Optical data transmission will remain a key enabling technology for the upgrading detectors at HL-LHC. In particular the inner tracking detectors will require low-mass, radiation tolerant optical transmit and receive modules for tight integration in the detector front-ends. We describe the development of such a module, giving details of the design, functional and environmental performance, as well as showing the feasibility of achieving small size, low-mass, and low-power operation.

Radiation-hard optical links are the backbone of read-out systems in high-energy physics (HEP) experiments at CERN. The optical components must withstand large doses of radiation and strong magnetic fields and provide high data rates. Radiation hardness is one of the requirements that become more demanding with every new generation of HEP experiment. Previous studies have shown that vertical cavity surface emitting lasers, on which the current optical links are based, will not be able to withstand the expected radiation levels in the innermost regions of future HEP experiments. Silicon photonics (SiPh) is currently being investigated as a promising alternative technology to address this challenge. We irradiated SiPh Mach-Zehnder modulators (MZMs) with different design parameters to evaluate their resistance against ionizing radiation. We confirm that SiPh MZMs with a conventional design do not show a phase shift degradation when exposed to a 20-MeV neutron fluence of 3 • 10 16 n/cm 2. We further demonstrate that custom-designed MZMs with shallow etch optical waveguides and high doping concentrations in their p-n junctions exhibit a strongly improved radiation hardness over devices with a conventional design when irradiated with X-rays. We also found that MZMs withstood higher radiation levels when they were irradiated at a low temperature. In contrast, larger reverse biases during irradiation led to a faster device degradation. Simulations indicate that a pinch-off of holes is responsible for the device degradation. Photodiodes (PDs) were also tested for their radiation hardness as they are needed in silicon photonic transceivers. X-ray irradiation of building-block germanium-silicon PDs showed that they were not significantly affected.

Optical links are vital components in the data transmission systems of high energy physics (HEP) experiments at CERN. With the ever higher beam fluxes achieved by the Large Hadron Collider, the optical components have to withstand higher radiation levels and handle ever-increasing data volumes. To face these challenges, the use of silicon photonics (SiPh) Mach-Zehnder modulators (MZMs) in the next generation of optical transceivers for HEP experiments is currently being investigated. In this paper, the dependence of the radiation hardness of custom-designed SiPh MZMs on temperature is reported, including the observed improvement in radiation tolerance at low operating temperatures that are closer to the typical temperatures found in HEP experiments. Furthermore, postirradiation annealing measurements of the devices were performed. An effective annealing method has been found by applying a forward current to the MZMs, leading to an almost immediate and full recovery of the device after irradiation up to 3 MGy. This enhanced device recovery method could effectively increase the radiation hardness tremendously in applications with low dose rates and periodic shutdown times.

The Versatile Link + project is about to enter its production phase, ready for the Phase 2 HL-LHC detector upgrades. We present the status of the front-end part of the Versatile Link + project: the Versatile Link + Transceiver, which provides a low-mass, radiation tolerant, optical transmit-and receive module for tight integration in the upgrading HL-LHC detectors. We describe the development and thorough testing carried out with the transceiver prototypes and their sub-components and the design decisions that have led to the final production-ready prototype. The planned production schedule is also presented.

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