Cancellation of lateral displacement noise of 3-port gratings for coupling light to cavities
2015
Reflection gratings enable light coupling to optical cavities without
transmission through substrates. Gratings that have three ports and are mounted in a
second-order Littrow configuration even allow the coupling to high-finesse cavities
using low diffraction efficiencies. In contrast to conventional transmissive cavity
couplers, however, the phase of the diffracted light depends on the lateral position
of the grating, which introduces an additional noise coupling. Here, we
experimentally demonstrate that this kind of noise cancels out once both diffracted
output ports of the grating are combined. We achieve the same signal-to-shot-noise
ratio as for a conventional coupler. From this perspective, three-port grating
couplers in a second-order Littrow configuration remain a valuable approach to
reducing optical absorption of cavity coupler substrates in future
gravitational-wave detectors.
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