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Simulation der Laserquelle
Translation into English coming soon!
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Simulation was done using the program LASCAD. A finite element analysis (FEA) for the thermal effects of the Nd:YVO4 crystal is possible with it, allowing to determine the exact geometry of the resonator. Especially the distances of mirror 1 and the uncoupling mirror to the crystal (see diagram 1) are important for the construction later on. Additionally the simulation program can calculate the average power in CW mode (continuous wave mode), as well as the peak power of the laser source when using the Q switch. The simulation was done without frequency doubler, as it is less important for the laser geometry.
In order to do the simulation with the used program, we had to simplify the laser setup. Firstly, the simulation was done with a rectangular crystal instead of a trapedzoid one. Secondly, the program assumed a 100% anti reflection in the crystal, being about 99,9% in reality. Additionally, it was not possible to simulate a bounce geometry in the crystal. Therefore the laser beam was set to the lower end of the crystal, as the temperature gradient in this area is comparable best with the real bounce geometry (see diagram 2). The thermal lens is quite large with the bounce geometry and extends through the whole beam path. As the thermal strain is uniform in every point in the beam path, the thermal lens is compensated well and a high radiated power can be achived. With a side pumped laser the thermal lens is quite strong and the thermal strain varies along the beam path, leading to a high power reduction of the generated laser beam. The efficiency then lies at about 10% only, whereas bounce geometry reaches an efficiency of 50%. Because of this we needed to do a better adaption for the simulation in order simulate the bounce geometry more precise. As the laser reaches the crystal with an inclined angle using the bounce geometry and is reflected with the same angle there, the laser geometry at that spot can be considered as done in diagram 3. Is the geometry mirrored at the crystal surface then, you get a laser running in a straight line with uniform distribution of thermal strain in the crystal. This can be simulated in the used program by pumping the crystal from the side at the lower end (see diagram 2). In this pumping area a smaller thermal lens is created, leading to thermal strains similar to the bounce geometry. Diagram 4 and 5 show the temperature distribution in a side pumped crystal and one based on the bounce geometry, respectively.
Diagram 2: Real (a) and simulated (b) path of the laser beam
Diagram 3: Scematic view of the beam path with bounce geometry (left)
and beam path adapted for simulation (right)
Diagram 4: Temperature distribution for side pumped crystal
Diagram 5: Temperature distribution for simulated crystal
Abbildung 2: Darstellung des a) realen und b) simulierten Verlauf des Laserstrahls
Abbildung 3: Schematische Darstellung des Strahlenverlaufs der Bounce-Geometrie (links) und
der, für die Simulation angepasste, gespiegelte Strahlenverlauf (rechts) am Kristall und die jeweilige
Verteilung der thermischen Spannung des Kristalls im Strahlengang
Abbildung 4 Temperaturverlauf des seitlich gepumpten Kristalls
Abbildung 5: Temperaturverlauf des simulierten Kristalls
Ergebnisse der Simulation
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