KAGRA Logbook

MIF (ITF Control)

yutaro.enomoto - 20:13 Friday 04 May 2018 (4946)

Preliminary Measurements for Schnupp Asymmetry Measurement

Nakano, Kokeyama, Izumi, Nagano, Enomoto

= Summary =
For the preparation of the Schnupp asymmetry measurement, we 1.) measured propotional constant between analog feedback voltage applied to AOM driver and ADC count that monitors the feedback voltage, and 2.) checked the polarity of the calibrated Michelson displacement signal (K1:CAL-CS_PROC_C00_DISPLACEMENT).

= Background =
In the Schnupp asymmetry measurement, we sweep the frequency of main laser from PSL room and look at the calibrated Michelson length signal:
δl_MICH / l_asym = δf / f_laser => l_asym = (δl_MICH / δf) * f_laser

To obtain l_asym from this TF (δl_MICH / δf), we need to know how much we are sweeping the frequency of the laser.
Since we know the actuator efficiency of AOM is 10.02 MHz/V (https://gwdoc.icrr.u-tokyo.ac.jp/cgi-bin/private/DocDB/ShowDocument?docid=7575), what we have to know is feedback voltage applied to AOM.

To know l_asym including the sign (X arm is longer or Y arm is longer), we have to know the sign of Michelson displacement signa, δl_MICH.

= Results =
1.) Propotional constant between analog feedback voltage and ADC count
We injected excitation from IMC-SERVO_EXC_B (amp. 5 at 1 kHz) and measured the amplitued of the feedback voltage both with an oscilloscope and a digital channel (IMC-SERVO_BTEST2_CON_IN is temporarily used).
The P2P amplitude of the analog voltage was 262 +/- 1 mV and the P2P amplitude of the ADC count was 466 +/- 1 counts, which means 5.71e-4 V/cnt.
Note 20/2¹⁵ = 6.10e-4 V/cnt was expected.

2.) Polarity of Michelson displacement signal
2-1. Polarity of BS TML actuation
Using the sign of oplev PIT and YAW, we checked the polarity of TML actuation. If you request "+" count in TM LOCK, BS will move in + L direction, which corresponds to increase of δl_MICH(= l_x - l_y).
2-2. Polarity of LSC-MICH1 filters
This is "+" in total at DC.
2-3. Polarity of LSC-MICH1_IN1
Combining 2-1 and 2-2, one can say that the polarity of the optical gain is "-", for the MICH loop to be negative feedback. In other word, LSC-MICH1_IN1 is propotional to - δl_MICH.

Comments to this report:

masayuki.nakano - 21:44 Friday 04 May 2018 (4947)

The transfer function from IMC-SERVO_BTEST2_CON_IN to CAL-CS_PROC_C00_DISPLACEMENT was measured

=== Method ===
Excited the MC length by injecting the signal to VIS-MCE_TM_TEST_L_EXC. The typical value of the excitation amplitude is 300 @ 100 Hz.
Measured the spectrum of each channel by the fft. Check the coherence and transfer function.

I measured at 7 frequencies in total. The result is listed below:

Frequency [Hz], Coherence, Gain [dB], Phase [deg], Reference set (diaggui)
50, 0.77, -33, -12, REF8-
80, 0.73, -31, -5.2, REF4-
100, 0.93, -30, -5.6, REF0-
190, 0.97, -29, -6.6, REF29-
310, 0.93, -30, -11, REF12-
380, 0.92, -31, -15, REF25-
500, 0.96, -34, -9.3, REF17-
610, 0.93, -33, -38, REF33
800,0.94, -33, -32, REF37

The data file is /users/Commissioning/Schnupp/Schnupp_20180504_MCexc_fft.xml

The expected gain is -24 dB and there are the factor of 2 difference. We will discuss tomorrow and calculate the schnupp asymmetry.

yutaro.enomoto - 11:41 Saturday 05 May 2018 (4949)

= Summary =
I calculated the Schnupp asymmetry from the yesterday's measurements.
The result is: l_asym = 1.33(7) m, although the designed value is 3.3298 m.
I do not yet know why they do not coincide. I will re-do the measurement once the interferomter gets healthy.

= Detail =
-- Simple average of the TF measured yesterday is: 0.027(2) nm/cnt
-- This can be converted to m/Hz as: (δl_MICH / δf) = TF_measured * 10^-9 / ( V2cnt conv. * A_aom) = 0.027 * 10^-9 / (5.71 * 10^-4 * 10.02 * 10⁶ )
-- l_asym = (δl_MICH / δf) * f_laser = 0.027 * 10^-9 / (5.71 * 10^-4 * 10.02 * 10⁶ ) * (3*108 / 1.064*10-6) = 1.33 m
-- Dominant error source is from the TF measurement, so l_asym = 1.33(7) m
-- Note on polarity: positive excitation in feedback signal to AOM gives positive frequency shift after AOM, and that results in negative frequency shift of main laser via RefCav loop, and then that is equivalent to negative displacement of l_mich if l_x > l_y.
Recalling current calibrated displacement channel moitors -l_mich, measured TF should be positive. The polarity is correct, accoriding to the phase of TF measured yesterday.

= Possible reason why they do not coincide =
-- Optical gain might have changed so much. --> I will repeat this TF measurement when IFO is quiet.
-- My calculation might be wrong --> Can someone do double checking???
-- Length between ends and center area might be wrong --> quiet unlikely.