A How produce twin photons in practice?

[Quant]

TL;DR Summary

Advice on production of twin photons

I would be very thankful if somebody helps me with some practical advices about producing twin photons?
I know that twins are produced after a laser ray is directed on a BBO crystal. After the BBO there are two rings of photons and the twins are in the intersection of the rings.
1. I wonder in order to get the two rings must I have the laser ray falling strictly at 90 degree to the BBO? My crystal is very small 3x3 mm so I intend to focus the laser on it which means I would have different angles of them on the BBO so I fear I would not get the rings but a smeared spot. Is this right and what to do? I would like the beam to fall on a point but to be perpendicular. Is there a way to do this?
2. I am not at all sure how to capture the light from the intersection of the rings where the entangled twins are. I will filter the incident beam 405 nm by a filter at 405 but how to collect only the intersection spots to proceed and to direct them to a beam splitter b.e.? To make a mask with two openings? Or fibers?
Thanks in advance.

 

[johnbbahm]

I am going to assume you are taking about a second harmonic crystal, and because double 405 nm is 810 nm
your laser source is a laser diode. As long as you power is low enough, you should be able to get
a lens to collimate the beam, and then bring the beam size down to the size of the crystal.

 

[Quant]

Thank you. I see. Collimating is ok but how to bring the beam size to 2 mm. Of course I will use a lense to focus the beam to a point but then I must use a lense with very short focus distance (maybe 1 mm) to get a parallel beam after it which will have a diameter of 2 mm. Are there such lenses?

 

[johnbbahm]

Beam ex panders work both ways, they can also make the beam diameter smaller.
Depending on the power level, you might want to avoid a real focal point, it takes less energy per square mm
to ionize air than you think.