Abstract

The detection of short duration electromagnetic pulses (EMPs) with a double Gaussian temporal profile of up to a few hundred nanoseconds duration and a central RF frequency in the GHz is achieved conventionally using an electrical antenna based sensor [1]. These sensors, which are capable of accurately measuring both the peak electric field strength of the EMP and the temporal profile of the pulse, are macroscopic systems (with sizes comparable to several wavelengths of the central RF frequency) and can introduce a significant perturbation of the EMP during measurement. The use of a sensor exploiting electro-optic materials and optical detection methods offers the possibility of a significantly smaller sensor package with minimal potential for EMP perturbation. Lithium Niobate (LiNbO₃), which has a large electro-optic tensor, is a suitable candidate for the fabrication of an all-optical sensor device. In this paper, we report, for the first time, the design and optimisation of a set of closely coupled single-mode waveguides in LiNbO₃ that, under the influence of the RF EMP, change the coupling of near infrared light between the adjacent waveguide channels. These waveguides were fabricated in z-cut LiNbO3 using direct electron beam lithography and reactive ion etching. The etched channels were then in-filled with RF sputtered amorphous Silicon with a controlled refractive index to allow optimised coupling and guiding between three waveguide channels. Following assembly and integration, with conventional single mode telecommunications fibre (both input and output), into full sensor packages (Fig. 1), the sensors were tested and shown to measure the EMP field strength and pulse shape successfully (Fig. 2).

© 2015 IEEE