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Encyclopedia of Laser Physics and Technology

© Dr. Rüdiger Paschotta

Last update: 2008-05-13

Definition: lasers based on semiconductor gain media

Semiconductor lasers are lasers based on semiconductor gain media, where optical gain is generally achieved by stimulated emission at an interband transition under conditions of a population inversion (i.e., high carrier density in the conduction band, compared with the valence band).

The physical origin of gain in a semiconductor (for the usual case of an interband transition) is illustrated by Figure 1. Without pumping, most of the electrons are in the valence band. A pump beam with a photon energy somewhat above the bandgap energy can excite electrons into a higher state in the conduction band, from where they quickly decay to states near the bottom of the conduction band. At the same time, the generated holes in the valence band move to the top of the valence band. Electrons in the conduction band can then recombine with these holes, emitting photons with an energy near the bandgap energy. This process can also be stimulated by incoming photons with suitable energy.

Most semiconductor lasers are laser diodes, which are pumped with an electrical current in a region where an n-doped and a p-doped semiconductor material meet. However, there are also optically pumped semiconductor lasers, where carriers are generated by absorbed pump light.

Common materials for semiconductor lasers (and for other optoelectronic devices) are

• GaAs (gallium arsenide)
• AlGaAs (aluminum gallium arsenide)
• GaP (gallium phosphide)
• InGaP (indium gallium phosphide)
• GaN (gallium nitride)
• InGaAs (indium gallium arsenide)
• GaInNAs (indium gallium arsenide nitride)
• InP (indium phosphide)
• GaInP (gallium indium phosphide)

These are all direct bandgap semiconductors; indirect bandgap semiconductors such as silicon do not exhibit significant light emission. As the emission wavelength is typically just above the bandgap wavelength, compositions with different bandgap energies allow for different emission wavelengths. While the most common semiconductor lasers are operating in the near-infrared spectral regions, some others (often based on gallium nitrides) generate blue or violet light, whereas quantum cascade lasers can emit at wavelengths beyond 10 μm. Another often used material for tunable mid-infrared emission is lead selenide (PbSe) (→ lead salt lasers).

Types of Semiconductor Lasers

There is a great variety of different semiconductor lasers, spanning wide parameter regions and many different application areas:

• Small edge-emitting laser diodes generate a few milliwatts (or up to roughly half a watt) of output power in a beam with high beam quality. They are used e.g. in laser pointers, in CD players, and for optical fiber communications.
• External cavity diode lasers contain a laser diode as the gain medium of a longer laser cavity. They are often wavelength-tunable and exhibit a small emission linewidth.
• Both monolithic and external-cavity low-power levels can also be mode-locked for ultrashort pulse generation.
• Broad-area laser diodes generate up to a few watts of output power, but with significantly poorer beam quality.
• High-power diode bars contain an array of broad-area emitters, generating tens of watts with poor beam quality.
• High-power stacked diode bars contain stacks of diode bars for the generation of extremely high powers of hundreds or thousands of watts.
• Surface-emitting lasers (VCSELs) emit the laser radiation in a direction perpendicular to the wafer, delivering a few milliwatts with high beam quality.
• Optically pumped surface-emitting external cavity semiconductor lasers (VECSELs) are capable of generating multi-watt output powers with excellent beam quality, even in mode-locked operation.
• Quantum cascade lasers operate on intraband transitions (rather than interband transitions) and usually emit in the mid-infrared region, sometimes in the terahertz region. They are used e.g. for trace gas analysis.

Typical Characteristics and Applications

Some typical aspects of semiconductor lasers are:

• Electrical pumping with moderate voltages and high efficiency is possible particularly for high-power diode lasers, and allows their use e.g. as pump sources for highly efficient solid-state lasers (→ diode-pumped lasers).
• A wide range of wavelengths is accessible with different devices, covering much of the visible, near infrared and mid infrared spectral region. Some devices also allow for wavelength tuning.
• Small laser diodes allow fast switching and modulation of the optical power, allowing their use e.g. in transmitters of fiber-optic links.

Such characteristics have made semiconductor lasers the technologically most important type of lasers. Their applications are extremely widespread, including as diverse areas as optical data transmission, optical data storage, metrology, spectroscopy, material processing, pumping solid-state lasers (→ diode-pumped lasers), and various kinds of medical treatments.

Pulsed Output

Most semiconductor lasers are generating a continuous output. Due to their very limited energy storage capability (low upper-state lifetime), they are not very useful for pulse generation with Q switching, but quasi-continuous-wave operation often allows for significantly enhanced powers. Also, semiconductor lasers can be used for the generation of ultrashort pulses with mode locking or gain switching. The average output powers in short pulses are usually limited to at most a few milliwatts, except for optically pumped surface-emitting external cavity semiconductor lasers (VECSELs), which can generate multi-watt average output powers in picosecond pulses with multi-GHz repetition rates.

Modulation and Stabilization

A particular advantage of the short upper-state lifetime is the capability of semiconductor lasers to be modulated with very high frequencies, which can be tens of gigahertz for VCSELs. This is exploited mainly in optical data transmission, but also in spectroscopy, for the stabilization of lasers to reference cavities, etc.

Bibliography

See also: lasers, laser diodes, external cavity diode lasers, broad-area laser diodes, diode bars, diode stacks, edge-emitting semiconductor lasers, surface-emitting semiconductor lasers, quantum cascade lasers, mode-locked diode lasers, semiconductor optical amplifiers

Category: lasers