Blazed micro-mechanical light modulator and array thereof

Details Blazed micro-mechanical light modulator and array thereof Blazed micro-mechanical light modulator and array thereof

2001-07-11

2003-09-09

Lester, Evelyn (Department: 2873)

Optical: systems and elements

Optical modulator

Light wave temporal modulation

C359S291000, C359S298000, C359S224200, C359S573000

Reexamination Certificate

active

06618187

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to micro-mechanical light modulators and to Spatial Light Modilators (SLMs) including arrays of such modulators.
BACKGROUND OF THE INVENTION
Various optical applications, such as projection, imaging and optical fiber communication, require light modulation and/or light beam steeped In optical applications where a plurality of optical beams should be handled simultaneously, the modulation can be achieved by using optical modulators called Spatial Light Modulators (SLMs) or Light Valves (LVs), which are arrays of individually controlled members. Distinctive class SLMs work in diffractive mode; An activated individual member of the SLM array diffracts the incoming light beam at a discrete multitude of angles, these angels being a function of the light wavelength and the dimensions of the modulator. Such modulators, based on Micro Elctro-Mechanical Systems (MEMS) technology and called Deformable Diffractive Gratings, are described, for example, in U.S. Pat. Nos. 5,311,360; 5,459,610 to The Board of Trustees of the Leland Stanford, Junior University; U.S. Pat. Nos. 5,629,801; 5,661,592 to Silicon Light Machines; U.S. Pat. No. 5,677,783 to The Board of Trams of the Leland Stanford, Junior University; U.S. Pat. Nos. 5,808,797; 5,841,579; 5,982,553 to Silicon Light Machines; U.S. Pat. No. 5,920,518 to Micron Technology, Inc.; U.S. Pat. No. 5,949,570 to Matsushita Electric Industrial Co.; U.S. Pat. No. 5,999,319 to InterScience Inc.; U.S. Pat. Nos. 6,014,257; 6,031,652 to Eastman Kodak Company.
In the conventional art Deformable Diffractive Gratings light modulation systems, the diffractive element is usually of “piston” type or cantilever mirror type. Both types of diffractive elements have some advantages, while suffering from some drawbacks. For example, a piston diffractive grating element is always faster than a cantilever mirror diffractive grating element, however, its efficiency is lower. Reference is made now to
FIGS. 1
,
2
a
,
2
b
and
2
c
, which show a typical conventional art design of a piston diffractive type element and demonstrate its operation. Throughout the figures, similar elements are noted with similar numeral references.
FIG. 1
is a schematic isometric view of a conventional art piston type deformable grating element
10
. The element
10
consists of several beams, noted
25
, created by a photolithographic process in a frame
20
. The beams
25
define a diffractive grating
22
, supported by the etched structure
30
. The bee
25
rest on a silicon substrate base
40
. Beams
21
of the beams
25
are movable and are suspended over gaps
41
, which are etched in the silicon substrate base
40
, while other beams
23
of the beams
25
are static. The beams
25
are coated with a reflective layer
60
. This reflective layer
60
is conductive and functions as an electrode. An opposite electrode
50
is deposited on the opposite side of the silicon substrate
40
.
FIGS. 2
a
and
2
b
show the A—A cross-section of the conventional art modulator
10
of
FIG. 1
in non-active and active states, respectively. In
FIG. 2
a
, no voltage is applied between the suspended beams
21
and the common electrode
50
. Accordingly, all the beams
21
and
23
are coplanar and the diffractive element works as a plane mirror, i.e. incident beam
70
and reflected beam
71
are in the exact opposite directions. When voltage is applied between the suspended beams
21
and the common electrode
50
, as shown in
FIG. 2
b
, the suspended beams
21
are deformed in the direction of the electrical field created by the applied voltage. Thus, the non-suspended beams
23
and the suspended beams
21
define a diffractive structure returing an incident beam
70
in directions
171
. The directions
171
and the direction
70
of the incident beam constitute an angle &PHgr; which follows the laws of diffractive optics and is called a diffractive angle. The angle &PHgr; is a function of the light wavelength &lgr; and the grating period d. The diffraction efficiency is a function of the grating amplitude. For piston type grating, the optimal amplitude for achieving optimal efficiency, is &lgr;/4, as illustrated in
FIG. 2
b
. In this example and the example below it is assumed that the light modulation system operates in air with refractive index n=1.
FIG. 2
c
shows the angular distribution of the light energy for non-active (thin line) and active (thick line) &lgr;/4 optimize piston type deformable grating light modulating element. The calculations are made for Fraunhofer diffraction of parallel light beam while &lgr;=830 nm and grating period d=10 &mgr;m, and while King into account the interference of two simultaneously working elements (i.e. 2d ‘UP’-‘DOWN’-‘UP’-‘DOWN’ structure). It can be seen from this figure that when the element is active, most of the energy is distributed in the +1
st
and −1
st
orders, while when it is non-active, most of the energy is distributed in the “zero” order (tinner line).
Commonly, there are two kinds of distinctive optical systems that utilize diffractive type light modulators: optical light systems having spatial filtering of the “zero” order, and optical light systems having spatial filtering of the ±1
st
and higher orders. When the “zero” order is filtered, the maximal theoretical energy efficiency is 70%, while when the ±1
st
and higher orders are filtered, the maximal theoretical energy efficiency can be as high as 90%. In both cases, the maximal theoretical contrast ratio (the ratio between the energies passing the spatial filter in the active and non-active states, respectively) that can be achieved is 1:12.
However, for most applications, such as pre-press imaging and projection displays, contrast ratio as low as 1:12 is unacceptable. An additional disadvantage of the piston type diffractive grating modulators, is that when in active state, the light energy is distributed symmetrically in the ±1
st
and higher orders, which in many cases can lead to a more complex optical system, as the light has to be cutoff from both sides of the maximum.
SUMMARY OF THE INVENTION
There is provided in accordance with an embodiment of the invention, a light valve of deformable grating type. The light valve includes at least three beams, one beam of being of a substantially fixed-position, and at least two beams being deformable by electrostatic force in a substantially staircase structure, each step of the staircase creating a predefined change in the phase of an impinging light beam, and first and second electrodes for transmitting electrostatic force to at least the deformable beams.
There is also provided in accordance with a further embodiment of the invention, a light valve of deformable grating type, which includes at least three beams, one beam being of a substantially fixed-position, and the three beams being deformable by electrostatic force in a substantially staircase structure, each step of the staircase creating a predefined change in the phase of an impinging light beam and a first electrode and a second electrode, the electrodes transmitting electrostatic force to the deformable beams.
In addition, there is also provided in accordance with an embodiment of the invention, a method for light modulation. The method includes the steps of:
providing a light valve of deformable grating type, the light valve includes at least three beams, at least the first beam of the at least three beams being of a substantially fixed-position, and at least two beams of the at least three beams being deformable by electrostatic force in a substantially staircase structure, each step of the staircase creating a predefined change in the phase of an impinging light beam;
illuminating the light valve,; and
applying voltage between the first electrode and the second electrode.
providing a light valve of deformable grating type, the light valve includes at least three beams, at least the first beam of the at least three beams being of a substantially fixed-po

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