[Enomoto, Kasuya, Yamamoto, Kokeyama, Nakano]

We locked the first cavity in bKAGRA!!!

The frequency stabilization loop was locked for some minutes and some transfer function measurement has been done. Also some parameters(optical gain, actuator efficiency etc) has been estimated. The estimated parameters are as below

Optical gain of the reference cavity : 130 mV/MHz (input power to RFPD : 8.9mV)
Actuator efficiency of broadband EOM : 230 Hz/V
UGF with common/fast gains of servo set to 500/250 : around 4kHz

Today we locked the cavity with using only laser PZT. We tried to use the broadband EOM as actuator but we didn't have enough time, so next step is using bEOM.

1. Reference cavity lock

The locking procedure is easy like below

(i) set the test/off/ramp switch on the TTFSS interface to test to open the control loop.
(ii) seek the resonant peak with sweeping the temperature of laser crystal. Laser temperature can be swept with a slow Fine nob on a TTFSS interface.
(ii) switch the test/off/ramp switch on the TTFSS interface. Then if the gain is adjusted well the reference cavity would be locked

Thanks to AEL team, we got new RF PD and it works well. (We still have some problem with RFPD and RFPD interface. Accidentally the RFPD interface can stop to supply -18V, and break the RFPD. We don't have any idea for that.) So we could lock the cavity very smoothly.

2. Optimization of the servo gain

To optimize the servo gain we estimated the optical gain. I already know the PZT response on laser (1.68MHz/V). The laser frequency swept with triangle wave signal with 1Hz 10Vpp, and measured the error signal at common OUT1 port on TTFSS. According to rough measurement with oscilloscope, error signal changed 45mV with 200mV actuation at near the resonance. From these result the optical gain can be calculated as

45[mV]/(1.68[MHz/V]*0.2[V]) ~ 130mV/MHz

Here the input power to RFPD was 8.9mW and the error signal size was 70mV (the difference between maximum and minimum voltage of error signal)

With this estimation, the servo gain was adjusted such to set the UGF at some kHZ. The common gain and fast gain were set to 500/250 each. Then the OLG was measured. The result is attached (We couldn't bring the data because of the old equipment. We didn't have a floppy disk) The UGF was around 4kHz

3. broadband EOM actuator efficiency measurement

The EOM actuator response was measured with using the reference cavity as a frequency sensor. The transfer function from broadband EOM input to FSS error signal (common OUT1 port on TTFSS) was measured and the result is attached.(We couldn't bring the data because......). The transfer function was -88dB@10kHz. from this number the EOM efficiency can be estimated as

10^(-90/20)/130e-9 ~ 230Hz/V 

From this and circuit transfer functions we could predict the optimal gain and tried to lock the cavity but failed. From some measurement the sign of EOM path seemed to be opposite, but surprisingly the circuit don't have the switch of the sign of EOM path. So we swapped the sign of EOM path by swapping the demodulation phase with extending the cable length of local oscillator signal. Tomorrow we would see it would work or not