Improvement in the shot noise of a laser interferometer gravitational wave detector by means of an output mode-cleaner

The sensitivity of a gravitational wave detector based on a laser interferometer is limited by photon shot noise for signals with frequencies above a few hundred hertz. The amplitude of this noise depends on the interferometer fringe contrast. By means of a complete simulation, we show that using a short optical cavity placed at the interferometer output port, the contrast can be significantly improved and the optimal sensitivity recovered. Such an output mode-cleaner has been realized and tested. The adopted design and the results of the tests are presented.