46
Remanufacturing and Advanced Machining
inversion. In effect, a high electrical to the optical power conversion efficiency of up
to 75% is reached, as well as high reliability, compactness, and low manufacturing
costs (Sobiesierski and Smowton, 2016). A semiconductor laser is very compact,
typically, the size of an active laser part is only 100 μm × 1,000 μm × 100 μm
(Razeghi, 2010).
The optical emission from semiconductor lasers arises from a radiative recombination of charge carrier pairs, i.e., electrons and holes in the active area of the device
(Koch and Hoffmann, 2018). In order to achieve carrier inversion, it is necessary to
excite sufficiently many electrons from the valence band into the conduction band,
which is referred to as pumping the semiconductor laser. This procedure can be
executed optically, but the possibility of electrical pumping with a few tens of milliamperes at voltages of a few volts can be considered another advantage of semiconductor lasers (Koch and Hoffmann, 2018). Moreover, output of a semiconductor laser
can be easily modulated by modulating its injection current, which further expands
the range of its applications (Dutta, 2003).
In the semiconductor structure, there are two types of bands: conduction bands,
which consist of unoccupied states; and valence bands, which consist of occupied
states. Most III–V and II–VI compounds (the numerals refer to columns in the
Periodic Table) are direct bandgap materials, where the conduction-band energy
minimum and the valence-band energy maximum have the same momentum. GaAs
is an example of a direct bandgap semiconductor. On the other hand, if the band
extrema occur at different momentum values, the semiconductor has an indirect
bandgap, e.g., SiGe (both column IV) and AlAs (III–V). In general, a semiconductor
electronic band structure has numerous bands with asymmetric shapes and sometimes several energy maxima and minima (Chow and Koch, 1999). A large variety of semiconductor materials are suitable for semiconductor lasers. For example,
materials covering the red are (AlGaIn)P, the near-infrared (AlGa)As or (GaIn)As,
the telecom range (GaIn)(AsP), infrared (GaIn)(AsSb), and even the blue-ultraviolet
range (GaIn)N. The most stringent requirement for a material is that it has a direct
bandgap, which tends to have a high radiation transition rate. This condition excludes
the elemental semiconductors, silicon and germanium, from use in semiconductor
lasers (Koch and Hoffmann, 2018).
Initially, the semiconductor laser was built with a simple p-n junction using the
same material for the active and surrounding layers, which is referred to as a homojunction. To increase laser efficiency, multiple layers with different optical properties
were used in the laser structure, forming heterojunction lasers. A further improvement can be obtained by sandwiching an active GaAs layer between two AlGaAs
layers. This structure is called a double-heterojunction (DH) or double-heterostructure To date, the most extensively used heterostructure semiconductor lasers are in
the GaAs-AlGaAs and GaAs-InGaAsP systems (Razeghi, 2010).
Color center lasers (CCL). Color centers are lattice vacancy defects trapping
electrons or holes, usually created in single crystals at room temperature under ionizing radiation (Courrol, 2006). Color centers in alkali halide crystals can be applied
as high-gain active materials in tunable solid-state lasers, cryogenically cooled and
optically pumped. CCLs have low threshold pump powers, relatively high output
Remanufacturing and Advanced Machining
inversion. In effect, a high electrical to the optical power conversion efficiency of up
to 75% is reached, as well as high reliability, compactness, and low manufacturing
costs (Sobiesierski and Smowton, 2016). A semiconductor laser is very compact,
typically, the size of an active laser part is only 100 μm × 1,000 μm × 100 μm
(Razeghi, 2010).
The optical emission from semiconductor lasers arises from a radiative recombination of charge carrier pairs, i.e., electrons and holes in the active area of the device
(Koch and Hoffmann, 2018). In order to achieve carrier inversion, it is necessary to
excite sufficiently many electrons from the valence band into the conduction band,
which is referred to as pumping the semiconductor laser. This procedure can be
executed optically, but the possibility of electrical pumping with a few tens of milliamperes at voltages of a few volts can be considered another advantage of semiconductor lasers (Koch and Hoffmann, 2018). Moreover, output of a semiconductor laser
can be easily modulated by modulating its injection current, which further expands
the range of its applications (Dutta, 2003).
In the semiconductor structure, there are two types of bands: conduction bands,
which consist of unoccupied states; and valence bands, which consist of occupied
states. Most III–V and II–VI compounds (the numerals refer to columns in the
Periodic Table) are direct bandgap materials, where the conduction-band energy
minimum and the valence-band energy maximum have the same momentum. GaAs
is an example of a direct bandgap semiconductor. On the other hand, if the band
extrema occur at different momentum values, the semiconductor has an indirect
bandgap, e.g., SiGe (both column IV) and AlAs (III–V). In general, a semiconductor
electronic band structure has numerous bands with asymmetric shapes and sometimes several energy maxima and minima (Chow and Koch, 1999). A large variety of semiconductor materials are suitable for semiconductor lasers. For example,
materials covering the red are (AlGaIn)P, the near-infrared (AlGa)As or (GaIn)As,
the telecom range (GaIn)(AsP), infrared (GaIn)(AsSb), and even the blue-ultraviolet
range (GaIn)N. The most stringent requirement for a material is that it has a direct
bandgap, which tends to have a high radiation transition rate. This condition excludes
the elemental semiconductors, silicon and germanium, from use in semiconductor
lasers (Koch and Hoffmann, 2018).
Initially, the semiconductor laser was built with a simple p-n junction using the
same material for the active and surrounding layers, which is referred to as a homojunction. To increase laser efficiency, multiple layers with different optical properties
were used in the laser structure, forming heterojunction lasers. A further improvement can be obtained by sandwiching an active GaAs layer between two AlGaAs
layers. This structure is called a double-heterojunction (DH) or double-heterostructure To date, the most extensively used heterostructure semiconductor lasers are in
the GaAs-AlGaAs and GaAs-InGaAsP systems (Razeghi, 2010).
Color center lasers (CCL). Color centers are lattice vacancy defects trapping
electrons or holes, usually created in single crystals at room temperature under ionizing radiation (Courrol, 2006). Color centers in alkali halide crystals can be applied
as high-gain active materials in tunable solid-state lasers, cryogenically cooled and
optically pumped. CCLs have low threshold pump powers, relatively high output