Generation of Short Pulses

The use of short-pulse lasers enables the generation of high peak power pulses, which are useful for the investigation of non-linear effects. Short pulses are useful for the investigation of time dependant effects e.g. time resolved spectroscopy. In order to achieve extremely high peak power up to the giga watt range, laser systems are applied, which possess long lived excited states able to store energy and to emit it in an extremely short time. One of such lasers is the Nd:YAG laser. With Q-switching in so-called active or passive mode, it is possible to generate such short pulses. Here, as the first step, the theory of laser operation with Nd:YAG is discussed 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.
In the experiment a mechanical chopper will be used to demonstrate the way towards an active Q-switch. In the second step, a saturable absorber for passive Q-switching is introduced. The dynamics of the pulse generation, like repetition rate, pulse width and peak power are determined.
The experiment consists of the laser diode pumped Nd:YAG - laser as a basic version with an additional passive Q-Switch (Cr:YAG) crystal.
The time dependant signals are displayed and evaluated using an optional oscilloscope. Besides 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.
The active Q-switch can be performed and explored by using the optional Pockel’s cell including the high voltage driver.
 

Examples of investigation and measurement


Nd:YAG laser
The experiments begin with the adjustment of the Nd:YAG laser to maximum output power. With regard to the basic properties of the laser, essential facts can already be picked up here. For problem-free operation of the saturable absorber, an optimum basic adjustment is required. This includes the cleaning of the resonator‘s components.

Mechanical chopper
By means of the mechanical light chopper, the „initial spike“ can be demonstrated. One can observe that the initial spike is not yet a pure giant pulse but the tendency of the system to store energy becomes obvious. With the internal modulator of the laserdiode controller, further spiking effects can be shown. In this case, the rise time is shorter than for the chopper.

Saturable absorber
After optimizing the output power of the Nd:YAG laser module L (Cr:YAG - crystal) is inserted into the resonator. After adjustment one observes a train of needle-like pulses on the oscilloscope. Groups of amplitudes of different height show that there are also transversal modes as the result of Q-switching. The repetition rate depends essentially on the losses and on the pump power. Its upper limit is limited by the lifetime of the initial level.

Lifetime of the Laser‘s initial energy level
The storage facility of the lasing medium is determined by the lifetime of the initial level. This one can be measured in a simple but impressive way. The diodelaser is periodically switched in and out by means of the internal modulator (upper trace of the oscilloscope). At the same time, one observes the fluorescent light intensity as seen by the photo detector. From the decay-curve (lower trace of the oscilloscope) one gets the value of the lifetime of about 250 µs.