From ajw@hep204.cithep.caltech.edu Tue Mar  7 15:24:24 2000
Date: Tue, 7 Mar 2000 15:22:58 -0800 (PST)
From: Alan Weinstein <ajw@hep204.cithep.caltech.edu>
To: ajw@caltech.edu, sanders@ligo.caltech.edu, barish@ligo.caltech.edu,
        dugolini@ligo.caltech.edu, steve@ligo.caltech.edu,
        ouimette_d@ligo.caltech.edu, seiji.kawamura@nao.ac.jp,
        keithr@mhpkeith.physics.lsa.umich.edu, mike@ligo.mit.edu,
        gustaf@fastloki.stanford.edu, worden@ligo.caltech.edu,
        kstrain@physics.gla.ac.uk, wea@ligo.caltech.edu, stan@ligo.caltech.edu,
        fjr@ligo.caltech.edu, nergis@ligo.caltech.edu,
        kells_b@ligo.caltech.edu, ganezer@dhvx20.csudh.edu,
        asiri_f@ligo.caltech.edu, smith_m@ligo.caltech.edu,
        lazz@ligo.caltech.edu, hiro@ligo.caltech.edu, jim@ligo.caltech.edu,
        willems@ligo.caltech.edu, jay@ligo.caltech.edu, rolf@ligo.caltech.edu,
        jordan@ligo.caltech.edu, fritschel_p@ligo.mit.edu,
        weiss_r@ligo.mit.edu, dhs@ligo.mit.edu
Subject: 40m vacuum mirror ringdown test
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Dear 40 Meter Interferometer Technical Advisory Committee
and interested parties,

Jordan Camp and Dennis Ugolini have tested two mirrors
>from Jordan's ringdown chamber, that were exposed to the
40m vacuum (in the BS chamber) for 28 days.

They see no evidence of enhanced losses,
absorption or otherwise, due to the exposure.
Their report is included below.

For more details on the apparatus/method, see
http://www.ligo.caltech.edu/docs/G/G990022-38.pdf
and J.Camp et al, Appl. Optics 38, 5378-5383 (1999).

Based on this, there is little evidence of
optical degradation due to contamination of the
40m vacuum by the viton springs or anything else.

SO, we may indeed consider NOT doing a bake-out,
and/or NOT replacing the viton with metal springs.

We may still want to do one or both of these things,
for other reasons (eg, seismic isolation performance),
but I have not heard a compelling reason.

NOTE that we DO have to build a seismic stack
(with metal springs) and bake out the output chamber.

NOTE also that Rai will do surface tests of some
microscope slides exposed to the 40m vacuum, as well.

I welcome your comments!!

Alan Weinstein

____________________________________________________________________
Date: Tue, 7 Mar 2000 14:26:34 -0800 (PST)
From: "Dennis Ugolini (40M)" <dugolini@ligo.caltech.edu>
Subject: Mirror measurement

I've just finished observing Jordan's ringdown measurements of
the vacuum-exposed mirrors.  Here's a summary of the procedure
and results.

The 1" diameter test mirrors were first measured in Jordan's
ringdown cavity, then placed in the 40m beam splitter chamber.
The 40m interferometer was kept at vacuum for 28 days (February
7th to March 6th).  We then vented, removed the mirrors, and
remeasured them today.

The ringdown cavity is a 0.5 meter tube with the test mirrors
clamped to either end.  The cavity is placed in a vacuum chamber,
and a roughly 45mW infrared laser beam is applied.  A 1 Hz interrupt
pulse is applied to break lock, and the storage time is measured
>from the exponential decay of the transmitted power.

Storage time before exposure:  31.8 usec
Storage time after exposure:   35.3 usec

These times can be converted to total losses by the equation
L(total) = 2l/ct (l = cavity length, t = storage time):

L(total) before exposure:  105 ppm
L(total) after exposure:    94 ppm

Jordan gives roughly +/- 10 ppm as the precision of this
cavity, so there is no change within the measurement's
uncertainty.

Absorption is measured by sweeping a sideband through a range
of frequencies to locate a resonance.  The laser power is then
increased about 50% by rotating a polarizer in the beam path.
This will increate heating and thermal lensing of the mirrors,
which shifts the resonance frequency.  The magnitude of the shift
will scale with the magnitude of absorption.

Before exposure:
  Laser power = 45 mW   f = 74.314 MHz
  Laser power = 70 mW   f = 74.310 MHz  delta(f) = 0.004 MHz

After exposure:
  Laser power = 42 mW   f = 74.391 MHz
  Laser power = 64 mW   f = 74.388 MHz  delta(f) = 0.003 MHz

Jordan says that these numbers are consistent with absorption
at the level of 1 ppm.

Thus there is no measurable contamination of these mirrors
over the course of 4 weeks exposure.  Or as Jordan put it,
"there's no noxious chemical fog rapidly attacking your mirrors."
Take it for what it's worth.

- Dennis