Laser power in optical tweezers

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SUMMARY

The discussion focuses on recalibrating an optical tweezer for an undergraduate thesis, specifically addressing power loss within a DIC microscope setup. Measurements indicate a significant drop in power from 105 mW before TL2 to 10-20 mW after L_BFP, raising concerns about the adequacy for basic applications, which typically require 20 to 100 mW. The participant also questions the optimal placement of a quadrant photodiode in relation to the focal point of L_BFP, emphasizing the importance of matching numerical apertures (NAs) of lenses to minimize power loss.

PREREQUISITES
  • Understanding of optical tweezers and their calibration processes.
  • Familiarity with DIC microscopy and its components.
  • Knowledge of numerical aperture (NA) and its impact on light propagation.
  • Experience with power measurement techniques in optical systems.
NEXT STEPS
  • Research the impact of numerical aperture matching in optical systems.
  • Learn about the calibration techniques for optical tweezers.
  • Investigate the specifications and performance characteristics of DIC microscope components.
  • Explore the role and placement of quadrant photodiodes in optical setups.
USEFUL FOR

Researchers, undergraduate students in optics or photonics, and professionals working with optical tweezers and microscopy techniques.

MadMarvin
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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?
 
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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.
 
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?
 

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