High packing density 2.5 THz true-time-delay lines using spatially multiplexed substrate guided waves in conjunction with volume holograms on a single substrate

A 3-bit true-time-delay lines device having a packing density of 5 lincs/cm² with a minimum delay step of ~100 ps is designed, fabricated and demonstrated. This device is based on substrate guided wave propagation combined with slanted photopolymer volume phase gratings. In this paper, we report the delay and bandwidth measurements for the 3-bit delay lines fabricated on BK-7 glass substrates with a substrate bouncing angle of 45°. The power fluctuation among the outputs due to the cascading fanout effect (a serious drawback for real system applications) is experimentally investigated as well. A power fluctuation controlled to within ±10% is achieved. A femtosecond laser pulse is sent through the device and a bandwidth measurement of up to 2.5 THz is obtained. The delay step is measured by employing an ultrafast photodetector together with a sampling scope. The true-time-delay device presented herein has the potential to be integrated with photodetector arrays due to its planar structure on a single substrate together with the surface normal fan-in and fan-out features