Participants: Eleonora, Raffaele, Matteo L.

We have installed in the end bench a thorlab passband filter (FL1064-10) on the path of the IR beam after the harmonic beam splitter. This allowed us to get rid of the residual green light and finally to monitor the beaviour of the IR light when the cavity is locked.

By changing the driving frequency of the AOM installed on the green path we were able to induce a frequency shift between the the green and the IR frequency in order to have both resonant at the same time.

NOTE THAT: since the AOM is put on the green path, the change in the frequency which it induces is compensated by the servo with a change on the IR which is half of the frequency change in the AOM. This means that a shift of 1 MHz in the driving frequency of the AOM corresponds to a shift of 500 kHz in the frequency of the IR light. 

In the following are reported some preliminary results that we were already able to obtain:

TEM00 resonance and FSR:  The resonance frequency of the TEM00 has been found at about 109.366 MHz. (The standard driving frequency of the AOM is 110 MHz). We have verified that it occurs every time we shift the AOM frequency of 1 MHz, meaning that the FSR is 500 kHz, as expected.

Cavity finesse for IR: We did a rough estimatin of the IR linewidth by slightly changing the AOM frequency in order to scan the IR resonance. We drove the AOM in order to find the maximum in the transmission (about 1.5 V) and then we checked the frequency shift necessary to get half of the maximum. Repeating this procedure few times, we found a FWHM of about 110 Hz +/- 10. This corresponds to a finesse of about 4500 +/- 450, which is consistent with what we expect. The big error is due to a large fluctuation in the trasmitted power.

HOM resonance frequency: The first order modes have been found at a AOM frequency of 108.970 MHz (and FSR multiples). It means that the the difference in the resonance frequency betwen the fundamental and the first HOM is about 198 +/-1 kHz. This value is in very good agreement with what expected, considering the cavity g-factors computed by using the RoC values measured at LMA.

According to LMA measurement, we have R_in = 436.7 m  and  R_end = 445.1 m which correspond to a frequency shift given by

δν = (FSR/pi)* acos (sqrt (1-L/R_in)*(1-L/R_end) = 198.1 kHz

We also observed that different modes of the first order resonate at slightly different frequencies, which is likely to be an effect due to the astigmatism. More investigations have to be done. 

Residual misalignment: by comparing the trasmitted power of the funtamental mode with that of the first HOM we estimated the misaligment to be between 20% and 25%. 

Lock accuracy: Since we don't have an error signal for the IR lock, it is not easy to evaluate the lock accuracy. Anyway the fact that resoanance can be scanned by shifting the AOM frequency gives us a lower limit that is surely better than few tens of Hz, that is much better than the accuracy on the green lock, estimated to be about 100 Hz. This is compatible with the hypothesis proposed by Matteo B, according to which the IR pole (at much lower frequency than the green one) contributes to filter the high frequency laser noise, improving the lock accuracy.

Stability: Once put on resonace the IR seems to be stable as long as the lock is mantained, anyway we observed that when the cavity unlocks and relocks it is not always possible to get the IR resonant again. This is probably due to the fact that only half of the green locking points are also good locking points for the IR.

The attached picture shows the transmitted beam at resonance for the IR (top screen) and green (bottom screen)