This is the memo about the FSS loop fundamental numbers with current settings .
1. the settings
laser curent : 1.997A
temperature : 23.40 degree The laser crystal temperature should not be around 25degree. There might be mode hop of the laser and makes transmitted power of the reference cavity unstable.
LDA temperature : 25.50 degree
LDB temperature : 24.16 degree
slow cource 658
slow fine 126
common gain : 100
fast:270
offset 437
2. the PD outputs at RefCav REFL and TRANS
TRANS DC PD(thorlabs PDA100, gain@10dB) : 724mV
REFL DC PD(thorlabs PDA100, gain@10dB) : 3V(locked), 3.96V(un-locked)
3. The power of TRANS and REFL
TRANS power with the FSS locked : 630uW
REFL power without the FSS locked : 4.6mW -> the PDA100 calibration factor is 3.96V/4.6mW = 1.16 V/mW
3. NPRO laser PZT efficiency measurement
The PZT efficiency, 1.6MHz/V, used for some calculation was not the number for a current NPRO but a old one used in iKAGRA.
The NPRO frequency was swept with exciting a PZT by 10Hz - 50Vpp triangular signal. And a error signal was measured and the 51.75 MHz sideband was used for a frequency ruler. 51.75MHz corresponds to 35 V excitation. With this measurement the PZT efficiency was estimated as
51.75MHz/35V = 1.48MHz/V
4.modulation depth measurement
the NPRO PZT was swept with same signal with the PZT efficiency measurement and measure the transmitted power. Then the resonant peak of a carrier and a sideband was compared and the modulation index was estimated. The height of each peak was 634mV and 29mV. The modulation signal injected to EOM was (18.9(function generator output) - 3(reduction by power splitter)) = 15.9dBm, so the injected voltage is sqrt(10^(15.9dBm/10)*50ohm*1e-3) = 1.39Vrms. The estimated modulation index is
(29mV/634mV)*2/(1.39Vrms*sqrt(2)) = 4.7e-2rad/V
According to the datasheet of newport 4003NF, modulation depth should be 0.1-0.3rad/V. So the result seems to be smaller than typical modulation depth
