CA2133335C - Modulating a light beam - Google Patents

Abstract

A modulator (10) for modulating incident rays of light, the modulator having several equally spaced beam elements (18), each having a light reflective planar surface. The beam elements are arranged and supported (12) parallel to each other, with their reflective surfaces parallel. During operation, the elements remain parallel, but the modulator moves the beams so that the per-pendicular spacing of their reflective surfaces changes between two configurations. In both configurations, the spacing equals m/4 times the wavelength of incident light. In the first configuration, m equals an even whole number or zero, and the modulator acts to reflect the incident rays of light as a plane mirror. In the second configuration, m equals an odd number and the modula-tor diffracts the incident rays as they are reflected.

Description

v -1- 21 3 3 3 3 5 1 Specification 6 Field of they Invention 7 This invention relates to a method and apparatus for 8 modulating << light beam and more particularly to the use 9 of a reflective,, deformable diffraction grating for performing such modulation.

:12 Brief Description of the Prior Art :l3 Devices which modulate a light beam, e.g. by altering :l4 the amplitude, frequency or phase of the light, find a :L5 number of applications. An example of such a device is a :l6 spatial light modulator (SLM) which is an electronically :l7 or optically controlled device which consists of one or .l8 two-dimensional reconfigurable patterns of pixel elements, 19 each of whi~~h can individually modulate the amplitude, :!0 phase or polarization of an optical wavefront.
~!1 These device- have been extensively developed, ~!2 particularly for applications in the areas of optical 23 processing and computing. They can perform a variety of 24 functions such as: analog multiplication and addition, 25 signal conversion (electrical-to-optical, incoherent-to-~;6 coherent, amplification, etc.), nonlinear operations and 2.7 short term storage. Utilizing these functions, SLMs have 28 seen many different applications from display technology 29 to optical sugnal processing. For example, SLMs have been 30 used as optical c:orrelators (e. g., pattern recognition 31 devices, programmable holograms), optical matrix 32 processors (e.g., matrix multipliers, optical cross-bar 33 switches wii=h broadcast capabilities, optical neural 34 networks, radar beam forming), digital optical 35 architecture:- (e.c~., highly parallel optical computers) 36 and displays.
37 The requirements for SLM technology depend strongly 38 on the application in mind: for example, a display 39 requires low bandwidth but a high dynamic range while SUBSTITU T E SH~E-i .~~I~~~ ~~/0~'~3~
213;335 - ~ "' ' ~p~19 N OV 1993 1 optical computers benefit from high response times but do 2 not require such high dynamic ranges. Generally, systems 3 designers require SLMs with characteristics such as: high 4 resolution, high speed (kHz frame rates), good gray scale, high contrast ratio or modulation depth, optical flatness, 6 VLSI compatible, easy handling capability and low cost.
7 To date, no one SLM design can satisfy all the above 8 requirement=_.. As a result, different types of SLMs have 9 been developed for different applications, often resulting in trade-offs.
11 Texas Instruments, for instance, has developed a 12 "Deformable Mirror Device (DMD)" that utilizes an 13 electromechanical means of deflecting an optical beam.
14 The mechanical motions needed for the operation of the DMD
are relatively large and, as a result, the bandwidths are 16 limited to tens of kilohertz. This device, however, gives 17 good contrast ratios and high-resolution and is, 18 furthermore, compatible with CMOS, and other low power 19 technologies.
Nematic and :Eerroelectric liquid crystals have also 21 been used as the active layer in several SLMs. Since the 22 electrooptic effect in liquid crystals is based on the 23 mechanical reorientation of molecular dipoles, it is to be 24 expected that liquid crystals are faster than the DMD-type devices. Modulators using ferroelectric liquid crystals 26 have exhibited moalerate switching speeds (150 ,sec to 100 27 nsec), low-power consumption, VLSI compatible switching 28 voltages (5-10 V), high extinction ratios, high resolution 29 and large a~~erturEas. However, these devices suffer from the drawbacks of limited liquid crystal lifetimes and 31 operating 1=emperature ranges. In addition, the 32 manufacturing process is complicated by alignment problems 33 and film thickness' uniformity issues.
34 Magnetooptic modulation schemes have been used to achieve faster switching speeds and to provide an optical 36 pattern memory cell.. Although these devices, in addition 37 to achieving fast: switching speeds, can achieve large 38 contrast ratios, they suffer from a low (<10%) throughput ~CTIU~ 9 3 ! 4 3 ~ 3 9 x;13 3 3 3 5 ~ ~p19 N OV 19g3 1 efficiency and are, therefore, often unsuitable for many 2 applications.
3 The need is 'therefore for a light modulation device 4 which overcomes these drawbacks.
Beside SLMs, another area of use of light modulators 6 is in fibcar optics. Fiber optic modulators are 7 electronically controlled devices that modulate light 8 intensity arid are designed to be compatible with optical 9 fibers. For high speed communication applications, lithium niobate (LiNbo3) traveling wave modulators 11 represent the state-of-the-art, but there is a need for 12 low power, High efficiency, low loss, inexpensive fiber 13 optic modulators, that can be integrated with silicon 14 sensors and electronics, for data acquisition and medical applications. A typical use of a modulator combined 16 with fiber optic technology, for example, is a data 17 acquisition system on an airplane which consists of a 18 central data processing unit that gathers data from remote 19 sensors. l3ecause of their lightweight and electro-magnetic immunity characteristics, fiber optics provide an 21 ideal communication medium between the processor and the 22 sensors which produce an electrical output that must be 23 converted to an optical signal for transmission. The most 24 efficient way to do this is to have a continuous wave laser at tree processor and a modulator operating in 26 reflection at the sensor. In this configuration, it is 27 also possible to deliver power to the sensor over the 28 fiber.
29 In thin> type of application the modulator should operate with high contrast and low insertion loss to 31 maximize the: signal to noise ratio and have low power 32 consumption. It should further be compatible with silicon 33 technology because the sensors and signal conditioning 34 electronics 'used in these systems are largely implemented in silicon.
36 Another use of a modulator combined with fiber optic 37 technology s in the monitoring of sensors that are 38 surgically implant=ed in the human body. Here optical 39 fibers are preferred to electrical cables because of their ~CTill,~ 9 3 / 0 3 9 3 2133335 . ~'I9Npy~

1 galvanic isolation, and any modulator used in these 2 applications. should exhibit high contrast combined with 3 low insertion loss because of signal to noise 4 consideratic>ns. Furthermore, as size is important in implanted devices, the modulator must be integratable with 6 silicon senscors and electronics.
7 There exist no prior art devices that have the 8 characteristics enumerated above. Modulators based on the 9 electro-optic, Franz-Keldysh, Quantum-Confined-Stark or Wannier-Stark effect in III-V semiconductors have high 11 contrast and. low insertion loss, but are expensive and not 12 compatible with ~~ilicon devices. Waveguide modulators 13 employing glass or epi-layers on silicon, require too much 14 area and too complex fabrication to be easily integratable with other ;silicon devices. Silicon modulators that do 16 not employ waveguides and that are based on the plasma 17 effect, require high electrical drive power and do not 18 achieve higr~ contrast.
19 The need is therefore for a light modulator which can be used with fiber optic technology with low power, high 21 efficiency, low :Loss, low cost and compatibility with 22 multimode optical fibers and silicon technology.

,PCT~I~ 93 /0383' 213 3 3 3 5 , ~P~A~ 19 N 0 V 19 9 3 2 Objects of the Invention 3 Accordingly, it is an object of this invention to 4 provide a light modulator which alone or together with other modulators exhibits most of the following 6 characteristics: high resolution, high speed (Khz frame 7 rates), gra~~ levels (100 levels), high contrast ratio or 8 modulation depth, optical flatness, VLSI compatible, easy 9 handling caF~abilit~y and low cost.
A further object of this invention is to provide a 11 light modulator which has a tolerance for high optical 12 power and good optical throughput.
13 Yet another object of this invention is to provide a 14 light modulator which is compatible with CMOS technology.
Still another object of this invention is to provide 16 a light modulator capable of use with fiber optic 17 technology.
18 A final object of this invention is to provide a 19 light modulator which is capable of modulating white light to produce colored light.

22 8ummarv 23 Briefly a presently preferred embodiment of this 24 invention includes a modulator for modulating incident beams of lic3ht, the modulator comprising a plurality of 26 equally spaced apart grating elements, each of which 27 includes a light reflective planar surface. The elements 28 are arranged parallel to each other with their light 29 reflective surfaces parallel to each other. The modulator includes means for supporting the elements in relation to 31 one another and means for moving the elements relative to 32 one another so tlhat the elements move between a first 33 configuration wherein the modulator acts to reflect the 34 incident beam of light as a plane mirror, and a second configuration wherein the modulator diffracts the incident 36 beam of light as it is reflected therefrom. In operation, 37 the light reflective surfaces of 2 ~ ;~ 3 ~ ;3 5 v PCTII~~ 9 3 l 0 3 9 3 '%
IPI:'~A~ 19 N O V 199 3 -6- ' 1 the elements remain parallel to each other in both the 2 first and the second configurations and the perpendicular 3 spacing between the reflective surfaces of adjacent 4 elements is equal. to m/4 times the wavelength of the incident beam of light, wherein m = an even whole number 6 or zero when the elements are in the first configuration 7 and m = an odd number when the elements are in the second 8 configuration.
9 One embodiment of this invention includes a reflective deformable grating light modulator, with a 11 grating amp7.itude that can be controlled electronically, 12 consisting of a reflective substrate with a deformable 13 grating sus~~ended above it. In its undeformed state, with 14 no voltage applied between the elements of the grating and the substrai~e, the grating amplitude is one half of the 16 wavelength of the incoming light. Since the round-trip 17 path difference between the light reflected from the top 18 and bottom of the grating is one wavelength, no 19 diffraction occurs. When a voltage is applied between the grating elements and the substrate, the electrostatic 21 force pull; the elements down to cause the grating 22 amplitude to become one quarter of the wavelength so that 23 reflections from the elements and the substrate add 24 destructive_Ly, causing the light to be diffracted. If the detection s~rstem for the reflected light has a numerical 26 aperture which accepts only the zero order beam, a 27 mechanical motion of only one quarter of a wavelength is 28 sufficient to modulate the reflected light with high 29 contrast.
Typically the grating is formed by lithographically 31 etching a film made of silicon nitride, aluminum, silicon 32 dioxide o:r any other material which can be 33 lithographic~ally etched.
34 The deformab~le grating modulator of this invention has the advantage that it is implemented in silicon 36 technology, using micromachining and sacrificial etching 37 of thin films to fabricate the gratings. Circuitry for 38 addressing and multiplexing can be manufactured on the 39 same siliccn substrate and thus be directly integrated PCTIU~ 9 3 I 0 3 9 3 9 213 3 3 3 5 ; ~p~~ 1~ 9 N OV 1993 1 with the mo<iulato:r. Direct integration with electronics 2 is an important. advantage over non-silicon based 3 technologie~~ like liquid crystal and electrooptic SLMs.
4 Moreover, the device demonstrates simplicity of fabrication and can be manufactured with only a few 6 lithographic: step:a.
7 A furt:.her advantage of the deformable grating 8 modulator i~, that because the deformable grating modulator 9 utilizes diffraction rather than deflection of a light beam, the requirs:d mechanical motions are reduced from 11 several microns (as in deformable mirror devices) to 12 tenths of a micron, thus allowing for a potential three 13 orders of magnitude in increase in speed. This speed is 14 comparable i~o the fastest liquid crystal modulators, but without the device suffering the same complexity in the 16 manufacturing process.
17 Still a further advantage of these devices is that 18 the require~3 motion of the grating elements is only one 19 quarter of a wavelength, which means that elements with high resonance frequencies can be used.
21 These and other objects and advantages of the present 22 invention will no doubt become apparent to those skilled 23 in the art after having read the following detailed 24 description of the preferred embodiment which is illustrated in the several figures of the drawing.

28 This invention will now be further illustrated with 29 reference to the accompanying drawing in which:
FIG. 1(a)-(d.) are cross-sections through a silicon 31 substrate illustrating the manufacturing process of a 32 reflective, deformable diffraction grating according to 33 one embodiment of the invention;
34 FIG. 2 is an isometric, partially cut-away view of the diffracaion grating, the manufacture of which is 36 illustrated in FIG. 1.
37 FIG. 3 illu:~trates the operation of the grating of 38 FIG. 2 in its "non-defracting" mode;

.'CTIU~ 9 3 I 0 s y s ~
' lPl~Ul~619 N OV X993 2133335 , 1 FIG. 4 illustrates the operation of the grating of 2 FIG. 3 in its "dit:fracting" mode;
3 FIG. 5 is a cross-section similar to that in FIG. 3, 4 illustrating an alternative embodiment of the grating in its "non-defracting" mode;
6 FIG. 6 is a cross-section similar to that in FIG. 4, 7 illustrating the grating in FIG. 5 in its "defracting"
8 mode;
9 FIG. 7 is a pictoral view illustrating a further embodiment of the grating;
11 FIG. 8 is a cross-section along line 8-8 in FIG. 7;
12 FIG. ~~ is a graphical representation of the 13 modulation of a laser beam by the grating of the 14 invention;
FIG. 10 is an illustration of how the diffraction 16 grating of the :invention can be combined with other 17 gratings to form a complex modulator; and 18 FIG. 11. illu:~trates the operation of the grating in 19 the modulation of white light to produce color.
21 DlsBCRIP'PION OF PREFERRED EMBODIMENTB
22 The fabrication steps required to produce a 23 reflective deforrnable grating 10 according to this 24 invention are illustrated in FIG. 1(a)-(d).
The first step, as illustrated in FIG. 1(a), is the 26 deposition of an insulating layer 11 made of stoichemetric 27 silicon nitride topped with a buffer layer of silicon 28 dioxide followed by the deposition of a sacrificial 29 silicon dio~:ide film 12 and a low-stress silicon nitride film 14, both 213 nm thick, on a silicon substrate 16.
31 The low-stress silicon nitride film 14 is achieved by 32 incorporating extra silicon (beyond the stoichiometric 33 balance) ini~o they film, during the deposition process.
34 This reducer the tensile stress in the silicon nitride film to roughly 200 MPa.
36 In the second step, which is illustrated in FIG.
37 1(b), the silicon nitride film 14 is lithographically 38 patterned into a grid of grating elements in the form of 39 elongate els~ments 18. In an individual grating, all the ~.r., PCTIt~ 9 3 l 0 3 9 3 9 1p~619 N OV 1993 _g_ 1 elements are. of the same dimension and are arranged 2 parallel to one another with the spacing between adjacent 3 elements equal to 'the beam width. Depending on the design 4 of the grating, however, the elements could typically be 1, 1.5 or 2u;m wide with a length that ranges from lO~Cm to 6 120~m. After this lithographic patterning process a 7 peripheral silicon nitride frame 20 remains around the 8 entire perimeter of the upper surface of the silicon 9 substrate 16. This frame 20 is further illustrated in :10 FIG. 2 and will be more fully described below with 11 reference to that figure.
12 After the patterning process of the second step, the 13 sacrificial silicon dioxide film 12 is etched in 14 hydrofluoric acid, resulting in the configuration illustrated in FI:G. 1(c). It can be seen that each 16 element 18 now forms a free standing silicon nitride 17 bridge, 213 nm thick, which is suspended a distance of 18 213nm (this being the thickness of the etched away 19 sacrificial film 7.2) clear of the silicon substrate. As can further be seen from this figure the silicon dioxide 21 film 12 is not entirely etched away below the frame 20 and 22 so the frame 20 is supported, a distance of 213 nm, above 23 the silicon substrate 16 by this remaining portion of the 24 silicon dio~:ide film 12. The elements 18 are stretched within the frame and kept straight by the tensile stress 26 imparted to the silicon nitride film 14 during the 27 deposition of that: film.
28 The last fabrication step, illustrated in FIG. 1(d), 29 is sputtering, through a stencil mask, of a 50 nm thick aluminum film 22 to enhance the reflectance of both the 31 elements 18 and the substrate 16 and to provide a first 32 electrode for applying a voltage between the elements and 33 the substrate. A aecond electrode is formed by sputtering 34 an aluminum film :?4, of similar thickness, onto the base of the silicon substrate 16.
36 The final configuration of the grating is illustrated 37 in FIG. 2. Here it can be seen that the elements 18 38 together with the frame 20 define a grating which, as will ~ PCTIU~ 9 3 / 0 3 9 3 9 2 33~~35 1f~619 N 0'~ 1993 1 be later explained, can be used for modulating a light 2 beam. Furthermore, and as can be gathered from the above PeT~ 9 3 I 0 3 9 3 9 ~p19 N OV 1993 1 described manufacturing process, the frame 20 is formed 2 integrally with t:he elements 18 and thus provides a 3 relatively rigid supporting structure which maintains the 4 tensile stress within the elements 18. In so doing, and as the frame 20 is supported by the remainder of the 6 silicon dioxide film 12 that was not etched away, the 7 elements are kept straight and a distance of 213 nm above 8 the surface ~of the silicon substrate 16.

9 The operation of the deformable grating 10, formed by the above process, is illustrated with reference to FIG.

11 3 and 4. Before commencing the description of how the 12 grating oper~3tes, however, it should be recalled that, in 13 this case, each oi: the elements 18 are 213 nm thick and 14 are suspended a distance of 213 nm clear of the substrate 16. This mEaans that the distance from the top of each 16 element to the top of the substrate is 426 nm. Similarly, 17 the distance between the top of the reflective surface on 18 the elements to the top of the reflective surface on the 19 substrate is also 426 nm. This distance is known as the grating amplitude.

:21 In FIG. 3 the grating l0 is shown with no voltage 22 applied bet~~reen the substrate 16 and the individual 23 elements 18, and with a lightwave, generally indicated as 24 26, of a wavelength ~1 - 852 nm incident upon it. The grating amplitude of 426 nm is therefore equal to half of 26 the wavelength of the incident light and, therefore, the 27 total path length difference for the light reflected from 28 the elements and from the substrate equals the wavelength 29 of the incident light. As a result, light reflected from the elements and from the substrate add in phase and the 31 grating 10 acts to reflect the light as a flat mirror.

32 However, as illustrated in FIG. 4, when a voltage is 33 applied between the elements 18 and the substrate 16 the 34 electrostatic forces pull the elements 18 down .onto the substrate 16, with the result that the distance between 36 the top of t:he elements and the top of the substrate is 37 now 213 nm. As this is one quarter of the wavelength of 38 the incident lights, the total path length difference for ~T~ 93/03939 21.3 3 3 3 5 ~ ~p~1' N O V 19 9 3 -loA
1 the light reflecaed from the elements and from the 2 substrate i:~

rc;i~u~ y3fOj93y IP~~, l s N o v 2133335 -11- ~3 1 now one half of t:he wavelength (426 nm) of the incident 2 light and the reflections interfere destructively, causing 3 the light to be diffracted, indicated as 28.

4 Thus, .if this grating is used in combination with a system, for detecaing the reflected light, which has a 6 numerical a~>erturEa sized to detect one order of diffracted 7 light from t:he gr<~ting e.g., the zero order, this grating 8 can be used to modulate the reflected light with high 9 contrast.

In FIGS. 5 and 6 an alternative embodiment of the 11 diffraction grating 30 of the invention is illustrated.

12 In this embodimeni~ the grating 30 consists of a plurality 13 of equally spaced, equally sized, fixed elements 32 and a 14 plurality of equally spaced, equally sized, movable elements 34 in which the movable elements 34 lie in the ~6 spaces between the fixed elements 32. Each fixed element 17 32 is supported on and held in position by a body of 18 supporting material 36 which runs the entire length of the 19 fixed element 32. The bodies of material 36 are formed during a 7_ithographic etching process in which the 21 material between 'the bodies 36 is removed.

22 As can be seen from FIG. 5 the fixed elements 32 are 23 arranged to be coplanar with the movable elements 34 and 24 present a flat upper surface which is coated with a reflective Layer 38. As such the grating 30 acts as a 26 flat mirror when i.t reflects incident light, however, when 27 a voltage is applied between the elements and an electrode 28 40 at the b<~se of the grating 30 the movable elements 34 29 move downwards as is illustrated in FIG. 6. By applying different vc>ltages the resultant forces on the elements 31 and, therefore, the amount of deflection of the movable 32 elements 34 can be varied. Accordingly, when the grating 33 amplitude (defined as the perpendicular distance d between 34 the reflective layers 38 on adjacent elements) is m/4 times the wavelength of the light incident on the grating 36 30, the grating 30 will act as a plane mirror when m = 0, 37 2, 4... (i.e. an even number or zero) and as a reflecting 38 diffraction grating when m - 1, 3, 5... (i.e. an odd 39 number) . In this manner the grating 30 can operate to P~'1'lU~ 9 3 / 0 3 9 3 9 213 3 3 3 5 ' ~p19 N OV 193 -12- , 1 modulate incident light in the same manner as the grating 2 10 illustrated in FIGS. 1 to 4.

3 Yet another embodiment of the diffraction grating of 4 the invention is illustrated in FIGS. 7 and 8. As with the grating l0 in FIGS. 1 to 4 this grating 41 consists of 6 a sacrificial silicon dioxide film 42, a silicon nitride 7 film 44 and a substrate 46. In this embodiment, however, 8 the substr~~te 46 has no reflective layer formed thereon 9 and only the silicon nitride film 44 has a reflective coating 45 formed thereon. As is illustrated in FIG. 7 11 the deformable elements 48 are coplanar in their 12 undeformed state and lie close to one another so that 13 together they provide a substantially flat reflective 14 surface. 'Ihe elements 48 are, however, formed with a neck 50 at either end,, which is off-center of the longitudinal 16 center line of each of the elements 48.

17 When a. uniformly distributed force, as a result of an 18 applied voltage for example, is applied to the elements 48 19 the resultant force F, for each element 48, will act at the geometric center 52 of that element. As each 21 resultant force F is off-set from the axis of rotation 54 22 (which coincides with the centerline of each neck 50), a 23 moment of rotation or torque is applied to each element 48 24 which resu7.ts in a rotation of each element 48 about its axis 54 to the position 48' indicated in broken lines.

26 This is known as "blazing" a diffraction grating.

27 As carp be seen from FIG. 8, the reflective planes 56 28 of the elements ~48 remain parallel to each other even in 29 this "blazed" configuration and therefore, the grating amplitude d is the perpendicular distance between the 31 reflective surfaces of adjacent elements. This "blazed"

32 grating will operate to diffract light in the same manner 33 as a sawtooth greeting.

34 Although note illustrated in any of FIGS. 1 to 8, it will be apparent that a deformable diffraction grating can 36 be constructed in which, in its undeformed state, all the 37 reflective elements are in the form of movable elements 38 arranged parallel., adjacent and coplanar with each other.

39 In this ty~~e of crating not only the grating PC'flU~ 9 3 / 0 3 9 3 21i 3 3 3 3 5 ' ~p~~ 9 N OV 1993 1 amplitude (i.e., the perpendicular distance between 2 adjacent re:Elective surfaces) can be varied but also the 3 average height of all the reflective surfaces can be 4 changed by movinc3 all the elements relative to a fixed datum. This arrangement has the advantage that both the 6 amplitude and the phase of the reflected/diffracted light 7 can be modulated .

8 The electrical, optical and mechanical 9 characterisi~ics of a number of modulators, similar in design to l~he modulator illustrated with reference to 11 FIGS. 1 to 4 but of different dimensions were investigated 12 by using a Helium Neon laser (of 633 um wavelength) 13 focused to a spot: size of 36um on the center portion of 14 each modulator. This spot size is small enough so that the curvature of the elements in the region where the 16 modulator w~~s illuminated can be neglected, but is large 17 enough to allow the optical wave to be regarded as a plane 18 wave and covering enough grating periods to give good 19 separation between the zero and first order diffraction modes resulting f~_-om the operation of the grating. It was 21 discovered that grating periods of (i.e.) the distance 22 between the centerlines of two adjacent elements in the 23 grating, 2,:3 and ~4 ~m and a wavelength of 633 um resulted 24 in first o~_der diffraction angles of 18', 14' and 9' respectivel~l.

26 One of these first order diffracted light beams was 27 produced by using a 120 ~m-long grating modulator with 1.5 28 ~.m-wide elements at atmospheric pressure together with a 29 HeNe light beam modulated at a bit rate of 500 kHz.

detected by a low-noise photoreceiver and viewed on an 31 oscilloscope. Tlhe resulting display screen 30 of the 32 oscilloscopE~ is illustrated in FIG. 9.

33 However, before proceeding with a discussion of the 34 features illustrated in this figure, the resonant frequency of the grating elements should first be 36 considered.

37 The re;:onant frequency of the mechanical structure of 38 the grating of the invention was measured by driving the 39 deformable grating modulator with a step function and PcTNS 93/0393' 21 . x"19 N OV X99:

1 observing t:he ringing frequency. The area of the aluminum 2 on the deformable grating modulator is roughly 0.2 cm2 , 3 which corre~spond:~ to an RC limited 3-dB bandwidth of 1 MHz 4 with roughly 100 ohms of series resistance. This large RC

time constant slowed down the step function, however, 6 enough powEar existed at the resonant frequency to excite 7 vibrations, even in the shorter elements. Although the 8 ringing could be observed in normal atmosphere, the 9 factor was. too low (approximately 1.5) for accurate measurements, so the measurements were made at a pressure 11 of 150 mbar. At this pressure, the Q-factor rose to 8.6, 12 demonstrating that air resistance is the major damping 13 mechanism, for <j grating of this nature, in a normal 14 atmosphere.

Nonetheless" it was found that due to the high 16 tensile stress in the elements, tension is the dominant 17 restoring force, and the elements could therefore be 18 modeled as vibrating strings. When this was done and the 19 measured and theoretically predicted resonance frequencies compared, it wa,s found that the theory is in good 21 agreement with the experimental values, particularly when 22 considering the uncertainty in tensile stress and density 23 of the elements. As it is known that the bandwidth of 24 forced vibrations of a mechanical structure is simply related to the resonance frequency and Q-factor, a Q-26 factor of 1.5 yis:lds a 1.5 dB bandwidth of the deformable 27 grating modulator 1.4 times larger than the resonance 28 frequency. The range of bandwidths for these gratings is 29 therefore from 1.8 MHz for the deformable grating modulator with 120 um elements to 6.1 MHz for the 31 deformable grating modulator with 40 um elements.

32 Returning now to FIG. 9, it should be noted that with 33 an applied voltage swing of 3 V, a contrast of l6dB for 34 the 120 um-long bridges could be observed. Here the term "modulation depth" is taken to mean the ratio of the 36 change in optical. intensity to peak intensity.

37 The input (lower trace 62) on the screen 60 38 represents a pseudo-random bit stream switching between 0 39 and -2.7 V across. a set of grating devices on a 1 cm by ..:~,CTIU~ 9 3 ~ 0 3 9 3 ' -15- ' ~. 9 N OV 1g9:
1 cm die. The observed switching transient with an initial 2 fast part followed by a RC dominated part, is caused by 3 the series resistance of the deformable grating modulator, 4 which is comparable to a 50 ohm source resistance.

The output (upper trace 64 ) on the screen corresponds 6 to the optical output of a low-noise photoreceiver 7 detecting th;e first diffraction order of the grating used.

8 The output (upper trace 64) from the deformable grating is 9 high when '~he elements are relaxed and low when the elements are deflected. Ringing is observed only after 11 the rising t:ransisant, because of the quadratic dependence 12 of the ele~ctro-static force on the voltage (during 13 switching from a voltage of -2.7 V to 0 V, the initial, 14 faster part of the charging of the capacitor corresponds to a larger change in electro-static force, than when 16 switching the oppc>site way) . This ringing in the received 17 signal indicates a decay close to critical damping.

18 Furthermore, it was found that because the 19 capacitance increases as the elementsa~are pulled toward the substrate, the: voltage needed for a certain deflection 21 is not a monotonically increasing function of this 22 deflection. At a. certain applied voltage condition, an 23 incremental increase in the applied voltage causes the t t b ll d t t t t th b l t l 24 o e pu s ra e ( s o e spon aneous o e su e emen y latch) and this voltage is known as the "switching 26 voltage" of the modulator. The switching voltage was 27 found to be 3.2 V for gratings with 120 ~m long elements 28 and, if it is assumed that tension dominates the restoring 29 forces, the switching voltage is inversely proportional to the element length and therefore, the predicted switching 31 voltage for 40 ~m long elements will be 9.6 V.

32 The importance of the switching voltage is that below 33 this voltage, the deformable grating modulator can be 34 operated in an analog fashion, however, if a voltage greater than the switching voltage is applied to the 36 modulator it. acts in a digital manner. Nonetheless, it is 37 important to noted that operating the modulator to the 38 point of contact is desirable from an applications point 39 of view, because as discussed above when the elements are PCTIt~fi 9 3 ~I 0 3 9 3 ' 213 3 3 3 5 -16_ ' Cpl 9 N OV 199 1 deflected E~lectr~ostatically, an instability exists once 2 the element def~~Lection goes beyond the halfway point.

3 This results in hysteretic behavior which will "latch" the 4 element in the down position. This latching feature gives the modulator the advantages of an active matrix design 6 without the need for active components. A further 7 advantage of this latching feature is that once the 8 element has "latched" it requires only a very small 9 "holding voltage"', much smaller than the original applied voltage, to keep the element in its latched configuration.

11 This feature is. particularly valuable in low power 12 applications where efficient use of available power is 13 very important.

14 Finally, it was discovered that when the elements of the modulators are brought into contact with the substrate 16 they could stick. This can be solved by adding small 17 ridges below the' elements to reduce the contact area 18 between the beams and the substrate and thereby reduce the 19 sticking problem.

The u;se of the modulator of this invention in 21 displays re~quire:a high yield integration of individual 22 modulator devices> into 2-D arrays of modulator devices.

23 The modulator devices may be comprised of a single grating 24 such as described above or may be comprised of multiple modulator components such as that illustrated in FIG. 10 26 which shows a plurality of grating modulator components 27 combined to form a single modulator device 65 which can be 28 used to provide a gray-scale operation. Each of the 29 individual modulator components 66, 68, 70, 72 consist of a number of elements and gray-scale can be obtained by 31 addressing each modulator component in a binary-weighted 32 manner. Th.e hysi~eresis characteristic for latching (as 33 described above) can be used to provide gray-scale 34 variation without analog control of the voltage supplied to individual grating modulator elements.

36 In FIG. 11 'the use of the grating, in combination 37 with other c~ratinc~s, for modulating white light to produce 38 colored light is illustrated. This approach takes 39 advantage oi_ the <ability of a grating to separate a light pC'CI~ 9 3 ~ 4 3 9 3' 2 'I ~ 3 3~ 3 5 -16A- ~ ' IPE~UI~ 19 N 0 V 199, 1 spectrum into its constituent colors. By constructing 2 modulator devices 73 including three separate red, green 3 and blue modulai=ion components 74, 76 and 78, each with a 4 grating designed to diffract the appropriate color into an optical s~~stem ('not shown), a color display which is white 6 light PCT'IU~ 9 3 / C~ 3 9 3 213 3 ;s 3 5 ; IP'~~ 19 N 4V '993 1 illuminated by a light beam 80 can be achieved. Although 2 shown separated for purposes of illustration, it will be 3 appreciated that: the three modulation components 74, 76 4 and 78 could be positioned contiguous to each other as are the components 66-72 in Fig. 10 to form a single modulator 6 device 73. This approach is attractive for large area 7 projection displays.

8 In summary, the reflective, deformable grating light 9 modulator of this invention is a device which exhibits high resolution ( 4 0 by 4 0 ~Cm2 to 100 ~Cm2 ) ; high response 11 times/large bandwidth (2 to 6 MHz); high contrast ratio 12 (close to :100% modulation with a 3V switching voltage); is 13 polarization independent and easy to use. This device 14 also has tolerance for high optical power, has good optical throughput, is simple to manufacture, CMOS

16 compatible, and has application in a wide range of fields 17 including use as an SLM and with fiber optic technology.

18 Although the present invention has been described 19 above in terms of specific embodiments, it is anticipated that alterations. and modifications thereof will no doubt 21 become apparent to those skilled in the art. It is 22 therefore intended that the following claims be 23 interpreted as covering all such alterations and 24 modifications as fall within the true spirit and scope of the invention.

26 What is claimed is:

Claims (44)

CLAIMS:

1. A modulator for modulating an incident beam of light, comprising:
a plurality of elongated elements, each including a light reflective planar surface, the elements being arranged in first positions parallel to each other and with the light reflective surfaces of the elements lying in one or more parallel planes;

means for supporting the elements in relation to one another; and means for moving a first set of the elements into second positions parallel to a second set of the elements, and between a first modulator configuration wherein said first and second sets act to reflect the incident beam of light as a plane mirror, and a second modulator configuration wherein said first and second sets diffract the incident beam of light as it is reflected from the surfaces of the elements.

2. A modulator as recited in claim 1, wherein the reflective surfaces of adjacent elements lie within planes separated by a distance equal to m/4 times a particular wavelength of the incident beam of light, wherein m is an even whole number or zero when the elements are in the first configuration and m is an odd whole number when the elements are in the second configuration.

3. A modulator as recited in claim 2, wherein the means for moving the elements operates to rotate both the first set of elements and the second set of elements about axes extending generally parallel to their longitudinal dimensions when moving them relative to one another.

4. A modulator as recited in claim 2, wherein alternate elements are fixed relative to the supporting means.

5. A modulator as recited in claim 1, wherein the means for moving the first set of elements comprises means for applying an electrostatic force to the first set of elements.

6. A modulator as recited in claim 5, wherein the reflective surfaces are formed by metallic layers deposited on the upper surfaces of said elements.

7. A modulator as recited in claim 6, wherein the means for applying electrostatic force includes a voltage supply coupled to the metallic layers.

8. A modulator as recited in claim 2, wherein the reflective surfaces of the plurality of elements are equal in dimensions and are substantially rectangular in plan.

9. A modulator as recited in claim 3, wherein the elements are resilient.

10. A modulator for modulating a beam of incident light, comprising;
means forming a first planar light reflective surface;
a grating having means forming a plurality of second planar light reflective surfaces, the grating being arranged with its second reflective surfaces lying in a plane parallel to and spaced from the first planar reflective surface in a direction normal thereto; and means for moving the grating in a direction normal to said first planar reflective surface and into a position closer thereto while maintaining the second reflective surfaces of the grating substantially parallel to the first planar reflective surface;
whereby, when the spacing between the respective first and second light reflective surfaces is equal to m/4 times a particular wavelength of the incident light and m is an even whole number or zero, the modulator acts to reflect the incident light as a plane mirror, and when m is an odd whole number the modulator diffracts the incident light as it reflects it, thereby providing modulation of the beam.

11. A modulator as recited in claim 10, wherein the grating is comprised of a plurality of equally sized and equally spaced apart elongated rectangular elements disposed parallel to each other.

12. A modulator as recited in claim 11, wherein the spacing between each of the elements is substantially equal to the transverse width of each of the elements.

13. A modulator as recited in claim 12, wherein the spacing between the first planar reflective surface and the second planar reflective surfaces of the unmoved grating is equal to one-half the wavelength of the beam of incident light.

14. A modulator as recited in claim 13, wherein the means for moving the grating towards the first planar reflective surface and into the position closer thereto comprises means for applying an electrostatic force between the 5 first planar reflective surface and the second planar reflective surfaces of the grating.

15. A modulator as recited in claim 14, wherein the thickness of each element is equal to one-half the wavelength of the beam of incident light.

16. A modulator as recited in claim 11 wherein each said element of the grating is comprised of a deformable resilient material.

17. A method of modulating a beam of light, comprising the steps of:

causing the beam to impinge upon a plurality of equally spaced apart elements, each including an elongated, light reflective planar surface, the elements being arranged parallel to each other and with the light reflective surfaces of the elements being parallel to each other; and moving some of the elements relative to others between a first configuration wherein the incident beam of light is reflected as by a plane mirror, and a second configuration wherein the incident beam of light is diffracted as it is reflected from the elements.

18. A method as recited in claim 17, wherein in the second configuration the said some of the elements are moved to positions wherein the spacing between the planes of the reflective surfaces of adjacent elements is equal to m/4 times a particular wavelength of the incident beam of light, wherein m is an even whole number or zero when the elements are in the first configuration, and m is an odd whole number when the elements are in the second configuration.

19. A method as recited in claim 18, wherein the thickness of each element is equal to one-half the wavelength of the beam of incident light.

20. A method as recited in claim 19, wherein the said some of the elements are caused to move relative to the other elements by applying electrostatic forces to said some of the elements.

21. A modulator for modulating an incident beam of light, comprising:
a plurality of equally spaced apart elements, each including a light reflective planar surface, the elements being arranged parallel to each other and with the light reflective surfaces of the elements lying in at least one first plane;
means for supporting the elements in relation to one another; and means for moving alternate ones of the elements relative to the other elements and between a first configuration wherein all elements lie in said first plane(s) and the modulator acts to reflect the incident beam of light as a plane mirror, and a second configuration wherein alternate elements lie in at least one second plane parallel to the first plane(s) and the modulator diffracts the incident beam of light as it is reflected from the planar surfaces of the elements.

22. A modulator as recited in claim 21, wherein said means for moving includes means for selectively: applying electro-static forces to said alternate ones of said elements.

23. A modulator as recited in claim 22, wherein the reflective surfaces of adjacent elements lie within planes separated by a distance equal to m/4 times a particular wavelength of the incident beam of light, wherein m is an even whole number or zero when the elements are in the first configuration and m is an odd whole number when the elements are in the second configuration.

24. A modulator as recited in claim 23, wherein the means for moving the elements operates to rotate both the first set of elements and the second set of elements about axes extending generally parallel to their longitudinal dimensions when moving them relative to one another.

25. A modulator as recited in claim 24, wherein said other elements are fixed relative to the support means.

26. A modulator as recited in claim 21, wherein the means for moving said first set of elements comprises means for applying an electrostatic force thereto.

27. A modulator as recited in claim 26 wherein the first set of elements are formed of a deformable resilient material.

28. A modulator as recited in claim 26 wherein said first set of elements has a hysteretic characteristic such that following application of a first electrostatic force thereto to move it into said second configuration, a second electrostatic force of lesser magnitude than that of said first electrostatic force can be used to hold said first set of elements in said second configuration.

29. A modulator as recited in claim 14 wherein said grating has a hysteretic characteristic such that following application of said electrostatic force said 4 grating can be held in the closer position by a reduced electrostatic force.

30. A modulator as recited in claim 22 wherein following application of a first electrostatic force to said alternate ones of said elements, application of a second electrostatic force less than said first electrostatic force can be used to hold said alternate ones in said second configuration.

31. A modulator as recited in claim 10 wherein said means for moving includes means for selectively applying particular electrostatic forces to discrete groups of said means forming said second reflective surfaces.

32. A modulator as recited in claim 10 wherein the grating is comprised of a plurality of equally sized rectangular elements configured such that an incident beam of light directed normal to said light reflective surfaces is reflected back along the incident ray path when said first set of elements are in said first configuration, and the incident beam is angularly diffracted relative to said incident ray path when said first set of elements are in said second configuration, the angularly diffracted beam lying in a plane including said incident ray path and transversely intersecting said elements.

33. A modulator as recited in claim 1 and further comprising means for moving a third set of the elements relative to a fourth set of the elements and between a third modulator configuration wherein said third and fourth sets act to reflect the incident beam of light as a plane mirror, and a fourth modulator configuration wherein said third and fourth sets diffract the incident beam of light as it is reflected from the surfaces thereof, wherein said means for moving includes means for selectively applying electrostatic forces to particular groups of said first set of elements.

34. A modulator as recited in claim 10 wherein an incident beam of light directed normal to said light reflective surfaces is reflected back along the incident beam path when said grating is in one position, and said incident beam of light is angularly diffracted relative to said incident beam path when said grating is moved to another position.

35. A modulator as recited in claim 21 wherein said means for moving includes means for selectively moving particular groups of said alternate ones of said beam elements such that a detector placed in the path of the diffracted beam will detect a beam intensity proportional to certain characteristics of the particular groups moved.

36. A modulator as recited in claim 21 wherein an incident beam of light directed normal to said light reflective surfaces is reflected back along the incident beam path when said alternate ones of said elements are in said first configuration, and said incident beam is diffracted at an angle to said incident beam path when said alternate ones of said elements are in said second configuration.

37. In a light modulating device including an array of individual light modulators for modulating an incident beam of light to generate an image, each pixel of which has a luminous characteristic corresponding to a physical characteristic of a corresponding modulator in the array, an improved modulator comprising:
a plurality of elongated elements, each including a light reflective planar surface, the elements being arranged parallel to each other and with the light reflective surfaces of the elements lying in one or more parallel planes;
means for supporting the elements in relation to one another; and means for moving a first set of the elements in a direction normal to said planes and relative to a second set of the elements and between a first modulator configuration wherein said first and second sets act to reflect the incident beam of light as a plane mirror, and a second modulator configuration wherein said first and second sets diffract the incident beam of light as it is reflected from the surfaces of the elements.

38. In a light modulating device including an array of individual light modulators for modulating an incident beam of light to generate an image, each pixel of which has a luminous characteristic corresponding to a physical characteristic of a corresponding modulator in the array, an improved modulator comprising:
means forming a first planar light reflective surface;

a grating having means forming a plurality of second planar light reflective surfaces, the grating being arranged with its second reflective surfaces lying in a plane parallel to and spaced from the first planar reflective surface in a direction normal thereto; and means for moving the grating in a direction normal to said first planar reflective surface while maintaining the second reflective surfaces of the grating substantially parallel to the first planar reflective surface;
whereby, when the spacing between the respective first and second light reflective surfaces is equal to m/4 times a particular wavelength of the incident light and m is an even whole number or zero, the modulator acts to reflect the particular wavelength of the incident light as a plane mirror, and when m is an odd whole number the modulator diffracts the particular wavelength of the beam of incident light as it reflects it, thereby providing modulation of the beam.

39. In a light modulating device including an array of individual light modulators for modulating an incident beam of light to generate an image, each pixel of which has a luminous characteristic corresponding to a physical characteristic of a corresponding modulator in the array, an improved modulator comprising:
a plurality of equally spaced apart elements, each including a light reflective planar surface, the elements being arranged parallel to each other and with the light reflective surfaces of the elements lying in a first plane;
means for supporting the elements in relation to one another; and means for moving alternate ones of the elements relative to the other elements and between a first configuration wherein all elements lie in the first plane and the modulator acts to reflect the incident beam of light as a plane mirror, and a second configuration wherein alternate elements lie in a second plane parallel to the first plane and the modulator diffracts the incident beam of light as it is reflected from the planar surfaces of the elements.

40. In a light modulating device including an array of individual light modulators for modulating an incident beam of light to generate an image, each pixel of which has a chromatic characteristic corresponding to a physical characteristic of a corresponding modulator in the array, an improved modulator including at least three modulation components each of which is comprised of:
a plurality of elongated elements, each including a light reflective planar surface, the elements being arranged parallel to each other and with the light reflective surfaces of the elements lying in one or more parallel planes;
means for supporting the elements in relation to one another; and means for moving a first set of the elements in a direction normal to said planes and relative to a second set of the elements and between a first modulator configuration wherein said first and second sets act to reflect the incident beam of light as a, plane mirror, and a second modulator configuration wherein said first and second sets diffract the incident beam of light as it is reflected from the surfaces of the several elements, each said modulation component being designed to diffract a different color into an optical system when illuminated with a beam of white light.

41. In a light modulating device including an array of individual light modulators for modulating an incident beam of light to generate an image, each pixel of which has a chromatic characteristic corresponding to a physical characteristic of a corresponding modulator in the array, an improved modulator including at least three modulation components each of which is comprised of:

means forming a first planar light reflective surface;
a grating having means forming a plurality of second planar light reflective surfaces, the grating being arranged with its second reflective surfaces lying in a plane parallel to and spaced from the first planar reflective surface in a direction normal thereto; and means for moving the grating in a direction normal to said first planar reflective surface while maintaining the second reflective surfaces of the grating substantially parallel to the first planar reflective surface;
whereby, when the spacing between the respective first and second light reflective surfaces is equal to m/4 times a particular wavelength of the incident light and m is an even whole number or zero, the modulator acts to reflect the particular wavelength of the incident light as a plane mirror, and when m is an odd whole number the modulator diffracts the particular wavelength of the beam of incident light as it reflects it, thereby providing modulation of the beam, each said modulation component being designed to diffract a different color into an optical system when illuminated with a beam of white light.

42. In a light modulating device including an array of individual light modulators for modulating an incident beam of light to generate an image, each pixel of which has a chromatic characteristic corresponding to a physical characteristic of a corresponding modulator in the array, an improved modulator including at least three modulation components each of which is comprised of:
a plurality of equally spaced apart elements, each including a light reflective planar surface, the elements being arranged parallel to each other and with the light reflective surfaces of the elements lying in a first plane;
means for supporting the elements in relation to one another; and means for moving alternate ones of the elements relative to the other elements and between a first configuration wherein all elements lie in the first plane and the modulator act to reflect the incident beam of light as a plane mirror, and a second configuration wherein alternate elements lie in a second plane parallel to the first plane and the modulator diffracts the incident beam of light as it is reflected from the planar surfaces of the elements, each said modulation component being designed to diffract a different color into an optical system when illuminated with a beam of white light.

43. A modulator for modulating a beam of incident light, the modulator comprising:
a substrate;
a planar light reflective surface formed on the substrate;
a deformable grating having a planar light reflective surface, the deformable grating being arranged with its reflective surface being parallel to and spaced from the planar reflective surface on the substrate;
means for moving the grating towards the substrate while at the same time maintaining the reflective surface of the grating substantially parallel to the planar reflective surface on the substrate; and means for preventing the grating from sticking to the substrate when the grating is moved towards the substrate;
whereby, when the perpendicular spacing between the respective light reflective surfaces is equal to m/4 times the wavelength of the incident light and m is an even whole number or zero the modulator acts to reflect the incident light as a plane mirror and when m is an odd whole number the modulator diffracts the incident light as it reflects it, thereby providing the modulation of the beam of light.

44. A modulator as recited in claim 43, wherein the grating comprises a plurality of equally sized and equally spaced apart parallel rectangular grating elements having an underside, and wherein the means for preventing the grating elements from sticking to the substrate includes ridges positioned between the underside of the grating elements and the substrate.