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ID Date Author Type Category Subject
11864   Tue Dec 8 15:57:16 2015 yutaroSummaryLSCPower recycling gain estimation from arm loss measurement

I estimated power recycling gain with the results of arm loss measurement.

From elog 11818 and 11857, round trip losses including transmittivity of ETM of Y arm and X arm (let us call them $T_\mathrm{loss,Y}$ and $T_\mathrm{loss,X}$) are 229+13.7=243 ppm and 483+13.7=495 ppm, respectively.

How I calculated:

I used the following formula.

Amplitude reflectivity of an arm cavity $r_\mathrm{FP}$

$r_\mathrm{FP}=\sqrt{1-\frac{4T_\mathrm{ITM}T_\mathrm{loss}}{T^2_\mathrm{tot}}}$   (see elog 11816)

Amplitude reflectivity of FPMI $r_\mathrm{FPMI}$

$r_\mathrm{FPMI}=\frac{1}{2}(r_\mathrm{FP,X}+r_\mathrm{FP,Y})$

With power transmittivity of PRM $T_\mathrm{PRM}$ and amplitude reflectivity of PRM $r_\mathrm{PRM}$, power recycling gain is

$\mathrm{PRG}=\frac{T_\mathrm{PRM}}{(1-r_\mathrm{PRM}r_\mathrm{FPMI})^2}$.

I assumed $T_\mathrm{ITM}\simeq T_\mathrm{tot}=\frac{2\pi}{401}=0.01566$$T_\mathrm{PRM}=0.05637$, and $r_\mathrm{PRM}=\sqrt{1-T_\mathrm{PRM}}$, and then I got

PRG = 9.8.

Since both round trip losses have relative error of ~ 4 % and PRG is proportional to inverse square of $T_\mathrm{loss}$ up to the leading order of it, relative error of PRG can be estimated as ~ 8 %, so PRG = 9.8 +/- 0.8

Discussion

According to elog 11691, which says TRX and TRY level was ~125 when DRFPMI was locked, power recycling gain was $\mathrm{PRG}=125\times T_\mathrm{PRM}=7.0$ at the last DRFPMI lock.

Measured PRG is lower than PRG estimated here, but it is natural because various causes such as mode mismatch between PRC mode and arm cavity mode, imperfect contrast of FPMI, and so on could decrease PRG, which Eric suggested to me.

If $T_\mathrm{loss,X}$ were as small as $T_\mathrm{loss,Y}$, PRG would be 16.0. PRC would be still under coupled.

11870   Thu Dec 10 12:33:04 2015 yutaroUpdateLSCstrange behavior of ASDC

I did additional tests for the strange behavior of ASCD. ETMY, ETMX and ITMY were misaligned so that only light reflected by ITMX went into AS port. I had done similar measurement before with ITMY YAW varied.

Attachment 1 shows how ASDC level changed when ITMX PIT varied.

Attachment 2 shows how ASDC level changed when ITMX YAW varied.

Attachment 3 shows how the power of light measured by a power meter just after the AS view port varied when ITMX YAW varied.

Comparing 1 & 2, we can say that this behavior is not unique to YAW direction.

From 2 & 3, we can say something strange is happening inside the chamber.

Attachment 1: 07.png
Attachment 2: 28.png
Attachment 3: ASDC.png
11871   Thu Dec 10 19:53:22 2015 yutaroUpdateLSCstrange behavior of ASDC

To check if the strange behavior of ASDC is caused by SR2/SR3 or not, I did the following measurement:

ASDC measures the power of the light reflected by ITMX. POXDC measures the power of the light reflected by ITMX and SRM successively. Then I varied the angle of ITMX in YAW direction and compared the behaviors of ASDC and POXDC.

The results are shown in Attachments 1-3.

As you can see in these figures, the strange up-and-down behavior appeared ONLY in ASDC. Therefore, the cause of this behavior exists between AS table and SRM (I had confirmed that the angle of SRM did not affect ASDC).

And this behavior is fringe-like, as can be seen in the figures (there seems to be 3 "peaks" and 2 "valleys"), so the cause could be interference between main path and not good AR reflection at a mirror after SRM before AS table (I suspect a mirror is flipped mistakenly).

Attachment 1: 30.png
Attachment 2: 11.png
Attachment 3: 49.png
11872   Fri Dec 11 09:35:44 2015 yutaroUpdateLSCPower recycling gain estimation from arm loss measurement

I took PR3 AR reflectivity and calculated PRG (PR3 is flipped and so AR surface is inside PRC).

As shown in attached figure, which shows AR specification of the LaserOptik mirror (PR3 is this mirror), AR reflectivity of PR3 is ~0.5 %. Since resonant light in PRC goes through AR surface of PR3 4 times per round trip, round trip loss due to this is ~2 %. Then I got

PRG = 7.8.

Attachment 1: LaserOptikAR.png
11874   Fri Dec 11 15:37:50 2015 yutaroUpdateLSCPower recycling gain estimation from arm loss measurement

Attached is the plot of relation between the average arm round trip loss and power recycling gain. 2 % loss due to PR3 AR reflection is taken into account.

Attachment 1: PRG_plot.png
11875   Fri Dec 11 16:16:36 2015 yutaroUpdateOptical LeversCalibration of oplevs for ITMX/ETMX

Based on calibration measurement I have done (elog 11785, 11831), I updated calibration factors of oplevs on medm screen as follows. Not to change loop gain oplev servo, I also changed oplev servo gain.

C1:SUS-ETMX_OL_PIT_CALIB, C1:SUS-ETMX_OL_PIT_GAIN

(45.1,16) => (200,3.5)

C1:SUS-ETMX_OL_YAW_CALIB, C1:SUS-ETMX_OL_YAW_GAIN

(85.6,8) => (222,3.0)

C1:SUS-ETMY_OL_PIT_CALIB, C1:SUS-ETMY_OL_PIT_GAIN

(26,-16) => (140,-3.0)

C1:SUS-ETMY_OL_YAW_CALIB, C1:SUS-ETMY_OL_YAW_GAIN

(31,-21) => (143,-4.5)

C1:SUS-ITMX_OL_PIT_CALIB, C1:SUS-ITMX_OL_PIT_GAIN

(110,8) => (122,7.2)

C1:SUS-ITMX_OL_YAW_CALIB, C1:SUS-ITMX_OL_YAW_GAIN

(81,-11) => (147,-6)

C1:SUS-ITMY_OL_PIT_CALIB, C1:SUS-ITMY_OL_PIT_GAIN

(159,15) => (239,10)

C1:SUS-ITMY_OL_YAW_CALIB, C1:SUS-ITMY_OL_YAW_GAIN

(174,-21) => (226,-16)

ELOG V3.1.3-