Phase locking to a LISA arm: First results on a hardware model

We present the first experimental confirmation of the so-called 'self-phase-locked delay interferometry'. This laser frequency stabilization technique consists basically in comparing the prompt laser signal with a delayed version of itself that has been reflected in another LISA satellite 5 × 10⁹ m away. In our table-top experiment, the phase of a voltage-controlled oscillator is stabilized by means of a control loop based on this technique. In agreement with the theory, the measured unity gain frequency is not limited by the inverse of the used delay (1.6 νs). In the time domain, the system also behaves as predicted, including the appearance of a quasi-periodic 'ringing' just after the lock acquisition, which decays exponentially. Its initial amplitude is smaller when the loop gain is slowly ramped up instead of suddenly switched on.