Selective focusing into rib waveguide arrays via large-aperture integrated lenses on LiNbO3

Abstract

The integrated optical synthetic aperture radar (IOSAR) processor is one of a number of potentially powerful optical signal processing components that combine many of the throughput characteristics of bulk optical processors with the cost, size, and power advantages of guided wave optical devices.1 As essential elements of such processors, we have developed large-aperture (2.5-mm) integrated lenses with short focal lengths (1.2–2.5 cm). These lenses were fabricated by masking desired concave-shaped lens regions of Ti:LiNbO3 with thin films of SiO2 followed by a proton exchange process.2 Titanium-indiffused proton exchanged (TIPE) slab waveguides were thus formed around the Ti:LiNbO3 concave lens, generating positive focal lengths. The measured throughput efficiencies for our lenses were of the order of 74%. Guided wave optical chips were developed on TIPE substrates with the following integrated elements: Tl lenses, large-area rib waveguide arrays (660 waveguides, 8 μm wide, 1 μm high, 1 cm long, with 2-μm separations), and uniform surface outcoupllng gratings on the rib waveguide arrays (4-μm grating spacing, 0.05 μm high, 1 mm long).1 Individual waveguides of the arrays were selectively coupled Into by the integrated lenses, and the guided modes were then outcoupled by the superimposed surface gratings to access the third processing dimension.