Laser diodes are semiconductor devices that emit coherent light and are classified into several types based on structure, emission direction, mode, and application.

Major Types of Laser Diodes

1. Edge-Emitting Diode Lasers (EELs) These lasers emit light from the edge of the semiconductor chip, parallel to the surface. They are typically made from materials like gallium arsenide (GaAs), indium phosphide (InP), or gallium nitride (GaN). Edge-emitting lasers provide high optical power, ranging from milliwatts to tens of watts, and are widely used in telecommunications, optical storage, barcode scanning, laser printing, medical equipment, and industrial systems . 2. Vertical-Cavity Surface-Emitting Lasers (VCSELs) VCSELs emit light perpendicular to the chip surface. They use a vertical cavity with distributed Bragg reflector mirrors and a central active region where electrons and holes recombine. VCSELs are compact, efficient, and commonly used in fiber-optic communications, sensing, and short-range data transmission . 3. Single-Mode Laser Diodes These have a narrow active region supporting only one optical mode, producing a highly focused, low-divergence beam with high coherence. They are ideal for precision applications like spectroscopy, fiber-optic communication, and sensing . 4. Multi-Mode Laser Diodes With a broader active region, these diodes support multiple optical modes, resulting in a wider beam with higher divergence and lower coherence. They are used in high-intensity applications such as laser cutting, welding, printing, and illumination . 5. Master Oscillator Power Amplifier (MOPA) Diodes MOPA diodes combine a single-mode oscillator with a multi-mode amplifier to increase output power while maintaining spectral quality. Applications include lidar, range finding, and medical imaging . 6. Distributed Feedback (DFB) Lasers DFB lasers incorporate internal gratings for wavelength stability, producing narrow-line, single-mode emission. They are widely used in telecommunications and sensing where precise wavelength control is critical . 7. External Cavity Diode Lasers (ECDLs) ECDLs combine a diode chip with an external cavity to achieve tunable, narrow-linewidth output. They are used in spectroscopy, precision measurements, and research requiring adjustable wavelengths . 8. Quantum Cascade Lasers (QCLs) QCLs operate using intersubband transitions in quantum wells, producing tunable mid- and far-infrared light. They are used in gas sensing, spectroscopy, and environmental monitoring . 9. Superluminescent Diodes (SLDs) SLDs provide broadband, low-coherence light, useful in imaging applications such as optical coherence tomography (OCT) and fiber-optic gyroscopes .

Summary

Laser diodes are versatile devices classified by emission direction (edge vs. surface), optical mode (single vs. multi-mode), feedback mechanism (DFB, ECDL), and specialized structures (QCL, SLD). Their applications span telecommunications, medical procedures, industrial processing, sensing, and scientific research, making them essential components in modern photonics .