How a zener and a schottky diodes protect laser diode?
Due to the Schottky's low forward voltage drop, any (positive) ESD on the left side of your diagram will cause the Schottky to be forward biased, preventing back biasing of the LD. The Zener will clamp the forward voltage of the the LD to the Zener's breakdown voltage, in this case 3.3V, preventing over voltage across the LD.