Gravitational Wave Group
Durability of the ultra-low loss mirror in the cryogenic environment.
Motivation
LCGT is characteristic to use cryogenic super-low loss mirrors as test masses
of a Fabry-Perot type laser interferometer. Such mirrors, however, are worried
to absorb molecules like a cryopump and to decrease their power reflection,
which results in low cavity reflectance and low power recycling gain.
In this experiment, we put a high-finesse Fabry-Perot cavity in a cryostat and
monitored its finesse change for over one month to examine whether drastic
contamination would take place or not.
Experimental setup
Clean room and housed cryostat
Optical setup
Result(preliminary)
Remarkable finesse change at 4 [k] temperature and 10-7 [torr] vacuum level
was not observed, exept for a change due to a low pressure (~10-3 [torr])
accident at 70[K] in 8th day.
The finesse recover in 36th day was obtained just by heating up the whole system
to 300[K]
Next step
In this expeiment, the 4[K] mirrors of the FP cavity are faced with each
other, and the space betwen them was almost closed by a rod. In LCGT,
each mirror faces with 300[K] vacuum and is expected to suffer from
the 300[K] molecules attack. In the next step, finesse change of a FP
cavity which is composed of one 4[k] mirror and one 300[K] mirror will be
monitored.