Pigtail PD is a device which can transform the optical signal to electronic signal. PD is worked under the effection of backward voltage, when there is no light shine, the  reverse current is very low, which called dark current; when there is light shine, the  reverse current suddenly increase to several microamp ere, which called light  electric current. The reverse current is big while the light power is high. The electric current changes according to the power of light changes, then the light signal can be changed to electrical signal, which called  photo-electronic light sensors.