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The physical principle behind the generation of laser light is the phenomenon of stimulated light emission.
In fact, LASER stands for Light Amplification by Stimulated Emission of Radiation. Thus, a laser may be
defined as light amplified by the emission of a chain of photons, originating from an initial photon (light
particle) that interacts with an excited atomic system and stimulates the emission of two additional photons,
which, in turn, interact with other atoms, giving rise to an avalanche effect.
To excite the atomic system an external energy supply capable of triggering the laser effect is required.
"Optical pumping", is obtained when the light emitted by a light source strikes the active material (material
capable of emitting laser light) causing the atoms to become excited through the absorption of light energy.
The laser effect is amplified by causing the emitted light to pass through the active material repeatedly.
This is accomplished by interposing the active medium between two opposing mirrors, i.e. constructing and
"aligning" what is known as a "resonator". When the resonator is perfectly "aligned", the crystal and the
mirrors are centered on the optical axis. The faces of the crystal, the front mirror and the face of the rear
mirror closest to the crystal lie parallel to each other. In this optical configuration the resonator extracts the
maximum laser energy, the beam is circular and approximately uniform in intensity. After being amplified,
the laser beam will be concentrated by a focusing lens that may have varying focal lengths and can be
deflected into the welding chamber by a 45° mirror. In this way the energy will be concentrated in an area of
just a few tenths of a millimetre, making it possible to melt metals.
The energy intensity of the concentrated laser beam is in the order of 800 – 1000 KW/cm. To get this into
perspective it is worth considering that, if the same lens were used to focus the sun's rays, the resulting
energy intensity would be 0.5 KW/cm.
THE PHYSICAL PRINCIPLE OF LASER WELDING
WELDING
Figure 49 – Laser welding diagram