I always wondered why the most common pinout for laser diodes with a monitoring photodiode was so strange. It wasn't sure how it was intended to be used until I ran into this circuit here:
In my mind, it seems a bit strange that the current source is shared by both the laser diode and the photodiode since it seems that this coupling would cause the laser diode and photodiode to distort each other's output.
The only reason I can think for this is to reduce component count by using the laser diode's own forward voltage voltage drop to reverse bias the phtoodiode to eliminate the need for a bias supply, and that the tiny photodiode current is considered negligible relative to the full laser diode current.
Is my reasoning correct? Or is there a useful purpose to this coupling?