Abstract:
An optical pickup device is positioned in confronting relationship to an optical disc for reading an information signal from and/or writing an information signal on the optical disc. The optical pickup device includes an optical unit having at least a light source for emitting a light beam and a photodetector for detecting a light beam returned from the optical recording medium, and a converging system for converging the light beam emitted from the light source onto a signal recording surface of the optical recording medium. The optical unit includes a light guide for guiding the light beam toward the converging system and a light-transmissive optical plate covering the light guide.

Description:
This application is a divisional of the patent application Ser. No. 08/997,327, filed Dec. 23, 1997, now U.S. Pat. No. 6,185,180. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an optical pickup device for reading an information signal from and/or writing an information signal on an optical recording medium such as an optical disc, and an optical disc device which incorporates such an optical pickup device. 
     2. Description of the Prior Art 
     Heretofore, there have been proposed optical recording mediums such as optical discs and optical pickup devices for reading an information signal from such optical recording mediums. The optical pickup devices comprise a light source such as a semiconductor laser and optical components including an objective lens for converging a beam of light emitted by the light source onto a signal recording surface of the optical recording medium, and a beam splitter and other optical elements. The optical pickup devices read an information signal recorded on the signal recording surface by applying a converged beam of light onto the signal recording surface and detecting a beam of light reflected from the signal recording surface with a light detector. 
     An optical disc as an optical recording medium is held at its central area on a disc table in a disc player which incorporates an optical pickup device, and rotated about its own axis by a spindle motor coupled to the disc table. In the disc player, the optical pickup device is positioned so as to direct the objective lens toward the signal recording surface of the optical disc which is rotated. The optical pickup device is supported so as to be movable radially across the optical disc. 
     An information signal is recorded on the optical disc along a spiral pattern of recording tracks that are substantially concentrically arranged on the signal recording surface. When the optical disc is rotated about its own axis, the optical pickup device reads the recorded information signal along the recording tracks. 
     During rotation of the optical disc, the readout position where the optical pickup device reads an information signal from any of the recording tracks is periodically reciprocally moved radially of the optical disc because of an off-center deviation of the center of curvature of the recording track from the center of rotation of the optical disc, i.e., the central axis of the drive shaft of the spindle motor. In addition, the readout position on the signal recording surface is also periodically reciprocally moved perpendicularly to the signal recording surface due to a distortion or planarity error of the disc board of the optical disc. 
     To cope with such an off-center deviation and planarity error, the optical pickup device usually has an objective lens drive mechanism comprising a two-axis actuator. The objective lens drive mechanism supports the objective lens for movement along both an optical axis thereof and a direction normal to the optical axis, i.e., a radial direction of the optical disc. The objective lens drive mechanism moves the objective lens based on signals (a tracking error signal and a focusing error signal) which are representative of deviations or distances of the spot of light converged on the signal recording surface by the objective lens from the recording track in both the radial direction of the optical disc and the direction normal to the signal recording surface. 
     The beam of light emitted from the light source is guided to the objective lens by a light guide mechanism which comprises optical components including a beam splitter and is housed in a cavity defined in an optical unit housing along the optical axis of the objective lens. 
     The objective lens is moved by the objective lens drive mechanism to keep the beam of light focused on the recording tracks at all times so as to follow periodical fluctuations of the recording tracks. 
     As described above, the objective lens is moved by the objective lens drive mechanism in both the direction of the optical axis of the objective lens and the direction normal to the optical axis. The objective lens and the two-axis actuator of the objective lens drive mechanism are usually enclosed by a cover which prevents dust particles from entering and being deposited on the objective lens and the two-axis actuator. The cover has a through hole, whose size is greater than the diameter of the objective lens, defined in its portion in front of the objective lens for passing the beam of light therethrough, and hence the objective lens is exposed through the through hole. The cavity which houses the light guide mechanism is open and spaced from the objective lens by a predetermined distance. Consequently, while the disc player is in use or not in use, i.e., is in storage or shipment, dust particles tend to be deposited on the surface of the objective lens, and find their way into the cavity, possibly resulting in a failure to read the information signal from the optical disc. When dust particles are deposited on the two-axis actuator (which includes a support art, etc.), the objective lens drive mechanism may not be properly operated. 
     Particularly, fine sand particles or dust particles can easily be deposited on the objective lens and the support arm of the two-axis actuator, posing problems on maintaining desired playback characteristics with respect to the optical disc. The surface of the objective lens may be cleaned by a brush or an air blower. However, since fine sand particles or dust particles are liable to stick to the brush, they cannot sufficiently be removed from the surface of the objective lens with the brush. If fine sand particles or dust particles deposited on the surface of the objective lens are dampened, then it is difficult to remove them from the objective lens. 
     When dust particles and fine sand particles enter and are deposited in the cavity of the optical unit housing, it is highly difficult to remove them with the brush or the air blower. 
     In some applications, the disc player described above is incorporated into stationary unitary audio playback systems where the disc player is housed in a common casing shared by an amplifier, a radio tuner, etc. The casing necessarily requires an opening or openings for radiating the heat from the amplifier, the radio tuner, etc., and the optical components in the cavity, particularly the objective lens, cannot be cleaned with ease because the optical pickup device is housed in the casing. Stated otherwise, despite the fact that the entry of dust and dirt into the casing cannot be avoided, it is practically impossible to clean the optical components, particularly the objective lens, of the optical pickup device in the stationary unitary audio playback systems. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide an optical pickup device which prevents dust particles from being deposited on optical components of a light guide mechanism housed in a casing for thereby maintaining good playback characteristics with respect to an optical recording medium. 
     Another object of the present invention to provide an optical disc device which incorporates such an optical pickup device. 
     According to the present invention, there is provided an optical pickup device adapted to be positioned in confronting relationship to an optical recording medium for reading an information signal from and/or writing an information signal on the optical recording medium, comprising an optical unit having at least a light source for emitting a light beam and a photodetector for detecting a light beam returned from the optical recording medium, and a converging system for converging the light beam emitted from the light source onto a signal recording surface of the optical recording medium, the optical unit including a light guide for guiding the light beam toward the converging system and a light-transmissive optical plate covering the light guide. 
     According to the present invention, there is also provided an optical disc device adapted to be positioned in confronting relationship to an optical disc for reading an information signal from and/or writing an information signal on the optical disc, comprising a drive mechanism for rotating the optical disc, an optical pickup comprising an optical unit having at least a light source for emitting a light beam and a photodetector for detecting a light beam returned from the optical recording medium, and a converging system for converging the light beam emitted from the light source onto a signal recording surface of the optical disc, and a moving mechanism for moving the optical pickup with respect to the optical disc in a radial direction of the optical disc, the optical unit including a light guide for guiding the light beam toward the converging system and a transparent optical plate covering the light guide. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a plan view of a disc drive which incorporates an optical pickup device according to the present invention; 
     FIG. 2 is a side elevational view of the disc drive shown in FIG. 1; 
     FIG. 3 is a schematic vertical cross-sectional view of the optical pickup device; 
     FIG. 4 is a perspective view of the optical pickup device, with a shutter being open; 
     FIG. 5 is a perspective view of the optical pickup device, with the shutter being closed; 
     FIG. 6 is an exploded perspective view of the optical pickup device; 
     FIG. 7 is a vertical cross-sectional view of an upper portion of the optical pickup device; 
     FIG. 8 is a perspective view of the optical pickup device and a shutter closing member; 
     FIG. 9 is an enlarged fragmentary sectional front elevational view of the optical pickup device; and 
     FIG. 10 is an enlarged fragmentary sectional side elevational view of the optical pickup device. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In a preferred embodiment of the present invention, an optical pickup device is incorporated in a disc drive for reading an information signal from an optical disc as an optical recording medium. Prior to describing the optical pickup device, the optical disc and the disc drive will first be described below. 
     The optical pickup device according to the embodiment of the present invention is illustrated as being used to read a recorded information signal. However, the optical pickup device may be used to write an information signal on an optical disc. 
     As shown in FIGS. 1 through 3, an optical disc  101  comprises a disc board  103  made of a transparent material such as polycarbonate and a signal recording layer  102  disposed on a principal surface of the disc board  103 . The disc board  103  may have a diameter of 64 mm, 80 mm, or 120 mm, and has a circular chucking hole (positioning hole)  101   a  defined centrally therein. 
     A boundary surface between the disc board  103  and the signal recording layer  102  serves as a signal recording surface. A focused beam of light for reading a recorded information signal is applied through the disc board  103  to the signal recording surface. A spiral pattern of substantially concentric recording tracks is formed on the signal recording surface. If an information signal is recorded as pits, the pits are arranged along the recording tracks. 
     A radially inner position indicated by A on the optical disc  101  is referred to as a lead-in position where the recorded information signal starts to be read, and a radially outer position indicated by B on the optical disc  101  is referred to as a lead-out position where the recorded information signal ends being read. A range R which extends radially from the lead-in position to the lead-out position serves as a signal recording area on the signal recording surface. 
     As shown in FIGS. 1 and 2, the optical pickup device  10  is disposed on a chassis  1  in a disc player for reproducing the recorded information signal from the optical disc, making up a disc drive. The chassis  1  comprises a socalled floating chassis which is mounted by a plurality of dampers  3  on a movable frame  2  which is pivotally attached at its rear end by a shaft  2   a  to a fixed frame (not shown) of the disc player. The movable frame  2  is positioned below a disc tray  4  which is horizontally slid into and out of an outer housing (not shown), and can be angularly moved from a horizontal position to a lower position about the shaft  2   a  by a cam mechanism  5 . 
     A spindle motor  6  is mounted on the chassis  1  near the free end of the movable frame  2  remotely from the shaft  2   a . The spindle motor  6  has a drive shaft  6   a  projecting upwardly of the chassis  1  perpendicularly to the principal surface of the chassis  1 . The drive shaft  6   a  has its proximal end rotatably supported by a bearing (not shown) disposed on the chassis  1 . A substantially circular disc table  7  is mounted on the distal end of the drive shaft  6   a , and has a conical centering pad  7   a  on an upper central area thereof. When the central region of the optical disc  101  is placed on the disc table  7 , the conical centering pad  7   a  is fitted in the chucking hole  101   a  thereby centering the optical disc  101  with respect to the disc table  7 , and a circumferential area of the optical disc  101  around the chucking hole  101   a  is placed on an outer circumferential flange of the disc table  7 . The optical disc  101  thus positioned on the disc table  7  is then chucked by a chucking mechanism  60 , and securely held on the disc table  7 . At this time, the disc board  103  of the optical disc  101  faces downwardly toward the chassis  1 . 
     When the spindle motor  4  is energized after the optical disc  101  is placed on the disc table  7 , the optical disc  101  is rotated in unison with the disc table  7  by the drive shaft  6   a . When the chassis  1  is angularly moved downwardly together with the movable frame  2  as indicated by the two-dot-and-dash lines in FIG. 2, the disc table  7  is retracted to a position below the disc tray  4 . In this position, the optical disc  101  is transferred from the disc table  7  onto the disc tray  4 , and can be unloaded as the disc tray  4  is ejected out of the outer housing. 
     As shown in FIG. 3, the optical pickup device  10  has an optical unit  11  including a housing  12  which accommodates a semiconductor laser  13  as a light source. A laser beam emitted from the semiconductor laser  13  passes through a diffraction grating  14 , is reflected by a semi-transmitting mirror (beam splitter)  15  to change its direction of travel by 90°, and enters an objective lens  18  held by a lens holder  17  of an objective lens drive unit (two-axis actuator)  16  which is positioned above the optical unit  11 . The diffraction grating  14  splits the laser beam into at least three beams (0th-order and ±1th-order beams). The objective lens  18  is positioned in confronting relationship to the signal recording surface  102  of the optical disc  101  that is held on the disc table  7 . The objective lens  18  serves to focus the applied beam through the disc board  103  onto the signal recording surface  102  of the optical disc  101 . 
     The beam focused onto the signal recording surface  102  is modulated in intensity by an information signal recorded on the signal recording surface  102  and then reflected by the signal recording surface  102 . The reflected beam travels back through the objective lens  18  to the semi-transmitting mirror  15 . The reflected mirror passes through the semi-transmitting mirror  15  and a detector system lens which comprises a concave lens, and is focused onto a photosensitive surface of a photodiode  19  as a photodetector. When the reflected beam passes through the semi-transmitting mirror  15 , the semi-transmitting mirror  15  causes astigmatism because it acts as a plane-parallel plate lying obliquely to the path of the beam. 
     The photosensitive surface of the photodiode  19  is divided into a plurality of facets capable of independently generating electric signals indicative of the levels of beam intensities falling on the facets. The signals generated by the facets of the photodiode  19  are processed into a readout signal (RF signal) representing the information signal recorded on the optical disc  101 , a focusing error signal, and a tracking error signal. 
     The focusing error signal is obtained by the astigmatic method, and indicates the distance between the beam spot focused by the objective lens  18  and the signal recording surface  102  of the optical disc  101  in the direction along the optical axis of the objective lens  18 . The tracking error signal is obtained by the three-beam method, and indicates the distance the beam spot focused by the objective lens  18  and a recording track on the optical disc  101  in the radial direction of the optical disc  101 , i.e., in the direction perpendicular to the optical axis of the objective lens  18 . 
     As shown in FIGS. 1 and 2, the optical pickup device  10  is movably supported on a guide shaft  8  disposed on and extending along an upper surface of the chassis  1 . The guide shaft  8  has opposite ends supported by support legs  8   a  mounted on the chassis  1 . The optical unit  11  has a support hole through which the guide shaft  8  extends, and is movable axially along the guide shaft  8 . The optical unit  11  is prevented from rotating about the guide shaft  8  by a stop bar  9  mounted on the chassis  1 , holding the optical axis of the objective lens  18  perpendicular to the chassis  1 . 
     A pickup feed mechanism  30  is mounted on the chassis  1  for moving the optical unit  11  along the guide shaft  8  on the chassis  1 . The pickup feed mechanism  30  comprises a motor  31  having a vertical drive shaft  32  with a drive gear  33  mounted on an upper end thereof. The drive gear  33  is held in driving mesh with an idle gear  34  rotatably supported on the chassis  1 . A small gear  35  coaxially coupled to the idle gear  34  is held in driving mesh with a driven gear  36  rotatably supported on the chassis  1 . A pinion gear  37  is coaxially coupled to the driven gear  36  and held in driving mesh with rack teeth  20  on one side of the optical unit  11 . The rack teeth  20  extend parallel to the guide shaft  8 , and are combined with a backlash correcting mechanism (not shown). 
     When the motor  31  of the pickup feed mechanism  30  is energized, the optical unit  11  moves along the guide shaft  8  in the range R, i.e., the signal recording area, shown in FIG.  1 . Upon movement of the optical unit  11 , the optical pickup device  10  moves radially over the optical disc  101  supported on the disc table  7 . The optical pickup device  10  can move radially outwardly beyond the lead-out position at the radially outer end of the range R or the signal recording area of the optical disc  101 . 
     When the optical disc  101  is rotated by the spindle motor  6 , the optical pickup device  10  reads an information signal recorded along the recording tracks of the optical disc  101  along the recording tracks. 
     When the optical disc  101  is rotated, the readout position where the optical pickup device  10  reads an information signal from any of the recording tracks is periodically reciprocally moved radially of the optical disc  101  because of an off-center deviation of the center of curvature of the recording track from the center of rotation of the optical disc  101 , i.e., the central axis of the drive shaft  6   a  of the spindle motor  6 . In addition, the readout position on the signal recording surface is also periodically reciprocally moved perpendicularly to the signal recording surface due to a distortion or planarity error of the disc board  103  of the optical disc  101 . 
     To cope with such an off-center deviation and planarity error, the optical pickup device  10  has the objective lens drive unit (two-axis actuator)  16 . The objective lens drive unit  16  supports the objective lens  18  for movement along both the optical axis (focusing direction) thereof and a direction (tracking direction) normal to the optical axis, i.e., a radial direction of the optical disc  101 . 
     As shown in FIGS. 6 and 7, the objective lens drive unit  16  has an actuator base  21  which comprises an upper panel of the housing  12  of the optical unit  11 , and a support block  22  disposed on the actuator base  21 . The support block  22  has a pair of support holes through which respective support pins  23  extend into the actuator base  21 , thus connecting the support block  22  to the actuator base  21 . An upstanding support wall  24  is mounted on a rear edge of the actuator base  21  and supports a rear face of the support block  22 . To a front face of the support block  22 , there is pivotally attached a proximal end of an arm  26  through a first hinge  25  parallel to an upper surface of the actuator base  21 . The arm  26  has a distal end to which there is connected a front plate through a second hinge  27  parallel to the upper surface of the actuator base  21 . An attachment plate is joined to a front face of the front plate through a third hinge  28  perpendicular to the upper surface of the actuator base  21 . 
     The components ranging from the support block  22  to the attachment plate are integrally formed of a flexible and resilient material such as a synthetic resin material. The arm  26  is angularly movable about the first hinge  25  to displace its distal end vertically. The front plate is vertically movable toward and away from the actuator base  21  by the first and second hinges  25 ,  27  while the third hinge  28  is lying perpendicularly to the upper surface of the actuator base  21 . The attachment plate is displaceable in directions parallel to the upper surface of the actuator base  21  by the third hinge  28 . 
     The lens holder  17  is attached to the attachment plate, and the objective lens  18  is mounted on the lens holder  17  near its front edge. Specifically, the objective lens  18  is fitted in a through hole defined in the lens holder  17  and has its outer circumferential edge supported by the inner circumferential edge of the through hole. 
     Two focusing coils  29  are mounted respectively on opposite sides of the lens holder  17 , and two pairs of tracking coils  40  are also mounted respectively on the opposite sides of the lens holder  17 . A pair of magnetic circuits is disposed on the actuator base  21  for placing the coils  29 ,  40  in magnetic fields. Specifically, the magnetic circuits comprise respective upstanding U-shaped yokes  41   a  mounted on respective opposite ends of a substantially U-shaped yoke base  41 , and respective magnets  42  attached to the respective yokes  41   a . Specifically, the magnets  42  are attached to respective arms of the yokes  41   a , which have other arms extend respectively into the focusing coils  29 . The magnetic circuits generate magnetic fields of looped fluxes extending from the magnets  42  through the arms of the yokes  41   a  and back to the magnets  42 . The coils  29 ,  40  are positioned in the magnetic fields thus generated by the magnetic circuits. 
     An adjustment plate  43  is connected between opposite rear ends of the yoke base  41  at the yokes  41   a , and the yoke base  41  is fastened by threaded pins  44  to the upper panel of the housing  12 . The support block  22  is fixed to the adjustment plate  43  by the support pins  23 . 
     When the focusing coils  29  are supplied with a drive current based on the focusing error signal, the lens holder  17  which supports the objective lens  18  moves along the optical axis of the objective lens  18 . When the tracking coils  40  are supplied with a drive current based on the tracking error signal, the lens holder  17  moves the objective lens  18  in a direction perpendicular to the optical axis of the objective lens  18 . 
     When the objective lens  18  is thus moved by the objective lens drive unit  16 , the beam spot focused by the objective lens  18  is positioned on a desired one of the recording tracks at all times while following periodical fluctuations of the recording tracks. 
     The objective lens drive unit  16  is enclosed by a cover  45  to prevent dust and dirt particles from being deposited on the movable components and the objective lens  15  thereof. As shown in FIGS. 4 through 7, the cover  45  is made of a synthetic material and comprises a downwardly open housing substantially in the shape of a rectangular parallelepiped. The cover  45  has a plurality of teeth  46  projecting downwardly from its lower edge. The teeth  46  are held in engagement with the yoke base  41  fixed to the actuator base  21 , thus securing the cover  45  to the actuator base  21  and hence the housing  12  of the optical unit  11 . 
     The cover  45  has a window hole  47  defined in an upper panel thereof at a position above the objective lens  18 . The window hole  47  has a diameter greater than the diameter of the objective lens  18 . The window hole  47  is large enough to cover the range in which the objective lens  18  is moved by the objective lens drive unit  16 . Therefore, the window hole  47  allows the laser beam emitted from the objective lens  18  to pass therethrough upwardly of the cover  45 . 
     The optical pickup device  10  has a shutter  48  angularly movably mounted on the cover  45  for selectively opening and closing the window hole  47 . As shown in FIGS. 4 through 8, the shutter  48  comprises a substantially crescent-shaped flat plate having an end pivotally connected to the cover  45  by a support shaft  49 . The shutter  48  is positioned over the upper panel of the cover  45 , and can be angularly moved about the support shaft  49  to selectively open and close the window hole  47 . 
     The shutter  48  has an engaging hook  50  extending downwardly from a free end thereof remote from the support shaft  49  and facing an outer peripheral surface of the cover  45 . An arcuate guide flange  51 , which extends about the support shaft  49 , projects horizontally from the outer peripheral surface of the cover  45  and extends from a front portion of one side of the cover  45  toward a front end of the cover  45 . The engaging hook  50  has a lower end slidably engaging the arcuate guide flange  51  for guiding the shutter  48  to turn about the support shaft  49  in substantial sliding contact with the upper panel of the cover  45 . When the shutter  48  is positioned over the window hole  47 , the shutter  48  closes and substantially seals the window hole  47 . 
     The shutter  48  is normally urged by a spring  52  in a direction to open the window hole  57  as indicated by the arrow A in FIG.  8 . 
     The spring  52  comprises a tension spring connected between a spring retainer  53  projecting from the pivoted end of the shutter  48  and spring retainer  54  projecting from a front portion of the other side of the cover  45 . The shutter  48  has an abutment stop  55  projecting laterally from the pivoted end thereof and normally held against the other side of the cover  45  for preventing the shutter  48  from angularly moving beyond a given angular position shown in FIG.  8 . 
     The shutter  48  also has an engagement finger  56  projecting downwardly from the pivoted end thereof. When the engagement finger  56  is engaged and pushed by a shutter closing member (described below), the shutter  48  is angularly moved about the support shaft  49  against the bias of the spring  52  for thereby closing the window hole  47 . 
     As shown in FIGS. 1,  2 , and  8 , a shutter closing member  57  for closing the shutter  48  is mounted on the chassis  1  near the range in which the optical pickup device  10  is movable, i.e., on a side of the chassis  1  extending from a region where the optical pickup device  10  is positioned at the radially inner end of the signal recording area on the optical disc  101  to the disc table  7 . 
     The shutter closing member  57  comprises an elongate slider  57   a  longitudinally slidably supported on the chassis  1  by a pair of longitudinally spaced support pins  58  projecting upwardly from the chassis  1  through respective longitudinally oblong holes defined in the slider  57   a . The slider  57   a  has an upwardly projecting boss  57   b  positioned substantially longitudinally centrally thereon for engagement with a front edge of the engagement finger  56 . A vertical sliding cam  57   c  is mounted upwardly on a front end of the slider  57   a  remotely from the support pins  58 . The sliding cam  57   c  has a slanted cam surface  57   c   1  on its front end which is inclined backward in the downward direction and projects forward beyond a front end (free end) of the movable frame  2 . 
     The fixed frame (not shown) of the disc player has a pusher  59  projecting toward the slanted cam surface  57   c   1  of the sliding cam  57   c.    
     When the optical disc  101  is ejected from the disc player after it has been played back, the optical pickup device  10  is moved radially inwardly of the signal recording area of the optical disc  101 . The movable frame  2  is angularly moved downwardly, and the chassis  1  is also lowered, depressing the disc table  7  below the disc tray  4 . The slanted cam surface  57   c   1  of the sliding cam  57   c  is also lowered and then engaged by the pusher  59 . 
     Further downward movement of the chassis  1  causes the slanted cam surface  57   c   1  to slide against the pusher  59 , pushing the sliding cam  57   c  and hence the slider  57   a  slidably backward as indicated by the arrow in FIG.  8 . 
     Upon the backward sliding movement of the slider  57   a , the boss  57   b  engages and pushes the front edge of the engagement finger  56 . The shutter  48  is now angularly moved about the support shaft  49  with respect to the cover  45  against the bias of the spring  52  for thereby closing the window hole  47 . Since the window hole  47  is closed by the shutter  48 , the objective lens  18  is protected against damage, smear, and deposits of dust and dirt particles. 
     When the disc table  7  on the chassis  1  is completely retracted downwardly of the disc tray  4 , the optical disc  101  is transferred from the disc table  7  onto the disc tray  4 , and can be unloaded as the disc tray  4  is ejected out of the outer housing. 
     After the optical disc  101  is ejected, the window hole  47  of the cover  45  remains closed by the shutter  48  insofar as the chassis  1  is kept in the lowered position, continuously protecting the objective lens  18  against damage, smear, and deposits of dust and dirt particles. 
     When the movable frame  2  is angularly moved upwardly, the chassis  1  is lifted, bringing the sliding cam  57   c  out of sliding engagement with the pusher  59 . Therefore, the engagement finger  56  is released from the boss  57   a , whereupon the shutter  48  is angularly moved back in the direction indicated by the arrow A in FIG. 8 under the bias of the spring  52 . The window hole  47  is now opened, exposing the objective lens  18  to make the optical pickup device  10  ready for playing back another optical disc. As the shutter  48  is angularly moved to open the window hole  47 , the boss  57   a  is pushed back by the engagement finger  56  for thereby moving the shutter closing member  57  slidably forward toward the pusher  59 . 
     In response to the ascent of the chassis  1 , the disc table  7  is elevated above the disc tray  4 , carrying an optical disc  101  transferred from the disc tray  4 . The optical disc  101  is then chucked on the disc table  7  by the chucking mechanism  60 . The optical disc  101  is now ready for being rotated by the spindle motor  6 . 
     The movable frame  2  is illustrated as being angularly moved to lift and lower the chassis  1 . However, the chassis  1  may be lifted and lowered by a movable frame which is vertically movable while lying horizontally. 
     As described above, when the window hole  47  is closed by the shutter  48 , the objective lens  18  is protected against damage, smear, and deposits of dust and dirt particles. When the window hole  47  is opened by the shutter  48 , the objective lens  18  can be cleaned to remove deposited dust and dirt particles. 
     As shown in FIG. 3, the semiconductor laser  13 , the diffraction grating  14 , and the semi-transmitting mirror (beam splitter)  15  are housed in a cavity  121  defined in the housing  12 , and the photodiode  19  is mounted on a lower surface of the housing  12  beneath a lower opening of the cavity  121 . The housing  12  also has a light guide hole  122  defined therein and extending upwardly from the cavity  121  for guiding the laser beam from the cavity  121  toward the objective lens  18 . The light guide hole  122  has an upper opening  123  which is fitted with a light-transmissive optical plate  124  that may comprise a transparent optical plate. 
     The light-transmissive optical plate  124  comprises a truly flat, rectangular plate of a transparent material such as glass, acrylic resin, or the like, having a thickness of 0.5 mm, for example. The light-transmissive optical plate  124  is fitted over the opening  123  of the light guide hole  122  and lies perpendicularly to the optical axis of the objective lens  18  held by the lens holder  17 . Therefore, the light guide hole  122  is sealed by the light-transmissive optical plate  124 . 
     More specifically, as shown in FIGS. 9 and 10, the light-transmissive optical plate  124  is securely fitted in a recess  125  defined in the upper surface of the housing  12  around the opening  123  of the light guide hole  122 . The recess  125  has a depth which is substantially the same as the thickness of the light-transmissive optical plate  124 , and an outer profile which is substantially the same as the outer profile of the light-transmissive optical plate  124 . The recess  125  has a flat bottom surface  125   a  lying perpendicularly to the optical axis of the objective lens  18  and surrounded by burr receptacle grooves  126  defined in the housing  12 . 
     The housing  12  also has a pair of adhesive filling grooves  127   a ,  127   b  defined in its upper surface, one on each side of the recess  125 , and a pair of shallow recesses  128   a ,  128   b  defined in its upper surface immediately outside of the adhesive filling grooves  127   a ,  127   b.    
     To install the light-transmissive optical plate  124 , the light-transmissive optical plate  124  is fitted in the recess  125 . At this time, burrs  124   a  remaining on peripheral edges of the light-transmissive optical plate  124  are received in the burr receptacle grooves  126  around the bottom  125   a  of the recess  125 . Therefore, the light-transmissive optical plate  124  is fitted in intimate contact with the bottom  125   a  of the recess  125 . 
     With the light-transmissive optical plate  124  closely fitted in the recess  125 , the adhesive filling grooves  127   a ,  127   b  are filled up with an adhesive  130 . The adhesive  130  may be an ultraviolet-curable adhesive, for example. When the filled adhesive  130  is cured, the light-transmissive optical plate  124  is firmly fixed in the recess  125 , i.e., to the housing  12 . Even if the adhesive  130  overflows the adhesive filling grooves  127   a ,  127   b , the excessive adhesive flows into the shallow recesses  128   a ,  128   b  immediately outside of the adhesive filling grooves  127   a ,  127   b . Because of the shallow recesses  128   a ,  128   b , therefore, the adhesive  130  when cured does not protrude upwardly from the upper surface of the housing  12 , which is thus kept flat. 
     The light-transmissive optical plate  124  fitted in the recess  125  sealingly closes the light guide hole  122  in the housing  12  to prevent ambient air from entering the cavity  121 , which houses the various optical components therein, while allowing the laser beam from the semiconductor laser  13  to pass therethrough toward the objective lens  18 . 
     Consequently, the semiconductor laser  13 , the diffraction grating  14 , the semi-transmitting mirror (beam splitter)  15 , and the photodiode  19  are protected against deposits of dust and dirt particles. The gap between the light-transmissive optical plate  124  and the objective lens  18  can easily be cleaned by a brush, for example, to keep clean the light-transmissive optical plate  124  which faces the objective lens  18 . Accordingly, the optical pickup device  10  can operate for a relatively long service life. 
     Inasmuch as the light-transmissive optical plate  124  is fitted in the recess  125  perpendicularly to the optical axis of the objective lens  18 , the light-transmissive optical plate  124  does not refract the laser beam emitted from the light guide hole  122 , but transmits the laser beam accurately along the optical axis to the objective lens  18 , which can thus focus the laser beam accurately and reliably onto the signal recording surface  102  of the optical disc  101 . 
     In the illustrated embodiment, the light-transmissive optical plate  124  seals the cavity  121  which houses the optical components and the light guide hole  122  connected to the cavity  121  to protect the optical components against deposits of dust and dirt particles, and the shutter  48  selectively closes the window hole  47  which passes the laser beam therethrough to prevent dust and dirt particles from being deposited on the objective lens  18  and the objective lens drive unit  16 . 
     However, the principles of the present invention are not limited to a combination of the light-transmissive optical plate  124  and the shutter  48 , but are applicable to an ordinary optical pickup device or an optical disc device which does not have a shutter with respect to the objective lens. 
     The configuration of the cavity  121  and the light guide hole  122  may be modified as desired depending on the layout of the optical components of the optical unit  11 . The light-transmissive optical plate  124  is not limited to the rectangular shape, but may be of a circular shape. The optical components of the optical unit  11  may be modified as desired. 
     According to the present invention, as described above, since the cavity  121  which houses the optical components and the light guide hole  122  connected to the cavity  121  are sealed and closed by the light-transmissive optical plate  124 , the optical components are protected against deposits of dust and dirt particles. Furthermore, the gap between the optical unit  11  and the objective lens  18  can be cleaned to remove any deposited dust and dirt particles therefrom for allowing the laser beam to be transmitted accurately and reliably between the optical unit  11  and the objective lens  18  for reading recorded information from the optical disc  101  highly accurately and reliably. In addition, the window hole  47  which passes the laser beam therethrough is selectively closed by the shutter  48  to prevent dust and dirt particles from being deposited on the objective lens  18  and the objective lens drive unit  16 . 
     Having described preferred embodiments of the invention with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments and that various changes and modifications could be effected by one skilled in the art without departing from the spirit or scope of the invention as defined in the appended claims.