Compact silicon-based integrated optical time-delay network

Conventional phased-array antennas are limited by the bandwidth and attenuation as well as mechanical rigidity of the coaxial cables employed to perform the microwave phase shift. Photonic technology offers advantages in distribution of the microwave signal including ligthweight, compact delay lines, immunity from electromagnetic interference, low rf transmission loss and possibility of optical signal processing on the microwave encoded optical beam. Fiber optic delay lines have been demonstrated already but one requires a precision cut of the fiber length ( accurate to a mm) to achieve accurate psec time delays. Guided-wave based approach allows precise definition of the waveguide delay line lengths. In addition, reproducible delays can be mass-proSPIEVol. 3160. 0277-786X197/$10.00 2 duced ensuring low cost modules. In this paper, we demonstrate the first integrated optical delay lines in the silicon-on-insulator (SOT) waveguide technology. An 8channel array with 12 ps incremental time delay has been demonstrated as shown in the photograph in Figi . The SOT rib waveguide delay lines were fabricated using TUE etch on a standard BESOT wafer with a 5tm silicon layer that was etched down to 2tm to define the rib waveguide. The experimental setup is shown in Fig.2. Light from an external cavity tunable laser was externally r.f. modulated at 220 GHz and was coupled into the time-delay chip and time/phase measurements were performed using a vector network analyzer. Fig. 3 shows the measured performance of the 8 channel time-delay array. The phase delay is measured with respect to the straight through delay-line. The results clearly demonstrate 8 channel true-time delay in 10 Ps increments that is broadband from 220 GHz. The variance in measured time-delay is due to the 0.5 ps timing jitter of the source. Due to the tight optical confinement afforded by the Si-Si02 interface (A n 2.0) we are able to define extremely sharp low-loss waveguide bends. In our design we chose a conservative bend radius of 5000 m. Further, due to the high refractive index of silicon (n 3.44) we obtain much higher time-delay for a unit length of the waveguide compared to other guided-wave time delay approaches based on silica (glass) waveguides (n = 1.46). This combined with the large available silicon substrates (8-12" diameter) promises extremely compact and low-cost time-delay modules that can further be readily integrated with emerging high-speed SOT CMOS electronic circuitry.