1. Field of the Invention
The present invention relates to a technique for forming devices and other articles in optical material wherein radiant electromagnetic energy removes selected regions of the material.
2. Description of the Prior Art
Lithium niobate and other birefringent crystals have been investigated for use in a wide range of optical systems. Recently, it has been proposed to use such crystals in the fabrication of electro-optic guided wave devices. These devices are used to perform various functions on optical energy, such as switching, polarizing, combining, separating, etc. This optical energy typically carries information transmitted over optical fibers. However, the chemically inert nature of most of the birefringent crystal materials has been an obstacle in attempting to generate a simple etchant. In particular, lithium niobate and other materials are highly resistant to etching in chemical systems and plasma systems, including reactive ion etching systems. The removal of material (i.e., etching), is an important operation for the production of many devices in optical materials. For example, a known type of optical switch utilizes a groove between parallel optical paths; see for example, "Efficient LiNbO.sub.3 Balanced Bridge Modulator/Switch With an Ion-Etched Slot" by M. Minakata, Applied Physics Letters, Vol. 35, pp. 40-42 (1979). As is noted therein, an ion beam can be used to etch material from the optical substrate. However, an ion beam system is not always practical in production. This is due in part to the need for processing in a vacuum, and the limited amount of beam energy that can be obtained at reasonable cost in an ion generator. For these and other reasons, it is apparent that it is desirable to have an improved technique for selectively removing material from optical substrates.
The use of laser energy is known for vaporizing various materials. In the case of highly reflective materials (for example, most metals) it is known to coat the material with an absorber, or otherwise modify its surface, to increase the absorbence of laser energy. In this manner, even highly reflective materials can be drilled or machined with laser energy. It is also known to indent the surface of a reflective metal conductor, in order to trap laser energy, and thereby heat bonding material more efficiently; see U.S. Pat. No. 4,320,281. In the field of optical information recording, it is known to coat a relatively transparent optical disc material with a more absorbent layer, in order to promote laser vaporization of pits in the transparent layer. However, this typically relies on the ability of a single laser pulse to effect the desired removal of the transparent material, since the absorbent layer is also removed by the laser pulse.