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.
OMC mirror specs
- 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 Mirror Diameter Thickness Side 1 Side 2 wedge Radius of curvature M1 7.75 mm 4 mm R > 99.9%, 0° inc R < 0.2% < 30 amin 1 m, CC/PL M2 1” 0.25” R ³ 99.97%, 0° inc R < 0.2% < 30 amin Flat PL/PL M3 1” 0.25” T ~ 1% , 45° inc, P-pol R < 0.2% 30 amin Flat PL/PL M4 1” 0.25” T ~ 1% , 45° inc, P-pol R < 0.2% 30 amin Flat PL/PL
[http://www.ligo.caltech.edu/~cit40m/Docs/DCdetect/40mDCdetect_mirrors.pdf OMC mirror spec document]
[http://www.ligo.caltech.edu/~cit40m/Docs/DCdetect/40m-Output_MC.pdf OMC mirror spec sheet]
[http://www.ligo.caltech.edu/~cit40m/Docs/DCdetect/CALTECH2357-02.pdf REO quote for input/output coupling mirrors (and more)]
[http://www.ligo.caltech.edu/~cit40m/Docs/DCdetect/40mDCdetect_OMC.pdf OMC design info]