User:Pranav Rathi/Notebook/OT/2010/04/13

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Power Step-Up Beam Profile experiment (BPE)


Correct beam profile is very important for optical trap; in our case its (0, 0) Gaussian mode. It looks like that at higher powers our laser moves to higher modes, which introduces some extra difficulties in focusing at getting diffraction limited spot size, and not good for the stiffness calibration. The motivation behind doing this experiment is to answer some questions.


Answering the following questions.

  1. At what output power the beam profile starts changing?
  2. Is this change slow with power?
  3. How much does it change at 4.1W in comparison to the power-point at; it just starts changing?
  4. What is the appropriate range of power for tweezing?


The profiling is done from .2W to 4.1W in .3W steps at 85 deg F, laser temperature. The beam was passed through AOM (AOM at CW mode) and 2nd order diffracted beam was picked for profiling. The profiling is done through a regular CCD camera with following set-up:


After AOM beam was passed through an IR mirror (at 45 deg maximum reflectance of IR), to control the transmission; reflected beam forward to beam stop. After IR it goes through a regular aperture just sized (not small enough to introduce diffraction) to fit the beam (aperture was used to save the filters from over exposure). Than a 3ND, a Plano-convex lean and a 2ND filter were used in series to project the beam on the CCD.

A series of profile pictures were taken starting at .2W with .3W step. The pictures were analyzed through Image-j. The analyzed image is 90 deg right rotated image of the original (this is done to enhance the surface plot profile), so the horizontal axis in the set-up plane is vertical axis in the image plane.


Just by looking the pictures, it is clear; what happens to the beam at higher powers and up-to what power the beam stays in the fundamental mode.
Steve Koch 01:11, 14 April 2010 (EDT): To me looks like between 1.7 and 2 Watts is when it gets bad?


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