The Laser Interferometer Space Antenna for

Gravitational Wave DetectionCody Arceneaux

Department of Physics and Astronomy, Louisiana State University

IntroductionThe Laser Interferometer Space Antenna (LISA) is a proposed mission of ESA and NASA to put a gravitational

wave detector in space. The detector will be made up of three satellites forming the vertices of an equilateral

triangle in orbit around the Sun. Each of the satellites will send an infrared laser beam to the other two to form an

interferometer. As a gravitational wave passes though the LISA constellation, the distance between the satellites

will change by a small amount.

The LISA Pathfinder is scheduled to launch in 2011. It is a single

satellite designed to test the concept and technology of the proposed

LISA mission. If it is decided that LISA will be built and launched, the

suggested launch date is 2020.

By putting the interferometer in space, it is able to avoid the noise

inherent to ground-based gravitational wave detectors such as LIGO

and VIRGO. In addition, the two different types of detectors will not

be redundant. LISA will be able to detect gravitational waves with frequencies between 0.1 mHz and 0.1 Hz. Ground-based

detectors are usually able to detect waves between 1 Hz and 1 kHz. This allows LISA to observe gravitational waves from

merging massive black holes, white dwarf binaries in our galaxy, neutronstar and black hole binaries, and hopefully unforeseen

sources.

References

Jennrich, O. LISA technology and instrumentation. Classical Quantum Gravity, 26:153001, 2009.

Ara ujo, H., et al. LISA and LISA PathFinder, the endeavour to detect low frequency GWs. Journal of Physics: Conference Series 66:012003, 2007.

Hecht, Eugene. Optics ,Fourth Edition, 2002.

ESA Science and Technology: LISA, ESA Portal, http://sci.esa.int/science-e/www/area/index.cfm?fareaid=27 (May 2, 2010)

Acknowledgments

I would like to thank Dr. Greg Stacy, for giving me the opportunity to research a

topic that I have a great deal of interest in, and I would like to thank Colleen Fava,

and my classmates in Dr. Stacy’s Optics coursefor their valuable input and

advice.

ConclusionsThe proposed LISA mission is a very complex and ambitious project. If it is successful, it will allow astronomers to observe with something other than the various

wavelengths of light. It could help confirm aspects of general relativity, provide a great deal of insight on the formation and growth of massive black holes, and survey

various binary systems.

LISA Constellation Major ComponentsThe optics on each of the LISA satellites can be broken down into three main components: the optical bench, the telescope, and the laser assembly.

The LISA mission in orbit around the Sun. Image from.ESA LISA

website.

Underlying ConceptsMichelson Interferometery

The LISA mission is basically a modified Michelson interferometer. In a standard Michelson interferometer, a laser from one of the

satellites is emitted and split by a beam splitter. The two resulting beams are bounced off of mirrors back to the source and then

joined together again. The combined beam is then analyzed. If the lengths of the paths the diverged beams took are not equal, phase

noise will be detected.

Interferometry with LISA

Instead of the lasers being aimed toward mirrors on the other satellites, they are instead aimed at free-falling test masses. The test

masses are set up so that the satellites containing the equipment shield them possible sources of noise and fly without disturbing them.

Sensors determine if the satellites will disturb the test masses a, and micro-thrusters keep them from interacting.

Due to the extreme length of the interferometer’s arms, the laser’s beam will diverge to being kilometers wide, and the laser’s power

reaching the second satellite will be greatly decreased. Because of this, transponders will be used. Light will not be reflected back from

the second satellite by the test mass like a standard Michelson interferometer’s mirror, the information about the phase will be read, and

light from the laser on the second satellite will be emitted with the phase data encoded into it..The orientation of the LISA satellites. Image

from Ara ujo, H., et al.

Comparison of possible LISA and LIGO targets.

Image from Ara ujo, H., et al.

Optical Bench

Due to the nature of the mission, the optical bench is

required to be complex. The optical bench will contain all of

the equipment for the interferometery needed for the

experiment. In addition to the various lenses and mirrors

used to move and focus the laser light, several other

important mechanisms are placed on the optical bench:

•Point-ahead angle mechanism (PAAM) – helps to correct for

the aberration in the laser due to movement of the satellites

perpendicular to the laser beam

•Optical truss – used to determine how stable the telescope

structure is

•Reference interferometer – used to determine the phase

noise between the primary and secondary laser

Telescope

The telescope is a 40 cm Cassegrain and is designed to take

in and re-emit the light coming in from another satellite.

There are two on each satellite

Laser

The laser assembly is be made up of the primary and

secondary laser and various instruments to control them

properly. The primary laser is a 1 W Yag-Neodymium

infrared laser with a wavelength of 1064 nm. A 100 mW

secondary laser is also present to be used for reference

purposes in the PAAM and the optical truss.

The optical bench in LISA. Image from Jennrich, O.