecting mirror device  301  is large, the optical deflecting mirror device  301  is greatly deformed in a bowl shape, so that the dynamic face-deflection peak-to-valley amount D is still large, i. e. , about 80 nm. Thus, the dynamic face-deflection peak-to-valley amount D of the optical deflecting mirror device  301  becomes larger than the required dynamic face-deflection peak-to-valley amount D R  (=45 nm) in optical scanners for high definition projectors. That is, D&lt;D R  is not satisfied, so that the optical characteristics of reflected light of the optical deflecting mirror device  301  do not satisfy the required optical characteristics in optical scanners for high definition projectors. 
     On the other hand, in the third prior art optical deflector of  FIG. 8 , as the ring-shaped reinforcement rib  301   b  is provided at the outer circumference of the mirror  301   a,  the moment of inertia of the optical deflecting mirror device  301  is still large, so that the resonant frequency “f” of the optical deflecting mirror device  301  is still small. For example, the resonant frequency “f” of the optical deflecting mirror device  301  is about 25.6 kHz, as stated above. Thus, the resonant frequency “f” of the optical deflecting mirror device  301  becomes lower than the required resonant frequency f R  (=26.5 kHz) in optical scanners for high definition projectors. That is, f&gt;f R  is not satisfied, so that it is impossible to drive the optical deflecting mirror device  301  at a higher speed than the required speed. 
     In  FIG. 9 , which is a rear-side perspective view illustrating a comparative example of the optical deflector of  FIG. 8 , the optical deflecting mirror device  301  of  FIG. 8  is replaced by an optical deflecting mirror device  301 ′ where a central reinforcement rib  301   c  along a Y-axis is added to the elements of the optical deflecting mirror device  301  of  FIG. 8 . Therefore, the rigidity of the optical deflecting mirror device  301 ′ is larger than the optical deflecting mirror device  301  of  FIG. 8 , to reduce the bowl-like face-deformation of the optical deflecting mirror device  301 ′. Thus, the dynamic face-deflection peak-to-valley amount D is about 12 nm, which becomes smaller than the required dynamic face - deflection peak-to-valley amount D R ; however, the resonant frequency “f” is about 25.3 kHz, which becomes lower than the required resonant frequency f R  (see:  FIG. 4 ). 
     According to the above-described prior art optical deflectors as illustrated in  FIGS. 6 ,  7  and  8  and the comparative example as illustrated in  FIG. 9 , the suppression of the dynamic face-deflection peak-to-valley amount D of an optical deflecting mirror device has a trade-off relationship with the increase of the resonant frequency “f” of the optical deflecting mirror device. 
     SUMMARY 
     The presently disclosed subject matter seeks to solve one or more of the above-described problems. 
     According to the presently disclosed subject matter, an optical deflecting mirror device includes a mirror with a reflective front surface, and a figure “8”-shaped reinforcement rib provided on a rear surface of the mirror symmetrically along a rocking axis of the mirror. The figure “8”-shaped reinforcement rib includes two ring-shaped reinforcement ribs coupled to each other along the rocking axis. 
     Thus, according to the presently disclosed subject matter, the suppression of the dynamic face-deflection peak-to-valley amount D can be realized simultaneously with the increase of the resonant frequency “f”. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other advantages and features of the presently disclosed subject matter will be more apparent from the following description of certain embodiments, taken in conjunction with the accompanying drawings, as compared with the prior art and comparative example, wherein: 
         FIG. 1  is a rear-side perspective view illustrating a first embodiment of the one-dimensional optical deflector according to the presently disclosed subject matter; 
         FIG. 2  is a rear-side perspective view illustrating a second embodiment of the one-dimensional optical deflector according to the presently disclosed subject matter; 
         FIG. 3  is a rear-side perspective view illustrating a third embodiment of the one-dimensional optical deflector according to the presently disclosed subject matter; 
         FIG. 4  is a diagram showing the dynamic face-deflection peak-to-valley amounts and resonant frequencies of the first, second and third embodiments as compared with the prior art and comparative example; 
         FIG. 5  is a front-side perspective view illustrating a two-dimensional deflector to which the first embodiment of the one-dimensional optical deflector of  FIG. 1  is applied; 
         FIG. 6  is a rear-side perspective view illustrating a first prior art optical deflector; 
         FIG. 7  is a rear-side perspective view illustrating a second prior art optical deflector; 
         FIG. 8  is a rear-side perspective view illustrating a third prior art optical deflector; and 
         FIG. 9  is a rear-side perspective view illustrating a comparative example of the third prior art optical deflector. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     In  FIG. 1 , which is a rear-side perspective view illustrating a first embodiment of the one-dimensional optical deflector according to the presently disclosed subject matter, this optical deflector is constructed by an optical deflecting mirror device  1 , a support frame  2  supporting the optical deflecting mirror device  1 , a pair of torsion bars  3 - 1  and  3 - 2  coupled between the support frame  2  and the optical deflecting mirror device  1 , and two pairs of piezoelectric actuators  4 - 1  and  4 - 2 ;  5 - 1  and  5 - 2  serving as cantilevers, coupled between the support frame  2  and the torsion bars  3 - 1  and  3 - 2 , for rocking the optical deflecting mirror device  1  around an X-axis through the torsion bars  3 - 1  and  3 - 2 . The optical deflecting mirror device  1  is coupled to the torsion bars  3 - 1  and  3 - 2  at their coupling portions C 1  and C 2 . 
     Also, the optical deflecting mirror device  1  includes a circular or elliptic mirror  1   a  with a reflective front surface, and a figure “8”-shaped reinforcement rib  1   b  provided on a rear surface of the mirror  1   a  symmetrical with respect to the X-axis serving as a rocking axis. In this case, the figure “8”-shaped reinforcement rib  1   b  includes ring-shaped reinforcement ribs  1   b - 1  and  1   b - 2  coupled to each other along the X-axis. The minimum distance between the figure “8”-shaped reinforcement rib  1   b  and the outer circumference of the mirror  1   a  is about 40 μm. Also, the thickness of the mirror  1   a  is the same as that of the torsion bars  3 - 1  and  3 - 2 , i. e. , about 40 μm. Further, the figure “8”-shaped reinforcement rib  1   b  is about 100 μm wide and about 150 to 200 μm thick. On the other hand, the support frame  2  is about 400 to 500 μm thick. 
     In  FIG. 1 , the central reinforcement rib  301   c  of  FIG. 9  and the farthest portions of the ring-shaped reinforcement rib  301   b  of  FIG. 9  from the X-axis are removed to thereby realize the figure “8”-shaped reinforcement rib  1   b  which has an X-shaped center portion formed by crossed beams sloped at an angle of ±15 to 30 degrees with respect to a Y-axis. Thus, the moment of inertia of the optical deflecting mirror device  1  is much smaller than that of the optical deflecting mirror device  301 ′ of  FIG. 9 . For example, if the sizes of the mirror  1   a  and the figure “8”-shaped reinforcement rib  1   b  are defined as stated above, the resonant frequency “f” of the optical deflecting mirror device  1  is about 26.8 kHz (see  FIG. 4 ), which would satisfy the resonant frequency requirement where f&gt;f R =26.5 kHz. Thus, the optical deflecting mirror device  1  can be driven at a higher speed than the required speed. 
     On the other hand, the rigidity of the optical deflecting mirror device  1  having the figure “8”-shaped reinforcement rib  1   b  is smaller than that of the optical deflecting mirror device  301 ′ of  FIG. 9  having the ring-shaped reinforcement rib  301   b  and the central reinforcement rib  301   c;  however, the reduction of the rigidity is compensated for by the X-shaped center portion of the figure “8”-shaped reinforcement rib  1   b.  Therefore, the reduction of the rigidity is small, so that the increase of the dynamic face-deflection peak-to-valley amount D is small. Actually, the dynamic face-deflection peak-to-valley amount D of the optical deflecting mirror device  1  is about 25 nm (see  FIG. 4 ) which is a little larger than the dynamic face-deflection peak-to-valley amount D (=12 nm) of the optical deflecting mirror device  301 ′ of  FIG. 9 , thus satisfying the dynamic face-deflection peak-to-valley amount requirement where D&lt;D R =45 nm. As a result, the optical characteristics of reflected light of the optical deflecting mirror device  1  would satisfy the required optical characteristics in optical scanners for high definition projectors. 
     Thus, according to the first embodiment, the suppression of the dynamic face-deflection peak-to-valley amount D of the optical deflecting mirror device  1  can be realized simultaneously with the increase of the resonant frequency “f” of the optical deflecting mirror device  1 . 
     In  FIG. 2 , which is a rear-side perspective view illustrating a second embodiment of the one-dimensional optical deflector according to the presently disclosed subject matter, linear reinforcement ribs  1   c - 1  and  1   c - 2  are added to the elements of the optical deflecting mirror device  1  of  FIG. 1 , to realize an optical deflecting mirror device  1 ′. That is, the linear reinforcement rib  1   c - 1  is coupled between the inner ends of the ring-shaped reinforcement rib  1   b - 1  along the X-axis, while the linear reinforcement rib  1   c - 2  is coupled between the inner ends of the ring-shaped reinforcement rib  1   b - 2  along the X-axis. Also, the width and thickness of the linear reinforcement ribs  1   c - 1  and  1   c - 2  are the same as those of the figure “8”-shaped reinforcement rib  1   b.    
     In  FIG. 2 , due to the presence of the linear reinforcement ribs  1   c - 1  and  1   c - 2 , the rigidity of the optical deflecting mirror device  1 ′ is larger than that of the optical deflecting mirror device  1  of  FIG. 1 , so that the dynamic face-deflection peak-to-valley amount D is decreased to about 20 nm as illustrated in  FIG. 4 . On the other hand, since the linear reinforcement ribs  1   c - 1  and  1   c - 2  are arranged along the X-axis (rocking axis), the increase of the moment of inertia of the optical deflecting mirror device  1 ′ as compared with that of the optical deflecting mirror device  1  of  FIG. 1  is small. As a result, the resonant frequency “f” of the optical deflecting mirror device  1 ′ is slightly smaller than that of the optical deflecting mirror device  1  of  FIG. 1 , i. e. , about 26.7 kHz as illustrated in  FIG. 4 . Therefore, in the optical deflecting mirror device  1 ′ of  FIG. 2 , although the resonant frequency “f” is slightly low, the dynamic face-deflection peak-to-valley amount D is improved. Thus, the optical characteristics of reflected light of the optical deflecting mirror device  1 ′ would satisfy the required optical characteristics where D&lt;D R =45 nm in optical scanners for high definition projectors, and the resonant frequency “f” of the optical deflecting mirror device  1 ′ would satisfy the resonant frequency requirement where f&gt;f R =26.5 kHz, thus, driving the optical deflecting mirror device  1 ′ at a higher speed than the required speed. 
       FIG. 3  is a rear-side perspective view illustrating a third embodiment of the one-dimensional optical deflector according to the presently disclosed subject matter. 
     In  FIG. 3 , an optical deflecting mirror device  1 ″ is constructed by the mirror  1   a  and a figure “8”-shaped reinforcement rib  1   b′.  In the figure “8”-shaped reinforcement rib  1   b ′, the ring-shaped reinforcement ribs  1   b - 1  and  1   b - 2  of  FIG. 1  are replaced by ring-shaped reinforcement ribs  1   b ′- 1  and  1   b ′- 2  whose end portions along the X-axis are recessed. Therefore, recess portions  1   b ″- 1  and  1   b ″- 2  are created at coupling portions C 1 ′ and C 2 ′ between the optical deflecting mirror device  1 ″ and the torsion bars  3 - 1  and  3 - 2 . The frontage of the recess portions  1   b ″- 1  and  1   b ″- 2  is about 200 to 250 μm long. As a result, a stress concentration at the coupling portions C 1 ′ and C 2 ′ can be relaxed, to thereby avoid the mechanical destruction thereof. 
     In the optical deflector of  FIG. 1 , note that, when the optical deflecting mirror device  1  is rocked through the torsion bars  3 - 1  and  3 - 2 , a stress concentration occurs at the coupling portions C 1  and C 2 . Concretely, the stress (in this case, the Mises stress) on the rear side of the coupling portions C 1  and C 2  is anisotropically large due to the presence of the figure “8”-shaped reinforcement rib  1   b,  so that a stress slope on the rear side of the coupling portions C 1  and C 2  is large. On the other hand, the stress (in this case, the Mises stress) on the front side of the coupling portions C 1  and C 2  is isotropically small due to the absence of the figure “8”-shaped reinforcement rib  1   b,  so that a stress slope on the front side of the coupling portions C 1  and C 2  is small. As a result, the stress distribution on the rear side of the coupling portions C 1  and C 2  is asymmetrical to the stress distribution on the front side of the coupling portions C 1  and C 2 , so that the coupling portions C 1  and C 2 , particularly, the torsion bars  3 - 1  and  3 - 2  would be mechanically broken. For example, when the rocking angle is larger than 10°, the torsion bars  3 - 1  and  3 - 2  would often be mechanically broken. Similarly, in the optical deflector of  FIG. 2 , the torsion bars  3 - 1  and  3 - 2  would be mechanically broken. 
     Conversely, in the optical deflector of  FIG. 3 , the stress (in this case, the Mises stress) on the rear side of the coupling portions C 1 ′ and C 2 ′ is isotropically large due to the presence of the recess portions  1   b ″- 1  and  1   b ″- 2  opposing the figure “8”-shaped reinforcement rib  1   b ′, so that a stress slope on the rear side of the coupling portions C 1 ′ and C 2 ′ is small. As a result, the stress distribution on the rear side of the coupling portions C 1 ′ and C 2 ′ is symmetrical to the stress distribution on the front side of the coupling portions C 1 ′ and C 2 ′, so that the coupling portions C 1 ′ and C 2 ′, particularly, the torsion bars  3 - 1  and  3 - 2  would not be mechanically broken. 
     In the optical deflector of  FIG. 3 , since the recess portions  1   b ″- 1  and  1   b ″- 2  are provided in the figure “8”-shaped reinforcement rib  1   b ′, the rigidity of the optical deflecting mirror device  1 ″ is slightly decreased to slightly increase the dynamic face-deflection peak-to-valley amount D to about 30 nm as illustrated in  FIG. 4 . Also, the moment of inertia is slightly increased, the resonant frequency “f” is slightly decreased to about 26.7 kHz, as illustrated in  FIG. 4 . However, the optical deflecting mirror device  1 ″ would satisfy the required dynamic face-deflection peak-to-valley amount D where D&lt;D R =45 nm and the required resonant frequency where f&gt;f R =26.5 kHz. 
     The one-dimensional optical deflectors of  FIGS. 1 ,  2  and  3  can be applied to a two-dimensional optical deflector. 
     In  FIG. 5 , which is a front-side perspective view illustrating a two-dimensional optical deflector to which the one-dimensional optical deflector of  FIG. 1  is applied, the support frame  2  of  FIG. 1  is replaced by an inner support frame  2 ′ whose thickness is the same as that of the torsion bars  3 - 1  and  3 - 2 . Additionally, an outer support frame  6  is provided to surround the inner support frame  2 ′. Further, coupled between the outer support frame  6  and the inner support frame  2 ′ are a pair of meander-type piezoelectric actuators  7   a  and  7   b.  In this case, the outer support frame  6  is about 400 to 500 μm thick. Thus, the meander-type piezoelectric actuators.  7   a  and  7   b  can rock the mirror  1   a  around the Y-axis through the inner support frame  2 ′. 
     In  FIG. 5 , instead of the meander-type piezoelectric actuators  7   a  and  7   b,  a pair of outer torsion bars can be coupled between the outer support frame  6  and the inner support frame  2 ′, and two pairs of piezoelectric actuators can be coupled between the outer support frame  6  and the outer torsion bars to rock the inner support frame  2 ′ through the outer torsion bars along the Y-axis. 
     Also, the one-dimensional optical deflector of  FIG. 2  or  3  can be applied to the two-dimensional optical deflector of  FIG. 5 . 
     Further, in the above-described embodiments, only one torsion bar can be provided instead of the pair of torsion bars for rocking the mirror around the X-axis. Similarly, only one meander-type actuator or only one outer torsion bar can be provided instead of the pair of meander-type actuators or the pair of outer torsion bars. 
     Furthermore, in the above-described embodiments, electrostatic actuators or electromagnetic actuators can be provided instead of the piezoelectric actuators. 
     It will be apparent to those skilled in the art that various modifications and variations can be made in the presently disclosed subject matter without departing from the spirit or scope of the presently disclosed subject matter. Thus, it is intended that the presently disclosed subject matter covers the modifications and variations of the presently disclosed subject matter provided they come within the scope of the appended claims and their equivalents. All related or prior art references described above and in the Background section of the present specification are hereby incorporated in their entirety by reference.