I’m currently projecting a laser and studying what to buy for my first prototype. I have a couple of questions on the best material to dope the nd on for the specs i m going for. So here are the points i would like opinionsand help on:
1-) I’m inclined to acquire a Nd:YVO4 crystal , for two reasons. First it has a much larger gain when compared to YAG , and second it has a birefrigent strucuture which favors the formation of a polarized beam. What i don’t quite understand is HOW exacly is the polarization of the output beam dependant on the input beam ? Do i get a “partially” polarized beam if i pump it with unpolarized light ?
2-) How long does my crystal have to be in order to be uniformly pumped by , lets say a 2 J , 10 ms pulse from a colimated 808 nm beam, coming out of a fiber ? I’ve never design a end pumped laser so i haven’t dealt with this in pratice.
I’m trying to give a short answer to your questions but struggle a bit because there is much more explanation needed than it seems at the first look.
If the crystal is birefringent, it has two optical axis. If there is an unpolarized beam entering the crystal it will split into two beams (ordinary and extraordinary) with perpendicular polarizations. The power ratio in these beams will depend on the crystal cut (axis) direction. To make the laser cavity prefer a particular polarization, the losses of the one polarization has to be larger than the other. This can be done by using a polarizer or PBS but you should also use the right crystal cut so the same polarization the PBS keeps in the cavity aligns with one of the crystal axes. The physical separation of the ordinary and extraordinary beams is negligible (they overlap) with the usual crystal lengths so most likely you need a PBS.
For this questions first you can simply plot with the Beer-Lambert exponent how much of the light is absorbed in the crystal. By changing the Nd concentration you can get more light absorbed on the same length but the distribution will be less even. I recommend to use a mirror on the other side and make the pump double pass to somewhat even out the distribution. Using the fluorescence lifetime, absorbed energy per seconds, absorption cross section, number of Nd ions per volume … you can calculate how many of the ions are in excited state with the provided pump power and calculate the gain for the given length of the crystal.