Transcript Slide 1

1.4 Pulsed operation
Pulsed lasers may be of three types: normal-mode pulsed lasers, Qswitched lasers, and mode-locked lasers
Normal pulsed mode
In a normal-mode pulsed laser, pumping is usually via a short pulse
that produces a short-lived population inversion.
The laser output has a similar duration to the pump pulse.
The laser output may actually be comprised of several small,
shorter pulses over the duration of the pump pulse.
Q-switched mode
Q-switched mode
Q-switching is a strategy to allow the energy stored in the laser
cavity to build up by preventing laser oscillation from taking place.
Doing so results in a larger population inversion to build up than
would otherwise be the case. Laser oscillation is then switched on,
which results in the emission of a short, high power laser pulse.
This process may be conceptualised as follows:
One cavity mirror is “removed”, resulting in large cavity losses
A pump pulse begins to excite the molecules in the gain medium
The large cavity loss suppresses laser oscillation while the pump
pulse increases the population of excited molecules
The cavity mirror is “replaced” – cavity losses are minimised and
laser oscillation rapidly depletes the population inversion,
resulting in a short laser pulse
Q-switched mode
Comparison of normal-mode and Q-switched mode:
How is Q-switching achieved?
Q-switched mode
Experimental realisation of Q-switching:
(a) A Pockel’s cell rotates the polarisation of light coming from the gain
medium so that it is perpendicular to the polariser after reflection.
(b) An acousto-optic modulator reflects the beam from the cavity.
When ON, both Pockel’s cell and AOM remove photons from the cavity,
thereby preventing laser oscillation.
When OFF, photons remain in the cavity and are amplified.
Mode-locking
Mode-locking
The technique of mode-locking is used to produce ultrashort pulses
(10-12 to 10-15 s)
Multi-mode operation of the laser is required for mode-locking. In
ordinary, multi-mode CW operation the phases of the modes are
random and the intensity varies with time.
If all cavity modes
have a suitable
amplitude and
phase relationship
to other modes, the
resulting output will
be a circulating
pulse in the cavity
Mode-locking
The time between mode-locked pulses is the same as the round-trip
time for a photon in the cavity:
tR = 2d / c
The width of the pulse is given by
Δt = 2π / [(2N+1)Δν]
Where (2N + 1) is the number of axial modes excited and Δν is
the frequency separation of modes