Elli, Daniel, Evan

Summary

We tried adding a dc bias to the PSL EOM in order to reduce the RFAM, with little success.

Details

At the top of ISC R1, we inserted a minicircuits bias tee into the 9 MHz drive that goes to the PSL EOM. With a DMM, we measured the input impedance of the 9 MHz drive to be an open, and the input impedance of the analogous point in the 45 MHz drive was 70 Ω. We take this to mean that the 9 MHz drive goes directly to the EOM, with no intervening amplifier.

To see the effect of dc bias, we first hooked up the dc port of the tee to ISC-EXTRA_C_AS_AO_4, which is a slow DAC channel on the ISC rack. We applied ±10 V dc offset while watching the time series of REFL_A 9I&Q, but saw no obvious changes.

So instead, we hooked up the dc port of the tee to LSC-EXTRA_AO_2 (a fast channel) and drove it with a 32700 ct, 444 Hz sine wave. Then we looked for this excitation in spectra of REFL_A RF9I&Q.

• We found that the signal showed up mostly in Q, at 0.014 ct/rtHz,with a bandwidth of 0.187 Hz. This corresponds to 6.1×10⁻³ ct_rms.
• Using 4.7×10⁶ ct/W as the end-to-end calibration of the PD (measured by Kiwamu), this is 1.3×10⁻⁹ W_rms.
• The DC power was 81 mW_dc, according to the calibrated output of LSC-REFL_A_LF. Since our bias voltage is V_bias = 32700 ct ≈ 10 V, the effect of our excitation is (1.6×10⁻⁹ W_rms/W_dc) / V_bias.

Previously (LHO#15681), the RFAM level has been pegged at 1.1×10⁻³ W_rms/W_dc. That means that the fractional RFAM change that can be effected by this bias tee configuration appears to be something like 1×10⁻⁶ / V_bias, which is way too small to be practical.