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