| Quote from #5914 |
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So I have added an SR560 in the other input of the Newfocus servo box to make the filter shape 1/f^2.
I will post the servo shape and diagram later.
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The Y arm green PDH servo is working fine with a sufficient amount of suppression.
(Servo filters)
As reported on the previous elog entry ( #5914) an SR560 was installed to provide one more pole-zero combination in the servo filter. Here is a plot showing the transfer function of the latest servo filter.
And the servo configuration looks like this :
The demodulated signal is split into two path; one goes directly to the Newfocus servo box and the other goes through SR560.
With the SR560 the two way summing path makes a pole at 1 Hz and zero at 100 Hz with when the SR560 has a gain of 100.
The overall gain is adjustable from a knob on the Newfocus servo box.
(the Error signal)
One of the reasons we wanted to increase the servo gain was that :
the laser frequency has to be tightly locked to the Y arm motion because the laser frequency must represent the arm motion in our scheme.
Our requirement for allowing a successful ALS is : RMS < 10 pm (1/100 of the cavity linewidth)
I took a spectrum of the error signal when the laser was locked to the Y arm and found that it meets the requirement.
In the plot I also put a dark noise from the PD to make sure the in-loop noise is above the dark noise.
Right now the power lines at 60 Hz and 180 Hz are lifting the RMS up.
Note that the UGF was at 20-30 kHz. |