PD.2 Electrooptic AlGaAs Digital Spatial Light Deflectsr/Modulator Based on a Grating Phased Array.

Concept. We propose a novel concept for optical beam steering, consisting of a diffraction grating output coupler integrated on an array of uncoupled rib waveguides. By properly adjusting the phase of each element of this array, beam deflection in the direction perpendicular to the rib waveguides can be achieved. To facilitate proper excitation of the individual rib waveguide modes, an input grating coupler is used. By apodizing the input or output grating, it is possible to taylor the intensity distribution in each waveguide and to control the far field emission of the array as a whole. Also, TE and TM polarizations as well as rib and slab waveguide modes can be easily discriminated by appropriatly choosing the incidence angle. Between the input- and output-sections, SnO2-doped In203 (ITO) electrodes serve as electrooptic phase modulators on the waveguides. Different phase shifts in each rib waveguide can be obtained either by applying different voltages on each electrode, or by varying the electrode length. Experiment. The asymetric waveguide was grown by MBE. It consists of an AZo,lsGao.s5As guiding layer on a Alo.25G~,75As cladding. Gratings and ribs were patterned using direct electron beam lithography. Input- and output-coupler grating periodicities were 298nm and 245nm respectively. For large numbers of rib waveguides, individual electrodes become unpractical. The phase gradient required for beam steering can be obtained by varying linearily the electrode length over the entire array '. If only one deflected beam position is of interest (digital beam steering), the required phase gradient is fixed. The electrode slope for a given bias voltage is then also hed. Using a sawtooth shape, we can reduce the overall electrode length by inducing a maxi" phase shift of 2x under each tooth. To balance electrode induced absorption and zero bias phasing, we use a second interdigitated electrode. Both electrodes have a sawtooth shape consisting of five sections 100pm wide and 1.56" long. The teeth are electrically connected together. The emitter array consists of 43 waveguides spaced 12pm center to center, under one large output grating. The far field lobes are essentially diffraction limited by the grating size. The fist side lobes are found at 75mrad on each side of the main lobe, and their intensity is 60% smaller than that of the main lobe at Xo = 900nm. The measured width of the main lobe is 1.8mrad (FWHM) which is close to the diffraction limit. The swithed beam angular separation is Smrad, the switching voltage is -21V. Outlook. A laser source (single laser and star coupler, or phase locked laser array) can replace the grating input coupler. In such a case, a fully integrated beam steering device can be realized.