New configuration of guided-wave electro-optic correlator for synthetic-aperture radar applications

In the last few years, a considerable effort in the optoelectronics research field has been spent for the development of a number of guided-wave active and passive components, such as laser diodes, electrooptic modulators, acoustooptic transducers, photodetectors, microlens array, and so on, for fabricating optical devices and circuits for signal processing and computing. The interest related to optical processors is particularly due to a lower power consumption, reduced size, cost and weight, and high throughput with respect to the corresponding electronic processors. In particular, synthetic aperture radar (SAR) applications are well suited for an optics-based processing technique implementation, because the synthesis of the object image, performed by correlating the received radar signal with a reference signal, is equivalent to the optical reconstruction of the Fresnel diffraction pattern of the same object, illuminated with coherent light. Guided-wave optical processors, including acousto-optic transducers and CCD cells, can be successfully applied to the reconstruction of two- dimensional images by using both spatial and time integration. In this paper, we present the theoretical investigation, design, and simulation of a new LiNbO3 guided-wave optical correlator suitable for real-time SAR applications. It is based on a complex interferometric structure, involving four aperiodic phase-reversal traveling wave modulators. The electrode structure is designed in order to reproduce the product signal between the received and reference voltages, which is then time-integrated by a suitable photodetector. The filtered signal coming from the detector is proportional to the final correlation function, which can be electronically registered and multiplexed on a two-dimensional matrix by sum-and-shift procedure. Thus, the processor performs the correlation function between the reference signal and the received signal when they are applied to laser diode and to the electrodes as driving voltage, respectively.