Optical coherent transient true-time delay generator programmed with linear phase chirp

Abstract

Summary form only given. The process of true time delay (TTD) has become a highly desired tool for processing of phased array antenna signal for beam steering and coherent detection. Typically phase shifting devices (PSD) are used in phased arrays, which produce beam squint when used at wide bandwidths (>1 GHz) and when fraction bandwidths approach 100%. The inherent problem with PSDs is that they do not impart each frequency component of the signal with the same time delay, whereas TTD device delay each frequency component by the same delay time. In this paper, we describe a novel optical coherent transient (OCT) technique for programming high bandwidth TTDs with large time-bandwidth products (TBP). Linear frequency chirped lasers significantly reduced the cost and delay programming complexity of OCT TTD, but these lasers currently lack the frequency stability needed for accumulated programming, a desired feature. Linear phase chirps, where a frequency stabilized external cavity diode laser is externally phase modulated by a chirped rf drive signal, offer the advantages of chirped lasers, plus excellent frequency stability. Linear phase chirps are easily created with integrated optical phase modulators, which can have broad bandwidths (0-40 GHz). To demonstrate the concept, we created up to 3 GHz linear phase chirps by driving our phase modulators with chirped square waves from a 12 Gbit/sec pulse pattern generator. Frequency shifts were introduced with acousto-optic modulators and time delays from 100 ns to 1 /spl mu/s were demonstrated.