After setting up the BBO-Cavity on a new breadboard we finally have 230nm-light again! We get this through frequency-quadrupling light from a 922nm-laser (not in one step but frequency doubling the light two times). For frequency-doubling a nonlinear medium (often a crystal) is needed to generate light with twice the frequency than the incidence light. What happens there? Two photons of frequency f "merge" together to a photon with frequency 2f. Since this is a nonlinear process a nonlinear environment is needed. In our first frequency doubling stage (922nm->461nm) it is a PPKTP-crystal (periodically poled potassium titanyl phosphate). To frequeny double this light (461nm->230nm) we use a brewster-cut BBO-crystal (beta barium borate).
The process of frequency doubling is a very ineffecient process. To increase the conversion efficiency one can build a cavity around the crystal such that the incindence light will pass the crystal multiple times and can be frequency doubled every time it passes the crystal. Adjusting such a cavity is not so easy since you have to get the right focal waist in the middle of the crystal and make sure that the beam is reflected in itself.
The light is needed for the detection and cooling of 115In+ in our multi-ion clock.
Our task now is to increase the power at 230nm to the maximum by adjusting the cavity and getting the mode-matching right! :)