Aspheric waveguide lenses for photonic integrated circuits

Indium phosphide is one of the most promising materials for photonic integrated circuits that combine passive optical components such as waveguides, couplers, and filters with active components such as lasers, detectors, and modulators all on the same chip. A waveguide lens is a very useful component for integrated optics and substantial work has indeed been devoted to its achievement. Many lens designs have been demonstrated in thin film and LiNbO, waveguides, including etched,’ geodesic,2’3 Luneburg,4 Fresnel,5Y6 grating,7-9 and aspheric” lenses. Some work has also been reported in semiconductor materials, mainly with gratings, ” or composite Fresnel-grating lenses.r2 Due to the high index of the semiconductor material, these approaches usually require writing very fine structures with direct e-beam lithography followed by dry etching technology to pattern the wafer. Moreover, the performance of the lens is limited by the diffraction efficiency of the grating and by the background light left by the undiffracted beam. In this letter, we report fabrication of an aspheric lens by wet etching using a standard photolithographic masking step. Wide aperture, very low insertion losses, and spot size close to the diffraction limit are demonstrated. An effective index change in a planar waveguide can be easily obtained by changing the thickness of the waveguide layer. For creating a precisely controlled index step in a planar waveguide, we can take advantage of wet etching and etch-stop fabrication techniques as shown in Fig. 1. In our sample, a 2000 A waveguide core layer of il= 1.3 pm quaternary material is grown on top of the InP substrate, followed by a 200 A InP etch-stop layer, and another 2000