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Figures 4 to 7 show a typical resonator or optical resonant cavity. This is where the flash lamp and the material selected (Nd:YAG crystal) are located. When intensive light is applied to the crystal, via a reflector, it initially produces non-directional light. For optimum utilization of the flash lamp light, both the laser crystal and the flash lamp are arranged just within the "focal point" of an ellipsoidal mirror. A semi-reflecting and a fully reflecting mirror are mounted outside the crystal.
Only those parts of the laser light that hit these mirrors and are reflected into the laser crystal can be amplified during the pulse of the flash lamp while passing through the crystal. The amplified laser light has the same properties as the original laser light; i.e. it has the same direction, the same wavelength, the same phase and the same polarization. The mirrors determine the highly directional propagation characteristics of the laser light.
Part of the laser light passes through the semi-reflecting mirror and is the laser light that performs the welding function. This process produces a very high energy density light beam, many times higher than is possible with normal light at the focal point of a lens. The energy -"hot light"- created at the focal point in a relatively short time (0.5 to 20 ms) heats the work piece beyond its melting point and thus enables a welding.
The area effected is in a limited range of only approximately 0.25 to 2 mm, depending on the material. The laser light welds two metals together and thus permits safe, durable, precise and non-warp joining of parts in the form of a spot or seam. Because of the very short time of the laser pulse the zone of heat influence is limited to the immediate vicinity of the welded spot or seam.
The characteristics of a laser pulse and thus the effect on the material can be influenced by the operating parameters VOLTAGE and PULSE LENGTH (width). The voltage has influence on the amplitude; the pulse length influences the width of the laser pulse.
In practice the effect of both parameters while welding metals is as follows:
- The voltage first influences the welding depth,
- The pulse length predominantly influences the diameter of the welding point.
- The focus influences the welding depth as well as the diameter of the welding spot. When increasing the diameter, the welding depth is reduced at the same time.
Background
The word "laser" stands for "light amplification by stimulated emission of radiation." Lasers are possible because of the way the light interacts with electrons. Electrons are atomic particles that exist at specific energy levels. These energy levels are unique and are different for every atom or molecule. The energy levels can be compared to orbits or rings around the sun or nucleus. Electrons in outer rings are at higher energy levels than those in the inner rings. A flash of light Fig. 1 can bump electrons to higher energy levels by the injection of energy. When an electron drops from an outer ring to an inner ring or level, the excess of energy is given off as light. The wavelength or the color emitted is related to the amount of energy released. Depending on the particular lasing medium or material, specific wavelengths are emitted.
The propagation of light through space can be described in terms of a traveling wave motion. The wave is composed of a combination of mutually perpendicular electric and magnetic fields; therefore the direction of propagation of the wave is at right angles to both filed directions. Figure 2 shows an electromagnetic wave
Fig. 2 The electromagnetic wave
The concept of laser light is better understood by the definition of its properties. Laser light has three properties: monochromatic, coherent, and collimated. When all emitted photons bear constant phase relationship with each other in both time and phase the light is said to be coherent. It is also monochromatic (one color) due to the specificity and purity of the medium used. Last, the light is contained in a very narrow pencil, almost collimated (see figure 3).
Fig. 3 | 
