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Choosing the Readout Quadrature

by Robert Ward — last modified 2010-10-07 15:42

In detuned interferometers, the GW signal is no longer purely in the phase quadrature. It now appears in a frequency-dependent manner in all quadratures, and so a choice must be made about which quadrature to detect (when using homodyne detection).


Homodyne Detection (DC readout)


Homodyne detection is what is currently implemented in GWINC.  It differs subtlely from DC readout.  In DC readout, the quadrature is decided by the relative sizes of the arm cavity loss imbalance and the arm cavity length offsets. 

GWINC ignores this, and just uses a specified quadrature.  This plot shows how the inspiral ranges vary with quadrature for an input power of 125W and this plot is the same for 30W. 

Lacking better information, the current reference quadrature is pi/2. We should be able to get arbitrarily close to this quadrature, so long as the differential losses in the arm cavities are low.

Heterodyne Detection


In principle, with heterodyne detection, unbalanced RF sidebands will allow simultaneous detection of all quadratures.  The achievable quantum-noise limited sensitivity is still worse than with homodyne detection, but this is still worth considering.  It's usually ruled out for technical noise reasons rather than quantum-noise reasons, however.