Laser diodes are one of the most important components of modern laser technology. Their compact design, high efficiency and flexible wavelength options have established them as the preferred light source for numerous technologies and they are used in a wide range of applications, from industrial material processing and optical communication to use as seed lasers for other types of laser.

A laser diode (LD) is an optoelectronic semiconductor component that is based on the principle of stimulated emission and emits coherent light. It belongs to the class of semiconductor lasers and is structurally similar to a light-emitting diode (LED), but differs in its ability to provide optical amplification and laser emission.

In contrast to other solid-state or gas lasers, the laser emission is generated by direct electrical injection, which enables the high efficiency and compactness of laser diodes.

The active zone of a laser diode consists of a pn junction layer, which serves as a light-generating medium. Here, charge carriers are injected by a current source. These recombine in the semiconductor layer and emit photons. The band gap of the semiconductor material determines the emission wavelength. By choosing the respective semiconductor material, a spectrum of ultraviolet (UV) to infrared (IR) radiation can be realized.

Like other lasers, the laser diode requires an optical cavity in which photons are amplified by stimulated emission. This is formed by two partially transparent mirrors arranged in parallel. As soon as the amplification in the cavity is greater than the losses, the diode starts to lase. Some highly developed variants, such as DFB (distributed feedback) laser diodes or VCSELs (vertical cavity surface-emitting lasers), use special resonator concepts to improve spectral purity and beam quality.

Available optical power of laser diodes

Due to the high power loss in the semiconductor during light generation, overheating of the crystal limits the beam power per single emitter to a few hundred microwatts up to 10 watts depending on the diode type. To achieve higher power levels, several diodes are operated in parallel on a chip as bars. This makes it possible to achieve laser outputs of up to 100W in the IR. For higher outputs of up to 10 kW or even 100 kW, several of these bars are stacked and combined as a diode laser.

Beam quality of laser diodes

Some important properties of laser diodes are determined by the geometry of the optical resonator. In a laser diode, the light is confined in a very thin layer, and the structure allows only a single optical mode in the direction perpendicular to the pn layers. In the transverse direction, the waveguide is wide compared to the wavelength of the light, so the waveguide can contain several transverse optical modes. Therefore, such a laser diode is called a transverse multimode laser. These transverse multimode lasers are suitable for applications where no small diffraction-limited beam is needed, but a high optical power.

For applications that require a small, focused beam, the waveguide must be narrow, on the order of the optical wavelength. In this way, only a single transverse mode is supported and a diffraction-limited beam is obtained.

Reliability of laser diodes

Laser diodes are sensitive to thermal overload, overvoltage and electrostatic discharges (ESD), as these can damage the semiconductor structure and significantly reduce the lifespan. Careful heat dissipation and operation within the specified current and voltage limits are crucial to ensure a long lifespan of up to 100,000 hours.

The performance and possible applications of a laser diode are specified by various electrical, optical and thermal parameters. These parameters describe the diode's efficiency, beam quality, modulation speed and temperature stability, and are crucial for selecting the appropriate laser diode for a specific application.

Laser diodes are used in a wide range of technological and industrial applications due to their compact design, high efficiency and versatile wavelength options. They enable precise and powerful solutions for a wide range of requirements.

Industrial manufacturing

Laser diodes are used in industrial material processing for applications such as laser cutting and laser welding. Since several laser diodes have to be combined to form a diode laser to achieve the necessary high laser power, the beam quality suffers. Diode lasers are therefore mostly used for less precise applications with high power requirements.

Medical technology

In medical technology, laser diodes are used, for example, in dermatological treatments, dentistry, surgery and photodynamic therapy (PDT). The wide range of available wavelengths allows for a precise selection for the desired medical effects, such as maximum blood coagulation, maximum penetration depth or maximum tissue ablation. The compact dimensions are also advantageous for many applications.

Measurement and sensing

Laserdioden sind essenziell für LIDAR-Systeme, Spektroskopie, Abstandsmessung und Gasanalytik, da sie präzise und kohärente Lichtquellen für hochauflösende Messungen bieten.

Communication

Laser diodes are the key components in optical data transmission. The compact and cost-effective devices enable stable and efficient signal transmission in optical fibers over long distances.

Consumer electronics

Laser diodes are used in laser pointers, laser projectors, Blu-ray players and other optical drives. Their high modulation speed and compact design enable innovative displays and high-resolution image and storage systems.