noisy signal processing question. signal in detector with slope shift in some areas

Hello,

I have several scans I did in a photodiode using an interferometer. The detector's size if very small so we see some drift in the slope of the zero (should be the offset) in some if the spectrum. I need the envelope above zero point. (there are some values below zero but I need those which are above)

Could someone help with this?

Attaching several text files with the examples. I have plenty more and need to understand how to process them.