Abstract:
A laser drive controller compensates for temperature-dependent effects of a temperature-sensitive laser. Temperature variations in the laser may be measured and/or predicted based on variable pulsed output. The controller may drive the laser to maintain temperature and/or to compensate for variations in temperature. The techniques may be applied to a laser scanner, scanned beam display, laser printer, laser camera, scanned beam imager, etc.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims priority benefit from the U.S. Provisional Patent Application Ser. No. 60/707,854, entitled METHOD AND APPARATUS FOR STABLE LASER DRIVE, filed Aug. 12, 2005, invented by Randall B. Sprague et al., commonly assigned herewith and hereby incorporated by reference. 
     
    
     BACKGROUND  
       [0002]     Laser beams are used across a range of applications. It is frequently desirable to generate a laser beam using a solid state device. Laser diodes, for example, are commonly used to generate infrared, red, and violet beams. Some intermediate wavelengths such as green have been difficult to achieve directly with a laser diode. An approach used to achieve green laser beam output is to provide an infrared (IR) beam coupled to a component that converts the input beam into a shorter wavelength. Such a component is frequently referred to as a frequency doubling crystal or second harmonic generator (SHG). One exemplary application of an SHG is to generate a 1064 nanometer (nm) IR laser beam with an IR laser diode and pass it through the SHG to convert the 1064 nm IR beam to a 532 nm green laser beam. Various architectures have been developed for this including diode-pumped solid state (DPSS) and other architectures that use external cavities and/or external wavelength converters.  
         [0003]     One type of device  101  for providing a green laser beam is shown schematically in  FIG. 1 . An infrared laser diode  102  is energized to output an infrared beam  104  at 1064 nm into an external cavity  106 . The external cavity  106  includes a SHG  108  comprising periodically poled lithium niobate (PPLN) (periodically polled LiNbO 3 ). The infrared beam  104  enters the crystal  108  and is doubled in frequency to produce a halved wavelength of 532 nm. An output face of the external cavity  106  includes a mirror  110  that reflects substantially 100% of infrared light and about 90% of green light. The 90% of reflected green light continues to pass back and forth through external cavity  106  to make multiple passes through the SHG  108 . The 10% of green light that passes through the mirror  110  is emitted as a green laser beam  112 .  
         [0004]     Another type of device  201  for providing a green laser beam is shown schematically in  FIG. 2 . An infrared laser diode  102  is energized to output a first infrared beam  202  at 808 nm into an external cavity  106 . The external cavity  106  includes a down-converting crystal  204  that down-converts the frequency of the input beam  202  to produce a longer wavelength second infrared beam  104  at 1064 nm. Second infrared beam  104  enters a frequency doubling crystal (SHG)  108  comprising PPLN. As in the device  101  shown in  FIG. 1 , the second infrared beam  104  enters the crystal  108  and is doubled in frequency to produce a halved wavelength of 532 nm. An output face of the external cavity  106  includes a mirror  110  that reflects substantially 100% of infrared light and a high amount but less than 100% of green light. The green light that passes through the mirror  110  is emitted as a green laser beam  112 .  
         [0005]     Another type of device  301  for providing a green laser beam is shown schematically in  FIG. 3 . An infrared laser diode  102  is energized to output an infrared beam  104  at 1064 nm wavelength. The infrared laser beam  104  enters a PPLN SHG with Bragg grating and waveguide  302 . A green laser beam  112  at 532 nm wavelength is emitted from the SHG with Bragg grating and waveguide  302 . Some experts consider device  301  to be a single-pass device because an external mirror for providing multiple passes of the green light is omitted.  
         [0006]     While the devices of  FIGS. 1-3  are illustrated as internally generating a 1064 nm beam to produce a 532 nm output beam, other wavelengths may similarly be used. For example, a 1080 nm IR beam may be generated internally to produce a 540 nm output beam. Moreover, the technique may be used to produce blue, red, or even hyperspectral wavelength outputs. The apparatus and methods taught for stabilization of such devices should not be considered limited to driving devices producing particular exemplary wavelengths.  
         [0007]     One consideration for using lasers such as devices  101 ,  201 ,  301 , and other types of lasers relates to maintaining a relatively constant temperature within the devices. Unintended temperature variations in such devices can result in unintended variations in beam  112  output power. Unfortunately, some desired applications of such devices use beam modulation patterns that can result in corresponding modulation of heat dissipation within the devices, which in turn can cause variations in device temperature. While fans, liquid cooling, heaters, and thermostatically-controlled thermo-electric-coolers have been used with such devices, the response time of such systems is often greater than the time associated with temperature variations arising from modulation pattern variation. In certain applications, such as scanned beam displays, a desired pixel cycle time (pixel period) is somewhat shorter than the time constant for variable output induced variable heating, while a desired line period is somewhat greater than the time constant for variable output induced variable heating.  
         [0008]      FIG. 4  illustrates the non-linearity of optical output power of a laser diode compared to driver current. A drive current trace  402  has a series of drive pulses  404 ,  406 ,  408 ,  410 ,  412 , and  414  having increasing drive current interleaved with off segments  416   a - 416   e . As may be seen by comparison of the heights and widths of the drive pulses to the line  418 , the drive pulses increase monotonically and evenly and correspond to an intended series of laser pulses that similarly increase monotonically and evenly. A laser power output trace  420  has a series of light output pulses  422 ,  424 ,  426 ,  428 ,  430 , and  432  corresponding to respective drive current pulses  404 ,  406 ,  408 ,  410 ,  412 , and  414  and interleaved with non emitting segments  434   a - 434   e . As may be seen by comparison of the heights and widths of the light output pulses to the line  436 , the relative brightness of the laser power output pulses  422 - 432  do not correspond closely to the driver pulses  404 - 414 . In particular, some pulses are considerably narrower than the drive current pulses and some pulses undershoot the intended output brightness. The combination of varying pulse heights and widths results in erratic apparent brightness of the pulses compared to the input energization signal  402 .  
       OVERVIEW  
       [0009]     One embodiment according to the invention relates to methods and apparatuses for scanning variably modulated beams of light emitted from a laser that is sensitive to variable modulation. According to one embodiment, such lasers may be stabilized by providing stabilization or thermal compensation pulses at a current or through a current path that does not result in substantial lasing, but does provide power dissipation, thus maintaining relatively constant heat flow through the device even in the absence of laser emissions. By maintaining a relatively constant heat flow, as measured over periods corresponding to one to several pulses, the temperature of the device may be maintained relatively constant, thus stabilizing optical power output.  
         [0010]     According to one embodiment stabilization pulses are provided through the normal laser modulation current path at a current below the lasing threshold current for the device. Such stabilization pulses may be provided, for example, during portions of the cycle lying between nominal light output portions, or in the case where no light is output during a given cycle, during the period when light output normally occurs.  
         [0011]     According to another embodiment, stabilization pulses are provided through the normal laser modulation current path at a current above a rollover threshold for the device. At high current levels above a rollover threshold, some devices do not emit substantial amounts of light and such current levels may be used to provide power dissipation with light pulses being enabled by modulating down from above the rollover threshold.  
         [0012]     According to another embodiment, stabilization pulses are provided through the normal laser modulation current path with a duration shorter than the rise time of the laser.  
         [0013]     According to another embodiment, stabilization pulses are made through a power dissipation current path different from the laser modulation current path, for example through a resistor held in close contact to the laser device. Such pulses may be made simultaneously with or sequential to laser modulation pulses.  
         [0014]     According to some embodiments, stabilization pulses are determined from the modulation data alone. According to other embodiments, a temperature sensor or other sensor may additionally provide input for determining appropriate stabilization pulses. Modulation data may be used at a single pulse level to determine a corresponding stabilization pulse. Alternatively or additionally, a series of modulation data may be analyzed to determine corresponding stabilization pulses. For cases where a series of modulation data is analyzed, such data may include future and past modulation activity.  
         [0015]     According to some embodiments, laser stabilization techniques may be combined with other approaches adapted to reducing or accommodating noise and other variability from laser sources. Some such other approaches are taught in U.S. patent application Ser. No. 10/933,033 entitled Apparatuses and Methods for Utilizing Non-ideal Light Sources, invented by Margaret Brown et al., filed Sep. 2, 2004 and hereby incorporated by reference.  
         [0016]     According to some embodiments a stabilized laser is used in a scanned beam display such as a head-up display, head-worn display, a microdisplay embedded in a device such as a cell phone or camera, a projection display such as a personal computer projector (beamer), a rear projection or front projection television, and other types of displays.  
         [0017]     According to other embodiments, a stabilized laser is used in a scanned beam image capture device such as a laser camera, a scanned beam endoscope, a bar code scanner, a confocal image capture device, and other types of image capture devices.  
         [0018]     Other aspects will become apparent to the reader through reference to the appended brief description of the drawings, detailed description, claims, and figures. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]      FIG. 1  is a diagram illustrating one type of laser that may be sensitive to temperature variations.  
         [0020]      FIG. 2  is a diagram illustrating another type of laser that may be sensitive to temperature variations.  
         [0021]      FIG. 3  is a diagram illustrating still another type of laser that may be sensitive to temperature variations.  
         [0022]      FIG. 4  is a storage oscilloscope output comparing the output power of a laser diode that is sensitive to temperature variations compared to input current, according to an embodiment.  
         [0023]      FIG. 5  is a flowchart showing a general method for providing a compensation signal to improve the correspondence of laser light output to intended brightness, according to an embodiment.  
         [0024]      FIG. 6  is a flowchart showing a variation of the method of  FIG. 5  wherein the compensation signal is combined with a laser modulation pattern, according to an embodiment.  
         [0025]      FIG. 7  is a diagram illustrating the optical output power of a pulsed laser as a function of drive current according to an embodiment.  
         [0026]      FIG. 8  illustrates a combined laser modulation pattern and thermal compensation or stabilization pattern according to an embodiment where stabilization pulses are made below a lasing threshold.  
         [0027]      FIG. 9  is a storage oscilloscope output comparing the output power of a laser diode compared to input pulses that include thermal compensation or stabilization pulses, according to an embodiment.  
         [0028]      FIG. 10  is a diagram illustrating a combined laser modulation pattern and a thermal compensation or stabilization pattern wherein the stabilization pulses are made above a rollover threshold of the laser, according to an embodiment.  
         [0029]      FIG. 11  is a diagram illustrating a combined laser modulation and thermal compensation pattern wherein the stabilization pulses have a duration shorter than the rise time of the laser, according to an embodiment.  
         [0030]      FIG. 12  is a diagram of a laser having a separate current path for power dissipation, according to an embodiment.  
         [0031]      FIG. 13  is a diagram illustrating a separate laser modulation and thermal compensation or stabilization patterns wherein stabilization pulses may be driven through a separate conduction path, according to an embodiment.  
         [0032]      FIG. 14  is a diagram illustrating a combined laser modulation pattern and thermal compensation or stabilization pattern wherein stabilization for a given pixel may be made over a plurality of pixel clock cycles, according to an embodiment.  
         [0033]      FIG. 15A  is a partial simplified compensation controller block diagram for the generation of distributed thermal compensation waveforms using future and/or past pixel values, according to an embodiment.  
         [0034]      FIG. 15B  is a partial simplified compensation controller block diagram for the generation of thermal compensation waveforms using the current pixel value, according to an embodiment.  
         [0035]      FIG. 16A  is a diagram illustrating some of the principal components of an RGB scanned laser beam display, according to an embodiment.  
         [0036]      FIG. 16B  schematically illustrates a scanned beam display system used as a head up display, for example as a heads-up display in a motor vehicle, according to an embodiment.  
         [0037]      FIG. 17  is a diagram illustrating some of the principal components of an RGB scanned laser beam image capture device, according to an embodiment.  
         [0038]      FIG. 18  is a diagram illustrating a field of view of a scanned beam system according to an embodiment. 
     
    
     DETAILED DESCRIPTION  
       [0039]      FIG. 5  illustrates, in flow chart form, a general approach to compensating to avoid unintended variations in light output, such as the variations illustrated by  FIG. 4 . In step  502 , a laser modulation pattern is received. Such a pattern may, for example, correspond to a series of intended grayscale pixel values in a scanned beam display. The first laser modulation pattern received in step  502  may comprise a single pixel value, or may comprise a sequence of pixel values, depending upon the embodiment. It may correspond to a single laser emitter or may correspond to a plurality of laser emitters, depending upon the embodiment.  
         [0040]     Proceeding to optional step  504 , a signal is received indicating a measured temperature, which may for example be a measured temperature of the laser to be driven, an ambient temperature, or another characteristic temperature such as the temperature of a heatsink. Step  504  is an option in some embodiments.  
         [0041]     In step  506 , a thermal compensation modulation pattern is determined from the first laser modulation pattern received in step  502 , and optionally from the temperature sensor signal received in step  504 . Generally speaking, a thermal compensation modulation pattern may be formed to have a relatively high amount of cumulative power output when the cumulative power output of the first laser modulation pattern is relatively low, and the thermal compensation modulation pattern may be formed to have a relatively low amount of cumulative power output when the cumulative power output of the first laser modulation pattern is relatively high. For embodiments where a temperature sensor signal is also received, the value may further inform the formation of the thermal compensation modulation pattern. For example, if the temperature is relatively high, the cumulative power output of the thermal compensation signal may be reduced or eliminated and when the temperature is relatively low, the cumulative power output of the thermal compensation signal may be increased.  
         [0042]     Finally, in step  508  the first laser modulation pattern and the thermal compensation modulation pattern are outputted. The signals may be outputted as separate signals or alternatively, may be combined into a single signal is indicated in  FIG. 6  below. According to some embodiments, the first laser modulation pattern may be varied responsively to the laser temperature signal. According to some embodiments, the first laser modulation pattern is a function both of a pattern of pulses emitted and a laser temperature signal.  
         [0043]      FIG. 6  is a flow chart illustrating a method wherein the first laser modulation pattern and the thermal compensation modulation pattern are combined into a single pattern, shown in step  602 , called the second laser modulation pattern. In the example of  FIG. 6 , a temperature sensor signal is omitted. In step  602 , the first laser modulation pattern and the thermal compensation modulation pattern are combined to form a second laser modulation pattern having combined attributes.  
         [0044]     Use of the signals output in step  508  of  FIGS. 5 and 6  will be described below.  
         [0045]     Turning now to  FIG. 7 , a curve  702  indicates an illustrative embodiment where the optical output power  704  as a function of drive current  706  under nominal pulsed operating conditions. One can see that below a threshold current I T    708 , substantially no light is output by the laser. Above the threshold current, light output power increases, for example, monotonically, with drive current until a rollover region is reached where output power no longer increases. At a high rollover current, light output may decrease to substantially zero.  
         [0046]      FIG. 8  illustrates an idealized combined laser modulation pattern and thermal compensation pattern according to one embodiment. A laser drive waveform  402  comprising a plurality of pulses is shown as a function of time along a time axis  802 . Laser drive output current is shown on the vertical axis  706 . As may be seen, a threshold current  708  is shown as a horizontal line across the graph. As may be appreciated from the discussion related to  FIG. 7 , portions of the waveform  402  that fall below the threshold current  708  do not result in light output from the laser.  
         [0047]     The plurality of laser modulation pattern pulses  804 ,  806 ,  808 ,  810 ,  812 , and  814  correspond to a plurality of intended laser output powers. As may be seen, pulse  804  corresponds to a relatively high desired brightness, pulse  812  to a somewhat lower brightness, pulses  808 ,  806  and  814  to successively lower brightness, and pulse  810 , to no light. As is shown, pulse  810  does not exceed the threshold current  708  and is therefore a zero-value pulse. It may also be seen that the pulses occur, in this example, at a fixed repetition frequency indicated by the vertical dashed lines and that they are separated by a corresponding series of interleaved periods  816 ,  818 ,  820 ,  822 ,  824 , and  826 . While the time periods represented by the pulse stream  402  are shown as uniform, it is not necessary for such time periods to be uniform. As will become clear from the discussion below, the method and apparatuses taught herein may be easily adapted to a non-constant pulse frequency.  
         [0048]     It may also be seen that the current of the interleaved periods are not uniform, but rather are varied in a manner inversely proportional to the height of the preceding modulation pattern pulse. The interleaved periods  816  to  826  are termed compensation pulses. Because the compensation pulses  816  to  826  are at a drive current that falls below the threshold current  708 , they do not produce light output. Instead, the compensation pulses  816  to  826  are used to create a relatively uniform rate of power dissipation in the laser, even though the current of the light modulation pulses is not uniform.  
         [0049]     Light output pulse  804  is at a relatively high level. Compensation pulse  816  is thus set to a relatively low level. In comparison, light output pulse  814  is at a relatively low level, and corresponding compensation pulse  826  is set to a relatively high level. Generally speaking, the compensation pulse current is selected to provide relatively uniform average power dissipation, and therefore relatively uniform average laser heating, over each period. Thus the integrated power dissipation during the first period comprising light output pulse  804  and compensation pulse  816  is substantially equal to the integrated power dissipation during the second period comprising light output pulse  806  and compensation pulse  818 . The subsequent periods similarly have substantially equal integrated power dissipation. The black pixel pulse  810  and its corresponding compensation pulse  822  are selected to be just below or at the threshold current  708  and therefore meld with one another with no apparent edge in between.  
         [0050]     According to some embodiments, the cumulative amount of power dissipated during each respective pixel cycle need not necessarily be maintained absolutely constant, but may rather be made relatively constant. That is, a particularly bright pixel may dissipate somewhat more power than the average pixel, even when the drive current is held at zero between pulses, and a particularly dark pixel may dissipate somewhat less power than the average pixel, even when the drive current is held just below the threshold current over the duration of the pixel period. The acceptable or desirable power dissipation variability during given pixel periods may vary according to the temperature sensitivity of the laser, the acceptable variability or noise in the output pixel brightness, the thermal time constant of the laser, the variability in the pixel period, etc.  
         [0051]     While the approach of  FIG. 8  and other embodiments shown herein depict compensation for a moderate number of finite laser brightnesses, embodiments of the invention are not so limited. For example, with appropriate controller hardware and logic, continuously varying brightnesses may be compensated for. Similarly, single-bit (on-off) brightnesses may be compensated for, for example by choosing illumination and compensation patterns corresponding to only the pulses  804 ,  816  corresponding to the first pixel period and the  810 ,  822  pulses corresponding to the fourth pixel period.  
         [0052]     Moreover, while the pixel periods are shown as constant in  FIG. 8  and  FIGS. 10-13 , pixel periods may be varied. For example, sinusoidal scan patterns typically use relatively shorter pixel periods near the center of a field of view and relatively longer pixel periods near the edges of the field of view. The schedule relating desired pixel brightness to thermal compensation may be varied across a scan line, for instance by maintaining a relatively constant power dissipation rate even though the pixel period is varied.  
         [0053]      FIG. 9  is a storage oscilloscope output showing a series of light output pulses  902  compared to input modulation  904  that includes stabilization or thermal compensation pulses. As may be seen, the resultant light output pulses maintain a relatively constant pulse width across a range of brightness levels and generally reach a desired peak brightness, as indicated by line  906 . Input modulation  904  includes laser modulation pulses  908 ,  910 ,  912 ,  914 ,  916 , and  918  corresponding to respective light output pulses  920 ,  922 ,  924 ,  926 ,  928 , and  930 . Thermal compensation or stabilization pulses in input modulation  904  may be seen by comparing to a zero output drive level  932 . Thus, the first, low power laser modulation pulse  908  is followed by a relatively high current stabilization pulse  934 . Similarly the second, somewhat higher power laser modulation pulse  910  is followed by a somewhat lower current stabilization pulse  936 . This trend may be seen to follow as progressively higher laser modulation pulses  912 ,  914 ,  916 , and  918  are followed by progressively lower respective stabilization pulses  938 ,  940 ,  942 , and  944 . As described above, the stabilization pulses  934 ,  936 ,  938 ,  940 ,  942 , and  944  are made inversely proportional to the associated laser modulation pulse and, being below the lasing threshold of the device, do not result in any substantial amount of laser radiation from the laser, but rather serve to provide power dissipation in an amount that results in substantially constant dissipation through the device, regardless of the pulse brightness from the laser. This allows, for example, a video signal having variable and arbitrary brightness pixels to be scanned across a field of view wherein the pixels reach their intended brightness, rather than a different or variable brightness.  
         [0054]      FIG. 10  is a diagram, according to an embodiment, illustrating a combined laser modulation and thermal compensation or stabilization pattern  1002  on current  706  versus time  802  axes, wherein the stabilization pulses are made above a rollover threshold I R    1004  of a laser. As mentioned above, some lasers have a maximum current at which they will emit light, wherein current applied above the maximum current, termed the rollover threshold, results in substantially no light being emitted. A series of laser modulation pulses  1006 ,  1008 ,  1010 ,  1012 ,  1014 ,  1016 , and  1018  are made according to a pixel clock illustrated by vertical dashed lines  1020 . As may be seen, the laser modulation pulses  1006 - 1018 , with the exception of black pulse  1012 , extend from above the rollover threshold  1004  down to a current between the threshold current I T    708  and the rollover threshold I R    1004 . That is, a given laser pulse (with the exception of black, non-illuminating pulses) is made at a current below the current dissipated by the laser between pulses.  
         [0055]     As may be seen, a relatively high brightness pulse  106  is followed by a compensation pulse  1022  that is relatively low, but above the rollover current  1004 . A somewhat lower brightness pulse  1010  is paired with a somewhat higher compensation pulse  1026 . This trend may be seen throughout the modulation and stabilization pattern  1002  as respective progressively dimmer (lower current) light modulation pulses  1008 ,  1018 ,  1014 , and  1016  are paired with progressively higher current compensation pulses  1024 ,  1034 ,  1030 , and  1032 . A black or null pixel is formed by a combined pulse  1012  and  1028  that holds the current just above the rollover threshold  1004  throughout the duration of the pixel period  1020 .  
         [0056]     Thus, in a manner akin to that shown in  FIG. 8 , the method of  FIG. 10  provides a relatively constant amount of power dissipation through a laser even thought the application calls for a variable pattern of light output. Relatively low optical power pulses such as  1016  and  1014  are paired respectively with relatively high power thermal compensation pulses  1032  and  1030 . Relatively high optical power pulses such as  1006  and  1010  are paired respectively with relatively low power thermal compensation pulses  1022  and  1026 . Black pulses such as  1012  are made, according to the illustrated embodiment, by keeping the drive current just above the rollover current  1004  such that an appropriate amount of power is dissipated over the period while substantially no light is emitted.  
         [0057]     In addition to being characterized by threshold current and rollover current, lasers may be characterized by bandwidth or maximum modulation frequency. Lasers may be designed to modulate below a given cut-off frequency but not output light when modulated above the cut-off frequency. The bandwidth characteristics of a given type of laser may be characterized by a rise time, wherein an energization pulse must be applied to the laser for a period at least as long as its rise time before any substantial amount of light is emitted.  FIG. 11  is a diagram illustrating a combined laser modulation and thermal compensation or stabilization pattern  1101  wherein the stabilization pulses have durations shorter than the rise time of the laser.  
         [0058]     As with  FIGS. 8 and 10 ,  FIG. 11  shows a combined laser modulation and thermal compensation or stabilization pattern  1101  on current versus time axes,  706  and  802 , respectively. A high brightness pulse  1102  precedes a medium brightness pulse  1104 . These pulses are followed respectively by medium-bright pulse  1106 , black or null pulse  1108 , high brightness pulse  1110 , low brightness pulse  1120 , and medium brightness pulse  1114  as time progresses from left to right. As may be seen, the high brightness pulse  1102  during the first illustrated full pixel period  1020  has sufficient power dissipation that substantially no additional compensation power is dissipated during subsequent compensation pulse  1116 . Subsequent medium brightness pulse  1104 , however, does not dissipate the desired amount of power during the subsequent period  1020  and additional compensation power is dissipated during subsequent compensation period  1118 . In contrast with the approaches shown above, however, the compensation pulse  1118  comprises a plurality of short on-off sub-pulses, with the on periods being held to durations shorter than the rise time of the laser. Thus, sub-pulses of compensation pulse  1118  are expressed in the laser substantially as heat dissipation rather than light emission (although the designer may opt to output some small amount of light during the heat dissipation pulses).  
         [0059]     Similarly, medium-bright pulse  1106  is paired with a compensation pulse  1120  that dissipates somewhat more thermal energy than the compensation pulse  1118  but less thermal energy than the compensation pulse  1116  paired with bright illumination pulse  1102 . The dark or null pixel  1108  is comprised only of short power dissipation on-off pulses that carry through its paired compensation pulse  1122 . This results in thermal power dissipation that is close to the amount of power dissipated during other pixel periods  1020 , but results in substantially no light emission. Bright pulse  1110  is similar to bright pulse  1102  in that it is paired with a similar low power compensation pulse  1124 , and low brightness pulse  1112  is paired with a relatively high power compensation pulse  1126 .  
         [0060]     Also shown in  FIG. 11  is a laser temperature curve  1128 , plotted as temperature T  1130  along a common time axis  802 . Pixel periods  1120  are shown extending from the pulse pattern curve to the laser temperature curve to illustrate the correspondence of the curves.  
         [0061]     In the example of  FIG. 11 , a laser device has a preferred temperature operating range between a minimum temperature T min    1132  and a maximum temperature T max    1134 . Thus, it is desirable to keep the temperature of the laser, indicated by curve  1128 , between these two extremes. As may be seen, the temperature rises during light emission pulses and falls during periods of non-energization. Compensation pulses are made in the during the times  1116 ,  1118 ,  1120 ,  1122 ,  1124 , and  1126  between light pulses as necessary to keep the temperature of the device above T min    1132 . As may be seen, high brightness pulses  1102  and  1110  create a relatively large corresponding temperature rise in the laser temperature curve  1128 . The lack of compensation pulses during the paired compensation periods  1116  and  1124  allows the laser to cool back down to a level closer to T min    1132 , thus preparing the laser for the next light emission pulse, which will again cause a temperature rise to some higher level. As may be seen, the amount of compensation energy dissipated through the laser is chosen to bring the temperature of the laser back to approximately the same level for the start of each light emission pulse. During the black pixel pulse  1108 , the laser may receive compensation pulses for the entire pixel period to maintain its temperature.  
         [0062]     While the temperature response  1128  is idealized in that the same temperature is returned to for each light emission pulse, the system need not necessarily be so constrained. As will be seen in conjunction with  FIG. 14 , it may be possible in some systems to allow for relatively wider swings in temperature and the system may optionally cause the temperature of the laser to return to a desired level over a series of pixel periods. The system may similarly prepare for a period of relative inactivity (low brightness or black pixels) by raising the temperature prior to the period such that a desired nominal operating temperature range is substantially maintained for the duration of the period of relative inactivity.  
         [0063]     While the on-off pulses made during the compensation pulses of waveform  1101  are shown as being about one-quarter the duration of the light emission pulses and the duty cycle during the compensation period of the compensation pulses is shown as being about 50%, other values may be selected according to the application. For example, the scan rate and addressability of a scanned beam system may be such that the pixel periods  1020  range from about 20 to 30 nanoseconds (nS), varying sinusoidally across the field of view. Light emission pulses may be chosen to last about 10 nS with the compensation pulses comprising the remainder of the pixel periods. The on sub-pulses during the compensation pulses may be chosen to last about 1 nS, or about one order of magnitude shorter than the light emission pulses. Accordingly, a laser bandwidth or cutoff frequency may be chosen to fall between a frequency corresponding to the illumination pulses and the compensation sub-pulses. In this example, the 10 nS illumination pulses correspond to a frequency of about 100 mega-Hertz (MHz) and the 1 nS sub-pulses correspond to a frequency of about 1 giga-Hertz (GHz). It is sometimes desirable for a device bandwidth to be at least three times a designed pulse frequency, so a suitable laser bandwidth for the example could be about 300-500 MHz, corresponding to a rise time of about 2 to 3 nS. The drive circuit may, for example, have a bandwidth of about 3 GHz to support the relatively high frequency of the sub-pulses. The ratio of frequencies of the compensation sub-pulses to the illumination pulses may be modified to further optimize the system, for example by shortening the sub-pulses to allow for dissipating heat through a laser having a bandwidth higher than about 3 GHz while still avoiding light output during the compensation period. Other ranges may be appropriate depending on things such as resolution, scan rate, number of beams, etc.  
         [0064]     As an alternative to dissipating compensation pulses through the same current path as the light emission pulses, a laser may be configured to have an alternative, non-illuminating current path. According to one example, diagrammatically shown in  FIG. 12 , a non-light emitting diode or power diode PD  1202  may be packaged in close proximity to a laser diode LD  1204  with separate or switched respective current sources  1206 ,  1208  and grounds  1210  and  1212 . As shown, the power diode  1202 , which may contain internal resistance corresponding to the resistance of the laser diode  1204 , is constructed from a separate die and bonded subjacent to and thermally coupled to the laser diode  1204 . Laser diode  1204  is aligned to emit an output beam  1214  through a lens  1216 . Alternatively, the power dissipation current path may, for example, be configured as a neighboring, on-die device with no lasing cavity, and/or no light guide, and/or no exit facet, with an oppositely oriented exit facet, etc. Alternatively, an alternative power dissipation path may comprise a resistor or other device thermally coupled to the laser.  
         [0065]      FIG. 13  is a diagram illustrating a separate laser modulation pattern  1302  and thermal compensation or stabilization pattern  1304 , wherein stabilization pulses are driven through a separate conduction path such as, for example, through a power diode  1202  as shown in  FIG. 12 . The waveforms  1302  are shown plotted as respective current  706  and  1306  along common time axes  802  aligned vertically. Pixel periods  1020  are shown extending between axes, illustrating the correspondence of the periods across both waveforms  1302  and  1304 .  
         [0066]     A high brightness, high current laser emission drive pulse  1308  is shown with a corresponding low current thermal compensation pulse  1310 . Since the pulse  1308  results in a target amount of thermal dissipation during the first pixel period  1020 , no additional thermal dissipation is desired, and as such thermal compensation pulse  1310  is kept at a very low level. A medium brightness emission pulse  1312  is paired with a medium thermal compensation pulse  1314 . The relative amounts of current dissipation may be chosen to provide current dissipation during the second period  1020  (i.e. during the period corresponding to laser pulse  1312  and thermal compensation pulse  1314 ) approximately equal to the current dissipated by the high brightness laser drive pulse  1308  during the first period  1020 . Similarly, a medium-bright laser emission pulse  1316  is paired with a medium-low thermal compensation pulse  1318 , again resulting in relatively constant total thermal dissipation during the period  1020 . A black or null pixel is driven by the laser emission pulse  1320  having substantially no height and is paired with a high current thermal compensation pulse  1322 , again resulting in substantially constant total thermal dissipation during the period  1020 . Following the dark pixel, respective high, low, and medium brightness laser emission pulses  1324 ,  1326 , and  1328  are paired with corresponding thermal compensation pulses  1330 ,  1332 , and  1334  in the manner shown.  
         [0067]     Thus, a relatively constant temperature is maintained in the laser by providing inversely proportional laser illumination and thermal compensation waveforms  1302  and  1304 . Although the respective laser illumination and corresponding thermal compensation pulses are shown as occurring simultaneously in  FIG. 13 , such pulses may alternatively be offset during respective periods. Alternatively, the thermal compensation pulses may be distributed across a sequence of pixel periods or arranged in other ways.  
         [0068]      FIG. 14  illustrates a waveform  1402  comprising a sequence of laser illumination pulses and thermal compensation pulses wherein the thermal compensation pulses corresponding to a given laser illumination pulse may be distributed across a sequence of pixel periods  1020 . A first, high brightness pixel illumination pulse  1402  is followed by a second high brightness pixel illumination pulse  1404 , which is followed by a low brightness, low power pixel illumination pulse  1406 . Corresponding thermal compensation pulses  1408 ,  1410 , and  1414  are shown. The first corresponding thermal compensation pulse  1408  is shown dissipating very little or zero power. Since it falls between two high power illumination pulses  1402  and  1404 , no additional power dissipation is needed to maintain the laser temperature in an appropriate range. The second thermal compensation pulse  1410 , however, is shown at a higher current than would normally be expected with respect to its corresponding laser illumination pulse  1404 . This is because the subsequent laser illumination pulse  1406  is low enough current that its corresponding thermal compensation pulse  1414  is incapable of outputting sufficient thermal compensation energy during its allotted period to maintain the desired laser temperature while remaining below the lasing threshold  708 . Instead, the controller looks ahead at the future (F 1 ) laser emission pulse and adjusts the present thermal compensation pulse  1410  upward to share some of the thermal compensation workload. Thus, the thermal compensation corresponding to the low brightness laser illumination pulse  1406  is spread over two successive thermal compensation pulses  1410  and  1414 . Taken together, the two successive thermal compensation pulses  1410  and  1414  are, according to the example, sufficient to provide thermal compensation for the low brightness laser illumination pulse  1406 .  
         [0069]     Proceeding to the right, bright pixel laser drive pulse  1416  is followed by a low power thermal compensation pulse  1418 . This is followed by a bright laser drive pulse  1420 . This time, however, the controller again looks ahead and sees that the next laser illumination pulse  1422  is a black or null pixel and so the thermal compensation pulse  1424  associated with laser illumination pulse  1420  is adjusted to a high level to begin compensation for black laser illumination pulse  1422 . Black laser illumination pulse  1422  is followed by a thermal compensation pulse  1426  set just below the lasing threshold. According to the example, the combination of thermal compensation pulses  1424  and  1426  are not quite sufficient to maintain the laser temperature at a nominal value. A subsequent high power laser illumination pulse would be sufficient to again raise the temperature to a desired range, but the pixel map instead calls for a medium power laser drive pixel  1428 . Thus, the controller looks back and determines that the thermal compensation pulse  1430  associated with medium power laser drive pixel  1428  should be adjusted upward a bit to provide extra power dissipation to compensate for the previous black pixel  1422 . Accordingly, thermal compensation pulse  1430  is set just below the lasing threshold  708  to raise the laser temperature back to its desired range.  
         [0070]     While the pattern indicated in  FIG. 14  illustrates the use of both look-ahead and look-back logic to determine the value of thermal compensation pulses, just one or just the other may suffice for a given application. Furthermore, compensation logic may be extended beyond only one future and/or one previous pulse in determining an appropriate thermal compensation pulse value.  
         [0071]     Moreover, while the combined laser illumination and distributed thermal compensation waveform  1402  is illustrated as corresponding to the approach of  FIG. 8 , the approaches of  FIGS. 10, 11 ,  13  or combinations thereof may be used.  
         [0072]     The look-ahead feature illustrated by  FIG. 14  may be used to advantage in intermittently used systems. For applications where no light need be emitted for extended periods, the laser may be allowed to cool. This may be used, for example, to reduce power consumption, increase laser life, increase safety, etc. When an indication of impending laser emission is received, for example as a new video frame begins to be received or when a trigger pull in a scanned beam imager is sensed, the compensation controller may transmit warm-up pulses to the laser to raise its temperature to or near an optimal or nominal operating temperature. This may be used even in systems that do not exhibit variable laser emissions during use, but rather may use a relatively constant duty cycle.  
         [0073]     Alternatively, the approaches described herein may be used in combination with image compensation. According to such an approach, thermal compensation may be used to improve the consistency of the laser temperature, but not necessarily maintain temperature closely enough to prevent mode-hopping or output variation altogether. In such a system, various approaches to compensation logic may be used. For example, a pixel-value-to-code look-up table may include a variable related to the mode (or the output efficiency) that the laser is in or predicted to be in. When the laser is in a low output state, the code value used to drive the laser drive amplifier may be increased somewhat to provide extra current or extra on-time sufficient to overcome the reduced output. Conversely, the designer may choose to operate the laser in a less-than-maximum efficiency mode. When the laser is heated so as to increase the efficiency above the nominal design efficiency, laser drive power or duration may be decreased correspondingly.  
         [0074]     The choice to drive the laser in a less than maximum-output mode may, of course, be implemented whether or not image compensation is used. Such an approach may be used to add control authority or range to a system, compensate for part aging, part-to-part variations, or system alignment, respond to brightness control input, etc.  
         [0075]      FIG. 15A  is a block diagram of a simplified controller adapted to compensate for laser illumination pulses over a plurality of compensation periods. An input data stream  1502 , which may for example be a video data stream, is received. A first pixel value is received in a memory array  1504  and loaded into a first partition F 2    1506 , the width of which is determined according to the bit depth of the pixel value. The contents of first partition  1506  represent the second future grayscale value to be output by a laser. At the beginning of the next pixel period, the contents of the first partition  1506  are shifted to a next memory partition F 1    1508  and the subsequent pixel value is loaded into first partition  1506 . This process continues, with new pixel grayscale values being received in the first memory partition F 2    1506 , then shifted sequentially through the second memory partition F 1    1508 , a third memory partition C  1510 , a fourth memory partition P 1    1512 , a fifth memory partition P 2    1514 , and then dumped with each new pixel period. According to the example of  FIG. 15 , F 2    1506  represents the second future pixel grayscale value desired to be produced by the laser; F 1    1508  represents the future or next pixel grayscale value, C  1510  represents the current pixel grayscale value, P 1    1512  represents the past pixel grayscale value, and P 2    1514  represents the second past pixel grayscale value.  
         [0076]     In some applications, and particularly in applications that use separate pixel illumination and thermal compensation current paths, the grayscale pixel illumination value held in the C or current memory partition  1510  can be read and the value used to drive an optional first digital-to-analog converter (D/A)  1516 , the signal from which is amplified by and optional first amplifier  1518  and used to drive the laser emission pulses of laser  1520 . Additionally or alternatively, the pixel values held in memory partitions  1506 - 1514  are read by a compensation processor  1522 . Compensation processor  1522  produces a series of digital pulses on output  1524  that are used to drive a second D/A  1526 . The output of D/A  1526  is amplified by amplifier  1528 . The amplified output of amplifier  1528  then drives a current dissipation path in laser  1520 .  
         [0077]     In cases where optional D/A  1516  and optional amplifier  1518  are not used, the output of amplifier  1528  is used to drive at least the light emission current path of laser  1520 . For cases where optional D/A  1516  and optional amplifier  1518  are not used, amplifier  1528  may optionally also be used to drive a second power dissipation current path in the laser  1520 . For cases where the optional D/A  1516  and optional amplifier  1518  are used, the laser light emission is driven from optional amplifier  1518  and the amplifier  1528  is used to drive the second power dissipation current path in laser  1520 .  
         [0078]     The compensation processor  1522  may be implemented in a variety of ways such as, for example, a programmable microprocessor or microcontroller, an application-specific integrated circuit (ASIC), a field-programmable gate array (FPGA), a programmable array logic device (PAL), a gate array, discrete circuitry, and/or other forms, along with associated circuitry.  
         [0079]     The memory array  1504  and associated pixel value shifting may be implemented in a variety of ways including as a single memory device or a portion of a single memory device and as separate discrete memory devices such as shift registers, FIFOs, etc. The location of a given pixel value may remain fixed with a rotating pointer determining the relative positions of pixel values or the data may be physically shifted from location to location. All or parts of the compensation system  1501  may comprise a portion of a larger controller or may comprise a purpose-specific controller.  
         [0080]     A simplified compensation controller block diagram is shown in  FIG. 15B . The compensation processor  1522  and other components of  FIG. 15B  operate in a manner similar to that described in conjunction with  FIG. 15A  except that past and future pixel activity is not considered in determining a compensation waveform.  
         [0081]     As with the arrangement of  FIG. 15A , compensation processor  1522  of  FIG. 15B  may optionally output separate laser illumination and laser thermal compensation waveforms, an arrangement that may be especially useful in conjunction with lasers having a separate heater conduction path, such as the example shown in  FIG. 12 . Such an embodiment may use a separate output (not shown) to carry a compensation waveform such as waveform  1304  of  FIG. 13 .  
         [0082]      FIG. 15B  additionally shows an optional pixel period input  1530  that may also be used in conjunction with the compensation controller of  FIG. 15A . Pixel period input  1530  may input a pixel clock, a scan velocity indication, a pixel location indication, etc. When a variable pixel clock is used, as described below, for example, the pixel period input  1530  may be used by the compensation processor  1522  to create a compensation waveform that provides substantially constant power dissipation through the laser per unit time, rather than per pixel clock cycle. For example, when the beam of a sinusoidally scanned system is near the center of the field of view, its velocity may be significantly higher than when the beam is near the edge of the field of view. In such a case, a reduced amount of compensation power may be applied during a pixel cycle corresponding to the location near the center of the field of view, compared to the amount of compensation power that is applied during pixel cycles corresponding to locations near the edge of the field of view, for example. By varying the relative amount of compensation energy in a manner proportional to the pixel cycle time (shorter cycles receive relatively less compensation energy, longer cycles receive relatively more compensation energy) the amount of power dissipation through the laser may be kept constant per unit time, thus keeping the temperature of the device more nearly constant.  
         [0083]     One application for a stable laser drive system is a scanned beam display, such as that described in U.S. Pat. No. 5,467,104 of Furness et al., entitled VIRTUAL RETINAL DISPLAY, which is incorporated herein by reference. As shown in  FIG. 16A , in a scanned beam display  1602 , a scanning source  1604  outputs a scanned beam of light that is coupled to a viewer&#39;s eye  1606  by a beam combiner  1608 . In scanned displays, a scanner, such as a scanning mirror or acousto-optic scanner, scans a modulated light beam onto a viewer&#39;s retina. An example of such a scanner is described in U.S. Pat. No. 5,557,444 to Melville et al., entitled MINIATURE OPTICAL SCANNER FOR A TWO-AXIS SCANNING SYSTEM, which is incorporated herein by reference. The scanned light enters the eye  1606  through the viewer&#39;s pupil  1610  and is imaged onto the retina  1612  by the cornea. In response to the scanned light the viewer perceives an image.  
         [0084]     Sometimes such displays are used for partial or augmented view applications. In such applications, a portion of the display is positioned in the user&#39;s field of view and presents an image that occupies a region of the user&#39;s field of view. The user can thus see both a displayed virtual image and background information  1614 . If the background light is occluded, the viewer perceives only the virtual image.  
         [0085]      FIG. 16B  shows some additional detail of some components of the scanning source  1604  of  FIG. 16A  in the context of a head up display in a motor vehicle  1615 , according to an embodiment. A controller  1616  receives information for display from interface  1618 . Such information may comprise video data or alternatively may comprise sensor data. In the case where the interface  1618  provides sensor data, the controller  1616  selects sensor data and formats it into video. The controller  1616  modulates light sources  1620 ,  1622 , and  1624 , which may be for example, a red laser diode, a green frequency doubled laser, and a blue frequency doubled laser, respectively. As is described above, the video format may comprise variable brightness pixels that nominally would create non-constant power dissipation in the light sources  1620 ,  1622 , and  1624 . The controller may format the video data in a manner that results in a relatively uniform distribution of pixel brightness across the field of view by selecting where and how to display information across the field of view. Alternatively or additionally, the controller may provide thermal compensation waveforms to one or all the light sources  1620 ,  1622 , and/or  1624  in manners described above. In the example where a red laser  1620  is not particularly sensitive to variations in temperature but a green laser  1622  and a blue laser  1624  are sensitive to variations in temperature, the controller  1616  may provide thermal compensation waveforms only to green laser  1622  and blue laser  1624 .  
         [0086]     The light sources  1620 ,  1622 , and  1624  emit modulated beams of light at respective wavelengths into a beam combiner  1626  that combines the modulated beams into a single modulated beam  1628 . A beam shaping optic  1630 , such as a collimator, a top-hat converter, an astigmatism corrector, etc. shapes the beam  1628  and directs it toward a scan mirror  1632 . Controller  1622  drives the scan mirror  1632  to, in combination with the light sources  1620 ,  1622 , and  1624 , provide a scan pattern that may be perceived by the user&#39;s eye  1606  as an image. Scan mirror  1632  thus creates a scanned beam of modulated light  1634 . Scanned beam  1634  is reflected by an optional mirror  1636  toward a final combining optic  1638 . In some cases, final combining optic  1638  may be the windshield of a motor vehicle. Thus the scan source  1604 , in combination with the optional mirror  1636  and final combining optic  1638  provides a see-through display to the user  1606 .  
         [0087]     In addition to finding application in scanned beam imaging systems such as those shown in  FIGS. 16A and 16B , embodiments of the method and apparatus for stable laser drive may be used in scanned beam image capture systems.  FIG. 17  is a diagram illustrating some of the principal components of an RGB scanned laser beam image capture device  1702  according to an embodiment.  
         [0088]     An illuminator  1704  creates a first beam of light  1706 . A scanner  1708  deflects the first beam of light across a field-of-view (FOV) to produce a second scanned beam of light  1710 , shown in two positions  1710   a  and  1710   b . The scanned beam of light  1710  sequentially illuminates spots  1712  in the FOV, shown as positions  1712   a  and  1712   b , corresponding to beam positions  1710   a  and  1710   b , respectively. While the beam  1710  illuminates the spots  1712 , the illuminating light beam  1710  is reflected, absorbed, scattered, refracted, wavelength shifted, or otherwise affected by the properties of the object or material to produce scattered light energy. A portion of the scattered light energy  1714 , shown emanating from spot positions  1712   a  and  1712   b  as scattered energy rays  1714   a  and  1714   b , respectively, travels to one or more detectors  1716  that receive the light and produce electrical signals corresponding to the amount of light energy received. The electrical signals drive a controller  1718  that builds up a digital image and transmits it for further processing, decoding, archiving, printing, display, or other treatment or use via interface  1720 .  
         [0089]     Light source  1704  may comprise multiple emitters such as, for instance, light emitting diodes (LEDs), lasers, thermal sources, arc sources, fluorescent sources, gas discharge sources, or other types of illuminators. In some embodiments, illuminator  1704  comprises a laser that is temperature-sensitive. In such embodiments, circuitry in the controller  1718  may provide thermal dissipation compensation signals as taught herein.  
         [0090]     In some embodiments, light source  1704  comprises a red laser diode having a wavelength of approximately 635 to 670 nm, a violet or blue laser diode or diode-pumped solid-state (DPSS) laser having a wavelength of approximately 415 to 473 nm, and a green laser providing a green laser beam having a wavelength of about 532 nm. The green laser may be a DPSS and/or a type of laser that uses second harmonic generation to convert 1064 nm light to 532 nm light, such as is shown in  FIGS. 1-3 . Other types of lasers may be interchanged and/or combined, wherein at least one of the lasers possesses a temperature sensitivity that is accommodated using compensation pulses. One or more of the lasers may optionally be externally modulated. In the case where an external modulator is used, it is considered part of light source  1704 . Similarly, light source  1704  may comprise other types of light emitters such as one or more light emitting diodes (LEDs).  
         [0091]     Light source  1704  may include, in the case of multiple emitters, beam combining optics to combine some or all of the emitters into a single beam. Light source  1704  may also include beam-shaping optics such as one or more collimating lenses and/or apertures. Additionally, while the wavelengths described in the previous embodiments have been in the optically visible range, other wavelengths may be within the scope of the invention.  
         [0092]     Light beam  1706 , while illustrated as a single beam, may comprise a plurality of beams converging on a single scanner  1708  or onto separate scanners  1708 .  
         [0093]     Some embodiments of scanned beam displays and scanned beam image capture systems use a MEMS scanner  1632 ,  1708 . A MEMS scanner may be of a type described in, for example; U.S. Pat. No. 6,140,979, entitled SCANNED DISPLAY WITH PINCH, TIMING, AND DISTORTION CORRECTION and commonly assigned herewith; U.S. Pat. No. 6,245,590, entitled FREQUENCY TUNABLE RESONANT SCANNER AND METHOD OF MAKING and commonly assigned herewith; U.S. Pat. No. 6,285,489, entitled FREQUENCY TUNABLE RESONANT SCANNER WITH AUXILIARY ARMS and commonly assigned herewith; U.S. Pat. No. 6,331,909, entitled FREQUENCY TUNABLE RESONANT SCANNER and commonly assigned herewith; U.S. Pat. No. 6,362,912, entitled SCANNED IMAGING APPARATUS WITH SWITCHED FEEDS and commonly assigned herewith; U.S. Pat. No. 6,384,406, entitled ACTIVE TUNING OF A TORSIONAL RESONANT STRUCTURE and commonly assigned herewith; U.S. Pat. No. 6,433,907, entitled SCANNED DISPLAY WITH PLURALITY OF SCANNING ASSEMBLIES and commonly assigned herewith; U.S. Pat. No. 6,512,622, entitled ACTIVE TUNING OF A TORSIONAL RESONANT STRUCTURE and commonly assigned herewith; U.S. Pat. No. 6,515,278, entitled FREQUENCY TUNABLE RESONANT SCANNER AND METHOD OF MAKING and commonly assigned herewith; U.S. Pat. No. 6,515,781, entitled SCANNED IMAGING APPARATUS WITH SWITCHED FEEDS and commonly assigned herewith; and/or U.S. Pat. No. 6,525,310, entitled FREQUENCY TUNABLE RESONANT SCANNER and commonly assigned herewith; all hereby incorporated by reference.  
         [0094]     A 2D MEMS scanner  108  scans one or more light beams at high speed in a pattern that covers an entire 2D FOV or a selected region of a 2D FOV within a frame period. A typical frame rate may be 60 Hz, for example. Often, it is advantageous to run one or both scan axes resonantly. In one embodiment, one axis is run resonantly at about 19 KHz while the other axis is run non-resonantly in a sawtooth pattern so as to create a progressive scan pattern. A progressively scanned bi-directional approach with a single beam scanning horizontally at scan frequency of approximately 19 KHz and scanning vertically in sawtooth pattern at 60 Hz can approximate an SVGA resolution. In one such system, the horizontal scan motion is driven electrostatically and the vertical scan motion is driven magnetically. Alternatively, both the horizontal and vertical scan may be driven magnetically or capacitively. Electrostatic driving may include electrostatic plates, comb drives or similar approaches. In various embodiments, both axes may be driven sinusoidally or resonantly.  
         [0095]     Several types of detectors may be appropriate, depending upon the application or configuration. For example, in one embodiment, the detector may include a simple PIN photodiode connected to an amplifier and digitizer. In this configuration, beam position information may be retrieved from the scanner or, alternatively, from optical mechanisms, and image resolution is determined by the size and shape of scanning spot  1712 . In the case of multi-color imaging, the detector  1716  may comprise more sophisticated splitting and filtering to separate the scattered light into its component parts prior to detection. As alternatives to PIN photodiodes, avalanche photodiodes (APDs) or photomultiplier tubes (PMTs) may be preferred for certain applications, particularly low light applications.  
         [0096]     In various approaches, simple photodetectors such as PIN photodiodes, APDs, and PMTs may be arranged to stare at the entire FOV, stare at a portion of the FOV, collect light retrocollectively, or collect light confocally, depending upon the application. In some embodiments, the photodetector  1716  collects light through filters to eliminate much of the ambient light.  
         [0097]     The scanned beam image capture system  1702  may be embodied as monochrome, as full-color, and even as a hyper-spectral. In some embodiments, it may also be desirable to add color channels between the conventional RGB channels used for many color cameras.  
         [0098]     In some embodiments, the illuminator may emit a polarized beam of light or a separate polarizer (not shown) may be used to polarize the beam. In such cases, the detector  1716  may include a polarizer cross-polarized to the scanning beam  1710 . Such an arrangement may help to improve image quality by reducing the impact of specular reflections on the image.  
         [0099]     High speed MEMS mirrors and other resonant deflectors may be characterized by sinusoidal scan rates, compared to constant rotational velocity scanners such as rotating polygons. To reduce power requirements and size constraints of the scanner, some embodiments may allow both scan axes to scan resonantly.  
         [0100]      FIG. 18  is an idealized diagram illustrating a field of view of a scanned beam system according to an embodiment.  FIG. 18  illustrates a two-dimensional (2D) beam scan pattern  1802 , illustrated by solid lines, overlaying a field of view  1804 . A variety of beam scan patterns may be used. The exemplary scan pattern is a Lissajous scan pattern that repeats several top-to-bottom and bottom-to-top vertical cycles per frame while a large number of horizontal cycles are repeated. The amplitudes of the scan pattern  1802  may be selected such that a portion of the scan pattern occurs within the field of view  1804  and other portions of the scan pattern  1806  and  1808  fall outside the field of view.  
         [0101]     For resonant scanning systems, constant frequency pulse modulation may be used with constant pixel clock rate and variable pixel spacing. In such a mode, it may be desirable to apply image processing to interpolate between actual sample locations to produce a constant pitch output. In this case, the addressability limit is set at the highest velocity point in the scan as the beam crosses the center of the FOV. More peripheral areas at each end of the scan where the scan beam is moving slower are over-sampled. In general, linear interpolation applied two-dimensionally has been found to yield good image quality and have a relatively modest processing requirement.  
         [0102]     Alternatively, constant pixel spacing may be maintained by varying pixel clocking frequency. Methods and apparatus for varying pixel clocking across a FOV are described in U.S. patent application Ser. No. 10/118,861, entitled ELECTRONICALLY SCANNED BEAM DISPLAY, filed Apr. 9, 2002, commonly assigned herewith and incorporated by reference.  
         [0103]     As noted above, compensation energy may be selected to provide relatively constant power dissipation in the laser per pixel cycle. Alternatively, and especially when pixel clocking frequency is varied, compensation may be selected to provide relatively constant power dissipation in the laser per unit time. Such a system may be implemented by providing to the compensation processor  1522  in  FIG. 15A  with information about the instantaneous scan rate and/or the scan position within the scan pattern. For multiple pixel implementations, such information may be combined with future and/or past pixel grayscale values to determine a compensation pattern.  
         [0104]     In addition to the continuous or pixel-by-pixel thermal compensation taught herein, scanned beam systems my use overscan areas such as areas  1806  and  1808  to provide additional thermal compensation. That is, for scanned lines that cumulatively provide more nominal heating of the laser than may be desired, the laser may be turned off in the overscan regions  1806  and  1808  to allow it to cool somewhat. Conversely, for scanned lines that cumulatively proved less nominal heating of the laser than may be desired, the laser may be turned on in the overscan regions  1806  and  1808  to allow it to heat somewhat. For applications where the appearance of light in the overscan regions is not objectionable, light emitted from the laser in the overscan regions may be allowed to pass through to a visible location. For applications where the appearance of light in the overscan regions may be objectionable, the overscan regions may be occluded such that light emitted therein is emitted toward a light block that does not allow the light to pass to a visible location.  
         [0105]     The preceding overview of the invention, brief description of the drawings, and detailed description describe exemplary embodiments according to the present invention in a manner intended to foster ease of understanding by the reader. Other structures, methods, and equivalents may be within the scope of the invention. For example, while the laser modulation pulses illustrated in the foregoing discussions use amplitude modulation to select a laser brightness, pulse width modulation may be similarly used. Moreover, the system may be used to compensate for the presence or absence of pixels in a substantially single-brightness (non-grayscale) system. One or more sensors may be combined to provide feedback to the system. For example, a temperature sensor may be used in combination with short term pixel-by-pixel compensation to provide noise reduction over extended periods of use, variable use environments, etc. Moreover, one or more optical detectors may be used to provide feedback to the system.  
         [0106]     As such, the scope of the invention described herein shall be limited only by the claims.