The dual-channel multi-mode 850 nm optical Miniature Transmitter (MTx) is developed for data transmission of the ATLAS LAr calorimeter readout at LHC. The MTx’s are exposed to the radiation field of proton–proton collisions, therefore, the tolerance in Total Ionizing Dose (TID) is required. The TID effects in the MTx are investigated with X-rays and Co-60 gamma-rays for the active components of VCSEL diodes, and the customized Link-on-Chip laser driver (LOCld) developed in 0.25μm Silicon-on-Sapphire CMOS technology. The irradiation tests were conducted at various dose rates. The responses to TID are observed with degradation of laser currents at initial dose of 10 to 100 Gy(SiO2), and partial recovery with additional TID to a stable output about 90% of the original. The optical eye diagrams of irradiated samples show slightly increased jittering, and are suitable for the ATLAS requirement of 5 Gbps applications. •Optical link for collider experiments.•Radiation hardness of optical transmitter, Total Ionizing Dose effects.•X-ray and Co-60 gamma irradiations on CMOS devices.