[Zach / Koji]

Measurement of the primary (CCW) cavity error / feedback signal.

Method:

- Lock the cavity. Measure the spectrum of the input monitor output.
- Close the laser shutter. Measure the same spectrum.

- Measure the feedback signal after the notch filter.

Result:

- The global shape of the error signal is a kind of flat. Though it's spiky due to mechanical resonances.
  Below 10Hz it got smoother but the actual shape is not obvious because of the rough resolution.
- At around 100Hz, the error is below the dark noise. This means the out-of-loop stability does not go below the dark noise level.

Thought:

- The broad peak at around 10Hz is the servo bump.
- Note that the signal is supposed to be amplified by a factor of +41. This means the dark noise floor level is ~25nV/rtHz.

To Do:

• Calibrate these spectra in Hz/rtHz.
  • How much is the optical gain in V/Hz or V/m.
• Convert this measurement into the noise budget of the Gyro signal.
• Make the unsuppressed spectrum
  • => How much cavity length fluctuation does the error signal feel if there is no feedback and the sensor is infinitely linear?
  • This requires the model of the open loop TF
  • Compare the consistency with the same quantity derived from the feedback signal
  • Compare the unsuppressed frequency noise spectra with the free running laser freq noise (see rana's phd thesis P.80).
  • Convert the spectrum into the displacement noise (m/rtHz) and compare with the displacement on the table.
• Measure the shot noise level
  • How much DC power do we typically have? (Actually we don't have the record during this measurement, but we can measure it again)
• Measure low frequency spectra with CDS
• How to improve the noise floor
  • Is this noise level disturbing in terms of the gyro requirement? How about in the low frequency?
  • How much is the demodulator noise? Put a 50-ohm terminator on the cable instead of the PD. Then measure the same signal.
  • How much is the gain of the input stage. It is supposed to be +41, but we are not sure.
    => Think about the noise level at the demodulator output.
  • How much is the noise level of the PD measured by an RF analyzer? Is it consistent with the above analysis?
  • Do we need a new resonant RF PD? How much should we improve the noise level with the new PD? And how much noise does the new one actually have?