THORLABS EDU-OT2 User Manual page 61

Portable Optical Tweezers Kit
Step 8: 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.
Page 58
Chapter 6: Setup and Adjustment
MTN012639-D02