Laser diode is a kind of light source semiconductor laser invented in the 1960s, also known as laser tube (Laser Diode)。LASER is an abbreviation of “Light Amplification by Stimulated Emission of Radiation”, usually abbreviated as LD.Since it can produce light with exactly the same properties such as wavelength and phase, high coherence is its biggest feature 。Laser diodes have been greatly developed in this century, and their applications have become more and more extensive, and their roles have become greater and greater. In general laser diodes, in addition to light-emitting LDs, photodiodes are usually packaged into laser diodes. Next, we will mainly introduce the role of photodiodes in laser diodes.


First, let’s see the common laser diodes on the market. A rough structure diagram is as such:

As can be seen from the above figure, the laser diode includes four parts: the first part is the laser emitting part (which can be represented by LD), and its function is to emit laser ;The second part is the photodiode, which is the laser receiving feedback device (it can be represented by PD). Its function is to receive and monitor the laser light emitted by the LD (If you don’t need to monitor the output of the LD, the PD part can not be used of course). In addition, these two parts also need a common electrode. Therefore, laser diodes usually have three electrodes;The third part is a lens made of glass cover, which is used for dust prevention and resonant cavity; the fourth part is a metal shell, which is mainly used for fixing and shielding external interference signals and heat dissipation.


Regarding the photodiode in the laser diode, its main function is to detect the output of the LD in the laser diode.


For the role of PD, there are mainly several aspects:

1.PD can be used to make a simple laser rangefinder. Laser diodes made by PD generally have good accuracy at short distances. When the distance is longer, the accuracy will be reduced.

2.The PD can be used to monitor the output of the LD, and the actual output power of the LD at this time can be known by detecting the size of the photocurrent in the PD. Through network adjustment, the continuous output of stable power of LD can be realized. Use this feature to make a constant power laser.


The main function of the PD in the laser diode is to detect the output of the laser diode, so as to control, adjust and detect the output of the LD.