Page 342 - Electrical Properties of Materials

324 Lasers

3.5

3.0

2.5

Wavelength (μm) 2.0

1.5

1.0
355 nm pump
266 nm pump
0.5
Fig. 12.24
Theoretical (solid lines) and
experimental ( and ) tuning
0.0
curves using BBO as the nonlinear 20 30 40 50
medium. Phase matching angle (degrees)

served to amplify a signal. This idea was resurrected in the mid 1980s, using
another rare earth element, erbium, as the dopant. Today, erbium-doped fibre
amplifiers (acronym EDFA) pumped by diode lasers are standard components
in an optical fibre communication system. Without them the World Wide Web
could have hardly come into existence.

12.11 Masers
The acronym stands for ‘microwave amplification by stimulated emission of
radiation’. It is like ‘laser’ but the word ‘light’ has been replaced by ‘mi-
crowave’. In fact, the microwave application was there first, well before lasers.
Masers represented the first two-state systems in which quantum-mechanical
principles were used to achieve amplification. They were invented independ-
ently by Townes in the United States and Basov and Prokhorov in the Soviet
Union. All three received a Nobel Prize in 1964. For a while masers were used
as low-noise amplifiers (e.g. in the first satellite communications across the
Atlantic), but nowadays they are only of historical interest. It is worthwhile,
though, to mention here one of their realizations as an example of magnetic
tuning.
When discussing paramagnetism in Section 11.7.2, we came across the
splitting of energy levels in an applied magnetic field. The possible energies
are given by eqn (11.41):

E =–M J gμ mB B, M J = J, J –1, ... ,–J. (12.52)