The present invention pertains to methods for producing non-linear optical waveguides and passive waveguides and more particularly pertains to methods for preparing non-linear optical (poled polymer) stripe waveguides and passive stripe waveguides and to the non-linear optical and passive stripe waveguides produced by those methods.
Optical stripe waveguides have been produced by subjecting a planar waveguide to a localized exchange or diffusion process. These "diffuse boundary" methods are commonly used for some semiconductors and ceramic materials such as LiNbO.sub.3 and glass. Optical stripe waveguides can also be produced by patterned etching or by localized growth or deposition processes. These "ridge" type methods are commonly used for semiconductors and for ceramics such as LiNbO.sub.3.
Organic polymers have been used in planar waveguides. Although some shortcomings are presented, such as changes in buffer layer and waveguide indices and permitivities with humidity, temperature and the like, there are a number of advantages in comparison to waveguide materials such as GaAs and LiNbO.sub.3. Organic polymers can show a large electro-optic effect, reduced coupling losses with optical fibers due to differences in refractive indexes, compatibility with thin film processing techniques, relatively low costs, and relative ease in integration.
Organic polymer stripe waveguides are not readily prepared by techniques suitable for other materials. Organic polymers are soluble in the solvents and other materials used in a number of standard processes. Non-organized organic polymers for non-linear waveguides, that is, everything except Langmuir-Blodgett polymer films, must be electrically poled to become ordered and electro-optically active. These organic polymers generally have low glass transition temperatures (T.sub.g) below about 160 degrees C. and after poling must not be heated above T.sub.g.
Organic polymer stripe waveguides have been prepared by some alternative techniques. Krchnavek et al in an article entitled: "Laser Direct Writing of Channel Waveguides Using Spin on, Polymers", in the Journal of Applied Physics 66(11), 1989, pp. 5156-5160, disclose a method in which an organic polymer stripe waveguide is prepared by photon-induced polymerization using a UV curable polymer and a scanned Argon-ion laser. Comparable techniques are described in Rochford, K. B. et al, "Fabrication of integrated optical structures in polydiacetylene films by irreversible photoinduced bleaching", Applied Physics Letters, 55(12), 18 Sep. 1989, pp. 1161-1163; Diemeer, M. B. J. et al, "Photoinduced Channel Waveguide Formation in Nonlinear Optical Polymers", Electronics Letters, 26(6), 15 Mar. 1990, pp. 379-380; Franke, H. et al, "Photo-induced self-condensation, a technique for fabricating organic lightguide structures", SPIE Volume 651 Integrated Optical Circuit Engineering III, 1986, pp. 120-125; Rochford, K. B. et al, "Waveguide channels and gratings in polydiacetylene films using photo-induced bleaching", SPIE Volume 1147 Nonlinear Optical Properties of Organic Materials II, 1989, pp. 279-284. These methods form the stripe waveguide by altering the polymer structure. This is a shortcoming where poled polymers are concerned, since electrically poled organic polymer waveguides have a chemical structure that must be maintained to maintain poling. U.S. Pat. No. 4,824,522 to Baker et al teaches a method for preparing a stripe organic waveguide by photolithography of a resist layer followed by reactive ion etching of an underlying organic polymer waveguide layer. All of these methods for producing stripe waveguides are slow, relatively complex and subject to a variety of technical limitations.
The use of a laser to remove material, that is, laser photoablation, is well known. Photoablation occurs when a material is excited with a high energy laser having a frequency greater than the bandgap of the material. Excited ions of the material are driven off and form by-products in the surrounding medium. Depending upon the wavelength and material, very little of the energy supplied by the laser is converted to heating the material that is not driven off. U.S. Pat. No. 4,414,059 to Blum et al described two different forms of laser photoablation. The first form utilizes relatively low intensity UV light at about 8 milliJoules per square centimeter (mJ/cm.sup.2) and is temperature dependent and not strictly dependent on the wavelength of UV light used. This form of laser photoablation requires oxygen and is relatively slow, particularly for some materials. The second form, referred to as "ablative photodecomposition" in Blum et al, is described as being critically dependent on wavelength and power density, but not dependent on oxygen, temperature, or the nature of the organic material to be ablated.
UV lasers have been used to ablate organic polymers. Blum et al (cited above) and U.S. Pat. No. 4,617,085 to Cole, Jr. et al, teach the use of a laser to photoablate a pattern in a film of organic resist materials such as polymethylmethacrylate (PMMA). U.S. Pat. No. 4,889,407 to Markle et al teaches the use of laser photoablation on an optical fiber waveguide to form chambers for an indicator material. U.S. Pat. No. 4,842,782 to Portney et al teaches the use of a focused excimer laser and semi-transmissive mask for photoablating implantable opthalmic lenses from polymethylmethacrylate stock.