User:Pranav Rathi/Notebook/OT/2012/11/13/Optical Tweezers Calibration

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To perform accurate measurements with optical tweezers, it is necessary to calibrate some experimental parameters and even before that study the active components of the optical tweezers setup. The parameters I want to calibrate: Stiffness; which measures the force that is exerts on the trapped bead, and Sensitivity; which measures the relative position of bead within the trap. The components I want to study: Laser, AOM (acoustooptic module), microscope objective, QPD (quadrant photo diode), z and x piezo. It is necessary to characterize these components first because not only they affect the experimental parameters but also reduce the chances to see something unexpected in data later on.


I characterized most of the active components time to time in the setup. The characterization also helped me in designing this tweezers.


I use two laser: 1064nm 2W ND:YAG crystalaser for tweezing ans 633nm 2mW He-Ne for surface detection and alignment purposes. It was important to know some facts about IR laser. I did not do a rigorous study (because it was not necessary), just specified few parameters like power output, beam waist and its location, polarization, beam mode profile and beam propagation factor. I used this information to design the tweezers expansion optics. The laser specification are given in the following link:



Acoustooptic modulator is used to modulate the laser intensity in the trap. So it is second most important component of the tweezers. AOM has two components: AOM driver and AOM module, I characterized both of them. The specifaction are given in the link:

  • AOM driver


  • AOM module


I use 1st order diffraction beam from AOM to feed the tweezers. NI-DAQ controlled by feedback main96 labview v7.1 program controls the AOM through analog input voltage to AOM driver. So there is a relationship between the analog input voltage and output laser power in 1st order diffraction beam. Unfortunately this relationship is not linear, it is some odd function (characteristic curve; see the second link). Once i know the curve i can calculate the laser power in the trap at particular input voltage. This information is very useful while calculating the stiffness from the cutoff frequency.

Microscope objective