/home/ajw/public_html/40m_optspecs.txt AJW, 10/24/01
Updated for as-installed MC1,MC2,MC3, AJW, 4/11/02
Specs for the 40m suspended optics:
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Core optics:
The the core optics coatings
are as specified for Advanced LIGO in:
http://www.ligo.caltech.edu/~ligo2/scripts/l2refdes.htm
and
LIGO-E000487-00-D (Helena) AdvLIGO Coating specs
and all the other good stuff in
http://www.ligo.caltech.edu/~gari/
Namely:
T_SRM = ( 7.0 +- 0.7) %
- For AdvLIGO, this value maximizes the CBI reach.
- For 40m, this gives a strong peak in the RSE response function,
without making the SRC be too high in finesse
to be easily controlled.
T_ITM = ( 0.50 +- 0.05)%
- For AdvLIGO, this value maximizes the CBI reach.
- For the 40m, we want the same finesse in the arms as in AdvLIGO.
- The common-mode tolerance is estimated by requiring
the reflectivity of the IFO to stay in the over-coupled region
for a fixed T_PRM of 7%.
- The diff-mode tolerance is estimated by requiring
the resulting contrast defect to be no bigger than 300ppm.
- A tighter tolerance of ( 0.50 +- 0.01)% would be desirable!
T_ETM = < 20 ppm, > 10 ppm (ie, 15 +- 5 ppm)
- For modeling, we assume that the total ETM loss,
including transmission, is 37.5 ppm.
Performance of the IFO is a strong function of this number!
- For AdvLIGO, spec is > 1 ppm, becuase it operates at high power;
the 40m needs to see the light even with 0.1 w of input light.
T_BS = (50 +- 1)% at 45degree incidence,
- taken from http://www.ligo.caltech.edu/docs/E/E000487-00.pdf,
assuming that that's a "do-able" tolerance.
- I'm sure that a tighter tolerance would be desirable!
For the PRM, we will have 3 specs:
T_PRM = (6.5+-0.7)%
(7.0+-0.7)%
(8.0+-0.7)%
- Note that http://www.ligo.caltech.edu/~ligo2/scripts/l2refdes.htm
says "T_PRM ~ 6% "
but my Twiddle simulations suggest that this would yield
an under-coupled IFO; T_PRM needs to be 6.5% or more
for an over-coupled IFO (r_front < r_back).
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AR coatings:
- We want pickoff beams from the BS and ITMs;
- and there's no harm in also having a pickoff beam at the PRM
just in case (although we also have an "initial pointing beam").
But we'll get sufficient beam with a standard <300 ppm coating.
- In principle, we should optimize the pickoff amount
(trading off PRC loss, which reduces PRC gain,
for sufficient pickoff signal).
- However, this is not a critical issue for the 40m,
so we can stick with the Initial-LIGO and AdvLIGO spec of 600 ppm.
- NOTE that this builds in 1200 ppm of loss, minimum,
in the PRC cavity. For modelling, assume 1500-2000 ppm of loss, total.
SO:
R_AR = (600+-100) ppm for BS, ITM
R_AR < 300 ppm for PRM, SRM, ETM
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Coating of the ITMs and ETMs should be over the central
60 mm diameter aperture of the 125 mm diameter optic;
this reduces the re-introduction of scattered light
back into the high-finesse arm cavities.
Noe that the 1ppm beam diameter is 16 mm in the ITM and
28 mm in the ETMs.
All other optics can be coated over the full 75 mm diameter optic.
Polarization:
In the main IFO, the beam is everywhere P-polarized
(parallel to the horizontal, such that the E-field
vector "pierces" beam-splitter).
In the Input mode cleaner, the beam is
S-polarized ("skimming" the flat mirrors).
The Faraday isolator rotates the polarization
by 90degrees in between the input mode cleaner
and the main IFO.
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Input mode cleaner optics
are as specified for Initial LIGO in the
Input Optics Final Design document
http://www.ligo.caltech.edu/docs/T/T980009-01.pdf
Table 17
MCFM1 & MCFM2:
T_HR = 2000 +- 100 ppm
R_AR < 300 ppm
MCCM :
T_HR = 10 ppm +0 - 10 ppm (ie, < 10 ppm)
R_AR < 300 ppm
Note that Dave Reitze suggests that we coat all optics
over the full 3" diameter surface,
(instead of the 28 mm aperture on the MCCM as in Initial LIGO)
in order to avoid alignment difficulties.
That sounds good to me.
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Summary table:
T_HR R_AR aperture angle
MCFM1,2 2000+-100ppm < 300ppm 75 mm full 45, S-pol
MCCM 10+0-10ppm < 300ppm 75 mm full 0.43,S-pol
PRM1 (6.5+-0.7)% < 300ppm 75 mm full 0
PRM2 (7.0+-0.7)% < 300ppm 75 mm full 0
PRM3 (8.0+-0.7)% < 300ppm 75 mm full 0
SRM (7.0+-0.7)% < 300ppm 75 mm full 0
BS (50+-1)% 600+-100 ppm 75 mm full 45, P-pol
ITMs 5000+-500ppm 600+-100 ppm 60 mm 0
ETMs 15+-5ppm < 300ppm 60 mm 0
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AS INSTALLED:
13 meter input mode cleaner optics installed on April 10, 2002
by Batsy Bland, Steve Vass, Dennis Ugolini.
PSL -> IOO -> Periscope -> M1 -> MC1 -> MC3 -> M2 -> M3 -> M4 -> M5
| ^
-> MC2 -|
MC1: SN: MCFM03 (DCC # E020437-00, 4/3/02.)
wedge: RIGHT (facing HR surface, thicker side is to the right;
scribe is on thin side, pointing to HR surface).
wedge: 30'
mass: 0.235 kg
ROC > 5625
T_HR = 2000+-100 ppm
R_AR < 300 ppm
aperture: 75 mm diameter, fully coated
thickness: 25 mm
spot w = 1.66 mm
angle: 45degrees, S-pol (perp to optical table)
OSEM SNs: S:229, UL:230, LL:242, UR:235, LR:238
pitch: 3.9 mrad LOW
magnets mis-centered in order to get pitch better.
defects: no defects in cental 10 mm diameter.
Small coating defect, ~0.52mm long,
8 mm from center, ~ 280degrees.
MC2: SN: MCCM01 (DCC # E020438-00, 4/3/02.)
wedge: LEFT
wedge: 30'
mass: 0.225 kg
ROC 18.4 m
T_HR = 10 ppm
R_AR < 300 ppm
aperture: 75 mm diameter, fully coated
thickness: 25 mm
spot w = 3.08 mm
angle: +-0.5 degrees from normal, S-pol
OSEM SNs: S:201, UL:203, LL:251, UR:219, LR:236
pitch: 3.25 mrad LOW
defects: no defects in cental 10 mm diameter.
Within the central 30mm dia, there is a round defect
~8mm from center,
area~434 um^2, less than the 500 um^2 spec.
Outside central 30 mm dia, defect ~1764 um^2.
MC3: SN: MCFM01 (DCC # E020436-00, 4/3/02.)
wedge: LEFT (facing HR surface, thicker side is to the left;
scribe is on thin side, pointing to HR surface).
wedge: 30'
mass: 0.235 kg
ROC > 5625
T_HR = 2000+-100 ppm
R_AR < 300 ppm
aperture: 75 mm diameter, fully coated
thickness: 25 mm
spot w = 1.66 mm
angle: 45degrees, S-pol (perp to optical table)
OSEM SNs: S:217, UL:218, LL:232, UR:223, LR:224
pitch: 2.35 mrad HIGH
defects: no point defects anywhere. Numerous sleeks.