Abstract
Laser frequency noise is a significant noise source which couples into the main science measurement of the Laser Interferometer Space Antenna via the mismatch between the interferometer arm lengths. In this paper we discuss the application of an unequal pathlength heterodyne Mach–Zehnder interferometer to measure and actively stablize the master laser frequency as used in LISA Pathfinder. In comparison with an optical cavity or atomic reference the technique has a wide operating range and does not require a complex lock acquisition procedure. Frequency tuning can be provided by purely electronic means and does not require physically changing the pathlength (or resonance frequency) of the frequency reference and can therefore be combined with arm locking in a straightforward manner.
Highlights
IntroductionThe Laser Interferometer Space Antenna (LISA) aims to detect gravitational waves in the 100 μHz to 1 Hz frequency band by measuring the distances between freely falling proof masses enclosed within three identical spacecraft in a triangular constellation [1]
Laser frequency noise is a significant noise source which couples into the main science measurement of the Laser Interferometer Space Antenna via the mismatch between the interferometer arm lengths
In this paper we discuss the application of an unequal pathlength heterodyne Mach–Zehnder interferometer to measure and actively stablize the master laser frequency as used in Laser Interferometer Space Antenna (LISA) Pathfinder
Summary
The Laser Interferometer Space Antenna (LISA) aims to detect gravitational waves in the 100 μHz to 1 Hz frequency band by measuring the distances between freely falling proof masses enclosed within three identical spacecraft in a triangular constellation [1]. Whilst the orbits of the spacecraft are designed to keep the separation between the three spacecraft equal and as constant as possible without station-keeping, a slowly changing mismatch on the order of ±60 000 km is unavoidable [2] This large arm length mismatch couples laser frequency noise with the differential single-pass. The reference interferometer measures the phase difference (φR) between the lasers on adjacent optical benches in one LISA satellite. To measure the frequency noise of the master laser, an additional interferometer with unequal pathlengths, as will be implemented in LISA Pathfinder [15, 16], can be placed on the optical bench. The laser from the adjacent optical bench (the slave laser) is offset phase locked to the master with high gain/bandwidth with a constant offset: νs = νM + fhet (4). The first loop only controls the frequency difference between the two lasers
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