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Description
This study details the experimental characterization of silicon photonic ring modulators (RMs) and silicon-germanium (SiGe) electro-absorption modulators (EAMs) exposed to 12 MGy(SiO2) total ionizing dose (TID) within INFN’s project FALAPHEL. We extensively report on the evolution of their key performance metrics as a function of TID. These trends are analyzed in relation to the underlying physical mechanisms responsible for TID-induced degradation. We demonstrate the TID tolerance of SiGe EAMs and the effectiveness of room-temperature forward-bias annealing as a technique for recovering RMs from TID damage, along with its potential as damage compensation if applied periodically during irradiation.
Summary (500 words)
Silicon photonics (SiPh) is under evaluation for developing next-generation radiation-hard electro-optical (EO) transceivers (TRXs) and replace the current state-of-the-art modules based on discrete III-V optical components (VTRx+) [1]. The latter may not meet the increasingly stringent requirements for radiation tolerance and readout data rate in upcoming particle physics experiments like HL-LHC and FCC. SiPh solutions offer inherent benefits in transmission bandwidth (> 50 Gb/s), power consumption (< pJ/bit) and electronic-photonic integration, enabling their deployment even within the innermost detector shells.
Preliminary studies suggest that SiPh devices can effectively function in environments with high levels of both ionizing and non-ionizing radiation, eventually exploiting radiation-hardening-by-design techniques such as waveguide and doping engineering [2]. While various alternatives exist for realizing on-chip EO modulation, compact PN junction-based devices are particularly attractive for high-energy physics (HEP) scenarios for the possibility of multiplexing few high-speed lanes and then reduce fiber counts and on-detector data aggregation requirements. For this reason, ring modulators (RMs), folded Mach-Zehnder modulators (MZMs), or silicon-germanium (SiGe) electro-absorption modulators (EAMs) emerge as appealing choices for developing next-generation EO TRXs for HEP [3].
Assessing the radiation tolerance of SiPh modulators is crucial to understand radiation damage mechanisms and design radiation-resistant building blocks. For this reason, two SiPh RMs and a SiGe EAMs designed for C-band operation are integrated on the same photonic integrated circuit (PIC) fabricated using Imec's iSiPP50G technology. RMs are equipped with deep-etched highly-doped phase shifters. One of them is custom-designed and the other belongs to Imec’s process design kit (PDK). Instead, doping and waveguide design are not shared for the EAM as it is an Imec’s intellectual property (IP). The devices have been exposed to over 12 MGy(SiO2) TID using a 10-keV X-ray irradiator while being kept at room temperature (RT). Their EO performances, primarily captured through optical spectra as a function of bias voltage, have been continuously monitored during the irradiation.
In the case of RMs, the observed trends in resonance coupling state and modulation efficiency confirm the expected radiation damage effect, i.e., the gradual electrical pinch-off of the P-doped side of the RM’s phase shifter. Data post-processing reveals significant reductions in both modulation efficiency (~50%) and intra-cavity optical propagation losses (~30 dB/cm). The latter effect significantly modifies the resonance states in RMs nominally designed in critical coupling.
RT post-irradiation annealing proves ineffective in recovering TID damage in RMs. Conversely, PN junction forward biasing efficiently anneals the accumulated ionizing damage. A second irradiation session, reaching up to 3 MGy(SiO2), further demonstrates that RMs’ radiation-induced degradation can be effectively limited by incorporating periodic forward bias steps during operation, confirming findings reported for MZMs in [4].
Regarding the SiGe EAM, an outstanding radiation resistance is reported up to 12 MGy(SiO2) with only minor changes to extinction ratio, optical insertion losses and leakage current. SiGe EAM devices thus seem to offer viable alternatives for developing next-generation radiation-hard SiPh-based TRXs.
References
[1] M. Zeiler, doi: https://doi.org/10.1109/TNS.2017.2754948
[2] M. Lalovic, doi: https://doi.org/10.22323/1.313.0048
[3] S. Cammarata, doi: https://doi.org/10.1088/1748-0221/19/03/C03009
[4] A. Kraxner, doi: https://doi.org/10.22323/1.343.0150
