KAGRA Logbook

Since the SNR in DC readout is proportional to the inverse of the square root of the DARM offset power (i.e., the OMC transmitted power), the smaller the offset power, the higher the SNR—provided the IFO lock is maintained. However, due to interferometer asymmetries, such as birefringence, the offset power has nonzero value even on perfect dark fringe; contrast defect. In principle, this defect limits how low the offset power can be. Near the dark fringe, OMC transmitted power is quadratically dependent on optical gain with contrast defect offset.

Now we have estimated the contrast defect and the value is 0.13 mW which is roughly consistent with Haoyu-san’s birefringence simulation.

We measured DARM LSC loop gain with various offset power. Offset changes are compensated by adjusting the gain at K1:LSC-OMC_DC to keep it locked, so the gain is roughly constant.

Fig1: K1:LSC-DARM1_IN1 for each DARM offset power

Optical gain is proportional to K1:LSC-DARM1_IN1/ K1:LSC-OMC_DC. We focused on 86.8 Hz and created a plot of the OMC transmitted power versus the optical gain (W/m).

Fig2: plot of the DARM offset(transmitted power) versus the optical gain(normalized to the value when offset power is 15 mW) @86.8 Hz, and its quadratic curve fitting
Fig3: enlargement of Fig2

In Fig2, the fitting curve shows contrast defect is ~ 0.13 mW.
Additionally, IFO lock is stable with 5 mW offset. This suggests that it is possible to keep the current SNR (with 15 mW offset) if the input laser power is tripled.