2 DOF Robotic Arm with a decoder(photodiode,laser) instead of a potentiometer

Hello everyone! For my final year I am planning to make a decoder for a 2 DOF robotic arm. Commonly, the angular movement of a robotic arm is controlled through potentiometer but this time i am planning to do it through a photodiode(utilizes current) and a laser that will be attached to the gears of a servomotor. Do you have any suggestion on how i will do it?

Do you have any suggestion on how i will do it?

Do what? Power the laser? Read the photodiode? What IS a photodiode?

What does “utilizes current” mean? Every electrical device on the planet utilizes current.