Radiation-induced Ionization Effects and Space Mission Requirements for Silicon Photonic Mach-Zehnder Modulators

Photonic integrated circuits have become essential for meeting the growing global demand for high-capacity information processing and transport. Assessing their radiation tolerance is essential for deploying systems in radiation prone environments - including in space, high-energy particle accelerators, and defense radiation testing facilities - where the performance and compactness of photonic integrated circuits are increasingly advantageous. This work investigates the analog and digital radio frequency electro-optic performance of Mach-Zehnder modulators (MZMs) subject to 10-keV X-ray irradiation, which mimics cumulative ionization effects in space flight. Silicon photonic MZMs serve as excellent exemplars since they are interferometric devices comprised of elements common to many integrated photonic circuits. Under standard bias conditions, the irradiated MZMs exhibited significantly reduced bandwidth, a corresponding eye closure and baud rate dependent increases in the estimated error rate. The observed performance degradation is attributed to total ionizing dose effects which leads to hole trapping at the silicon/silicon dioxide waveguide interfaces as well as fast traps with energies near the conduction band edge. Notably, when MZMs were irradiated with all leads grounded, no radiation sensitivity to the electro-optic response was observed highlighting the importance of testing under standard operating conditions for ground-based radiation testing as well as on-orbit studies. Understanding the radiation induced performance degradation of MZMs and other integrated photonic devices is increasingly important for space and accelerator environments as performance requirements and deployment opportunities increase.