Fibre-Coupled Oscillator-Driven Electro-Optic Spectral Interferometry as a Longitudinal Bunch Profile Monitor

Electro-optic (EO) diagnostics offer non-destructive methods to resolve the longitudinal charge profile of highly relativistic bunches without the need for complex calibrations or ambiguous phase recovery techniques. The most common technique, EO spectral decoding (EOSD), is favoured for its simplicity, reliability, and straightforward output interpretation, however its resolution is constrained to the geometric mean of the transform-limited and chirped probe laser durations. We have introduced a new technique utilising spectral interferometry (EOSI), which overcomes this limitation by adding a single optical element to an EOSD setup, with successful measurements shown previously on the CLARA accelerator at Daresbury Laboratory. To further explore this technique, we have performed EOSI on the CLEAR accelerator at CERN (160-200 MeV, 15-150 pC and 0.2-1.5 ps RMS) utilising a nJ oscillator and optical fibre-coupled transport. Our results highlight the potential for low power, turn-key operation and flexible integration of EOSI systems for single-shot ultrashort bunch length and arrival time monitoring.