Why do laser diodes have two parts: LD and PD

A laser diode is a photoelectric device that consists of two main parts: a laser emitting part and a laser receiving part. These two parts share a common electrode, so the entire device has three electrodes.
During operation, under forward bias conditions, the photoactive semiconductor between the junctions is excited and emits light. This light interacts with the optical resonant cavity, further enhancing the specific wavelength of light emitted from the junction. The properties of this light are determined by the physical properties of the material.

Although laser diodes have many advantages, such as small size, light weight, low power consumption, simple driving circuit, convenient modulation, resistance to mechanical shock and vibration, they are very sensitive to overcurrent, overvoltage and electrostatic interference.
Therefore, special care must be taken when using it to ensure that its working parameters do not exceed the maximum allowable values. Specifically, the following measures can be taken:
Using a DC constant current source to drive the laser diode;
Connect current limiting resistors in series and bypass capacitors in parallel on the laser diode circuit;
Due to the increase in temperature of the laser diode, the current flowing through it will increase. Therefore, appropriate heat dissipation measures must be taken to ensure that the device operates within the appropriate temperature range;
To prevent damage to the laser diode due to excessive reverse voltage, a fast silicon diode can be anti parallel connected at both ends.