Formation of Planar Waveguide Lenses and Lens Arrays in GaAs Using Ion Milling*

Abstract

Waveguide lenses are among the essential components in integrated optic modules or circuits such as spectrum analyzers,(1) correlators,(2, 3) and computers.(4, 5). Various types of waveguide lens have been fabricated in LiNbO3 and glass substrates. These lens types include Luneburg, geodesic, index refraction via TIPE or two layers construction, chirp grating, and Fresnel. The material constraints such as a very high refractive index and high brittleness, and the relatively small reduction in refractive index in Ga1-xAlxAs for a desirable fractional composition x have thus far prevented the first three lens types from being fabricated in GaAs waveguides. Although fabrication of AlGaAs graded-index waveguide lenses using laser-assisted chemical vapor deposition was proposed most recently,(6) to the best of our knowledge, no working lens has been reported. However, the positive or negative index-changes required in both the chirp grating and Fresnel lenses may be produced through either deposition of a higher-index cladding material, or reduction of the waveguide thickness via etching or ion milling. As a high-quality higher-index cladding material for GaAs is yet to be grown, we have chosen the later technique through ion milling to fabricate the negative-index change lenses. Most recently, we have successfully applied this lens fabrication technique to fabricate lenses of high-efficiency and diffraction limited characteristics in LiNbO3.(7) In a continuing effort we have successfully produced single lenses and lens arrays of the types including analog Fresnel, chirp grating, and a hybrid combination of the two in GaAs waveguides, and have obtained equally encouraging results. In this paper, design, fabrication, and measured performances of single microlenses and microlens arrays of the aforementioned types are presented.