JP2017078727A - Light source drive device and image display device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a light source drive device with which it is possible to display a low gradation level with high accuracy without causing the laser output power of a laser beam to exceed rated laser output power at a high gradation level.SOLUTION: A light source drive device 250 comprises a signal generation unit 40 for generating a pixel signal in pixel units that corresponds to the gradation level of pixels constituting an image, a light source 51 for emitting a laser beam of intensity that corresponds to the pixel signal, and a signal superposition unit 60 for superposing a high-frequency signal on the pixel signal inputted to the light source 51. The frequency of the high-frequency signal is greater than or equal to a reciprocal of a generation cycle on which the signal generation unit 40 generates the pixel signal, and the current amplitude of the same equals the width of a pixel signal drive current value in a region where LED light emission and LD light emission coexist in the I-L characteristic of the light source 51. The signal superposition unit 60 superposes the high-frequency signal on the pixel signal when the pixel signal drive current value lies in at least the coexisting region.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a light source driving device for driving a semiconductor laser and an image display device using the light source driving device.

  A scanning-type image display device that emits laser light whose intensity is modulated according to an image signal from a semiconductor laser (LD) and scans the laser light in a horizontal direction and a vertical direction to display an image on a projection target. Are known. In a semiconductor laser, there is a relationship called IL characteristics between the drive current value of an input pixel signal and the laser output power (intensity) of emitted laser light. According to the IL characteristic, the laser output power increases linearly when the drive current value is increased. Some semiconductor lasers have a kink region where the IL characteristic abruptly changes and the linearity is lost in a region where the drive current value is small.

  A semiconductor laser having a kink region may not be able to accurately display a low gradation level. In Patent Document 1, in order to accurately display such a low gradation level, the frequency of the pixel signal is equal to or greater than the reciprocal of the generation period of the pixel signal, and the amplitude (current amplitude) is equal to or greater than the width of the kink region. A high frequency signal is superimposed.

JP 2011-079557 A

  In a semiconductor laser having a kink region, if a pixel signal to which a gradation level is assigned is input to the semiconductor laser on the assumption that the IL characteristic is linear, gradation collapse may occur at a low gradation level. When a high frequency signal having an amplitude equal to or larger than the width of the kink region is superimposed on the pixel signal as in Patent Document 1, the laser output power varies according to a change in the amplitude of the superimposed high frequency signal. The brightness of the pixel is the brightness corresponding to the laser output power obtained by averaging the fluctuating laser output power. The relationship between the drive current value of the pixel signal and the average laser output power is made close to linear, and the IL characteristic in the kink region is made to appear to be almost linear, thereby suppressing occurrence of gradation collapse at a low gradation level. be able to.

  However, in the technique described in Patent Document 1, the laser output power of the laser beam exceeds the rated laser output power when a high frequency signal having an amplitude equal to or larger than the width of the kink region is superimposed on the pixel signal at a high gradation level. May end up. When the laser output power of the laser light exceeds the rated laser output power, the life of the laser is remarkably shortened.

  The present invention has been made in view of the above background, and a light source capable of accurately displaying a low gradation level without causing the laser output power of the laser light to exceed the rated laser output power at a high gradation level. An object is to provide a drive device and an image display device.

  A light source driving device according to the present invention includes a signal generation unit that generates a pixel signal corresponding to a gradation level of a pixel constituting an image for each pixel, and a semiconductor laser that emits a laser beam having an intensity corresponding to the pixel signal. A signal superimposing unit that superimposes a high-frequency signal on the pixel signal input to the semiconductor laser, and the high-frequency signal has a frequency equal to or higher than a reciprocal of a generation cycle in which the signal generation unit generates the pixel signal. And the current amplitude is a width of the drive current value of the pixel signal in a region where LED light emission and LD light emission are mixed in the IL characteristic of the semiconductor laser, and the drive current value of the pixel signal is at least the mixed The signal superimposing unit superimposes the high-frequency signal on the pixel signal when it is in the region to be processed.

  Furthermore, an image display device according to the present invention includes the above-described light source driving device, and an optical deflector that displays an image by deflecting laser light emitted from the semiconductor laser in a horizontal direction and a vertical direction.

  According to the present invention, a low gradation level can be accurately displayed without the laser output power of the laser light exceeding the rated laser output power at a high gradation level.

It is a block diagram which shows schematic structure of the image display apparatus concerning this Embodiment. The IL characteristic which the semiconductor laser which is any one of R light source, G light source, and B light source used with the image display apparatus concerning this Embodiment has is shown. It is a figure which shows typically the state which superimposed the high frequency signal on the pixel signal, when the drive current value of a pixel signal exists in a threshold current. It is a figure which shows typically the state which superimposed the high frequency signal on the pixel signal when the target laser output power is near rated laser output power.

Embodiments of the present invention will be described below with reference to the drawings.
The image display apparatus according to the present embodiment is used for an in-vehicle head-up display (HUD) or the like. FIG. 1 is a block diagram showing a schematic configuration of an image display apparatus 500 according to the present embodiment. As shown in FIG. 1, the image display device 500 includes a light source driving device 250 and an optical deflector 100.

  The light source driving device 250 includes a signal generation unit 40, a laser light generation unit 50, and a control unit 200. The signal generation unit 40 includes an R light source signal generation unit 40r, a G light source signal generation unit 40g, and a B light source signal generation unit 40b that generate a pixel signal that is a pixel unit signal of each of the three primary colors. The R light source signal generation unit 40r sequentially generates a pixel signal having a drive current value corresponding to the gradation level of the red component of each pixel, and inputs the pixel signal to the R light source 51r. Similarly, the G light source signal generation unit 40g sequentially generates a pixel signal having a drive current value corresponding to the gradation level of the green component of each pixel for each pixel, and inputs the pixel signal to the G light source 51g. Further, the B light source signal generation unit 40b sequentially generates a pixel signal having a driving current value corresponding to the gradation level of the blue component of each pixel for each pixel, and inputs the pixel signal to the B light source 51b.

  The laser light generation unit 50 includes a light source 51 as a semiconductor laser, prisms 52 to 54, a mirror 55, and a lens 56. The light source 51 is an R light source 51r that emits red (R) laser light, a G light source 51g that emits green (G) laser light, and B that emits blue (B) laser light, which are semiconductor lasers. And a light source 51b. When a pixel signal is input from the R light source signal generation unit 40r, the R light source 51r sequentially emits red laser light whose intensity is modulated according to the gradation level of the red component of each pixel in units of pixels. Similarly, when a pixel signal is input from the G light source signal generation unit 40g, the G light source 51g sequentially emits green laser light whose intensity is modulated according to the gradation level of the green component of each pixel in units of pixels. . Further, when a pixel signal is input from the B light source signal generation unit 40b, the B light source 51b sequentially emits blue laser light whose intensity is modulated according to the gray level of the blue component of each pixel in units of pixels.

  The prism 52 bends the optical path of the R laser beam emitted from the R light source 51r by 90 degrees. The prism 53 bends the optical path of the G laser light emitted from the G light source 51g by 90 degrees and synthesizes the R laser light with the G laser light. The prism 54 bends the optical path of the B laser light emitted from the B light source 51b by 90 degrees, and combines the R laser light and the G laser light with the B laser light. The mirror 55 reflects the combined light obtained by combining the R, G, and B laser beams output from the prism 54. The lens 56 collects the combined light from the mirror 55 and makes it incident on the mirror 12.

  The signal superimposing unit 60 superimposes the high frequency signal on the pixel signal generated by the signal generating unit 40. Specifically, the R light source signal superimposing unit 60r, the G light source signal superimposing unit 60g, and the B light source signal superimposing unit 60b are generated by the R light source signal generating unit 40r, the G light source signal generating unit 40g, and the B light source signal generating unit 40b. A high frequency signal is superimposed on each pixel signal. Details of the high frequency signal will be described later.

  The optical deflector 100 includes a mirror 12, a horizontal drive unit 11H, and a vertical drive unit 11V. The mirror 12 is a MEMS (Micro Electro Mechanical System) mirror, for example, and the angle changes according to the drive signal. The mirror 12 is oscillated so that the laser beam is scanned in the horizontal direction of the screen 70 by the horizontal driving unit 11H, and is oscillated so that the laser beam is scanned in the vertical direction of the screen 70 by the vertical driving unit 11V. An image based on the image signal is displayed on the screen 70 by scanning the laser beam in the horizontal direction and the vertical direction by the optical deflector 100.

  The control unit 200 has an APC (Auto Power Control) circuit, and controls the drive current value of the pixel signal input to the semiconductor laser so that laser light having a desired laser output power (intensity) is emitted (APC). control). Specifically, the control unit 200 drives the R light source 51r, the G light source 51g, and the B light source 51b according to the respective pixel values in the input image signal, and the drive current value of the pixel signal corresponding to the desired laser output power. R light source signal generation unit 40r, G light source signal generation unit 40g, and B light source signal generation unit 40b are controlled. In the APC control, a pixel signal corresponding to a desired laser output power is not input based on the IL characteristic of the semiconductor laser to be used, instead of inputting a pixel signal assigned a gradation level on the premise of linearity to the semiconductor laser. Since the signal is generated and input to the semiconductor laser, unlike the technique described in Patent Document 1, the IL characteristic need not be linear.

  The control unit 200 generates a horizontal drive signal based on the input horizontal synchronization signal and supplies it to the horizontal drive unit 11H. The horizontal drive unit 11H swings the optical deflector 100 in the horizontal direction based on the horizontal drive signal. Further, the control unit 200 generates a vertical drive signal based on the input vertical synchronization signal and supplies the vertical drive signal to the vertical drive unit 11V. The vertical drive unit 11V swings the optical deflector 100 in the vertical direction based on the vertical drive signal.

  FIG. 2 shows an example of IL characteristics of a semiconductor laser that is one of the R light source 51r, the G light source 51g, and the B light source 51b. As shown in FIG. 2, in the region where the drive current value of the pixel signal is small, the IL characteristic changes sharply and the linearity is lost (the laser output power increases linearly with respect to the drive current value of the pixel signal). Not) is a kink region. The kink region further includes a transition region in which LD light emission (laser light emission) and LED light emission (natural light emission) coexist (laser output power is around 1 mW). The drive current value of the pixel signal in this transition region is the threshold current Ith. The threshold current Ith has a certain width (threshold current width ΔI). In the semiconductor laser shown as an example in FIG. 2, the minimum value of the threshold current Ith is 84.4 mA, and the maximum value is 84.84 mA. Therefore, the threshold current width ΔI is 0.44 mA. The rated laser output power is around 5 mW.

  When the drive current value of the pixel signal is within the threshold current Ith, the laser output power becomes unstable because the semiconductor laser goes back and forth between the LED emission region and the LD emission region. The threshold current width ΔI is very small compared to the drive current width Ic in a region (kink region) where the IL characteristic changes sharply and becomes nonlinear. This threshold current width ΔI is substantially constant even when various conditions such as temperature change, and is uniquely determined by the type of laser.

Here, the high frequency signal superimposed on the pixel signal will be described below.
As described above, when the drive current value of the pixel signal is within the threshold current Ith, the semiconductor laser goes back and forth between the LED emission region and the LD emission region, so that the laser output power becomes unstable and the target laser output I can't get power. When the drive current is within the threshold current Ith in at least one of the R light source 51r, the G light source 51g, and the B light source 51b, the target laser output power cannot be obtained with the light source whose drive current is within the threshold current Ith. Therefore, a color shift different from the color originally intended to occur occurs.

  FIG. 3 is a diagram schematically illustrating a state in which a high-frequency signal is superimposed on the pixel signal when the drive current value of the pixel signal is within the threshold current Ith. In the figure, the pixel signal is indicated by a one-dot chain line, and the high-frequency signal is indicated by a solid line. As shown in FIG. 3, the pixel signal is a pulse wave, and the generation cycle is T. Further, it is assumed that the drive current value of the pixel signal corresponding to the target laser output power is the drive current value Ip within the threshold current Ith.

  The frequency of the high-frequency signal superimposed on the pixel signal is equal to or greater than the reciprocal (1 / T) of the generation period T of the pixel signal. The current amplitude of the high frequency signal is a threshold current width ΔI. In addition, although the high frequency signal in a figure is a sine wave, it is not restricted to this, For example, a pulse wave and a triangular wave may be sufficient. When a signal in which a high-frequency signal having the above-described frequency and current amplitude is superimposed on a pixel signal is input to the semiconductor laser, a laser output power obtained by averaging the laser output power that varies periodically can be obtained. Thereby, even when the drive current value of the pixel signal is within the threshold current Ith, the output power of the laser beam can be stabilized, so that a low gradation level can be accurately displayed.

  The life of the semiconductor laser is remarkably shortened when it is used beyond the rated laser output power. For this reason, it is necessary to prevent the peak of the laser output power when the high frequency signal is superimposed on the pixel signal from exceeding the rated laser output power. In the image display device 500, the laser output power of the highest gradation is set slightly lower than the rated laser output power in order to achieve both high brightness of the highest gradation and long life of the semiconductor laser.

  FIG. 4 is a diagram schematically showing a state in which a high-frequency signal is superimposed on a pixel signal when the target laser output power is near the rated laser output power. In the figure, the pixel signal is indicated by a one-dot chain line, and the high-frequency signal is indicated by a solid line. As shown in FIG. 4, the current amplitude of the high frequency signal superimposed on the pixel signal is very small with respect to the drive current value of the pixel signal, and therefore corresponds to the peak of the drive current value in the signal after the high frequency signal is superimposed. The laser output power will not exceed the rated laser output power. For this reason, the lifetime of the semiconductor laser is not significantly shortened compared with the case where no high-frequency signal is superimposed.

  As described above, even when the target laser output power is near the rated laser output power, the high-frequency signal can be superimposed on the pixel signal without significantly shortening the life of the semiconductor laser. That is, a high-frequency signal having a constant current amplitude can be superimposed on pixel signals corresponding to laser output power at all gradation levels. As described above, when superimposing a high-frequency signal having a constant current amplitude in pixel signals corresponding to laser output powers of all gradation levels, the high-frequency signal is changed depending on the magnitude of the gradation level when changing the gradation level. There is no need to perform complicated processing such as determining whether to superimpose or changing the current amplitude of the superimposed high-frequency signal.

  As described above, in the present invention, at least when the driving current value of the pixel signal is in a region where LED light emission and LD light emission are mixed in the IL characteristic of the semiconductor laser, the above-described high frequency signal is superimposed on the pixel signal. The laser output power can be prevented from becoming unstable at a low gradation level, and the low gradation level can be accurately displayed. Further, even if the above-described high-frequency signal is superimposed on the pixel signal at a high gradation level, the laser output power of the laser light does not exceed the rated laser output power.

  Note that the present invention is not limited to the above-described embodiment, and can be changed as appropriate without departing from the spirit of the present invention.

40 signal generator 40r R light source signal generator 40g G light source signal generator 40b B light source signal generator 50 laser light generator 51 light source 51r R light source (semiconductor laser)
51g G light source (semiconductor laser)
51b B light source (semiconductor laser)
60 signal superimposing unit 60r R light source signal superimposing unit 60g G light source signal superimposing unit 60b B light source signal superimposing unit 70 screen 100 optical deflector 200 control unit 250 light source driving device

Claims (3)

A signal generation unit that generates pixel signals corresponding to the gradation levels of the pixels constituting the image in units of pixels;
A semiconductor laser that emits laser light having an intensity corresponding to the pixel signal;
A signal superimposing unit that superimposes a high-frequency signal on the pixel signal input to the semiconductor laser,
The high-frequency signal has a frequency that is equal to or greater than the reciprocal of the generation cycle in which the signal generation unit generates the pixel signal, and the current amplitude is a mixture of LED light emission and LD light emission in the IL characteristics of the semiconductor laser. The width of the drive current value of the pixel signal in the region,
The light source driving device in which the signal superimposing unit superimposes the high-frequency signal on the pixel signal when the driving current value of the pixel signal is at least in the mixed region.   The light source driving device according to claim 1, wherein the signal superimposing unit superimposes the high-frequency signal on the pixel signal at all gradation levels. The light source driving device according to claim 1 or 2,
An image display device comprising: an optical deflector that displays an image by deflecting laser light emitted from the semiconductor laser in a horizontal direction and a vertical direction.

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