Hi PHK, everyone,
On 2016-02-14 0:30:22, Poul-Henning Kamp wrote:
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In message <1E75A9592178425ABD11390EB725D060@pc52>, "Tom Van Baak" writes:
Yes, the interferometer is 4 km in length but they bounce the beam back
and forth 400 times so the effective length is more like 1600 km. They
keep the mirrors stationary to "picometers". They use hundreds of clever
tricks to pull this off.
It's actually more amazing than that, each arm is a resonant cavity,
so while the actual laser is only about 10W, they have about 20 kW of
photons inflight at any one time.
With 20 kiloWatt of light safety-glasses are not _that_ important any more.
This number keeps getting repeated, but I have some doubts there.
The 'finesse' of the cavity is about 1000. The view that the photons
keep bouncing back and forth seems a bit simplistic, wouldn't it be more
like a standing wave?
The cavity acts as a resonator, and although the instantaneous power
would indeed be 20kW, as soon as you load that cavity, its stored energy
would be dissipated in whoever was unlucky enough to end up in the beam.
Given a length of 4km, it would take no more than 13 us to empty the
cavity. And 13us times 20kW gives an energy of only 0.27 J.
The part that I am still having trouble understanding is why the two
cavities in the arms of the interferometer help increase the
sensitivity. Are they modulating the reflectivity of the mirrors on the
inner testmasses so they can 'dump' both beams at the same time back
into the half-silvered mirror?
Cheers, Paul - 73 de PEaNUT.
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