Radiation hardness evaluation and phase shift enhancement through ionizing radiation in silicon Mach-Zehnder modulators

Data acquisition systems in High Energy Physics (HEP) experiments rely on tens of thousands of radiation hard optical links based on high data rate, low power transmitters which also have to be able to withstand high levels of different types of radiation. Radiation hardness is one of the requirements that becomes more demanding with every new generation of experiment. Previous studies have shown that there is currently no qualified technology for optical transmitters able to withstand operation in the innermost regions of upgraded LHC experiments at CERN. Silicon photonic Mach-Zehnder Modulators (MZMs) are being investigated as one of the promising technologies to address this challenge. We designed MZMs with different design parameters and exposed them to ionizing radiation in order to assess how their performance changes. We demonstrate that the etch depth of the MZM waveguides and the doping concentration in the waveguides strongly impact the response of the MZMs. In particular, a shallow etch depth and increased doping concentrations help to mitigate the detrimental effects of ionizing radiation. MZMs fabricated with these design parameters are found to show a post-irradiation phase shift enhancement compared to the pre-irradiation values. The improved radiation resistance is high enough that such devices could potentially be installed in future HEP experiments or in other fields of application sensitive to radiation.