CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims the priority of Korean Patent Application No. 2003-4839, filed on Jan. 24, 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 pickup, and, more particularly, to an optical pickup including a front photo-detector to detect power of light incident on an objective lens. 
   2. Description of the Related Art 
     FIG. 1  schematically shows the structure of an optical pickup used in an optical disc drive. Referring to  FIG. 1 , the optical pickup includes a light source  1 , a grating  2 , a collimating lens  3 , an objective lens  9 , and a main photo-detector  7 . The light source  1  emits a light beam having a predetermined wavelength. The grating  2  diverges 0 th -order and ±1 st -order beams from the light beam. The collimating lens  3  transforms the divergent beams emitted from the light source  1  into parallel beams. The objective lens  9  enables the light beam to be focused as a light spot on a recording surface of an optical disc  10 . The main photo-detector  7  receives a light beam reflected from the recoding surface of the optical disc  10  to detect an information signal and/or an error signal. 
   Reference numeral  4  denotes a beam splitter that enables the light beam emitted from the light source  1  to be incident on the objective lens  9 , and that guides the light beam, which has been reflected from the optical disc  10  and passed through the objective lens  9 , toward the main photo-detector  7 . A concave lens  6  may be interposed between the beam splitter  4  and the main photo-detector  7  to prevent astigmatism. 
   A light beam having a predetermined power level should be incident on the optical disc  10  so as to record information and/or reproduce information from the optical disc  10 . As shown in  FIG. 1 , the optical pickup further includes a beam splitter  5  and a front photo-detector (FPD)  8 . The beam splitter  5  splits the light beam, which has passed through the beam splitter  4 , into two beams so as to be incident on the objective lens  9  and the FPD  8 , respectively. The FPD  8  detects power from the incident beam portion. When the detected light power is lower or higher than a predetermined level, the power of the light source  1  is adjusted so that a light beam having a predetermined power level is incident on the optical disc  10 . 
   As described above, optical elements of an optical system are installed on a pickup base which moves in a radial direction of an optical disc. When an optical pickup is manufactured, it is necessary to adjust locations of the optical elements and angles among them when installing the optical elements. In other words, the installation locations and angles for the optical elements must be precisely adjusted to focus the light beam emitted from the light source  1  as a light spot of a predetermined size on a predetermined portion of the optical disc  10 . 
   The FPD  8  should always be located relative to the light source  1  to precisely adjust the light power of the light source  1  to a predetermined level, so that the light power detected by the FPD  8  is proportional to the light power of the light source  1 . However, as described above, when the locations of the light source  1  and the FPD  8  are separately adjusted, the locations of light source  1  and the FPD  8  may vary when an optical pickup is manufactured. In this case, the relationship between the light power of the light source  1  and the light power detected by the FPD  8  may vary when an optical pickup is manufactured. Thus, it is quite difficult to adjust the light power of the light source  1  to a predetermined level. Accordingly, whenever the location and angle of the light source  1  are adjusted, the location and angel of the FPD  8  must be adjusted. As a result, the cost of manufacturing an optical pickup may increase. 
   SUMMARY OF THE INVENTION 
   The present invention provides an improved optical pickup in which a front photo-detector and a light source are incorporated into an assembly so that the front photo-detector is always located relative to the light source. 
   Additional aspects and/or 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, there is provided an optical pickup mounted on a pickup base moving relative to an optical recording medium and used to record information on and/or reproduce information from the optical recording medium. The optical pickup includes a first optical module, an objective lens to focus a first light beam emitted from the first optical module on the optical recording medium, and a first front photo-detector to monitor power of the first light beam emitted from the first optical module; wherein the first optical module is coupled to the front photo-detector to be installed on the pickup base. 
   The optical pickup may further include a first collimating lens between the first optical module and the objective lens to transform the first light beam into a parallel beam; wherein the first front photo-detector is provided between the first optical module and the first collimating lens. 
   The first optical module may comprise a light source to emit the first light beam; and a main photo-detector to receive the first light beam after being reflected from the optical recording medium to detect an information signal and/or an error signal. 
   The optical module may include a second optical module to emit a second light beam, wherein the first and second light beams have different wavelengths. One of the first and second light beams may have a first wavelength so as to record information on and/or reproduce information from a digital versatile disc, and the other one of the first and second light beams has a second wavelength so as to record information on and/or reproduce information from a compact disc. 


   
     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 embodiments taken in conjunction with the accompanying drawings in which: 
       FIG. 1  is a schematic view showing the structure of a conventional optical pickup used in an optical disc drive; 
       FIG. 2  is a perspective view of an optical disc drive adopting an optical pickup according to an embodiment of the present invention; 
       FIG. 3  is an exploded perspective view of the optical pickup of  FIG. 2 ; 
       FIG. 4  is a schematic view of an optical system of the optical pickup of  FIG. 3 ; and 
       FIG. 5  is a view showing the relationship between an optical module and a front photo-detector. 
   


   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Reference will now be made in detail to the embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The embodiments are described below to explain the present invention by referring to the figures. 
     FIG. 2  is a perspective view of an optical disc drive adopting an optical pickup according to an embodiment of the present invention. Referring to  FIG. 2 , a tray  30  is installed in a housing  20  so as to slide in a direction indicated by arrow A. A spindle motor  31  and a pickup base  33  are installed in the tray  30 . The spindle motor  31  rotates an optical disc  10 , and has a shaft at which a turntable  32  is installed. The optical disc  10  is placed on the turntable  32 . The pickup base  33  slides in a radial direction of the optical disc  10  as indicated by arrow B. An optical pickup  40  is installed on the pickup base  33 . The optical pickup  40  accesses a recording surface of the optical disc  10  to record information and/or reproduce information from the optical disc  10 . 
     FIG. 3  is an exploded perspective view of the optical pickup of  FIG. 2 , and  FIG. 4  is a schematic view of an optical system of the optical pickup of  FIG. 3 . Here, the optical pickup is constituted so as to perform recording on and/or reproduction from optical discs such as compact discs (CDs) and digital versatile discs (DVDs). 
   Referring to  FIG. 4 , the optical pickup includes optical modules  100  and  101 , collimating lenses  120  and  121 , a reflecting mirror  140 , and an objective lens  150 . The optical disc  10  is used as an optical recording medium. 
   Although not shown in  FIG. 4 , in each of the optical modules  100  and  101 , a light source and a main photo-detector are incorporated into an assembly. Here, the light source emits a light beam having a predetermined wavelength, and the main photo-detector receives a light beam reflected from the optical disc  10  to detect an information signal and/or an error signal. 
   When three beams are to be used to detect a tracking error, the optical modules  100  and  101  may serve to diverge 0 th -order and ±1 st -order beams from a light beam. 
   However, in a case where an optical module which only emits a light beam is used, instead of the optical modules  100  and  101 , the main photo-detector  7  and the beam splitter  4  as shown in  FIG. 1  may be further installed. When the three beams are used to detect the tracking error, the grating  2  as shown in  FIG. 1  may be further installed. 
   In this embodiment, the optical module  100  emits a light beam having a red wavelength, i.e., a wavelength of 650 nm, so as to record information and/or reproduce information from a DVD. The optical module  101  emits a light beam having an infrared wavelength, i.e., a wavelength of 780 nm, so as to record information and/or reproduce information from a CD. In this case, the optical pickup of the present invention can be used to record information and/or reproduce information from CD-family optical discs and DVD-family optical discs. 
   The collimating lenses  120  and  121  transform divergent beams emitted from the optical modules  100  and  101  into parallel beams. For the transformation purpose, the optical modules  100  and  101  must be located at focuses of the collimating lenses  120  and  121 , respectively. 
   The objective lens  150  focuses the light beam as a light spot of a predetermined size on the optical disc  10 . An objective lens driver (not shown) is further installed to precisely adjust the location of the objective lens  150  so that the light beam is accurately incident on a predetermined portion of the optical disc  10 . 
   Reference numeral  110  denotes a reflecting mirror which reflects the light beam emitted from the optical module  100  toward the collimating lens  120 . Reference numeral  130  denotes an optical path changer which transmits the light beam emitted from the optical module  100  and reflects the light beam emitted from the optical module  101 . 
   Reference numeral  160  denotes an FPD which detects power of the light beam emitted from the optical module  101 . A light beam having a predetermined power level should be incident on the optical disc  10  so as to record information and/or reproduce information from the optical disc  10 . When the light power detected by the FPD  160  is lower or higher than a predetermined level, the light power of the optical module  101  is adjusted so that a light beam having a predetermined power level is incident on the optical disc  10 . 
   The FPD  160  may serve to detect light power from the parallel beam which passes through the collimating lens  121 . In the conventional optical pickup, as shown in  FIG. 1 , the beam splitter  5  is needed to diverge a portion from the parallel beam before the parallel beam is incident on the objective lens  9 . Also, the FPD  8  is installed in the rear of the beam splitter  5 . In this case, the beam splitter  5  must be coated so as to transmit a predetermined amount of light toward the FPD  8 . Thus, it is difficult to coat the beam splitter  5 . In addition, since the light source  1  is distant from the FPD  8 , it is not easy to incorporate the light source  1  and the FPD  8  into an assembly. 
   However, in this embodiment of the present invention, the FPD  160  is installed between the optical module  101  and the collimating lens  121  so as to detect a portion of the divergent beam. Thus, the beam splitter  5  as shown in  FIG. 1  is not necessary. 
   Referring to  FIG. 3 , the optical elements shown in  FIG. 4  are installed on the optical base  33  which slides in a radial direction of the optical disc  10 . The locations of the optical elements and the angles among them are precisely adjusted so that the light beams emitted from the optical modules  100  and  101  are exactly incident on the objective lens  150 . 
   The optical module  101  is combined with one end of a holder  170 . The FPD  160  is combined with the other end of the holder  170  at a predetermined distance from the optical module  101 . The optical module  101  and the FPD  160  are incorporated into the holder  170  to constitute an assembly  200 . The assembly  200  is installed on the pickup base  33 . The location of the assembly  200  is adjusted so that the optical module  101  is located at the focus of the collimating lens  121 . As previously described, in the conventional optical pickup, the location of the light source  1  is adjusted, and then the location of the FPD  8  is adjusted. However, in the optical pickup of the present invention, the entire assembly  200  is moved to adjust the locations of the optical module  101  and the FPD  160 . Nevertheless, the relative locations of the optical module  101  and the FPD  160  do not vary. Thus, the optical system may be completely adjusted by performing an adjustment process only one time. 
   As shown in  FIG. 5 , the FPD  160  detects the light power of the optical module  101  from a portion of the divergent beam emitted from the optical module  101 . The divergent beam has a Gaussian distribution according to an angle with respect to an optical axis. Thus, the FPD  160  must always keep a predetermined distance L from the optical module  101 , and a predetermined angle C with the optical axis, so that the light power of the optical module  101  can be accurately calculated from the light power detected by the FPD  160 . 
   In the conventional optical pickup, the location of the light source  1  is adjusted, and then the location of the FPD  8  is adjusted to the location of the light source  1 . Thus, the relative locations of the light source  1  and the FPD  8  may slightly vary according to a process of manufacturing an optical pickup. Therefore, it is difficult to accurately calculate the light power of the light source  1  from the light power detected by the FPD  8 . In particular, it is quite difficult to detect light power from a divergent beam. 
   In the optical pickup according to the present invention, the relative locations of the optical module  101  and the FPD  160  are constant. Thus, the light power of the optical module  101  can be accurately calculated from the light power detected by the FPD  160 . As a result, the light power of the optical module  101  can be precisely adjusted. 
   Light power of an optical module is particularly important when recording information on an optical recording medium. Thus, in the present invention, the case in which the optical module  101  is used to record information on a CD has been described. However, this does not limit the scope of the present invention to this embodiment. Although not shown, another FPD can be further included to detect the light power of the optical module  100 . Even in this case, it is preferable that the optical module  100  is combined with the FPD. 
   Also, in the present invention, the case in which an FPD is installed between an optical module and a collimating lens has been described. However, the FPD and the optical module may be incorporated into an assembly wherein the FPD is installed to detect light power from a parallel beam that has passed through the collimating lens. 
   Also, an optical pickup according to the present invention can be used to record information on and/or reproduce information from a CD and a DVD. The optical pickup can also be used to record information on and/or reproduce information from other types of optical recording media. 
   In addition, since the optical module is combined with the FPD, the locations of the optical module and the FPD can be simply adjusted when manufacturing the optical pickup. As a result, productivity can be improved and costs can be reduced. 
   Although a few embodiments of the present invention have been shown and described, it would be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.
An optical pickup used to record information on and/or reproduce information from an optical recording medium, the optical pickup including an optical module to emit a light beam; and a front photo-detector to monitor power of the light beam; wherein the optical module is coupled to the front photo-detector so that a fixed distance is maintained between the optical module and the front photo-detector.