Liquid crystal modulator including a diffuser with plural phase shifting regions

Light emitted from a coherent light source undergoes fine adjustment of its polarizing direction by a polarizing device and is directed to a liquid crystal device through a collimating optical system. The polarizing device is capable of fine-adjusting the polarizing direction in accordance with the wavelength of the incident light, thereby providing a high contrast ratio. When an image is recorded by holography using such a liquid crystal spacial light modulator, it is possible to obtain an image of high quality.

BACKGROUND OF THE INVENTION 
1. Field of the Invention: 
The present invention relates to a liquid crystal spacial light modulator 
used in display panels, image information recording, etc., and to a 
holographic image information recording apparatus using the same. 
2. Description of the Prior Art: 
Researches on image information recording and reproduction techniques using 
holography have been under way since the 1960s. Especially, 
three-dimensional information recording and reproduction apparatus have 
recently begun to attract attention from the fields of medicine, art, etc. 
as the application value of such apparatus is recognized. Active matrix 
liquid crystal display (LCD) panels, for use in portable TV, 
projection-TV, etc., using twisted nematic liquid crystals have also been 
actively developed, and high performance, high resolution products have 
begun to be marketed. Under these circumstances, the holographic image 
recording system has been proposed which uses an LCD panel as the spacial 
light modulator (SLM) and displays a computer-processed image on the 
liquid crystal SLM to produce object light. 
In the holographic image recording system, the performance of the liquid 
crystal SLM plays an important part in determining the quality of recorded 
images. Besides, anti-noise measures are an essential requirement, which 
is not the case with conventional LCD panels illuminated with incoherent 
light. To specifically describe the measures, it is required to suppress 
speckle noise, interference noise, etc., which are caused as a result of 
the illumination of the liquid crystal SLM by coherent light. 
In a conventional LCD panel, as shown in FIG. 1, polarizing plates 2a and 
2b are respectively attached to the front and back surfaces of a liquid 
crystal device 1. Light emitted from a light source 10 is passed through a 
lens 20, a pinhole 23, and a lens 21, and is polarized by the polarizing 
plates 2a and 2b in the rectilinear direction coinciding with the 
anchoring direction of the liquid crystal device 1. The liquid crystal 
device 1 rotates the direction of polarization of the incident light to 
modulate the intensity of the outgoing light emitted through the 
polarizing plate 2b on the back side. However, such a construction is 
disadvantageous in that the performance, including the contrast ratio, of 
the liquid crystal device 1 cannot be fully utilized, being restricted by 
the performance of the polarizing plates 2a and 2b. 
To describe in detail, the polarizing plates have the shortcomings in that 
the light transmittance is low, that the extinction ratio is relatively 
low, and that the polarizing direction is not uniform but dispersed within 
the surface. Furthermore, there is a possibility that the polarizing 
direction may slightly deviate from the proper angle because of an error 
in attaching the polarizing plates to the liquid crystal device. As a 
result, the contrast ratio of the SLM is reduced. Another factor 
contributing to the reduced contrast ratio is the inability to obtain the 
proper polarizing direction for incident light other than that of a 
particular wavelength because the proper polarizing direction with the 
liquid crystal device 1 varies depending on the wavelength of the light 
entering the SLM. As a result, the contrast ratio of the SLM is reduced. 
Further, in the conventional LCD panel, film-like polarizers are used. The 
problem is that the light is reflected on the surface of the polarizers, 
thus generating interference noise. To prevent the reflection of light, an 
anti-reflection film can be provided on the surface of the polarizer, but 
it is difficult to provide a durable anti-reflection film on the surface 
of a film-like polarizer. 
For the above reasons, in order to obtain a high quality image using a 
liquid crystal SLM, it is necessary to adjust the polarizing direction of 
the incident light without providing a polarizer on the front side of the 
liquid crystal panel. To achieve this purpose, it is possible to collimate 
the linearly polarized light from a laser light source for direct entry to 
the liquid crystal device, but in such a construction, if the laser light 
source itself is to be rotated through a minute angle for fine adjustment 
of the polarizing direction of the laser beam, a relatively large-sized 
mechanism will be required for the rotation of the laser light source. On 
the other hand, a mirror may be used so as to reduce the size of the 
entire optical system, but the light reflected by a mirror generally 
becomes elliptically polarized and is therefore not desirable. 
Furthermore, when the liquid crystal SLM is applied to holography, the 
problem is that the reference light separated from the object light by a 
half mirror also becomes elliptically polarized. Thus, in the liquid 
crystal SLM, it is not easy to properly design an optical system capable 
of fine-adjusting the polarizing direction of the incident light. 
SUMMARY OF THE INVENTION 
The liquid crystal spacial light modulator of the present invention, which 
overcomes the above-discussed and other numerous disadvantages and 
deficiencies of the prior art, comprises a coherent light source; a 
polarizing device capable of fine-adjusting the polarizing direction of 
the light emitted from the coherent light; a collimating optical system 
into which the light with its polarizing direction adjusted by the 
polarizing device is introduced; and a liquid crystal device into which 
the light passing through the collimating optical system is introduced for 
transmission therethrough and which modulates the intensity of the 
transmitting light in accordance with image data. 
In an embodiment, a polarizing plate is provided on the back light side of 
the liquid crystal device. 
In an embodiment, a diffuser having phase shift regions arranged at a 
prescribed pitch is disposed on the front side of the liquid crystal 
device, a plurality of phase shift regions of the diffuser being arranged 
in a manner to correspond to one pixel of the liquid crystal device. The 
phase shift regions of the diffuser is (0, .pi./2, .pi., 3.pi./2) or (0, 
.pi./3, 2.pi./3). 
In an embodiment, an anti-reflection plate is provided on the back side of 
the liquid crystal device. The anti-reflection plate consists of a 
transparent plate whose front and back surfaces are parallel to each other 
and an anti-reflection film attached in a contacting relationship to the 
back surface of the transparent plate, the anti-reflection plate being 
disposed in such a way that the back surface of the transparent plate is 
parallel to the back surface of the liquid crystal device. Alternatively, 
the anti-reflection plate consists of a transparent plate whose back 
surface is slanted with respect to the front surface thereof, the 
anti-reflection plate being disposed in such a way that the back surface 
thereof is slanted with respect to the back surface of the liquid crystal 
device. Alternatively, the anti-reflection plate consists of a transparent 
plate whose front and back surfaces are parallel to each other, the 
anti-reflection plate being disposed in such a way that the back surface 
thereof is slanted with respect to the back surface of the liquid crystal 
device. 
The holographic image information recording apparatus of the present 
invention comprises a coherent light source; a beam splitter that splits 
the light from the coherent light source into two beams, an object beam 
and a reference beam; a polarizing device capable of fine-adjusting the 
polarizing direction of the object beam separated by the beam splitter; a 
collimating optical system into which the object beam with its polarizing 
direction adjusted by the polarizing device is introduced; a liquid 
crystal device into which the object beam passed through the collimating 
optical system is introduced for transmission therethrough and which 
modulates the polarization of the transmitting beam in accordance with 
image data; and an interference optical system which causes the light 
emitted from the liquid crystal device to interfere with the reference 
beam separated by the beam splitter; and a means for recording the light 
produced as a result of the interference by the interference optical 
system. 
In an embodiment, the holographic image information recording apparatus 
further comprises a polarizing device for fine-adjusting the polarizing 
direction of the reference beam separated by the beam splitter. 
Thus, the invention described herein makes possible the following 
objectives. 
The polarizing plate previously provided on the incident side of the liquid 
crystal SLM is replaced by a polarizing device which is provided in the 
path of the light entering the liquid crystal device and which is capable 
of fine-adjusting the polarizing direction. This serves to eliminate the 
problem associated with an error in attaching the polarizing plate, a 
problem inherent in the prior art liquid crystal SLM. Moreover, since the 
polarizing direction can be adjusted according to the wavelength of the 
incident light, a high contrast ratio can be constantly obtained even when 
the wavelength of the incident light varies. Furthermore, since the 
polarizing device is provided on the incident side of the collimating 
optical system, it is only necessary for the polarizing device to have a 
transmitting face of the order of a few mm in diameter, which allows the 
use of a polarizing device, such as a Gran-Thompson prism, which has a 
high transmittance and a high distinction ratio. Thus, according to the 
present invention, holographic image recording with good image quality can 
be performed without providing a polarizer on the back side required in 
the conventional liquid crystal SLM.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
EXAMPLE 1 
FIG. 2 shows the construction of a liquid crystal SLM using a TN liquid 
crystal device of the present invention. Light emitted from a coherent 
light source 10 passes through a polarizing device 40 and enters a 
collimating system consisting of a lens 20, a pinhole 23 and a lens 21. 
The collimating system enlarges the incident light and directs it to a TN 
liquid crystal device 1. The TN liquid crystal device 1 works to modulate 
the polarization of the transmitting light in accordance with the image 
information supplied from a pattern generator 70. The light with modulated 
polarization passes through a polarizing plate 2 and is emitted as 
spacially modulated light 100. 
The important point in the above-mentioned construction is that when a 
device, such as a Gran-Thomson prism, having a high distinction ratio is 
used as the polarizing device 40, a further complete linearly polarized 
light can be obtained and also the polarizing direction of the light can 
be changed as desired. As a result, the polarizing direction is almost 
uniform within the surface of the polarizing device 40 as compared with 
the conventional liquid crystal SLM using polarizing plates shown in FIG. 
1, so that the plane-polarized light that contains fewer unwanted 
components of polarization enters the liquid crystal device 1. resulting 
in a spacially modulated light 100 with a high contrast ratio. 
Furthermore, since the polarizing direction can be adjusted according to 
the wavelength of the incident light, it is possible to constantly obtain 
a high contrast ratio regardless of the variation in the wavelength of the 
light emitted from the light source. 
The contrast ratio was measured with a liquid crystal SLM having the 
construction of FIG. 2. A He-Ne laser of a wavelength 633 nm was used as 
the light source 10, and a Gran-Thomson prism mounted on a rotating stage 
was used as the polarizing device 40. Light emitted from the light source 
10 was polarized by the polarizing device 40 so that the polarizing 
direction thereof coincided with the anchoring direction of the liquid 
crystal device 1. The light passing through the polarizing device 40 was 
directed through the lens 20, pinhole 23 and lens 21 of the collimating 
system to enter the liquid crystal device 1 perpendicularly, and the light 
transmitted through the liquid crystal device 1 was emitted as the 
spacially modulated light 100 through the polarizing plate 2. Then, while 
monitoring on an optical power meter the power of the light transmitted 
through an area of about 30.times.30 mm.sup.2 of the liquid crystal device 
1, the angles of the polarizing device 40 and the polarizing plate 2 were 
readjusted so that the light transmission was at its minimum when no 
voltage was applied to the liquid crystal device 1. In this situation, the 
transmittance of the liquid crystal device 1 was measured. The results are 
shown in Table 1. In the conventional liquid crystal SLM with a polarizing 
plate attached to the front side of the liquid crystal device, the 
contrast ratio is of the order of several tens even when a plane-polarized 
laser light is used. On the other hand, in the liquid crystal SLM of the 
present invention, a high contrast ratio of over 600 (ratio of maximum to 
minimum transmittance, 140/0.23&gt;600) was obtained. 
TABLE 1 
______________________________________ 
Applied Transmitted Transmitted light 
voltage (mV) 
light power (UW) 
power ratio 
______________________________________ 
0 0.23 1 
195 3.48 15 
400 55.0 239 
610 110.3 480 
920 140 609 
______________________________________ 
EXAMPLE 2 
Example 1 discloses a liquid crystal SLM designed for production of an 
image of high contrast ratio, but for holographic recording of a higher 
image quality, it is necessary to suppress speckle noise generated from 
the coherent optical system. For that purpose, a practical holographic 
recording system requires, as shown in FIG. 3, the provision of a diffuser 
200 on the front side of a TN liquid crystal device 300. 
In this embodiment, the diffuser 200 is brought into contact with a glass 
plate 304 of the liquid crystal device 300 into which a collimated laser 
beam 101 is introduced. In the liquid crystal device 300, the glass plate 
304 having cell electrodes that is disposed on the front side of the 
liquid crystal device 300, is disposed at an appropriate space from a 
glass plate 305 that is disposed on the back side, by means of a spacer 
303, and the space is charged with a TN liquid crystal 301. The glass 
plate 305 on the back side of the liquid crystal device 300 is provided 
with an anti-reflection plate. The anti-reflection plate comprises a glass 
plate 400, which is a transparent plate whose front and back surfaces are 
parallel to each other, and an anti-reflection film 401 attached to the 
back surface of the glass plate 400, and the glass plate 400 is attached 
to the glass plate 305 on the back side of the liquid crystal device 300, 
by means of a matching liquid 402 having a refractive index equal to that 
of the glass plates 400 and 305. The spacially modulated light 100 is 
emitted through the anti-reflection film 401. 
The diffuser 200 is a glass plate having a pattern of protrusions and 
recesses formed at a prescribed pitch on one surface thereof to give a 
prescribed pseudo-random phase distribution to the light entering in the 
form of a plane wave. The amount of phase shift is determined by the depth 
of the said uneven pattern formed on the surface of the diffuser 200, each 
pattern area forming a phase shift region. The phase distribution given to 
the transmitting light by the diffuser 200 is determined by the pitch of 
the phase shift regions and the amount of phase shift at each phase shift 
region; for example, a four-level pseudo-random phase system is used. In 
the four-level pseudo-random phase system, the incident light is subjected 
to any one of four phase shift, 0, .pi./2, .pi., or 3.pi./2 at any phase 
shift region, the phase shift between adjacent phase shift regions being 
.pi./2. 
FIG. 3b shows the pixel configuration of the liquid crystal device 300, and 
FIG. 3c shows the phase distribution on the diffuser 200 corresponding to 
the pixel configuration of the liquid crystal device 300. The diffuser 200 
and the liquid crystal device 300 are drawn on the same scale, nine phase 
shift regions, 201, 202, 203, . . . , 209 corresponding to one pixel 391 
of the liquid crystal device. As described above, each phase shift region 
201-209 provides any one of the four phase shift levels, 0, .pi./2, .pi., 
or 3.pi./2 to the incident light, the phase difference between adjacent 
phase deviation regions being .pi./2. 
Alternatively, the pseudo-random phase system can be so configured as to 
provide three phase shift levels, 0, .pi./3, and 2.pi./3, to the incident 
light, with a .pi./3 phase difference between adjacent phase shift regions 
(see FIG. 3d). 
By using the diffuser 200 of the above-mentioned construction, the 
spacially modulated light emitted from the liquid crystal SLM is diffused, 
thereby achieving holographic image recording of high quality with little 
speckle noise. If the diffuser 200 is not provided, holographic recording 
can still be performed by slightly deviating the photographic plate from 
the focal point of a Fourier-transform lens, but in this case, the 
coherent light having almost planar wavefronts and striking the liquid 
crystal SLM interferes with the scattered light produced by dust and other 
particles adhering to the lens, etc., thereby generating concentric 
speckle noise. In contrast, when the diffuser 200 is provided, almost no 
interference noise patterns appear on the spacialy modulated light 100 by 
the particles adhering to the lens, etc. because the coherent light 
undergoes a phase shift at the same pitch as that of the protrusions and 
recesses (the pitch of the phase shift regions) formed on the diffuser 
200. In this case, even though the coherent light diffused by the diffuser 
200 interferes with the scattered light, the noise pattern produced by the 
interference is a pattern with a fine pitch equivalent to the pitch of the 
protrusions and recessed formed on the diffuser 200. Therefore, when the 
pitch of the protrusions and recesses on the diffuser 200 is made 
sufficiently smaller than the size of the pixel 391 of the liquid crystal 
device 300, the noise pattern can be suppressed to a negligible level. As 
a result, an image of higher quality and better S/N ratio can be obtained 
in image recording when the phase shift regions are provided at a high 
density on the diffuser 200 than when they are provided one each 
corresponding to one pixel 391 of the liquid crystal device 300. Another 
advantage of using the high density diffuser is that moire fringes 
produced by the diffuser 200 and the liquid crystal device 300 are less 
likely to occur, which facilitates the positional alignment of the 
diffuser 200 with respect to the liquid crystal device 300. 
Moreover, in the liquid crystal SLM of the present embodiment, an 
anti-reflection plate is disposed on the surface of the glass plate 305 
placed on the front side of the liquid crystal device 300. The 
anti-reflection plate comprises a glass plate 400 with an anti-reflection 
film 401 attached thereto. The glass plate 400 is in contact with the 
glass plate 305 of the liquid crystal device 300 with a matching liquid 
402 interposed therebetween. This serves to suppress the Fresnel 
reflection at the back surface of the liquid crystal device 300, thereby 
reducing the interference fringes to be superimposed on the output image. 
The above description has illustrated how the improvement in image 
recording can be achieved by the provision of the diffuser 200 and the 
anti-reflection arrangement on the back surface of the liquid crystal 
device 300. To verify the effect of the diffuser 200, we measured the S/N 
ratio of the image produced by the liquid crystal SLM shown in FIG. 3. A 
TN liquid crystal panel (pixel pitch of about 90 .mu.m) for a projection 
TV was used as the liquid crystal device 300, and the diffuser 200 having 
phase shift regions with a pitch of about 15 .mu.m was attached in a 
contacting relationship to the incident light surface of the liquid 
crystal device 300. For comparison, a diffuser having phase shift regions 
with a pitch of about 90 .mu.m was attached to the front surface of the 
liquid crystal device of another liquid crystal SLM. The reconstructed 
image from holograms recorded with each liquid crystal SLM was focused 
through the lens system onto the CCD element, and the output signal from 
the CCD was converted from analog to digital to measure and compare the 
S/N ratios of images produced by the respective liquid crystal SLMs having 
different pitches of phase shift regions. We defined the S/N ratio as the 
ratio of the standard deviation of fluctuation .sigma. of the output light 
intensity to the average image intensity I when a consistent white image 
was input to the liquid crystal SLM. The result was that the S/N ratio of 
the reconstructed image was 26 dB with the 15 .mu.m-pitch diffuser, while 
the S/N ratio of the reconstructed image was 20 dB with the 90 .mu.m-pitch 
diffuser. This means that the S/N ratio was improved by 6 dB by reducing 
the pitch of the phase shift regions of the diffuser to 1/6. Also, the 
output image of sufficiently high quality was obtained by installing the 
diffuser in a contacting relationship to the liquid crystal device. 
EXAMPLES 3 and 4 
FIGS. 4a and 4b, respectively, show other examples of the anti-reflection 
plate to be provided on the back surface of the liquid crystal device 300. 
The embodiment shown in FIG. 4a uses as the anti-reflection plate, a glass 
plate 400 of a constant thickness, whose front and back surfaces are 
parallel to each other, with no anti-reflection film attached on the back 
surface thereof. Using a matching liquid 402, the glass plate 400 is 
attached to back surface of the liquid crystal device 300, in a manner to 
slant at an angle greater than a prescribed angle with respect to the back 
surface of the liquid crystal device 300. In this case, a spacer 403 is 
provided between the glass plate 400 and the glass plate 305 on the back 
side of the liquid crystal device 300 to provide a prescribed slanting 
angle to the glass plate 400. 
In the embodiment of FIG. 4b, a glass plate 400 having wedge-shaped cross 
section with its back surface slanted with respect to its front surface is 
attached to the glass plate 305 on the back side of the liquid crystal 
device 300 by using a matching liquid 402, the back surface of the glass 
plate 400 being slanted with respect to the back surface of the liquid 
crystal device 1. 
In either embodiment, the Fresnel reflection at the back surface of the 
glass plate 400 is not reduced, but the optical axes of reflected light 
103 and transmitted light 104 are shifted from each other. Therefore, by 
determining the slanting angle of the back surface of the glass plate 400 
so that the optical axes are shifted by an angle greater than a specific 
angle, it is possible to sufficiently reduce the spacial frequency of 
interference fringes occurring in the outgoing light, thereby preventing 
the deterioration of the final image produced. 
EXAMPLE 5 
FIG. 5 shows a holographic image recording optical system which uses the 
liquid crystal SLM of the present invention. Light emitted from a light 
source 10 is passed through a shutter 41 and is directed to a half mirror 
31 acting as a beam splitter. The light directed at the half mirror 31 and 
reflected by it is used as a reference beam, which is directed to a 
polarizing device 42 and then enters a Fourier-transform lens 50 after 
passing through the polarizing device 42 and a lens 22. The light allowed 
to pass through the half mirror 31 is used as an object beam, which is 
directed to the liquid crystal SLM. In the liquid crystal SLM of the 
present embodiment, a diffuser 80 is provided on the front surface of a TN 
liquid crystal device 1, and the light transmitted through the half mirror 
31 is passed through a polarizing device 40 of the liquid crystal SLM and 
is directed at a collimating system consisting of a lens 20, a pinhole 23, 
and a lens 21. The object beam passed through the collimating system 
enters the liquid crystal device 1 through the diffuser 80. The liquid 
crystal device 1 is driven in accordance with image data supplied from a 
pattern generator 70, to spacially modulate the object beam passing 
through the liquid crystal device 1. The object beam passing through the 
liquid crystal device 1 enters the Fourier-transform lens 50. Interference 
occurs between the object beam and the reference beam also entering the 
Fourier-transform lens 50, thereby forming an interference pattern on a 
photographic plate 60. 
The holographic image recording optical system shown in FIG. 5 uses the 
Fourier-transform lens 50 as the interference optical system in 
consideration of the suitability for high density image recording, but 
other interference optical systems can also be used depending upon the 
recording and reproduction method employed; for example, a cylindrical 
lens can be used for holographic stereogram recording. 
The thus constructed holographic image recording optical system of the 
present invention is characterized, among others, by the provision of the 
polarizing device 40. In the conventional holographic image recording 
optical system, the construction is such that polarizing plates are 
disposed on the front and back sides of the liquid crystal device 1 or 
that a rotatable polarizer is disposed on the back side of the SLM. The 
former construction has had problems with contrast ratio, etc. as 
previously mentioned. On the other hand, the latter construction has 
required the provision of a polarizer having an aperture size equal to or 
larger than the image area of the SLM and has also had problems in terms 
of the maximum contrast ratio, because the outgoing light from the SLM 
becomes elliptically polarized when the wavelength of the light emitted 
from the light source varies. According to the construction of the 
holographic image recording optical system of the present invention, since 
the polarizing device 40 is provided, there is no need to provide a 
polarizing plate on the front side of the liquid crystal device 1, and the 
outgoing light from the SLM is provided with a high contrast ratio. 
Moreover, since the polarizing device 42 is provided in the light path of 
the reference beam, the reference beam, elliptically polarized by the half 
mirror, can be polarized back to the plane-polarized light, so that an 
interference pattern of good visibility can be obtained. 
Another advantage of the holographic image recording optical system of the 
present invention is that there is also no need to provide a polarizing 
plate on the back side of the liquid crystal device 1. When a polarizing 
plate is provided on the back side of the liquid crystal device, the 
intensity of the outgoing light from the SLM is modulated, but when such a 
polarizer is not provided, the outgoing light has its polarizing direction 
phase-modulated. This results in the generation of components whose 
polarizing direction is perpendicular to the reference beam, but in this 
case, only bias components are applied to the interference pattern on the 
photographic plate, as compared with the case when the polarizing plate is 
provided, and the interference pattern shape remains unaffected. Moreover, 
since the polarizing plate is not provided, an almost constant exposure 
amount can be obtained regardless of the pattern shape, and accordingly an 
excellent holographic recording can be performed. 
It is understood that various other modifications will be apparent to and 
can be readily made by those skilled in the art without departing from the 
scope and spirit of this invention. Accordingly, it is not intended that 
the scope of the claims appended hereto be limited to the description as 
set forth herein, but rather that the claims be construed as encompassing 
all the features of patentable novelty that reside in the present 
invention, including all features that would be treated as equivalents 
thereof by those skilled in the art to which this invention pertains.