Many people ask that the laser diode has three pins, so how to connect them? Let's take a look at it from the perspective of Xiaobian.

The three pins of a laser diode are generally the positive electrode (LD+), the negative electrode (LD-) and the photosensitive monitoring terminal (PD).

Of course, the packaging methods of different manufacturers are slightly different, such as TO-18, TO-56, TO-5 or C-Mount, butterfly packaging and so on, and the pin distribution will also change, so we should not only look at the number of pins, but also look at the pin definition diagram. But no matter what kind of structure, the principle is the same.

LD anode and cathode

In fact, the inside of a laser diode is a bit like an ordinary diode, both of which are PN junctions, except that this PN junction is designed to generate stimulated radiation in the reflection cavity.

When you apply a forward current to it, electrons and holes recombine and a laser is emitted.

So the two most critical pins are LD+ (anode) and LD- (cathode), which directly determine whether the laser can be emitted normally.

In many common package (such as TO-18),

The anode is usually connected to an independent pin on the outside of the shell,

The negative electrode is sometimes directly connected to the shell, and sometimes led out separately.

The third foot: photosensitive monitoring terminal (PD)

The third pin is generally the signal terminal of monitoring Photo Diode (PD).

Don't underestimate this little foot, its role is crucial.

The position of PD is usually on the back of the laser chip, which is used to detect the laser output power.

By feeding back the current, the current laser intensity can be judged, thus realizing automatic power control (APC).

Simply put, PD will "tell" the strength of laser output to the driving circuit in real time, and the driving circuit will automatically adjust the current according to the feedback to keep the output power stable.

This is especially useful in high-precision scenes, such as optical communication, ranging module, medical laser instrument, etc., all of which require that the output should not drift with temperature and current.

Pin position difference of different packages

Several common packages are like this:

To-18 (φ 5.6 mm) package:

Generally, there are three legs, the shell is connected with LD negative electrode, and the two independent legs are LD positive electrode and PD respectively.

To-56 (φ 9mm) package:

Similar structure, but larger and higher power.

Butterfly package:

Usually there are several pins, besides LD and PD, there are also temperature control (TEC) power pins and thermistor pins for precise temperature control.

Three pins of the laser diode, one for emitting light (LD+/-) and the other for monitoring (PD).

The first two determine whether it can be bright, and the latter determines whether it can be stable.

If it is only used as an ordinary diode, it is easy to burn;

But if you know how to use the third foot for feedback control, it can output stably for a long time.