Many people are puzzled when they buy lasers for the first time. Why are laser diodes so complicated? Today, Xiaobian gives you a brief talk about the schematic diagram of diode laser, and why laser diodes have PD and LD.

In modern optoelectronic technology, Laser Diode (LD) has become one of the indispensable core devices. Whether it is optical storage in consumer electronics, printers, industrial processing, medical treatment, ranging communication, laser diodes play a vital role. For many engineers and users who are new to laser devices, a common question is: What is the schematic diagram of laser diode? Why is there a PD (Monitoring Photodiode) besides LD itself?

In this paper, the basic structure, working principle, and the relationship between LD and PD will be deeply analyzed to help everyone fully understand this optoelectronic device.

First, the basic structure of diode laser schematic diagram

The schematic diagram of laser diode is not a simple "diode symbol", but consists of several key parts. Generally speaking, the representation of the laser diode module on the circuit diagram roughly includes the following elements:

1. LD laser emission core

Essentially, it is a PN junction semiconductor device. When a forward current is applied, carriers (electrons and holes) recombine in the active region to generate stimulated radiation, thus forming a highly coherent laser output.

In the schematic diagram, LD is generally represented by a symbol similar to a diode, but the word "Laser" will be marked in the direction of the arrow to distinguish it from ordinary LED.

2. PD monitoring photodiode

It is attached to the back end (back) of LD chip and used to monitor the luminous intensity of LD in real time.

On the schematic diagram, PD is usually represented by the symbol of photodiode, and marked with LD module to form a whole of "LD+PD".

3. Pin structure

Generally, a laser diode has three pins:

LD Anode (Anode)

LD Cathode

PD output (for detecting feedback signal)

Therefore, in the complete schematic diagram of laser diode, we will see a combination of diode (LD) and diode (PD), not just a single diode symbol.

Second, why do laser diodes have PD and LD?

Many users will find that there is an extra PD pin besides the main LD pin when they get the laser diode. Why do you need this photodiode?

The main reasons are as follows:

1. Stable output power

Laser diodes are very sensitive to current. Slight current fluctuation will lead to obvious changes in output optical power and even damage devices.

The function of PD is to monitor the light intensity output by LD in real time and feed it back to the driving circuit.

The driving circuit will perform Automatic Power Control (APC) according to the PD signal, thus ensuring the stability of laser output.

2. Extend the service life of the device

If LD is in an over-current or over-power state for a long time, its life will be greatly shortened. PD monitoring and feedback mechanism can prevent LD from being over-driven, thus significantly improving reliability and service life.

3. Ensure communication and measurement accuracy.

In optical communication, laser ranging and other application scenarios, the stability of output power is directly related to signal quality and measurement accuracy.

The feedback control provided by PD can ensure the stability of output waveform and power curve and reduce errors.

4. Necessary conditions for industrial and scientific research applications

Many scientific and industrial laser systems must meet the conditions of long-term continuous and stable light emission. It is difficult to guarantee this by current drive alone, and the feedback system of PD+LD becomes the standard scheme.

Third, the cooperative working principle of LD and PD

A complete laser diode drive system usually adopts APC (Automatic Power Control) mode:

1. Current drives LD→ LD to emit laser.

2. PD receives backscattered light → generates a current signal proportional to the laser intensity.

3. The feedback circuit adjusts the driving current to ensure that the LD output is stable at the set power.

Iv. Application examples of LD and PD

1. Optical communication: PD feedback ensures the power stability of each laser pulse and avoids signal distortion.

2. Laser ranging/sensing: stabilize the output beam and improve the detection accuracy.

3. Medical and cosmetic laser: It is necessary to continuously stabilize energy to ensure safe and effective treatment.

4. Industrial processing: In high-power laser diode array, PD feedback system can prevent overload and ensure reliable operation.

The schematic diagram of laser diode is not a single diode, but a combination structure of LD+PD.

LD(Laser Diode) is responsible for emitting laser, and PD(Photodiode) is responsible for monitoring the output power in real time.

Both of them form a closed-loop control through a feedback circuit, which ensures that the laser output is stable, safe and accurate.

It is precisely because of the cooperative work of LD and PD that laser diodes can be widely used in industry, scientific research, medical care and daily life, and become the backbone of modern optoelectronic technology.