Abstract:
A MEMS oscillating laser scanning unit (LSU) composed of a MEMS Control Module, a Pre-scan Module and a Post-scan Module is disclosed. The MEMS Control Module consists of a laser source and a MEMS oscillating mirror. The laser source and the MEMS oscillating mirror both are aligned with the same side, opposite to target surface so that laser beam emits from the side of the target surface, reverses by a reflection mirror of the Pre-scan Module and then moves along a plane formed by a central axis as well as an oscillatory rotary axis of the MEMS oscillating mirror, enters center of the MEMS oscillatory mirror. Thus, scanning spots on the target surface are all symmetrical to the central axis. Thus effective area of the MEMS oscillating mirror is reduced and further reduce the cost as well as improve scanning efficiency. Moreover, design of the fθ Lens is simpler and the volume of the LSU is reduced.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to a Micro Electronic Mechanical System (MEMS) oscillating laser scanning unit (LSU), and more particularly, to a laser scanning unit that optically scans laser light and projects to target object drum used in a laser printer, a scanner, and a multi-function printer (MFP) using the same. 
         [0002]    Most of LSU available now uses a polygonal mirror rotating at high speed to control reflection direction of laser beam. However, due to working rotational speed limits, high manufacturing cost, high noises and crawling start-up, such LSU is unable to meet requirements of high speed and high precision. 
         [0003]    In recent years, torsion oscillators are getting known yet are not progressively applied to LSU of an imaging system, a scanner, a laser printer or a multi-function printer (MFP). The main cause is they still have some problems such as resonant frequency instability. However, the MEMS (micro electronic mechanic system) oscillatory mirror developed based on principle of torsion oscillators has higher scanning efficiency than conventional polygon mirror. Due to advantages of compact, light, rugged and fast resonance frequency, it is expected that the polygon mirror is going to be replaced by MEMS oscillating mirror in near future. 
         [0004]    Refer to  FIG. 1  &amp;  FIG. 2 , in a laser scanning unit (LSU) a Micro Electronic Mechanical System (MEMS) oscillating mirror mainly includes circuit board, torsion oscillators and reflection mirror. The reflection mirror driven by resonance magnetic field oscillates along X-axis with Y-axis as axis of symmetry. When a laser beam emits to the reflection mirror surface of the MEMS oscillating mirror, the MEMS oscillating mirror reflects the incident laser beam toward the Z-axis at different angles along with different rotating angles of the mirror surface that changes with time. Thus features, of high resolution and large rotation angle are achieved. Therefore, it has been applied broadly such as in U.S. Pat. No. 5,408,352, U.S. Pat. No. 5,867,297, U.S. Pat. No. 6,947,189, U.S. Pat. No. 7,190,499, TW Patent M253133 and JP 2006-201350. 
         [0005]    There are two placements for laser beam incident to the polygon mirror or the MEMS oscillating mirror, respectively having its shortcomings: 
         [0000]    (1) laser light is obliquely incident to the polygon mirror or the MEMS oscillating mirror, as shown from  FIG. 1  to  FIG. 4 : 
         [0006]    Refer to Taiwanese Patent No. M253133, U.S. Pat. No. 7,184,187, U.S. Pat. No. 7,190,499, U.S. Pat. No. 6,956,597 and US Pub. App. No. 2006/0050346, in the devices disclosed, the laser beam is obliquely focused onto the polygon mirror or the MEMS oscillating mirror. In the US Pub. App. No. 2006/0033021, the laser beam is reflected by a reflection mirror and then is obliquely incident to the MEMS oscillating mirror (or polygon mirror). There are two concerns that result in deviation of the reflected laser beam. The first concern is assembly tolerance between laser source and MEMS oscillating mirror (or polygon mirror) that leads to the inconsistence incident angle. Furthermore, after scanning through the polygon mirror or the MEMS oscillating mirror, deviation of the scanning beam is generated. The prior techniques to deal with this are to calibrate the emitting angle of light with the laser source by a plurality times of precise alignment. That&#39;s waste time and money. The second concern is the relationship between the scanning angle and time. After being reflected by the polygon mirror, the relationship between the scanning angle of the laser beam and time is linear. However, after being reflected by the MEMS oscillating mirror, the relationship between the scanning angle and time is intrinsic non-linear. Refer from  FIG. 1  to  FIG. 4 , the laser beam P 1  reflected by a reflection mirror of a Pre-scan Module and then is obliquely incident to the MEMS oscillating mirror P 2  for reflective scanning. Then the scanning beam P 3  enters the fθ or f-sin θ lens P 4  and projects onto a target surface P 5  for performing scanning. Because incident angle of the scanning beam P 3  on right and left sides of a central axis P 6  are different while entering the fθ or f-sin θ lens P 4 , this is called deviation of the Y axis, as shown in  FIG. 4 , θ 1 ≠θ 2 . The prior techniques way to eliminate the deviation is by means of various curved surfaces that form optical surfaces on the right and left sides. A linear fθ lens is designed and is manufactured for compensation, as disclosed in U.S. Pat. No. 6,330,524 or TW Patent No. I250781. Yet there is still problems of skew or bow generated. Refer to U.S. Pat. No. 6,232,991, the prior art is tried to solve the bow. However, both difficulties in manufacturing of the lens and cost are increased. 
         [0000]    (2) laser light is frontal incident to the polygon mirror or the MEMS oscillating mirror: 
         [0007]    Refer to JP Patent No. 08-334716, JP Patent No. 2006-276133, U.S. Pat. No. 6,690,498, and US Pub. App. No. 2.007/0002446, the laser light through the reflection mirror is frontal incident to the polygon mirror. But the polygon mirror, generally is hexagonal mirror, is disposed on outer edge of the rotary axis. Once the laser light is frontal incident to the polygon mirror, the distance between each point on the mirror and the rotary axis is unequal so that reflective point of the laser beam is not the same point. This causes deviation of the Y axis. Moreover, refer to US2006/0279826, although the laser light is directly focused into the MEMS oscillating mirror. Because the MEMS oscillating mirror is a prism, the laser beam with a Gaussian distribution projects into top of the oscillatory prism and is reflected into two light beams. Due to displacement of the top of the prism, the reflected light beam is with new Gaussian distribution. And the reflective point as well as size of the reflected light beam changes along with movement of the reflection mirror. 
         [0008]    Offset in Y axis will lead to asymmetry of spots to the central axis of the MEMS oscillating mirror. Thus cause different resolution on the right and left sides of the scanning image. A fθ or f-sin θ lens may be used to form different optical surface for right and left sides for compensation. However, there are still problems of skew or bow, as mentioned in U.S. Pat. No. 6,232,991. As to light spot deviation, it is unable to be compensated by means of optical surface formed by the fθ lens. 
         [0009]    In addition, the LSU applied to color printers or scanners requires four sets of scanning optical elements for displaying four colors-black, magenta, yellow and cyan. For example, a device disclosed in US 2006/027982 includes two sets of laser sources and two sets of MEMS oscillating mirror. Refer to Taiwanese Patent No. I268867, the device revealed consists of four sets of laser sources and four sets of MEMS oscillating mirror. Due to high cost of MEMS oscillating mirror, there is a need to develop a colorful laser scanner with only one MEMS oscillating mirror. 
       SUMMARY OF THE INVENTION 
       [0010]    Therefore it is a primary object of the present invention to provide a MEMS oscillating laser scanning unit consisting of a MEMS Control Module, a Pre-scan Module, a Post-scan Module and a housing. The MEMS Control Module is composed of a laser source, and a MEMS oscillating mirror. The laser source as well as the MEMS oscillating mirror are arranged on the same side, opposite to target surface so that laser beam incidents in reverse direction by a reflection mirror of a Pre-scan Module, along a plane formed by a central axis and an oscillatory rotary axis of the MEMS oscillating mirror, enters center of the MEMS oscillatory mirror. Then the reflected laser beam enters fθ Lens set inside the said Post-scan Module in a scanning way symmetrical to the central axis of the MEMS oscillating mirror, and size of the spots of laser beam is symmetrical to the axis of the MEMS oscillating mirror. Thus design of the fθ lens set may be simplified and the volume of the device may be reduced. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a schematic drawing showing top view of an MEMS oscillating LSU of a prior art; 
           [0012]      FIG. 2  is a perspective view of another MEMS oscillating LSU of a prior, art; 
           [0013]      FIG. 3  is a perspective view of a further MEMS oscillating LSU of a prior art; 
           [0014]      FIG. 4  is a schematic drawing showing asymmetrical laser beam formed by the MEMS oscillating mirror in  FIG. 3 ; 
           [0015]      FIG. 5  is a schematic drawing showing a side view of an embodiment (single color) according to the present invention; 
           [0016]      FIG. 6  is a schematic drawing showing upper part of a top view of the embodiment in  FIG. 5 ; 
           [0017]      FIG. 7  is a schematic drawing showing lower part of a top view of the embodiment in  FIG. 5 ; 
           [0018]      FIG. 8  is a perspective view of the embodiment in  FIG. 5 ; 
           [0019]      FIG. 9  is a perspective view showing the laser beam in the embodiment in  FIG. 5  is projected directly into the MEMS oscillating mirror; 
           [0020]      FIG. 10  is a perspective view showing a symmetrical laser beam formed by the MEMS oscillating mirror of the embodiment in  FIG. 5 ; 
           [0021]      FIG. 11  is a schematic view showing a side view of a reflection cylinder lens in the embodiment (single color) in  FIG. 5 ; 
           [0022]      FIG. 12  is a schematic view showing a side view of another embodiment (multiple color) according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0023]    Refer from  FIG. 5  to  FIG. 10 , a MEMS oscillating LSU according to the present invention comprises of a MEMS control module  1 , a Pre-scan Module  2 , a Post-scan Module  3 , and a housing  4 . The MEMS control module  1  comprises of a laser source  11 , a MEMS oscillating mirror  12 , a sensor  14  and a control board (printed circuit board)  13  while the Pre-scan Module  2  comprises a collimator lens  21 , a cylinder lens  22 , and a reflection mirror  23 . The present invention is characterized in that: the laser source  11  and the MEMS oscillating mirror  12  are disposed on the same side, opposite to a target surface  5  so that laser light  111  emitted from the laser source  11  passes the collimator lens  21  to form parallel light beam, through the cylinder lens  22  for being focused, and then being projected onto the reflection mirror  23 , as shown in  FIG. 5  &amp;  FIG. 6 . Next, direction of the laser light  111  is reversed by the reflection mirror  23  so as to form a laser beam  112 . The laser beam  112  incidents along a plane (Y-Z plane) formed by a central axis  121  (Z axis) of the MEMS oscillating mirror  12  and an oscillatory rotary axis  123  (Y axis) of the MEMS oscillating mirror, enters and focus onto the center  122  of the MEMS oscillatory mirror  12 . After being scanned, the laser beam  112  becomes into a scanning beam  113  that enters into a fθ Lens  31  ( 32 ) of the a Post-scan Module  3 , as shown in  FIG. 5  &amp;  FIG. 7 . 
         [0024]    Refer to  FIG. 5 ,  FIG. 6  &amp;  FIG. 7 , the reversed direction means the axis of the laser beam  112  from the reflection mirror  23  to the center  122  of the MEMS oscillatory mirror  12 ; and the axis of the laser light  111  from the laser source  11 , through the collimator lens  21  or the cylinder lens  22  to the reflection mirror  23  are located on the same Y-Z plane, without x-axis deviation. 
         [0025]    The Post-scan Module comprises fθ Lens  31  ( 32 ) and a Synchronizing Mirror ( 34 ). The fθ Lens  31  ( 32 ) is used to covert the Scanning Beam formed by the MEMS oscillating mirror  12  into an Imaging Beam  114  in which the scanning angle and time are converted linearly. The image is formed on a target surface  5 . An Synchronizing mirror  33  ( 34 ) is for reflecting the Synchronizing scanning beam  115 / 116  out of image range of the target surface  5  back to the MEMS Control Module  1 , as shown in  FIG. 7 . The sensor  14  ( 15 ) turns the reflected light beam into electrical signal that is processed and transmitted by the MEMS Control Module  1 . Moreover, the fθ Lens  31  ( 32 ) can be designed into a single piece type, a plurality piece type having a first fθ lens  31  and a second fθ lens  32 , as shown in the figure. Similarly, the Synchronizing mirror  33  ( 34 ) can be a single piece type, a plurality piece type having a first Synchronizing mirror  33  and a second Synchronizing mirror  34 , as shown in the  FIG. 6  &amp;  FIG. 7 . The number of the sensor  14  ( 15 ) is corresponding to the number of the Synchronizing mirror  33  ( 34 ). The sensor  14  ( 15 ) can be a single one, two sensors, corresponding to the first sensor  14  and the second sensor  15 , and is disposed on the MEMS Control Module  1 . The housing  4  is used to accommodate of all components, locate the components and isolate the components for maintaining their positions and precision. 
         [0026]    The relationship between clear aperture D of the MEMS oscillating mirror and beam size of incident laser light d is as following: 
         [0000]    
       
         
           
             
               
                 
                   
                     D 
                     = 
                     
                       d 
                       
                         sin 
                          
                         
                           ( 
                           Φ 
                           ) 
                         
                       
                     
                   
                   , 
                 
               
               
                 
                   ( 
                   I 
                   ) 
                 
               
             
           
         
       
     
         [0027]    Wherein, Φ is the angle between the laser beam  112  and the MEMS oscillating mirror  12 ; 
         [0028]    Hence, this invention, the laser beam  112  is vertically projected to the MEMS oscillating mirror  12  so that is the angle Φ is close to 90 degrees is and D is close to d. Thus the reflective surface of the MEMS oscillating mirror  12  can be quite small size to elevate the reliability. On the other hand, once the laser light is obliquely incident into the MEMS oscillating mirror  12 , the angle Φ is less than 90 degrees and the clear aperture D of the MEMS oscillating mirror  12  is larger than d. Thus the reflective surface of the MEMS oscillating mirror  12  can&#39;t be diminished size. 
         [0029]    The present invention has at least following advantages: 
         [0000]    (1) As shown in  FIG. 11 , asymmetry problem arises when the laser beam  111  is obliquely incident to the MEMS oscillating mirror  12  realized as enlarged spots or difficulty in optical design; instead of this invention, the laser beam  111  is frontal incident to the MEMS oscillating mirror  12  leading in symmetry along the z axis.
 
(2) The clear aperture (D) of the MEMS oscillating mirror  12  is smaller than the effective diameter (D) of the design of obliquely incident to the MEMS oscillating mirror. Thus manufacturing cost of the MEMS oscillating mirror  12  is reduced. Moreover, the scanning frequency is also accelerated due to reduction of the reflection surface and elevated the reliability.
 
(3) Because the laser source  11 , the MEMS oscillating mirror  12  and the sensor  14  ( 15 ) are all arranged on the same side so that they can be assembled on one Control board  13  to form an integrated MEMS Control Module  1 . Therefore, manufacturing, assembling, calibrating and maintenance operation can be simplified and the cost is reduced more effectively.
 
         [0030]    Standard assembling and aligning procedures of the MEMS oscillating LSU with a MEMS Control Module  1  composed of a laser source  11 , a MEMS oscillating mirror  12 , a control board  13  and a sensor  14  include following steps: 
         [0000]    assembling in alignment of the laser source  11 , the MEMS oscillating mirror  12 , the control board  13  and the sensor  14  ( 15 ) according to designed angles and positions; and then adjust the laser source  11  as well as the collimator lens  21  by optical instruments for calibration to form a calibrated module;
 
calibrating the collimator lens  21  and the cylinder lens  22  for aligning with the reflection mirror  23 ;
 
adjusting reflection angle of the reflection mirror  23  so as to make the laser light incident in reverse direction and then to perform calibration so as to make the laser beam incidents along a plane (Y-Z plane) defined by a central axis  121  (Z-axis) of the MEMS oscillating mirror  12  and an oscillatory rotary axis  123  (Y-axis) of the MEMS oscillating mirror  12  and enters a center  122  of the MEMS oscillating mirror  12 ;
 
then adjusting the central axis of the fθ Lens  31  (such as the first fθ Lens  31  and the second fθ Lens  32 ) for aligning with a central axis of the MEMS oscillating mirror  12  and adjust an axial surface of the fθ Lens  31  for aligning with reflective surface of the MEMS oscillating mirror  12 ;
 
at last, adjusting the Synchronizing mirror  33  ( 34 ) and the sensor  14  ( 15 ) for aligning with each other so that the laser light is reflected to the sensor  14  ( 15 ) on the Control board  13 .
 
         [0031]    The assembling method as mentioned above has at least following advantages: 
         [0000]    (1) The complicated and repeated calibration of conventional assembling way is avoided so that both assembling and calibration (alignment) are more convenient and fast.
 
(2) The alignment of the MEMS Control Module  1  with the collimator lens  21  is not affected by volume of the LSU so that the module can be calibrated in advance before being assembled. Thus assembling of the LSU is more fast and convenient.
 
(3). As to colorful LSU, laser lights emitted from a plurality of sets of laser sources (as shown in  FIGS. 11 ,  11   a ˜ 11   d ) are reversed and are projected to the MEMS oscillating mirror  12 . Thus it takes only one MEMS oscillating mirror  12  to scanning the four colors. The four colors MEMS Control Module  1  can be calibrated before assembled. Therefore, cost of optical elements is reduced dramatically.
 
         [0032]    Refer to  FIG. 8 , said the cylinder lens  22  and said the reflection mirror  23  can be integrated in designed a reflection cylinder lens  24 . One side of the reflection cylinder lens  24  is concave cylindrical lens while the other side is coated with reflective film so that it has both reflecting and focusing functions. While being assembled, the reflection cylinder lens  24  is aligned so as to make the laser beam  112  move along the plane (Y-Z plane) defined by the central axis (Z-axis)  121  of the MEMS oscillating mirror  12  and the oscillatory rotary axis (Y-axis)  123  of the MEMS oscillating mirror  12  and enters the center  122  of the MEMS oscillating mirror  12 . Because the reflection cylinder lens  24  has functions of the cylinder lens  22  as well as the reflection mirror  23  so that it can effectively shorten light path with fewer optical elements. Thus not only volume of the LSU is correspondingly reduced but also cost is saved. 
         [0033]    The position for disposition of the MEMS oscillating mirror  12  is located on the same side of the laser source  11  (the X-Y plane), same placement of Z-axis. The MEMS oscillating mirror  12  and the laser source  11  can be arranged on the same control board  13  or respectively arranged on the same side of different Control board  13 . 
         [0034]    While designing the LSU, the position and angle of each optical element arranged inside the housing  4  are determined according to the optical path. That means according to calculation results of the optical path, slots  41  or pedestals  42  of the optical elements are preset inside the housing  4 , as shown in  FIG. 5 . Thus each optical element is mounted on each slot  41  or the pedestal  42  so that they can be assembled quickly and located remaining within tolerance. 
         [0035]    The MEMS oscillating mirror  12  oscillates on resonant frequency that is easy to be affected by temperature. Thus heat generated by the fθ lens  31  inside the MEMS oscillating LSU of the present invention should be released. The pedestal  42  of the fθ lens  31  in the housing  4  is made by metal with high heat dissipation efficiency such as aluminum and is connected with a base of the metal housing  4  so that heat generated by the fθ lens  31  is conducted through the aluminum pedestal  42  to the housing  4  for dissipation. 
         [0036]    Refer to  FIG. 12 , a MEMS oscillating LSU of the present invention applied to color laser printers or scanners includes a precision housing  4  for accommodating the MEMS Control Module  1 , a Pre-scan Module  2 , a Post-scan Module  3 , and other elements. The MEMS Control Module  1  is composed of a Control board  13 , laser sources  11   a ˜ 11   d  and a MEMS oscillating mirror  12 . The Pre-scan Module  2  is composed of a plurality of collimator lenses  21 , a plurality of cylinder lenses  22 , and a plurality of reflection mirrors  23 ; the Post-scan Module  3  is composed of a plurality of fθ lenses  31   a ˜ 31   d . The laser sources  11   a ˜ 11   d  and the MEMS oscillating mirror  12  are disposed on the same side, opposite to target surfaces  5   a ˜ 5   d , and are respectively above or below the MEMS oscillating mirror  12 . 
         [0037]    Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details, and representative devices shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.