New measurement and control technology in tunnelling - Tunnel Engineering: Basics and Additional Services for Design and Construction - page 250

Figure 6-5 Principle of a helium-neon laser, from [29].
Helium-neon lasers deliver continuous light with powers of 0.5 to 50 mW, and the effi-
ciency of the systems is 0.1 %. The wide prevalence of this type is due to the long lifetime
of the tubes with low manufacturing costs, and also that no special cooling is necessary.
Semiconductor lasers. The semiconductor laser (also called diode laser or injection laser)
has a much more simple construction although its manufacture demands highly developed
technology. In an isolating base material are various elements, some of which emit elec-
trons and some of which can absorb electrons. With the application of an external voltage,
the electrons are recombined, which reverses spontaneously leading to the emission of
radiation. The construction of a semiconductor laser is shown in Fig. 6-6.
Figure 6-6 Construction
of a semiconductor laser.
Optical resonance occurs through partial reflection at the outer edges. Semiconductor la-
sers emit solely in the invisible infrared range. The other properties are:
- Simple excitation of extremely short pulses (less than 200 ns) while simultaneously
permitting continuous operation,
- Extremely small dimensions of the element,
- Low coherence,
- No bundling of the light.
Powers of up to 10 kW are normal in continuous operation, and with short pulses with
relatively long pauses, peak pulse powers of up to 100 kW are possible, although the aver-
age power has to correspond to that of continuous operation (danger of overheating). The
supply voltage is only a few volts (in contrast to the helium-neon laser with many kV).