Patent Publication Number: US-2009219964-A1

Title: laser diode driver

Description:
FIELD OF THE INVENTION 
     The present invention relates to activating a laser diode by applying a high current during a transient rise time for the laser, and a lower voltage during operation of the laser. 
     BACKGROUND OF THE INVENTION 
     A common electrical circuit to start a laser diode is shown in  FIG. 1 . A prior art electrical circuit design for a laser diode includes a voltage source  12 , which is applied to start laser diode  10  as well as to supply the power required for conducting continuous operation of the laser diode. 
     A computer  18  initiates transmission of digital data to be imaged using laser diode  10 . The data is supplied in synchronization with voltage source  12  through a digital control unit  15 , typically a field programmable gate array (FPGA). The current supplied to the laser diode  10  is controlled by current control unit  17 . 
     Because of the expense of laser diodes used in printing heads it is desirable to extend the life expectancy of the laser diodes. Some of the factors which may contribute to the shorter life expectancy for the diodes includes high operating temperatures and rough voltage transitions. 
     SUMMARY OF THE INVENTION 
     Briefly, according to one aspect of the present invention an apparatus for driving a laser diode includes a first voltage source for powering the laser diode during a transient period of operation of the laser diode. A second voltage source, having a lower voltage than the first voltage source, powers the laser diode during continuous operation of the laser diode. 
     The present invention describes an apparatus and method for invoking a laser diode having a short rise/fall time while maintaining high system power efficiency. In one embodiment, the invention utilizes two different power voltage sources and one current drive module in conjunction with the laser diodes. A higher power voltage source is used during the transient rise time, and thus, for a short period. Once the laser diode is energized, the lower power voltage source is used for continuous laser diode operation. 
     Using two power supply sources, the higher power supply source level will be used to drive the current during the transient time of the laser diode operation, at the stage of starting the laser diode, and not during normal continuous laser diode operation. During the normal operation of the laser diode a lower power supply source will be used. The use of the dual power supply sources described above is advantageous. The use of high power supply source for a short period of time for starting the laser diode, and the lower power supply source for continuous operation of the laser diode, have a major role in extending the life expectancy of the laser diode. 
     Additional features and advantages of the invention will become apparent from the following drawings and description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a prior art schematic illustrating a laser diode voltage driver control; 
         FIG. 2  is a schematic illustrating a laser diode dual voltage driver control each using a dedicated power supply external power supply; 
         FIG. 3  is a schematic illustrating a laser diode dual voltage driver control using a single external power supply for both voltage drivers; and 
         FIG. 4  is a schematic illustrating laboratory results for laser driver power supply simulation. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention discloses methods and apparatus for a laser diode driver. A laser driver, according to the present invention, has the following advantages:
         a) The laser driver has a short rise time, typically less than 250 nanoseconds, using a high power voltage source.   b) The life expectancy of the laser diode power voltage source is extended by using a lower power voltage source than the one applied for starting the laser diode, during the continuous operation of the laser diode.   c) A smooth transient of current through the diode prevents high temperature from developing in the diode.       

     Referring to  FIG. 3 , computer  18  sends data via the expose bus to be imaged by laser diode  10 . Laser diode  10  is started by the transient voltage source  13 . The level of the transient voltage  13  is controlled by the digital control unit  15 , which applies the required voltage level (usually between −3 v to −5 v) through the voltage boost control line  14  activating voltage boost circuit  19 , synchronized with the data received from computer  18 . In parallel, digital control unit  15  controls the digital to analog converter  16 , to apply the required voltage level for starting laser diode  10 . After laser diode  10  is started, digital control unit  15  opens switch  11  to connect external voltage source  12  to supply the voltage laser diode  10  for continuous diode operation, typically in voltage levels of −2.5 v. 
       FIG. 4  shows voltage and current curves at the boost circuit  19 , shown in  FIG. 3 , during operation of the voltage for laser diode  10 . An advantage in this configuration is the generation of a smooth transition between the transient voltage and continuous voltage sources applied on the laser diode  10 . The smooth voltage transition extends the life expectancy of the laser diode  10 . 
     In order to achieve a smooth transient current  33  and smooth transient voltage  34 , good timing synchronization should be achieved between the current command  35 , triggered by the image data sent by computer  18 , and the activation or deactivation of the boost circuit  19 . Good synchronization will result in controlling the rise time value  31  and the current overshoot profile  33 A developed on laser diode  10 . The current overshoot profile  33 A is essentially characterized by two factors: the current slew rate (measured by current value divided by the rise time value  31 ) and the switching point to the voltage source  12 , used during continuous operation of laser diode  10 . 
     At the time current command  35  is detected the boost circuit is already at the state of maximum duty cycle value, according to the setting received from digital control unit  15 . At this point, high voltage values are applied to laser diode  10 , enabling the smooth transient current  33  to reach to maximum values in a very short rise time  31 .  FIG. 4  shows a measured rise time  36  for a current increase from 0 A to 15 A to be achieved in 250 nanoseconds. 
     Just before the current values reaches the maximum value, the continuous voltage source  12  takes control.  FIG. 4  shows the smooth transition between the two voltage sources as it is indicated in the charts showing smooth transient current  33  and smooth transient voltage  34 . 
     When the current command  35  is off, the current in the diode falls as well, fall time  32  is measured at approximately  250  nanoseconds. At this stage the boost circuit  19  will start reloading again. The boost circuit  19  is used for supplying current to laser diode  10  only during rise time  31 , following that the current will be supplied by the continuous voltage source  12 . 
       FIG. 3  shows transient voltage source  13  generated by the same external power supply, amplified by a voltage boost circuit  19 .  FIG. 2  depicts use of two distinct power supplies, external voltage source  12  for supplying continuous voltage lower than the voltage generated by power supply  23  for transient voltage purpose. The switching between the two power supplies is performed by opening switch  11  and closing switch  24  for transient voltage selection. The activation of voltage source  12  is done by closing switch  11  and opening switch  24 . 
     The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the scope of the invention. 
     PARTS LIST 
     
         
           10  laser diode 
           11  switch 
           12  voltage source 
           13  transient voltage source 
           14  voltage boost control line 
           15  digital control unit 
           16  digital to analog converter 
           17  current control unit 
           18  computer 
           19  boost circuit 
           23  transient power supply 
           24  switch 
           31  rise time value 
           32  fall time 
           33  smooth transient current 
           33 A current overshoot profile 
           34  smooth transient voltage 
           35  current command 
           36  measured rise time