Abstract:
An optical head includes a substrate, a laser diode, a photodetector, an objective lens, a prism, and an optical element. The laser diode is installed on the substrate and emits light. The photodetector is installed on the substrate and receives the light. The objective lens is installed on a first side of the substrate and focuses the light emitted from the laser diode onto a recording surface of a disc. The prism is installed on a second side of the substrate, transmits the light emitted from the laser diode toward the objective lens, and transmits the light reflected from the recording surface toward the photodetector. The optical element adjusts an optical path formed between the substrate and the prism.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims the priority of Korean Patent Application No. 2003-14480, filed on Mar. 7, 2003, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an optical head used in an optical pickup device and a method of adjusting an optical path thereof. 
   2. Description of the Related Art 
   An optical pickup device generally includes an optical head having a laser diode (LD) emitting light to a recording medium and a photo diode receiving the light reflected from the recording medium. 
     FIG. 1  illustrates a structure of a conventional optical head disclosed in PCT International Publication No. WO 01/43126 A2. The conventional optical head includes a substrate  8 , an LD  5 , a mirror  6 , a prism  4  having a polarized light division function, a quarter wave plate  3 , an objective lens  1 , and a photodetector (PD)  7 . In the above structure, light emitted from the laser  5  is reflected from the mirror  6  and reflective surfaces  4   a  and  4   b  of the prism  4  and then is converted into circularly polarized light by the quarter wave plate  3 . The circularly polarized light is condensed by the objective lens  1  and then reaches a recording surface of a disc D. Thereafter, the light reflected from the recording surface of the disc D is converted into linearly polarized light by the quarter wave plate  3 . Next, the linearly polarized light passes through the reflective surface  4   a , is reflected from a reflective surface  4   c , and returns to the PD  7 . A height H of the optical head is about 3 mm. 
   The optical head is generally assembled according to the following process. As shown in  FIG. 2 , the prism  4 , the quarter wave plate  3 , a spacer  2 , and the objective lens  1  are stacked to be attached to each other, thereby completing a first assembly body  10 . As shown in  FIG. 3 , the first assembly body  10  is combined with a second assembly body  20 , which is completed by assembling the substrate  8 , the LD  5 , and the PD  7 . Reference numeral  9  denotes a radiating plate. 
   However, as is observed during a test for emitting and receiving the light after the assembling process, the light frequently deviates from a designed optical path. Stacking and assembling optical elements of the optical head are precisely performed using an align mark as a zero reference. However, since a tolerance exists in manufacturing the optical elements, although the optical elements are precisely assembled, the light may deviate from the optical path. Accordingly, relative positions of the first and second assembly bodies  10  and  20  have to be adjusted when being assembled in order to compensate for the deviation of the light from the optical path. In other words, the relative positions of the first and second assembly bodies  10  and  20  have to be adjusted so that the light emitted from the LD  5  is reflected from the recording surface of the disc D and then reaches a center of the PD  7 . As shown in  FIG. 4 , the adjustment of the relative positions of the first and second assembly bodies  10  and  20  is achieved by rotating the first assembly body  10  around a Z-axis (corresponding to the light reflected from the mirror  6 ) and detecting changes in a state of an optical spot focused on the PD  7 . A trajectory of the optical spot focused on the PD  7  is indicated by a dashed line in FIG.  5 . 
   However, when the assembling process is carried out with a signal sensed by the PD  7 , the first assembly body  10  rotates around the Z-axis. Thus, the assembling process does not affect the optical path through which the light is transmitted, i.e., the optical path from the LD  5  to the objective lens  1 . However, if the trajectory of the optical path fails to pass through the center of the PD  7  and shifts due to the manufacturing tolerance, the first assembly  10  has to horizontally move toward an X-axis or a Y-axis of  FIG. 4  so that the trajectory passes through the center of the PD  7 . Then, the optical path through which the light is transmitted, re-deviates during adjusting of the optical path through which the light is focused onto the PD  7 . Thus, it is quite difficult to correct the light deviating from the optical path. Also, since the optical head has a structure in which the optical elements are stacked on the substrate  8 , the height of the optical head becomes higher. 
   Accordingly, an optical head having a new structure is demanded to solve these problems. 
   SUMMARY OF THE INVENTION 
   The present invention provides an optical head having an improved structure which is appropriate for making a product slim and which allows an easy adjustment of an optical path, and a method of adjusting the optical path thereof. 
   Additional aspects and advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention. 
   According to an aspect of the present invention, an optical head includes a substrate, a laser diode, a photodetector, an objective lens, a prism, an optical element. The laser diode is installed on the substrate and emits light. The photodetector is installed on the substrate and receives the light. The objective lens is installed on a first side of the substrate and focuses the light emitted from the laser diode onto a recording surface of a disc. The prism is installed on a second side of the substrate, transmits the light emitted from the laser diode toward the objective lens, and transmits the light reflected from the recording surface toward the photodetector. The optical element adjusts an optical path formed between the substrate and the prism. 
   According to another aspect of the present invention, a method of adjusting an optical path of an optical head includes preparing the optical head so as to include a substrate, installing a laser diode on the substrate to emit light, installing a photodetector on the substrate to receive the light, installing an objective lens on a first side of the substrate to focus the light emitted from the laser diode onto a recording surface of a disc, installing a prism that on a second side of the substrate to transmit the light emitted from the laser diode toward the objective lens and to transmit the light reflected from the recording surface toward the photodetector, and forming an optical element to adjust the optical path formed between the substrate and the prism. A first optical path through which the light is transmitted to the disc is adjusted by moving a first assembly body including the substrate and first optical elements assembled on the first side of the substrate relative to a second assembly body including second optical elements assembled on the second side of the substrate. A second optical path through which the light is received from the disc is adjusted by adjusting a position of the hologram and quarter wave plate with respect to the first assembly body. 
   According to another aspect to the present invention, an optical head includes a substrate mounted with a laser diode and a photodetector, a first assembly body disposed on a first side of the substrate, and having an objective lens, and a second assembly body disposed on a second side of the substrate, and having a prism. 
   According to another aspect to the present invention, an optical head includes a substrate mounted with a laser diode and a photodetector, a first assembly body disposed on a first side of the substrate, and having an objective lens movably mounted on the first side of the substrate to adjust a first optical path from the laser diode to an outside of the objective lens, and a second assembly body having a prism and a hologram element disposed on a second side of the substrate to adjust a second optical path from the objective lens to the photodetector. 
   According to another aspect to the present invention, a method of adjusting first and second optical paths in an optical head includes causing a first assembly body having an objective lens to be disposed on a first side of a substrate mounted with a laser diode and a photodetector, causing a second assembly body having a prism to be disposed on a second side of the substrate, causing a hologram element to be disposed between the second side of the substrate and the prism, moving the first assembly body to adjust the first optical path from the laser diode to an outside of the objective lens, and moving the hologram element to adjust the second optical path from the objective lens to the photodetector. 
   According to another aspect to the present invention, a method of adjusting one of first and second optical paths in an optical head includes causing a first assembly body having an objective lens to be disposed on a first side of a substrate mounted with a laser diode and a photodetector, causing a second assembly body having a prism to be disposed on a second side of the substrate, causing a hologram element to be disposed between the second side of the substrate and the prism, and moving the hologram element to adjust the second optical path from the objective lens to the photodetector without adjusting the first optical path from the laser diode to an outside of the objective lens. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and/or other aspects and advantages of the invention will become apparent and more readily appreciated from the following description of the preferred embodiments, taken in conjunction with the accompanying drawings of which: 
       FIG. 1  illustrates a conventional optical head; 
       FIGS. 2 and 3  are views explaining a process of assembling the optical head shown in  FIG. 1 ; 
       FIG. 4  is a view explaining a method of adjusting an optical path of the optical head shown in  FIG. 1 ; 
       FIG. 5  is a view illustrating a trajectory of an optical spot passing through a PD when adjusting the optical path of the optical head shown in  FIG. 4 ; 
       FIG. 6  is an exploded perspective view of an optical head according to an embodiment of the present invention; 
       FIG. 7  is a view illustrating an optical path of the optical head shown in  FIG. 6 ; and 
       FIGS. 8A through 8C  are views illustrating changes in an optical spot formed when adjusting the optical path of the optical head shown in FIG.  6 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Reference will now be made in detail to the present preferred embodiment of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The embodiment is described in order to explain the present invention by referring to the figures. 
     FIGS. 6 and 7  illustrate an optical head according to an embodiment of the present invention. Referring to  FIGS. 6 and 7 , the optical head includes a substrate  140 , which is made of silicon-on-bench (SiOB) having a good thermal conductivity, an LD  141  and a PD  142  which are attached to a bottom surface of the substrate  140 , an objective lens  110  which is installed over the substrate  140 , a prism  180  which is installed under the substrate  140  and has a polarized light division function, a hologram element and quarter wave plate  170  which is installed between the substrate  140  and the prism  180  to be connected to the objective lens  110  via a through hole  143  formed in the substrate  140  so as to form an optical path. Reference numerals  120 ,  150 , and  160  denote spacers, reference numeral  130  denotes a radiating plate, and reference numeral  190  denotes a plate spring which supports the optical head so that the optical head elastically moves and is used for transmitting and receiving signals between controllers (not shown) of the LD  141  and the PD  142 . In other words, in this structure, optical elements are not stacked on the substrate  140  but arranged on first and second sides of the substrate  140 . Here, first optical elements including the substrate  140 , assembled on an upper (first) side of the substrate  140  are referred to as a first assembly body  100 , and second optical elements assembled on a lower (second) side of the substrate  140  are referred to as a second assembly body  200 . 
   The optical head having the above-described structure forms the optical path as shown in FIG.  7 . In other words, light emitted from the LD  141  is reflected from a mirror  144  of the substrate  140  and then from a polarized surface  181  of the prism  180 . The reflected light is then reflected from a mirror  182 , is converted into a circularly polarized light by the hologram element and quarter wave plate  170 , and becomes incident on a recording surface of a disc D via the objective lens  110 . The light reflected from the recording surface of the disc D is converted into a linearly polarized light by the hologram element and quarter wave plate  170 , is reflected from the mirror  182 , passes through the polarized surface  181  of the prism  180 , is then reflected from the mirror  183 , and becomes incident on the PD  142 . 
   According to the above-described structure, a height of the optical head according to the present invention can be reduced compared with the height of a conventional optical head shown in FIG.  1 . In other words, the optical path of the conventional optical head ranges from an LD installed on a substrate to an objective lens layer by layer. However, in the present invention, the light emitted from the LD  140  installed beneath the substrate  140  proceeds downward and then upward to form a U-shaped optical path. Thus, when a first object distance from the LD  141  to the objective lens  110  of the optical head according to the present invention is equal to a second object distance from the LD  141  to the objective lens  110  of the conventional optical head, the structure of the optical head according to the present invention can be made slimmer. In addition, arranging the radiating plate  130  on a top layer, such as the objective lens  13 , of the optical head is helpful to manufacture a slim optical head. In other words, in the conventional optical head, a radiating plate is attached onto a bottom surface of the substrate and thus is formed as an additional layer while in the optical head according to the present invention, the radiating plate  130  is attached onto a vacancy (space) next to the objective lens  110  over the substrate  140 . Thus, the radiating plate  130  is not formed as the additional layer and shares a layer on which the objective lens  110  is installed, thereby reducing the height of the optical head. Like this, when the radiating plate  130  is installed on the top layer on which the objective lens  110  is installed, a radiation efficiency can be further increased. In other words, the disc D rotates at a high speed over the optical head during recording and/or reproducing data on and/or from the disc D using the optical head. In this case, when the radiating plate  130  is positioned on the top layer, the radiating plate  130  is exposed to an air current formed during a rotation of the disc D, which results in maximizing an air cooling efficiency. As a result, the radiating plate  130  can rapidly radiate heat generated by the LD  141 . 
   Moreover, when the optical head is assembled, it is very easy to adjust a first optical path through which the light is transmitted to the disc D, and a second optical path through which the light is received from the disc D. In other words, when assembling the optical head, the first assembly body  100  and the second assembly body  200  are separately assembled and then combined. In this process, when the first optical path through which the light is transmitted to the disc D, i.e., the first optical path ranging from the LD  141  to the objective lens  110 , is desired to be adjusted, the first optical path is adjusted by moving the first assembly body  100  relative to the second assembly body  200 . Also, the second optical path through which the light is received from the disc D, i.e., the second optical path through which the light is reflected from the disc D toward the PD  142 , is adjusted by adjusting a position of the hologram element and quarter wave plate  170  with respect to the first assembly body  100 . For this, the hologram and quarter wave plate  170  should be designed so as not to affect the first optical path through which the light is transmitted to the disc D although the position thereof is changed. In other words, a hologram element, which is designed to create astigmatism only for the light reflected from the disc D not for the light incident on the disc D, is used in the hologram and quarter wave plate  170 . As shown in  FIG. 8A , an optical spot having sub-optical spots with the same intensity is supposed to be focused on quarterly divided sensors of the PD  142  in an on-focus state due to the hologram element. The astigmatism occurs when a distance between the objective lens  110  and the recording surface of the disc D becomes too larger or too small. Thus, as shown in  FIGS. 8B and 8C , the optical spot deforms. Therefore, the second optical path through which the light is received is adjusted with the hologram element and quarter wave plate  170  displaced so that the optical spot shown in  FIG. 8A  is formed. 
   As a result, even if the first optical path through which the light is received is adjusted by the hologram element and quarter wave plate  170  after adjusting the second optical path through which the light is transmitted, the first optical path through which the light is transmitted and the second optical path through which the light is received, can be independently adjusted without interference between the first and second optical paths. In other words, unlike the conventional optical head, in the present invention, when one of the first and second optical paths is adjusted, the other one of the first and second optical paths is not distorted. 
   It is possible that a wavelength of the light emitted from the LD  141  is within a range of 400 nm-800 nm and a numerical aperture (NA) of the objective lens  110  is within a range of 0.43-0.85 or is 0.85 or more. For example, when the LD  141  emits the light having an infrared wavelength of 780 nm and the NA of the objective lens  110  is within a range of 0.4-0.5, the optical head according to the present invention can be used for recording data on and/or reproducing data from a compact disc (CD). Also, when the LD  141  emits the light having a red wavelength of 650 nm and the NA of the objective lens  110  is within a range of 0.6-0.7, the optical head according to the present invention can be used for compatibly recording data on and/or reproducing data from a digital versatile disc (DVD) and the CD. Furthermore, when the LD  141  emits the light having a blue wavelength of 405 nm and the NA of the objective lens  110  is 0.85, the optical head according to the present invention can be used for compatibly recording data on and/or reproducing data from a high-density disc, the DVD, and the CD. Moreover, the optical head according to the present invention can be used in an optical system having an NA of 0.85, such as an SIL or the like. 
   As described above, in the optical head according to the present invention, the substrate can be arranged in a middle of an optical head assembly body of the optical head so that the U-shaped optical path is formed. Thus, the same object distance can be secured and the optical head can be made slimmer. 
   Also, since a hologram, which affects only changes in the second optical path through which the light is received, can be used, the first optical path through which the light is transmitted, and the second optical path through which the light is received, can be separately adjusted. Thus, an adjustment process can be conveniently performed. 
   Furthermore, the radiating plate, which radiates the heat generated by the LD, can be installed on the top layer on which the objective lens is installed so as to be exposed to the air current formed during the rotation of the disc. As a result, the radiation efficiency can be maximized and the optical head can be made slimmer. 
   While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims and their equivalents.