Keita, Elli
Continuing with Evan's and my attempts to lock the auxiliary laser or IOT2 to the PSL with a frequency offset in order to continue the PRC measurements started by Paul Fulda...
Today Keita and I got the auxiliary laser locked for periods of ~20s. Then PLL servo control signal would reach its maximum of 5V and the lock would drop. The beat note between the aux laser and the PSL was continually drifting in one direction- this may have been due to temperature instability on the table (the fan was off to start with and then I turned it on for a while when adjusting the Aux laser temperature.) I am watching
The PLL is controlled by an SR560 servo controller with 10Hz low-pass filter cutoff and 500 gain. A 7dBPm signal is demodulated with the beat signal between the PSL and the auxiliary laser for the error signal.
Today the frequency drift of the auxiliary laser was much slower, I was able to lock it the the PSL and sweep it across 2MHz a few times before the SR560 output railed. I moved the 1611 photodiode back a few cm to where the beam is more tightly focused, increasing the beat note power from -42dBm to -33dBm.
I will switch from using the SR560 to the New Focus LB1005 servo controller. I also plan to add an amplifier with gain 25 output voltage +/- 125 V after the LB1005, to increase the voltage driving the PZTs.
10Hz single pole, combined with a integration of PLL, means that the OLG phase is already -135deg at 10Hz, so the UGF could not possibly be much more than 10Hz.
But it easily acquires lock and stays locked until the output rails. Anyway this is just the first setting that worked while I was changing things non-systematically. Students will improve it to perfection.
As for the 5V output rail of SR560, using something else to go 10V would only be a marginal improvement as the drift is big. The frequency shifted by at least 200MHz while I was watching and it was not slowing down.
According to Evan, offloading to temperature was attempted without much success in the past, but it seems to me that it is still the way to go.
In preparation of allowing a larger offset frequency, I gave Elli a ZFM-2 that is a level 7 mixer for 1MHz to 1GHz.
We should have a New Focus/Newport LB1005 in the lab which is a proper PI controller for laser locking. We also should have +/-120V PZT driver for the JDSU NPROs.