Abstract:
A system and method for controlling the operating parameters of a laser diode ( 20 ) is provided. The laser control system ( 10 ) automatically optimizes the laser diode ( 20 ) operating characteristics while maintaining a safe peak power for pulse duration and pulse repetition frequency (PRF). The controlled level of output power is based on the laser diode gain determined during calibration of each laser diode projector as well as using the application of predetermined laser safety formulas. The laser control system ( 10 ) includes a laser diode ( 20 ) that is powered by a laser drive current. The laser diode ( 20 ) has a laser output having a peak power level. A detector ( 28 ) is coupled to the laser diode ( 20 ) for sensing the laser output. A laser driver ( 18 ) including a primary control loop ( 44 ) is operable, in response to the sensed laser output and a reference ( 43 ), to control the laser drive current such that the output power corresponds to the reference ( 43 ). A controller ( 52 ) is coupled to the laser driver ( 18 ). The controller ( 52 ) includes a laser settings module ( 40 ) for generating the reference ( 43 ) in response to a laser output setting ( 42 ) such that the laser output characteristic level is approximately a predetermined output level. The output characteristic of the laser diode ( 20 ) is maintained within the predetermined standard. Another aspect of the invention provides an independent safety monitoring function ( 68 ) based on the laser settings.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of the filing date of U.S. provisional application No. 60/147,913 filed Aug. 9, 1999. 
    
    
     BACKGROUND AND SUMMARY OF THE INVENTION 
     The present invention relates generally to the output power control of eye safe structured laser light. Such structured laser light is used to illuminate features which are then imaged with a one-dimensional or two-dimensional imaging array (camera). In particular, the present invention takes advantage of laser safety standards which allow for greater peak power levels for pulsed lasers. Particularly, in applications requiring high speed “stop action” imaging the present invention will, transparent to the user, facilitate optimization for best signal level at selected integration times. 
     Traditionally, for any commercial laser projector systems, laser output power is set at a single fixed “safe” power level during manufacture of the laser projection device. The laser power setting for commercial laser products is established per applicable U.S. and international safety standards. For example, CDRH 21 CFR Part 1040.10 requires that commercial CW laser systems rated at Class II emit no more than 1 milliwatt of visible laser power into a 7 millimeter aperture at a specified distance from the laser source. For pulsed laser systems, the same 1 mW power level rating is in effect, but the power level is now considered average rather than continuous power. Average power for a pulsed laser is computed as the peak pulse power multiplied by the duty cycle (percentage of on-time of the pulse) of the pulse train. 
     For pulsed laser systems, international laser safety standards also allow for greater peak pulse power levels as long as the 1 mW average power setting is not violated. A formula for visible laser radiation and appropriate to the intended use of the laser establishes the maximum allowable peak power. In particular, EN 60825-1 standard for Class 3A rated laser systems requires that the following formula for visible light with pulse duration greater than 1.8 microseconds be used to establish the maximum peak power level using the 7 mm aperture:              Pp   =     0.7       (     N   *     t   p       )     0.25                     where        :                   Pp     =     peak                 pulse                 power                 in                 mW                   N   =     number                 of                 pulses                 in                 0.25                 second                 interval                                t   p     =     pulse                 duration                 of                 an                 individual                 pulse                 in                 seconds                                  
     In this case, depending on the pulse duration and pulse duty cycle, it is possible to have a peak pulse power level much greater than the CW power setting. For example, for a pulse duration of 0.001 seconds and N=1 pulse train (one pulse every 0.25 seconds), the maximum allowed peak power would be almost 4 mW with a consequential average power of approximately 0.016 mW. 
     Laser safety standards require that adequate safeguards be designed into the laser drive circuitry to ensure that safe power settings are not exceeded. This is usually implemented in commercial CW laser projectors using a potentiometer which is adjusted to the proper resistance setting resulting in a value less than 1 mW output power into the specified aperture. In the case of pulsed laser projectors, the output power is normally set at a fixed level corresponding to the maximum peak power level at a particular pulse duration at a fixed pulse clock frequency. Analog circuitry is used to monitor the pulse signal to ensure that the pulse parameters are maintained within safe limits. 
     For applications such as the imaging of reflected structured laser light pulsing of the laser projector has been used to improve “stop-action” imaging of moving targets of interest. Stop-action or strobing of the target by the laser projector is used to avoid blurring resulting from the relative motion during the imager integration period. The application of strobing over short durations also results in very low relative signal levels due to the short camera exposure intervals of the imager. This problem is exaggerated if the targets of interest have very low optical reflectance properties thus giving extremely low signal levels acquired by the imager during the exposure time. 
     The present invention provides a method and system for incorporating into a laser driver circuit a technique to automatically maximize the operating parameters of a laser diode within the constraints of a predetermined standard. The laser diode operating characteristic is based on the laser diode gain which is determined during calibration of each laser diode projector as well as application of the specified laser safety formulas. 
     The present laser control system provides a system and method for controlling the operating parameters of a laser diode. The preferred embodiment of the laser control circuit includes a laser diode that is powered by a laser drive current. The laser diode has a laser output having a peak power level. A detector is coupled to the laser diode for sensing the laser output. A laser driver including a primary control loop is operable, in response to the sensed laser output and a reference, to control the laser drive current such that the output characteristic corresponds to the reference. A controller is coupled to the driver. The controller includes a laser settings module for generating the reference in response to a laser output setting such that the laser output characteristic is maintained at approximately a predetermined output level. The output level of the laser diode is maintained within a predetermined safety standard. Another aspect of the invention provides an independent safety monitoring function based on the laser settings. 
    
    
     For a more complete understanding of the invention, its objects and advantages, reference may be had to the following specification and to the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a block diagram of a laser control system in accordance with the teachings of the invention; 
     FIG. 2 illustrates an entity relationship diagram of a laser control system in accordance with the teachings of the invention; 
     FIG. 3 illustrates a block diagram of a presently preferred embodiment of a laser control system in accordance with the teachings of the invention; 
     FIG. 4 illustrates a graphical representation of the relationship 5 between laser output and detector current; 
     FIG. 5 illustrates a Laser Gain Lookup Factors Table; and 
     FIG. 6 is a flow diagram illustrating calibration and operational modes of a laser control system in accordance with the teachings of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, a laser control system  10  according to the present invention is shown. While the present invention is shown and described as being separated into multiple assemblies, it will be appreciated that the particular division of functions is merely exemplary, and the laser control system  10  could be mounted on a single assembly or multiple assemblies. The laser control system  10  provides a method for maximizing the operational characteristics of a laser diode while ensuring compliance with governing standards such as safety standards and customer imposed standards. A host sensor microprocessor  11  such as a Motorola ColdFire communicates to the laser control system  10  through a serial link  13 , a RESET signal  15 , and a CNTRL signal  17  during several operating modes such as calibration, operating mode selection, and initial system set up. 
     The laser control system  10  includes a microcontroller  12  for executing a program stored in system memory  14 . In the presently preferred embodiment of the invention, a Motorola microcontroller is employed as the microcontroller  12 . The Motorola microcontroller includes an on-board 8 bit analog to digital converter (ADC), an 2k EEPROM for system memory  14 , and a serial interface  13 . One skilled in the art will readily recognize that other microcontrollers, ADCs, and quantities and types of system memory  14  can be utilized. A crystal oscillator  16  coupled to the microcontroller  12  provides an accurate, stable clock reference. In the presently preferred embodiment, two laser drivers  18  are coupled to the microcontroller  12  via power, ground, and three control lines. However, the scope of the invention includes powering one or more laser drivers  18  as illustrated in FIG.  1 . Each laser driver  18  provides a gain stage for powering a laser diode  20 . A digital to analog converter (DAC)  22 , converts a digital signal from the microcontroller  12  to an analog reference voltage for driving the laser driver  18 . The reference signal  24  is coupled to the noninverting input of an amplifier  26  used within a primary control loop for regulating the output of the laser diode  20 . The inverting input of the amplifier  26  is coupled to the output of an internal photodetector detector  28  that senses the optical output of the laser diode  20 . A capacitor (not shown) is connected from the amplifier output to the inverting input to provide stabilizing feedback. Those skilled in the art will readily recognize that numerous other feedback circuits may be employed to stabilize the amplifier  26 . The output of the amplifier  26  is coupled to a buffer  30  that is controlled by an ENABLE signal  32  through a softstart circuit  34 . The softstart circuit  34  disables the laser diode  20  until stable circuit power level has been reached and a no fault condition exists The first buffer  30  provides a sufficient laser driver current and as well provides an independent path for disabling power to the laser diode  20  as in the above referenced examples. A second buffer  36  is coupled to the output of the internal photodetector  28  to provide a buffered representation of the laser diode output to the microcontroller  12 . 
     Referring to FIG. 2, a functional block diagram of the laser control system  10  is illustrated. The laser control system  10  includes a laser settings module  40  for determining the operational characteristics of the laser diode  20  from predetermined output settings  42 . The laser diode operational characteristics are selected by the laser settings module  40  so that the peak power of the laser diode  20  is maximized while remaining within a predetermined safety standard limiting output characteristics such as average power or pulse energy. Examples of such safety standards include customer defined safety standards, U.S. safety standards, and the international safety standard for Class 3A rated laser systems, EN 60825-1. For example, a user may select a pulse duration and the number of pulses per interval to be applied to the laser diode  20 ; in response the laser settings module  40  determines the maximum peak power the laser diode  20  may be set to while still complying with the safety standard. By maximizing the peak power emitted by the laser diode  20 , measurement errors associated with detecting low levels of reflected light are minimized. Likewise, one of the other output settings such as the pulse duration may be the determined variable, while the peak power and number of pulses are initially selected. In each case, the laser settings module  40  selects a value for the determined variable that optimizes the laser diode output while complying with the predetermined safety standard, represented for example, by the formula presented earlier, thereby reducing measurement errors. To control the value of the laser diode peak power, PRF, and pulse duration, the laser settings module  40  sets the value of a reference (REF)  43  that is coupled to a primary control loop  44  of the laser driver  18 . 
     The primary control loop  44  ensures the laser output tracks the corresponding reference level by controlling the quantity of current flowing to the laser diode  20 . The detector  46  provides a feedback signal to the primary control loop  44  indicating the intensity of the laser diode output. The primary control loop  44  compares the actual intensity of the laser output to the reference level supplied by the laser settings module  40  and adjusts the laser drive current in response. 
     The sensed laser output of the detector  46  is additionally coupled to an input of a secondary control loop  48  that provides redundant control of the laser output. The secondary control loop  48  disables the laser diode output in response to conditions such as detecting a fault. 
     Referring to FIG. 3, a block diagram of a presently preferred embodiment of a laser control system  50  in accordance with the principles of the invention is illustrated. The laser control system  50  includes a laser driver  18  that is similar to previously described embodiments. The laser driver is coupled to a controller  52  through a set of control and power lines. The controller  52  of the presently preferred embodiment includes the functional features of the previous embodiments with additional calibration and fault detection capabilities. 
     The control lines include a reference (REF)  54 , an ENABLE  56 , and a sensed laser output signal (PD)  58 . The reference  54  is a voltage signal that sets the laser diode operating level. The ENABLE  56  is a redundant signal path to control the application of power to the laser diode  20  to ensure the laser diode  20  can be disabled if a failure occurs within the laser control system  50 . The sensed laser output signal  58  is used for fault detection to determine whether the laser diode operating parameters are within the predetermined safety standard as well as within predicted operating limits over changing environmental conditions such as temperature and duration of use. 
     The controller  52  includes a laser settings module  60  for setting the operating parameters of the laser diode  20 . The laser settings module  60  implements a peak power formula for the pertinent laser safety standard. In the presently preferred embodiment, the peak power formula includes three variables; pulse duration, pulse repetition frequency (PRF), and peak power. PRF is related to N, the number of pulses in 0.25 seconds, by the relation PRF=4N. In response to selection of two of the three variables such as pulse duration and PRF, the laser settings module  60  determines the remaining variable which in this case is the maximum peak power for the laser output. The laser settings module  60  queries a Laser Gain Lookup Factors Table  62  to set an output signal to a level corresponding to the determined peak power. In the presently preferred embodiment, the laser settings module  60  computes the peak power using a safety formula like that presented earlier with the pulse duration and PRF as inputs. However, it is within the scope of the invention to employ a look up table for determining the peak power or a parameterized representation of the safety formula or to use other operating parameters such as pulses per interval. It is additionally within the scope of the invention to specify the peak power and control either the pulse duration or PRF. A DAC  64  converts the output signal to an analog level that is coupled through the reference  54  to the laser driver  18 . 
     A calibration fixture  66  determines the gain characteristics of the laser diode  20  during the laser driver manufacturing process. In the presently preferred embodiment, the calibration fixture  66  comprises a personal computer, a separate laser power meter, and a communications link to the laser controller. However, the scope of the invention includes locating portions of the calibration fixture elsewhere such as within the sensor host processor. The gain characteristics are represented by a gain curve  61  that describes the gain characteristic of the laser diode  20  from minimum to maximum power levels (see FIG.  4 ). Two or more power measurements are made at low, nominal and high power settings to characterize the gain curve. In the presently preferred embodiment, the relationship between the output power and the feedback monitoring current is described by a linear approximation, however the scope of the invention includes using other curve fitting techniques to represent the gain curve  61 . The linear approximation is described by a set of parameters that are stored, via the communications link, in a Laser Gain Lookup Factors Table  62  (see FIG.  5 ). 
     The Laser Gain Lookup Factors Table  62  contains device characteristic information associated with each laser diode  20  such as points describing the gain curve at nominal and half-nominal output power settings. In addition, the Laser Gain Lookup Factors Table  62  includes scaling factors associated with each laser diode  20  that account for changes in the current-gain curve over temperature and time, and tolerances of the instruments used to perform the calibration. 
     Referring to FIG. 3, a fault detection scheme is illustrated. The fault detection scheme ensures that the peak power of the laser output does not exceed a predetermined limit if a single point failure occurs within the laser control system  50 . The fault detection scheme includes a fault detector  68  for detection and response to faults, a watchdog timer  72 , and redundant control paths for disabling the laser diode  20 . 
     The fault detector  68  monitors the pulse parameters in a failsafe mode to insure that the predetermined safety standards are not exceeded. Pulse parameters such as the pulse duration, power output, and PRF are monitored to ensure the laser control system  50  does not improperly or by external effects violate laser safety levels due to a single failure. In the event that a requested pulse parameter exceeds a safe threshold, the fault detector  68  overrides the request from the control link and interrupts current flowing to the laser diode  20 , thereby disabling the laser output. The controller  52  preferably communicates an error message to the host sensor identifying the nature of the failure. To reactivate the laser diode  20 , the host sensor processor  11  communicates with the microcontroller  12  via the serial link  13 . 
     The fault detector  68  additionally monitors the watchdog timer  72  and other elements of the controller  52  to ensure the laser control system  50  is in an active operating state. The watchdog timer  72  monitors execution of the system program. The watchdog timer  72  activates if the system program has stopped executing. In response to the watchdog timer  72  becoming active, the fault detector  68  disables the laser diode  20  via the ENABLE signal. 
     Referring to Table I, the response of the laser control system  50  to several failure modes is described. 
     
       
         
               
               
               
             
               
               
               
               
               
               
             
               
               
               
             
           
               
                 TABLE I 
               
               
                   
               
               
                 DESCRIPTION 
                 SYSTEM RESPONSE TO 
                 SYSTEM STATUS 
               
               
                 OF FAILURE 
                 THE FAILURE 
                 AFTER RESPONSE 
               
               
                   
               
             
             
               
                 RESET signal 
                 Controller upon reset sets 
                 Laser is disabled after 
               
               
                 grounded 
                 output ENABLE to low 
                 internal time delay not to 
               
               
                   
                 state 
                 exceed 16 ms 
               
               
                 Crystal oscillator 
                 Internal Controller 
                 Laser is disabled after 
               
               
                 disabled 
                 hardware timer is initiated 
                 internal time delay not to 
               
               
                   
                 upon removal of master 
                 exceed 16 ms 
               
               
                   
                 clock At timeout, ENABLE 
               
               
                   
                 transitions from high to the 
               
               
                   
                 low state 
               
               
                 Reference (REF) 
                 Controller senses out of 
                 Laser is disabled after 
               
               
                 disabled 
                 tolerance measurement 
                 internal time delay not to 
               
               
                   
                 from detector and disables 
                 exceed 16 ms 
               
               
                   
                 the laser by setting 
               
               
                   
                 ENABLE to low state 
               
               
                 Main program 
                 Internal Controller watch 
                 Laser is disabled after 
               
               
                 stops executing 
                 dog timer times out when 
                 internal time delay not to 
               
               
                   
                 the main program 
                 exceed 16 ms 
               
               
                   
                 execution is halted and 
               
               
                   
                 ENABLE transitions from 
               
               
                   
                 high to low state. Under 
               
               
                   
                 normal conditions, timer is 
               
               
                   
                 reset during main program 
               
               
                   
                 execution continuously 
               
             
          
           
               
                 1. 
                 When in CW 
                 1. 
                 Controller identifies a 
                 1. 
                 Laser is disabled 
               
               
                   
                 mode, any 
                   
                 change to laser operating 
                   
                 upon receipt of new 
               
               
                   
                 pulse 
                   
                 mode without the 
                   
                 commands 
               
               
                   
                 parameters 
                   
                 appropriate change to the 
                 2. 
                 Laser is disabled 
               
               
                   
                 sent to laser 
                   
                 control line /CTRL. The 
                   
                 after 5 pulses are 
               
               
                   
                 controller 
                   
                 laser is disabled by setting 
                   
                 sampled on /CTRL 
               
               
                   
                 while /CTRL 
                   
                 ENABLE to a low state and 
               
               
                   
                 is enabled 
                   
                 Reference (REF) to 0 VDC 
               
               
                 2. 
                 When in 
                 2. 
                 Controller identifies an 
               
               
                   
                 PULSE 
                   
                 incorrect configuration for 
               
               
                   
                 mode, any 
                   
                 the PULSE mode and 
               
               
                   
                 incorrect 
                   
                 disables the laser by 
               
               
                   
                 parameters 
                   
                 setting ENABLE to the low 
               
               
                   
                 sent to laser 
                   
                 state and Reference to 0 
               
               
                   
                 controller 
                   
                 VDC 
               
               
                   
                 while /CTRL 
               
               
                   
                 is strobing 
               
             
          
           
               
                 GROUND to 
                 Removal of circuit ground 
                 Laser is disabled 
               
               
                 controller board 
                 return 
                 immediately upon 
               
               
                 and driver 
                   
                 removal of ground 
               
               
                 floating 
               
               
                 V CC  (power) for 
                 Removal of circuit power 
                 Laser is disabled upon 
               
               
                 both controller 
                   
                 removal of power 
               
               
                 and driver is an 
                   
                 immediately 
               
               
                 open 
               
               
                 V CC  (power) for 
                 Removal of circuit power 
                 Laser is disabled upon 
               
               
                 just the driver is 
                   
                 removal of power 
               
               
                 an open 
                   
                 immediately 
               
               
                 GROUND to just 
                 Removal of circuit ground 
                 Laser is disabled 
               
               
                 the driver floating 
                 return 
                 immediately upon 
               
               
                   
                   
                 removal of ground 
               
               
                 Open detector 
                 Controller senses loss of 
                 Laser stays at constant 
               
               
                 connection from 
                 feedback signal and 
                 power and is disabled 
               
               
                 driver to 
                 disables the laser through 
                 after internal time delay 
               
               
                 controller 
                 ENABLE and Reference 
                 not to exceed 16 ms 
               
               
                 REF saturated at 
                 Controller senses out of 
                 Laser power increases 
               
               
                 maximum 
                 tolerance increase in laser 
                 to the full power of the 
               
               
                 voltage output of 
                 power by reading detector 
                 laser and then within a 
               
               
                 5 VDC 
                 and disables laser through 
                 millisecond turns off 
               
               
                   
                 ENABLE and Reference if 
                 laser diode 
               
               
                   
                 it can&#39;t reach acceptable 
               
               
                   
                 levels 
               
               
                 ENABLE signal 
                 Soft start circuit requires 
                 Laser is disabled 
               
               
                 grounded 
                 5VDC on ENABLE to 
                 immediately 
               
               
                   
                 activate the laser 
               
               
                 Detector 
                 Controller senses out of 
                 Laser power increases 
               
               
                 feedback in 
                 tolerance increase in laser 
                 to the full power of the 
               
               
                 primary control 
                 power by reading detector 
                 laser and then within a 
               
               
                 loop is an open 
                 and disables laser through 
                 millisecond turns off 
               
               
                   
                 ENABLE and Reference if 
                 laser 
               
               
                   
                 it can&#39;t reach acceptable 
               
               
                   
                 level 
               
               
                 Detector 
                 Controller senses out of 
                 Laser power increases 
               
               
                 feedback in 
                 tolerance decrease in laser 
                 to the full power of the 
               
               
                 primary control 
                 power by reading detector 
                 laser and then within a 
               
               
                 loop shorted to 
                 and disables laser through 
                 millisecond turns off 
               
               
                 ground 
                 ENABLE and Reference if 
                 laser diode 
               
               
                   
                 it can&#39;t reach acceptable 
               
               
                   
                 level 
               
               
                 Feedback on 
                 Controller senses out of 
                 Laser power increases 
               
               
                 laser drive 
                 tolerance increase in laser 
                 to the full power of the 
               
               
                 amplifier stage is 
                 power by reading detector 
                 laser and then within a 
               
               
                 an open 
                 and disables laser through 
                 millisecond turns off 
               
               
                   
                 ENABLE and Reference if 
                 laser diode 
               
               
                   
                 it can&#39;t reach acceptable 
               
               
                   
                 level 
               
               
                 Laser drive 
                 Controller senses out of 
                 Laser power increases 
               
               
                 amplifier output 
                 tolerance increase in laser 
                 to the full power of the 
               
               
                 is shorted 
                 power by reading detector 
                 laser and then within a 
               
               
                   
                 and then disables laser 
                 millisecond turns off 
               
               
                   
                 through ENABLE and 
                 laser diode 
               
               
                   
                 Reference if it can&#39;t reach 
               
               
                   
                 acceptable level 
               
               
                   
               
             
          
         
       
     
     The laser settings module  60  may additionally modify the value of the reference  54  to ensure that system delays in responding to a fault do not cause the laser output to exceed the predetermined safety standard. For example, when the pulse width selected by the laser settings module  60  is less than the fault response time of the laser control system  50 , the determined value peak power could lead to an excessive laser output. In the presently preferred embodiment, the maximum fault response time is 16 msec as shown in Table I. Therefore, the maximum peak power is limited to a value that in combination with a 16 msec pulse width and the selected PRF, does not exceed the predetermined safety standard. Those skilled in the art will readily recognize that the fault response time may change depending on oscillator frequency, control chip selection, and future improvements in technology. 
     Referring additionally to FIG. 6, the operational control algorithm of the laser control system  50  is illustrated. The operating mode of the laser control system  50  is set through the communication link and the laser diode  20  is activated upon receiving the CTRL input. The controller  52  then automatically modifies the laser power setting consistent with the loaded laser safety standard. The operating mode is preferably limited to being changed only using the communication link. The host processor communicates the desired diode operating parameters, such as PRF and the pulse duration, to the controller  50 . Based on the communicated values, the controller  50  determines the remaining diode operating parameters, such as the peak power level, and sets the laser driver  18  to the new power level upon receiving the CTRL active signal from the sensor host controller. The controller  50  also records the values of PRF, pulse duration, and power level for the particular pulse mode and then monitors the laser diode  20  in real-time for laser safety 
     At step  80 , the laser diode gain characteristics are calibrated; preferably with an external power meter, personal computer and communications link to the controller. Two or more power measurements are made at low, mid and high power settings. The resolution of the power setting adjustments is equal to the resolution of the DAC scaled to cover the full range of operating optical power. A relationship between the diode optical output power and the feedback monitoring current is determined and at step  82 , a corresponding diode gain curve representation is stored into the Laser Gain Lookup Factors Table  62  via the communications link  13  during manufacturing. Other device characteristic information such as scaling factors are additionally entered into the Laser Gain Lookup Factors Table  62 . 
     At step  84 , the predetermined safety standard representation is downloaded into the laser control system non-volatile memory as the laser settings module  60 . Although in the presently preferred embodiment, the laser settings module  60  is downloaded into the laser control system, it is within the scope of the invention to maintain selectable laser setting modules for each safety standard. 
     The laser settings module  60  accesses the output settings module  70  to load the laser diode output settings corresponding to the operating mode of the laser control system  50 , at step  86 . An output setting value, in this example a peak power setting, for the laser diode  20  is computed based on the predetermined safety standard, at step  88 . The output setting value is modified if necessary to ensure the laser output does not exceed the predetermined safety standard under fault conditions, at step  90 . The device characteristic information is retrieved from the Laser Gain Lookup Factors Table  62 . The computed output setting value is modified as necessary and converted to a reference value corresponding to the diode characteristics and scaling factors stored in the Laser Gain Lookup Factors Table  62 . The reference value is converted to an analog signal and coupled to the laser driver amplifier  26  which generates an output in response to comparing the reference to a laser output sense signal. The amplifier output is converted to a laser drive current by buffer  30 . During initial turn-on of the laser control system  50 , the laser drive current is ramped up in response to a positive value of the REF voltage as long as the ENABLE signal  32  is provided through the softstart circuit  34 . During normal operation, the softstart circuit  34  does not affect the laser drive current. The laser drive current flows through the laser diode  20  powering the laser output. A detector  28  senses the laser output and generates the laser output sense signal. The laser output sense signal is coupled to the controller  52  through a buffer  36 . 
     At step  94 , the fault detection scheme monitors the laser output and the components of the laser control system  50  to ensure the laser output does not exceed the predetermined safety standard should a fault occur. 
     The laser control system and method of the present invention controls the operational characteristics of a laser diode employed in an imaging system. The laser control system optimizes the laser output of a laser diode over selectable operating modes while ensuring the laser output remains within the limits of a predetermined safety standard. Additionally, the system accounts for changes in system operating characteristics. Also, the laser output is constrained from exceeding the predetermined safety standard during single point failures. 
     Further, while the preferred embodiment of the laser control system employs a microcontroller as part of the laser driver assembly, the functions performed by the microcontroller could be equivalently performed by a programmable logic device (produced, for example, by Lattice, Inc.), a timer, discrete ADC and DACs, a serial interface chip, and a non-volatile storage resource such as flash memory or an EEPROM. 
     Additionally, while the preferred embodiment of the laser control system employs a microcontroller as part of the laser driver assembly, the functions performed by the microcontroller could be equivalently performed by the sensor host processor with additional resources such as ADCs and DACs. 
     A further embodiment of the laser control system employs a digital potentiometer, produced, for example, by Dallas Semiconductor, to replace the DAC, EEPROM and serial interface chip. 
     Thus it will be appreciated from the above that as a result of the present invention, a laser control system for a laser imaging system is provided by which the principal objectives, among others, are completely fulfilled. It will be equally apparent and is contemplated that modification and/or changes may be made in the illustrated embodiment without departure from the invention. Accordingly, it is expressly intended that the foregoing description and accompanying drawings are illustrative of preferred embodiments only, not limiting, and that the true spirit and scope of the present invention will be determined by reference to the appended claims and their legal equivalent.