Features

• Trap Laser
• SM Fiber Coupled DFB Laser, 14-Pin Butterfly Package
• 975 nm, 330 mW (Max)
• Integrated TEC Element for Temperature Stabilized Output
• LD Controller and Mount Included
• Nikon 100X Oil Immersion Objective
• Inverted Light Microscope Design
• 3-Axis Sample Positioning Stage
• CCD Camera with USB Interface for Video Imaging
• Position-Sensing Detector Module Available

The OTKB (OTKB/M) Optical Tweezer Kit consists of five pre-assembled segments with which optical tweezers, also known as an optical trap, can be constructed. The advantage to purchasing and assembling this kit is the flexibility it provides over other closed optical tweezer systems. Since the optical tweezer system is built using standard Thorlabs' components, it is easy to modify or upgrade the system using other standard components. The kit is shipped in pre-assembled segments and will only require final coupling of segments and alignment. A step-by-step assembly instruction manual is provided with each kit. Once constructed, we suggest our OTKBTK Sample Preparation Kit, which provides users with everything necessary to prepare a sample and determine that their optical tweezer kit has been properly assembled and functioning. For customers who prefer to have the system shipped assembled, aligned, and tested on a breadboard, we are able to provide this at an additional cost. We also offer a module that adds computer-controlled trap positioning to the optical tweezer system by integrating our  2D galvo mirror system. This is described under the Applications Module tab above. We also describe a module that enables you to add fluorescence spectroscopy to the system. If you are interested in either the assembled version, the steering module, or fluorescence module, and would like to receive a quotation, please contact  David Fairbank.

S. Wasserman, D. Appleyard and M. Lang at the Department of Biological Engineering, MIT published an article (Optical Trapping for Undergraduates, Am. J. Phys. 75 (1), January 2007) on an optical tweezer system that they designed and built for use in teaching labs. Thorlabs, in collaboration with the aforementioned, has developed this kit so that others may build optical tweezers with similar capabilities as the system published in the American Journal of Physics. Thorlabs' optical tweezers, or optical traps, have been employed in numerous experiments and have been utilized in many graduate-level instructional laboratories; for details, please see the Video and Publications tabs.

One of the principal advantages of the OTKB (OTKB/M) optical tweezer kit over a black box system is the ease with which the design of the optical trap can be modified to add functionality. For example, the OTKBFM is a position sensing module that can be easily attached to the base OTKB (OTKB/M) kit. Unlike the fully functional OTKB (OTKB/M) kit, an additional DAQ card and software will be required to calibrate the position, stiffness, and force of the optical trap using the OTKBFM position sensing module. The OTKBFM position sensing module does provide the user with the components needed to add calibration capability to the OTKB (OTKB/M) optical trap.

Please contact David Fairbank for more information. If you have a modification suggestion that you would like to share, please consider using the Feedback tab.

Laser Safety

Whenever operating a laser, it is imperative to pay attention to the classification of the laser and observe appropriate laser safety procedures. Thorlabs' Optical Tweezer Kit uses a 975 nm, 330 mW laser. As the laser beam is predominantly encased by lens tubes, not operated near its maximum power, and the only exposed beam is divergent after the sample, the system may be considered Class 1 when fully assembled and functioning properly. However, if the trap is not assembled or exposure to the laser beam is possible, then the system should be treated as a Class 3B laser. In this instance appropriate safety procedures and care must be taken including, but not limited to, the use of laser safety glasses.

Stage

The sample stage consists of a microscope slide holder mounted to a 3-axis (X, Y, Z) translation stage. This stage is then mounted on a 1-axis long-travel translation stage and then to a translating breadboard, which results in the following capabilities:

1. 2" (50 mm) and 2.4" (60 mm) of travel perpendicular to the beam path. This makes it easy to load the sample and coarsely position it near the trap.
2. 4 mm of travel in the X, Y, and Z directions using the NanoMax stage with differential micrometer drives. The coarse adjustment knobs provide 0.5 mm/rev.
3. 300 µm of travel in the X, Y, and Z directions using the differential knobs (50 µm/rev) on the NanoMax stage.
4. 20 µm of travel in the X, Y, and Z directions using the piezo actuators on the 3-axis stage. 20 nm resolution is possible without using feedback from the internal strain gauge sensors, while 5 nm resolution can be achieved using the internal strain gauges for positional feedback. Three T-Cube Piezo Drivers (TPZ001) are included in the kit. Two T-Cube Strain Gauge Readers (TSG001) are included with the OTKBFM Force Module.

Laser Input

A FiberPort (PAF-X-7-B) collimates the output of the trapping laser. The FiberPort is a versatile collimator since it allows the aspheric collimation lens to be precisely positioned along 5 axes (X, Y, Z, Pitch, and Yaw). For polarization-sensitive applications, the keyway on the FiberPort can be rotated about the optical axis so that the orientation of a linearly polarized collimated beam can be set.

The 975 nm trapping laser source is a pigtailed Fiber Bragg Grating (FBG) stabilized single mode laser diode in a hermetically sealed 14-pin butterfly package. The integrated TEC element and thermistor in the butterfly package allow the temperature of the laser to be precisely controlled when mounted in the LM14S2 laser diode mount. Laser current is controlled using an LDC210C controller and temperature is controlled using a TED200C. This laser, mount, and controller combination was chosen to ensure that the output power of the laser will be very stable, which is important for maintaining a constant trapping force.

Force Module

The optional OTKBFM module contains components that can be used to calibrate the tweezers using positional detection of the back-focal plane of the condenser. By placing the Quadrant Position Detector (QPD) in a plane conjugate to the back focal plane of the condenser, the signal generated by the QPD is sensitive to the relative displacement of the trapped particle from the laser beam axis. As a result, the output of the detector can be used to calibrate the position, stiffness, and force of the optical tweezers. The detector is connected to the cage cube above the condenser. A TQD001 T-Cube Quadrant Detector Reader and two TSG001 T-Cube Strain Gauge Readers are the main components included in this module. For high-bandwidth measurements, the QPD signal can be read out from the controller cube directly via a DAQ card (not included). Due to the sensitivity of these measurements, we usually recommend using an active isolation support for mounting the system.

Beam Expander

This beam expander is based on Galilean expansion, which minimizes overall space while providing an expansion factor of 3. We use anti-reflection coated achromatic doublets ACN254-050-B and AC254-150-B that are computer-optimized at infinite conjugate ratios to expand the collimated trapping laser beam. The cage rods are assembled in such a way that they allow the user to optimize the focus of the trapping beam on the camera by allowing up to 12.5 mm adjustment along the optical axis.

Camera Module

Visible light from the LED source illuminates the sample and is then imaged on the 1280 x 1024 pixel color CCD camera (DCU224C) using an achromatic doublet (AC254-200-A). The dichroic mirror in the light path in combination with a short pass filter prevents backscattered light from the 980 nm laser from saturating the CCD detector. With the camera, you can acquire high-quality still images or video both in color or B&W.

Application Modules

Thorlabs offers three modules that are designed to enhance the functionality of our Optical Tweezer Kit. For information about these modules or to request a quotation, please contact David Fairbank.

The following modules are available:

• Galvo Steering Module
• Fluorescence Spectroscopy Module
• Raman Tweezers Module

Galvo Steering Module

Automated trap positioning capability can be added to the optical tweezer kit by integrating a 2D galvo mirror (GVS002). The galvo mirror replaces the turning mirror at the fiber input and is positioned in a plane conjugate to the back aperture of the objective. Drive voltages are applied to the galvo mirror controller boards via a DAQ card, allowing the user to position the trap while the sample stage remains stationary. Due to the optical path length between the galvo mirror and the back aperture of the objective, only small angle adjustments are allowed, which means that the galvo mirror can operate at its maximum bandwidth of 1 KHz. By moving the beam back and forth between two positions with an appropriate dwell time at each position, it is possible to create two stable traps from a single laser beam. Please contact David Fairbank for more information and to receive a quotation for this configuration.

Thorlabs would like to thank Professor Padgett from the University of Glasgow, for his help creating the steerable version of the optical tweezers.

Coupling Input Beam to Galvo Mirrors

The laser source is coupled to the galvo mirrors with a FiberPort collimator that comes with the basic optical trapping kit (OTKB). An iris (SM1D12D) will spatially filter the input beam. The combination of a cage plate (CP02), cage plate mount (CPB1), and cage rods (ER2) provides stability to the cage cube (GCM002) that holds the mirrors. If you already have a laser source that is collimated, the FiberPort will not be needed. The maximum beam diameter for the galvo mirrors is 5 mm.

Galvo Beam Expander Segment

The beam expander is based on a Keplerian beam expansion system with achromatic doublet lenses (AC254-060-B and AC254-200-B). The first achromatic doublet is mounted into an adjustable lens tube (SM1V10) and is positioned one focal length away from the center of the scanning mirrors. Lens tubes are then used to completely enclose the trapping laser.

The first half of the galvo steering beam expander segment is connected to the cage cube (C6W) of the optical trapping kit (OTKB) through a cage plate (CP02T) and two cage rods (ER1). The second achromatic doublet is then mounted to this cage cube (C6W) using an adjustable lens tube (SM1V05) such that the separation between the two achromatic doublets is the sum of the focal lengths of the achromatic doublets. This combination provides an expansion factor of 3.3.

The camera is connected to the module in a similar manner as described in the optical trapping kit manual (OTKB), but in this case, using just four cage rods (ER05) and a cage gasket (CPG3). The second achromatic doublet lens (AC254-200-B) in the beam expander segment is used as the imaging lens for the camera.

Fluorescence Spectroscopy Module

Click to Enlarge

By combining fluorescence spectroscopy with optical tweezers, researchers can visualize, manipulate, and rapidly characterize the properties of various samples and cellular structures. Such a technique can be used to detect the arrival of a single molecule into a small volume, detect the conformational changes of cellular structures or bacteria, study elastic properties of single DNA, demarcate different parts of a larger molecular complex, and measure the response of each to an applied force. Thorlabs provides a tested set of components as a module that enables the addition of such functionalities to our Optical Tweezers Kit, as shown in the photo to the right.

This module includes the high-power, broad-spectrum plasma source HPLS243, which couples light into the system via a Ø3 mm liquid light guide. Also needed is a plan fluorite or semi-apochromatic objective such as the RMS100X-PFO or N100X-PFO (both sold below). These objectives have a higher numerical aperture than the standard objective included with the OTKB, allowing them to trap particles like polystyrene that are functionalized and typically used as a probe.

As an application example, a sample consisting of a diluted solution of 1.0 µm uniformly dyed polystyrene beads, with an excitation wavelength of 480 nm and an emission wavelenght of 520 nm, was used. The excitation light is selected from the HPLS243 using a MF475-35 excitation filter, which has a transmission of more than 85% in the 470 - 490 nm range. The light is then coupled into the tweezers system using an MD499 dichroic mirror, which reflects light in the 470 - 490 nm range and transmits light in the 508 - 675 nm range. As with any standard epi-fluorescence technique, the fluorescence light emitted by the sample will be collected by the objective together with any reflected excitation light, which gives a better signal-to-noise ratio than a transmissive detection scheme. The signal then goes back through the dichroics and an MF530-43 emission filter which has a center wavelength of 530 nm and a 43 nm FWHM bandwidth, and is then detected by a CCD camera. The CCD camera can also be replaced by a photodiode for quantitative measurements.

If you are interested in our Fluorescence Spectroscopy Module and would like to receive more information or a quotation, please contact David Fairbank.

Raman Spectroscopy Module

For customers who are considering adding Raman spectroscopy to their optical tweezers, please see this application example from CREOL: Multispectral optical tweezers for molecular diagnostics of single biological cells. If you are interested in receiving more information, please contact David Fairbank.

MATLAB GUI

The following MATLAB-based graphical user interface (GUI) was developed through a collaboration with Massachusetts Institute of Technology and allows calibration of Throlabs' Optical Tweezer system. It includes position calibration in X and Y, and trap stiffness determination using PSD-rolloff, Stokes Drag, and Equipartition (see Appleyard et al). It also includes an example DNA tether assay. This involves stretching a piece of DNA that is attached to a coverslip on one side and a bead on the other side, thereby allowing the determination of the DNA tether length.

The files below are necessary files and manual to install the software. Please open the manual first and follow it before installing the other files. Note that this GUI is unsupported by Thorlabs; it is offered as an example for customers who wish to operate their Optical Trapping Kit in MATLAB. Additionally, customers are free to modify the code for their own personal use. Additional items that you will need with this GUI include a differential amplifier and a DAQ card.

Additional information on the software and the calibration may be found by visiting the MIT Optical Trap and Aligning the Optical Trap webpages.

Optical Trapping of Nanoparticles

Bergeron, J., Zehtabi-Oskuie, A., Ghaffari, S., Pang, Y., Gordon, R. Optical Trapping of Nanoparticles. J. Vis. Exp. (71), e4424, doi:10.3791/4424 (2013).

In the video below, a modified OTKB system is utilized to trap 20 nm polystyrene nanospheres. This is accomplished by using a double nanohole structure, which produces a large trapping force that allows nanometer-sized particles to be trapped.

The following papers describe research and education using Thorlabs' OTKB Modular Optical Tweezers. To learn how our tweezers might benefit your research, please contact applications@thorlabs.com.

Research Articles

Corey Butler, Shima Fardad, Alex Sincore, Marie Vangheluwe, Matthieu Baudelet, and Martin Richardson. Multispectral optical tweezers for molecular diagnostics of single biological cells. Proc. of SPIE Vol. 8225  82250C-1, 2012.

Ana Zehtabi-Oskuie, Jarrah Gerald Bergeron, and Reuven Gordon. Flow-dependent double-nanohole optical trapping of 20 nm polystyrene nanospheres. Scientific Reports 2, 966, 2012.

Yuhang Jin and Kenneth B. Crozier. An optical manometer-on-a-chip. Proc. SPIE 8097 80971U, 2011.

Educational Materials

Robert Kammel. Optical Tweezers. Abbe School of Photonics, Friedrich-Schiller-Universität, Physikalisch-Astronomische-Fakultät, 2010.

Aleksandra Radenovic. Optical Trapping. Advanced Bioengineering Methods Laboratory, École Polytechnique Fédérale de Lausanne.