Optical: systems and elements – Optical modulator – Light wave directional modulation
Reexamination Certificate
2001-04-13
2003-04-22
Epps, Georgia (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave directional modulation
C359S291000, C359S247000
Reexamination Certificate
active
06552842
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to optical devices, and more particularly to spatial light modulators.
BACKGROUND OF THE INVENTION
Spatial light modulators (SLMs) are currently used in a variety of optical systems. They can be used for controlling (modulating) the intensity of an incident optical signal. SLMs operate as either as transmissive or reflective devices. A flat panel laptop computer screen is an example of a two-dimensional array of transmissive SLMs. Most SLMs have relatively slow switching speeds. For example, SLMs made with liquid crystals generally have speeds from a hundred hertz to a few kilohertz. The speed of liquid crystal SLMs are limited by the time required to align (transmissive) or twist (reflective) liquid crystals under the influence of an applied electrical field.
Acoustic-optic modulators (AOMs) use acoustical standing waves to modulate light and can have switching speeds of up to a few megahertz, but these devices are also bulky. Relatively new micro electromechanical structure (MEMS) micro-mirror SLM switches can operate in the kilohertz to megahertz range, but are usually bulky as well.
Small optical structures have been developed which rely interactions between a periodic medium which has a periodicity comparable to (or smaller than) the wavelength of electromagnetic waves traveling therethrough. In this case, the medium begins to significantly inhibit the wave's propagation. Sub-wavelength grating structures (SWS) are a type of optical structure that rely on this sub-wavelength effect.
An example of the formation and several applications for a SWS is described in U.S. Pat. No. 6,035,089, to Grann, et. al. (“Grann”), which is assigned to Lockheed Energy Research Corporation, predecessor to the assignee of this application. The entire contents of U.S. Pat. No. 6,035,089 is hereby incorporated by reference. Grann describes a subwavelength resonant grating structure (SWS) formed from a single subwavelength resonant grating layer (SWL), which functions as a zeroth order diffraction grating. Grann's SWS uses periodically spaced high refractive index “posts” embedded in a lower refractive index dielectric waveguide material to form an extremely narrowband resonant reflector that occupies a very small area.
A SWS which functions as a zeroth order diffraction grating can be represented by an effectively uniform homogeneous material having an effective refractive index (neff). Under particular incident wave configurations, such as a substantially normal incident beam, and certain structural constraints, such as the refractive index of the medium surrounding the SWS<refractive index of the waveguide<refractive index of the posts, a SWS may exhibit a resonance anomaly which results in a strong reflected beam which is limited to an extremely narrow bandwidth. If the incident radiation is not within the SWS resonant bandwidth, most of the energy of the incident beam will propagate through the grating in the form of a transmitted beam.
This resonance phenomenon occurs when electromagnetic radiation is trapped within the grating material due to total internal reflection. If this trapped radiation is coupled into the resonant mode of the SWS grating, the field will resonate and redirect substantially all of the electromagnetic energy backwards. This resonance effect results in a nearly total reflection of the incident field from the surface, which may be designed to be extremely sensitive to wavelength.
If SLMs could be configured using SWS, improved SLM performance could result. This could benefit existing SLM applications and provide new applications for SLMs that were never before possible.
SUMMARY OF THE INVENTION
A reflective coherent spatial light modulator (RCSLM) includes a subwavelength resonant grating structure (SWS). The SWS is formed from at least one subwavelength resonant grating layer (SWL), the SWLs having a plurality of areas defining a plurality of pixels. Each pixel represents an area capable of individual control of its reflective response. The pixels are adapted to produce a resonant reflective response characterized by reflecting a band of incident light while transmitting light outside the reflective band. A structure for modulating the resonant reflective response of at least one pixel is also provided.
The structure for modulating can include at least one electro-optic layer, the electro-optic layer disposed in optical contact with at least one SWL. In this embodiment, the RCSLM can include a structure for modulating the refractive index of the electro-optic layer.
The RCSLM can include a first electrically conductive layer and a second electrically conductive layer, the electro-optic layer positioned substantially between the electrically conductive layers. At least one of the electrically conductive layers can include a plurality of plates, the plurality of plates substantially electrically isolated from one another and adapted to control the resonant reflective response of individual ones of the plurality of pixels.
The RCSLM can be formed on a die and can have electronic components provided thereon, including control electronics and a high speed voltage driver circuit, to permit the plurality of pixels to be separately switchable. Switching can be accomplished by application of a voltage signal generated by the high speed voltage driver circuit to the plurality of plates. The plurality of plates can be adapted for connection to the electronic components through back plane via connections. The RCSLM can include an anti-reflective coating layer disposed on the surface of the RCSLM.
In the embodiment having electrically conductive electrodes and at least one electro-optic layer therebetween, an anti-reflective coating layer can be disposed on the surface of the RCSLM and a buffer layer can be disposed between the anti-reflective coating layer and the second conductive layer. The buffer layer can be selected from air, SiO
2
, TiO
2
, and WO.
SWLs can include a plurality of features, at least a portion of the features having asymmetric exposed surfaces. In this embodiment, The RCSLM can include an electronically controllable quarter wave plate.
In the embodiment having electronic components formed on die, the electronic components can also include an amplitude controller for controlling the amplitude of light reflected from the plurality of pixels. The amplitude controller can include a feedback and control system. The feedback and control system can be adapted to separately control the amplitudes of light reflected by the pixels.
A method for forming a reflective coherent spatial light modulator (RCSLM), includes the steps of selecting a waveguide material having a first refractive index, and forming at least one subwavelength resonant grating structure (SWS), the SWS formed from at least one subwavelength resonant grating layer (SWL) in the waveguide material. The SWLs each have a plurality of areas defining a plurality of pixels, the plurality of pixels adapted to produce a resonant reflective response characterized by reflecting a band of incident light while transmitting light outside the reflecting band. The SWL can include a plurality of features formed from at least one material having a second refractive index greater than the first refractive index of the waveguide material.
The method can include the step of providing at least one electro-optic layer, the electro-optic layer disposed in optical contact with at least one SWL. The method can include a first electrically conductive layer and a second electrically conductive layer, the at least one electro-optic layer positioned substantially between the electrically conductive layers. At least one of the electrically conductive layers can include a plurality of electrically conductive plates, the plates substantially electrically isolated from one another and adapted to control the resonant reflective response of individual pixels.
The method can include the step of providing a bulk substrate material having a plurality of die, wherein a
Hutchinson Donald P.
Richards Roger K.
Simpson John T.
Simpson Marcus L.
Abutayeh M.
Akerman & Senterfitt
Epps Georgia
UT-Battelle LLC
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