Gautam and I looked into the DAC noise contribution to the noise budget for homodyne detection at the 40m. DAC noise is currently the most likely limiting source of technical noise.
Several of us have previously looked into the optimal SRC detuning and homodyne angle to observe pondermotive squeezing at the 40m. The first attachment summarizes these investigations and shows the amount of squeezing below vacuum obtainable as a function of homodyne angle for an optimal SRC detuning including fundamental classical sources of noise (seismic, CTN, and suspension thermal). These calculations are done with an Optickle model. According to the calculations, it's possible to see 6 dBvac of squeezing around 100 Hz.
The second attachment shows the amount of squeezing obtainable including DAC noise as a function of current noise in the DAC electronics. These calculations are done at the optimal 0.45 deg SRC detuning and 97 deg homodyne angle. Estimates of this noise are computed as is done in elog 13146 and include dewhitening. It is not possible to observe squeezing with the current 400 Ω series resistor which corresponds to 30 pA/rtHz current noise at 100 Hz. We can get to 0 dBvac for current noise of around 10 pA/rtHz (1.2 kΩ series resistor) and can see 3 dBvac of squeezing with current noise of about 5 pA/rtHz at 100 Hz (2.5 kΩ series resistor). At this point it will be difficult to control the optics however.
So it seems reasonable to reduce the DAC noise to sufficient levels to observe squeezing, but we will need to think about the controls problem more.
