Quadrant diodes are for centering the beam if you know that the maximum of throughput is where the center of your diode is. And you need 4 piezomotors for aligning that.

In my problem I dont know at which input position of the laser-fiber coupling the maximum is, nor do I know the value of this maximum. The position of the output spot of my fiber does not change its position on my photodiode. Only the reading of my photodiode changes as I move the inputpart of the fiber with my two piezos around in the laserbeam.
The two piezos can move the input of my fiber in a plane (x any y) which is perpendicular to the laser beam.
I need to find now the position x and y for my two piezos where I have maximum throughput.

I am really sorry for posting such a wrong description of my problem.
Thank you for you pacience,

Klaus