What is a semiconductor laser?

Like LEDs, lasers are light source devices, but compared to LEDs, lasers have advantages such as directionality, monochromaticity, and high output, which have led to their use in a variety of products that LEDs cannot cover.
Lasers can be divided into various types depending on the laser medium, such as gas, liquid, and solid, but semiconductor (solid) lasers in particular are being considered for use as sensing light sources in surrounding monitoring systems for autonomous driving vehicles.
This column explains the basics of semiconductor lasers.

A semiconductor laser is a semiconductor element that can emit laser light by passing electricity through it.
Like an LED, it emits light when a forward current flows through the pn junction.
What makes it different from an LED is that it has a light-emitting layer called an active layer between p-type and n-type semiconductors, which generates laser light. Compared to an LED, it can generate strong light with a consistent phase, so it is used in a wide range of applications, including sensing, material processing, and communications.

The general structure of a semiconductor laser is shown in the diagram.
It is formed by a structure (double heterostructure) in which an emission layer is sandwiched between n-type and p-type cladding layers.
As with an LED, when a forward bias is applied to a semiconductor laser, electrons in the n-type cladding layer and holes in the p-type cladding layer recombine in the active layer, causing light to be emitted. This light is confined within the active layer as both end faces act as reflective mirrors, and travels back and forth while being amplified. This is called stimulated emission, and when the light is sufficiently amplified, it becomes a strong, phase-aligned light that is emitted externally.

The following sections provide diagrams of different types of laser diodes.

There are two main types of semiconductor lasers.

• FP (Fabry-Perot) type

FP semiconductor lasers confine light between two opposing parallel mirrors to produce stimulated emission, which produces a laser beam with a certain degree of spectrum width and multiple wavelengths (multimode).

• DFB (Distributed Feedback) type

This is a semiconductor laser with a diffraction grating structure at the boundary between the active layer and the cladding layer.
The diffractive structure allows it to oscillate at only a specific wavelength (single mode).

Generally, multimode lasers have high optical output, making them suitable for drawing and processing products.
On the other hand, single-mode lasers have low output but have excellent wavelength uniformity, making them suitable for optical communications and other products.