Pre-Phase A Report - Lisa - Nasa

Chapter 3
Experiment Description
3.1 The interferometer
3.1.1 Introduction
When a gravitational wave passes through the plane of the LISA antenna it can be regarded
as changing the geometry of the antenna. The precise length of each arm is defined by
the distance between the front faces of proof masses positioned inside the three drag free
spacecraft. The changes in proof-mass separations are determined by measuring the phase
delays for laser beams which have traversed the arms of the interferometer. Due to the
wide spread of even a well collimated laser beam over the arm length of 5×106 km, the
beam cannot simply be reflected at the far spacecraft. Rather, the beams are ‘amplified’
with the help of a laser at each craft. So the interferometry is done by comparing the
phase of an infra-red laser beam being transmitted out to a far mass with that of the
beam transponded back.
In this section, we discuss the layout of the optical system, the performance requirements
it must meet and the current plans for measuring the phase of the various heterodyne
signals. In Section 4.3 we explain in more detail the planned methods for correcting for
the laser phase noise and for removing the effects of clock noise.
To describe the method for measuring the distance changes, it is useful to have a nomenclature
for referring to the different spacecraft, optical benches, and arms. The three
spacecraft (S/C) are labelled A, B and C, and each S/C contains two optical benches,
labelled A1, A2, B1, B2 and C1, C2. The arms of the interferometer are defined between
the optical benches, e.g. arm 1 is between optical benches A1 and C2. The whole nomenclature
is shown in figure 3.1. Note that all designations progress counterclockwise around
the triangle.
The laser associated with optical bench A1 will serve as the master laser for the whole
system and will be locked to an on-board reference cavity. All the other lasers in the
system will be phase locked to this master laser. The lasers on a spacecraft can thus
be considered as essentially identical and the three spacecraft can thus be thought of as
forming a Michelson interferometer with an extra arm. Signal information from the other
two S/C will be sent back to S/C A by modulation put on the laser beams travelling
Corrected version 2.08 53 3-3-1999 9:33