Patent Publication Number: US-2006002439-A1

Title: Laser output control apparatus and method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This application claims the benefit of Korean Patent Application No. 10-2004-0051186, filed on Jul. 1, 2004, 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 laser output control, and more particularly, to a laser output control apparatus capable of controlling a laser output regardless of a connection structure of a laser diode and a photo diode that constitute a semiconductor laser device.  
      2. Description of the Related Art  
      A semiconductor laser device can be extensively applicable to office appliances, such as a laser printer, a laser facsimile, or a multi-functional peripherals, as well as an optical recording medium, such as a Compact Disc Recordable (CD-R), a Compact Disc Rewritable (CD-RW), and a Digital Versatile Disc (DVD). Two semiconductor elements, i.e., a laser diode and a photo diode, are installed in the semiconductor laser device. The laser diode and the photo diode are constructed such that they are biased in a forward direction and a reverse direction, respectively. A semiconductor laser applicable to an optical recording medium generates a laser beam using a continuous wave technique. A connection of a laser diode and a photo diode of the semiconductor laser applicable to an optical recording medium may be variously structured. For instance, an anode of the laser diode and a cathode of the photo diode are commonly connected to each other, a cathode of the laser diode and the cathode of the photo diode are commonly connected to each other, the cathode of the laser diode and an anode of the photo diode are commonly connected to each other, or the anode of the laser diode and the anode of the photo diode are commonly connected to each other. The semiconductor laser device for an optical recording medium is standardized in that the anode of the laser diode and the cathode of the photo diode, or the cathode of the laser diode and the cathode of the photo diode are commonly connected to each other.  
      A semiconductor laser device applicable to an office appliance generates a laser beam according to a high-speed on/off switching method using a high-frequency pulse. The semiconductor laser device for the office appliance is constructed such that an anode of the laser diode and a cathode of the photo diode are connected to each other as a common electrode. Since only a semiconductor laser device with such a connection is applicable to the office appliance, versatility of used parts is limited, thereby increasing manufacture costs.  
     SUMMARY OF THE INVENTION  
      Additional aspects, features, and/or advantages of the invention will be set forth in part in the description which follows and, in part, will be apparent from the description, or may be learned by practice of the invention.  
      The present invention provides a laser output control apparatus that is applicable to a semiconductor laser device with at least one connection structure of a laser diode and a photo diode so as to control a laser output thereof.  
      According to one aspect of the present invention, there is provided a laser output control apparatus including: a laser unit including a laser diode and a photo diode having at least one connection structure therebetween; a power controller maintaining power of a laser beam emitted by the laser diode constant in accordance with a feedback voltage applied from the photo diode; and a bias voltage application unit selectively applying a bias voltage to the photo diode according to the connection structure so that the photo diode is biased in a reverse direction.  
      According to another aspect of the present invention, there is provided a laser output control apparatus including: a laser unit including a laser diode and a photo diode having at least one connection structure therebetween; a power controller maintaining power of a laser beam emitted by the laser diode constant in accordance with a feedback voltage applied from the photo diode; a bias voltage generator generating at least one bias voltage that makes the photo diode biased in a reverse direction; and a selector selecting a bias voltage, which is to be applied to the photo diode, from at least one bias voltage according to the at least one connection structure.  
      According to yet another aspect of the present invention, there is provided a laser output control apparatus including: a laser unit constructed according to one of a first connection structure in which a cathode of a laser diode and an anode of a photo diode are commonly connected, and a second connection structure in which the cathode of the laser diode and a cathode of the photo diode are commonly connected; a power controller maintaining power of a laser beam emitted by the laser diode constant in accordance with a feedback voltage applied from the photo diode; a bias voltage generator generating a first bias voltage and a second bias voltage that make the photo diode biased in a reverse direction; and a selector selectively applying one of the first and second bias voltage to the photo diode according to one of the first and second connection structures.  
      According to yet another aspect of the present invention, there is provided a method for controlling a laser beam emitted by a laser control apparatus, including: providing a laser diode and a photo diode having first and second connection structures therebetween; generating a first bias voltage and a second bias voltage; and selectively applying one of the first and second bias voltage to the photo diode according to one of the first and second connection structures to make the photo diode biased in the reverse direction. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      These and/or other aspects, features, and advantages of the invention will become apparent and more readily appreciated from the following description of the exemplary embodiments, taken in conjunction with the accompanying drawings of which:  
       FIG. 1  is a block diagram of a laser output control apparatus according to an exemplary embodiment of the present invention;  
       FIGS. 2A and 2B  illustrate examples of a connection structure of a laser diode and a photo diode that constitute a laser unit of  FIG. 1 ;  
       FIG. 3  is a circuit diagram of a power controller of  FIG. 1  according to an exemplary embodiment of the present invention;  
       FIG. 4  is a circuit diagram of a bias voltage application unit of  FIG. 1  according to an exemplary embodiment of the present invention;  
       FIG. 5  is a block diagram of a second bias voltage generation unit of  FIG. 4  according to an exemplary embodiment of the present invention;  
       FIG. 6  is a detailed circuit diagram of the second bias voltage generation unit of  FIG. 4  according to an exemplary embodiment of the present invention; and  
       FIGS. 7A through 7F  are waveform diagrams of the operations of elements of  FIG. 6 .  
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      Reference will now be made in detail to exemplary 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 exemplary embodiments are described below to explain the present invention by referring to the figures.  
      Referring to  FIG. 1 , a laser output control apparatus according to an exemplary embodiment of the present invention includes a laser unit  110 , a power controller  120 , and a bias voltage application unit  130 .  
      The laser unit  110  includes a laser diode (LD) acting as a light-emitting unit and a photo diode (PD) acting as a light-receiving unit. A terminal of the LD and a terminal of the PD are commonly connected to each other. Examples of a connection of the LD and the PD that constitute the laser unit  110  will be described with reference to  FIGS. 2A and 2B .  
      A laser unit  110  of  FIG. 2A  includes an LD  210  that is biased in a forward direction and a PD  220  that is biased in a reverse direction. The laser unit  110  of  FIG. 2A  has a first connection structure in which a cathode of the LD  210  and an anode of the PD  220  are connected to each other at a common terminal and the common terminal is grounded. In the first connection structure, a feedback voltage detected from a cathode of the PD  220  is applied to the power controller  120  of  FIG. 1 .  
      A laser unit  110  of  FIG. 2B  includes an LD  230  that is biased in a forward direction and a PD  240  that is biased in a reverse direction. The laser unit  110  of  FIG. 2B  has a second connection structure in which a cathode of the LD  230  and a cathode of the PD  240  are connected to each other at a common terminal and the common terminal is grounded. In the second connection structure, a feedback voltage detected from an anode of the PD  240  is applied to the power controller  120  of  FIG. 1 .  
      As in Automatic Power Control (APC), the power controller  120  changes the size of a driving current supplied to the LD according to the size of the feedback voltage applied from the laser unit  110 , thereby maintaining the power of a laser beam emitted by the LD constant. The feedback voltage is determined by a monitoring current and a bias voltage. The monitoring current is obtained by converting a laser beam output from the PD with respect to a laser beam output from the LD into a current. The bias voltage is used to make the PD biased in the reverse direction. In the case of the laser unit  110  with the first connection structure ( FIG. 2A ), the bias voltage is a positive voltage. In the case of the laser unit  110  with the second connection structure ( FIG. 2B ), the bias voltage is a negative voltage.  
      The bias voltage application unit  130  applies a bias voltage that makes the PD biased in the reverse direction to the laser unit  110 . Whether a positive voltage or a negative voltage is applied to the PD is determined depending on a connection structure of the LD and the PD of the laser unit  110 . When the laser unit  110  has the first connection structure shown in  FIG. 2A , a voltage obtained by adjusting a positive voltage is applied to the cathode of the PD. When the laser unit  110  has the second connection structure of  FIG. 2B , a voltage obtained by adjusting a negative voltage is applied to the anode of the PD.  
       FIG. 3  is a circuit diagram of the power controller  120  of  FIG. 1  according to an exemplary embodiment of the present invention. Referring to  FIG. 3 , the power controller  120  includes a comparator  310 , an automatic power controller  320 , a driving unit  330 , and an LD  340 . The comparator  310  compares an input signal and a feedback voltage V f , and provides a control signal representing the result of comparison to the automatic power controller  330 . The comparator  310  may be embodied as a NAND gate. When applied to a laser scanning unit, the input signal is image data.  
      The automatic power controller  320  generates a driving signal having a current and a voltage determined by the control signal and provides the driving signal to the driving unit  330  so as to maintain the power of a laser beam emitted by the LD  340  constant. The automatic power controller  320  may be an Application-Specific Integrated Circuit (ASIC) chip that has been used widely.  
      The driving unit  330  generates a predetermined driving current in response to the driving signal generated by the automatic power controller  320  and provides the driving current to the LD  340  that is biased in the forward direction. Then, the LD  340  emits a laser beam whose magnitude is proportional to that of the driving current.  
       FIG. 4  is a circuit diagram of the bias voltage application unit  130  of  FIG. 1  according to an exemplary embodiment of the present invention. Referring to  FIG. 4 , the bias voltage application unit  130  includes a first bias voltage generating unit  410 , a second bias voltage generating unit  420 , a selector  430 , a variable resistor  440 , and a PD  450 .  
      When the laser unit  110  of  FIG. 1  has the first connection structure shown in  FIG. 2A , the first bias voltage generating unit  410  generates a first bias voltage V cc  that makes the PD  450  biased in a reverse direction. The first bias voltage V cc  is a positive voltage, e.g., a power supply voltage of +5V that is generally applied to a laser scanning unit.  
      When the laser unit  110  has the second connection structure of  FIG. 2B , the second bias voltage generating unit  420  generates a second bias voltage −V ee  that makes the PD  450  biased in a reverse direction. The second bias voltage −V ee  is a negative voltage, e.g., −5V. Since a negative voltage is not applied to the laser scanning unit from an external source, the negative voltage is generated and used in the laser scanning unit.  
      The selector  430  selects and outputs one of the first bias voltage V cc  generated by the first bias voltage generating unit  410  and the second bias voltage −V ee  generated by the second bias voltage generating unit  420 . The selector  430  may be embodied as a switch having an output contact point T 1 , and first and second input contact points T 2  and T 3 . During a manufacture process, the selector  430  may be manipulated in advance according to a connection of the LD and the PD of the laser unit  110 . For instance, when the laser unit  110  has the first connection structure, the selector  430  may be installed such that the output contact point T 1  contacts the first input contact point T 2 . When the laser unit  110  has the second connection structure, the selector  430  may be installed such that the output contact point T 1  contacts the second input contact point T 3 .  
      The variable resistor  440  generates a feedback voltage V f  by dropping the first or second bias voltage in accordance with a monitoring current whose magnitude is proportional to that of a light emitted by the PD  450 , and provides it to the power controller  120 .  
       FIG. 5  is a block diagram of the second bias voltage generating unit  420  of  FIG. 4  according to an exemplary embodiment of the present invention. Referring to  FIG. 5 , the second bias voltage generating unit  420  includes an oscillator  510 , a current controller  520 , and a negative voltage generator  530 . The operations of these elements of the second bias voltage generating unit  420  will now be described with reference to  FIGS. 6 and 7 A through  7 F.  
      The oscillator  510  oscillates at a predetermined oscillation frequency to obtain a first clock signal and a second clock signal with different phases. For instance, the oscillator  510  performs oscillation using a Schmidt trigger circuit such as that shown in  FIG. 6  at an oscillation frequency of 22 KHz obtained by adjusting a value of a resistor or a value of a capacitor. Referring to  FIG. 6 , an oscillator  510  generates a first clock signal V 1  whose waveform is illustrated in  FIG. 7A  and a second clock signal V 2  whose waveform is illustrated in  FIG. 7B . The phase difference between the first clock signal V 1  and the second clock signal V 2  corresponds to 90 degrees.  
      The current controller  520  generates a third clock signal V 3  and a fourth clock signal V 4  using the first and second clock signals V 1  and V 2  so as to increase the current capacity for the second bias voltage −V ee . Referring to  FIG. 6 , a current controller  520  uses two transistors in which the first and second clock signals V 1  and V 2  are input to their base terminals, respectively, and a third clock signal V 3  whose waveform is illustrated in  FIG. 7C  and a fourth clock signal V 4  whose waveform is illustrated in  FIG. 7D  are output from their emitter terminals, respectively. In this case, a transistor that has a comparatively high current gain h fe  and a comparatively low collector-emitter saturated voltage V ce(sat)  with respect to a rated current of the PD  450  actually used, is preferably used.  
      The negative voltage generator  530  generates a fifth cock signal V 5  with a negative double voltage using the third and fourth clock signals V 3  and V 4  and generates the second bias voltage −V ee  using the fifth cock signal V 5 . The waveform of the fifth cock signal V 5  is illustrated in  FIG. 7E , and the waveform of the second bias voltage −V ee  is illustrated in  FIG. 7F . The fifth cock signal V 5  may be obtained using a charge pumping circuit such as that shown in  FIG. 6 . The second bias voltage −V ee  may be obtained by rectifying the fifth cock signal V 5  generated by the charge pumping circuit using a diode and a capacitor shown in  FIG. 6 .  
      The present invention is not limited by the structure of the second bias voltage generator  420  shown in  FIGS. 5 and 6 . A negative voltage required by the present invention can be generated using one of various circuits. In this disclosure, the second bias voltage −V ee  of −5V is used for convenience. That is, the second bias voltage −V ee  is not limited, and can be generated using an external negative voltage.  
      A laser output control apparatus according to the present invention may be installed in a laser scanning unit applicable to office appliance, such as a laser printer, a laser facsimile, or a multi-functional peripherals, that requires high-speed on/off switching.  
      As described above, in a laser output control apparatus according to the present invention, a different bias voltage is selectively applied to a photo diode according to a connection structure in which a laser diode and the photo diode of a semiconductor laser device are connected to each other. Accordingly, it is possible to use a semiconductor laser device in office appliance regardless of a connection structure of the semiconductor laser, thereby allowing versatility of machine parts and reducing manufacture costs.  
      Although a few exemplary 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 exemplary embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.