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MIF (ITF Control)
yutaro.enomoto - 1:23 Sunday 09 December 2018 (7307) Print this report
X arm cavity characterization with good spot positions

[K Yamamoto, Enomoto]

Today we characterized the cavity, including cavity scan, reflectivity measurement, and ring down measurement.
As we suspected that the beam spot position was bad when we did the similar measurements, we centered the beam spot on each test mass with the best effort, and did the measurement.
Here we summarize the numbers we got today, and put the pictures that show the spot positions.

Modulation index of f1 @MIF: 0.24(1)
Modulation index of f2 @MIF: 0.053(2)
Mode matching ratio: 91(1)%
Carrier power / total power: 88(1)%

Roundtrip loss (estimated from reflectivity): 86(3) ppm

Finesse: 1411(2)
Roundtrip loss (estimated from ring down): 9 +/-18 ppm

* Better spot position gave us better loss, but values of loss inferred from two independent measurement still do not match.

= Cavity scan (as a preparation for the loss measurement) =

We scan the X arm cavity by sweeping the PLL offset frequency, as we did the day before yesterday.
Note that we increased the offset frequency as time past, and so increased the main laser frequency (since AUX X frequency is locked slightly lower than that of main laser).
Here is the result. The list of peak positions and heights are also attached.

From this we get the follwoing information:
the height of f1 sidebands relative to the carrier (beta2/4): 0.014(1)
f1 modulation index (beta): 0.24(1)
the height of f2 sidebands relative to the carrier (beta2/4): 0.00070(7)
f2 modulation index (beta): 0.053(3)
Mode matching ratio: 91(1) %
Ratio of carrier power: 88(1) %


= Cavity reflectivity =

From the reflectivity of the cavity, the loss of the cavity (more strictly, loss * ratio of carrier power) can be obtained (see 7291).
The following figures show the overview of this measurement. The reflection decreases by ~ 10 % when the X arm is locked.

We did the same measument twice, and obtained these results.
Tloss (1st): 88.5(9) ppm
Tloss (2nd): 82.7(9) ppm
From these, Tloss = 86(3) ppm.


= Ring down =

To know the finesse, we repeated this three times. Here are the results and how the ring down and its fitting looks like.
F (1st): 1409.7 +/- 1.0
F (2nd): 1415.4 +/- 1.1
F (3rd): 1408.0 +/- 0.9
As a whole, F = 1411 +/- 2

Using the measured parameters for ETMX/ITMX, Tloss is estimated as
Tloss = 9 +/- 18 ppm


= Note =
Loss estimation from the ring down gave us totally nonsense value. This implies that the finesse might be systematically over-estimated from the ring down.
In fact, we did not take the effect of IMC into account. I will think about that.

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