Calibration of the characteristic frequency of an optical tweezer using an adaptive normalized gradient approach

We describe the use of a normalized gradient approach for calibration of the characteristic frequency of an optical tweezer. Unlike existing methods that require purely off-line data analysis, the method discussed here can be performed on-line, provided a running average is periodically computed to reduce the effects of Brownian motion. We describe the experimental setup used for calibration of the optical tweezers and calculate the characteristic frequency of a trapped, 10-micron diameter polystyrene bead by processing experimental data using the normalized gradient algorithm and two existing off-line calibration methods. The three methods yield similar values for the characteristic frequency.