Abstract:
A spatial light modulator applied to the collinear volume holographic storage system uses a hollow phase modulator to modulate the surrounding portion of an incident light to be a reference light, and the center portion of the incident light is modulated by an amplitude modulator to be a signal light. Thus, the spatial light modulator can enhance the convergence of the point spread function of the system.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a spatial light modulator applied to a collinear volume holographic storage system, and, especially, to a spatial light modulator that can enhance the convergence of the point spread function of this system. 
     2. Description of the Prior Art 
     The collinear holographic storage system is the main development in these years, since the collinear holographic technique has the features of high stability, high reliability and high miniaturability. 
     In the writing process, the collinear holographic storage system uses a spatial light modulator to generate a signal light and a reference light as a laser light passes the spatial light modulator. After the signal light and the reference light pass a phase modulating mask, the interference will be focused by a lens and recorded homogeneously on a recording medium. In the reading process, the laser light is modulated to a reference light and irradiates on the recording medium to restore the signal via the recorded interference. Finally, the restored light irradiates on a photo-detector and can be read. 
       FIG. 1   a  is employed to illustrate the light path of the system in the writing process. The spatial light modulator  200  modulates the incident light  100  to a signal light  110  and a reference light  120 , and a lens  300  focus the interference of the signal light  110  and the reference light  120  on a recording medium  400  to be recorded. 
       FIG. 1   b  is employed to illustrate the light path of the system in the reading process. The spatial light modulator  200  modulates the incident light  100  to a reference light  120  only, and, after the reference light  120  passes the lens  300 , the reference light  120  irradiates on the recording medium  400  to restore the signal light  110  and the reference light  120  via the recorded interference. The restored interference is focused on and irradiates on a photo-detector  600  and can be read. 
     The structure of a reflective holographic storage system is different from the transmissive system, abovementioned system, but in the similar theory. The rear face of the recording medium  400  is coated with a reflective material  410 , and a splitter  700  is set in the light path. In the writing process, the signal light  110  and the reference light  120  pass the splitter  700  and lens  300  to irradiate on the recording medium  400 . In the reading process, the reference light  120  is reflected by the reflective material  410  after passing the splitter  700  and lens  300  to restore the interference. The restored interference passes the splitter  700  and will be reflected to a photo-detector  600  for reading. The light paths in the writing process and reading process are shown in  FIG. 2   a  and  FIG. 2   b.    
     The above mentioned spatial light modulator has a disadvantage of poor convergence of the point spread function. 
     SUMMARY OF THE INVENTION 
     It is an object to provide a spatial light modulator for improving the convergence of the point spread function. The mean is to set a phase modulator surrounding an amplitude modulator. 
     In the writing process, the spatial light modulator modulates the surrounding portion of the incident light to a reference light and the center portion to a signal light. In reading, the center part of the spatial light modulator is blocked for avoiding generating the signal light, and the reference light will irradiate on the recording medium to restore the recorded interference. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1   a  and  FIG. 1   b  show the light paths of a transmissive holographic storage system of a prior art in the writing and reading process, respectively. 
         FIG. 2   a  and  FIG. 2   b  show the light paths of a reflective holographic storage system of a prior art in the writing and reading process, respectively. 
         FIG. 3   a  and  FIG. 3   b  show the light paths of a transmissive holographic storage system employing a spatial light modulator according to an embodiment of this invention in the writing and reading process, respectively. 
         FIG. 4   a - 4   g  show spatial light modulators according to the preferred embodiments of this invention. 
         FIG. 5   a - 5   k  show phase modulators according to the preferred embodiments of this invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Spatial light modulator includes a amplitude modulator and a phase modulator with a hollow, and the amplitude modulator and the phase modulator can be arranged in compact or in separation. The surrounding portion of an incident light is modulated into a reference light by the phase modulator and the center portion to a signal light by the amplitude modulator. The interference of the reference light and the signal light will be recorded on a recording medium after passing a phase modulating mask. Accompanying with  FIG. 3   a  and  FIG. 3   b , the description of the light paths of the transmissive holographic storage system is following. 
     In the writing process, as  FIG. 3   a , a hollow phase modulator  820  is adhered to a amplitude modulator  810  to form a spatial light modulator  800 , and a phase modulating mask  900  is adhered to the spatial light modulator  800 . The incident light  100  passes the spatial light modulator  800  and the phase modulating mask  900  to form a reference light  120  on the surrounding part of the spatial light modulator  800  and to form a signal light  110  in the center part. The interference of the reference light  120  and the signal light  110  is recorded on a recording medium  400  after passing a lens  300 . 
     In the reading process, as  FIG. 3   b , the center part of the spatial light modulator  800  is covered. The incident light  100  can not pass the center part of the spatial light modulator  800 , and the surrounding portion of the incident light  100  is modulated to a reference light  120  after passing the spatial light modulator  800  and the phase modulating mask  900 . The reference light  120  irradiates on the recording medium  400  after the reference light  120  passes the lens  300  to restore the signal light  110 , and the restored signal light  110  will pass a second lens  500  to be detected by a photo-detector  600 . 
     Reflective holographic storage system works in a similar way to the transmissive holographic storage system so the detailed description is omitted here except for the difference. The difference between these two systems is that the former has a reflective material coated on the rear face of the recording medium, a splitter is set in the light path and the second lens (the second lens  500  in previous embodiment) can be omitted. The splitter will guide the restored interference light to the photo-detector. 
     The phase modulator and the amplitude modulator of the spatial light modulator can be set in compact or in separation, and even more, the phase modulator and the amplitude modulator can be set in interlace between the other optical components of the system. The preferred embodiments are showing in  FIG. 4   a - FIG. 4   h.    
       FIG. 4   a  shows an embodiment, where the phase modulator  820  is adhered to a amplitude modulator  810 .  FIG. 4   b  shows another embodiment, where the phase modulator  810  and the amplitude modulator  810  clip the phase modulating mask  900 . 
     For the embodiments of  FIG. 4   c  and  FIG. 4   d , the phase modulator  820  is adhered to the phase modulating mask  900  and separated from the amplitude modulator  810 , and a lens  910  is placed in between them. No specific order is required for these optical components. 
     For the embodiment of  FIG. 4   e , the amplitude modulator  810  is adhered to the phase modulating mask and separated from the phase modulator  820 , and a lens  910  is placed in between them. No specific order is required for these optical components. 
     For the embodiments of  FIG. 4   f  and  FIG. 4   g , the phase modulator  820 , the amplitude modulator  810  and the phase modulating mask  900  are separated from each other totally. Lenses  910 ,  920  are placed between two components thereof. No specific order is required for these components. 
     The phase modulators  802  in abovementioned embodiments can be made from a lens with a hollow. The  FIG. 5   a - FIG. 5   k  are the preferred embodiments of the phase modulator according to this invention. 
       FIG. 5   a  and  FIG. 5   b  are a recto-ring-like and a circle-ring-like array of spherical lens with a hollow, respectively.  FIG. 5   c  and  FIG. 5   d  are a recto-ring-like and a circle-ring-like array of pillar lens with a hollow, respectively.  FIG. 5   e  and  FIG. 5   f  are recto-ring-like and circle-ring-like lenses with a hollow, respectively.  FIG. 5   g  and  FIG. 5   h  are recto-ring and circle-ring lenses with a hollow, respectively.  FIG. 5   i  and  FIG. 5   j  are recto-ring-like and circle-ring-like loop array of coned lenses with a hollow, respectively.  FIG. 5   k  is recto array of pillar lenses with a hollow. 
     Although this invention has been explained in relation to its preferred embodiment, it is to be understood that modifications and variation can be made without departing the spirit and scope of the invention as claimed.