Abstract:
The presently claimed invention provides a phase modulator for a see-through display, and the corresponding fabrication methods. The phase modulator comprises a liquid crystal layer having at least two types of domains including a first domain having a first refractive index and a second domain having a second refractive index. The phase modulator is able to increase field of view without inducing the problem of the fringe field effect between two adjacent pixels.

Description:
COPYRIGHT NOTICE 
       [0001]    A portion of the disclosure of this patent document contains material, which is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates to a see through display, more particularly, the present invention relates to a new phase modulator used for holographic see through display. 
       BACKGROUND 
       [0003]    Nowadays, head mount display (HMD) and head up display (HUD) being essentially wearable intelligent devices, or other kind of displays are capable of displaying images, inter alia, on glasses lenses or screens oriented in front of a user&#39;s eyes, among other things. More and more HMDs adopt see-through display to allow full or partial views of the user&#39;s surroundings. For instance, GOOGLE GLASS® is one HMD device that resembles a pair of glasses with a computing device built directly into the frame, and includes an optical structure to direct visible light into the eye of a user to display a variety of information. HMD devices, such as GOOGLE GLASS®, may provide users with a wearable computing device capable of providing visible overlays while still allowing the user to view his or her surroundings. HUD are systems which also adopts see through display onto which images could be projected such that it allows the viewer to maintain a posture in which the gaze is directed forward rather than downward to a display or instrument panel. Head-up displays are used in various environments such as motor vehicles, aircraft, helmets and other situations in which it is important that the viewer not divert his gaze. Therefore, the use of HUD could prevent a driver from taking his eyes off the road, i.e., reducing distraction for safe driving, and could reduce eye strain for comfortable driving. 
         [0004]    Currently, amplitude-modulated display technologies are commonly used for the see-through display, e.g., Thin Film Transistor (TFT) Liquid Crystal Display (LCD)+ Light Emitting Diode (LED) backlight (Dominant technology), Digital Light Processing (DLP) projection or Liquid Crystal on Silicon (LCoS) projection (Emerging technologies). However, for Amplitude-modulated display, since there is always very a small image area (always &lt;10%) to be used for display, most light is absorbed and creates heat for application in large Augmented Reality Head-up Display (AR-HUD) and large space is required for heat dissipation. Therefore, the light efficiency is very low, i.e., less than 10%. To solve such a problem, Phase only holographic projection display is an alternative solution for the see-through display. Holographic projection steers the coherent light to where an image needs to be displayed and in principle, no much light lost, just energy redirection. Therefore, the light efficiency could be increased to more than 90%. 
         [0005]    However, challenges exist for LCoS phase modulator for holographic projection display. For example, the small diffractive field-of-view (FOV) is limited by the phase modulator&#39;s pixel size.  FIG. 1A  shows the structure of the LCoS phase modulator comprises glass substrate, transparent electrode, liquid crystal layer, pixel reflective electrode, and silicon substrate from top to bottom, wherein pixel reflective electrodes represent for multiple pixels for the display. According to  FIG. 1B , diffractive angle θ=sin−1[λ/(2*Pitch)]. Normally, the pixel size of current LCoS phase modulator is between 6.4-32 m, and the diffractive FOV is less than 6 degree. In order to increase the diffractive FOV, the conventional solution is to further reduce the pixel size. However, due to the fringe field effect between two small adjacent pixels, if the pixel size is further decreased, the diffraction contrast and efficiency will be also decreased. 
         [0006]    There is a need in the art to have a phase modulator for see-through display providing a large field of view without inducing the problem of the fringe field effect between two adjacent pixels. 
       SUMMARY OF THE INVENTION 
       [0007]    Accordingly, the presently claimed invention provides a phase modulator for see-through display providing a large field of view without inducing the problem of the fringe field effect between two adjacent pixels. 
         [0008]    In accordance to an embodiment of the presently claimed invention, a phase modulator for a display, comprises: a liquid crystal layer; an electrode layer disposed on a first side of the liquid crystal layer for allowing light to pass through; and a plurality of pixel electrodes disposed on a second side of the liquid crystal layer and being operable with the electrode layer for supplying electric potential across the liquid crystal layer; wherein on each of the pixel electrodes, the liquid crystal layer comprises at least two types of domains including a first domain having a first refractive index and a second domain having a second refractive index; and wherein the first reflective index is different from the second reflective index. 
         [0009]    Preferably, the first domain of the liquid crystal layer comprises aligned liquid crystal molecules, and the second domain of the liquid crystal layer comprises non-aligned liquid crystal molecules. 
         [0010]    Preferably, the phase modulator further comprises an alignment layer located on the pixel electrodes and/or the electrode layer for forming the aligned liquid crystal molecules. 
         [0011]    Preferably, the first domain of the liquid crystal layer comprises aligned liquid crystal molecules having a first orientation, and the second domain of the liquid crystal layer comprises aligned liquid crystal molecules having a second orientation, wherein the first orientation is different from the second orientation. 
         [0012]    Preferably, the phase modulator further comprises an alignment layer located between the pixel electrodes and the liquid crystal layer, wherein the alignment layer comprises two different alignment directions on each of the pixel electrodes for forming the first domain of the liquid crystal layer and the second domain of the liquid crystal layer. 
         [0013]    Preferably, the phase modulator further comprises an alignment layer located between the electrode layer and the liquid crystal layer, wherein the alignment layer comprises two different alignment directions for forming the first domain of the liquid crystal layer and the second domain of the liquid crystal layer. 
         [0014]    Preferably, the phase modulator further comprises a polymer material penetrated into the liquid crystal layer to improve thermal stability of the liquid crystal layer. 
         [0015]    Preferably, the phase modulator further comprises a polymer material enclosing the alignment layer to improve thermal stability of the alignment layer. 
         [0016]    Preferably, the pixel electrodes are addressable. 
         [0017]    A further aspect of the present invention is to provide a method for fabricating the phase modulator. 
         [0018]    In accordance to an embodiment of the presently claimed invention, the alignment layer is formed by steps of: coating photo-sensitive alignment material on each of the pixel electrodes; placing a photo mask on the alignment material; and illuminating the alignment material with UV light without shielding by the photo mask to form the alignment layer. 
         [0019]    In accordance to an embodiment of the presently claimed invention, the alignment layer is formed by steps of: coating photo-sensitive alignment material on each of the pixel electrodes; placing a first photo mask on the alignment material; illuminating a first part of the alignment material with light having a first polarized direction, wherein the first part of the alignment material is not shielded by the first photo mask; placing a second photo mask on the alignment material; and illuminating a second part of the alignment material with light having a second polarized direction to form the alignment layer comprising two different alignment directions, wherein the second part of the alignment material is not shielded by the second photo mask. 
         [0020]    In accordance to an embodiment of the presently claimed invention, the alignment layer is formed by steps of: coating photo-sensitive alignment material on each pixel electrode; placing a photo mask on the alignment material; illuminating a part of the alignment material with light, wherein the part of the alignment material is not shielded by the photo mask; forming the alignment layer from the alignment material after light illumination; illuminating the second part of the pixel electrode with a first wavelength UV light; filling in the liquid crystal layer between the opposing electrodes, the liquid crystal layer including liquid molecules, and monomers; and polymerizing the monomer with a second wavelength UV light. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    Embodiments of the present invention are described in more detail hereinafter with reference to the drawings, in which: 
           [0022]      FIG. 1A  shows a structure of a LCoS phase modulator in the prior art; 
           [0023]      FIG. 1B  shows pixel electrodes for diffracting incident beam in the prior art; 
           [0024]      FIG. 2A  shows a pixel pattern of a LCoS Phase modulator in the prior art; 
           [0025]      FIG. 2B  shows same alignment direction of the liquid crystal molecules in the prior art; 
           [0026]      FIG. 3  shows one pixel optically separated into several sub-pixels by non-aligned liquid crystal molecules according to an embodiment of the presently claimed invention; 
           [0027]      FIGS. 4A-C  illustrate a photo alignment process for optically separating one pixel into several sub-pixels according to an embodiment of the presently claimed invention; 
           [0028]      FIG. 5  shows alignment domain configured to be different between two adjacent sub-pixels according to an embodiment of the presently claimed invention; 
           [0029]      FIGS. 6A-C  illustrate a photo alignment process for optically separating one pixel into several sub-pixels according to an embodiment of the presently claimed invention; 
           [0030]      FIG. 7A  shows a phase modulator having a liquid crystal layer incorporated with polymer networks according to an embodiment of the presently claimed invention; and 
           [0031]      FIG. 7B  shows a phase modulator having a polymer network formed on the alignment surface according to an embodiment of the presently claimed invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0032]    In the following description, a LCoS phase modulator and the corresponding fabrication methods are set forth as preferred examples. It will be apparent to those skilled in the art that modifications, including additions and/or substitutions may be made without departing from the scope and spirit of the invention. Specific details may be omitted so as not to obscure the invention; however, the disclosure is written to enable one skilled in the art to practice the teachings herein without undue experimentation. 
         [0033]    In the light of the foregoing background, it is an object of the present invention to provide a new LCoS phase modulator with particular structure to efficiently increase the diffraction of FOV so as to increase the FOV for information displayed. 
         [0034]      FIG. 2A  shows a pixel pattern of the LCoS Phase modulator. There are Y rows and X columns pixel electrodes  21  arranged above the Silicon substrate of the modulator. The pixel electrodes are reflective and electrically isolated from each other. Diffraction spatial pitch P  22  is the distance between the centers of the two pixels. Inter pixel gap  23  exists between every two pixel electrodes  21 . Normally in one pixel, the refractive index is the same with the same alignment direction as shown in  FIG. 2B . There is a plurality of liquid crystal molecules  24  formed on the pixel electrode  21 . In the pixel, there are a transparent electrode  25  and a reflective electrode  26 . An alignment layer  27  is formed on the transparent electrode  25  and the reflective electrode  26 . The liquid crystal molecules  24  are located between the transparent electrode  25  and the reflective electrode  26  to form a liquid crystal layer  28 . As the liquid crystal molecules  24  are aligned in the same direction due to the alignment layers  27 , the refractive index within the liquid crystal layer  28  is the same. 
         [0035]    According to the present invention, in order to decrease the diffraction spatial pitch without affecting the efficiency, each pixel is divided into two or more sub-pixel areas that are optically isolated from each other. In one embodiment of the present invention, as shown in  FIG. 3 , one pixel  31  is optically separated into several sub-pixels  32 , e.g., four sub-pixels, by non-aligned liquid crystal molecules  33 . The sub-pixels  32  comprise the aligned liquid crystal molecules which can be horizontal aligned or vertical aligned. A gap  34  between two sub-pixels could be the same as the inter pixel gap. The non-aligned liquid crystal molecules  33  are formed on the transparent electrode  35  and the reflective electrode  36  without the presence of alignment layers  37 . As such, new diffraction spatial pitch is reduced to p/2 and the diffraction of FOV can be increased about two times. 
         [0036]      FIGS. 4A-C  illustrate a photo alignment process for optically separating one pixel into several sub-pixels for the embodiment of  FIG. 3 . In  FIG. 4A , an alignment layer  401  is arranged on multiple pixel electrodes  402  that are configured above a silicon substrate  403 . Then, a photo mask  404  is configured on the alignment layer  401  at the silicon substrate side before 1st UV light  405  exposure along a specified direction. After the 1st UV light  405  exposure, the alignment layer  401  with liquid crystal molecules will be well aligned except the area under the mask. In  FIG. 4B , an alignment layer  406  is arranged on a transparent ITO electrode  407  that is configured above a glass substrate  408 . Then, a photo mask  409  is configured on the alignment layer  406  at the glass substrate side before the UV light  410  exposure in which the UV light  410  is same as the 1st UV light  405  in terms of wavelength and direction. After the 2nd UV light  410  exposure, the alignment layer  406  with liquid crystal molecules will be well aligned except the area under the mask. After the photo masks  404  and  409  are removed, in  FIG. 4C , a silicon substrate portion  411  and a glass substrate portion  412 , formed from the above steps, are assembled to form a phase modulator  413  wherein each pixel is separated into several sub-pixels  414  by non-aligned liquid crystal molecules  415  formed on the un-aligned areas  416  of the two alignment layers  401 . 
         [0037]    In an alternative embodiment of the present invention, as shown in  FIG. 5 , a pixel  51  is equally divided into four sub-pixels  52   a ,  52   b ,  52   c , and  52   d . The alignment domain of the liquid crystal molecules is configured to be different between two adjacent sub-pixels, such that the two adjacent sub-pixels are optically isolated to each other. For example, the sub-pixel  52   a  is optically different from sub-pixels  52   b  and  52   c . Such configuration is achieved by forming two types of alignment layers  55   a  and  55   b , having different orientations, on a transparent electrode  53  and a reflective electrode  54  of the pixel  51 . The alignment layers  55   a  and  55   b  can be formed from AZO dye and their thickness can be in a range of several nanometers to hundreds of nanometers. The alignment layer  55   a  is assisted to form the sub-pixels  52   a  and  52   d  having liquid crystal molecules  57  aligned with a first orientation while the alignment layer  55   b  is assisted to form the sub-pixel  52   b  and  52   c  having liquid crystal molecules aligned with a second orientation. As the first orientation of the liquid crystal molecules  57  is different from the second orientation of the liquid crystal molecules  57 , the refractive index of the sub-pixel  52   a  is different from that of the sub-pixels  52   b  and  52   c . Under such arrangement, new diffraction spatial pitch is reduced to p/2 and the diffraction FOV can be increased about two times. 
         [0038]      FIGS. 6A-C  illustrate a photo alignment process for optically separating one pixel into several sub-pixels for the embodiment of  FIG. 5 . Similar as  FIG. 4A and 4B , a first alignment layer is arranged on the multiple pixel electrodes that are configured above the silicon substrate and a second alignment layer is arranged on the transparent ITO electrode that is configured above the glass substrate. As shown in  FIG. 6A , 1st photo masks  61   a  and  61   b  are arranged to cover the 1st sub-pixel area  62   a  of each pixel  63  on both the first alignment layer  64   a  and second alignment layer  64   b . Then a 1st UV light  65   a  is illuminated on the 1st and 2nd alignment layers  64   a  and  64   b  in a perpendicular oriented direction  66   a . After that, as shown in  FIG. 6B , the 1st photo masks  61   a  and  61   b  are taken away, and 2nd photo masks  67   a  and  67   b  are arranged to cover the 2nd sub-pixel area  62   b  of each pixel  63  on both of the first and second alignment layers,  64   a  and  64   b . In one embodiment, the 1st and 2nd sub-pixel areas  62   a  and  62   b  are adjacent to each other. Then, a UV light  65   b , having the same wavelength as the 1st UV light  65   a , is illuminated on the 1st and 2nd alignment layers  64   a  and  64   b  in a parallel oriented direction  66   b . After the 2nd photo masks  67   a  and  67   b  are removed, as shown in  FIG. 6C , a silicon substrate portion  68   a  and a glass substrate portion  68   b , formed from the above steps, are assembled to form a phase modulator  69  wherein each pixel  63  is separate into sub-pixels  63   a  and  63   b  that are optically isolated to each other due to different alignments of the liquid crystal molecules. 
         [0039]    In actual, there are several methods to make the alignment for a phase modulator. In one embodiment, mechanical rubbing could be used to make the alignment layer. However, the produced alignment layer may have scratches and contamination. Furthermore, this method can&#39;t realize multi-domain alignment in one pixel. In an alternative embodiment, the present invention could use UV light for photo-alignment as described above. The advantage of photo-alignment is the ease to get sub-micro multi-domain alignment in one pixel. However, thermal stability issue should be solved to satisfy the auto-grade standard. 
         [0040]    In order to improve the thermal stability of the photo-alignment layer, the polymer network can be penetrated into the liquid crystal layer to strengthen the alignment energy so as to improve alignment layer thermal stability. As shown in  FIG. 7A , firstly reactive monomers material  71  are mixed into the liquid crystal layer  72 . The monomers material  71  can be RM257, C12A, TMPTA, or NVP. Then, the monomers material  71  polymerizes together to form the polymer material for improve the thermal stability. In one embodiment, monomers&#39; concentration is less than 1 wt %. In  FIG. 7B , during the 2nd UV light exposure, monomers such as RM257, C12A, TMPTA, or NVP are polymerized on the alignment surface  73  previously formed under a 1st UV light to form a polymer network  74 . The 2nd UV light has different wavelength from that of the 1st UV light. 
         [0041]    The foregoing description of the present invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many modifications and variations will be apparent to the practitioner skilled in the art. 
         [0042]    The embodiments were chosen and described in order to best explain the principles of the invention and its practical application, thereby enabling others skilled in the art to understand the invention for various embodiments and with various modifications that are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalence.