It's possible to subtract a lot of laser noise from DARM using H1:LSC-REFL_A_RIN_OUT_DQ as a witness. Another channel that works (but not as well) is H1:LSC-REFL_SERVO_ERR_OUT_DQ.

DARM coherence with laser noise is reduced to almost zero.

We can even improve a bit the noise all the way down to 1 kHz.

One question to address is the stability over time of the subtraction parameters. I expect they might change with thermalization and if we change TCS settings.