United States Patent (19) 11 Patent Number: 4,878,225 Aiba Et Al

United States Patent (19) 11 Patent Number: 4,878,225 Aiba et al. 45 Date of Patent: Oct. 31, 1989

54 CONTROLLINGAPPARATUS FOR LASER 56 References Cited DODE U.S. PATENT DOCUMENTS (75) Inventors: Masahiko Aiba, Nara; Yukio Mizuno, 4,237,427 12/1980 Holland ...... 372/29 Yamatokoriyama, both of Japan Primary Examiner-William L. Sikes 73 Assignee: Sharp Kabushiki Kaisha, Osaka, Assistant Examiner-B. R. R. Holloway Japan Attorney, Agent, or Firm-Birch, Stewart, Kolasch & Birch 21 Appl. No.: 326,453 57 ABSTRACT 22 Filed: Mar 20, 1989 A controlling apparatus for a laser diode which is used in a laser printer or the like to transmit laser beam light rays. According to the output characteristics of the Related U.S. Application Data laser diode, a bias is applied in the current region (from 63 Continuation of Ser. No. 89,532, Aug. 26, 1987, aban 0 to Ixm) of approximately 0 in radiation output. The doned. control object period (from Ix to Ixm) is divided into many stages to perform the radiation output control so 30 Foreign Application Priority Data as to effect the high-precision output control without Aug. 26, 1986 JPl Japan ...... 6-2004.04 the provision of a more complicated circuit. The central Aug. 27, 1986 JP Japan ...... 61-2O2467 processing unit outputs the control signal into the duty controlling means, so that the output of the laser diode 5ll Int. Cl“...... HOS 3/OO may be automatically held constant in accordance with 52 U.S. C...... 372/38; 372/29; the predetermined process or the operation arranged in 372/30 advance. 58 Field of Search ...... 372/8, 9, 25, 29, 30, 372/33, 38 13 Claims, 5 Drawing Sheets

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U.S. Patent Oct. 31, 1989 Sheet 2 of 5 4,878,225 Afg. 2

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Current

reference VOue U.S. Patent Oct. 31, 1989 Sheet 3 of 5 4,878,225. Afg. 3

START

V n APCEN F. Low

2 Counter A - 28

Counter BE 28 n3

counter B=B/2 5 6

n 8

NO Ounter B= in 9

YES t no APCEN High

END U.S. Patent Oct. 31, 1989 Sheet 4 of 5 4,878,225 s

VOO ssale 33

i s s U.S. Patent Oct. 31, 1989 Sheet S of 5 4,878,225

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PCON

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6 LMNT s 4. R7 A

3 4,878,22 1. 2 Also, the present invention comprises a duty control CONTROLLINGAPPARATUS FORLASER DODE ling means which is capable of varying the duty into the pulse of a laser power control signal in accordance with This application is a continuation of application Ser. a control signal to be inputted, a smoothing means for No. 089,532 filed on Aug. 26, 1987, now abandoned. s smoothing the laser power control signal controlled in duty by the duty controlling means, a current conver BACKGROUND OF THE INVENTION sion means for feeding into the laser diode the signal The present invention generally relates to a control smoothed by the smoothing means, a limiting means for ling apparatus for a laser diode which is used in a laser restricting the top limit of the current to be fed into the printer or the like to transmit laser beam light rays. O laser diode, a comparing means for comparing the mon Generally the output characteristics of the laser diode itor output of the laser diode with the predetermined are shown in FIG. 2. Conventionally, the current from reference voltage, and a central processing unit which 0 to In has been divided into many stages to control the responds to the output from the comparing means to radiation output. Such output control as described here guide a control signal into the duty controlling means to inabove has been performed by a circuit or the like. 15 variably instruct the pulse duty of the laser power con In the output control of the conventional laser diode, trol signal. the control object period is from 0 to In of the output Accordingly, the duty controlling means varies the characteristics of the laser diode, so that the high-preci pulse duty of the laser power control signal in accor sion controlling operation cannot be performed by the dance with the control signal to be inputted from the circuit or the like, and a complicated circuit is required 20 central processing unit for outputting the signal into the to perform this high-precision control. smoothing means. The smoothing means smooths the laser power control signal into the DC analog signal to SUMMARY OF THE INVENTION feed the current into the laser diode through the current Accordingly, an object of the present invention is to conversion means. When the light output that has been provide a controlling apparatus for a laser diode. 25 provided from the laser diode is greater than or equal to Wherein according to the output characteristics of the a predetermined value, the circuit to feed the current laser diode, the bias applied in the current region (from into the laser diode is compulsorily cut out. Also, the 0 to Ixm) is approximately 0 in radiation output, and the monitor output of the current-fed laser diode is con control object period (from Ixm to In) is divided into pared with the predetermined reference voltage by the many stages for performing the radiation output control 30 comparing means. According to this result, the central so the high-precision output is performed without the processing unit outputs the control signal into the duty provision of a more complicated circuit. controlling means, so that the output of the laser diode In accomplishing the object, according to one pre may be automatically held constant in accordance with ferred embodiment of the present invention, a control the operation arranged in advance. ling apparatus for a laser diode is provided which com 35 prises a duty controlling means capable of varying the BRIEF DESCRIPTION OF THE DRAWINGS duty into the pulse of a laser power control signal in These and other objects and features of the present accordance with a control signal to be inputted, a invention will become apparent from the following smoothing means for smoothing the laser power control description taken in conjunction with the preferred signal controlled in duty by the duty controlling means, 40 embodiment thereof with reference to the accompany a current conversion means for feeding the signal ing drawings, in which: smoothed by the smoothing means into the laser diode, FIG. 1 is a circuit block diagram of the controlling an applying means for applying a given bias current into apparatus for a laser diode of the present invention; the laser diode, a comparing means for comparing the FIG. 2 is a chart showing the relationship between monitor output of the laser diode with the predeter 45 the light output and the current value to be fed into the mined reference voltage, and a central processing unit laser diode of the apparatus; which responds to the output from the comparing FIG. 3 is a flow chart showing the automatic power means to guide a control signal into the duty controlling control operation of the apparatus; means to variably instruct the puise duty of the laser FIG. 4 is a time chart during the automatic power power control signal. Accordingly, the duty controlling 50 control operation of the apparatus; means varies the pulse duty of the laser power control FIG. 5 is a chart showing the state of the counter A signal in accordance with the control signal to be input during the automatic power control operation; and ted from the central processing unit to output the signal FIG. 6 is a block diagram showing the concrete con into the smoothing means. The smoothing means struction of the essential portion of FIG. 1. smooths the laser power control signal into the DC 55 analog signal to feed the current into the laser diode DETALED DESCRIPTION OF THE through the current conversion means. Also, as the INVENTION predetermined current is normally applied upon the Before the description of the present invention pro laser diode by the bias applying means, the bias current ceeds, it is to be noted that like parts are designated by and the current from the current conversion means are like reference numerals throughout the accompanying added. The monitor output of the current-fed laser drawings. diode is compared with the predetermined reference Referring now to the drawings, there is shown in voltage by the comparing means. FIG. 1 an automatic output controlling apparatus for a According to this result, the central processing unit laser diode using a laser printer according to one pre outputs the control signal into the duty controlling 65 ferred embodiment of the present invention. The appa means, so that the output of the laser diode may be ratus includes a central processing unit (CPU) 1, and a automatically held constant in accordance with the programmable duty control circuit 2. The programma predetermined process. ble duty control circuit 2 to programmably control the 4,878,225 3 4. pulse duty of the laser power control signals (hereinaf count value N (for example, 128) corresponding to the ter referred to as PCON) into many stages in accor predetermined duty at a counter A, a counter (n2, n3). dance with control signals to be inputted from the CPU Then, upon setting of the counter A at the count 1. A smoothing circuit 3 is provided to smooth the value N (n4), the laser power control signal (PCON) of duty-controlled laser power control signals into the DC the duty based on the count value N is transmitted (1 of analog signals as pulse wide modulation signals (herein the PCON, FIG. 4) by the programmable duty control after referred to as PWM). A current conversion circuit ling circuit 2. Also, the counter B sets a control numeri 4 controls the service current into a laser diode 5 which cal value for the automatic power control, the control transmits laser beams. The current conversion circuit numerical value B halves the N (n5) to add this value to biases the lowest threshold value current of the laser 10 the value of the counter A or subtract the halved value diode 5 and also feeds a predetermined current into the from the value of the counter A as described herein laser diode 5. Also, the laser diode 5 has a PIN diode 5 above. The value of the counter A may be gradually therein, which is a light receiving element for monitor approached into the object value through the repetition ing purposes. A comparison circuit 6 compares an oute of the na through n9 loop. put voltage from the PIN diode 5 with a reference S The PCON signal which has been generated by the voltage. The outputs from the comparison circuit 6 are programmable duty controlling circuit 2 is smoothed inputted into the central processing unit 1 as the refer into a DC analog signal by the smoothing circuit 3 so as ence signal LMNT. The pulse duty of the PCON signal to control the feed current into the laser diode 5 by the is controlled by the duty controlling circuit 2 in accor current conversion circuit 4. dance with the control signal to be inputted from the 20 The output from the laser diode 5, i.e., the output of CPU 1. An image signal conversion circuit latches the the PIN diode 5’ is compared (né) with the reference VIDSO signal for image printing by a D-type flip-flop voltage by the comparison circuit 6. As a result, the so as to output the VIDEO signal as the printing signal counter A of the programmable duty controlling circuit DATA/. An AND gate 8 and a transistor 9 turn the 2 is set (nl) to a value (N-(N/2)), which is provided laser diode 5 on and off by the LDATA or the APCEN 25 through the subtraction of the control numerical value signal. The APCEN signal becomes "high” during an (N/2) from the count value N in accordance with the automatic power control period. The flowing into the control signal to be inputted from the CPU 1 when the laser diode 5 increases as the "high” period of the reference signal LMNT is "high”. Also, when the refer PCON signal becomes longer. Accordingly, the ON ence signal LMNT is “low”, the counter A is set (n3) to duty of the PCON signal is increased so that the current 30 (N--N/2). The calculating operation is repeated before flowing into the laser diode 5 may increase when the the value of the counter B becomes 1, the smallest output of the laser diode 5 has been lowered, whereby value. When the value of the counter B becomes 1 (n9), the contrast laser output is held. The laser power con the signal of B= 1 is inputted into the CPU 1 by the trol signal PCON is inputted into the smoothing circuit programmable duty controlling circuit 2, the APCEN 3 by the programmable duty controlling circuit 2, and is 35 signal becomes "high” (n10) allowing the signal outputted into the current converting circuit 4 as the LDATA from the progress control unit 7 to turn the pulse wide modulation signal PWM so as to control the laser diode 5 on and off. current of the laser diode 5. The current conversion FIG. 5 shows the operation of the automatic power circuit 4 steadily adds the given bias into the laser diode control, with the axis of ordinates showing the value of 5. The light output of the laser diode 5 is approximately 40 the counter A, and the quadrature axis showing the zero in the low current region (to Ixm from 0) as shown frequency in of the automatic power control operation. in FIG. 2, with the actual control object period being When the output voltage of the laser diode 5 is smaller applied only to the Yportion (from Ixm to In). There in accordance with the value of the counter Athan the fore, it is necessary for the bias to be added in advance reference value as illustrated, the reference signal only for the Yportion. 45 LMNT becomes “low”. When the output voltage of the Also, the VIDEO signal is latched by the D type laser diode 5 is larger in accordance with the value of flip-flop of the process control unit 7 so as to be output the counter A, the reference signal LMNT becomes ted as the LDATA. When the LDATA is "high” in the "high”. As illustrated, the value of the counter A gradu AND gate 8, the current of the laser diode 5 bypasses a ally becomes closer to the target value through the transistor 9 to turn off the laser so as to form a non 50 repetition of the automatic power control operation. exposure portion on the sensitive member. The laser As described hereinabove, the present invention com diode 5 has a PIN diode 5' for monitoring use as de pares the reference value with the output of the PIN scribed hereinabove so that the output current of the diode for built-in monitoring use of the laser diode 5 to PIN diode 5 is used to adjust the light output and to keep the output of the laser diode constant in accor prevent the failure with respect to the excessive radia 55 dance with the operation arranged in advance through tion. The reference signal LMNT for the automatic the reference value. power control is generated after the output current of It is to be noted that only the prior processing before the PIN diode has been converted to a voltage and the printing starts is described in the present embodi compared with the reference voltage by the comparison ment. The comparison circuit 6 normally compares the circuit 6. reference voltage with the output voltage of the laser FIG. 3 is a flow chart showing the operation of the diode 5. The automatic power control operation may be automatic power control, and FIG. 4 is a time chart for performed with the APCEN signal being made “low” the operation thereof. and the image not being formed, i.e., when the LDATA At first, the CPU 1 turns the enable signal APCEN is “high”. for the automatic power control into a low state (n1) 65 Referring to FIG. 6, there is shown the concrete and the CPU 1 also outputs a control start signal into construction of the essential portion of FIG. 1. The the programmable duty controlling circuit 2. At this concrete operation will be described hereinafter with time, the programmable duty controlling circuit 2 sets a reference to the drawing. 4,878,225 5 6 The bias current up to the current Ixm of FIG. 2 is pulse of a laser power control signal in accordance with normally flowing through a transistor 10, and a resistor a control signal to be inputted, a smoothing means for R1 into the laser diode 5. The output from the program smoothing the laser power control signal controlled in mable duty controlling circuit 2 which programmably duty by the duty controlling means, a current conver controls the pulse duty of the laser power control signal sion means for feeding the signal smoothed by the into many stages in accordance with the control stages smoothing into the laser diode, a limiting means for to be inputted from the CPU 1 is fed into the current restricting the top limit of the current to be fed into the conversion circuit 4. The current conversion circuit 4 is laser diode, a comparing means for comparing the mon composed of a transistor 11, a transistor 12 and a resistor itor output of the laser diode with the predetermined R5 through R5.The current conversion circuit 4 is fed O reference voltage, a central processing unit which re by the smoothing circuit 3 which is composed of resis sponds to the output from the comparing means to tors R2, R3, R4, capacitors C1, C2. The laser diode 5 is guide a control signal into the duty controlling means to controlled by the current conversion circuit 4. variably instruct the pulse duty of the laser power con The light output from the laser diode 5 is detected by trol signal. Therefore, when a light output that is the diode 5' for feedback control use so as to be inputted 15 greater than or equal to a predetermined value has been into the comparison circuit 6 for comparison with the provided from the laser diode, the detected light output reference voltage. The reference voltage circuit which compulsorily cuts out a circuit which feeds the current develops the reference voltage is composed of the resis to the laser diode. Thereby, a laser diode controlling is tors R6, RT, R8, with the comparison value being input provided to prevent the control elements and so on ted into the CPU 1. 20 from failing without the provision of a complicated Also, at this time, when the output of the diode 5' for feedback controlling use is detected to be greater than circuit. or equal to the reference voltage by a zener diode 13, a Although the present invention has been fully de transistor 14 is turned on so as to force the base electric scribed by way of example with reference to the accom potential of the transistor 11 to be lowered by the auto 25 panying drawings, it is to be noted here that various matic power control (APC) to turn off the transistor 11. changes and modifications will be apparent to those Therefore, the operation of the current conversion cir skilled in the art. Therefore, unless such changes and cuit 4 is prohibited and the application of current that is modifications depart from the scope of the present in greater than or equal to a predetermined value of cur vention, they should be construed as being included rentis prevented from being applied to the laser diode 5. 30 therein. w In addition, a data signal which is actually related to the What is claimed is: record is fed into the laser diode 5 through the AND 1. A laser diode controlling apparatus for controlling gate 8 and the transistor 9. a current supplied to a laser diode that is used for print Accordingly, when the light output of that is greater ing in a laser printing apparatus provided with a moni than or equal to a predetermined value has been pro 35 tor that monitors light intensity of the laser diode by vided from the laser diode 5, the detected light output developing a monitoring signal related thereto, com forces the current conversion circuit 4, which feeds the prising: current into the laser diode 5, to break off and prevent comparing means for comparing the monitoring sig the control elements from failing. mal developed by the laser diode with a predeter As described hereinabove, the bias current is more 40 mined reference intensity signal and developing a mally applied upon the laser diode 5 and therefore, the reference signal in response thereto; control object period may be made Y (from Ixm to In), central processing means for developing a pulse sig as only the control object period Y is divided into many nal representative of a desired light intensity level stages for output control operation, so that the precise in response to said reference signal received from laser diode controlling operation may be effected. 45 said comparing means; As is clear from the foregoing description, the present duty cycle controlling means responsive to said pulse invention comprises a duty controlling means which is signal for developing a duty cycle control signal capable of varying the duty into the pulse of a laser having a duty cycle related to the current to be power control signal in accordance with a control sigo supplied to the laser diode; mal to be inputted, a smoothing means for smoothing the 50 smoothing means for developing a DC analog signal laser power control signal controlled in duty by the by smoothing said duty cycle control signal; and duty controlling means, a current conversion means for current conversion means for supplying a drive cur feeding the signal smoothed by the smoothing means rent to the laser diode related to the DC analog into the laser diode, an applying means for applying a signal, the laser diode developing light intensity given bias current into the laser diode, a comparing 55 related to said drive current; means for comparing the monitor output of the laser said central processing means developing said pulse diode with the predetermined reference voltage, a cene signal and thereby enabling adjustment of light tral processing unit which responds to the output from intensity of the laser diode only during an initializa the comparing means to guide a control signal into the tion period preceding printing by the laser diode. duty controlling means to variably instruct the pulse 60 2. A laser diode controlling apparatus as defined in duty of the laser power control signal. Therefore, the claim 1, further comprising: radiation output controlling operation may be effected applying means for applying a bias current to said only through the dividing of the control object period laser diode in response to a video signal from said into many stages without a complicated circuit so as to central processing means that switches said laser provide a high-precision laser diode controlling appara 65 diode on and off during printing by the laser diode tus. after said initialization period is completed. Also, the present invention comprises a duty control 3. A laser diode controlling apparatus as defined in ling means which is capable of varying the duty into the claim 1, wherein said initialization period is developed 4,878,225 7 8 in response to an automatic power control signal gener said central processing means that switches said ated by said central processing means. laser diode on and off during printing by the laser 4. A laser diode controlling apparatus as defined in diode after said initialization period is completed. claim 1, wherein said central processing means com 9. A laser diode controlling apparatus as defined in prises: 5 claim 7, wherein said duty cycle control signal is stored (a) means for setting an initial count value corre by said duty controlling means after said non-printing sponding to an initial value of said duty control period is completed; and signal; said duty cycle control signal being constantly sup (b) means for initialization said count value as an plied to the laser diode during said printing period. initial iteration value; O 10. A laser diode controlling apparatus as defined in (c) means for dividing said iteration value by a prede claim 5, wherein said central processing means com termined iteration value to develop a current iterae prises: tion value; (d) means for adding or subtracting said current itera (a) means for setting an initial count value corre tion value to said count value in response to the 5 sponding to an initial value of said duty control value of said reference signal for revising said ini signal; tial duty cycle signal; and (b) means for initialization said count value as an (e) means for repeating steps (c) and (d) to develop initial iteration value; increasingly accurate control of said current for (c) means for dividing said interation value by a pre producing light intensity of the laser diode at a 20 determined iteration value to develop a current predetermined constant intensity level, said steps of iteration value; repeating being discontinued when said iteration (d) means for adding or subtracting said current itera value executes a predetermined number of itera tion value to said count value in response to the tions. value of said reference signal for raising said initial 5. A laser diode controlling apparatus for controlling 25 duty cycle signal; and a current supplied to a laser diode that is used for print (e) means for repeating said steps (c) and (d) to de ing in a laser printing apparatus provided with a monio velop increasingly accurate control of said current tor that monitors light intensity of the laser diode by for producing light intensity of the laser diode at a developing a monitoring signal related thereto, come predetermined constant intensity level, said steps of prising: 30 repeating being discontinued when said iteration comparing means for comparing the monitoring sig value executes a predetermined number of itera nal developed by the laser diode with a predeter tions. mined reference intensity signal and developing a 11. A laser diode controlling apparatus for control reference signal in response thereto; ling a current supplied to a laser diode that is used in a central processing means for developing a pulse sig 35 laser printing apparatus, said laser diode being provided nal representative of a desired intensity level in with a monitor that monitors light intensity output from response to said reference signal received from said the laser diode by developing a monitoring signal re comparing means; lated thereto, said controlling apparatus varying the duty cycle controlling means responsive to said pulse current during a non-printing period and disabling mon signal for developing a duty cycle control signal itoring of light intensity output during a printing period, having a duty cycle related to the current to be comprising: supplied to the laser diode, said duty cycle control signal being stored by said duty controlling means; comparing means for comparing the monitoring sig smoothing means for developing a DC analog signal nal devleoped by the laser diode with a predeter by smoothing said duty cycle control signal; and 45 mined reference intensity signal and developing a current conversion means for supplying a drive cure reference signal in response thereto; rent to the laser diode related to the DC analog central processing means for developing a pulse sig signal, the laser diode developing light intensity nal representative of a desired intensity level in related to said drive current; response to said reference signal received from said said central processing means developing said pulse 50 comparing means; signal and thereby enabling adjustment of light duty cycle controlling means responsive to said pulse intensity of the laser diode only during an initializa signal for developing a duty cycle control signal tion period preceding printing by the laser diode. having a duty cycle related to the current to be 6. A laser diode controlling apparatus as defined in supplied to the laser diode; claim 5, further comprising: 55 smoothing means for developing a DC analog signal applying means for applying a bias current to said by smoothing said duty cycle control signal; and laser diode in response to a video signal from said current conversion means for supplying a drive cur central processing means that switches said laser rent to the laser diode related to the DC analog diode on and off during printing by the laser diode signal, the laser diode developing light intensity after said initialization period is completed. 60 related to said drive current; 7. A laser diode controlling apparatus as defined in said central processing means developing said pulse claim 5, wherein said initialization period is developed signal and enabling adjustment of light intensity of in response to an automatic power control signal gener the laser diode only during an initialization period ated by said central processing means. corresponding to said non-printing period preced 8. A laser diode controlling apparatus as defined in 65 ing said printing period. claim 7, further comprising: 12. A laser diode controlling apparatus as defined in applying means for applying a bias current to said claim 11, wherein said central processing means com laser diode in response to a data print signal from prises: 4,878,225 10 (a) means for setting an initial count value corre operation performed in a central processing means com sponding to an initial value of said duty control prising the steps of: signal; (a) setting an initial count value corresponding to an (b) means for initializing said count value as an initial initial duty cycle signal; iteration value; (b) initializing said current value as an initial iteration (c) means for dividing said iteration value by a prede value; termined iteration value to develop a current itera (c) dividing said iteration value by a predetermined tion value; iteration value to develop a current iteration value; (d) means for adding or subtracting said current itera (d) developing a reference signal having a value in tion value to said count value in response to the 10 dicative of whether the current signal supplied to value of said reference signal for revising said ini the laser diode is greater than the predetermined tial duty cycle signal; constant intensity level; (e) means for repeating said steps (c) and (d) to de (e) adding or subtracting said current iteration value velop increasingly accurate control of said current to said count value in response to the value of said for producing light intensity of the laser diode at a 15 reference signal for revising said initial duty cycle predetermined constant intensity level, said steps of signal; and repeating being discontinued when said iteration (f) repeating said steps (c)-(e) to develop increasingly value executes a predetermined number of itera accurate control of said current for producing light tions. intensity of the laser diode at the predetermined 13. A method for maintaining a current to a laser 20 constant intensity level, said step of repeating being diode that outputs light at a predetermined constant discontinued when said iteration value executes a intensity level by developing a current signal to be predetermined number of iterations. supplied to the laser diode in response to an interation 25