Speaker
Description
Summary
In view of proposed upgrades to CMS, work has started at CERN to investigate
optical link components for new high speed links, expected to be up to 10Gbit/s
capacity. The radiation environment in the upgraded experiment is expected to be
~10 times more intense than that we have considered in the current generation of
the optical links.
Testing is therefore expected to be required at dose levels of 1MGy and fluences of
10^15particles/cm^2, where the dominant, most damaging particle species is
expected to remain charged pions with energies around 200MeV as in the current
generation Tracker.
A first test has been made with neutrons to very high fluences, in order to try to
measure the ultimate lifetime of some laser and photodiode samples, as well as to
gain some experience of high-fluence testing with a view to propose an appropriate
test procedure for the future. Two types of laser and photodiode were irradiated
with up to 10^16n/cm^2, using ~20MeV neutrons at the CRC facility in Louvain-la-
Neuve. The first type of laser was that used in the current generation of CMS
optical links and the second type was a similar device, but with larger bandwidth.
Likewise, the first type of photodiode tested was that used in the CMS Tracker
control links. The second type was very similar, from the same supplier, with this
time a smaller active diameter, which meant also a higher bandwidth.
Both types of laser exhibited similar effects. There was the usual close-to-linear
increase in threshold current and decrease in efficiency, which was ultimately
stopped when the efficiency reached zero. There was significant annealing of the
damage. The photodiodes also exhibited the expected increase in leakage current and
loss of response (which decreased eventually to zero), but with more limited
annealing. Both sets of devices were therefore killed by the test and, as such,
their ultimate lifetime was observed for the first time.
Interestingly, the point at which this the lasers stopped working appeared to be as
much affected by the internal (junction) temperature of the laser, as by the
radiation induced change in threshold current or efficiency. The effects are
synergistic; after being damaged, more current is required to drive the laser and
this, in turn, heats the device. For these InGaAsP/InP lasers it is well known that
the performance is strongly temperature dependent and under these test conditions
this sensitivity ultimately determined the lifetime of the lasers. Thorough tests
of the thermal behaviour of the current generation of lasers have since been made,
with measurements of thermal resistance and junction temperature. The combined
results from the radiation damage and thermal tests suggest that a streamlined
method could be used for future radiation hardness validation tests such that the
radiation damage would be measured at lower fluences (an easier, cheaper test) in
parallel with full thermal characterization of the device. These two measurements
combined could predict the effects of a high fluence test, with the result being
that fewer higher fluence tests would be necessary. In addition, these results also
suggest that R&D into improved sub-mounts for the lasers, to maximize heat transfer
away from the laser, would also be very worthwhile.
