I checked the linearity of the system by changing the incident power of the HeNe laser.

The PD DET10A gives a DC signal proportional to the laser power. And the AC signal obtained after the lock-in demodulation is proportional to the absorption signal and to the DC. So if there is some non-linearity (for example saturation effects) it should cause the AC/DC signal to be not constant.

I put an optical density wheel to reduce HeNe power. I rotated it a bit to change the DC level, acquired some points of AC and DC in that condition, and rotated again the wheel a bit to a different DC value and so on.

The plot shows good linearity.