Interferometry in Space

The LISA mission comprises three identical spacecraft located 5 million km apart, forming an equilateral triangle. The center of the triangle lies in the ecliptic plane and follows the Earth around the Sun. The corners are defined by free floating test masses within the spacecraft. LISA is basically a giant Michelson interferometer in space. The distance between the spacecraft determines the frequency range LISA can observe. It is chosen so that it complements the range of the ground-based interferometers, allowing to observe the most interesting sources of gravitational radiation.

Work in Hannover so far has concentrated on the LISA Pathfinder mission. Here the "Optical Metrology System" was designed, a laser interferometer that will monitor the distance between the free floating test masses with pm resolution. The interferometry for LISA itself presents a number of new , even more challenging problems such as the fact that only a few 10 pW of light arrive at the other end, the initial lock acqusition, wavefront sensing to optimize the beam curvature and the need to stabilize the laser frequency many decades better than has ever been demonstrated. New developments are necessary for several key components on the optical bench, such as angular actuators to compensate the point-ahead angle and length actuators for the reference cavity.

Big Bang Observer, the LISA follow-on mission will need a number of completely new technologies to achieve the desired sensitivity. These include in particular high-power lasers and local optical readout of the test mass orientation within its enclosure.