Yuhang, Marc, Michael

We once again tried to find transmission sidebands at 88 MHz for the SHG locking but could not see them, so we had to rethink the lock scheme.

We had some discussion of how to resolve the BAB direct reflection issue (in fact, the beam is forming an interferometer with OPO HR and SHG input as the two arms, and SHG refl as the output port).

Our first thought was to put a Faraday isolator to decouple the two beams.  For the SHG input path, there is not very much space for an FI, while on the BAB path the beam is quite large and would require a redesign of the mode matching telescope for BAB to OPO.

We had some thought about trying to decouple them using polarization. But given that SHG must be p-pol and BAB must be s-pol, we cannot decouple them using just HWPs and one PBS, and besides, we need those polarization optics for the Taiwan and Korea activities in ATC.

We then decided to try a pick-off from the SHG reflection before it reaches the SHG/BAB separation beam splitter and recombines with BAB. Since the current SHG refl PD has an ND2 attached and has about ~ 100 mW incident, we only really need about 1 mW of power to get a good signal. At this point we remembered that there are two dichroic mirrors on the green output of the SHG that reflect away residual IR into beam dumps. Actually, they reflect quite a lot, we measured 3.6 mW infrared reflected from the first dichroic after green generation.

And then we found that our attempts to see the transmission sidebands on the transmission PD were separately mistaken. On the first attempt, a low bandwidth PD was used in reflection, which is not fast enough to see RF sidebands. On the second attempt, an ND2 filter was left screwed on to the SHG refl RFPD when it was moved to the transmission side. So in both cases the sidebands were suppressed. Correctly implementing an RFPD in transmission of the SHG with sufficient incident power resulted in recovery of the error signal. So we didn't really have to change the locking scheme very much if at all.

We decided to put both SHG lock PDs in transmission. We selected a BSF10C beam sampler with an output wedge - this has ~ 1% reflection for p-pol at 45 degrees AOI, and maybe 5% at imperfect angle. To improve the visibility of the small transmitted power, the SHG correction signal SERVO OUT was disconnected, and an offset applied to the SHG PZT to see strong green generation. Then the IR transmission also becomes stronger, to about the level we would expect during SHG lock - about 3 mW transmission. In 99% transmission of the beam sampler I placed the RFPD Thorlabs PDA05CF2 which goes to the SHG error signal. In 1% reflection I placed the DCPD Thorlabs PDA36A-EC switchable gain, which goes to the servo threshold check and TRANSMIS OUT. I looked at TRANSMIS OUT on the oscilloscope -it was quite small so I increased the DCPD gain to 30 dB and it became about 550 mV. Then I checked the RFPD signal going to the mixer, which has 4.5 V max signal. I could see 95.7 % mode matching (4480 TEM00 + (96+56+48) HOMs). Then I placed a large beam dump at the former SHG refl PD point, which has quite large infrared power.

I set the threshold to slightly less than half of the DCPD signal (about 250 mV) and tried to lock, but the SHG would still only lock to a bad point (i.e. not to the level of the TEM00 peaks on TRANSMIS OUT), I looked at the error signal and tried to optimize the SHG demodulation phase but could not. From the reference signal on the wiki, it should have jump from a large negative point to a high positive point with a strong positive slope at the lock point (once again the NAOJ elog image upload issue is not very nice). However, instead it goes from large negative peak -> small positive peak -> small negative peak -> large positive peak, essentially making a small bump in the middle of the error signal with the opposite sign compared to what the polarity of the large peaks should give. I cannot get rid of this by changing the demodulation phase. Tomorrow we will try using the dichroic reflection for RFPD to see if that fixes the error signal.