Laser detector parts
Oh, @tomnexus gave me a clue... I think there is only one photodiode (PD) and as he said the rings scatter light that is not "on axis".  So you find a signal maximum when the laser light is perfectly aligned, (cute).   

And to answer the question: Yes I think you can use a commercial PD.  You'll want the PD area to fill the end area of the rod... there might be some diffusor in the middle there between the PD and rod.