I have read papers that use LIDAR-like and LIDAR-Raman systems that detect elements.

I want to measure the vapor water (H2O) and N2 of trees/agriculture using Arduino and low cost elements (DIY).

I am thinking of using 405 nm 500 mW laser. The problem is how can I detect the returned wavelength?

What I am thinking:
1) A CCD sensor?
2) A Photodiode array?
3) A modified webcam?
4) A photodiode with a filter?

My question is what do I measure with the wavelength sensor? The time from when I send the pulse of the laser and the time it returns in the wavelength sensor? Only the wavelength? Should I send a specific coded message with the laser? For example 101010011 ?

Thank you