Abe, Marc

On Monday we decided to further increase the laser power by tuning the HWP.

In order to avoid burning dust and/or stains, we decided to do smaller map (radius = 9 mm) centered at X = 324 mm and Y = 134 mm.

Indeed, in this area no spikes were present in the previous measurement (see entry 2589).

The resulting map with Pin = 0.6236 W is presented in figure 1. In this measurement the lockin amplifier sensitivity was set to 500 uV.

There was some spikes saturating but we thought it could be good enough to estimate the viewport surface background absorption.

The resulting absorption is presented in figure 2. Again, the background level seems lower than the spare viewport one (see entry 2585). Another difference is the presence of many spikes (~2 times larger absorption than overall area).

This lower absorption could be explained by several possibilities (too low power to distinguish absorption, misalignment of probe and/or pump lasers, viewport tilted, properties of spare and cleaned viewport are differents, cleaning damaged the surface, ....)

To eliminate possibilities, we increased the power to Pin = 1.2503 W (HWP angle = 55 deg). and got the results reported in figure 3. The background absorption level stayed the same, meaning that we are indeed sensing absorption.

(note that the spike absorption level is not meaningful because the locking amplifier was saturating on purpose).

To eliminate the possibility of a viewport tilt, we then started to do Z scan to check the surface position across the viewport surface.

We changed back the laser power to Pin ~ 0.6W and did a Z scan at the top position of entry 2585. Result is presented in figure 4.

While the phase exhibits the expected behavior, I'm a bit more surprised by the AC and DC shapes...