[Enomoto, Sugimoto, Yokogawa]

(Yesterday work)

We tried automating the PLL and succeeded!

The procedure of PLL;

1. bring the beatnote frequency close to 40 MHz.
2. trun on SLOWBOOST filter of MC servo. (Set INPUT GAIN of MC servo as -23 dB.)
3. sweep the PROMETHEUS temperature so that the beatnote is locked to the local frequency(40 MHz).
4. trun on COMMON CONPESASION filter of MC servo.
5. trun on COMMON BOOST filter of MC servo.
6. close the temperature loop. (Refer to log6543 for setting of low path filter of the temperature loop)

It is difficult to judge the lock because the Phase Frequency Discriminator (PFD) outputs a sawtooth wave. (The output signal of the PFD is an error signal for PLL.) PFD outputs -4 V when the beatnote frequency is higher than the local frequency and outputs +4 V when the beatnote frequency is lower than the local frequency. If the beatnote frequency is close to the local frequency, the PFD outputs a sawtooth wave.

So, we decided that PLL is locked when the PROMETHEUS frequency is 40MHz lower than the main laser frequency and the beatnote frequency approaches the local frequency from the lower ferequency side by sweeping the PROMETHEUS temperature. Specifically, when the current and the previous value of the error signal and the direction of sweeping temperature satisfy the following expression, PLL is locked.

( (Error signal CURRENT value) - (Error signal PREVIOUS value) ) * (Temperature sweep direction) < 0