LIGO Caltech 40 meter Upgrade Test mass decision

Laser Interferometer Gravitational-Wave Observatory

LIGO Caltech 40 meter prototype Upgrade

Core optics questions

Garilynn has been waiting (for months, now!) to know the core optics sizes, radii of curvature, optical quality (Corning or Heraeus SV), and coatings, to start ordering.

AJW has done some more careful noise studies to determine whether we should use old-40m-style 4"diameter 3.5"thick core optics (1.56 kg, which require already-engineered scaled SOS/LOS suspension), or LIGO 3"diameter 1"thick core optics (0.25 kg, using LIGO SUS suspensions, identical to the MC suspended optics).
- It appears that the thermal noise in the mass itself, which scales very weakly with mass, dominates over suspension noise (which does depend on mass of optic) everywhere except for a few violin-mode spikes; see below.
- Smaller optics presumably cost less and take less time to grow.
- Smaller optics require a suspension with a smaller footprint on the already very crowded chamber tables.
- I believe that even 3inch optics have sufficient aperture, once the OSEMs are taken into account, to cover all but ~ 1ppm of the beam power (1.4" diameter).
Other than noise issues, (and cost and growth time), are there any other good criteria for deciding between these very different test mass sizes? Advice on how to make this decision would be appreciated!
We have specified all the radii of curvature, under two options (both with g_arms = 1/3):
- arm beam waist at the middle of the arm (ROC_ITM = ROC_ETM = 90.5 meters, ROC_RM = ROC_SM = 60.3 meters);
- or at the ITM (ROC_ITM = infty, ROC_ETM = 57.375 meters,)
Since beam spots (amplitude radius rms) are everywhere < 5mm, as far as I can tell, the only significant difference between these two options are the practical issues of cost and ease of making these ROCs, which may not be significant. Advice on how to make this decision would be appreciated!
It seems reasonable to go with higher optical quality (Heraeus SV) for all transmissive optics (RM, BS, ITMs, MC1, MC2) and Corning for all purely-reflective optics (ETMs, MC3), as long as lead times are not too different. It's twice as expensive, but not a very big expense anyway.
The LSC AIC and other LIGO II thinkers are converging on specs for the LIGO II T_ITM, T_RM, T_SM. To minimize extrapolation from 40m to LIGO, it makes sense to use these same values (same cavity finesses, power gains, etc; of course, 100 times shorter storage time, larger arm cavity pole frequency, etc). The numbers, currently, are the same as LIGO I: T_ITM = 3%, T_ETM = 15 ppm, T_RM = 2.44%. Advice on how to make this decision would be appreciated!
The 12m suspended mode cleaner will be very much like the LIGO I version.
Mode matching telescopes will be done with transmissive optics (lenses on rails). Any problem with this?

Here's some noise curves:

shot noise has P_laser = 1 watt, PRC power gain of 89, RSE tune of -0.6 radians, T_ITM = 3%, T_ETM = 15 ppm, T_RM = 2.44%, T_SRM = 1.7%.
Internal test mass noise uses Yury Levin formula, rbeam = 1.5 mm (power radius), Q = 1E5.
Suspension noise uses fpend = 1 Hz, mass = 0.25 kg or 1.56 kg, phi_pend = 3e6, phi_violin = 2*phi_pend.
Seismic noise is without active (STACIS) damping.

Optics parameters for flat ITMs:

Optics parameters for symmetric arms:

Optics parameters for 12m MC

email discussion: http://www.ligo.caltech.edu/~ajw/40m_testmass.txt

Thanks for looking at this!

To 40 meter Upgrade page.
To LIGO home page at Caltech, and to LIGO home page at MIT.

URL: http://www.ligo.caltech.edu/~ajw/40m_testmass.html
This page last modified July 25, 2000.
Questions? Contact Alan Weinstein