100 Years of Relativity Space-Time Structure: Einstein and Beyond ...

Receiving Gravitational Waves 241too — particularly the requirement for the very precise control ofthe wavelength of the laser and of the lengths of the cavities. Indeedwith long cavities of the high finesse desirable here exceptionalshort-term wavelength stability is required from the laser.The heart of the difficulty is that, unlike a delay line, a Fabry-Perotcavity stores light because it is in itself an interferometer — the trapping ofthe light for many round trips comes about only by careful adjustment of thephases of the superposed beams. This can only occur when the wavelengthof the light and the length of the cavity are in resonance, that is matched sothat an integer number of waves fits into the cavity. Very near the resonancecondition, the phase of the output light varies with mirror separation in thesame way as the light that has traveled through a delay line. To achievethis condition, the light and the arm have to be locked together by a servosystem. Drever’s lecture goes on to describe the style of servo required,one that he and his group developed in conjunction with John Hall’s groupat the Joint Institute for Laboratory Astrophysics in Boulder, Colorado. 18This servo design has its roots in an analogous microwave device developedby Robert Pound. 19While the essence of the difficulty was thus solved, in practice the useof Fabry-Perot cavities has additional complications. One is due to the factthat when a cavity is not very close to resonance, the phase of the outputlight has almost no dependence at all on the separation of the mirrors,thus making it very hard to generate the sort of signal necessary to acquirethe lock on resonance in the first place. An additional level of complicationcomes when the arm cavities are assembled into a complete Michelsoninterferometer, since the interferometers within the interferometer need tobe separately controlled without degrading the function of the main instrument.Solving these sorts of problems robustly proved to be challengingwork. That it now has been done successfully in LIGO is a great engineeringtriumph.Drever goes on to show another important improvement in sensitivitythat can be achieved with Fabry-Perot technology. It starts from recognizingan opportunity in operating the interferometer so that, in the absenceof a gravitational wave, no light exits the interferometer toward the outputphotodetector; instead, all of the light returns toward the laser. Weiss hadproposed this operating point as a way of minimizing excess noise in theinterferometer readout. Drever’s insight was that, given the ultra-low levelsof absorption then becoming available in mirror coatings, the power in that