JP2016100536A - Drive method of semiconductor laser and display system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a drive method of semiconductor laser and a display system capable of reducing speckle noises without increasing the cost.SOLUTION: A semiconductor laser is driven a drive current of a rectangular wave with a pulse width of 0.4-2 ms. Since the pulse width is narrow, the temperature of an active layer in a semiconductor laser does not reach steady temperature during supplying the pulse of drive current. Therefore, the temperature of the active layer and the oscillation wavelength continuously increase during driving the semiconductor laser. As a result, since the half width of the spectrum gets wider, speckle noises can be reduced without increasing the cost.SELECTED DRAWING: Figure 3

Description

  The present invention relates to a driving method and a display system for a semiconductor laser used as a light source for a cinema or a projector.

  A semiconductor laser has advantages such as small size, good color reproducibility, low power consumption, and high brightness as compared with other light sources, and is expected as a light source for projection displays such as projectors and cinemas. However, when the surface to be irradiated is irradiated with laser light, a speckle pattern called speckle noise appears and the image appears to flicker (for example, see Patent Documents 1 and 2). This is caused by interference caused by a single laser beam having a high coherence. Speckle noise causes discomfort and eye fatigue for video viewers, and reduction is desired.

International Publication No. 2009/069282 International Publication No. 2011/016170

  In order to reduce speckle noise, there are a method of vibrating the irradiated surface and a method of providing a diffusion plate on the optical path between the semiconductor laser and the irradiated surface. However, there is a problem that the configuration of the display system becomes complicated and the cost increases.

  The present invention has been made to solve the above-described problems, and an object thereof is to obtain a semiconductor laser driving method and a display system that can reduce speckle noise without increasing cost. .

  In the semiconductor laser driving method according to the present invention, the semiconductor laser is driven by a rectangular-wave drive current having a pulse width of 0.4 ms to 2 ms, and the active layer of the semiconductor laser is supplied while the drive current pulse is supplied. The temperature is not a steady temperature.

  In the present invention, since the pulse width of the drive current is narrow, the temperature of the active layer of the semiconductor laser does not reach a steady temperature while supplying the drive current pulse. Therefore, the temperature of the active layer continues to increase during driving, and the oscillation wavelength continues to increase. As a result, the full width at half maximum of the spectrum is widened, so that speckle noise can be reduced without increasing the cost.

It is a figure which shows the display system which concerns on embodiment of this invention. It is a figure which shows the relationship between the pulse width of the drive current in the comparative example, and the temperature of an active layer. It is a figure which shows the relationship between the pulse width of the drive current in embodiment of this invention, and the temperature of an active layer. It is a figure which shows the relationship between the half value width of an oscillation spectrum at the time of driving a red semiconductor laser oscillating at 642 nm by a rectangular wave pulse so that it may become 35 degreeC and optical output 1.5W, and the pulse width of a drive current. It is a figure which shows an oscillation spectrum in case the pulse width of a drive current is 1.0 ms. It is a figure which shows an oscillation spectrum in case the pulse width of a drive current is 5.0 ms.

  FIG. 1 is a diagram showing a display system according to an embodiment of the present invention. In this system, red, green, and blue semiconductor lasers 1a, 1b, and 1c are driven by driving circuits 2a, 2b, and 2c, respectively, and these laser beams are condensed by a lens 3 to be dichroic mirror 4 and a spatial light modulator. 5 is a laser projection system that reflects the light at 5. The drive circuits 2a, 2b, and 2c drive the semiconductor lasers 1a, 1b, and 1c with a rectangular-wave drive current having a pulse width of 0.4 ms to 2 ms.

  Each semiconductor laser 1a, 1b, 1c has a semiconductor laser chip, a submount, and a stem of φ5.6 mm. For example, the red semiconductor laser 1a is obtained by die-bonding a semiconductor laser chip having AlGaInP as an active layer to a sintered aluminum nitride (AlN) submount and mounting it on a copper block in the stem. The spatial light modulator 5 is a DMD (Digital mirror device) or the like.

  Subsequently, the effect of the present embodiment will be described in comparison with a comparative example. Here, there is a relationship of Cs∝1 / √Δλ between the magnitude Cs of speckle noise and the half-value width Δλ of the oscillation spectrum of the laser beam. Therefore, in order to reduce speckle noise, it is effective to widen the half width of the oscillation spectrum.

  When a pulse-driven semiconductor laser is used as a projector light source, one frame is formed by sequentially lighting red, green, and blue semiconductor lasers. When the frame rate is 120 Hz, the oscillation time of each color semiconductor laser is about 2.7 ms, which is 1/3 of 1/120 Hz. In the comparative example, the pulse width w of the drive current is 2.7 ms.

  FIG. 2 is a diagram showing the relationship between the pulse width of the drive current and the temperature of the active layer in the comparative example. Since the pulse width of the comparative example is longer than the thermal time constant in the vicinity of the active layer of the semiconductor laser, the temperature of the active layer increases with the input of the pulse, leading to saturation. This can be interpreted by dividing into heat generation of the active layer of the semiconductor laser and heat conduction to the entire device. At the beginning of the rise, the temperature increases with a thermal time constant determined by the thermal resistance and thermal capacity near the active layer of the semiconductor laser chip. The temperature in the steady state depends on the thermal time constant determined by the thermal resistance of the entire device. Here, considering that the oscillation wavelength of the semiconductor laser becomes longer as the temperature increases, the oscillation wavelength of the pulse-driven semiconductor laser becomes longer with time. Therefore, the oscillation spectrum obtained from the pulse-driven semiconductor laser is the time average of the spectrum that changes with time. Accordingly, when driving with a sufficiently long driving current pulse width, the time during which the temperature of the active layer is constant occupies most of the driving current pulse width. As a result, the half width of the oscillation spectrum is narrowed, and speckle noise is likely to occur.

  FIG. 3 is a diagram showing the relationship between the pulse width of the drive current and the temperature of the active layer in the embodiment of the present invention. In this embodiment, the pulse width w of the drive current is as narrow as 2.0 ms. Therefore, the temperature of the active layer of the semiconductor laser does not reach a steady temperature while the drive current pulse is supplied. Therefore, the temperature of the active layer continues to increase during driving, and the oscillation wavelength continues to increase. As a result, the full width at half maximum of the spectrum is widened, so that speckle noise can be reduced without increasing the cost.

  FIG. 4 is a diagram showing the relationship between the half-value width of the oscillation spectrum and the pulse width of the drive current when a red semiconductor laser oscillating at 642 nm is driven with a rectangular wave pulse so as to have an optical output of 1.5 W at 35 ° C. is there. When the pulse width of the drive current is decreased from 5 ms, the half-value width starts increasing from 2.0 ms or less, and has a maximum value at the drive current pulse width of 1 ms. When the pulse width of the drive current is further decreased, the half width starts to decrease.

  FIG. 5 is a diagram showing an oscillation spectrum when the pulse width of the drive current is 1.0 ms. The half width is 3.9 nm. FIG. 6 is a diagram showing an oscillation spectrum when the pulse width of the drive current is 5.0 ms. The half width is 1.7 nm. When the pulse width of the drive current is 5.0 ms compared to 1.0 ms, the half width of the oscillation spectrum decreases.

  When the pulse width of the drive current is long, the time during which the temperature becomes constant in the pulse becomes long, so that the wavelength component at this temperature increases and the wavelength width becomes small. On the other hand, when the pulse width of the driving current is short, the amount of change in the temperature of the active layer during driving is small. For these reasons, the pulse width of the rectangular wave drive current is preferably 0.4 ms to 2 ms. In addition, since the time constant of the temperature rise of the semiconductor laser active layer does not greatly depend on the material and shape of the semiconductor laser chip, submount, and stem, the pulse width of the drive current is set to 0.4 ms to 2 ms in any form. Thus, the effects of the present invention can be obtained.

1a, 1b, 1c Semiconductor laser, 2a, 2b, 2c drive circuit, 3 lens, 4 dichroic mirror, 5 spatial light modulator

Claims (3)

  The semiconductor laser is driven by a rectangular wave drive current having a pulse width of 0.4 ms to 2 ms, and the temperature of the active layer of the semiconductor laser does not become a steady temperature while the pulse of the drive current is supplied. Semiconductor laser driving method.   2. The semiconductor laser driving method according to claim 1, wherein the semiconductor laser is a red semiconductor laser.   A display system using the driving method according to claim 1.

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