OUTPUT MODE CLEANER
The Output Mode Cleaner (OMC) takes the beam coming out of the anti-symmetric (AS) port of the interferometer and cleans it of all but the fundamental (TEM00) transverse mode. This means that it strips off junk which is not Gaussian; all of the ripply junk light which comes from a constrast defect; imbalances in the wavefronts of the two arms.
Reducing this junky, useless light reduces the overall light power detected and so also reduces the shot noise. 
In addition, the OMC has a narrow frequency passband (~1 MHz) and so it also reflects all the RF sidebands which are normally used to lock the interferometer.
By rejecting all the Higher Order Modes (HOMs), it is hoped that the OMC output will be stable enough to use as the gravitational wave error signal and also be used to lock the differential arm length (L-) degree of freedom.
The Output Mode Cleaner is mounted into a monolithic copper block cavity, with three fixed mirrors and one PZT length-controlled mirror.
OMC specs
http://www.ligo.caltech.edu/~cit40m/Docs/DCdetect/OMC_params.m
OMC mirror specs
Mirror |
Diameter |
Thickness |
Side 1 |
Side 2 |
wedge |
Radius of curvature |
comments |
M1 |
7.75 mm |
4 mm |
R > 99.9%, 0° inc |
R < 0.2% |
< 30' |
1 m, CC/PL |
PMC spare from P.King |
M2 |
1” |
0.25” |
R > 99.97%, 0° inc |
R < 0.2% |
< 30' |
Flat PL/PL |
Newport HR supermirrors |
M3 |
1” |
0.25” |
T ~ 1.5% , 45° inc |
R < 0.2% |
30' |
Flat PL/PL |
LHO spare, REO part #31301 |
M4 |
1” |
0.25” |
T ~ 1,5% , 45° inc |
R < 0.2% |
30' |
Flat PL/PL |
LHO spare, REO part #31301 |
- Input and output couplers: Spares from LHO. REO part #31301 – PL/PL – 1”dia. x .250” thick; Super-polished fused silica, wedged (30 min. arc +/-15 min); Coating Wavelength: 1064 nm; Angle of incidence: 45° - “P” Polarization; Side 1: HR – T ~1% (matched as close as possible among the 3 mirrors in the order); Side 2: AR – R = 0.2%.
- From: Helena Armandula; Date: Oct 19, 2005 9:51 AM; Subject: Mirrors tested at Caltech; The mirrors sent by Betsy were measured by Liyuan.; ... two 1" dia., coated at 45 degrees, P-pol, measured: T at 45 degrees P-pol is 15,500 ppm +/-200 ppm (or 1.55+-0.02%).
- The 1" flat reflector is one of a pair ordered and received by Steve, broadband HR super-mirrors from Newport. Steve and Liyuan tested them. The transmission was 600 ppm, at 0 and 5degrees incidence.
the curved mirror: M1: 7.75 mm dia; 4 mm thick; side 1: R > 99.9%, 0° inc; side 2: R < 0.2% < 30 amin; ROC: 1 m, CC/PL
REO quote for input/output coupling mirrors (and more) This quote has nothing to do with anything we actually have. Instead we ended up just scrounging crap from whoever would crap on us. See above table. 
SW Output Mode Cleaner Dwgs
The SW drawings are attached below.
LIGO-D050379-C.pdf LIGO-D050366-B.pdf LIGO-D050369-C.pdf LIGO-D050370-B.pdf LIGO-E050336-00.pdf