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EleonoraCapocasa - 16:33, Tuesday 26 June 2018 (850)Get code to link to this report
Green mode cleaner trasmission issue

[ Yuhang, Matteo, Eleonora]

After locking the Green mode cleaner we measured a trasmissivity below 50% which is much lower than what we expected.

An extremely rough power budget gave us: 

P_in = 22 mW

P_tra = 8.5 mW

P_ref = 10.5 mW

P_tra from end mirror = 0.5 mW 

Missing = 2.5 mW

We investigated some of the following possibilities:

 

Mirrors transmissitvity

For a triangular cavity, as our modecleaner,  the transmission is given by:

T = (t_in*t_out)/1-r_in *r_out *r_end)^2 

according to the spec for the mirrors used (see pic 1) :

R_in = R_out = 0.992  and R_end > 0.995  (measured from the producer 0.9993)

= > T = 0.92  (taking R_end  0.9993)

Considering an error of +/- 0.003 in all the three nominal transimissivity, the expected cavity trasmissivity is 0.92 +/- 0.33 

 

Matching and alignment

The optical spectrum of the cavity is shown in pic.1. The alignement seems good. The sidebandes at 78 MHz ( used for the lock of the MC) are not visible while we can see the 15.2 MHz modulation that we know to be high. 

[ Note that we are sending at the 78 MHz resonant EOM, a driving  RF signal with amplitude 1 V pp which should correspond to a modulation depth of 0.185 rad. this means that the expected power in the sidebads is 0.0086]

 

input beam polarization

Yuhang used a PBS for 532nm and verified that the light is almost all in s-pol,  as it should be.

 

Conclusions:  The origin of the low MC transmissivity is not clear but the most probable hypotesis, among those considered,  is that the effective transmission of the mirrors are a bit different from the nominal ones.

Images attached to this report
850_20180626093234_mcm.png 850_20180626093311_mcscan.png
Comments related to this report
RaffaeleFlaminio - 23:36, Wednesday 27 June 2018 (854)
From the power transmitted behind the end mirror (0.5 mw) and behind the output mirror (8.5 mW) it seems that the end mirror transmission is about 17 time smaller than the output mirror. Assuming that the output mirror transmission is 0.8%, one would deduce that the end mirror transmission is of the order of 0.05% in good agreement with the expected value. If so, the large reflection should come from an asymmetry between the input and output mirrors (which seems difficult to imagine).
On the other hand there is a strange "large" peak in the middle of the FSR. Where is that coming from? Is there a polarization problem? In the entry it is written that the polarization is OK.
Question: did somebody already check that the input and output mirrors are mounted with the HR side facing the inside of the cavity?
YuhangZhao - 09:39, Wednesday 04 July 2018 (866)

I checked the input and output mirror. From the point view of marker on the side of mirror, I am sure the mirror is installed in a correct way.

I checked also the mirror from the same box, this arrow points to the HF side of this mirror.

RaffaeleFlaminio - 20:30, Tuesday 10 July 2018 (877)
At the last weekly meeting doubts were raised on the measured powers.
Are the values given above now confirmed?
P_in = 22 mW
P_tra = 8.5 mW
P_ref = 10.5 mW
P_tra from end mirror = 0.5 mW
Missing = 2.5 mW
Are the 2.5 mW still missing?