Page 339 - Electrical Properties of Materials

Laser modes and control techniques 321

L

Tilted etalon

Laser gain medium
Fig. 12.22
Schematic representation of a laser
oscillator in which single mode
operation is achieved with the aid of
M M
1 2 an etalon.

12.8.3 Q switching
This is a method for concentrating a large amount of power into a short time
period. It is based on the fact that for the build-up of oscillations a feedback
mechanism is needed, usually provided by mirrors. If pumping goes on, but we
spoil the reflectivity of one of the mirrors (i.e. spoil the Q of the resonator) by
some means, then there will be a lot of population inversion without any output.
If the reflectivity is restored (i.e. the Q is switched) for a short period to its
normal value, the laser oscillations can suddenly build up, resulting in a giant
pulse output. The pulse duration might be as short as a few nanoseconds, the
10
power as much as 10 W, and the repetition frequency may be up to 100 kHz.
The easiest, though not the most practical, way of spoiling the Q is by rotating
the mirror. The Q is then high only for the short period the mirrors are nearly
parallel.

12.8.4 Cavity dumping

This is another, very similar method for obtaining short pulses also based on
manipulating the Q of the resonator (called also ‘cavity’; that’s where the name
comes from). We let the pump work and make the reflectivity 100% for a cer-
tain period, so the oscillations can build up but cannot get out. If we now lower
the reflectivity to zero, all the accumulated energy will be dumped in a time
equal to twice the transit time across the resonator. The method may be used
up to about a repetition rate of 30 MHz.

12.8.5 Mode locking
We have implied earlier that it is undesirable to have a number of axial modes
in a laser. This is not always so. The large number of modes may come useful if
we wish to produce very short pulses of the order of picoseconds. The trick is to
bring the various axial modes into definite relationships with each other. How
will that help in producing short pulses? It is possible to get a rough idea by
doing a little mathematics. Let us assume that there are N +1 modes oscillating
at frequencies ω 0 + lω, where l =(–N/2, ... ,0, ... , N/2), that they all have
the same phase and amplitude, and they all travel in the positive z-direction