LASER-8

power can be rather low. Then we must consider that all the energy is concentrated within the narrow
spectrum of the active atomic transition. This means that the spectral brightness (i.e. the intensity in
the beam divided by the width of the emission line) is even higher in comparison with a white light
source like a light bulb. For example, the spectral brightness of a 1 mW laser beam could easily be
millions of time greater than that of a 100 W light bulb.
Properties 1-4 are common to all lasers. There is a fifth property which is found in only some:
5. ULTRASHORT PULSE GENERATION.
Lasers can be made to operate continuously or in pulses. The time duration of the pulses tp is
linked to the spectral band width of the laser light ?? by the "uncertainty" product ?t ?? ~1:
??
>
~ 1
pt . (1.2)
This follows from taking the Fourier transform of a pulse of duration tp. As an example, the bandwidth
of the 632.8 nm line in the HeNe laser is 1.5 GHz (see table 1.1 above), so that the shortest pulses that
a HeNe can produce would be 0.67 ns long. This is not particularly short by modern standards. Dye
lasers typically have gain bandwidths greater than 1013 Hz, and can be used to generate pulses shorter
than 100 fs (1 fs = 10-15 s). This is achieved by a technique called "mode-locking". The present world
record for a light pulse is less than 1 fs. These short pulsed lasers are very useful for studying fast
processes in physics, chemistry and biology, and are also being developed for high data rate
transmission using optical fibre networks.
Essential operating principles of a laser
light amplification + positive optical feedback
Light amplification is achieved by stimulated emission. (