Working Principle and Applications of Laser Diodes

Laser Diode, abbreviated as LD, is an electronic device that uses semiconductor materials to convert electrical energy into laser light.

The fiber-optic communication equipment, laser rangefinders, barcode scanners, laser printers, and even some medical devices we use in daily life may all employ laser diodes.

Its greatest features are: small size, low power consumption, fast response speed, ease of integration, and the ability to produce highly directional laser light.

I. What Does a Laser Diode Look Like?

What does a laser diode look like?

Common laser diodes come in three appearances:

TO package: commonly used in small lasers and ranging devices.

Butterfly package: commonly used in optical communications and high-stability laser systems.

Bare chip: mainly used in scientific research, optoelectronic integration, and device development.

II. How Does a Laser Diode Produce Laser Light?

To understand a laser diode, you first need to know its core structure.

How laser light is produced

A laser diode mainly consists of the following parts:

Structure and function

P-type semiconductor: provides holes

N-type semiconductor: provides electrons

Active layer: electrons and holes recombine to produce light

Waveguide layer: confines light to propagate within a specific region

Resonant cavity: provides optical feedback and enhances specific modes

Electrodes: supply current to the device

Step One: Powering the Laser Diode

When we apply a forward voltage to the laser diode:

Electrons in the N-type region move toward the active layer.

Holes in the P-type region move toward the active layer.

Electrons and holes meet within the active layer.

You can think of it this way: the current continuously delivers the carriers needed to produce light into the active layer.

Step Two: Electrons and Holes Recombine to Produce Photons

Electrons and holes recombine to produce photons

When electrons and holes recombine, they release energy, part of which is released in the form of photons.

The energy of a photon is related to the laser wavelength:

The energy of a photon is related to the laser wavelength

Where:

E: photon energy.

h: Planck's constant.

v: frequency of light.

c: speed of light.

λ: wavelength of light.

For example, laser light with a wavelength of 1550 nm has a photon energy of approximately 0.8 eV.

Step Three: Stimulated Emission Produces Laser Light

Stimulated emission produces laser light

This is the most core working principle of a laser diode.

Photons already present in the active layer can induce other excited-state electrons to recombine and produce new photons.

The newly generated photons have the same frequency, phase, propagation direction, and polarization state as the original photons.

This is stimulated emission.

As stimulated emission continues to occur, the optical signal is amplified.

Step Four: Resonant Cavity Feedback Outputs Laser Light

Resonant cavity feedback outputs laser light

Laser diodes typically use the reflective surfaces at both ends of the chip to form a resonant cavity.

Light propagates back and forth within the cavity, continuously passing through the active layer and being amplified.

When the gain exceeds the internal loss and output loss, the laser reaches threshold and begins to stably produce laser output.

This also explains why laser diodes differ from ordinary LEDs:

LEDs mainly rely on spontaneous emission to emit light.

Laser diodes mainly rely on stimulated emission to produce laser light.

III. What Are the Main Types of Laser Diodes?

Different laser diodes differ in their internal structure, output wavelength, and application scenarios.

FP Laser Diode (Fabry-Perot LD)

It uses the reflective surfaces at both ends of the chip to form a resonant cavity. The structure is relatively simple, and it is commonly used as a general light source and in optical communications.

DFB Laser Diode

It has a periodic Bragg grating inside, enabling stable single-longitudinal-mode output. It is commonly used in 1550 nm and 1310 nm optical communications.

VCSEL Laser Diode

The laser is emitted perpendicular to the chip surface. It offers advantages such as easy array formation and low power consumption, and is commonly used in 3D sensing and short-distance optical communications.

High-Power Edge-Emitting Laser Diode

It can output relatively high optical power and is commonly used in industrial processing, laser pumping, and medical equipment.

IV. What Are the Applications of Laser Diodes?

Laser diodes have a very wide range of applications. Six important fields are introduced below.

Fiber-Optic Communications

This is a very important application of DFB laser diodes.

Working process: the laser diode generates laser light → optical lens coupling → enters the optical fiber → optical signal transmission → the photodetector at the receiving end receives it.

Common wavelengths:

Wavelength | Main application

850 nm | Short-distance multimode fiber-optic communication

1310 nm | Fiber-optic communication

1550 nm | Long-distance fiber-optic communication

In particular, 1550 nm DFB lasers are widely used in high-speed optical communication systems.

LiDAR

Laser diodes can serve as the emission light source for LiDAR.

By emitting laser light and receiving the light reflected back from the target, it measures the target distance.

Applications include: autonomous vehicles, drone obstacle avoidance, industrial robots, and 3D environmental scanning.

Industrial Laser Processing

High-power laser diodes can be used directly for material processing, or as pump sources for other solid-state lasers.

Common applications: metal welding, plastic welding, laser cladding, surface heat treatment, and laser cutting.

Medical Aesthetics

Depending on wavelength and power, laser diodes can be used in different medical scenarios.

For example: skin treatment, hair removal devices, dental treatment, vascular-related treatment, and medical photodynamic devices.

3D Sensing and Facial Recognition

VCSEL lasers are often used in mobile phones and consumer electronic devices.

Together with receivers, they can enable: facial recognition, depth measurement, structured light scanning, proximity sensing, and gesture recognition.

Laser Sensing and Fiber-Optic Detection

Laser diodes can also serve as light sources for fiber-optic sensing systems.

For example: fiber-optic temperature sensing, fiber-optic strain sensing, fiber-optic vibration detection, fiber-optic gas sensing, and fiber-optic hydrophones.