Electro-Optical Light Modulator for Protection of Optical Systems Against Pulsed Lasers

FIGS. 1A and 1B are exemplary protective electro-optic shutter devices for monocular and binocular systems, respectively.

FIG. 2 schematically illustrates structure of an electro-optic shutter.

FIG. 3 illustrates a block diagram of a circuit to control the alternation of clear and dark states of a shutter to protect a monocular system.

FIG. 4 is a graphic illustration of alternation between clear and dark states of a monocular electro-optic shutter device.

FIG. 5 is a graphic illustration of alternation of clear and dark states of the two shutters of a binocular electro-optic device.

FIG. 6 illustrates a circuit including a delay circuit for shutters to protect a binocular system.

FIG. 7A illustrates the operation of the shutter during a dark state.

FIG. 7B illustrates the operation of the shutter during a clear state.

Referring now to the drawings wherein like numbers refer to like parts, FIGS. 1A and 1B depict typical shapes of monocular and binocular embodiments, respectively, of the invention. Electro-optic shutters 2,3, and 4, each is positioned between the optical system to be protected and the oncoming threat laser beam. Housing 8 of FIG. 1A and earpieces 10 and 11 of FIG. 1B contain the circuits which, by applying electric field at set time intervals to the shutters, control the alternation of the clear and dark states of each shutter. In a binocular shutter device, the housing also contains circuitry to control staggering of the clear-dark cycles of the shutters, so that the two shutters do not become clear simultaneously. This affords a binocular system, such as a pair of human eyes, an added degree of protection since at any given moment, at least one eye is protected from laser pulses. In FIG. 1A, strap 6 is slipped over the optical system to be protected and holds the electro-optic shutter in place. In FIG. 1B, the earpieces 10 and 11, in addition to housing the control circuitry, are perched over the ears to hold the shutters securely before the eyes.

As can be seen in FIG. 2, shutters 2, 3 and 4 are each made of two light polarizers 24 and 26 and a liquid crystal layer 29 located therebetween. The plane of polarization of one of the polarizers is at 90 degrees to that of the other polarizer. Liquid crystal layer 29 in the middle is further made up of panels 30A and 30B of transparent electrically conductive material and a liquid crystal material 28 sandwiched between them. Panels 30A and 30B are required to be transparent to admit light and to be electrically conductive to respond to application of electric field by the control circuits. The liquid crystal layer may also be made of a porous block or a matrix of transparent electrically conductive material and droplets of liquid crystal material filling the pores or distributed throughout the matrix, respectively. Whatever the particular structure of the liquid crystal layer is the layer is selectively between 5 .mu.m and 1 mm to provide a requisite thickness to cause polarization rotation by 90 degrees of radiation passing through it, when electric field is absent.

The circuit to control the monocular electro-optic shutter device is illustrated in FIG. 3. A power source 15 such as a battery drives a clock 16 which generates pulses at a preset frequency selectively between 30 Hz and 100 Hz. These pulses are fed into a pulse stretcher 18 comprised of a monostable multivibrator 19 and a darkness control mechanism 20. The pulses are then stretched into the series of square pulses 17 as illustrated. The stretched square pulses 17 are fed into the shutter containing the liquid crystal layer to cause the shutter to alternate between the clear and dark states. The series of square pulses 17 is shown in an enlarged mode in FIG. 4. When electric field from pulses 17 is applied across panels 30A and 30B, the shutter becomes dark and blocks radiation from its path to the optical system. As can be seen in FIG. 4, the shutter is in the dark state most of the time, so that for any particular laser pulse which may be incident on the optical system, the probability of transmission of that pulse is low and substantial protection is provided to the optical system. In a binocular device, as illustrated in FIG. 5, the clear-dark cycles are staggered between the two shutters so that even when a laser pulse such as d arrives at a time when one shutter is clear, one eye is protected since the other shutter shielding this eye is in a dark state blocking the laser pulse transmission. The circuitry for a binocular device with staggering is shown in FIG. 6 where delay circuit 22 delays the entrance of clock-produced pulses into pulse stretcher 27 so that one series of stretched square pulses 40 reach shutter 21A before the other series of stretched square pulses 42 reach 21B. Pulse stretcher 25 functions in the same way as pulse stretcher 18 in FIG. 3. Such alternation in the square pulses results in staggering of clear-dark cycles in the two shutters.

FIG. 7A shows how the shutter remains dark in the presence of electric field. For purposes of explanation, it is assumed that light travels from left to right and that polarizer 24 is a vertical polarizer whereas polarizer 26 is a horizontal polarizer. However, the position of the polarizers may be reversed as long as the plane of polarization of one polarizer is 90 degrees from that of the other polarizer. Light in a random polarization state enters the vertical polarizer which lets through only the vertically polarized portion of the light. This vertically polarized light portion next enters the liquid crystal layer 28. When electric field is present, the molecules of the liquid crystal material are horizontally aligned and have no effect on the plane of polarization of incident light, i.e. the molecules cause no polarization rotation to the light passing through. Therefore, the vertically polarized light portion passes through the liquid crystal layer with no change wrought on it. Then the light comes to the horizontal polarizer. Since the vertically polarized light portion has no horizontal component, no light passes through the horizontal polarizer and thus the shutter stays dark. In FIG. 7B, the electric field is absent from liquid crystal layer 28. In the absence of the electric field, the molecules of the liquid crystal material are in a random state and some have no effect on the incident light while others rotate the plane of polarization of the light so that the light leaving the liquid crystal layer is something other than completely vertically polarized light, i.e. the light leaving the liquid crystal layer has a horizontal component. This horizontal component passes through the horizontal polarizer and thus the shutter becomes clear.

Although a particular embodiment and form of this invention has been illustrated, it is apparent that various modifications and embodiments of the invention may be made by those skilled in the art without departing from the scope and spirit of the foregoing disclosure. Accordingly, the scope of the invention should be limited only by the claims appended hereto.