The future High-Luminosity era of the Large Hadron Collider, with its unprecedented instantaneous luminosity, will impose new challenges on the LHC experiments. ATLAS will replace its inner detector with a new all-silicon Inner Tracker (ITk), whose innermost layers will be based on pixel technology and are expected to produce a data output of about 11 Tb/s. A high-speed transmission chain with many parallel lines running at 1.28 Gb/s will transmit data from the detector to an opto-electrical conversion system. This Optosystem features custom-designed radiation-hard electronics devoted to signal equalisation, aggregation (to 10.24 Gb/s) and optical-electrical conversion.

After Run III the ATLAS detector will undergo a series of upgrades to cope with the harsher radiation environment and increased number of proton interactions in the High Luminosity LHC. One of the key projects in this suite of upgrades is the ATLAS Inner Tracker (ITk). The pixel detector of the ITk must be read out accurately and with extremely high rate. The Optosystem performs electrical-to-optical conversion of signals from the pixel modules. We present a general overview on the design of the Optosystem and recent results related to the performance of the data transmission chain, pivoted on the Optoboards, and to the radiation hardness of the ASICs housed on it.