This article describes work undertaken at the University of Bern to model the cavity of a side-pumped crystal laser. The radiation transferred from the pumping diode array to the laser rod was modeled using the non-sequential mode of ZEMAX. The absorbed flux was then transferred to the LASCAD program where it was transformed into a volumetric gain profile, and the subsequent laser modes were predicted. Excellent agreement between predicted and experimentally obtained results was achieved. 

The schematic of the laser is as follows:

A Brewster-angled rod of Nd:YVO₄ is placed in an standard hemispherical cavity consisting of a spherical 100% reflective mirror and a plane output coupler (OC on drawing). It is pumped from the side by an array of diodes. The laser rod has a high-reflectivity coating placed on its far side so that pump light which is not absorbed by the rod is reflected back into the rod. Here is the side-pumped model in ZEMAX: 





The system components are as follows:

• The light source is modeled using a Source_File object. It represents a laser diode bar of 10mm in length. The source file was created in Matlab (generating a .dat-file). The Matlab file is based on real intensity profile measurements of the laser diode. For more information on this technique, see this article.
• The light is collimated by a gradient-index glass rod of index given by n(r) = 2 -0.6r², where r is the radial coordinate. This is set under the GRIN tab of the object properties:

• The Nd:YVO₄ material is entered as a new material in a custom glass datalog. Both the refractive and absorption properties are entered.
• A rectangular bar of Nd:YVO₄ is co-located with a Detector_Volume. This consists of 100 x 100 x 50 = 500,000 voxels. The Detector_Volume records the absorbed flux in each volume
• The laser rod has a high-reflectivity coating on its last face, so that un-absorbed light is reflected back into the laser rod.

100,000 rays were traced, and the absorbed flux in the laser rod was measured. Here is the data showing the absorved flux in a single slide of the rod:

As the crystal rod is optically dense the absorption as a function of distance can be clearly seen.

The absorption data was exported as a text file, for subsequent analysis by LASCAD. 

LASCAD is custom software for analysing laser cavities. Given the absorption profile in the rod, it can calculate the gain profile, resonator stability, mode structure etc.

The absorption data was entered into LASCAD as follows:

As an example of LASCAD's capabilities, the temperature distribution inside the laser rod based on the absorbed energy computed by ZEMAX was simulated and is shown here:



Note the orientation of the crystal is reversed from the ZEMAX drawings, so the crystal is being illuminated from the right hand side in this graphic.

Here are the results of a comparison between the ZEMAX-LASCAD simulation and the experimentally measured results:

Parameter	ZEMAX-LASCAD Simulation	Measurement	Difference
M2x	1.85	1.8	3%
M2y	3.95	3.8	4%
Power	20.5W	18.7 W	9%

It can be seen that the comparison between the simulated and experimentally obtained results are very good.