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Description
Gravitational-wave detectors require a purely Gaussian beam in output, which ensures a smooth distribution of the optical power across the transversal plane, and the lowest angular divergence. For this reason, an Output Mode Cleaner (OMC) optical cavity is placed before the detection photodiode, to filter out the control sidebands created with phase modulation, and spurious higher-order modes (HOM) that can arise from optical mismatch and misalignment in the interferometer. The performance of an OMC is evaluated in terms of HOM suppression factor, Gaussian mode transmittivity and noise from back-reflected stray light.
The present contribution illustrates the current status and plans for optical and mechanical design of the OMC for the ETpathfinder experiment, at a wavelength of 1550 nm. The chosen cavity layout is the so-called “bow-tie”, with two highly reflecting mirrors and two input and output couplers. All the mirrors will be glued to a base plate of the same material (fused silica), to reduce cavity length noise due to thermal expansion. At the moment, the initial assessments on the input beam specifications, available space on the optical bench, and target performance requirements are ongoing. Simulations in FINESSE and OSCAR are used to build the best optical configuration. Once the optical layout will be fixed, the mechanical design of the base plate and mirrors will begin. Future steps include the whole process of characterization of the single optical elements, mounting, installation and commissioning of the OMC at the ETpathfinder site.