Construction and testing of Resistive Plate Chambers with optical readout

Resistive Plate Chambers (RPCs) are among the core technologies in modern collider experiments. In their traditional form, RPCs operate on the principle of charge induction: ionization within the gas gap induces signals on an external anode plane segmented into pads or strips. Recent advances — particularly the development of one-glass and hybrid RPCs — have enabled significant modifications to this architecture, creating opportunities for functional coatings or embedded readout structures that can enhance signal detection and expand the range of measurable phenomena. It is well established that, in addition to avalanche charge, scintillation light is also produced during the electron multiplication process in the gas gap. Measuring this light offers a unique avenue to obtain precise timing and spatial information. Building on this insight, our team has been developing RPCs with optical readout, designed to exploit scintillation light generated within the gas gap. A number of small-scale RPCs (10 cm × 10 cm, 1.3 mm gas gap) were constructed with SiPMs directly in the gas layer which also includes a hybrid RPC with four SiPMs whose windows were coated with TPB (tetraphenyl butadiene), a wavelength shifter that converts UV scintillation photons into the visible range. In parallel, a larger hybrid RPC (32 cm × 48 cm) was constructed, incorporating four TPB-coated quartz fibers embedded in the chamber and read out at both ends by external SiPM assemblies. This configuration demonstrates a scalable optical readout concept for future large-area RPC systems. All of the RPC prototypes are designed for simultaneous detection of scintillation light and avalanche charge, offering the potential for improved spatial and temporal resolution as well as a deeper understanding of signal formation in gaseous detectors. In this report, we describe the construction of these optically read out RPCs and present recent test results from the different designs.