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Precisometer
Optical tweezers trapping a bead inside a microscope sample chamber
Technical guides

Optical trapping guide

Building an optical-tweezers setup

From a stable laser and filled objective pupil to bead detection, calibration, and safety—a practical path from optical diagram to measurable force.

Optical layoutTrap physicsInteractive design labProduct links

Start with the measurement

A trap is an optical, mechanical, and detection system—not just a focused laser.

A single-beam gradient trap requires a steep intensity gradient around the focus. The objective must redirect enough light that the gradient force pulling a dielectric bead toward the focus overcomes radiation pressure, Brownian motion, fluid drag, and external disturbances.

Define bead material and diameter, medium viscosity, chamber geometry, force range, bandwidth, and measurement method first. These choices determine wavelength, objective, power, detector, mechanics, and calibration.

Core architecture

Six subsystems that must work together

Trapping laser

1064 nm is a common starting point

Choose low intensity noise, stable pointing, TEM₀₀ beam quality, and enough power after every optic—not only at the source.

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Beam conditioning

Isolator · expander · steering

Protect the source, control polarization, expand the beam to fill the objective pupil, and provide two-axis alignment.

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High-NA objective

NA typically ≥1.2 for a strong 3D trap

Match immersion, working distance, correction collar, sample chamber, and beam diameter to the objective pupil.

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Sample positioning

Coarse XY/Z plus fine motion

Use a rigid low-drift stage with enough travel for loading and enough resolution to position the trap relative to the sample.

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Imaging path

Camera + illumination + dichroic separation

Separate trapping and imaging wavelengths, then choose field of view, sampling, and frame rate for bead acquisition.

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Position detection

QPD, condenser, and fast electronics

Forward-scattered light on a quadrant detector enables bead-position measurement beyond camera frame rates.

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Interactive trap-design lab

See how power, NA, bead size, and viscosity interact

Relative stiffness

0.050 pN/nm

Thermal RMS motion

9.1 nm

Corner frequency

844 Hz

Trap centreMotion envelope ∝ √(kBT/κ)

Educational estimate only. Stiffness uses a relative scaling model, not a component guarantee. Calibrate every real trap with the actual bead, medium, depth, and optical train.

Precisometer optical tweezers prototype setup

Alignment sequence

Build and validate one function at a time

  1. 1

    Establish the beam

    Measure power, polarization, beam quality, pointing, and noise before the microscope.

  2. 2

    Conjugate the steering plane

    Move the trap without walking the beam across the objective pupil.

  3. 3

    Fill and centre the pupil

    Expand and align the beam through the back aperture without clipping.

  4. 4

    Form and observe the trap

    Use known beads and verify stable 3D confinement away from surfaces.

  5. 5

    Align detection

    Image the condenser back focal plane onto the QPD and centre sum/difference signals.

  6. 6

    Calibrate

    Determine volts-to-position and stiffness under the actual measurement conditions.

Detection and calibration

Convert detector volts into bead motion and force

A camera is excellent for finding beads. A QPD provides the bandwidth for thermal spectra and active microrheology. Calibrate position sensitivity first, then determine stiffness by equipartition, power-spectrum fitting, viscous drag, or active response.

Read the QPD signal-chain note

Laser safety is part of the design

  • Enclose the beam and sample reflections wherever practical.
  • Use wavelength-appropriate eyewear from a documented hazard analysis.
  • Provide shutters, interlocks, warning indicators, and controlled alignment power.
  • Treat invisible 1064 nm radiation as hazardous even when no beam is visible.
  • Follow institutional procedures and the responsible laser-safety officer.

Two ways to continue

Build openly, or configure a complete research instrument.

Explore the OpenFlexure optical-tweezers project, or discuss a stable 1064 nm trapping and microrheology platform with our team.

Open-source build Explore MicroRheo