Difference between revisions of "Optical trap"

Contents 1 Introduction 2 Instrument overview 2.1 Block diagram 2.2 Physical layout 3 Summary of calibration methods 4 Setup and alignment 4.1 Connecting the electronics 4.2 Remove the optics 4.3 Collimating and adjusting the fiber port 4.4 Initial laser alignment 4.5 Beam expander coarse adjustment 4.6 Condenser adjustment 4.7 Connecting the piezo stage 4.8 Fine adjusting the beam expander 5 OTKB software 5.1 Starting the software 6 Calibration 6.1 Measuring R by scanning a stuck bead 6.2 PSD method 6.3 Equipartition method 6.4 Stokes method

Laser tweezers based on the ThorLabs OTKB optical trap kit.

Introduction

Optical tweezers can exert and measure forces on micron-scale dielectric particles. This capability offers a unique and valuable tool for manipulating and measuring cell components at the single molecule level. For example, optical traps have been used extensively to investigate the mechanical properties of biological polymers and the force generation mechanisms of molecular motors. In many studies, optical tweezers apply force to functionalized microspheres, which act as convenient handles attached to molecules of interest.

To make quantitative force measurements, the instrument records the displacement of a trapped microsphere over time. For small displacements, the exerted force is very nearly proportional to displacement, so the trap can be modeled as a linear spring. Accurate force and position measurements depend on careful calibration of the position detector responsivity, G, and the trap stiffness α, also called the spring constant. The stiffness is a function of trapping laser power, bead size, bead composition, and optical properties of the sample.

This page has tips for setting up and aligning an optical trap. It discusses three methods for obtaining the spring constant and two methods for measuring α.

Instrument overview

Block diagram

Optical trap block diagram.

A high-NA objective lens (L1) performs the dual functions of imaging the sample and focusing the trapping laser. L1 is a 1.25 NA, 100X, infinity corrected, oil immersion objective lens with a working distance of 0.65 mm. Its back aperture is 5mm in diameter.

L7 focuses an image of the LED illuminator in the backplane of the condenser lens L2 to provide collimated transillumination in the sample plane. L2 is a .25 NA, 10X, infinity corrected objective lens with a 7 mm working distance. Steering mirror M1 and tube lens L3 complete the imaging path. L3 is a 1" diameter, uncoated plano convex lens with a focal length of 200 mm. L3 forms an image of the sample plane on the detector of a CCD camera placed at a distance of 200 mm. (For best performance, the distance between L3 and the CCD imager must be accurate.)

The trapping laser beam emerges from a fiber coupler with a diameter of approximately ??mm. Lenses L4 and L5 (focal lengths ?? and ??, respectively) implement a Galilean telescope that expands the beam by a factor of 4. To minimize aberrations and reflections, L4 and L5 are IR-coated achromatic doublet lenses. Dichroic mirror D1 deflects the collimated trapping beam toward L1.

Physical layout

Optical trap layout.

Summary of calibration methods

Setup and alignment

Connecting the electronics

Piezo driver connection diagram for X axis. Piezoelectric elements in the stage move the sample over a range of approximately 20 microns. The piezo driver applies a voltage across the piezoelectric element to achieve the desired position. The driver is a closed-loop system that uses a strain gauge displacement sensor to generate a feedback signal that depends linearly on the stage position.

Many optical trap measurements (such as Stokes calibration, DNA tether measurement, and force clamping) require precise, computer-controlled positioning of the sample. Although the Thorlabs piezo driver can be controlled directly by computer, it does not allow the smooth, complex motions required for some procedures. To facilitate precise computer control of the stage position, an instrumentation amplifier is inserted in the piezo driver's control loop. The amplifier subtracts a voltage generated by a digital to analog converter under software control from the displacement sensor's ouput.

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Remove the optics

Collimating and adjusting the fiber port

Initial laser alignment

Beam expander coarse adjustment

Condenser adjustment

Connecting the piezo stage

Fine adjusting the beam expander

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OTKB software

Starting the software

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Calibration

Measuring R by scanning a stuck bead

PSD method

Equipartition method

Stokes method

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