Yuhang and Michael

We measured some more points of the green power versus crystal temperature characteristic for the new OPO. However, this time we put a laser line filter after the OPO to cut off any transmitted infrared beam from reaching the power meter. We had a suspicion that this was causing an offset in the measurement. We did a small comparison with some values of T that were already measured as well. With the laser line filter, the green power meter reading is much lower, even though it is supposed to be switched to the "532nm" setting. We checked if this was a linear or nonlinear offset by looking at some values in the flat, steep and peak parts of the green vs T curve. As we can see in figure 1, subtracting the power offset between the measurement sets works well for the green vs T peak. There is a bit of a discrepancy in the points connecting the left side "shoulder" of the peak, but looking at my recordings I think this was a set of points from before we optimised the locking of the OPO (i.e. unlocking and relocking between each temperature change). Anyway, we can see that the peak phase matching occurs at 7.26 kOhm thermistor resistance, which corresponds almost exactly to the old OPO. Also I made a mistake in reporting the amount of power in 3018, it is actually about 357 mW of infrared being sent to the new OPO.

Then we measured the beam profile of the generated green coming from the OPO, in order to find the actual beam position inside the OPO assembly. The measurement is set so that z = 0 corresponds to the metal edge of the OPO holder as shown in figure 2. We used the beam profiler shown in figure 3, removed the ND filter and turned the lights off since the generated green power is very low. The beam profiler software gives us the "13.5%" Gaussian beam width. We used an f = 500 mm lens placed at z = 8 cm with the intent of making it easier to measure the beam over a longer distance, since the OPO beam waist is of the order of 20 µm. The beam profile measurement begins at z = 10cm, so the calculated waist here is for the beam after the lens, as shown in figure 4. I will add a comments later with a JaMMT calculation using the fitted beam and actual lens parameters (Thorlabs LA4181).

However, even using a f = 500 mm lens, we couldn't measure over a large range of distance values, because the beam quickly became too large for the beam profiler. Looking at the graph it looks like low z values are above the fit line while high z values are below, and the beam isn't really focusing. We should probably change the lens or lens position.