Question

Why is photodiode operated in the reverse bias?

Answer 1

Because its not much use in the forward bias direction, it will conduct pretty much like a normal diode. The magic happens in the reverse bias condition where the reverse current (in a normal diode you might call it leakage current) is dependent upon and proportional to the amount of light power hitting the junction. There is also a component called dark current which is the tiny leakage current which flows when there is no light. The current can be readily converted to a voltage using a transimpedance amplifier.