The present invention relates generally to optical limiting devices and more particularly to a solid-state optical limiting device and to a method for preparing the device.
Optical sensors are important, light sensitive devices that are used in light detection. They can, however, be damaged if they are exposed to a high enough intensity of light. Dynamic and passive forms of optical sensor protection are known. One form of sensor protection may include, for example, special glasses that a welder uses to protect the welder""s eyes from the high intensity light generated by the welder""s arc. Another form of sensor protection may include an optical sensor that can sense high intensity light and communicate electronically to an actuator to block at least some of the light, thereby reducing its intensity and minimizing damage to the sensor.
Optical limiting devices placed between the light source and the sensor can provide the sensor with protection from high intensity light. These protective devices absorb or scatter high intensity light while transmitting low intensity light from the light source to the sensor. Since no communication from the sensor or from any other device is required for optical limiting devices to become active, they provide a type of xe2x80x9csmartxe2x80x9d protection; they are always active, and they respond almost instantly to high intensity light. The increased speed is particularly important for applications where the sensor may be exposed to sudden bursts of high intensity light. Optical limiting devices are used, for example, to protect optical sensors in pulsed laser systems.
Optical limiting devices have been recently reviewed by Lee W. Tutt, et. al. in xe2x80x9cA Review of Optical Limiting Mechanisms and Devices Using Organics, Fullerenes, Semiconductors, and Other Materials, Prog. Quant. Electr, 1993, vol. 17, pp. 299-338, hereby incorporated by reference. A subset of devices described therein employ optically sensitive materials that exhibit an accumulative optical absorption effect known as xe2x80x9creverse saturable absorptionxe2x80x9d (RSA). The RSA effect, first reported by C. R. Guiliano and L. D. Hess in IEEE Journal of Quantum Electronics, 1967, vol. 3, p. 358, was discovered when various dye molecules did not bleach, i.e. become more transparent, when exposed to high intensity light but instead became less transparent.
The RSA effect for dye molecules arises when the excited state absorption cross-sections for the molecules are larger than the ground state absorption cross-sections. As these types of molecules absorb light, their first excited state populations increase and contribute to the total absorption cross-section. Since the first excited state cross-section is larger than the ground state cross-section for an RSA absorber, the total absorption increases as the excited state becomes more populated and the molecules become less transparent as they absorb light. In contrast, xe2x80x9csaturable absorbersxe2x80x9d are molecules having a first excited state cross-section that is smaller than the ground state cross-section. When these types of molecules absorb light, they bleach, i.e. they become more transparent to light.
A wide variety of materials potentially usable as optical limiting materials have been tested. These materials include transition metal cluster compounds, such as iron carbonyl cluster compounds reported in U.S. Pat. No. 5,283,697 to L. W. Tutt et al., which were blended into a polymer to provide an optical limiting device. They also include fullerenes, described in U.S. Pat. No. 5,391,329 to T. K. Dougherty et al. in xe2x80x9cProcess for Making a Solid Optical Limiter Containing a Graded Distribution of Reverse Saturable Materialxe2x80x9d, and in U.S. Pat. No. 5,741,442 to D. W. McBranch et al. entitled xe2x80x9cOptical Limiting Materialsxe2x80x9d. The strongest RSA effects to date have been observed for metalloporphyrin and metallophthalocyanine complexes, e.g. lead tetrakis(4-cumylphenoxy) phthalocyanine. For example, see J. S. Shirk et al., xe2x80x9cOptical Limiter Using a Lead Phthalocyaninexe2x80x9d, Appl. Phys. Lett. (63)14, 1993, pp. 1880-1882; and U.S. Pat. No. 5,805,326 to A. W. Snow et al. entitled xe2x80x9cOptical Limiter Structure and Methodxe2x80x9d. Some of these complexes were dissolved in (poly(methylmethacrylate)) (PMMA) and in polystyrene to provide xe2x80x9coptical limiter structuresxe2x80x9d. While these structures can be used as high intensity light attenuators, they are also subject to damage from high intensity light.
Thus, there remains a need for optical limiting devices with superior properties. Therefore, an object of the present invention is to provide a method for preparing optical limiting devices that exhibit a strong RSA effect and less subject to damage from high intensity light than known devices.
Yet another object of the invention is to provide an optical limiting device with a strong RSA effect and less subject to damage from high intensity light than known devices.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
In accordance with the purposes of the present invention, as embodied and broadly described herein, the present invention includes a method of preparing an optical limiting device, comprising the steps of dissolving an RSA material having reverse saturable absorption properties in a first solvent to produce a first solution; dissolving polyvinylbutyral into a second solvent to produce a second solution; mixing the solutions together to produce a third solution and stirring the third solution to remove air bubbles therefrom; layering the third solution onto one side of a transparent substrate; and evaporating the solvent from the third solution to produce a solid layer of polyvinylbutyral/RSA material that is adhered to the substrate and thereby form the optical limiting device.
The invention also includes a method of forming an optical limiting device, comprising the steps of dissolving an RSA material having reversible saturable absorption properties in a first solvent to produce a first solution; dissolving polyvinylbutyral into a second solvent to produce a second solution; mixing the solutions together to produce a third solution and stirring the third solution to remove air bubbles therefrom; layering a portion of the third solution onto one side of a transparent substrate; evaporating the evaporating the solvent from the third solution produce a first solid layer of polymer/material that is adhered to the substrate; layering another portion of the third solution onto another side of the substrate plate; and evaporating the solvent from-other portion to produce a second solid layer of polymer/RSA material having reversible saturable absorption properties that is adhered to the substrate and thereby form the optical limiting device.
The invention also includes an optical-limiting device produced by the method of preparing an optical limiting device, comprising the steps of dissolving an RSA material having reverse saturable absorption properties in a first solvent to produce a first solution; dissolving polyvinylbutyral into a second solvent to produce a second solution; mixing the solutions together to produce a third solution and stirring the third solution to remove air bubbles therefrom; layering the third solution onto one side of a transparent substrate; and evaporating the solvent from the third solution to produce a solid layer of polyvinylbutyral/RSA material that is adhered to the substrate and thereby form the optical limiting device.
The invention also includes optical limiting device comprising a transparent substrate and a layer comprising a homogeneous mixture of an RSA material and polyvinylbutyral, the layer being attached to the substrate.
The invention also includes an optical limiting device comprising a transparent substrate comprising a first substrate surface and a second substrate surface; a first layer comprising a homogeneous mixture of an RSA material and polyvinylbutyral, the.first layer being attached to the first substrate surface; and a second layer comprising a second homogeneous mixture of a second an RSA material and polyvinylbutyral, the second layer being attached to the second substrate surface.