With the popularity of laser diodes, many people are curious, why do laser diodes have three feet? What is the working principle of laser diode? Let's take a look at it together

1. Why does a laser diode have three feet?

Common laser diodes (especially models with monitoring and detection functions) usually have three pins, which correspond to each other:

1. LD(Laser Diode)

This is the main part of the laser diode, and the pin is used to drive the laser to emit light. Generally, constant current drive is needed, and the current directly determines the luminous intensity.

2. PD(Photodiode)

A photodiode is integrated internally to monitor the laser output power. Because the laser diode is affected by temperature and current fluctuations, the light intensity may be unstable. Through PD real-time monitoring, it can be fed back to the driving circuit, and the current can be automatically adjusted to ensure the stability of laser output.

3. GND (common ground/negative pole)

This is the common reference terminal, which is connected with the power ground or the circuit ground to ensure the normal operation of the circuit.

The three-pin laser diode is actually "emission+monitoring+ground". Compared with ordinary diodes, the laser tube with PD can realize closed-loop control and ensure stable output, which is especially suitable for applications that require strict optical power (such as optical communication, sensors, medical instruments, etc.).

Second, the working principle of laser diode

The working principle of Laser Diode (LD) is similar to that of light-emitting diode (LED), which is based on the electroluminescence of semiconductor PN junction, but laser diode realizes stimulated radiation, so it can output laser with excellent monochromaticity and strong directivity.

The principle is divided into three steps:

1. Carrier recombination luminescence

When the laser diode is forward biased, electrons are injected from the N region to the P region, and holes are injected from the P region to the N region, and photons are released when they recombine at the PN junction.

2. Stimulated radiation amplification

Inside the diode, the material is specially designed (quantum well structure). When a photon passes by, it will induce more electron-hole pairs to recombine and release photons with the same phase, direction and energy, thus realizing "optical amplification".

3. The optical resonator forms laser.

The two ends of the laser diode chip are processed into parallel mirrors (usually one end is highly reflective and the other end is partially transmissive). Photons are reflected and amplified back and forth in the cavity, and finally output from the transmission surface, forming a laser with high intensity and good monochromaticity.

LED: electronic recombination, casual light, scattered light.

LD: electron recombination+photon stimulated radiation+resonant cavity beam limiting → output "ordered" laser.

Third, application examples

Because of the design with three pins (LD+PD+GND), laser diode is very suitable for application in:

Optical fiber communication: stable light source is needed, and PD feedback can ensure constant power.

Ranging/sensing: such as laser rangefinder and displacement sensor.

Industrial detection: laser marking, precision measurement.

Medical equipment: laser therapeutic instrument, beauty instrument.