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Abstract

Sagnac speed meter (SSM) topology is known as an alternative technique to
reduce quantum back-action in gravitational-wave interferometers. However, any
potential imbalance of the main beamsplitter was shown to reduce the quantum
noise superiority of speed meter at low frequencies, caused due to increased
laser noise coupling to the detection port. In this paper, we show that
implementing balanced homodyne readout scheme and for a particular choice of
the local oscillator (LO) delivery port, the excess laser noise contribution to
quantum noise limited sensitivity (QNLS) is partly compensated and the speed
meter sensitivity can outperform state-of-the-art position meters. This can be
achieved by picking the local oscillator from interferometer reflection
(\textit{co-moving} LO) or the main beamsplitter anti-reflective coating
surface (BSAR LO). We also show that this relaxes the relative intensity noise
(RIN) requirement of the input laser. For example, for a beam splitter
imbalance of $0.1 \%$ in Glasgow speed meter proof of concept experiment, the
RIN requirement at frequency of 100Hz decreases from $4\times
10^{-10}/\sqrt{\rm Hz}$ to $4\times 10^{-7}/\sqrt{\rm Hz}$, moving the RIN
requirement from a not practical achievable value to one which is routinely
achieved with moderate effort.