Abstract:
A front photo detector (FPD) of an optical pick-up for an optical recording and playing apparatus having no gain selection switch, a simplified configuration, an improved prevention of an abnormal operation due to an error in gain selection, and improved reliability. The front photo detector includes a photo diode unit, a current-voltage amplifying circuit, and a voltage amplifying unit. The photo diode unit outputs current proportional to the power of the light emitted from one of a plurality of laser diodes. The current-voltage amplifying circuit converts the current output from the photo diode unit into a voltage. The voltage amplifying circuit amplifies the voltage output from the current-voltage amplifying circuit by a predetermined gain.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims the priority of Korean Patent Application No. 2003-14477, filed on Mar. 7, 2003, in the Korean Intellectual Property Office, the disclosure of which is hereby incorporated by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a front photo detector (FPD) for an optical pick-up, and more particularly, to an FPD for an optical pick-up that does not need a gain selection switch and thus does not need to control gain selection and prevents an abnormal operation due to gain selection, thereby having improved reliability. 
   2. Description of the Related Art 
   A laser diode is used as a light source for an optical pick-up of an optical recording and playing apparatus such as a compact disc (CD) player, a CD-Recordable drive, or a CD-ReWritable drive. To ensure the smooth operation of the optical recording and playing apparatus, the power of the laser diode should be stabilized; however, this is difficult because the power of the laser diode changes significantly with temperature and period of use. To maintain the power of the laser diode at a proper level, power control is required; therefore a laser diode power control device is included in the optical pick-up. 
   The laser diode power control device in the optical pick-up of the optical recording and/or playing apparatus includes an FPD and an automatic power control (APC) circuit. The FPD receives a portion of the light emitted from the laser diode of the optical pick-up by using a photo diode, converts current that is generated from the photo diode in proportion to the power of the received light into a voltage by using a current-voltage amplifying circuit, amplifies the converted voltage by using a voltage amplifying circuit, and outputs the amplified voltage to the APC circuit. The FPD is generally used to obtain a monitoring voltage for recording power control. 
   The APC circuit receives an output voltage of the FPD, compensates for a difference between the output voltage and a predetermined reference voltage, and outputs a proper laser diode driving current. 
     FIG. 1  is a circuit diagram of a conventional FPD. 
   Referring to  FIG. 1 , an FPD  100  includes a photo diode  101 , a current-voltage amplifying circuit  110 , and a voltage amplifying circuit  120 . The photo diode  101  receives a portion of a light emitted for recording from a laser diode and generates a current that is proportional to the power of the received light. The current-voltage amplifying circuit  110  converts the current generated by the photo diode  101  into a voltage. The voltage amplifying circuit  120  amplifies the input voltage from the current-voltage amplifying circuit  110  by a predetermined gain. 
   Vref denotes a reference voltage input to amplifiers  125  of the current-voltage amplifying circuit  110  and the voltage amplifying circuit  120 . 
   A gain of the voltage amplifying circuit  120  can be adjusted by adjusting input resistances that are adjusted by adjusting a variable resistor VR disposed at an input terminal of the voltage amplifying circuit  120 . The variable resistor VR is adjusted before being released into the market. Such adjustment is designed to output the same voltage to an APC circuit when the same light power is input to the FPD  100 . 
     FIG. 2  is a circuit diagram of a conventional FPD of an optical pick-up in which two laser diodes are used. 
     FIG. 1  shows the FPD  100  of an optical pick-up that uses a single laser diode, e.g., an optical pick-up that only performs recording/reproducing with respect to CDs. In contrast,  FIG. 2  shows an FPD  200  of an optical pick-up in which two laser diodes that emit lights of different wavelengths are selectively used to perform recording/reproducing with respect to CDs and DVDs. 
   As shown in  FIG. 2 , the FPD  200  includes a photo diode  201 , a current-voltage amplifying circuit  210 , and a voltage amplifying circuit  220 . The photo diode  201  receives a portion of light from a selected laser diode that emits light for recording and generates current that is proportional to the power of the light emitted from the selected laser diode. The current-voltage amplifying circuit  210  converts the current generated by the photo diode  201  into a voltage. The voltage amplifying circuit  220  amplifies the voltage input from the current-voltage amplifying circuit  210  by a predetermined gain. 
   Unlike the current-voltage amplifying circuit  110  of  FIG. 1 , a current-voltage amplifying circuit  210  of  FIG. 2  includes two feedback resistors R 1  and R 2  that can be selectively used. Also, unlike the voltage amplifying circuit  120  of  FIG. 1 , a voltage amplifying circuit  220  of  FIG. 2  includes two input variable resistors VR 1  and VR 2  that can be selectively used. 
   Selecting one of the two feedback resistors R 1  and R 2  or one of the two variable resistors VR 1  and VR 2  is performed by a gain selection switch  230 . The gain selection switch  230  operates with a selection signal input from a control unit of the optical recording and playing apparatus. The selection signal is output from the control unit to the gain selection switch  230  after the control unit identifies the type of a currently used recording medium, so that a gain corresponding to the identified recording medium can be selected. 
   In  FIG. 2 , the feedback resistor R 2  and the input variable resistor VR 2  are selected. Alternatively, the feedback resistor R 1  and the input variable resistor VR 1  may be selected. 
   The wavelength of the light emitted from a laser diode for CDs is approximately 650 nm, while the wavelength of the light emitted from a laser diode for DVDs is approximately 780 nm. Also, the power of the emitted light for the laser diode for CDs is different from the laser diode for DVDs. Thus, the sensitivity of a conventional FPD  200  should be changed according to whether the laser diode for CDs or the laser diode for DVDs is being used. That is, the voltage output from FPD  200  should be different even if the same power is input. Therefore, during product manufacturing, an input variable resistor for CDs and an input variable resistor for DVDs are separately adjusted to respectively control gains depending on whether the laser diode for CDs is used or the laser diode for DVDs is used. 
   As shown in  FIG. 2 , the conventional FPD  200  necessarily includes the gain selection switch  230  and inputs the selection signal output from the control unit to the gain selection switch  230  after the type of the currently used recording medium is identified; thereby, selectively using one of the feedback resistors R 1  and R 2  and one of the input variable resistors VR 1  and VR 2  that are suitable for the currently used recording medium. 
   Thus, the gain selection switch  230  has to be included in the conventional FPD  200  and a separate signal line has to be connected to the gain selection switch  230  from the control unit, resulting in a complicated configuration. Further, due to possible noise in the selection signal, there may be an abnormal operation in gain selection. 
   SUMMARY OF THE INVENTION 
   The present invention provides an FPD that does not need a gain selection switch, thereby having a simple configuration. 
   The present invention also provides an FPD that prevents an abnormal operation due to an error in gain selection, thereby having improved reliability. 
   According to one aspect of the present invention, there is provided a front photo detector (FPD) for an optical pick-up, wherein the front photo detector comprises a photo diode unit that outputs current proportional to a power of light emitted from one of a plurality of laser diodes, a current-voltage amplifying circuit that converts the current output from the photo diode unit into a voltage, and a voltage amplifying circuit that amplifies the voltage output from the current-voltage amplifying circuit by a predetermined gain. 
   The photo diode unit comprises an optical device that changes a direction of the light emitted from one of the plurality of laser diodes and photo diodes that receive lights that are emitted from each of the plurality of laser diodes and pass through the optical device. 
   The current-voltage amplifying circuit comprises current-voltage amplifying units, each of which converts current output from each of the photo diodes into a voltage. 
   The voltage amplifying circuit separately controls a gain with respect to the current output from each of the current-voltage amplifying circuits. 
   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. 

   
     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 circuit diagram of a conventional FPD of an optical pick-up in which a single laser diode is used; 
       FIG. 2  is a circuit diagram of a conventional FPD of an optical pick-up in which two laser diodes are used; 
       FIG. 3  is a circuit diagram of an FPD in which two laser diodes are used, according to an embodiment of the invention; and 
       FIG. 4  is a circuit diagram of an FPD in which two laser diodes are used, according to another embodiment of the invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The present invention will now be described more fully with reference to the accompanying drawings, in which embodiments of the invention are shown. Throughout the drawings, 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. 3  is a circuit diagram of an FPD  300  according to an embodiment of the present invention. 
   The FPD  300  of the present invention is applied when a plurality of laser diodes is used. In  FIG. 3 , for example, two laser diodes, i.e., a first laser diode and a second laser diode are used. 
   In this embodiment, the FPD  300  includes a photo diode unit  310 , a current-voltage amplifying circuit  320 , and a voltage amplifying circuit  330 . The photo diode unit  310  outputs a current that is proportional to the power of the light emitted from the first laser diode or the second laser diode. The current-voltage amplifying circuit  320  converts the current output from the photo diode unit  310  into a voltage. The voltage amplifying circuit  330  amplifies the voltage output from the current-voltage amplifying circuit  320  by a predetermined gain. 
   The photo diode unit  310  includes an optical device  311  for changing the direction of the light emitted from the first laser diode or the second laser diode. 
   The optical device  311  operates because the wavelengths of the lights emitted from the first laser diode and the second laser diode are different from each other and, as a result, within the same medium, the refractive indices of the lights emitted from the first laser diode and the second laser diode are also different from each other. 
   The optical device  311  may include a prism that directs lights incident through the same path to travel in different paths according to the wavelengths of the lights. Also, the optical device  311  may include a hologram or a grating that functions similarly to the prism. 
   The light that is emitted from the first laser diode and passes through the optical device  311  is received by a first photo diode  312 . The light that is emitted from the second laser diode and then passes through the optical device  311  is received by a second photo diode  313 . 
   The current-voltage amplifying circuit  320  includes a first current-voltage amplifying unit  321  and a second current-voltage amplifying unit  322 . The first current-voltage amplifying unit  321  converts a current output from the first photo diode  312  into a voltage and the second current-voltage amplifying unit  322  converts a current output from the second photo diode  313  into a voltage. 
   The voltage amplifying circuit  330  is configured to separately set a gain with respect to an output of the first current-voltage amplifying unit  321  and a gain with respect to an output of the second current-voltage amplifying unit  322 . 
   In  FIG. 3 , a single amplifier  335  is used, and a first input variable resistor VR 1  connected to the output of the first current-voltage amplifying unit  321  and a second input variable resistor VR 2  connected to the output of the second current-voltage amplifying unit  322  are different. 
   Meanwhile, as shown in  FIG. 4 , two amplifiers  335   a ,  335   b  may be used. In this case, a first voltage amplifying circuit  331  is connected to the first current-voltage amplifying unit  321  and a second voltage amplifying circuit  332  is connected to the second current-voltage amplifying unit  322 . 
   Hereinafter, the operation of the FPD  300  will be described. 
   The first laser diode may be used to emit a light for CD recording and the second laser diode may be used to emit a light for DVD recording. During manufacturing of the optical pick-up, the gain of the FPD  300  is properly set. 
   The first laser diode emits light, the direction of the emitted light changes while passing through the optical device  311 , and the light is received by the first photo diode  312 . The first photo diode  312  outputs a current that is proportional to the power of the received light, and the output current is converted into a voltage by the first current-voltage amplifying unit  321 . 
   The voltage output from the first current-voltage amplifying unit  321  passes through the first input variable resistor VR 1  and is then amplified while passing through an amplifier  335  of the voltage amplifying circuit  330  in  FIG. 3 . In  FIG. 4 , the voltage output from the first current-voltage amplifying unit  321  is amplified while passing through the first voltage amplifying circuit  331 . During these procedures, the gain with respect to the voltage output from the first current-voltage amplifying unit  321  is appropriately controlled by adjusting the first input variable resistor VR 1  of the voltage amplifying circuit  330  or an input variable resistor VR 1  of the first voltage amplifying circuit  331 . 
   After the gain associated with the first laser diode is controlled, the gain associated with the second laser diode is controlled. 
   The second laser diode emits light, the direction of the emitted light changes while passing through the optical device  311 , and the light is received by the second photo diode  313 . The second photo diode  313  outputs a current that is proportional to the power of the received light, and the output current is converted into a voltage by the second current-voltage amplifying unit  322 . 
   The voltage output from the second current-voltage amplifying unit  322  passes through the second input variable resistor VR 2  and is then amplified while passing through an amplifier  335  of the voltage amplifying circuit  330  in  FIG. 3 . In  FIG. 4 , the voltage output from the second current-voltage amplifying unit  322  is amplified while passing through the second voltage amplifying circuit  332 . During these procedures, the gain with respect to the voltage output from the second current-voltage amplifying unit  322  is appropriately controlled by adjusting the second input variable resistor VR 2  of the voltage amplifying circuit  330  or an input variable resistor VR 2  of the second voltage amplifying circuit  332 . 
   When the optical pick-up is used after gain control is completed, the light emitted from the first laser diode is received by the first photo diode  312  through the operation of the optical device  311  passes through the first current-voltage amplifying unit  321 , the first input variable resistor VR 1  (in the case of  FIG. 3 ) or the first voltage amplifying circuit  331  (in the case of  FIG. 4 ), and is then output after being amplified by the gain controlled in relation to the first laser diode. Similarly, the light emitted from the second laser diode is also output after being amplified by the gain controlled in relation to the second laser diode. During these procedures, there is no need for a gain selection switch that operates with a separate input of a gain selection signal from a control unit. 
   In the above embodiments, two laser diodes are used. However, it is obvious to those skilled in the art that the present invention can be applied to a case where three or more laser diodes are used, by including photo diodes and current-voltage amplifying circuits corresponding to respective laser diodes. 
   According to the invention, the FPD has a simple configuration because since the gain selection switch is not needed. Also, since a signal is amplified while traveling in a path physically determined according to the wavelength of the incident light without a need for gain selection, an abnormal operation due to an error in gain selection can be prevented, thereby improving the reliability of the FPD. 
   While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those skill in the art that changes may be made in this embodiment without departing from the spirit and principles of the invention, the scope of which is defined in claims and their equivalents.