Hi, I am doing experiments to make SHG beam from an ultrashort pulse laser.
I use BBO crystal and lens to focus the beam into the crystal.
I found that the output beam is a very asymmetric (looks like a line shape) shape. It was not good beam quality.
How do people make good quality SHG beam shape after frequency conversion? Is there any simple but good method?
Hi,
As a generic approach, beam shaping optics (optics transparent to the wavelength under consideration) are employed to obtain specific shape of the beams. Depending upon which shape one desires, related optics are used. One downside of using beam shaping optics is that it attenuates the light whilst it is shaping it. Unless until you have sufficient power in the upconverted light, use the output beam without beam shaping optics to do the research work. Do the beam shaping only when it is utmost necessary.
Regards
IMNSHO, the most important part is the beam shape before the SHG crystal. It should be cylindrically symmetric with monotonic decrease of the power density from the center to outer parts of the beam. Then, chirp control is also important; this includes avoiding all transparent media in the optical path wherever possible. And of course, the SHG itself. A SHG for femtosecond pulses is much thinner than for nanosecond of CW light; it is to be cut to work at incident angles close to normal. It's quite important to have possibility of traversal motion of the crystal in the beam, to see that no microscopic defect is in the beam.
In brief, a nice shape of the second harmonic beam corresponds to a well tuned laser system.
Hi, it is called walk-off in SHG in BBO crystal. A way to to compensate it is described by Prof. Shoji in Japanese.