Page 341 - Electrical Properties of Materials

Optical fibre amplifiers 323

Laser Focusing Optic axis
oscillator lens of crystal ω , ω , ω
1 2 3

ω 1
q
ω 3
ω 3 ω 3 ω 2
Laser ω 3
medium Nonlinear
crystal
R ≅ 100% R (high but <1)
1
1
R ≅ 100% R (high but <1)
2
2
R = 0% R = 0%
3
3
Fig. 12.23
Schematic representation of a tuneable parametric oscillator. R 1 , R 2 ,and R 3 are the reflectivities of the mirrors at frequencies
ω 1 , ω 2 , ω 3 respectively.
which may resonate at ω 1 and ω 2 . If the waves at these frequencies satisfy both
the ω 3 = ω 1 + ω 2 and the k 3 = k 1 + k 2 (k = propagation coefficient as in our
classical studies of Chapter 1) conditions, then there is a parametric interaction
between the waves. The power at ω 1 and ω 2 builds up from the general noise
background at the expense of pump power. Thus, we have output at all three
frequencies. But why is this set-up tuneable? Because of the particular prop-
erties of the chosen nonlinear medium. It is a crystal in which the dielectric
constant is dependent on the direction of propagation. By rotating the crystal,
the matching condition for the propagation constants is satisfied at another set

of frequencies, ω and ω , still obeying ω 3 = ω + ω .

1 2 1 2
The crystal used most often is barium borate (BBO), produced abundantly
in the People’s Republic of China. It may be used in a pulsed parametric oscil-
lator, pumped by either the third harmonic (355 nm) or by the fourth harmonic
3+
(266 nm) of the 1.064 μ m radiation from a Nd :YAG laser. The tuning range
for either pump wavelength is remarkably large as shown in Fig. 12.24. Re-
member, for a dye laser with a given dye, we might have a tuning range in the
vicinity of 10%, but now we have a device which can tune wavelength by a
factor of 7 between the highest and the lowest wavelengths. The price we pay
for it is the necessity to use an additional resonator with a piece (in practice
usually two pieces) of crystal in it.

12.10 Optical fibre amplifiers
It is quite obvious that amplifiers can be built on the same principles as oscillat-
ors, but usually there is less need for them. A field, however, in which amplifiers
have crucial importance is long-distance communications. One might be able
to span the oceans of the world using optical fibres without the need to regen-
erate the signal in repeaters, if the signals propagating in the fibres could be
amplified.
The idea of using fibre amplifiers is just about as old as the oldest laser.
There were experiments in the early 1960s with fibres doped with Nd. Popula-
tion inversion could be achieved by pumping it with a flash-lamp, which then