Participants:  Eleonora, Matteo L., Raffaele, Tomura

In the past days we kept working on the characterization of the IR lock, with the main purpose of measuring the cavity losses and the decay time.

ROUND TRIP LOSSES

We observed that in good alignement condition (IR trasmission above 1.5 V), the fluctuations of the transmitted and reflected power were much less than what observed before.

In this condition we were able to measure a change in the cavity reflectivity when the cavity is resonant and when it is not and give a preliminar estimation of the round trip losses (RTL)

In Fig.1 the trasmission and the reflection of the IR are shown when a set of lock/unlocks of the cavity was done. The reflected light has been focused on a photodiode using a lens with f = 50 mm. Et the beginning of the measurement the IR light has been blocked to measure possible offsets of the photodiodes.

The technique used to switch from resoant to not resoant state was to suddenly change the driving frequency of the AOM of 5 kHz. By using the values of the reflected power in the two states (resonant and not resonant) as explained in detail in the attached pdf we estimated the RTL to be about 80 +/-12 ppm, corresponding to 0.26 ppm/m. The error is mainly do to the residual fluctuations of the refected power when the cavity is locked. The associated squeezing degradation is reported in FIg 2.

The presence of light not coupled in the cavity (mismatching/misalignement) normaly reduces the measured losses and has to be compensated in order to have a real estimation of them. In the previous calculation I assumed a mismatching of 15%. 

[An idea of the impact of the mimastch compensation: assuming no mismatching the computed losses are 70 ppm while with 20% of mismatching they becomes 85 ppm. (Details about this can be found in my thesis at pag.101)]

According to the simulation we expected about 55 ppm of RTL  (40 ppm from flatness, 10 ppm from rougness/point defects and 5 ppm from trasmission and absorpition). Note that losses from small angles scattering (between mrad and few degrees) have not been considered in this loss budget.

DECAY TIME and FINESSE

A preliminar estimation of the decay time has also been done. To do that we used different tecniques: bringing the cavity suddenly out of resonance (by stopping the lock with the servo or changing the AOM driving frequency) or cutting the light in input. The transmission and reflection in this 3 cases are reported in the second pdf attached) 

A fit of the transmitted power for the first measurement shows a decay time of 0.0027 s, corresponding to a finesse of 4250  (Finesse = pi*FSR*decaytime ). See third figure attached.

Assuming the nominal reflectivity of the mirrors, this value is compatible with RTL of about 100 ppm.

A better analysis of the decay time, with an estimation of the error bars will be done soon.