Radiation damage studies of optoelectronic components for the CMS tracker optical links

Optical links are being developed to transfer analogue tracking data and digital timing and control signals in the future Compact Muon Solenoid (CMS) experiment at CERN. The radiation environment inside the CMS tracker will be extreme, with hadron fluences up to ~10¹⁴/cm² and ionising doses of ~100 kGy over the experimental lifetime. Prototype link elements, consisting of commercially available 1310 nm multi-quantum-well InGaAsP lasers and InGaAs p-i-n photodiodes, have been irradiated in a fully packaged form with ~6 MeV neutrons to 10¹⁵ n/cm², 24 GeV protons to 4×10¹⁴ p/cm² and ⁶⁰Co-gammas to 100 kGy. Three types of single-mode optical fiber, two pure-silica core and one Ge-doped core, were irradiated in several stages with ⁶⁰Co-gammas to a total dose of ~90 kGy. Neutron and proton damage induced large increases in laser threshold and significant decreases in light output efficiency. P-i-n leakage current increased by up to 6-7 orders of magnitude for neutron and proton damage. P-i-n response was relatively unaffected until ~2×10¹⁴ n/cm ², or ~4×10¹³ p/cm, after which the photocurrent decreased rapidly. Gamma damage after 100 kGy was minor in comparison to hadron damage in both the lasers and p-i-n photodiodes. The radiation induced attenuation at 1300 nm in the optical fibers was dependent upon the fiber type, with losses of 0.08 dB/m for the pure-silica core fiber and 0.12 dB/m in the Ge-doped core fiber, after ~90 kGy. The annealing in one of the pure-silica core fibers was found to be temporary in nature