# Why do LIGO use a quarter wavelength for detecting gravitational waves?

I have already researched into this and I am left slightly confused still. I have gathered that the use of a quarter wavelength is to turn a linearly polarised wave into a circularly polarised wave. But why? I appreciate any feedback.

• One-quarter of the wavelength is just an estimate because the frequency or wavelength isn't sharply determined for a signal. But the reason why LIGO is highly sensitive when $L=\lambda / 4$ is exactly analogous to the reason why the electric dipole antenna is best at $L=\lambda / 2$. – Luboš Motl May 26 '16 at 14:47
• Can you be more specific? Is there some image or text where they specifically mention the use of a quarter wavelength at LIGO? If you mean quarter waveplates, they are used in any large optics experiment. Note that at LIGO, you would find these typically on a laser bench, but it is not a key component in the interferometer itself. – Bas Swinckels May 31 '16 at 16:11

## 1 Answer

I believe you mean "quarter wave plate" rather than quarter wave length: https://en.wikipedia.org/wiki/Waveplate

Quarter wave plates are sometimes used after polarized beam splitters (PBS). Suppose you have a PBS illuminated by a source. One polarization (say linear horizontal) goes one way and the other (linear vertical) goes the other way. Now suppose you want to split the linear vertical light a second time. If you put it through another PBS then the light won't be split; all the light will come out in the vertically polarized direction. So you insert a quarter wave plate, which turns the linear vertical light into circular, after which the second PBS splits the light in half once more.

To get a more specific answer, give us a link to a picture of the interferometer setup you are are trying to understand and we can answer what specific purpose it plays there.