Main Types of Semiconductor Lasers

1. Representative semiconductor lasers

Semiconductor lasers can be broadly divided into two types: edge-emitting lasers and surface-emitting lasers.
VCSELs, which have been increasingly adopted as light sources for sensing in recent years, are a type of surface-emitting laser.
An overview of each is as follows:

Edge-emitting laser (EEL)

Laser chipThis is a laser that emits light horizontally to the substrate surface.
The resonator is placed parallel to the semiconductor substrate, and the cleaved end facets (side faces) are used as resonators to emit light. Semiconductor lasers with this structure are called edge-emitting lasers (EELs).

Surface-emitting laser (SEL)

Laser chipThis is a laser that emits light perpendicular to the substrate surface.
There are two types of lasers: one that uses mirrors or diffraction gratings to change the direction of light extraction while keeping the resonator structure of an edge-emitting laser, thereby emitting light vertically, and one that has a vertical resonator structure itself that emits light vertically. The latter type of laser is called a VCSEL (Vertical Cavity Surface Emitting Laser).

VCSEL (Vertical Cavity Surface Emitting Laser)

This is a type of surface-emitting laser that emits light vertically.
Vertical light emission is achieved by forming a cavity perpendicular to the substrate and sandwiching the active layer between two cavity mirrors. The cavity mirrors use a distributed Bragg structure DBR (Distributed Bragg Reflector) made of a semiconductor or dielectric layered structure.

2. Main structure

The main structures of edge-emitting lasers and surface-emitting lasers are shown below.

FP (Fabry-Perot) type

This simple double heterostructure is widely used in edge-emitting lasers.
Since oscillation is possible at low current density, continuous wave oscillation is possible at room temperature.

Vertical Cavity Surface Emitting Laser (VCSEL) Structure

By placing reflectors made of multilayer films called DBRs (Distributed Brag Reflectors) above and below the active layer, a cavity is formed perpendicular to the substrate. Because the active region is small, the threshold current required for laser oscillation can be lowered. It is also possible to obtain high output by densely integrating the light-emitting parts in an array.

3. Main packages used

Semiconductor lasers are available in a variety of packages, but here we will introduce three major types.

TO-CAN

Laser chipThis is a laser that emits light horizontally to the substrate surface.
The resonator is placed parallel to the semiconductor substrate, and the cleaved end facets (side faces) are used as resonators to emit light. Semiconductor lasers with this structure are called edge-emitting lasers (EELs).

VCSEL

SMD packages are widely used and are similar in shape to LEDs, making them easy to mount.
In some cases, an element called a diffuser is placed on the top surface of the package to control light distribution.
In addition, by using a metal with high thermal conductivity (CERAMICS, etc.) for the package, heat dissipation can be improved, reducing changes in laser characteristics due to temperature rise.

CoS (Chip on Submount)

Unlike the above two, this laser is not packaged, but simply has the laser chip mounted on a submount. It is extremely small and can be mounted with a high degree of freedom to suit the customer's optical specifications. However, since the laser chip and bonding are exposed, specialized knowledge is required for handling and mounting.