Page 348 - Electrical Properties of Materials

330 Lasers

12.13.14 Machining
A laser beam can of course be focused not only upon human tissue but upon
inanimate matter as well, making possible laser machining and welding.The
same technique may also be useful for writing patterns on high-resolution
photographic plates (possibly to be further reduced) used in integrated circuit
technology.

12.13.15 Sensors

An interesting application is for navigation, which necessitates the measure-
ment of rotation. Lasers can detect rotational movement as low as a thousandth
of a degree per hour. The basic principles may be understood from Fig. 12.26.
This is a so-called ring laser, in which resonance is achieved by a ray biting
its own tail. The condition of resonance is now that the total length around the
ring should be an integral multiple of the wavelength. When the system is at
rest (or moving with uniform velocity) the clockwise and anticlockwise paths
are equal, and consequently the resonant wavelengths are equal too. However,
angular rotation of the whole system makes one path shorter than the other one,
leading to different frequencies of oscillation. The two beams are then incident
upon a photodetector, which produces a current at the difference frequency.
The rate of rotation may be deduced by measuring this difference frequency.
A simpler variant, aiming to do the same thing, uses a cylinder upon which
hundreds of metres of optical fibres are wound. If the cylinder rotates, then
the light path going clockwise is different from the light path going counter-
clockwise. The rotation rate may then be determined by measuring the path
difference.
Talking of fibres, I must mention that rotation rate is just one of the numer-
ous physical quantities which can be measured with the aid of light propagating
in fibres. Sensors have already been built for measuring angular position, tem-
perature, pressure, strain, acceleration, magnetic field, etc. The availability of

Photo- Δ f
detector meter
(Mirror) M
Beam combining optics

Gas
laser

M M

Fig. 12.26
Waves travelling
Schematic representation of a laser clockwise and
rotation sensor. counterclockwise