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.

[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)]