Optical Engineer Explains How to Clean LIGO's Unique Optics
To search for gravitational waves, the Laser Interferometer Gravitational-wave Observatory (LIGO) uses a laser beam that is split in two and travels down perpendicular 2.5-mile arms containing mirrors at their far ends. The beam reflects off the mirrors and bounces back to converge where the arms meet. A passing gravitational wave will stretch and squeeze space itself, causing the distance a light beam travels to increase or decrease ever so slightly and this changes the way the split beams ultimately converge. But if any component has so much as a piece of dust on it, it might contaminate LIGO's optics and diminish the signal of a gravitational wave. Advanced LIGO Senior Optical Engineer GariLynn Billingsley at Caltech describes the work she does to characterize and clean the optics that go into LIGO.
Transcript
00:00:05 LIGO optics are characterized at Caltech; we want to understand everything we can about the physical characteristics of these amazing objects. Knowing their physical dimensions helps us to install them in exactly the right position in the interferometer. We clean the optics carefully before examining them. Our cleaning process involves a solvent-based polymer liquid called First Contact. Once the layer dries, it leaves a pristine mirror surface. After an optic has been cleaned we can look for any defects that might scatter light and spoil the interferometer signal. Our test masses are the most closely scrutinized of all the LIGO optics - these are the mirrors that hang at each end of our four kilometer long arms and form the resonant cavity which grows longer
00:00:57 and shorter with each passing gravitational wave cycle. We measure the surface profile of the test masses in many different orientations using a Fizeau interferometer. We need to align each map with the next by using fiducials that we place on the surface using a very special ink pen. The individual surface maps are combined to create a map that is accurate to about one angstrom. These maps are used to create a software model of the entire interferometer. One of the most daunting elements of the project is working with these very large, very expensive mirrors. We are really glad when they're safe and sound inside their containers.