48
Remanufacturing and Advanced Machining
emitted light depends on a gas mixture excited by a high voltage electrical discharge,
typically 2–4 kV. The mixture normally consists of an inert gas that absorbs the
discharge energy and transfers it to an active gas atom, whose acceptable energy
levels determine the emission energy. A large variety of gas mixtures are used in
lasers and power conversion efficiency can range from 0.01 to 15% (Razeghi, 2010).
Lasers with a gaseous active medium are advantageous with their wide tunability,
high flexibility, relatively low cost, beam quality, and power scalability. However,
the recent popularity of semiconductor lasers seems to have overshadowed gas lasers
(Endo and Walter, 2006).
Excimer lasers are pulsed gas lasers with an efficient and powerful broadband
emission in several spectral ultraviolet regions with typical spectral widths of ca.
2 nm (Sze and Harris, 1995). An exception to this categorization is the XeF laser
with its broadly tunable C→A transition (approximately 50 nm) in the visible spectrum. Two basic formation channels for the excited state can be named: (1) recombinations of positive rare gas ions with halide ions, and (2) reactions of excited rare
gas atoms with halogen compounds. The laser excitation techniques are primarily
as follows: high-energy electron beams, electron beam sustained discharge, neutron
pumping from reactors, preionized avalanche discharges, and microwave excitation.
The pulsed electron beam and preionized avalanche discharge techniques appear the
most useful. Among the best-known excimer lasers are ArF, KrF, XeCl, and XeF
(Sze and Harris, 1995).
Chemical lasers use a pumping method based on energy released during a chemical reaction. The reactions most commonly used in commercially available chemical lasers consist in the generation of excited HF* and DF* molecules. HF and DF
lasers can produce radiation up to several hundreds of watts in the far-infrared and
infrared regions (1.3–11 μm) as well as in the visible range (Abramczyk, 2005). The
most efficient high-energy chemical laser with the shortest wavelength is the chemical oxygen-iodine laser (COIL). It operates on a near-infrared radiation of atomic
iodine, λ = 1.315 μm. The laser medium, I*, is produced through a pumping reaction
between singlet oxygen and ground state iodine (Hu et al., 2007). Comparing the
lasing power for transonic and supersonic injection schemes, the output power and
chemical efficiency for the latter were about 20% higher than for the transonic mixing scheme (Barmashenko et al., 2003).
The main component of a free-electron laser (FEL) is an undulator magnet that
forces a large number of electrons to emit their radiation coherently. Both the role of
the active laser medium and the energy pump are taken over by a relativistic electron
beam. The FEL radiation is almost monochromatic and well collimated. Among
the important advantages of the FEL is the free tunability of the wavelength by
simply changing the electron energy (Schmüser, 2003). Other benefits are as follows
(National Research Council, 1994):
• High peak power up to gigawatt order is achievable.
• Flexible pulse structure. Not only picosecond pulses with sub-picosecond
jitter can be produced, but also the interval between pulses can be varied,
offering the option of generating complicated pulse structures.
Remanufacturing and Advanced Machining
emitted light depends on a gas mixture excited by a high voltage electrical discharge,
typically 2–4 kV. The mixture normally consists of an inert gas that absorbs the
discharge energy and transfers it to an active gas atom, whose acceptable energy
levels determine the emission energy. A large variety of gas mixtures are used in
lasers and power conversion efficiency can range from 0.01 to 15% (Razeghi, 2010).
Lasers with a gaseous active medium are advantageous with their wide tunability,
high flexibility, relatively low cost, beam quality, and power scalability. However,
the recent popularity of semiconductor lasers seems to have overshadowed gas lasers
(Endo and Walter, 2006).
Excimer lasers are pulsed gas lasers with an efficient and powerful broadband
emission in several spectral ultraviolet regions with typical spectral widths of ca.
2 nm (Sze and Harris, 1995). An exception to this categorization is the XeF laser
with its broadly tunable C→A transition (approximately 50 nm) in the visible spectrum. Two basic formation channels for the excited state can be named: (1) recombinations of positive rare gas ions with halide ions, and (2) reactions of excited rare
gas atoms with halogen compounds. The laser excitation techniques are primarily
as follows: high-energy electron beams, electron beam sustained discharge, neutron
pumping from reactors, preionized avalanche discharges, and microwave excitation.
The pulsed electron beam and preionized avalanche discharge techniques appear the
most useful. Among the best-known excimer lasers are ArF, KrF, XeCl, and XeF
(Sze and Harris, 1995).
Chemical lasers use a pumping method based on energy released during a chemical reaction. The reactions most commonly used in commercially available chemical lasers consist in the generation of excited HF* and DF* molecules. HF and DF
lasers can produce radiation up to several hundreds of watts in the far-infrared and
infrared regions (1.3–11 μm) as well as in the visible range (Abramczyk, 2005). The
most efficient high-energy chemical laser with the shortest wavelength is the chemical oxygen-iodine laser (COIL). It operates on a near-infrared radiation of atomic
iodine, λ = 1.315 μm. The laser medium, I*, is produced through a pumping reaction
between singlet oxygen and ground state iodine (Hu et al., 2007). Comparing the
lasing power for transonic and supersonic injection schemes, the output power and
chemical efficiency for the latter were about 20% higher than for the transonic mixing scheme (Barmashenko et al., 2003).
The main component of a free-electron laser (FEL) is an undulator magnet that
forces a large number of electrons to emit their radiation coherently. Both the role of
the active laser medium and the energy pump are taken over by a relativistic electron
beam. The FEL radiation is almost monochromatic and well collimated. Among
the important advantages of the FEL is the free tunability of the wavelength by
simply changing the electron energy (Schmüser, 2003). Other benefits are as follows
(National Research Council, 1994):
• High peak power up to gigawatt order is achievable.
• Flexible pulse structure. Not only picosecond pulses with sub-picosecond
jitter can be produced, but also the interval between pulses can be varied,
offering the option of generating complicated pulse structures.