Portable Optical Tweezers Kit
Chapter 5: Setup and Adjustment
Rev B, February 15, 2022
Page 57
Step 6: Insert Beam Expander
•
Adjust the distance between the lenses: the
distance between the CP45(/M) cage plates
should be 12 cm (distance between adjacent
edges).
•
Place the beam expander section back into the
cage segment.
•
Turn on the laser. It should look like the photo
below. Note that the elliptical beam is oriented
with the long arm vertical.
Step 7: Final Adjustment
•
Increase the diode current to approximately 95 mA.
•
Observe the expanded laser spot on the objective
aperture. Adjust the upper alignment mirror to move
the laser spot into the center of the objective
aperture.
Note: Ideally, an optical tweezers system consists of a single mode laser that enters the
objective as centered and vertical as possible. Also, the laser beam should be as parallel
as possible, with a small divergence angle. In our setup, this is not the case: After following
the instructions above, remove the end cap at the beamsplitter and examine the beam
shape using a viewing card or piece of paper. Notice that it converges at a distance of
about 1 to 2 meters. In other words, the light is not very parallel.
However, keep in mind that we do not have a single mode laser. Since we optimized the
setup for low cost, the laser diode used is relatively cheap. The intensity distribution of the
laser diode is elliptical and differs quite a bit from the ideal, Gaussian case. In our trials,
the trapping was clearly better when the laser showed the described convergence.
Therefore, the adjustment instructions above were written to yield optimal trapping results
rather than a minimum divergence angle.
Summary of Contents for EDU-OT3
Page 1: ...EDU OT3 EDU OT3 M Portable Optical Tweezers User Guide...
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