Gradient Index of Refraction for Missile Seekers

Gradient index glass has been used to make the "Woods's Lens". In the prior art, the index of refraction of a flat piece of glass is increased toward the center, in such a way that the flat glass focuses the light. In this application, a reversed index is used to make a curved ogive have the optical properties.

Current laser designator weapon systems are required to use hemispherical shaped windows on the seeker to obtain the necessary optical quality for guidance. The aerodynamic performance of the missile is reduced because the hemispherical shape introduces a large drag coefficient.

The seeker window of the present invention is constructed with an index of refraction which varies as a function of position off the axis of the cylindrical missile.

The window as set forth herein reduces the drag coefficient on laser designator weapons and increases their effective range without a reduction in the optical performance.

A missile having a sensor element in the nose portion for receiving electromagnetic radiation. The radiation is emitted from the target and the missile homes in on the radiation to impact with the target. An ogive shaped transparent window encloses the sensor. The window is found with a non-uniform index of refraction that varies at different positions on the window so that the ogive window appears to the seeker as a hemispherical window.

FIG. 1 is an elevational diagrammatic view illustrating the missile nose enclosing the sensor.

FIG. 2 is a view similar to FIG. 1 illustrating the effect of two rays striking the ogive window.

As shown in FIG. 1, a missile 10 includes a transparent nose 12 which defines a window for enclosing a sensor 13 mounted in gimballed relation along the missile axis 14. A ray of light 16 is illustrated as passing through the window to strike sensor 13. The light is received from a target (not shown) which has been illustrated by a laser designator.

As seen in FIG. 2, ray B strikes at a more oblique angle .theta., than ray A, at angle .phi.. If the surface has uniform thickness, ray B is delayed more than ray A, so the wavefronts are bent or distorted. If the material has a varying index of refraction n, so that n.sub.a is greater than n.sub.b to the extent that each ray is delayed in time the same amount, then the wavefront is not distorted (Fermat's principle). In practice the optical design will consider both the shape of the inside and outside surface, the thickness as a function of location, and the index of refraction gradient to trade-off distortion and field of view.

The ogive shaped sensor window is constructed in accordance with the required aerodynamic performance of the missile. The index of refraction of the sensor window is not uniform over the window but rather assumes different values at different positions on the window. The gradient of the index of refraction (i.e., the change in index of refraction from point to point) will be determined by both the geometrical shape of the window and the required optical performance of the window. For example, current missile systems use windows with rotational symmetry, thus, the gradient required will also have rotational symmetry. As an improvement to current missile systems, the optical performance of the window must be such that it appears to the seeker as if it were a hemispherical window.

A gradient index may be formulated in glass by heating the glass in contact with a salt, so that an ion-exchange diffusion takes place. The biggest change takes place close to the surface, so a gradient of the index created. A gradient index may be introduced in a plastic by photopolymerization. A plastic (poly-methyl-methacrylate) can be sensitized with a dye. Exposure to light then effects the polymeric bonds, changing the size of the molecules of polymer, and thus the index of refraction.

Gradient indexes can also be created by neutron irradiation, chemical vapor deposition, and ion implantation.

The gradient index window is designed to replace several correcting elements needed in the current design of a laser designator. This results in a weight savings as well as providing improved performance. The simplest implementation is to design the gradient index window to provide optical performance equivalent to a hemispherical window. To use the device, the conventional hemispherical window would be removed and the ogive shaped gradient index window would be installed.