  TileCal, the central hadronic calorimeter of the ATLAS detector is composed
of plastic scintillators interleaved by steel plates, and wavelength shifting
optical fibres. The optical properties of these components are known to suffer
from natural ageing and degrade due to exposure to radiation. The calorimeter
was designed for 10 years of LHC operating at the design luminosity of
$10^{34}$cm$^{-2}$s$^{-1}$. Irradiation tests of scintillators and fibres have
shown that their light yield decrease by about 10% for the maximum dose
expected after 10 years of LHC operation. The robustness of the TileCal optics
components is evaluated using the calibration systems of the calorimeter:
Cs-137 gamma source, laser light, and integrated photomultiplier signals of
particles from proton-proton collisions. It is observed that the loss of light
yield increases with exposure to radiation as expected. The decrease in the
light yield during the years 2015-2017 corresponding to the LHC Run 2 will be
reported. The current LHC operation plan foresees a second high luminosity LHC
(HL-LHC) phase extending the experiment lifetime for 10 years more. The results
obtained in Run 2 indicate that following the light yield response of TileCal
is an essential step for predicting the calorimeter performance in future runs.
Preliminary studies attempt to extrapolate these measurements to the HL-LHC
running conditions.
