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Digital archive of theses discussed at the University of Pisa


Thesis etd-02072024-215702

Thesis type
Tesi di dottorato di ricerca
Thesis title
Radiation-Hard Silicon Integrated Photonics for High-Speed Data Transmission
Academic discipline
Course of study
tutor Prof. Saponara, Sergio
  • electro-absorption modulators
  • electro-optical packaging
  • electro-optical testing
  • folded Mach-Zehnder modulators
  • high-energy physics
  • high-speed electro-optical communication
  • microwave printed circuit boards
  • photonic integrated circuits
  • radiation damage
  • radiation hardness
  • radio-frequency
  • ring modulators
  • silicon photonics
  • total ionizing dose
  • traveling-wave Mach-Zehnder modulators
Graduation session start date
Release date
Radiation is ubiquitous in a broad ensemble of environments, from natural manifestations like material radioactivity to more extreme artificial situations, e.g., space missions, nuclear reactors or particle accelerator facilities. High energy physics (HEP) detectors, such as those in the Large Hadron Collider (LHC) at CERN, are among the most demanding environments in terms of radiation exposure as they normally rely on particles collisions accelerated at enormous energy levels. While being used as a tool to catch glimpses of new physics, radiation could also be acknowledged as a tremendous hazard. On-detector solid state devices and integrated circuits are indeed exposed to a mix of particle-matter interactions which can determine degradations in their functional properties and, ultimately, potential system failures. Radiation-induced hazards thus requires proper radiation-hardened technologies to ensure flawless operation.

HEP experiments generate vast amounts of data volumes, which are managed through high-speed fiber-optic communication links. Current data readout systems rely on electro-optical transceivers (EO TRXs) based on bulk radiation-hardened directly-modulated vertical-cavity surface-emitting lasers (VCSELs). However, they face significant limitations in terms of long-term radiation tolerance, transmission speeds, integration and power consumption scaling. With the evolution of particle colliders, the expected trends for both radiation and data production levels foresee challenges with ever increasing complexity. Consequently, novel technological platforms are highly demanded to advance the field of radiation-hardened high-speed communications.

Silicon photonics (SiPh) shows promise to replace existing EO radiation-hard technologies, offering established capabilities of high-speed and power-efficient communication together with promising radiation tolerance, demonstrated by few pioneering experiments in the last decade. This research aims to explore the design space of SiPh devices to provide high-speed EO modulating components for ultra-high radiation-rich environments. A full-custom photonic integrated circuits (PIC) has been designed during the PhD program for this purpose. It encompasses a wide variety of EO modulator designs to assess the tradeoffs involved in utilizing SiPh technology in harsh environments typical of HEP experiments. Two distinct research directions have been followed in conceiving device types and design variations. First, customized SiPh modulators were developed to outline design flows for attaining high-speed data signaling up to the technological process limits, while also considering aspects related to the integration with driving electronics, e.g., impedance matching, modulation efficiency and power consumption. Then, custom modulators were specifically designed following radiation-hardening techniques to target efficient EO modulation even after radiation exposure up to the foreseen damaging conditions relative to the upcoming experimental runs of the LHC accelerator complex.

Among the broad set of custom-designed SiPh devices, including traveling-wave and folded lumped-element Mach-Zehnder modulators (MZMs), ring modulators (RMs) and electro-absorption modulators (EAMs), transmission speeds ranging from 25 Gb/s up to 50 Gb/s have been captured with power consumption ratings between few pJ/bit down to even 10 fJ/bit depending on the specific device and operating conditions.

Three X-rays irradiation experiments were conducted to assess SiPh modulators’ operation up to 1.2 Grad(SiO2) total ionizing dose (TID) levels. Radiation-hardened MZMs with novel phase shifter designs demonstrated comprehensive trade-offs between radiation-hardness, optical losses and modulation efficiency, preserving also high-speed wideband operation. RMs exhibited significant resonance coupling condition transformation with the accumulation of TID. Forward-bias annealing on RM’s PN junction-based phase shifter has been shown to completely recover RM’s TID-induced damage. Silicon-germanium (SiGe) EAMs instead showed no appreciable degradation up to the inspected TID levels, allowing to add another option to the set of SiPh devices suitable for the realization of radiation-hard TRXs in extreme radiation-rich environments.

In summary, this dissertation lays the groundwork for the development and the experimental evaluation of custom SiPh modulators that seamlessly combine high-speed capabilities with radiation tolerance, addressing the multifaceted requirements of communication links in particle physics experiments and beyond.