Operating LISA as a Sagnac interferometer

Abstract

A phase-locking configuration for LISA is proposed that provides a significantly simpler mode of operation. The scheme provides one Sagnac signal readout inherently insensitive to laser frequency noise and optical bench motion for a non-rotating LISA array. This Sagnac output is also insensitive to clock noise, requires no time shifting of data, nor absolute arm length knowledge. As all measurements are made at one spacecraft, neither clock synchronization nor exchange of phase information between spacecraft is required. The phase-locking configuration provides these advantages for only one Sagnac variable yet retains compatibility with the baseline approach for obtaining the other TDI variables. The orbital motion of the LISA constellation is shown to produce a 14 km path length difference between the counter-propagating beams in the Sagnac interferometer. With this length difference a laser frequency noise spectral density of 1 Hz/ √ Hz would consume the entire optical path noise budget of the Sagnac variables. A significant improvement of laser frequency stability (currently at 30 Hz/ √ Hz) would be needed for full-sensitivity LISA operation in the Sagnac mode. Alternatively, an additional level of time-delay processing could be applied to remove the laser frequency noise. The new time-delayed combinations of the phase measurements are presented.

Synthetic LISA: Simulating time delay interferometry in a model LISA

“…Time Delay Interferometry [3,4,5,6,16,17,19,20,21,22,23,24,25,26,27] is a technique to combine the basic Doppler variables y slr and z slr into composite observables that are sensitive to GWs, but that are free of the otherwise overwhelming laser frequency noise (they are also free of optical-bench and fiber noise, as discussed above). To understand how TDI works, it is useful to tie the algebraic representation of the TDI observables to a visual picture of the path traveled by light between the LISA spacecraft.…”

Erratum: Optimal filtering of the LISA data [Phys. Rev. D70, 022003 (2004)]

“…Recently, it was pointed out [7,8,9,10] that the rotational motion of the LISA array around the Sun and the time dependence of light travel times introduced by the relative (shearing) motion of the spacecraft have the effect of preventing the suppression of laser frequency fluctuations, at least under the current stability requirements, to the level of the secondary noises in the TDI observables as derived for a stationary array. This problem was addressed by devising new combinations that are capable of suppressing the laser frequency fluctuations below the secondary noises for a rotating LISA array [7,8], and for a rotating and shearing LISA array [9,10].…”

“…This problem was addressed by devising new combinations that are capable of suppressing the laser frequency fluctuations below the secondary noises for a rotating LISA array [7,8], and for a rotating and shearing LISA array [9,10]. In this context, the original stationary-array combinations are sometimes known as "TDI 1.0" (or first-generation TDI ), the rotating-LISA combinations as "TDI 1.5," and the rotating and shearing-LISA combinations as "TDI 2.0;" following Ref.…”

Principal component analysis for LISA: The time delay interferometry connection

“…These are linear combinations of the six LISA data streams, suitably offset in time to cancel the noise contributions from the three lasers. TDI variables have received detailed attention in the literature and there is now a sophisticated understanding of their generation and properties [2,6,7,8,14,21], extending to 'second-generation' TDI variables that take account of the slight relative motion of the spacecraft [13,17].…”