I did the same measurement for the 4th time but this time I covered better the pump path so that the scattered light is less.

The absorption value now is 4ppm/cm with a precision of 0.8 ppm/cm (after filtering).

This means that the pump stray light (1064nm) from outside the box gives a contribution of at least 2ppm/cm on the measure.

I couldn't cover the sample, but also the scattered light from the sample could give a significant contribution. So I would like to put a filter in front of the photodetector, to make only the probe light (633 nm) pass.