This work presents, for the first time, the experimental demonstration of the differential-field detection mode \(\Delta\) as a key component in the ongoing development of a novel interferometric Electro-Optic Beam Position Monitor (EO-BPM), capable of high-bandwidth monitoring of \(1\,\textrm{ns}\) -long HL-LHC ultra-relativistic proton bunches. Through the utilization of an innovative fibre-coupled Mach-Zehnder detection scheme, in its first experimental implementation, this study proves that the new field-focusing pickup design engineered to facilitate long-distance and high-bandwidth single-pass detection can deliver a sub-millimetric detection resolution while keeping an ultrafast time response below the HL-LHC goal of \(50\,\textrm{ps}\) . The transverse-position and time-resolution capability of the system were addressed at HiRadMat and CLEAR beamlines, respectively. The transverse position study was performed within a \(\pm 20\,\textrm{mm}\) range at \(3\,\textrm{GHz}\) acquisition bandwidth for SPS-like parameters ( \(4\sigma \approx 1.5\,\textrm{ns}\, \& \,1.2\times 10^{11}p^{+}\) ), whereas a \(33\,\textrm{GHz}\) response was achieved by detecting short CLEAR electron bunches ( \(4\sigma \approx 20\,\textrm{ps}\) ). In addition, the stability of the signals acquired under high levels of back-scattering radiation also proves that, due to the optical nature of the device, the EO-BPM differential-field mode is unaffected, and therefore very suitable for such environments.
