[ 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.