1.  Field of the Invention
The present invention relates to an optical pickup device and, more particularly, to an optical pickup device capable of optically reproducing, recording, or erasing information to/from an optical recording medium such as a CD or DVD. 
2.  Description of the Background Art
Since a large amount of information can be recorded/reproduced at high density to/from an optical recording medium such as a CD or DVD, the range of uses including an audio-visual system and a computer is very wide.  In particular, recently, it is requested to establish, practically use, and spread techniques of recording information to a high-density optical recording medium.  Among the techniques, the performance of an optical pickup device used for recording/reproducing information to/from an optical recording medium is being increased.  Therefore, it is requested not only to enhance the performance of each of parts of the optical pickup device but also to assure high positional precision of each of the parts so that light travels along a predetermined optical axis (design optical axis). 
FIG.  14 is a perspective view showing an optical pickup device 1 of Prior Art 1.  FIG.  15 is a perspective view showing a part of a housing 7 in which a light sensing device 5 and an opposed lens (hereinbelow, called a spot adjustment lens 6) in Prior Art 1 are provided.  FIG.  16 is a cross section showing a part of the housing 7 in Prior Art 1.  FIG.  17 is a perspective view showing a part of the housing 7 before the light sensing device 5 is provided in Prior Art 1.  The optical pickup device 1 of Prior Art 1 is constructed by including a light source 2, a condensing lens 3, a light leading part 4, and a light reception part having the light sensing device 5.  Light (laser beam) emitted from the light source 2 is condensed onto a not-shown optical recording medium (optical disk) by the condensing lens 3 and reflected by the optical recording medium.  The light reflected from the optical recording medium is made converged light by the spot adjustment lens 6 of the light leading part 4 facing the light sensing device 5 and is led to the light sensing device 5.  The spot adjustment lens 6 and the light sensing device 5 are provided for the housing 7.  An optical path 8 extending between the spot adjustment lens 6 and the light sensing device 5 is open without being surrounded by the housing 7.  Also in the other conventional optical pickup devices, although the light leading means is housed in a casing, the optical path extending between a lens opposed to a light sensing device and the light sensing device is open without being surrounded, for example, because of a through hole for leading light to a condensing lens (refer to, for example, Japanese Unexamined Patent Application Nos.  6-309813, 7-320293, and 11-149659). 
FIG.  18 is a plan view showing an attachment state of a hologram laser unit in an optical pickup device of Prior Art 2 to a housing (refer to, for example, Japanese Unexamined Patent Application No.  2000-21013).  A hologram laser unit 71 is a semiconductor device in which a light outgoing part and a light receiving part are integrated.  The optical pickup device is held in a housing 70 in a state where the hologram laser unit (hereinbelow, also called a laser unit) 71, a dichroic prism 85, and a collimator lens 84 are disposed on the same optical axis.  The laser unit 71 of the optical pickup device is attached to the housing 70 as follows.  First, the laser unit 71 is inserted into a laser holder 77 and is pressed against the housing 70 by a laser press spring 75.  By inserting screws 79 to holes 78 formed at both the ends of the laser press spring 75, each of the screws 79 is screwed into a screw hole 80 formed in the housing 70.  In an FPC (Flexible Pattern Circuit) 81 in which an operation circuit of the optical pickup device is formed, a plurality of holes are formed and a conduction land exists in the periphery of each of the holes.  A plurality of terminals 71d at the ends of the laser unit 71 are inserted into the holes of the FPC 81 and the conduction lands and the terminals 71d are soldered.  Subsequently, the positions of the laser unit 71 and the laser holder 77 are adjusted so that information of an optical recording medium can be read accurately.  After that, the laser unit 71 and the laser holder 77 are fixed in adhesion positions 82, and the housing 70 and the laser holder 77 are fixed in adhesion positions 83 by an adhesive.  In FIG.  18, reference numeral 71b denotes a diffraction device at the tip of the laser unit 77.  The arrow X indicates the lateral direction, the painted circle Y expresses the vertical direction, and the arrow Z expresses the optical path direction. 
In Prior Art 1, however, the optical path extending between the light sensing device and the spot adjustment lens is in an open state.  When a foreign matter such as dusts from the outside enters the optical path as an area where light is converged depending on use environments, light collides with the foreign matter and the travel of the light is deviated from a predetermined optical axis (design optical axis).  It causes a problem that the performance of the optical pickup device deteriorates terribly. 
In the case of the optical pickup device of Prior Art 2, it is requested to improve the pickup precision by holding the interval between the collimator lens 84 having the role of converting diffusion light emitted from the laser unit 71 into parallel light and the laser unit 71 at high precision.  However, the structure of the optical pickup device is insufficient.  Specifically, in the case of using the laser holder 77 formed by using a member having strength as high as the housing 70, at the time of assembling the optical pickup device, when the laser unit 71 is attached into the laser holder and the position adjustment is performed in the optical path direction (Z direction), the diffraction device 71b at the tip of the laser unit 71 may come into contact with a wall of the laser holder 77 by mistake and damage the diffraction device.  Consequently, it is difficult to attach the laser unit 71 and perform the positional adjustment in the optical path direction.  Therefore, it is difficult to make light emitted from the light source of the laser unit 71 toward the optical recording medium and light reflected from the optical recording medium match with a predetermined optical axis (design optical axis) at high precision and to display high-precision optical pickup performance. 
As described above, the optical pickup devices of Prior Arts 1 and 2 have a problem such that their high-precision optical pickup performance cannot be displayed due to their structures.