We developed a new directional borehole radar system with an array of optical electric field sensors as receivers. The system was tested in a field test site in order to assert its ability to detect three dimensional (3D) location of a subsurface tunnel. In this paper, the result is shown with a comparison to a result obtained by RAMAC directional borehole radar system. It is found that the optical electric field sensors are able to obtain highly coherent signals, that is necessary for the accurate estimation of a target azimuth direction, over the RAMAC system. The mutual coupling effect that deteriorates the coherence is also discussed. The azimuth estimation of the subsurface tunnel, which is located 5.5m apart from the tunnel, showed good agreement with the actual direction to an error of within 10 degree.