  LISA is an upcoming ESA mission that will detect gravitational waves in space
by interferometrically measuring the separation between free-falling test
masses at picometer precision. To reach the desired performance, LISA will
employ the noise reduction technique time-delay interferometry (TDI), in which
multiple raw interferometric readouts are time shifted and combined into the
final scientific observables. Evaluating the performance in terms of these TDI
variables requires careful tracking of how different noise sources propagate
through TDI, as noise correlations might affect the performance in unexpected
ways. One example of such potentially correlated noise is the relative
intensity noise (RIN) of the six lasers aboard the three LISA satellites, which
will couple into the interferometric phase measurements. In this article, we
calculate the expected RIN levels based on the current mission architecture and
the envisaged mitigation strategies. We find that strict requirements on the
technical design reduce the effect from approximately 8.7 pm/rtHz per
inter-spacecraft interferometer to that of a much lower sub-1 pm/rtHz noise,
with typical characteristics of an uncorrelated readout noise after TDI. Our
investigations underline the importance of sufficient balanced detection of the
interferometric measurements.
