Dave O, Stefan, Elli
Yesterday Stefan and Kiwamu turned on the ITMX CO2 laser was turned on to 440mW and the simple michelson was left locked. See alog 15328.
Today we looked at the effect this had on the contrast defect. When the CO2 laser was turned on, the contrast defect improved untill it was almost zero after about 35 mins. The contrast defect then got worse as the ITM continued to heat up. The ITM takes >4hrs to reach thermal equilibrium after the CO2 laser is switch on, as measured in alog 14742.
Using Aidan's measurments from alog 14742, we re-calculated the power needed to minimise the contrast defect to be 255mW. To do this, we fitted a curve to the time dependent behaviour of the spherical power (see picture). This fit was
y=aexp(-bx)+c
where
a=-8.14e-5
b=-4.27e-4
c=7.9e-6
We adjusted the ITMX CO2 laser power to 255mW. The michelson broke lock early this morning possibly due to strong winds. We were unable to relock the Michleson interfereometer possilbly because of ongoing strong winds and hence could not see the change in the contrast defect, that is the next step. We have left the CO2 power on at 255mW and so the next time the michelson locks we will have the optimum contrast defect.
Relocked the dark Michelson at 2:55pm. But it still often drops lock due to the current wind conditions. Attached is another plot that better shows the y-axis scale. For reference: the dark counts in H1:LSC-ASAIR_B_LF_OUT_DQ are -1.78cts. The best values in the cureve are about -0.2cts. The freely swinging Michelson peaks at 1890cts, suggesting an achievable contrast defect of 8.5e-4.
The winds are gusting to 55-60mph. I leave the MICH loop off, because it just keeps saturating.
With the new heating (0.255W) H1:LSC-ASAIR_B_LF_OUT_DQ bottoms out at 0.25cts. The dark offset is -1.8cts. We started off (before the heating exercise) with about 3cts. Thus we improved the contrast by a factor of (0.25+1.8)/(3+1.8)=0.43 = 1/2.3.
Expected sideband power:
tM = sin 2πfMla/c
PM = Γ2/2 tM2 Pin
With la = 0.05 m, fM = 45.5 MHz, Γ = 0.28, and Pin = 1890 cts, we get
tM = 0.048,
PM/Pin = 90 ppm and
PM = 0.17 cts.
This gives a contrast defect of 0.99 x 10–3 at 225 mW.