# Patent

## Title
Asymmetrical deformable diffractive grating modulator

## Abstract

A method for modulating light using a micro-electro-mechanical structure includes providing a plurality of deformable mirror elements (30) having an L-shaped cross section. Each of the deformable mirror elements is comprised of a pedestal (32) and an elongated ribbon (33). Each of the ribbons has a reflective surface (35). A beam of light is directed on the deformable mirror elements. The deformable mirror elements is flexed about an axis parallel to a long dimension of the ribbons to vary a curvature of at least one of the reflective ribbons.

## Background
CROSS REFERENCE TO RELATED APPLICATIONS
Reference is made to commonly-assigned copending U.S. patent application Ser. No. 13/737,983, filed Jan. 10, 2013, entitled ASYMMETRICAL DEFORMABLE DIFFRACTIVE GRATING MODULATOR; by Nissim Pilossof; the disclosure of which is incorporated herein.
FIELD OF THE INVENTION
The present invention relates in general to micro-mechanical structures and in particular to diffractive grating modulators.
BACKGROUND OF THE INVENTION
Micro-electro-mechanical systems (MEMS) are used in many devices which require modulation of light. For example, projectors may use a digital micromirror device (DMD), which has thousands of micromirrors. A cantilever or hinged mirror 10 of this type, shown in FIG. 1, rotates on an axis 11 reflect a beam of light when in an on position. A disadvantage of this type of device is the relatively slow response time,  ̃10 μs, due to the low natural frequency of each single hinged mirror.
In another type of modulation device, the deformable diffractive grating light modulation systems, the diffractive element is formed as a long narrow ribbon. In one design, the ribbon bends along the long axis of the ribbon thus forming a “piston” type switching diffractive element. Typical piston type diffractive elements are shown in U.S. Pat. Nos. 5,311,360; 5,459,610; and 5,677,783.
In another design, shown in FIG. 2a, the cross section of the diffractive ribbon is T-shaped and the ribbon bends along its short axis. FIG. 2a illustrates two pixel elements 21 and 22 of a spatial light modulator (SLM) 20 built on a silicon substrate 23, with each pixel comprised of three diffractive elements 210 and 220 respectfully. The pixels are driven using electrodes 24. Pixel 21 is shown in energized state while pixel 22 is in a non-energized state. Resulting diffraction distribution of the two pixels is shown on FIG. 2b. In energized (diffracting) state, a pixel produces symmetrical angular distribution of light intensity consisting of many diffraction maximums. T-shaped ribbon types of diffractive elements are shown, for example, in U.S. Pat. Nos. 6,661,561; 6,836,352; and 6,856,448.
Both types of diffractive elements have some advantages, while suffering from some drawbacks. In both types, however, the width of one ribbon, the pitch, determines the grating period d, and the diffracted light is distributed within multiple diffractive orders symmetrically in both directions. See FIG. 2b. The diffraction efficiency of a single element is very low with typical contrast of about 50%. Therefore, in optical systems using deformable ribbons light modulators, more than one ribbon is used for forming an optical pixel, rendering the minimum optical pixel size to two times the grating period or 2d.
SUMMARY OF THE INVENTION
Briefly, according to one aspect of the present invention a method for modulating light using a micro-electro-mechanical structure includes providing a plurality of deformable mirror elements having an L-shaped cross section. Each of the deformable mirror elements is comprised of a pedestal and an elongated ribbon. Each of the ribbons has a reflective surface. A beam of light is directed on the deformable mirror elements. The deformable mirror elements are flexed about an axis parallel to a long dimension of the ribbons to vary a curvature of at least one of the reflective ribbons.
The invention and its objects and advantages will become more apparent in the detailed description of the preferred embodiment presented below.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a prior art cantilever mirror.
FIGS. 2a and 2b show a prior art SLM based on T-shaped ribbons and corresponding angular distribution of light intensity.
FIG. 3 shows an L-shaped ribbon according to the present invention.
FIG. 4 is a cross-sectional view of a plurality of diffractive elements according to the present invention.
FIG. 5 shows intensity versus diffraction for L-shaped ribbons based SLM according to the present invention.
FIG. 6 is a schematic view of an optical system according to the present invention.



DETAILED DESCRIPTION OF THE INVENTION
The present invention will be directed in particular to elements forming part of, or in cooperation more directly with the apparatus in accordance with the present invention. It is to be understood that elements not specifically shown or described may take various forms well known to those skilled in the art.
One embodiment of the present invention is a diffractive grating consisting of mirrors shaped as ribbons with asymmetrical cross-section as shown in FIG. 3. The single diffractive element 30 will have approximately the same diffraction efficiency as symmetrical ribbons, but at half the pitch value, allowing for two times higher system resolution, all other optical conditions being equal. Fabrication methods for deformable mirrors is not within the scope of the present invention, but it is well known and described in details in U.S. Pat. Nos. 5,311,360 and 5,661,592 and can be adapted to fabrication of the mirrors of the present invention.
Referring again to FIG. 3, the deformable mirror element 30 is fabricated for example on a silicon substrate 31. A deformable mirror element 30 is comprised of a pedestal 32 which supports elongated ribbon 33. Either the pedestal 32 or the ribbon 33 or both are made electrically insulating material. The ribbon 33 has an unsupported elongated portion laterally extending along the pedestal 32. The surface of the ribbon 33 is coated with a highly reflective layer forming a mirror surface 35. For clarity the mirror is shown smaller than the ribbon width, but it is appreciated that it should cover whole width of the ribbon.
The choice of the coating material depends on the wavelength of light the SLM is intended to work with. For example SLM intended to work with visible light may be coated with aluminum while SLM intended to work with near infrared light may be coated with gold. In an embodiment wherein the ribbon is made of electrically insulating material like silicon nitride and the reflective coating is not electrically conductive, a special conductive coating beneath the reflective coating should be considered. Thus, in FIG. 3 element 35 represents an electrical electrode and reflective coating at the same time.
A second electrode 34 is formed on the surface of the substrate. Applying voltage between electrodes 34 and 35 energizes the ribbon and due to electrostatic forces, it bends along its shorter axis assuming position 36. When the ribbon is bent, its surface profile in the shorter direction (X-direction in FIG. 3) follows a profile function F(x). F(x) is taken into account when calculating the light intensity versus angle in diffracting state and depends on the dimensions of the ribbon, the mechanical properties of the material it builds from, and the applied voltage. Methods of calculating F(x) and the intensity angular distribution in diffracting state are well known in the art. Generally, the closer F(x) is to a straight line, the higher contrast ratio the SLM can achieve. Therefore it is desirable to have the thickness t of the post 32 as small as possible.
It is appreciated that the above discussed way of selecting and energizing a ribbon by means of electrostatic forces is only exemplary. Other methods like magnetic, thermal, etc. can also be used.
Reference is now made to FIG. 4 which shows a portion of SLM 40 with L-shaped deformable ribbons. The number of ribbons in a SLM can be hundreds or even thousands. The distance d between the ribbons is called the grating constant and determines the diffracting power of the device, while the ratio d/g, wherein g is the gap between the ribbons, is called the fill factor and affects the overall efficiency of the device. The higher the fill factor, the higher the device efficiency.
It is well known in the art that the diffraction efficiency depends on the number of individual elements contributing to the process. If a single pixel is formed from only one diffracting element the maximum achievable contrast is about 50%, which is not enough for most applications. Therefore individual pixels are usually formed from two or more diffracting elements. It is appreciated that the simultaneous wiring of four ribbons illustrated in FIG. 4 is only exemplary. The SLM can be built by pre-wiring groups of ribbons to form a pixel (as illustrated in FIG. 4) or by wiring each individual ribbon and later forming pixels by simultaneous addressing of adjacent ribbons by the SLM driving electronics.
For simplicity, FIG. 4 illustrates only two pixel elements 41 and 42 and each pixel element consists of four ribbons (diffracting elements) 410 and 420 respectfully. Pixel 41 is in diffracting state and pixel 42 is in non-diffracting state. The incident light beam 46 from light source 43 lies in a plane at incidence angle α44 and strikes the SLM plane at angle Θ relative to its normal 49. The light falling at the non-energized (inactive) pixel element 42 will experience a simple reflection 47 and will propagate in the plane of reflection β45 at angle Θ relative to the SLM 40 plane normal 49. Light falling on the energized (active) pixel after reflection will propagate in different directions 48 governed by the laws of diffraction. Different directions of propagations constitute different diffraction orders as only one order, called “Zero order,” will propagate in the plane of reflection 45 at angle Θ relative to the SLM 40 plane normal 49, i.e. will obey the law of simple reflection. All other orders first, second, etc. will generally propagate in different planes and at different angles relative to the plane normal 49.
For specific profile function F(x) the angle of incidence Θ can be chosen in a way that the light propagating in Zero order direction is minimized and the diffracted light is concentrated predominantly in one of the higher diffracting orders first, second, etc. FIG. 5 illustrates such optimized angular distribution of light for pixels in diffracting and non-diffracting states. It can be seen that the diffracted light is concentrated predominantly in one diffraction maximum at one side of the grating, i.e. the grating is “blazed.” This “blazing” property of the grating means that the diffracted light will be with approximately the same brightness as the non-diffracted.
The angular modulation of the light achieved by the SLM can be converted into spatial modulation. This is explained with the help of FIG. 6. In an exemplary optical system 600, a SLM 60 with pixel elements built and arranged as described above and shown in FIGS. 3-5 is illuminated with beam of light 64 using a prism 62 with two reflective surfaces 63. It is appreciated that instead of a prism, a system of mirrors and direct illumination can be used. The illumination angle Θ is the same angle discussed above. After reflecting from the SLM, the light propagates within two beams 65 and 66. Beam 65 originates from non-energized pixels while beam 66 originates from energized pictures. Element 61 is a diffractive element.
Beams 65 and 66 pass through lens 67 the focal plane (the system's Fourier plane) contains a blocker 68 which stops the light from beam 66. After the blocker 68, only light from non-energized pixel element will propagate through the rest of the optical system, i.e. only the Zero diffraction order will be allowed.
The next element downstream is lens or group of lenses 69 which together with lens 67 constitute an imaging system with object plane the SLM and image plane 70, i.e. the SLM and plane 70 are conjugate. As all light from energized pixels will be stopped at the blocker 68 and the image 71 will contain only images of non-energized pixels. It is appreciated that it is possible to place the blocker 68 in such a way that it will stop the Zero Order and allow the higher diffractive orders. In such case the image plane will contain images of all energized pixels.
The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the scope of the invention.
PARTS LIST


    10 hinged mirror
    11 axis
    20 spatial light modulator (SLM)
    21 pixel element
    22 pixel element
    23 substrate
    24 electrode
    30 deformable mirror element
    31 substrate
    32 pedestal
    33 ribbon
    34 electrode
    35 mirror surface
    36 axis assuming position
    40 spatial light modulator (SLM)
    41 pixel element
    42 pixel element
    43 light source
    44 incidence angle α
    45 reflection β
    46 incident light beam
    47 reflection
    48 direction
    49 plane normal
    60 spatial light modulator (SLM)
    61 ribbon (diffractive element)
    62 prism
    63 reflective surface
    64 beam of light
    65 beam
    66 beam
    67 lens
    68 blocker
    69 group of lenses
    70 image plane
    71 image
    210 diffractive element
    220 diffractive element
    410 ribbon (diffracting element)
    420 ribbon (diffracting element)
    600 optical system

## Claims

1. A method for modulating light using a micro-electro-mechanical structure comprising:
providing a plurality of deformable mirror elements having an L-shaped cross section;
wherein each of said deformable mirror elements is comprised of a pedestal and an elongated ribbon attached to the pedestal on a long axis of the ribbon;
wherein each of said ribbons has a reflective surface;
directing a beam of light on said deformable mirror elements; and
flexing one or more reflective ribbons about one or more long axes of one or more reflective ribbons to vary a curvature of one or more reflective ribbons.

2. The method of claim 1 wherein said reflective surface has one or more reflective segments.

3. The method of claim 1 wherein each of said reflective ribbons flexes independently.

4. The method of claim 1 wherein each of the pedestals is anchored to a substrate parallel to each adjacent pedestal.

5. The method of claim 1 further comprising:
providing an activation device on each of said ribbons.

6. The method of claim 5 further comprising:
activating said activation device to induce curvature in a pixel group of said ribbons.

7. The method of claim 5 further comprising:
providing complimentary activation devices on a substrate that anchors the pedestal on each of said deformable mirror elements to induce curvature in said ribbons.

8. The method of claim 5 wherein said activation device is selected from a group comprising electrical, magnetic, or heating elements.

9. The method of claim 1 wherein the reflective surface is conductive.

10. The method of claim 1 wherein the reflective surface is matched to a wavelength of the light.

11. The method of claim 1 wherein the reflective surface is aluminum and the wavelength of the light is in the visible range.

12. The method of claim 1 wherein the reflective surface is gold and the wavelength of the light is in the infrared range.