(Alexa, Sheila, Rana)
PLL Servo Board as per previous alogs..
PDH Servo Board Settings:
- Input 1 enable: ON
- Input 1 polarity: POS
- Input 1 gain: -7dB
- Common Comp: ON
- Boost 1: ON
- Boost 2: ON unless specified
Transfer Functions:
- Frequency: 24.407363 MHz, Demod phase: 120.7 deg (228 steps): SCRN0343.TXT (mag), SCRN0344.TXT (phase)
- Frequency: 24.407363 MHz, Demod phase: 140.7 deg (266 steps): SCRN0345.TXT (mag), SCRN0346.TXT (phase)
- Frequency: 24.407363 MHz, Demod phase: 100 deg (190 steps): SCRN0348.TXT (mag), SCRN0347.TXT (phase)
- Frequency: 24.407363 MHz, Demod phase: 80 deg (152 steps): GIF 349 ---- each measurement was different
- Frequency: 24.402363 MHz, Demod phase: 120.7 deg (228 steps): SCRN0354.TXT (mag), SCRN0355.TXT (phase)
- Frequency: 24.401363 MHz, Demod phase: 120.7 deg (228 steps): SCRN0356.TXT (mag), SCRN0357.TXT (phase)
- Frequency: 24.387319 MHz, Demod phase: 120.7 deg (228 steps): SCRN0358.TXT (mag), SCRN0359.TXT (phase) --- bistable mode, hops to 01 mode
I have attached two plots summarzing the above results. One plot consists of OLTFs where the modulation frequency was held at 24.407363 MHz and the demon phase as was adjusted (EX_PDH_OLTF_DiffDemodPhase.pdf), while the other plot has the demod phase held at 120.7deg (228 steps), while the modulation frequency was adjusted (EX_PDH_OLTF_DiffModFreq.pdf).
Amplitude Spectrum (from IMON) --- Freqency: 24.407363 MHz, Demod phase: 120.7 deg (228 steps)
- Boost 2 Off: EX_PDH_NoiseSpectrum_BoostOff.TXT
- Boost 2 On: EX_PDH_NoiseSpectrum_BoostOn.TXT, EX_PDH_NoiseSpectrum_BoostOn_Low.TXT
Here I've replotted the TF plot, but with a linear X scale so that the dips from the HOM resonances are more apparent.
If I use Daniel's X-FSR (37526 Hz) instead of Stefan's, then the initial frequency of 24,407,363 Hz is 650.41 FSR away from resonance.
As we tuned the modulation frequency down, there is some chance of accidental resonance. The following list is of how far the SB has been shifted from the initial position in units of the FSR.
GREEN = - 0.13
RED = -0.16
CYAN = -26.987
My interpretation of this plot is that the first two frequency shifts moved us into the range where we were having some accidental HOM resonances. These are visible as dips in the transfer function and corresponding kinks in the phase. The CYAN one, on the other hand, is almost at the same place (in terms of SB resonance) and so there are no phase dips. Instead, the overall gain is reduced due to the RF modulation frequency being detuned by 1 MHz from the narrow EOM resonance.
So, this technique seems reliable. We tune to the place on the EOM where we have a high optical gain and the shift to the SB frequency where we are 0.4 FSR away from resonance. In this spot (assuming a 5 kHz HOM spacing) we could get a resonance of the TEM03 mode of the lower sideband, but the TEM04 mode of the upper sideband would be 2 kHz off resonance. Good parking spot.