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The difference between a semiconductor laser and a solid-state laser lies in the working material, price, and excitation source.
Common working materials of Semiconductor Lasers are gallium arsenide (GaAs), cadmium sulfide (CdS), indium phosphide (InP), zinc sulfide ( ZnS ), etc. The working substance commonly used in Solid-state Lasers is composed of optically transparent crystal or glass as the host material, doped with activated ions or other activated substances.
The price of Semiconductor Laser is low. Solid-state Lasers are more expensive due to the complicated preparation of the working medium.
There are three main excitation methods of Semiconductor Lasers, namely electric injection type, optical pump type and high-energy electron beam excitation type. Electric injection Semiconductor Lasers are generally semiconductor junction diodes made of gallium arsenide (GaAs), cadmium sulfide (CdS), indium phosphide (InP), zinc sulfide (ZnS) and other materials. The injected current is excited to produce stimulated emission in the junction plane region.
Optically pumped Semiconductor Lasers generally use N-type or P-type semiconductor single crystals (such as GaAS, InAs, InSb, etc.) as the working substance, and use the laser light emitted by other lasers as the optical pump excitation. High-energy electron beam-excited semiconductor lasers generally use N-type or P-type semiconductor single crystals (such as PbS, CdS, ZhO, etc.) as the working material, and are excited by injecting high-energy electron beams from the outside.
Solid-state Lasers use light as the excitation source. Commonly used pulse excitation sources are xenon-charged flash lamps; continuous excitation sources include krypton arc lamps, iodine tungsten lamps, potassium rubidium lamps, etc. In small long-life lasers, semiconductor light-emitting diodes or sunlight can be used as excitation sources. Some new Solid-state Lasers also use laser excitation.
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Semiconductor lasers are solid-state lasers based on semiconductor gain media, where optical amplification is usually achieved by stimulated emission at an interband transition under conditions of a high carrier density in the conduction band.
The physical origin of gain in an optically pumped semiconductor (for the usual case of an interband transition) is illustrated in Figure 1. Without pumping, most of the electrons are in the valence band. A pump beam with a photon energy slightly above the band gap 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 holes generated 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. A quantitative description can be based on the Fermi&#;Dirac distributions for electrons in both bands.
Note that this process works well only for a semiconductor with a direct band gap. In indirect band gap semiconductors (e.g. silicon), the conduction band electrons in the holes acquire substantially different wavenumbers, and that does not allow for optical transitions due to problems with momentum conservation.
Figure 1:
Physical origin of gain in a semiconductor.Most semiconductor lasers are laser diodes, which are pumped with an electric 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, and quantum cascade lasers, where intraband transitions are utilized.
Common materials for semiconductor lasers (and for other optoelectronic devices), all having a direct band gap, are
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As the photon energy of a laser diode is close to the bandgap energy, compositions with different bandgap energies allow for different emission wavelengths. For the ternary and quaternary semiconductor compounds, the bandgap energy can be continuously varied in some substantial range. In AlGaAs = AlxGa1-xAs, for example, an increased aluminum content (increased x) causes an increase in the bandgap energy.
While the most common semiconductor lasers are operating in the near-infrared spectral region, some others generate red light (e.g. in GaInP-based laser pointers) or blue or violet light (with gallium nitrides). For mid-infrared emission, there are e.g. lead selenide (PbSe) lasers (lead salt lasers) and quantum cascade lasers.
Apart from the above-mentioned inorganic semiconductors, organic semiconductor compounds might also be used for semiconductor lasers. The corresponding technology is by far not mature, but its development is pursued because of the attractive prospect of finding a way for cheap mass production of such lasers. So far, only optically pumped organic semiconductor lasers have been demonstrated, whereas for various reasons it is difficult to achieve a high efficiency with electrical pumping.
Even for a given semiconductor material basis, there is a great variety of different semiconductor lasers, spanning wide parameter regions and many different application areas:
Some typical aspects of semiconductor lasers are:
Such characteristics have made semiconductor lasers the technologically most important type of lasers. Their applications are extremely widespread, including areas as diverse as optical data transmission, optical data storage, metrology, laser spectroscopy, laser material processing, pumping solid-state lasers (&#; diode-pumped lasers), and various kinds of medical treatments.
Most semiconductor lasers generate a continuous output. Due to their very limited energy storage capability (low upper-state lifetime), they are not suitable 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-gigahertz repetition rates.
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 frequency stabilization to reference cavities, etc.
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