Abstract:
A laser diode driving device capable of obtaining a stable pulse emission state even when variation in the current-light amount characteristic of a laser diode thereof is caused by environmental changes. A photodiode detects the amount of light emitted from the laser diode. A laser controller determines the amount of light to be emitted from the laser diode. Further, the laser controller controls the laser diode to emit light in the determined light amount. A bias current value-determining section determines a bias current value based on results of light emission performed by the laser diode in three or more kinds of light amounts determined by the laser controller.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a laser diode driving device and an optical scanning device. 
   2. Description of the Related Art 
   There has been proposed a technique related to a laser diode driving device, in Japanese Patent Laid-Open Publication No. H11-245444, in which the value of a bias current to be applied to a laser diode is determined based on two amounts of laser emission and current values corresponding to the respective amounts of laser emission. 
   However, the conventional technique has a problem with a method of calculating the bias current to be applied to the laser diode. 
   A laser diode, particularly a VCSEL (semiconductor vertical-cavity surface-emitting laser) sometimes has a current-light amount characteristic which assumes an extreme value as shown in  FIG. 6 . In a case where the current-light amount characteristic assumes an extreme value, when a threshold current value Ithc of the laser diode is calculated based on the two amounts of laser emission and the current values corresponding thereto, the threshold current value Ithc deviates from an actual threshold current value Ith by ΔI as shown in  FIGS. 7A and 7B . 
   A bias current value Ib is calculated based on the obtained threshold current value Ithc, and hence the value Ib to be set close to the target value Ith deviates therefrom. When the bias current value deviates from the target value, the light emitting characteristics of the laser become unstable. For example, when the laser is used in an optical communication apparatus, troubles occur in data transmission. Further, when the laser is used in an image forming apparatus, variation occurs in output of highlight-side density depending on the environment. 
   SUMMARY OF THE INVENTION 
   The present invention provides a laser diode driving device and an optical scanning device which are capable of obtaining a stable pulse emission state even when variation in the current-light amount characteristic of a laser diode thereof is caused by environmental changes. 
   In a first aspect of the present invention, there is provided a laser diode driving device comprising a light amount-detecting unit adapted to detect an amount of light emitted from a laser diode, a light amount-determining unit adapted to determine an amount of light to be emitted from the laser diode, a light amount control unit adapted to control the laser diode to emit light in the light amount determined by the light amount-determining unit, and a bias current value-determining unit adapted to determine a bias current value based on results of tight emission performed by the laser diode in three or more light amounts determined by the light amount-determining unit. 
   In a second aspect of the present invention, there is provided an optical scanning device comprising a light amount-detecting unit adapted to detect an amount of light emitted from a laser diode, a light amount-determining unit adapted to determine an amount of light to be emitted from the laser diode, a light amount control unit adapted to control the laser diode to emit light in the light amount determined by the light amount-determining unit, and a bias current value-determining unit adapted to determine a bias current value based on results of light emission performed by the laser diode in three or more light amounts determined by the light amount-determining unit. 
   The laser diode driving device and the optical scanning device are provided with the light amount-detecting unit adapted to detect the amount of light emitted from a laser diode, the light amount-determining unit adapted to determine the amount of light to be emitted from the laser diode, and the light amount control unit adapted to control the laser diode to emit light in the light amount determined by the light amount-determining unit. Further, the devices have the bias current value-determining unit adapted to determine a bias current value based on results of light emission performed by the laser diode in three or more light amounts determined by the light amount-determining unit. 
   With this configuration, a stable pulse emission state can be obtained even when variation in the current-light amount characteristic of the laser diode is caused by environmental changes. 
   The features and advantages of the invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram of a laser diode driving device according to an embodiment of the present invention. 
       FIG. 2  is a timing diagram of a control sequence executed by the laser diode driving device in  FIG. 1 . 
       FIG. 3  is a table of laser control modes performed by the laser diode driving device in  FIG. 1 . 
       FIG. 4  is a flowchart of a bias current-determining process executed by the laser diode driving device in  FIG. 1  so as to determine a bias current to be applied to an LD. 
       FIG. 5  is a diagram showing a characteristic of a semiconductor laser (laser diode) appearing in  FIG. 1  (No.  1 ). 
       FIG. 6  is a diagram showing a characteristic of the semiconductor laser (laser diode) appearing in  FIG. 1  (No.  2 ). 
       FIGS. 7A and 7B  are diagrams showing characteristics of a conventional laser diode. 
   

   DETAILED DESCRIPTION OF THE EMBODIMENTS 
   The present invention will now be described in detail with reference to the drawings showing an embodiment thereof. 
     FIG. 1  is a block diagram of a laser diode driving device according to the embodiment of the present invention. 
   Connected to the laser diode driving device  11  are a semiconductor laser (laser diode: hereinafter abbreviated as “the LD”)  12 , a photodiode (hereinafter abbreviated as “the PD”)  13 , and an image controller  14 . 
   The laser diode driving device  11  is comprised of a laser controller  20  and a laser driving circuit  21 . A drive current to be supplied to the LD  12  is controlled by the laser driving circuit  21 , whereby the LD  12  is caused to constantly emit a predetermined amount of light. 
   The laser controller  20  functions as a light amount-determining unit for determining the amount of light to be emitted from the LD  12 . The laser controller  20  also functions as a light amount control unit for controlling the LD  12  so as to cause the same to emit an amount of light determined by the light amount-determining unit. 
   The PD  13  as a light amount-detecting unit for monitoring a laser beam output from the LD  12  (i.e. detecting the amount of light from the laser diode) outputs an electric current corresponding to the light amount of the monitored laser beam. A light amount-adjusting variable resistor  23  performs adjustment such that the LD  12  emits a predetermined amount of light. 
   The electric current (PD current)  22  output from the PD  13  according to the light amount of the monitored laser beam is converted into voltage by the light amount-adjusting variable resistor  23  and is output as a PD voltage signal  24 . The PD voltage signal  24  is input to a sample/hold circuit  27  together with a light amount-determining signal  26  output from the laser controller  20 . 
   When a sample/hold (hereinafter abbreviated as S/H) control signal  29  output from the laser controller  20  requests sampling, the sample/hold circuit  27  makes a comparison between the PD voltage signal  24  and the light amount-determining signal  26 . 
   Then, when the PD voltage signal  24  is lower than the light amount-determining signal  26 , the PD voltage signal  24  is charged in a hold capacitor  51 , whereas when the PD voltage signal  24  is higher than the light amount-determining signal  26 , the PD voltage signal  24  is discharged from the hold capacitor  51 . Thus, a voltage value  28  dependent on the PD current  22  output from the PD  13  is caused to increase or decrease, whereby the LD  12  is controlled to the predetermined amount of light. 
   When the S/H control signal  29  requests holding, the voltage value  28  determined based on a result obtained depending on the PD current  22  when sampling was requested is held in the hold capacitor  51 . 
   A current mirror circuit  31  as a current control circuit is comprised of transistors  31   a  and  31   b . The mirror ratio of the current mirror circuit  31  is set to e.g. approximately 40. When data output is requested, the voltage value  28  output from the sample/hold circuit  27 , which is dependent on the PD current  22  output from the PD  13 , is input to the positive input terminal of an operational amplifier  52  in response to the S/H control signal  29  input from the laser controller  20 . As a consequence, an electric current output from the emitter of a transistor  53  flows through a resistor  54 . 
   It should be noted that the value of voltage generated across the resistor  54  is output as an LD current detection signal  56  to the laser controller  20  via an operational amplifier  55 . Since the mirror ratio of the current mirror circuit  31  is set to approximately 40, a laser drive current  32  output from a collector side of the transistor  31   a  of the current mirror circuit  31  becomes approximately 40 times larger than the current flowing through the resistor  54 . 
   A differential receiver (LVDS)  35  having a differential input receives a non-inverted data signal  33  and an inverted data signal  34  each input from the image controller  14 . An output selection circuit  38  outputs a first switching signal  39  or a second switching signal  40  determined by the S/H control signal  29  or a data output control signal  30 . 
   A current driver  41  has transistors  41   a  and  41   b  and is configured as a differential amplifier by connecting emitter terminals of the respective transistors  41   a  and  41   b  to each other. The transistor  41   a  switchingly drives the LD  12  based on the first switching signal  39 , using the laser drive current  32 . Similarly, the transistor  41   b  switchingly drives a load resistor  42  based on the second switching signal  40 , using the laser drive current  32 . 
   A bias current-determining section  57  outputs an electric current according to a bias current-setting signal  50  such that the ratio of a value of a bias current supplied to the LD  12  to a laser threshold current value Ith of the LD  12  or the amount of the bias current does not change. 
   The function of the bias current-determining section  57  as a bias current value-determining unit will be described in more detail. 
   The bias current-determining section  57  determines a bias current value based on the values of respective electric currents applied to the LD  12  when the LD  12  is caused to emit light in two different amounts. 
   Further, the bias current-determining section  57  determines a bias current value through comparison between bias current values obtained from the values of the respective electric currents applied to the LD  12  when the LD  12  is caused to emit light in the two different amounts. 
   Furthermore, the bias current-determining section  57  determines a bias current value based on the values of respective electric currents applied to the LD  12  when the LD  12  is caused to emit light in two amounts except a maximum amount which are selected from a three or more amounts. 
   Now, various kinds of laser control modes performed by the laser diode driving device  11  in  FIG. 1  will be described with reference to  FIGS. 2 and 3 . 
   (1) When the laser diode driving device  11  is in a laser control mode in which the laser driving circuit  21  is set to perform automatic light amount control (“APC (Automatic Power Control)), and the sample/hold circuit  27  is in a sampling state, the output selection circuit  38  forcibly outputs ON data to cause the LD  12  to emit light, irrespective of a receiver non-inverting output signal  36  and a receiver inverting output signal  37  output from the LVDS  35 , to thereby perform, using a signal corresponding to a difference between the PD voltage signal  24  and the light amount-determining signal  26 , control for adjusting the amount of light emitted from the LD  12  to a predetermined light amount as follows: 
   In the case of PD voltage signal  24 &gt;light amount-determining signal  26 : It is determined that the light emission amount of the LD  12  is larger than the predetermined light amount, and the hold capacitor  51  is discharged. As a consequence, the voltage value  28  dependent on the PD current  22  output from the PD  13  is lowered to reduce the laser drive current  32 , whereby the light emission amount of the LD  12  is reduced. 
   In the case of PD voltage signal  24 &lt;light amount-determining signal  26 : It is determined that the light emission amount of the LD  12  is smaller than the predetermined light amount, and the hold capacitor  51  is charged. As a consequence, the voltage value  28  dependent on the PD current  22  output from the PD  13  is raised to increase the laser drive current  32 , whereby the light emission amount of the LD  12  is increased. 
   In the case of PD voltage signal  24 =light amount-determining signal  26 : It is determined that the light emission amount of the LD  12  is equal to the predetermined light amount, and the hold capacitor  51  is neither charged nor discharged. As a consequence, both the voltage value  28  dependent on the PD current  22  output from the PD  13  and the laser drive current  32  are neither increased nor reduced. 
   (2) When the laser diode driving device  11  is in a laser control mode in which the laser driving circuit  21  is set to forcibly turn off the laser (OFF), and the sample/hold circuit  27  is in a holding state, the voltage value  28  set by the sample/hold circuit  27  depending on the PD current  22  output from the PD  13  is held, and the output selection circuit  38  forcibly outputs OFF data to turn off the LD  12  to stop light emission, irrespective of the receiver non-inverting output signal  36  and the receiver inverting output signal  37 . 
   (3) When the laser diode driving device  11  is in a laser control mode in which the laser driving circuit  21  is set to output data (DATA OUTPUT), and the sample/hold circuit  27  is in the holding state, the output selection circuit  38  outputs a signal corresponding to the receiver non-inverting output signal  36  or the receiver inverting output signal  37 , using voltage value  28  set by the sample/hold circuit  27  depending on the PD current  22  output from the PD  13 , whereby an electric current is caused to flow through the LD  12  or the resistor  42 . 
   (4) When the laser diode driving device  11  is reset, the laser driving circuit  21  is brought into a reset state, and the electric current set in the sample/hold circuit  27  is initialized, and at the same time, the output selection circuit  38  forcibly outputs OFF data to turn off the LD  12 . 
     FIG. 4  is a flowchart of a bias current-determining process executed by the laser diode driving device in  FIG. 1  so as to determine a bias current to be applied to the LD. 
   First, a description will be given of steps S 101  to S 105 . 
   I. The above-mentioned automatic light amount control is executed so as to adjust the light emission amount of the LD  12  to a light amount P aPc  for data light emission. 
   First in the step S 101 , n is set to 1, and then in a step S 102 , APC light emission is executed with the light amount P aPc . Voltage having a value corresponding to the value of electric current applied to the LD  12  in this state is output between the resistor  54  and ground GND. At this time, the light amount-determining signal  26  is set to V ref-aPc . When the resistance value of the resistor  54  is represented by R 54 , and an electric current flowing through the LD  12  by I aPc , a voltage V 54aPc  generated between the resistor  54  and the GND can be calculated by the following equation:
 
 V   54aPc   =I   aPc   ×R   54 /40  A
 
   The voltage value of V 54aPc  is output to the laser controller  20  via the operational amplifier  55 . In the step S 105 , the electric current I aPc  is calculated from the equation A, and the calculated current value is stored in a memory of the laser controller  20 . In this connection, the current value is calculated by the following equation:
 
 I   aPc =40× V   54aPc   /R   54  
 
   II. APC is executed with a light amount P aPc /2 which is half the light amount set in the stage I. 
   When the light amount-determining signal  26  is set to V ref-aPc /2 in the step S 103  so as to cause the LD  12  to emit light in the light amount P aPc /2 which is half the light amount set in the stage I, the PD voltage signal  24  becomes higher than the light amount-determining signal  26 , whereby the electric current flowing to the LD  12  is reduced as described in the case (1). A voltage V541/2aPc generated between the resistor  54  and the GND at this time can be calculated by the following equation:
 
 V   541/2aPc   =I   1/2aPc   ×R   54 /40  B
 
   The voltage value of V 541/2aPc  is output to the laser controller  20  via the operational amplifier  55 . In the step S 105 , the electric current I 1/2aPc  is calculated from the equation B, and the calculated current value is stored in the memory of the laser controller  20 . Incidentally, the current value is calculated as follows:
 
 I   1/2aPc =40× V   541/2aPc   /R   54  
 
   III. APC is executed with a light amount P aPc /3 which is one third of the light amount set in the stage I. 
   When the light amount-determining signal  26  is set to V ref-aPc /3 in the step S 104  so as to cause the LD  12  to emit light in the light amount P aPc /3 which is one third of the light amount set in the case I, the PD voltage signal  24  becomes higher than the light amount-determining signal  26 , whereby the electric current flowing to the LD  12  is reduced as described in the case (1). A voltage V541/3aPc generated between the resistor  54  and the ground GND in association with current flowing through the LD  12  at the time can be calculated by the following equation:
 
 V   541/3aPc   =I   1/3aPc   ×R   54 /40  C
 
   The voltage value of the voltage V 541/3aPc  is output to the laser controller  20  via the operational amplifier  55 . In the step S 105 , the electric current I 1/3aPc  is calculated from the equation C, and the calculated current value is stored in the memory of the laser controller  20 . In this connection, the current value is calculated by the following equation:
 
 I   1/3aPc =40× V   541/3aPc   /R   54  
 
   Next, a description will be given of a step S 106  for calculating the laser threshold current value Ith from the light amounts and the current values. 
   IV. The laser threshold current value Ith is calculated from the relationship between the light amounts P and the electric currents I associated with the LD  12 , which are calculated in the stages I, II, and III. 
   A laser threshold current value Ith (A) is tentatively calculated from the relationship between the light amounts P aPc , P aPc /2, and P aPc /3 and the electric currents I aPc , I 1/2aPc , and I 1/3aPc  corresponding to the respective light amounts. Since the light amount P and the electric current I are calculated as P aPc  and I aPc  in the stage I and as P aPc /2, and I 1/2aPc  in the stage II, the laser threshold current value Ith (A) can be calculated by the following equation:
 
 Ith ( A )= I   aPc   −P   aPc ×( I   aPc   −I   1/2aPc )/( P   aPc   −P   aPc /2)  D
 
   Then, a laser threshold current value Ith (B) is tentatively calculated from the relationship between the light amounts P and the electric currents I in the stages II and III. Since the light amount P and the electric current I are calculated as P aPc /2 and I 1/2aPc  in the stage II, and as P aPc /3 and I 1/3aPc  in the stage III, the laser threshold current value Ith (B) can be calculated by the following equation:
 
 Ith ( B ) =I   1/2aPc   −P   aPc /2×( I   1/2aPc   −I   1/3aPc )/( P   aPc /2 −P   aPc /3)  E
 
   Finally, a description will be given of steps S 107  to S 109 . 
   V. The two laser threshold current values calculated in the stage IV are compared with each other to thereby determine a bias current to be applied to the LD  12 . 
   If it is determined in the step S 107 , based on the result of comparison between the laser threshold current values Ith (A) and Ith (B), that an error of Ith (A) with respect to Ith (B) is e.g. within 5%, it is judged that the relationship between the light amount and the electric current corresponds to that between linear expressions denoted by E 1  and D 1  in  FIG. 5  (which means that the relationship between the light amount of which values are P aPc , P aPc /2, and P aPc /3 and the current of which respective associated values are I aPc , I 1/2aPc , and I 1/3aPc  can be expressed by a single linear expression). In this case, it can be said that the calculated laser threshold current value Ith and the actual laser threshold current value Ith of the LD  12  are approximately identical in characteristic, and therefore in the step S 108 , the bias current value is determined using the laser threshold current value Ith (A). 
   Now, the reason for determining the bias current value using the laser threshold current value Ith (A) will be explained. The voltage V 54  generated between the resistor  54  and the GND is used as a value for use in calculating the laser threshold current value Ith, but when the light amount of the LD  12  is small, due to a low value of the voltage V 54 , an error increases in the calculating the laser threshold current value Ith with high accuracy. 
   Another reason is that when the LD  12  is a VCSEL, the rated amount per se of light that can be emitted is small, and hence not only further reduction of the value of the voltage V 54 , but also reduction of the output of the PD  13  occur, which causes an increase in error in the APC. 
   The bias current Ib is calculated in the step S 108 , provided that the bias current value is set to 90% of the laser threshold current value Ith (A), by the following equation:
 
 Ib= 0.9× Ith ( A )
 
   On the other hand, if it is determined in the step S 107 , based on the result of comparison between the laser threshold current values Ith (A) and Ith (B), that the error of Ith (A) with respect to Ith (B) is more than 5%, it is judged that the relationship between the light amount and the electric current corresponds to that between linear expressions denoted by E 2  and D 2  in  FIG. 6  ((which means that the relationship between the light amount of which values are P aPc , P aPc /2, and P aPc /3 and the current of which respective associated values are I aPc , I 1/2aPc , and I 1/3aPc  cannot be expressed by a single linear expression). In this case, it can be said that the calculated laser threshold current value Ith and the actual laser threshold current value Ith of the LD  12  are largely different in characteristic. 
   Therefore, in the step S 109 , n is incremented by 1 so as to reduce the light amount to a lower level than the light amount P aPc /3 in which light was emitted in the step S 104 , and then in the step S 104 , APC is executed with a light amount P aPc /4 which is one forth of the light amount P aPc . Then, in the step S 106 , a laser threshold current value Ith (C) is calculated from the light amounts P aPc /3 and P aPc /4 and the currents I 1/3aPc  and I1/4aPc corresponding to the respective light amounts. Further, in the step S 107 , the laser threshold current values Ith (B) and Ith (C) are compared in the same manner as described above, to thereby determine whether or not the error between the two values falls within 5%. 
   This step is repeatedly carried out until the error between the two values falls within 5%. By doing this, it is possible to set the bias current accurately even when the laser diode has a non-linear characteristic having a maximum output light amount on its non-linear portion as shown in  FIG. 6 . 
   It should be noted that APC is always executed with the light amount P aPc  during a time period over which the LD  12  performs data output, e.g. in sampling timing in  FIG. 2  by way of example. Further, the steps S 103  et seq. for determining the bias current value Ib may be carried out during a time period over which data output is not performed as in  FIG. 2 . In such a case, when n is set to 1 and it is determined in the step S 107  that the error is more than 5%, and when the sequence for determining the bias current value Ib is to be operated again, the operation may be started by setting n to 2. Although in the present embodiment, control is performed for a single laser diode, it is possible to control a plurality of laser diodes. 
   As described heretofore, by calculating a bias current value from electric current values corresponding, respectively, to three or more kinds of light amounts of a laser diode, it is possible to set the bias current value Ib close to the threshold current value Ith of the laser diode even when the light amount-electric current characteristic is like the one shown in  FIG. 6 . 
   Therefore, trouble in data transfer in an optical communication apparatus or degradation of image quality in an image forming apparatus can be suppressed. Further, since the bias current value can be set close to the threshold current value Ith even when the light amount-electric current characteristic has no linear region, it is possible to perform precise light amount control e.g. even for a VCSEL (semiconductor vertical-cavity surface-emitting laser) having one or more light emitting points. 
   While the present invention has been described with reference to an exemplary embodiment, it is to be understood that the invention is not limited to the disclosed exemplary embodiment. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all modifications, equivalent structures and functions 
   This application claims priority from Japanese Patent Application No. 2007-179915 filed Jul. 9, 2007, which is hereby incorporated by reference herein in its entirety.