Abstract:
A projection unit comprising a coherent light source such as a laser diode and a beam shaping element forming a light beam which is directed towards a display, characterized in that it comprises a laser speckle suppression device which is positioned between the laser source and the display and wherein the laser speckle suppression device is a diffusing element actuated randomly by a piezoelectric vibrating structure.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a laser based projection unit for diffractive head up display having a laser speckle suppression device based on piezoelectric actuating. 
       BACKGROUND OF THE INVENTION 
       [0002]    Diffractive head up displays use coherent type light source in order to illuminate a display, and then to project a virtual image through a diffractive combiner. The usage of diffractive technology in this kind of application implies a necessity to have an adapted projection unit able to interact correctly with the diffractive combiner especially concerning the image quality. 
         [0003]    The phenomenon of speckle appears when we illuminate a display or a naturally rough surface with a light spatially and temporally coherent as the light generated by a laser source. This phenomenon can be useful for certain applications such as measuring deformation of objects. But in other applications such as projection systems, including those associated with head-up display application using diffractive components, the suppression of this phenomenon is very important to improve the quality of the virtual image perceived by user. 
         [0004]    As mentioned above, the statistical properties of the speckle generally depend from one hand on the coherence of the incident beam and on the other hand on the diffused mediums crossed by the light beam. In a projection system, including systems used in head-up display based on diffractive elements, the average size of speckle depends also on the optical system used after the display (lens, diffractive combiner and in some cases human eye). 
         [0005]    Up to today, almost all the currently patented speckle reduction devices uses complex optical systems and requires precise adjustments that can not be easily implemented for industrial application. We cite for example patents U.S. Pat. No. 4,155,630; US2008/0055698A1; WO 00/65401 and EP1257869 B1, U.S. 2009/0231720A1, all of which are incorporated herein by reference. 
         [0006]    All these are based on averaging of phase (case of patents U.S. Pat. No. 4,155,630; US2008/0055698A1 and) using rocking motion of a mirror; or on averaging the polarization of the light beam (case of EP1257869 B1). In all cases the usage of minors presents many limitations:
       The application of a movement of translation or tilting on the minor involves only a translation of wave front. In other words the averaging is done on several identical shifted wave fronts, this averaging reduces slightly the speckle but has no effect on the uniformization of the beam.   The usage of minors requires a very precise control to create a movement profile of movement adapted to averaging;   The most important weakness of mirror based solution is that this solution is available only for a speckle characterised by small amplitude.       
 
         [0010]    The purpose of the present invention is to give a solution to all the above described limitations of speckle reduction devices, by proposing a simple and small instrument for speckle reduction that can be integrated in any projection system without any need to modify the optical design of the projection system. Also the purpose of the present invention is a laser based projection system having the proposed speckle suppression device. 
       SUMMARY OF THE INVENTION 
       [0011]    The present invention proposes a projection unit for diffractive head up display comprising a coherent light source such as a laser diode and a beam shaping element forming a light beam which is directed towards a display, characterized in that it comprises a laser speckle suppression device which is positioned between the laser source and the display and wherein the laser speckle suppression device is a diffusing element actuated randomly by a piezoelectric vibrating structure. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The present invention is now described by way of example with reference to the accompanying drawings in which: 
           [0013]      FIG. 1  is a schematic view showing a projection unit comprising a laser speckle suppression device according to the invention. 
           [0014]      FIG. 2  is a schematic view showing a first embodiment of the laser speckle suppression device of  FIG. 1 . 
           [0015]      FIG. 3  is a schematic view showing how the laser speckle suppression device of  FIG. 2  is working. 
           [0016]      FIG. 4  is a schematic view of a second embodiment of the laser speckle suppression device of  FIG. 1 . 
           [0017]      FIG. 5  is a cross section view showing the second embodiment of the laser speckle suppression device. 
           [0018]      FIG. 6  shows the electrical configuration of the second embodiment of the laser speckle suppression device. 
           [0019]      FIG. 7  shows a projection unit incorporated in a diffractive head up display and using the speckle suppression device according to  FIG. 2 . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0020]    The speckle suppression device presented in the invention is based on moving directly a diffusing medium according to a composite vibration mode. This composite vibration mode (for example bending+rotation or +shearing) associated with the structural features of the diffuser will enable to generate for each position of the diffuser a corresponding wavefront and thus a special speckle noise. The averaging of these wavefronts will enable a real random spatial phase averaging, which can reduce the speckle, whatever its magnitude, and at the same time to correct the uniformity of the beam. 
         [0021]    As can be seen on  FIG. 1 , the projection unit  10  is constituted of a laser type source  12  generating a coherent light beam, a beam shaping element  14  (diffractive or not) and a display  16 . A laser speckle suppression device  18  is positioned between the laser source  12  and the display  16 . 
         [0022]    In a first embodiment of the laser speckle suppression device  18 , as shown on  FIG. 2 , the laser speckle suppression device  18  is constituted of a vibrating metallic structure  20  on which an optical diffusing element  22  is fixed through which the laser beam  24  passes. Piezoelectric plates  26  are fixed on the vibrating structure  20 . An electronic oscillating circuit  28  is designed to drive the vibrations in the piezoelectric elements at the desired frequency and at the desired vibration mode. This method enables to apply directly a vibrating movement to the diffusing element  22 . 
         [0023]    In a second embodiment of the laser speckle suppression device  18 , as shown on  FIGS. 4 to 6 , the laser speckle suppression device  18  is constituted of a quartz plate  30  on which a diffusion layer  32  is deposited. Electrodes  34  are directly deposited on the quartz plate surfaces. These electrodes  34  are used in order to generate the required vibrating modes at the required frequency. The quartz plate is fixed for example on a support surface  36  on the electronic PCB including the driving electronics  28 . This embodiment uses directly the piezoelectric properties of the quartz in order to generate the required vibrating modes at the desired frequency, and enables a simple integration with the associated driving electronics  28 . Also the optical characteristics of the quartz plate  30  (transparent material with a refraction index about 1.458) enable to use it directly for both functions: optical function and piezoelectric function. Also in this version the speckle is reduced due to the random averaging of the phase, and also the uniformization of the beam is obtained. 
         [0024]    For both embodiments of the laser speckle suppression device  18 , the electronic circuit  28  is designed in order to assure the vibration of the structure at the required vibration frequency. In fact, this frequency is related to the chosen vibrating mode. 
         [0025]    As explained above the vibration of the diffusing element  22  randomly averages the image and thus reduces the speckle by virtually creating a mean value of the laser wavelength. Generally it is enough to drive the vibration at a frequency higher than that of the human eye sensitivity with sufficient amplitude. The random vibrations at that frequency will enable the eye to see a mean value of the images superposed on the diffusing element  22 . As the piezoelectric actuating according to the present invention is specifically designed to generate the composite vibration mode at the desired frequency and amplitude, the frequency is chosen with respect to the eye sensitivity criterion and to comfort requirements (no audible noise is generated by the device). 
         [0026]    This kind of projection systems  10  having such speckle suppression device  18  can be suitable for application such as head up displays based on diffractive (or holographic) combiners. In such kind of head up displays, it is necessary to reduce the speckle effect in order to improve the quality of the virtual image. In fact the speckle effect is generally magnified by the diffractive combiner. 
         [0027]    According to the present invention, the projection unit  10  for the diffractive head up display  42  presented on  FIG. 7 , comprises a laser source  12 , a diffractive beam shaping element  14 , two mirrors  38 ,  40 , and a display  16 . The diffractive beam shaping element  14  is designed to obtain a light spot having the same shape and dimensions as the display  16  with a high uniformity and a high energetic efficiency. The laser speckle suppression device  18  shown corresponds to the first embodiment of the invention. The laser speckle suppression device  18  can be easily integrated within the projection unit  10  and within the head up display  42  without any need to modify the optical design initially defined. 
         [0028]    Patents U.S. Pat. No. 4,155,630; US2008/0055698A1; WO 00/65401; EP1257869 B1; US2009/023172; and U.S. Pat. No. 6,317,169 B1 are hereby incorporated herein by reference.