1.4 Pulsed operation

Normal pulsed mode

In a normal-mode pulsed laser, pumping is usually via a short pulse that produces a short-lived population inversion.

Pulsed lasers may be of three types: normal-mode pulsed lasers, Q-switched lasers, and mode-locked lasers

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-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 lossesA pump pulse begins to excite the molecules in the gain mediumThe large cavity loss suppresses laser oscillation while the pump

pulse increases the population of excited moleculesThe 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

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)

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

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.

The time between mode-locked pulses is the same as the round-trip time for a photon in the cavity:

The width of the pulse is given by

Mode-locking

Where (2N + 1) is the number of axial modes excited and Δν is the frequency separation of modes

tR = 2d / c

Δt = 2π / [(2N+1)Δν]