Optical trap, choosing the right condenser NA illuminate QPD

[donroy81]

Hi, I am looking to illuminate a quadrant photo diode using a condenser/ lens combination in a optical trap setup. I see thorlabs has a kit that has a 100x objective as a trap and a 10x objective as a condenser. Does that mean that one doesn't need to collect all the laser light scattered from a 1.25 NA objective, because surely a 0.25 NA condenser cannot do that. I have always thought it is critical to image the whole laser onto the QPD but maybe it is not. Any inputs?

 

[Andy Resnick]

Hi, I am looking to illuminate a quadrant photo diode using a condenser/ lens combination in a optical trap setup. I see thorlabs has a kit that has a 100x objective as a trap and a 10x objective as a condenser. Does that mean that one doesn't need to collect all the laser light scattered from a 1.25 NA objective, because surely a 0.25 NA condenser cannot do that. I have always thought it is critical to image the whole laser onto the QPD but maybe it is not. Any inputs?

We are currently doing exactly the same thing (except my own scope instead of a kit).

The QPD does not need to collect the entire amount of light, only enough to detect the change in diffraction pattern as the trapped particle moves. We're still debugging the QPD data analysis software, but we seem to be able to generate useful datasets fairly easily.

My setup uses either a 63X NA 0.9 dipping or a 100X NA 1.47 immersion objective lens, and we defocus the condenser (NA 0.85 IIRC) slightly off Kohler conditions to (we hope) increase the sensitivity at the QPD.

PM me if you'd like to discuss more, let me know how it goes!