Page 337 - Electrical Properties of Materials

Laser modes and control techniques 319

energy level on the other side to tunnel into. The drop across semiconductor
B being equal to V B ensures that the electron can tunnel from level 1 of semi-
conductor A on the left-hand side of B to level 2 of the next semiconductor A
on the right-hand side of B, and then the whole thing starts again. The electron
descends, emits a photon, tunnels across, descends, emits a photon, and so on
until it finds the last semiconductor A. If there are 50 layers of semiconductor
A, then a single electron will produce 50 photons. From the point of view of
the electron, this is like a cascaded ornamental waterfall. By the end the elec-
tron will have lost all its energy. From the point of view of the photon, this is
an exercise in gathering strength.
The energy difference between levels 1 and 2 depends on the thickness of
semiconductor A. Hence, the laser wavelength can be changed by choosing the
appropriate material thickness. The wavelength range quantum cascade lasers
can cover is large, from about 3 to 17 μm.
The principles upon which quantum cascade lasers work were enunciated
in the 1970s but they have only recently entered the market place. Why? You
can appreciate the reasons: it is the extreme accuracy required. Each layer must
have a certain number of atoms, not an atom more not an atom less.
As you can see, semiconductor lasers of all kinds have made and are mak-
ing great leaps forward. They are poised to acquire the same dominance in
lasers as other semiconductor devices enjoy in generating and amplifying lower
frequency signals and in the field of switching.

12.8 Laser modes and control techniques
Having discussed the principles of operation of a large number of lasers, let
us see now in a little more detail how the electric field varies inside a laser
resonator and describe a few methods of controlling the mode purity and the
duration of laser oscillations.

12.8.1 Transverse modes
What will be the amplitude distribution of the electromagnetic wave in the laser
resonator? Will it be more or less uniform, or will it vary violently over the
cross-section? These questions were answered in a classical paper by Kogel-
nik and Li in 1966, showing both theoretically and experimentally the possible
modes in a laser resonator. The experiments were performed in a He–Ne laser,
producing the mode patterns of Fig. 12.20. For most applications we would like
a nice clean beam, as shown in the upper left-hand corner. How can we elim-
inate the others? By introducing losses for the higher-order modes. This may
be done, for example, by reducing the size of the reflector. Since the higher-
order modes have higher diffraction losses (they radiate out more), this will
distinguish them in favour of the fundamental mode. However, this will influ-
ence laser operation in the fundamental mode as well; thus a more effective
method is to place an iris diaphragm into the resonator, which lets through the
fundamental mode but ‘intercepts’ the higher-order modes.