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MIF (ASC)

shinji.miyoki - 9:52 Sunday 26 January 2020 (12643)

Some reviews about Power Recycling Gain

[miyoki, Nakano, Kokeyama]

About PRG

We checked the ideal power recycling gain (PRG) and compare the present obtained PRG (~ over 10 ) in KAGRA.

Power Reflectance and Transmittance Setting

PRM: r = sqrt(0.9), t=sqrt(0.1)

FP Cavity Power Reflectance : 0.99 (for X), 0.9917 (for Y) assuming 0.44% and 0.48 transmittance for ITMX, ITMY (Kagra Doc 9173) and 0.9999 for both ETMs with loss less. I used 0.99 in the following cal.

Loss due to AR surface of ITMs and BS : 0.002 * 4 (for X), 0.002*2 (for Y). I used 0.008 in the following cal..

Rough Estimation

PRG = 28. We originally have factor 3 times margin. According to preliminary report from nakano-kun, the power enhancement in Y FP was 12, 10 for X.

The corresponding loss is 0.075, for Y, 0.09 for X.

According to 10369, 0.095 for Y and 0.07 for X due to birefringence ! There seemed to be swapping ??

Some Questions

P can resonate inside FP ?

P can also resonate in PRC ? Because of low finess of PRG, P has a room to resonate on the resonance shoulder if P part has extra phase shift due to BS reflection/transmission compared with S part.

Although we almost confirmed S -> P conversion, how about P -> S conversion during circulation inside PRC ? This is very confusing to estimate the power enhancement in PRC and FP cavity.

Comments to this report:

tomotada.akutsu - 16:48 Sunday 26 January 2020 (12644)

Thank you so much for your great and patient efforts!

About the difference of the transmission power, probably we might like to check the power distribution (or power budget) along the TMSs. Unfortunately, the optical components installed in the both TMSs did not take theP/S mixture into consideration, so saying somthing without assumptions would be difficult, even though we might actually do some measurements later. (For Y-end, the chamber for TMS is still opening, but closed for Xend). In the chambers, some of the large optics for folding the light beams have relatively small angle of incidence with respect to the beams, so we might not need to care about the polarization until the beam would meet smaller optical components that have larger angles of inicident; it might be possible to put a PBS and a power meter on the beam line after the beam get shrinked at least for the Yend in the chamber to check the ratio of P/S; but don't touch the inside of the TMSY chamber... the alignement is hell, and if required, I'll come to do that.

shinji.miyoki - 11:11 Monday 27 January 2020 (12653)

Sorry for the following correction

(1) Actual PRG

(wrong) the power enhancement in Y FP was 12, 10 for X.

(right) the power enhancement in Y FP was 10, 12 for X.

So "There seemed to be swapping ?? " -> qualitatively. consistent.

(2) Cavity Reflectance

To realize 1300 finesse in Y, one mirror loss should be around 8 ppm assuming the reported 0.48% ITMY transmittance . This setting results in 0.98% of Y cavity reflectance, 30ppm loss for 0.475% ITMY transmittance results in 0.95 of Y cavity reflectance, keeping finesse of 1300.

To realize 1450 finess in X, the reported 0.44% ITMX transmittance cannot reach 1450 finesse. Assuming 0.431% ITMX transmittance can reach 1450 finesse assuming loss less. Assuming 30ppm loss for one mirror, the 0.426% transmittance can realize finess of 1450. Resulting X cavity reflectance = 0.94.

The measured finesse seems to be higher than we expected from ITMs transmittance (smaller ITMY transmittance is required to assume reasonable loss around several decades ppm loss ).

I might have to set the loss in ITM should be smaller than ETM because of less layers. However, it also depends on cleaness on the surface.

shinji.miyoki - 16:46 Monday 27 January 2020 (12659)

Assuming 0.94 cavity reflectance for both arms, the loss of 0.024 in PRC results in PRG=12 (->X arm enhancemnet ), and the loss of 0.042 results in PRG = 10 ( -> Y arm enhancement).