Spatially Graded Optical Switch

The present invention relates generally to a solid state optical device capable of actively modifying the intensity distribution across the entrance aperture of an electro-optic sensor to reduce saturation of the entire image by a bright point source. The invention may preferably be used in conjunction with a spatial light modulator (which can selectively block a region in an image) placed at an intermediate image plane or with a detector array capable of handling large over saturation without affecting the performance of the surrounding detector elements.

A problem with existing electro-optical systems is that the entrance aperture for collecting light from the scene has a uniform intensity profile. That is, within the entrance aperture transmission into the system is a fixed percentage, t, and outside the aperture the transmission is zero. The edge of the entrance aperture, where transmission goes from zero to t, creates diffraction within the electro-optic system Under normal operating conditions, where the intensity of light from the scene is within an order of magnitude or two, diffraction effects within the system do not cause problems, However, if a brightpoint source, of greater intensity than about three orders of magnitude above detector saturation, is within the system field of view, diffraction of the bright source by the edges of the entrance aperture will cause saturation of the surrounding regions of the image.

The invention substantially reduces effects of a bright source on an electro-optic system having conventional optical switching devices by providing an optical switch comprising a layer of optical switching material with optical thickness which is a preselected function of switch radius. The invention may be switched between a normal highly transparent window and soft aperture with graded attenuation. In the unswitched (OFF) state for the switching material, the switch is a high transmission (or reflection) window (or mirror). In the switched (ON) state, attenuation of transmission (or reflection) varies smoothly radially across the switch width, from substantially zero at the center of the switch to substantially 100% at the outer edge. The thickness profile may be selected according to an attenuation profile required for a specific system.

An optical switch structured according to the invention, being achromatic, protects against all types of high intensity sources. In operation, the switch modifies the transmittance function incident on the entrance aperture of a protected optical system. With the invention in place, a system may operate normally without sacrificing operational throughput. The invention can be retrofit to existing systems.

It is therefore a principal object of the invention to provide an optical switching device.

It is a further object of the invention to provide a spatially graded optical switch.

These and other objects of the invention will become apparent as a detailed description of representative embodiments proceeds.

In accordance with the foregoing principles and objects of the invention, a spatially graded optical switch is described which comprises a transparent substrate, a layer of optically switchable material deposited on the substrate and having optical thickness which is a preselected function of radial distance from the center of the substrate, and electrical or thermal means for switching the material between an OFF state characterized by high transmission (or reflection) and an ON state at which transmission (or reflection) is attenuated smoothly with radius across the switch, according to a preselected profile, from substantially zero at the center to about 100% at the outer edge.

The invention will be more clearly understood from the following detailed description of representative embodiments thereof read in conjunction with the accompanying drawings wherein:

FIGS. 1a and 1b are graphs of relative intensity versus radius of Airy disk intensity profiles generated by a circular aperture in front of an electro-optic system on a log scale for 10 micron (.mu.) radiation at the focal plane of a diffraction limited system with system f-numbers of 2.5 and 5, respectively;

FIG. 2a is a schematic plan view of a representative optical switch embodiment according to the invention;

FIG. 2b is a view of the FIG. 2a switch taken along line B--B;

FIG. 3 is a view (not to scale) of a representative substrate, switch material layer and antireflective coating structure for the FIG. 2a,2b switch; and

FIG. 4 is a graph of optimum switching material thickness versus radial position in a switch of the invention utilizing vanadium dioxide switching material for long wavelength infrared operation and mid-range wavelength infrared operation, overlaid with the transmission profile for the switch in the 3-5.mu. and 8-12.mu. spectral regions.

Referring now to the drawings, FIGS. 1a and 1b show graphically the Airy disk intensity profiles 11,13 on a log scale at the focal plane of a diffraction limited system with a circular aperture and system f-number of 2.5 (FIG 1a) and 5 (FIG 1b) for 10.mu. incident radiation. FIGS. 1a and 1b show that in an f/2.5 optical system, viewing a bright source having brightness 10.sup.4 greater than the detector saturation level, detector pixels up to 250.mu. from the source location would be saturated because of diffraction. At greater source intensities the entire image may be saturated.

In accordance with a governing principle of the invention, diffraction rings in the point spread function at the focal plane can be reduced or eliminated by modifying the transmission function at the entrance aperture of the system. For example, if the entrance aperture has a Gaussian transmission function, then the point spread function at the image plane is also Gaussian. FIGS. 1a,1b show a comparison of the point spread functions generated by a hard circular aperture and an aperture with the transmission which varies radially as a Gaussian function; energy which causes detector saturation away from the central intensity peak can be significantly reduced with a soft aperture.

FIG. 2a shows a plan view of a representative optical switch 20 of the invention. FIG. 2b is a view in axial section of switch 20 taken along line B--B of FIG. 2a. In accordance with a principal feature of the invention, switch 20 comprises a layer of thermochromic or electrochromic material, described more fully below, which transitions (switches) between transmissive and reflective states upon application or removal of sufficient heat or electric field. Switch 20 may therefore comprise a substantially transparent substrate 21 having deposited thereon an active coating 23 having desired switching characteristics, as described more fully below in relation to FIG. 3. Switch 20 defines transmission aperture 24 of size preselected to accommodate a particular system application. Substrate 21 may preferably comprise silica based glasses, plastics and the like for operation in the visible to near infrared spectral range (0.4-1.5.mu.); silicon, germanium, zinc sulfide, zinc selenide, Alon.TM., Spinnel.TM., lanthanum doped yttria, gallium arsenide, diamond or the like for operation in the mid-infrared (1.5-5.mu.); and zinc selenide germanium or diamond for operation in the long infrared (5-15.mu.). Generator means 26 is operatively connected to coating 23 at contacts 27a,b through suitable connecting means 28a,b to provide a source of thermal or electrical energy for active transition control of coating 23 material. Antireflective coatings 30,31 of conventional composition applied conventionally may be included to enhance transmissibility of switch 20.

Referring now to FIG. 3, shown schematically therein is a view, substantially enlarged in the axial direction, of representative structure for substrate 21 and coating 23 of switch 20. According to a principal teaching of the invention, coating 23 comprises a layer 33 of switchable material graded in optical thickness radially outwardly from the center. Antireflective coatings 30 may be applied to the outer surface of layer 33. The specific design of the antireflective layers is dependent on the specific wavelength band and the selected switchable material for layer 33, as might occur to the skilled artisan in the thin film interference field.

Numerous thermochromic and electrochromic materials known in the applicable art having desired switching characteristics and electrical and thermal properties may be used in the structure of switch 20, the specific material selection not being considered limiting of the invention. Suitable thermochromic materials for use in the invention (and characteristic transition temperatures) may include vanadium dioxide (68.degree. C.), V.sub.2 O.sub.3 (.about.-130.degree. C.), Ag.sub.2 S (177.degree. C.), SmS, VO (.about.-250.degree. C.), Ag.sub.2 Se (.about.150.degree. C.), Ag.sub.2 Te (.about.80.degree. C.), Cu.sub.2 S (.about.100.degree. C.) and selected mixtures of the foregoing. Electrochromic materials suitable for use may include nickel hydroxide, tungsten oxide and other dielectric, organic and organometallic thin film materials.

In structures according to the invention which comprise a thermochromic material, generator means 26 may include a suitable heater operatively connected to switch 20 in order to controllably heat switchable layer 33 between transparent and reflective states. For example, the thermochromic materials may be switched by electrically (ohmically) heating the substrate to a temperature above the transition temperature of the switchable material. In structures utilizing electrochromic materials, transparent thin film electrodes (contacts 27a,b) may be incorporated into the layered structure of switch 20 for electrical connection to generator means 26 comprising a generator for applying an electric field across the electrochromic material, in manner known to the skilled artisan, to alter the complex refractive index of the material.

When the material comprising layer 33 switches, the complex portion of the refractive index increases; for VO.sub.2 the increase is from about 0.1 to more than 7. The exact values of the complex refractive index for the switchable materials are strongly dispersive through the infrared. A thickness gradient results in graded transmission or reflection across the aperture when switched. The specific optical thickness gradient must be calculated for each specific switchable material and wavelength region. For example, each 100 nm of VO.sub.2 results in roughly one optical density unit in the 3-5.mu. spectral band and 0.5 optical density unit in the 8-12.mu. spectral band. Deposition methods for thin films with specific thickness gradients are known in the applicable art. FIG. 4 is a graph of optimum switching material thickness versus radial position in a 50 mm diameter switch structured according to the invention utilizing VO.sub.2 switching material for long wavelength infrared operation (graph 41) and mid-range wavelength infrared operation (graph 43), overlaid with the transmission profile (graph 45) for the switch in the middle wavelength infrared and long wavelength infrared spectral regions, respectively. The Gaussian transmission profile 45 is truncated to 4% of the peak transmission at the outer diameter, resulting in 30% transmission in the switched state as compared to a clear aperture. Truncation of the Gaussian transmission profile will introduce some diffraction into the system, but at intensity levels many orders of magnitude lower than a conventional fixed clear aperture.

It is noted that an embodiment of the invention herein may comprise a layered structure including a switchable layer having a radially graded optical thickness profile and which is highly reflective in a normal (OFF) state and absorptive in the switched (ON) state. Under normal operation this embodiment is uniformly reflective and may be switched to a soft aperture by grading the reflectivity to near zero at the outer edge of the device.

It is noted further that a switch according to the invention may be constructed using switchable material selected for passive switch operation, wherein no heat or electric field is applied in operation, but the switchable material transitions from a normal operation substantially transparent state to a switched reflective state upon absorption of heat from the invasive radiation.

The invention therefore provides an improved optical device comprising a spatially graded optical switch having resistance to damage from invasive radiation. It is understood that modifications to the invention may be made within the scope of the appended claims as might occur to one with skill in the field of the invention. All embodiments contemplated hereunder which achieve the objects of the invention have therefore not been shown in complete detail. Other embodiments may be developed without departing from the spirit of the invention or from the scope of the appended claims.