photodiodes are shown enclosed in a vacuum system, initial commissioning will use in air photodiodes. The diagram also details the posit.ions of opt.ional subsystems - a fixed spacer reference cavity, laser mode cleaner, and power control - which will be installed as required. The interferometer has been modelled with the Optickle software package, paying particular attention to the DC offset and the PRM transmission. The Optickle ut parameters are given in Table 1, note that a small arm length asymmetry is assumed in order to include the effect of laser noise couplings. Figure 7 shows the variance of key interferometer parameters as a function of the PRM transmission, TpRM, and the static DARM offset, 8x. The inter­ ferometer noise floor is determined by the quadrature sum of shot noise, Johnson thermal noise in the transimpedance resistor, and voltage noise for the first amplifier. Parameter ut Laser Power Arm length BS-EM Arm asymmetry (X-Y) PRC length PR-BS End Mirror Transmission Beam splitter Transmission AR reflectivity Mirror loss (PR, BS, EM) Differential arm loss Substrate loss Transimpedance resistor Voltage noise Value 0.75 W