Yuhang and Michael

We did some more inspection of the OPO replacement setup at ATC. Perhaps we will need some more low f lenses.

I recalculated the error signal and mode matching shown in 2469.

We can obtain a much larger error signal in reflection when the meniscus is used as the input mirror. Even with <10 mW incident power it should not be a problem. Here I use 4 mW incidence, 40 MHz modulation and 0.3 modulation depth to achieve ~160 mVpk error signal in the linear range. Figure 1 shows the error signals and power outside of the cavity. Figure 2 shows the scan of demodulation phase versus reflection and transmission photodetector signals. Figure 3 shows the circulating power of 160 mW, for 4 mW input.

I also show a beam profile with a more complete mode matching telescope into the OPO. I did a mode matching telescope calculation using JamMT and the database of Thorlabs lenses. We wish to obtain a beam waist of 20.66 µm within the OPO cavity using two lenses. Judging by the OPO setup in TAMA, it would be good to leave approximately 15cm OPO to lens and lens to lens.

In 2469 I used an f = 100mm lens placed at 175mm from the last preset optic on the ATC bench. The beam waist of 150µm is located within 1cm of the last preset optic on the ATC bench This creates another 150µm beam waist 175mm from the lens. The rationale for doing this was:

i) to have a couple of steering mirrors before this lens, so that the beam would be level at 76mm height when going through the FI/EOM/AOM. However, Yuhang suggested to simply move the 100mm lens to achieve beam levelling.

ii) to reduce the size of the beam waist, and make the beam less than 1/5th of the EOM aperture size for a reasonable distance. 

This time I used an f = 100mm lens placed 125mm from the last preset optic (as per photograph and measurement in 2439). The mode matching telescope, calculated by JamMT, is shown in figure 4. We should have enough space for the modulators between the f = 100mm and f = 40mm (90mm FI, HWP, 56mm EOM, 22mm AOM). Steering mirrors can fit in the space between the f = 40mm and f = 75mm, and then there should be a beam splitter between the f = 75mm and the OPO cavity.