Fibre Ring Laser

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.
 

Principle of Operation

Fibre Laser are a special class of Lasers differing from the “classic design”. The optical resonator consists of an optical fibre which can be coiled to a drum providing extreme long amplification lengths.

Within this experiment the students are introduced to the basics of optical pumping with subsequent application by means of an Erbium doped optical fibre (EDF). This type of fibre is commonly used as amplifier in long distance telecommunication as so called EDFA (Erbium doped Fibre Amplifier). Due to its particular properties the EDF is a promising candidate also as Laser source for telecommunication. Furthermore it is also used as an active fibre Laser gyroscope.

This experiment allows to study the EDF in a linear as well as ring configuration. As pump source a diodelaser emitting around 100 mW at a wavelength of 980 nm is applied. Via a wavelength division multiplexer (WDM) the pump light is coupled to the EDF. The other end of the EDF is connected to a beam expander which delivers a parallel light bundle which is coupled after a distance of 100 mm back into a patch fibre.

To close the ring this fibre is connected to the WDM. In common applications this “free space” propagating part is not used. However, for experimental purposes a variety of demonstrations can be done. By means of a thin glass plate - used as output coupler - a small fraction of the ring laser radiation is coupled out for further analysis. Herewith also counter propagating ring modes can be verified. Additional optical components can be inserted like passive q-switch, birefringent tuner etc. With the provided photo detector and the modulator the time response like spiking, life time of excited states can be studied