Yuhang and Michael

We finished setting up the modulators.

Set up and aligned FI: 5.15 mW transmission, 5.54 mW incident (92.96% tranmission vs Thorlabs 92% specification)

Set up HWP and PBS to check vertical polarisation required for EOM: 5.17 mW transmit, 0.03 mW reflect, 5.35 mW incident (PBS), 5.97 mW incident (HWP)

Set up EOM: 5.63 mW transmission, 6.04 incident. 

At this point the power meter was seen to be not particularly reliable for alignment, since it seems to register too much power at the edge of the detector compared to the center. The EOM was centered by using the IR detector card and seeing that the beam passes through the center. Then, we added AOM.

Afterwards, we measured the beam profile coming out of the modulators. In 2421, the beam profile was measured coming from the lens that was previously fixed on the bench, as indicated in that entry. From that measurement I used a beam waist of 150 µm with position z = 0 defined by the position of the last preset lens. However, that beam waist was an average of the horizontal and vertical beam waist measurements, which were quite different. But perhaps that could have also been due to the alignment of the beam on the beam profiler, which in 2425 was seen to be off centered in the measurements close to z = 0. Using Jammt with Thorlabs lens database, an f = 100 mm lens placed at z = 0.125 m gives a waist of 218 µm at z = 0.272 (figure 1). However, as mentioned in 2482, the lenses shown were slightly adjusted in order to properly level the beam at 76 mm height. So the prediction may not be completely accurate anymore.

The measured beam profile is shown in figure 2. The predicted beam seems to be a bit too far forward and too large.

Readjusting the mode matching telescope with a waist of 193 µm at 13.3 cm with respect to the f = 100 mm lens gives the result shown in figure 3. The spacing between the two modematching lenses (f = 40 mm and f = 75 mm), as well as the space from the f = 75 mm to the OPO waist should still be sufficient.