It does seem to suggest that the source of the low frequency noise is something that is not unique to either the transmission or reflection readout (ie not the MZ phase noise or PLL phase noise), and is not something that should be suppressed by the loop gain (like input optics noise or AOM oscillator phase noise) in the transmission readout.
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Below are the (delayed) results of Friday's oscillator noise measurements, calibrated to gyro units and plotted alongside the gyro noise. The traces are:
- Gyro noise from AOM actuation
- Gyro noise from PLL actuation
- Two marconis with fcarr = 100 MHz and dev = 100 kHz/V beating together, with the beatnote fed back to one and the other's dev input shorted. This is divided by sqrt(2) to estimate the noise from the PLL oscillator alone in gyro mode. It agrees pretty well with (2) in the region we think is dominated by oscillator noise.
- Two marconis with fcarr = 50 MHz and dev = 100 kHz/V beating together, with the beatnote fed back to one and the other's dev input shorted. This is multiplied by 2 (i.e., 2/sqrt(2)) to estimate the noise from the AOM actuator alone in gyro mode, taking into account the double pass. This agrees pretty well with (1) in the region we think is dominated by oscillator noise
- This isn't really an oscillator phase noise measurement. It is the feeback signal to the PLL oscillator with the modulation on the AOM off (i.e., no gyro signal). This is significant in that, since it is roughly equal to the estimated PLL oscillator noise at low frequency, it rules out the possibility that the excess LF noise comes from phase noise in the output MZ. Therefore, the excess noise must be noise imparted on the CW light by the secondary loop. This helps narrow things down.
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