I checked the slow servo and the PLL of 80MHz VCO using the real green beat note signal.
The end laser is not locked to the cavity, so basically the beat signal represents just the frequency fluctuation of the two freely running lasers.
The PLL was happily locked to the green beat note although I haven't fedback the VCO signal to ETMX (or the temperature of the end laser).
It looks like we still need some more efforts for the frequency counter's slow servo because it increases the frequency fluctuation around 20-30mHz.
(slow servo using frequency counter)
As Yuta did before (see his entry), I plugged the output of the frequency counter to an ADC and fedback the signal to the end laser temperature via ezcaservo.
The peak height of the beat note is bigger than before due to the improvement of the PMC mode matching.
The peak height shown on the spectrum analyzer 8591E is now about -39dBm which is 9dB improvement.
The figure below is a spectra of the frequency counter's readout taken by the spectrum analyzer SR785.
When the slow temperature servo is locked, the noise around 20-30 mHz increased.
I think this is true, because I was able to see the peak slowly wobbling for a timescale of ~ 1min. when it's locked.
But this servo is still useful because it drifts by ~5MHz in ~10-20min without the servo.
Next time we will work on this slow servo using Aidan's PID control (see this entry) in order to optimize the performance.
In addition to that, I will take the same spectra by using the phase locked VCO, which provides cleaner signal.
(acquisition of the PLL)
In order to extract a frequency information more precisely than the frequency counter, we are going to employ 80MHz VCO box.
While the beat note was locked at ~ 79MHz by the slow servo, I successfully acquired the PLL to the beat signal.
However at the beginning, the PLL was easily broken by a sudden frequency step of about 5MHz/s (!!).
I turned off the low noise amplifier which currently drives the NPRO via a high-voltage amplifier, then the sudden frequency steps disappeared.
After this modification the PLL was able to keep tracking the beat signal for more than 5min.
(I was not patient enough, so I couldn't stand watching the signal more than 5min... I will hook this to an ADC)
Some more details will be posted later.