Laser power in optical tweezers

[MadMarvin]

Hi,
I'm currently recalibrating an optical tweezer for my undergraduate thesis. The alignment did turn out well so far but I'm a little bit concerned about the power loss within the microscope (here's a picture of the relevant part, without prisms).
I read in a paper that 20 to 100mW are sufficient for most basic applications. I measured the power at different points (the condenser is not fully aligned yet):

Before TL2: 105 mW
After L_BFP: 10-20 mW, depending on how well the immersion oil works.

Is that enough? I use a DIC microscope, so the light passes two DMs, two prisms, objective, condenser and their respective apertures. The beam has been expanded to roughly 3,5mm, so I did not expect the power after L_BFP to be very high. But 10 mW seems too low.

Also, I haven't inserted the quadrant photo diode yet. My advisor thinks it should be placed a good deal before the focal point of L_BFP - shouldn't it be placed a little bit behind?

 

[Claude Bile]

My first instinct would be to make sure that the NAs of the various lenses are matched properly. If you propagate from high NA to low NA you will lose a chunk of power.

Claude.

 

[MadMarvin]

Make sense, problem is: I do not even know the manufacturers of all the parts, let alone the properties of each one. I do not mind the power loss as long as the back focal plane interferometry still works so I will have to try that out, first (the diode is being repaired right now).

Anyway, thanks for your advice. Could someone elaborate on my last question, please?