Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
1999-07-01
2002-06-18
Epps, Georgia (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
C359S321000, C359S016000, C359S254000
Reexamination Certificate
active
06407849
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to optical systems and more particularly relates to an optical system for illuminating a spatial light modulator (SLM).
BACKGROUND OF THE INVENTION
Optical printing head systems are known in the art and are currently being used in a variety of applications. One way of constructing optical printing head systems is by using one or more high power laser diode bars (LDB) or laser diode arrays (LDA).
Laser diode bars (LDB) are used as light sources in imaging systems, like thermal recording systems. The emitters of the laser diode bar are all operated simultaneously in a continuous operation mode, thus the LDB can not be modulated. In order to produce the light modulation needed for creating a desired image, the light beams emitting from the laser diode bar can be transmitted to a multichannel spatial light modulator (SLM), which modulates the light according to the image information.
In a regular operation mode, the light emitting from the emitters of the LDB reaches many of the pixels of the SLM. In this way redundancy is built in the system, in the sense that if one of the emitters of the LDB fails to work, the system will still operate properly.
A conventional art spatial light modulator system is described in U.S. Pat. No. 5,521,748 and shown in FIG.
1
. Referring to
FIG. 1
, the system employs a laser diode bar
10
in conjunction with a microlenses lenslet
12
, the microlenses of the lenslet
12
having the same spacing as the emitters of the LDB
10
. The light from the emitters of the LDB
10
passes through the lenslet
12
and a field lens
14
, which is used to focus the respective light beams on a modulator
16
. The light beams, after passing through elements of the modulator area
16
, are imaged by imaging optics
18
onto the film plane
20
.
U.S. Pat. No. 5,517,359 provides a spatial light modulator system wherein the microlenses have a pitch which is less than but substantially equal to the pitch of the emitters of the LDB, as shown in
FIG. 2. A
laser diode
21
emits a light beam
22
which is collimated in the vertical dimension by a cylindrical lens
23
. A second microlens
24
is a linear array of cylindrical lenslets aligned with the emitters of the laser diode. The light from the lenslets of microlens
24
is collimated by cylindrical lens
25
and imaged on a line of linear light valve
26
. A polarizer prism
27
transmits the light of horizontal polarization and reflects the light
31
whose polarization was changed by passing through activated PLZT (lead-lanthanum zirconate titanate ceramic) cells, that are used as the linear light valve
26
. An imaging lens
28
images light valve
26
onto heat sensitive (or light sensitive) material
29
, forming an image
30
.
Spatial Light Modulators may be of various types. Some SLMs operate in a reflective mode, using an array of micromirrors (for example, the Deformable Mirror Device from Texas Instruments incorporated of Texas US), or use deformable membranes reflective elements, like those of Optron Systems, Inc. Bedford, Mass. U.S.A. and Silicon Light Machines, Inc. Sunnyvale, Calif. U.S.A. Other SLMs are based on polarization rotation, like Liquid Crystal Display (LCD) devices.
Other known SLMs are based on electro-optics devices like PLZT. Electro-optical materials, like PLZT or KPT (potassium titanyl phosphate crystal), are used to modulate the light. The operation is based on the modification of the polarization state of the light when it passes through the crystal, while an electric field is applied to the crystal. These devices have the advantage of having a very fast response time, since small size devices have small capacitance and can easily switch polarization state for modulation in 1 ns or even faster. These modulators can be built in arrays (as in U.S. Pat. No. 5,521,748 mentioned above).
A major problem that exists in illumination systems employing an SLM is crosstalk between adjacent channels of light, which occurs if the SLM is not properly illuminated. This will take place, for example, if light entering a certain pixel of the SLM leaves the SLM through another pixel. Obviously, crosstalk results in a blurry and inaccurate image.
FIG. 3A
is a schematic illustration of a light beam reaching a pixel
32
located at the middle of an SLM
33
in a conventional art system. The interaction length L of the pixel
32
is chosen such that a light beam enters the SLM through the pixel
32
and exits through the same pixel
32
. In this way, no crosstalk occurs between the channels.
FIG. 3B
is a schematic illustration of a light beam reaching a pixel
34
located close to the edge of an SLM
33
, in a conventional art system. A light beam
37
having the same divergence as in
FIG. 3A
is shown, the light beam
37
having an axis
35
at an angle a with respect to the optical axis
36
. It can be seen that the upper ray
37
depicting the light beam enters the pixel
34
through a neighboring pixel
38
, and leaves the pixel
34
through the neighboring pixel
39
. This is an example of crosstalk.
In both conventional art patents described above, no optimization of the divergence of the light beams reaching the pixels of the SLM is performed. In particular, the angles of the light beams reaching the pixels at the edges of the SLM are larger than the angles of the light beams reaching the pixels at the center of the SLM, thus increasing the possibility of crosstalk between adjacent channels of light.
A possible known way to solve the crosstalk problem is by narrowing the depth of the SLM, thus shortening the path of the light beam through the SLM and decreasing the possibility of crosstalk to occur.
The main disadvantage of this solution is that by shortening the path of the light beam through the SLM, the voltage which is needed in order to modulate the light by using the electro-optic effect increases. This is because the electro-optic effect is proportional to the product of the distance the light beam passes through the medium and the voltage used. Therefore, a decrease in the distance requires an increase in the voltage.
This disadvantage becomes a major barrier in illumination systems that require a substantial interaction length between the medium and the light to produce the modulation effect. For example, in an illumination system employing an PLZT SLM, an interaction length of about 200 &mgr; is required between the crystal and the light in order to produce the modulation effect at voltages on the order of 50V to 80V. Shortening the path of the light in order to prevent crosstalk from occurring will substantially limit the modulation rate.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a system and a method for illuminating a spatial light modulator, that reduces the crosstalk between adjacent channels, by reducing the divergence of the illumination of the SLM. The present invention is to be used preferably in conjunction with SLMs requiring long interaction length.
There is thus provided in accordance with a preferred embodiment of the present invention, a system including a linear array of light sources for generating a plurality of light beams, a linear array of microlenses, each of the microlenses receiving light from a corresponding light source of the array of light sources, an optical element for receiving light from the array of microlenses and for redirecting it and a spatial light modulator including an array of pixels for modulating the light. The distance between the array of microlenses and the optical element is such that all the pixels of the SLM are illuminated symmetrically with respect to the optical axis of the optical element.
Moreover, in accordance with a preferred embodiment of the present invention, the distance between the array of microlenses and the optical element is set according to the equation:
D
≈
f
1
*
H
SLM
E
wherein D represents the distance between the array of microlenses and the optical element, f
1
represents the focal length of each of the mic
CreoScitex Corporation Ltd
Eitan Pearl Latzer & Cohen-Zedek
Epps Georgia
Thompson Tim
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