Calibration & Measurement Module for OTKB/OTKBFM Chapter 5: Operation
Page 13 Rev C, August 2, 2018
5.3. Sample Preparation
Follow the steps below to prepare a sample for measurement.
1. Using the OTKBTK, prepare a sample with 1 um or 2 um beads. In most experiments, the trapped objects
are typically microspheres because of their symmetry and standardized characteristics, such as refractive
index, size, shape, etc. In this case, the QPD’s calibration is done with silica beads of 1 or 2 um in
diameter.
2. The sample solution can be loaded into the channel using a microscopy slide with built-in channel
(offered via our optical trapping accessories kit, Thorlabs item number OTKBTK, sold separately), or you
can build a simple channel by placing double-sided tape on a standard slide, and adding a cover glass on
top. Liquid can be pipetted in-between. The two open sides can be sealed off with nail polish, to prevent
the sample from drying out. (Hint: To obtain a sample with many stuck beads, add a high concentration of
NaCl (table salt) to the solution. Salt will reduce the Debye screening length between the beads and the
glass surface, thereby increasing the probability of beads sticking onto the glass surface.)
3. Place the slide onto the sample holder and carefully place the slide between objective and condenser.
Make sure to either use immersion oil on the bottom of the slide or to apply it to the objective before trying
to image the sample.
4. Assess the quality of the sample. There should be a small number of stuck beads in most fields of view. It
should take at least a minute to find a free bead. Too many free microspheres will make it difficult to trap
only one sphere for the duration of a measurement.
5.4. Saving Data
If the “Stream to Disk” flag is set on the Data Tracking screen, a set of files will be saved. Files can be named
automatically using a time stamp or the user can provide a file name. The file will include the data acquisition rate
used during the measurement, followed by the raw detector voltage data X,Y and SUM.
On the calibration tab it is further on possible to save the data acquired during the PSD Roll-Off Calibration. Three
sets of files are saved and can be identified by their file extension:
“.TDdat”: Time domain data file. It includes in the header the sample rate, number of samples, number
of averages. After the header four columns include the acquired data. Column 1 is time in
seconds, Column 2 to Column4 are X, Y and SUM detector voltage data. The X and Y
detector voltage signals are normalized by the SUM before saving to the file.
“.FDdat”: Fourier domain data file. It includes in the header the number of frequencies and the
measurement time used during the calibration. After the header four columns include the data.
Column1 is the frequency in Hertz, Column 2 to Column 4 are power spectral data based on
the detector’s X, Y and SUM signal. The data is derived from the time domain data using a
discrete FFT transformation. The raw FFT data is converted to absolute squared values and
devided by the measurement time. For background details please refer to publications, such
as “K. Berg-Sorensen et al., Rev.Sci.Instrum., Vol75, N0. 3, March 2004”
“.LSdat”: Line Scan data file. It includes in the header the number of samples and the number of
averages acquired. After the header four columns include the data. Column 1 is the stage
position in μm, Column 2 to Column 4 includes the detector X, Y and SUM voltage data. X and
Y detector voltage signals are normalized by the SUM before saving to the file.
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