Radiation-hard Silicon Photonics for Future High Energy Physics Experiments

Collisions of proton beams in the Large Hadron Collider at CERN produce very high radiation levels in the innermost parts of the particle detectors and enormous amounts of measurement data. Thousands of radiation-hard optical links based on directly-modulated laser diodes are thus installed in the particle detectors to transmit the measurement data to the processing electronics. The radiation levels in the innermost regions of future particle detectors will be much higher than they are now. Alternative solutions to laser-based radiation-hard optical links have to be found since the performance of laser diodes decreases beyond the operation margin of the system when irradiated to sufficiently high radiation levels. Silicon Photonics (SiPh) is currently being investigated as a promising alternative technology. First tests have indeed shown that SiPh Mach-Zehnder modulators (MZMs) are relatively insensitive to a high neutron fluence. However, they showed a strong degradation when exposed to ionizing radiation. A SiPh test chip that includes MZMs with varied design parameters was designed and fabricated at imec to identify parameters that improve the resistance of MZMs to ionizing radiation. The performance of the various MZMs was tested before, during and after irradiation with x-rays. An MZM design that can withstand ionizing radiation levels 5x higher than the initially tested devices was identified. Eye diagrams of these MZMs showed no significant difference after irradiation compared to reference samples. A model for the Optical Modulation Amplitude (OMA) of MZMs was developed to determine the radiation levels up to which SiPh-based optical links would work reliably. The analysis showed that a sufficiently large OMA could be sustained up to higher radiation levels than the upcoming Versatile Link system was designed. At the same time, the electrical power consumption of the proposed system was estimated to be similar.