Michael and Yuhang

We performed a ringdown measurement of OPO after replacing the OPO transmission PDA36A2 detector with PDA05CF2. The PDA05CF2 detector is made from InGaAs and has a bandwidth of 150MHz. (rise time is 0.35/150MHz = 2.3ns)

After this replacement, we locked OPO and use RF switch to turn on-off AOM to have OPO on-off resonance. We got a "ring down" of OPO transmission as the attached figure.

We can see that this is not an exponential decay. In addition, the observed decay time is much smaller than what is calculated in elog2784 of around 4us. So I double checked my calculation, and I found I made a mistake in the past about decay time. The ring down time of OPO should be 7.7ns, which is a number we are not able to measure. The limitation of this measurement comes from RF switch and AOM. The RF switch (M3SWA-2-50DRB+) has fall time of 4.6ns. The AOM rise time is 0.66*(beam diameter)/(acoustic velocity of TeO2 4200m/s), to achieve AOM rise time of 0.77ns, the beam diameter needs to be 4.9 um. Therefore, we are currently limited by the switch off time of AOM.

The fall time in the attached figure is 66ns, which is the time signal goes from 90% to 10%. Considering the current beam diameter inside AOM, the rise time of AOM is 0.66*550e-6/4200 = 86ns. There is a discrepancy of 20ns.

Note: the AOM fall time and rise time are limited by the transit time of the acoustic wave propagation across the optical beam. Thus fall time should be the same with rise time.