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Summary
The upgrade from LHC to High-Luminosity LHC will increase the luminosity of the LHC by a factor of 5-10; thus imposing even more stringent requirements on the optoelectronic components used in the front-ends of data-transmission links at the HL-LHC. Not only must they operate in harsher radiation environments, but they must also cope with the increase in data-rates. Components with lower power consumption, higher-speeds, and smaller sizes than those currently used in the optical links installed at the LHC are being investigated as possible candidates for HL-LHC data transmission links. In this paper we investigate the radiation tolerance of devices from two different “families” of technologies which fulfill these criteria: multi-channel transmitters and devices which can form the building blocks of a Si-photonics optical link.
The multi-channel transmitters tested consisted of a 12-channel InGaAs VCSEL array and a 4-wavelength Coarse-Wavelength-Division-Multiplexing (CWDM) module based on VCSELs. InGaAs lasers are a new material which we have not previously exposed to radiation testing, and are of interest because of their high reliability and efficiency, and their low power consumption. CWDM modules are interesting candidates for future optical links because they allow the transmission of multiple wavelengths over a single optical fibre which could allow us to increase the fibre-bandwidth of the currently installed fibre plants. The building blocks of a Si-photonics link tested were hybrid silicon lasers (Edge Emitting Lasers (EELs) built from type III-V semiconductor material on an SOI waveguide), SiGe photodiodes, and Si-based Mach–Zehnder Interferometer (MZI) modulators. The integration of these building blocks into a silicon-photonics link that could generate, modulate, process and detect light signals - all in a small Si-based package - would provide front-end components which are perfectly suited for HL-LHC applications.
The devices were irradiated with a 20 MeV neutron for 24 hrs at the University Cyclotron in Louvain-La-Neuve, Belgium and received a total fluence of 1.7-7.2x1015 n/cm2 depending on their position relative to the target. The static performance of the devices was monitored during the test and their recovery was monitored for ˜350 hrs after the end of the irradiation period. A decrease in the slope efficiency and an increase in threshold current was observed in the lasers, while an increase in leakage current and a decrease in responsivity was observed in the photodiodes. The hybrid silicon lasers were observed to stop lasing before a fluence of 1x1014 and therefore are not suitable for operation in the harshest radiation environments of the experiments (e.g. tracker-type applications). The SiGe photodiodes exhibited a smaller relative increase in dark current compared to InGaAs devices also tested. Finally the the leakage and forward current of the modulators was found to increase during the irradiation period.
This paper will present the results from this radiation test, and examine the effect of the radiation on the performance of the silicon-based modulators and photodiodes in more detail, and will conclude by presenting the potential HL-LHC applications for these types of devices.
