FPGA-Based Real-Time Feedback Control of Optical Tweezers

Author(s):
Heikki Ojala - University of Helsinki
Anders E. Wallin - University of Kelsinki

Industry:
Life Science, University/Education, Research

Products:
PXI/CompactPCI, LabVIEW

The Challenge:
Optical tweezers, or traps, are based on an emerging micromanipulation technique using focused intense (MW/cm2) laser light to trap dielectric particles. Stiffly trapping small particles typically requires more than 10 mW laser power. However, when used with biological specimens, the laser light causes adverse effects due to heating and optical damage, or “opticution.”

The Solution:
Using an FPGA-based, real-time controller, we connected back-focal-plane interferometric detection of trapped particles with fast trap steering using acousto-optic deflectors. Employing a simple proportional control algorithm, we achieved a four-fold increase in effective trap stiffness without increasing laser power – thus significantly reducing optical damage to biological specimens.

Using an FPGA-based, real-time controller, we connected back-focal-plane interferometric detection of trapped particles with fast trap steering using acousto-optic deflectors. Employing a simple proportional control algorithm, we achieved a four-fold increase in effective trap stiffness without increasing laser power – thus significantly reducing optical damage to biological specimens.

"The NI-7833R FPGA-based data acquisition card matched our requirements well, and we envision many future variations on the real-time nanoscale control concept introduced here."

In optical tweezers, optical damage and associated heating restricts sensitive single-molecule experiments. By connecting position detection and trap steering using a real-time controller, we have built a closed-loop instrument. A proportional gain position-clamp algorithm provides an approximately four-fold increased effective trap stiffness without increased laser power, which enables the execution of a broader range of experiments.

Optical tweezers trap micron-sized dielectric particles using focused intense light. Laser light creates a harmonic trap with approximately 0.1 pN/nm stiffness that can be used to exert or measure one to 100 pN forces. Nanoscale sensing and actuation in this force range facilitates experiments with single biomolecules, where a molecule is bound to a trapped latex or silica microsphere (1 μm in diameter). With back-focal-plane interferometry, we can measure the position of a trapped sphere with millisecond temporal and subnanometer spatial resolution.

These experiments that elucidate the molecular architecture of life usually determine the microsphere position while a controlled force is exerted on the bead, or while the biomolecule is chemically modified. However, nanoscale measurements are impeded by thermal noise. A trapped particle experiences frequent collisions with solvent molecules (water) that, due to the minuscule particle mass, randomly kick the particle around in the trap. This thermal motion of the microsphere degrades the position resolution of the instrument. Thermal noise can be reduced by employing a stiffer trap, which requires higher laser intensity. However, for biological specimens, any increase in laser intensity causes adverse effects due to heating and optical damage, or “opticution.”

Therefore, to increase the effective trap stiffness and reduce the thermal noise, we combined high-resolution position detection with rapid trap steering to create a feedback-controlled optical tweezers instrument. For control, we chose a real-time reprogrammable FPGA-based data acquisition card.

Instrument Description

Our single-beam optical tweezers are built around an inverted microscope with a 100X, 1.3 NA objective. A high-power infrared continuous wave laser (4 W, 1064 nm) produces a beam that is collimated and isolated, before passing through acousto-optic deflectors (AODs), which provide +/-16 mrad deflection in both the x- and y-axis. After 3X beam expansion, we can translate the laser focus by +/-11 µm in the specimen plane. The AODs are driven by digital direct synthesizers (DDSs) that accept a digital 30-bit control word. The least significant bit of the 30-bit DDS control word corresponds to 0.02 pm of movement, much below the current detection level. A separate HeNe (633 nm, 6 mW) laser provides back-focal-plane interferometric detection. A duo-lateral photodiode produces +/-10 V analog position detection signals.

Our controller requirements were a minimum of 60-bit digital I/O to control two DDS; three or more ADC inputs to collect analog position data; and rapid signal processing to implement real-time feedback algorithms. Based on these requirements, we chose the NI-7833R data acquisition card, because it had the required inputs and outputs, and with the FPGA module, it could be programmed in NI LabVIEW, which we use daily. The AD-conversion (4 μs) and the acoustic time-of-flight in the AOD (4.5 μs) are the major contributors to the loop delay of 8.5 μs. The instrument response is thus significantly faster than most biological processes of interest.

Results

We first implemented, calibrated, and tested LabVIEW host and FPGA target VIs for open-loop control. The instrument operator can steer the trap position using onscreen controls or mechanical hand-wheels that output quadrature signals to the FPGA. This manual control mode is useful at the initial stages of an experiment when a suitable microsphere must be found and trapped.

Next, we implemented a proportional gain feedback loop. For example, any bead motion to the left is counteracted by trap movement to the right. This algorithm is termed a position clamp, because it strives to immobilize the bead at the set-point position. We then measured the position fluctuations while varying the loop gain and found that the effective trap stiffness can be increased by up to a factor of approximately four. By using moderate negative feedback, we could also decrease the effective trap stiffness.

Instead of the position signal from the trapped bead, we also can use the force signal as input to the feedback loop. This creates a force clamp that maintains constant tension in the biomolecule under study. A constant tension is required in stepping-assays where the biomolecule length changes. To measure only structure-related length changes, unwanted length changes in the biomolecule due to stretching must be avoided by maintaining constant tension in the assay.

By using real-time feedback control, we have shown that the effective stiffness of optical tweezers can be increased approximately four fold without an increase in laser power. These discoveries mean we can use a quarter of the laser power necessary without feedback control. The NI-7833R FPGA-based data acquisition card matched our requirements, and we envision many future variations on the real-time nanoscale control concept introduced here.

We acknowledge the pleasant and professional cooperation of the whole NI Finland team.

For more information, contact:

Anders Wallin

University of Helsiniki

Department of Physical Sciences

Gustaf Hallstominkatu 2

00014, Finland

Tel: +358408240925

Email: Anders.wallin@helsinki.fi

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