|
Basic Concept
Optical ultra high peak power
emitted within a very short time is a frequent demand of
industry and research. Besides the Excimer laser, the commonly
used Nd:YAG laser does not have the inherent property to operate
in such a pulsed mode. However, it bears the potential due to
the relative long lifetime of its excited state. To run an
Nd:YAG, or other suitable source in the desired pulsed mode, the
so-called Q-switch technique is applied. A quite similar process
is well known in electronics, where e.g. a capacitor is used to
store energy which can be extracted in a short time like flash
lamps or resistance welding circuits. In these cases, a
capacitor is loaded and discharged in a short time by switching
a shunt with a low resistance parallel to the capacitor. In
order to achieve extremely high peak power up to the giga watt
range, laser systems are used, which possess long lived excited
states able to store energy and to emit it in an extremely short
time. One of such lasers is for e.g. the Nd:YAG laser. With
Q-switching in so called active or passive mode, it is possible
to generate such short pulses.
Within this experiment, the first step discussed the theory of
laser operation with Nd:YAG and the steady state as well as the
time dependent solution of the four level rate equation is
analysed. A two level rate equation model is introduced to
explain the saturation behaviour of an optical absorber applied
as passive Q-switch.
In the second step, a saturable absorber for passive Q-switching
is introduced. The dynamics of the pulse generation, like the
repetition rate, pulse width and peak power are determined. In
the final step, Pockel’s cell as an active Q-switch is applied.
The experiment consists of the laser diode pumped Nd:YAG laser,
the SMC - 03 model with an additional passive (Cr:YAG) and
active Q-switch (Pockels’s cell).
The time dependant signals are displayed and evaluated using an
optional oscilloscope. Beside the generation of short pulses,
the behaviour of the Nd:YAG laser can also be the subject of
additional investigations, like measuring the threshold, slope
efficiency, etc.
Experimental Set-up
The light of the pump laser is transferred via a fibre cable to
the fibre telescope (FT-1) which transforms the beam to an
almost parallel beam. The lens (C) focuses the radiation into
the Nd:YAG rod, which has a mirror coating on its back side and
forms the cavity with the laser mirror. The generated laser
emission at 1064 nm passes the filter (Fi) and the residual pump
light is blocked. The passed laser emission can be transferred
by means of the fibre telescope (FT-2) either to the optical
multi-channel analyser (OMA) or to the photodetector (PD). When
the laser mirror is removed, the excitation spectra can be
recorded by means of the OMA. By varying the temperature of the
laser diode its wavelength will change. This effect can be
studied first and then exploited to obtain the absorption
spectrum of the Nd:YAG crystal. By modulating the pump laser,
the time resolved emission spectrum allows the measurement of
life-time of the excited state.
By inserting the active or passive Q-sqwitch inside cavity the
laser starts to operate in pulsed mode. The subsequent
measurements can be performed either in local mode or through
computer control via the USB connection of the base housing.
This set-up is ideally suited to demonstrate the fundamental
behaviour of a solid state laser system, its excitation process
as well as its spectroscopic characteristics. Furthermore, the
active and passive Q-switching technique is demonstrated. |
