Patent Document

FIELD OF THE INVENTION 
     The present invention relates to an apparatus and method for controlling optical power stability a laser diode array, and more specifically an array of vertical cavity surface emitting lasers (VCSEL). 
     BACKGROUND OF THE INVENTION 
     The optical-power generated by a laser-diode is primarily a function of junction temperature and current. During laser-diode operation the junction-temperature increases and the optical-power decreases. 
     Certain applications, such as computer-to-plate (CTP), require that the optical power is kept constant during the operation of the system. In order to prevent optical power variations, the system should be equipped with the ability to monitor either the optical power or the junction temperature; this will allow applying necessary corrections through appropriate modulation of the current. 
     Continuous power measurement is not recommended as it disturbs the beam path and accurate temperature measurements of the individual junctions are practically impossible for laser diode arrays. Thus, such measurements cannot be practically used to control the optical power of multiple emitters. 
     SUMMARY OF THE INVENTION 
     Briefly, according to one aspect of the present invention an apparatus for controlling optical-power stability of emitting laser diodes, the laser diodes exhibiting temperature changes at the laser diode junctions, the temperature changes are predicted according to the laser diodes duty cycle. The apparatus includes, laser diodes arranged to emit light on a target, a data stream analyzer which is configured to receive incoming data stream and analyze it to produce an image data occurrence factor representing streams of data larger than zero, and an optical power stabilizer configured to control current intensity applied on a laser diode according to the image data occurrence factor. 
     These and other objects, features, and advantages of the present invention will become apparent to those skilled in the art upon a reading of the following detailed description when taken in conjunction with the drawings wherein there is shown and described an illustrative embodiment of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The subject matter regarded as the invention will become more clearly understood in light of the ensuing description of embodiments herein, given by way of example and for purposes of illustrative discussion of the present invention only, with reference to the accompanying drawings (Figures, or simply “FIG.”), wherein: 
         FIG. 1  is a schematic of a computer-to-plate (CTP) imaging head; 
         FIG. 2  is a schematic of a laser diode array arranged in a mechanical assembly; 
         FIG. 3  is a schematic of an electronic current controller for a laser diode array; and 
         FIG. 4  is a schematic of a block diagram of a laser diode controller according to image data stream. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the disclosure. However, it will be understood by those skilled in the art that the teachings of the present disclosure may be practiced without these specific details. In other instances, well-known methods, procedures, components and circuits have not been described in detail so as not to obscure the teachings of the present disclosure. 
     The present invention discloses a method to estimate changes in junction-temperature based on the mean operation duty-cycle of each laser-diode emitter and of its neighbors. 
       FIG. 1  shows a typical laser based CTP imaging system  10 , mounted to rotating drum  12 . A printing plate  14  is mounted on the drum. As drum  12  rotates under the control of positioning controller  22 , plate  14  rotates together. The direction of rotation  26  of drum  12  is called the “fast scan direction”. System  10  can be configured, in accordance with the invention, to print in a swath that expands and contracts. 
     Imaging head  16 , which includes an array of lasers, is coupled to optical head  18 , which positions imaging head  16  with respect to drum  12 . Optical head  18  can move laterally with respect to drum  12 . The direction in which optical head  18  can move is the same direction  28  as the axis of rotation of drum  12 , and is called the “slow scan direction”. Imaging head  16  may print parallel swaths in fast scan direction  26  or slow scan direction  28 , but usually imaging head  16  prints swaths helically by printing in fast scan direction  26  and slow scan direction  28  simultaneously. 
     Positioning controller  22  regulates the rotational speed of drum  12  and the position of optical head  18 . By rotating drum  12  and/or moving optical head  18 , positioning controller  22  can bring one or more lasers to bear upon substantially any point on the surface of printing plate  14 . 
     Lasers in imaging head  16  are under the control of laser controller  20 , and are modulated by image data  24  supplied to laser controller  20 . Positioning controller  22  typically sends a ting signal to laser controller  20  so that the correct image data will be supplied to the lasers when the laser array is in the correct position relative to printing plate  14 . Laser controller  20  may be, for example, a computer such as a personal computer, a microcomputer or an embedded processor or microcontroller. 
       FIG. 2  shows a laser diodes  204  arranged in a mechanical assembly  208  equipped with a cooling element  212  attached to the mechanical assembly  208 . The increase in the junction temperature correlates to the digital data content of the exposed image. As the operation time of a relevant laser diode increases, or in other words the number of ‘0’s in the data decreases per a time period per emitter, the temperature at the junction of a specific emitter will increase. The rise in junction temperature causes reduction in the optical power generated by the respective laser diode. In order to stabilize the intensity of emitted light rays it is needed to increase the outgoing current intensity, in order to achieve power intensity stability for the imaging laser diodes, thus resulting in a stable image on the printing plate. 
     The method comprises several steps:
     1. Estimate the temperature change AT by, for example, a weighted-average of the emitter duty-cycle state:
 
Δ T   n   =β·ΔT   n-1 +(1−β)·state n  
 
The parameter β, 0&lt;β&lt;1, is a measure of the longest time-interval that is required by the system in order to release the thermal energy generated by the operating laser diode and stored close to the emitter location.
   

     “State n ” represents the image data value in a specific pixel. In the case when State n =0 no laser diode will be invoked to image that pixel on the substrate. For values of State n &gt;0, a laser diode is invoked with a power intensity corresponding to value State n    
     Certain imaging devices will use only two pixel data values i.e. ‘no image data’=‘0’ and ‘image data’=‘1’. Other imaging devices will use plurality pixel data values .e.g. 16 values (0 to 15) or more. 
     The quantity ΔT n  is estimated for each emitter separately; ΔT n  is a measure of the heat absorbed at the junction in time slot n, e.g. one micro second separates between two consecutive time slots. ΔT n  is proportional to the local temperature change. 
     Typically, the temperature in the junction reaches back the nominal value about 3 to 4 milliseconds from the time the diode stops operating.
     2. Modify the emitter-current to compensate for the effect of the temperature-change. The emitter current is increased by an amount ΔI proportional to the temperature-change. The proportionality coefficient α is emitter specific.   

               Δ   ⁢           ⁢   I     =               ⅆ     ⅆ   T       ⁢   P         ⅆ     ⅆ   I       ⁢   P       ·   Δ     ⁢           ⁢   T     =       α   ·   Δ     ⁢           ⁢   T             
The above expression indicates that the proportionality coefficient is linear with the ratio between the power-derivative with respect to emitter-temperature and the power-derivative with respect to emitter current.
     3. Update current values periodically, preferably, the updates are performed at intervals significantly shorter than the time-interval associated with the parameter β.   

     The method can be further developed to compensate for thermal cross-talk between adjacent emitters. In this case the quantity ΔT is replaced by a weighted sum of the respective ΔT quantities of the emitter and its immediate neighbors:
 
Δ T   n   =a   −1 ·(Δ T   n ) (−1)   +a   0 ·(Δ T   n ) (0)   +a   1 ·(Δ T   n ) (1)  
 
The parameters β and α may are expected to be wafer dependent; thus, they may require adjustment whenever a particular LDA is replaced with another LDA produced from a different wafer.
 
     In order to achieve power-stability based on the method described herein, correction coefficients α are to be estimated for each of the emitters. A three step procedure is proposed:
         a) Each individual emitter is operated for intervals short enough such that the junction-temperature, T 1  is much lower compared to the temperature reached in continuous operation. The current is tuned to a certain value I 1  necessary to generate a preset optical-power.   b) The same procedure is repeated with each emitter operating for long intervals, such that the junction reaches the temperature T 2  associated with continuous operation and the same optical-power is generated for a certain current value I 2 .   c) Parameter α for each emitter is then estimated according to the formula shown below:       

       FIG. 3  shows a typical laser diode electrical circuit  316 . Image data stream  324  is applied on circuit  316 , when the incoming data value is larger than 0 (zero value), the circuit opens and PN junction  312  causes photon  320  to be emitted. 
       FIG. 4  shows a block diagram of the method and apparatus. Image data generator  404  supplies an image data stream  324  into a data stream analyzer  408 . The image data stream  324  represents an image data to be imaged on plate  14 . 
     Analyzer  408  analyzes data stream  324  and finds the larger than 0 values occurrence factor  424  in data stream  324 . Factor  424  is provided to optical power stabilizer  412 . Stabilizer  412  controls the current intensity  428  applied on laser diode  416 , according to the generated factor  424 . 
     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 
                 imaging system 
               
               
                 12 
                 rotating drum 
               
               
                 14 
                 plate 
               
               
                 16 
                 imaging head 
               
               
                 18 
                 optical head platform 
               
               
                 20 
                 laser controller 
               
               
                 22 
                 positioning controller 
               
               
                 24 
                 image data 
               
               
                 26 
                 fast scan direction 
               
               
                 28 
                 slow scan direction 
               
               
                 204 
                 laser diodes 
               
               
                 208 
                 mechanical assembly for laser diodes 
               
               
                 212 
                 cooling element 
               
               
                 304 
                 P 
               
               
                 308 
                 N 
               
               
                 312 
                 PN junction 
               
               
                 316 
                 electrical circuit 
               
               
                 320 
                 emitted photon 
               
               
                 324 
                 image data stream 
               
               
                 404 
                 image data generator 
               
               
                 408 
                 data stream analyzer 
               
               
                 412 
                 optical power stabilizer 
               
               
                 416 
                 a laser diode 
               
               
                 424 
                 larger than zero value occurrence factor 
               
               
                 428 
                 current intensity

Technology Category: 7