A New Easier-to-Align Setup: Collinear Down-Conversion
In the process of finding ways to simplify the experiments we found that the easiest way to get down-conversions is have the down-converted photons come collinear out of the crystal, as shown in the diagram below.
The pump is eliminated by the use of cross polarizers. It can be worked out that half of the pairs split at the first beam splitter after the crystal. An overall picture of the setup is shown below.
Individual parts are shown below:
A HeNe laser is needed to do the initial alignments. We incorporate it into the path by use of a "flipper mirror". The Mach-Zehnder interferometer shown has two non-polarizing cube beam splitters. The corner mirrors are mounted on translation stages. One stage has a piezo as a spacer for fine changes in the optical path length. The second stage is for larger changes in the difference in optical path length. Also shown in the interferometer are two irises, that we use to balance the intensities of both arms. They are not vital. The round circle around the BBO crystal is a gasket. When the apparatus is not in use we put a small glass bell-jar on top with desiccant. The gasket provides a seal. The desiccant provides a dry atmosphere for the crystal. We found out (the hard way) that if the BBO crystal is left exposed to the room's humidity it absorbs water vapor and fogs up. Polishing the crystal costs as much as ordering a new one!