New measurement and control technology in tunnelling - Tunnel Engineering: Basics and Additional Services for Design and Construction - page 251

The possibilities of pulse modulation have led to applications of semiconductor lasers in
optical data transmission and distance measurement.
Laser light is subject to the same laws as natural light of the same wavelength. In stable air
layering, beams can be deflected, which can be much more significant in tunnels than in the
open air due to temperature differences. Deflection of a laser beam by 5 cm at a distance of
100 m has been demonstrated for pipe jacking [47]. Air turbulence causes the beam to spread,
which can also hinder surveying. Nonetheless, when appropriate methods are used, the bundle
of rays can be concentrated at the intended contact point. Dust and water droplets lead to scat-
tering, which also results in beam spread and energy loss. The wandering of a beam, described
as drift, is essentially due to temperature dependency of the oscillation of the optical resonator.
The magnitude of this is insignificant for surveying technology in construction.
6.2.3 Optical components for laser devices
Beam‑widening optics reduce the divergence of the laser beam exactly to the degree to
which they enlarge the beam diameter near the device. The construction is similar to that
of a refracting telescope, with the beam entering at the eyepiece side. The widening of the
beam from 1 to 10 mm can theoretically reduce the spot size at a distance, for example
from 20 to 2 mm. Fig. 6-7 (top) shows this conversion qualitatively.
Figure 6-7 Effect of
beam widening optics
(top). Horizontal scanning
(bottom) creates a reference
level for levelling with a
detector array.
There are also widening optics, which can be focussed. These permit the bundling of
the rays to be set for the relevant distance, so that the spot size is reduced to the same
magnitude as the source (1 mm) or the residual beam widening is due to refraction and
spread. The helium-neon devices that are common in construction have non-focussing
beam widening to enable simple operation.