Characteristics of photonic time shifters based on switched gratings

A cascade of n independently controlled gratings can be used to route an optical carrier through one of 2(n) evenly spaced time delay paths. The resulting optical systems include digital time shifters for phased arrays with the potential for improving attainable performance in terms of insertion loss, crosstalk, and compactness. We describe results from an effort in which these characteristics of free-space optical time delay systems based on switched-volume-diffraction gratings were modeled and investigated experimentally. In one experiment, a 1x4 router, which constitutes the front end of a 2-bit photonic time delay circuit, was used to validate the low insertion loss and miniaturization capabilities of this technology. We fabricated electrically switched gratings which demonstrated 20 dB contrast (optical) and a response time of 15 microseconds. Realistic loss and crosstalk parameters were used in detailed system modeling to show that practical systems can be built using this technology with very low insertion loss and crosstalk. Various configurations are described, including a multi-pass device that may replace many single channel time shifters with a single optical system.