Laser interferometry

(our tool for detection gravitational waves)

Modern gravitational wave detectors are enormous scientific instruments, with laser interferometers at their heart. Light has both particle and wave characteristics. It transfers energy like a particle but propagates like a wave. Interferometry can display the wave-like nature of light. We make use of light waves to measure length between mirrors to an extremely high precision. A passing gravitational wave would change the observed distance between the mirrors and can thus be measured.

They key to the successful design and construction of such a laser interferometer, is to ensure that other disturbances, such as environmental noise or technical noise, do not disturb the interferometer signals. The individual elements that compose the interferometers, including mirrors, beam splitters, lasers, modulators, various polarising optics, photo detectors and so forth, are individually well described by relatively simple, mostly-classical physics. However, a new complexity arises from the special application and combination of these elements which is different from other applications. Thus optical design tasks are based on relatively simple mathematics but due to the complexity of the overall system, numerical simulations have to be used.

More information about the mathematical framework for describing the optical systems can be found in this online review article. Expert readers might be interested in this more the 400 pages long `first chapter' on optics in the so-called Virgo Book of Physics by Jean-Yves Vinet.

We make use of several computer simulations and related tools for designing, analysing and optimising laser interferometers. You can find several of these simulation programs on the Wiki pages of the GEO Simulation Group. On gwoptics.org we provide two software packages related to optical simulations:

• SimTools A collection of Matlab functions and scripts related to optical and numerical simulations.
• Finesse An interferometer simulation program.

Finesse is free to download and use. It comes with an extensive manual and can be used to model user defined interferometers: from very simple interferometer to the most advanced laser interferometric gravitational wave detectors today.