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Description
Laser noise can be modelled as laser amplitude and/or phase modulation sidebands that enter the interferometer. In an ideal dark-fringe condition, they interfere destructively at the dark port. But in reality, asymmetries between the two interferometer arms result into these laser noise sidebands competing against the gravitational wave phase modulation sidebands at the interferometer readout and can potentially mask a signal. The presence of arm and recycling cavities, radiation pressure effects, balanced homodyne readout and its beam-splitter asymmetry further complicate the laser noise coupling mechanisms which motivate their investigation and precise calculation of the laser noise requirements for the Einstein Telescope. We have used a Python-based frequency-domain interferometer simulation software, Finesse (version 3.0b3), to calculate these requirements. Here, we have assumed 1% asymmetry between the arm cavities, a small Schnupp asymmetry and 0.5% balanced homodyne beam-splitter asymmetry. The local oscillator for homodyne readout in our case is a pick-off from the power recycling cavity. For the high-frequency interferometer's input, the most stringent requirement calculated for laser frequency noise is $2\times10^{-7}\:\text{Hz}/\sqrt{\text{Hz}}$ around 50 Hz and that for laser relative power noise is $8\times10^{-10}\:1/\sqrt{\text{Hz}}$ around 10 Hz. Also, the readout phase of the local oscillator strongly influences the later requirement.