New measurement and control technology in tunnelling - Tunnel Engineering: Basics and Additional Services for Design and Construction - page 249

6.2.2 Lasers
Since the construction of the first laser in 1960, many laser devices and applications have
been developed at a rapid rate. The types that are useful in construction will be described
here. Laser light differs from normal light in the following properties:
1. Temporal and spatial coherence, which means that there is a fixed phase relationship
between the individual rays of light; they oscillate their emission in unison and main-
tain this property as they propagate through space.
2. Monochrome, which means the light is exclusively of one wavelength, or displays a
very narrow spectrum.
Laser light can also have the following additional properties:
- Bundling: laser light is tightly bundled (but not in every case).
- Many laser sources emit polarised light.
Construction and function of a laser source. Every laser device consists of a gain me-
dium, which can be a gas (for example argon) or gas mixture (for example helium-neon), a
solid body (for example ruby) or a semi-conductor (for example gallium-arsenide). This is
excited by an energy source (pumped) so that the energy state of the atomic shells is not in
thermal equilibrium. The most important types of excitation are optical (flash lamps) and
electrical (voltage supply). The decay of the excited atoms to their normal level, in other
words that corresponding to the temperature, results in the emission of radiation, which is
used directly as laser light or excites another part of the gain medium, which then emits
laser light while “jumping back”.
The emission of light is partially spontaneous, but mostly stimulated; only the latter part
leads to the creation of laser light. The stimulation is performed by the laser light itself:
this is reflected back and forth between two reflectors and equal-phase emissions are pro-
duced with each passage through the medium. These contribute to further equal-phase
emissions as they pass through the medium. This process is called optical resonance and
is the precondition for the creation of laser light.
The laser light can be radiated continuously or in short pulses. Pulse operation is normally
only used in laser systems, in which the pulses are exactly controlled by external action,
in the simplest form by the excitation (for example in a semiconductor laser). In surveying
laser technology for construction, the helium-neon laser has been prevalent for a long time
but semiconductor lasers have become more common recently.
Helium‑neon laser. The construction of a helium-neon laser is shown in Fig. 6-5. The
helium-neon gas mixture is enclosed in a glass tube; the excitation occurs at the anode
and the cathode through the application of a voltage of many kV. The angled ends permit
loss-free passing of the radiation through the windows for linear vertically polarised light
(Brewster windows). The convex outer mirrors reflect the light so that resonance can oc-
cur. The laser light passes through one of the two concave mirrors, which is for example
1 % partially transparent. The internal beam is then about 100 times stronger than the
emitted beam. The radiation of a helium-neon laser has extreme coherence and frequency
stability; the most-used wavelength is 632.8 nm (red light; other oscillation modes with
different wavelengths are also possible). The light is linearly polarised.