ESC471H1
Engineering Science Capstone Design
Optical Tweezers Preliminary Design Report
Date
October 22nd, 2010
Group Members
Maryam Badakhshi
Shannon O’Keefe
Laura Poloni
Hasmita Singh
Instructors
Professor Foster
Professor Nogami
Table of Contents
1. INTRODUCTION / EXECUTIVE SUMMARY ................................................................................................. 2
2. LASER SAFETY DESIGN............................................................................................................................... 2
2.1 – Requirements and Constraints......................................................................................................... 2
2.2 – Fibre Optic Enclosure ....................................................................................................................... 3
2.3 – Laser Beam Enclosure ...................................................................................................................... 3
2.4 – Safety Interlock / Emergency Switch ............................................................................................... 3
3. LABORARY EXPERIMENT ........................................................................................................................... 3
3.1 – Requirements, Constraints and Criteria........................................................................................... 3
3.2 – Improvements from Old  New ..................................................................................................... 6
3.3 – Deliverables ...................................................................................................................................... 6
3.3.1 Student Materials - Protocol ........................................................................................................ 7
3.3.2 Instructor/TA Materials................................................................................................................ 7
3.3.3 Safety Sheet ................................................................................................................................. 7
3.3.4 Safety Sign .................................................................................................................................... 7
3.3.4 Lecture Presentation .................................................................................................................... 7
4. BUDGET ..................................................................................................................................................... 7
4.1 – Laser Safety ...................................................................................................................................... 7
4.1.1 Fibre Optic Enclosure ................................................................................................................... 7
4.1.2 Laser Beam Enclosure .................................................................................................................. 8
4.1.3 Safety Interlock / Emergency Switch ........................................................................................... 8
4.2 – Experiment ....................................................................................................................................... 8
4.2.1 Experimental Materials ................................................................................................................ 8
4.3 – Overall Cost ...................................................................................................................................... 8
5. SCHEDULE ................................................................................................................................................. 8
6. REFERENCES .............................................................................................................................................. 9
APPENDIX A – Assessment of Alternative Solutions ..................................................................................... 9
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ESC471 – Optical Tweezers Preliminary Design Report
1. EXECUTIVE SUMMARY
This report outlines the laser safety design and experimental revisions that will be applied
to the Optical Tweezer system for the Advanced Physics Laboratory. The provision of laser
safety through the design will enable 3rd and 4th year undergraduate students in the Advanced
Physics Laboratory to utilize this cutting-edge research apparatus. This will ensure that students
who have not taken the full day laser safety training are suited for use of this equipment. As well,
the current experiment will be revised so as to make it usable by a modern student.
2. LASER SAFETY DESIGN
The existing optical tweezers system necessitates the inclusion of enclosures during full
power operation and associated interlocks.
2.1 – Requirements and Constraints
Table # lists the technical constraints for the optical tweezers apparatus, developed in
consultation with Professor Bailey, Professor Ryu, and Dr. Sandu Sonoc (Certified Laser Safety
Officer, Radiation Protection Service):
Table 1 Technical Constraints
Item Description of Constraint
ID
T1
The Optical Tweezers experiment must be made safe for undergraduate students who have not
taken the full day laser safety course and should be classified as a Class 1 laser.
The optical tweezers system employs a 980nm laser with a maximum power output of 330mW. If
the laser is exposed, it is considered a Class 3B laser. Appropriate measures must be taken to
enclose the optical path of the beam at the region where the laser is exposed. If the enclosure is
opened for beam alignment, the laser power output must be reduced to 5mW (Class 1) or the
laser must be completely shut off.
T2
The light path where the beam is exposed to air should be enclosed.
T3
The fibre optic should be enclosed. It would be helpful to be able to observe the fibre optic under
the enclosure.
T4
The reflected beam should not leak out from the sample stage, and the user must be able to
move the sample stage using the knobs.
There are x, y, and z knobs for sample stage movement. These must be accessible when the
beam is on.
T5
Budgetary constraints: Additional components added onto the existing apparatus should not
exceed CDN $1000.
T6
The design must prevent students from inserting an object into the path of the laser which could
cause the beam to diverge.
T7
Interlocks should not automatically reset. If an interlock is opened and the beam is turned off,
the beam should not turn back on automatically when the interlock is closed again; the laser
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ESC471 – Optical Tweezers Preliminary Design Report
should need to be turned back on.
2.2 – Fibre Optic Enclosure
An enclosure will be placed over the fibre optic wire to protect it from potential damage. As this
region poses no laser safety challenges, any transparent material would be sufficient for the
coverage and at the same time would allow the fibre optic to be visible to users. Access to this
wire is not necessary, thus the enclosure will be locked in place with screws. The table that the
apparatus rests on is able to facilitate the insertion of screws as it contains holes for such
purposes throughout the table.
-
Give dimensions, materials, screws – details, pictures
2.3 – Laser Beam Enclosure
The laser beam enclosure will be constructed of an optical density filter, as the exposed laser
beam passes through this region. The 980nm laser would require a neutral density filter with an
optical density of 8 (OD8+) in order to provide proper protection against the infrared beam.
http://www.lasersafetyindustries.com/1064_nm_Nd_YAG_Laser_Safety_Goggles_p/100-25-125.htm
The screws that will be utilized will contain a unique head such that students cannot easily
disassemble this safety enclosure.
2.4 – Safety Interlock / Emergency Switch
3. LABORARY EXPERIMENT
The optical tweezers experiment is designed for use by students in the Undergraduate Advanced
Physics Laboratories (APL). The existing Optical Trap experiment, created in Professor Ryu’s
lab by Jimmy Shen will be revised based on the requirements, criteria and constraints that have
been developed. The experiment consists of several sub-experiments that aim to empirically
determine the stiffness of the optical trap. The focus will be placed performing one method of
obtaining the optical trap.
3.1 – Requirements, Constraints and Criteria
The current Optical Trap experiment consists of a general description of each experiment,
questions to be answered by the student as well as an appendix containing additional details
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ESC471 – Optical Tweezers Preliminary Design Report
about the various components of the apparatus as well as some procedural details. However,
there are numerous limitations to this experiment. The main limitations are summarized below:
 Insufficient procedural instructions to carry out the experiments
 Lack of a Teaching Assistant (TA) manual providing further details
 Inclusion of safety precautions and laser safety details in the lab manual
In order to address the above limitations, each of the above have been included in the criteria for
the revised experiment. In addition, the Experience Design approach will be employed/concepts
from the Experience Design approach will be employed. <explain>.
o
o
o
o
-
Kolb Learning Cycle
Experience Design
Maximizing Retention Time
Design for cognition
The experience design approach will be utilized …explain.
http://en.wikipedia.org/wiki/Experience_design
As well, the goals for the APL include presenting the student with the “opportunity to work on
interesting and challenging experiments, deepen their understanding of the underlying Physics,
and to further develop laboratory, analysis and communication skills.” [H1]. Additionally, the
“experiments in this course are designed to form a bridge to current experimental research. A
wide range of experiments are available using contemporary techniques and equipment. Many of
the experiments can be carried out with a focus on instrumentation.” [H2]. As such, the
following development of constraints that constitute a “good laboratory experiment” ranging
from experimental constraints, to the experimental protocol and method of evaluation will reflect
the objectives of the APL as well as approaches considered in educational journals.
From Proposal: **table numbers will have to be edited.
Table 2 lists the Experimental Constraints for the proposed solution.
Table 2 Experimental Constraints
Item Description of Constraint
ID
E1
The experimental procedure should be within 18 hours over the course of 3 weeks.
Advanced Physics Laboratory structure.
E2
The room door must be closed and locked with a temporary “Laser Work in Progress” sign placed
outside the room door.
This sign can be provided by Sandu Sonoc, the Senior Radiation Safety Officer.
E3
Appropriate laser safety glasses must be worn at all times when using the equipment.
E4
The beam will not be pre-aligned prior to the student commencing the lab.
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ESC471 – Optical Tweezers Preliminary Design Report
E5
E6
E7
The beam should be turned off when changing samples.
Changing the sample involves moving somewhat reflective material in and out of the beam path.
The beam must be turned off when this occurs. Normally, no light should reflect out when
changing the sample, but there is a chance that if someone sticks something in (eg. A pen to
indicate the beam position), then inserting an object may cause the beam to reflect wildly.
The laser power supply key should be kept by the technologist.
The experiment should be carried out individually.
In accordance with APL structure.
Table 3 lists the Student Assessment Criteria for the experiments to be carried out on the Optical
Tweezers System.
Table 3 Student Assessment Criteria
Item Description of Criteria
ID
SA1 The mark composition for the laboratory should not be completely data-driven, and should
include a combination of data analysis within the student’s lab notebook as well as more general
questions pertaining to the lab that assess the student’s understanding. It should also include a
discussion with the TA/Professor regarding the results obtained.
This is in accordance with APL structure, and the consultation with the TA/Professor would
enhance oral and written communicability of technical material [H3].
SA2 Adequate time must be provided to the student for completion of the lab write-up.
SA3 The write-up should include a brief discussion of a research paper that utilizes Optical Tweezers
to bridge the experiment to current experimental research [H2].
Table 4 outlines the criteria for the Experimental Protocol that will accompany any experiment to
be carried out on the Optical Tweezers System.
Table 4 Experimental Protocol Criteria
Item Description of Criteria
ID
EP1 Clear and succinct safety instructions must be given to the students through the experimental
write-up, from the instructor, and displayed on the wall near the equipment.
EP2 The experiment should provide more detailed procedural steps to facilitate the execution of the
experiment. This will ensure that proper procedure is followed and leaves less room for
erroneous conduct around a high-powered laser.
EP3 The entire experiment should be divided into sections or a series of experiments that investigate
different concepts such that they can be carried out over the course of the 3-week lab.
The experiment should not be repetitive or tedious.
EP4 A more detailed experimental protocol should be provided to the TA/Professor to enable them to
assist the students when necessary. This should detail the potential pitfalls that may be
encountered [H4].
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ESC471 – Optical Tweezers Preliminary Design Report
EP5
EP6
EP7
The protocol should include the background and theory pertaining to the lab, as well as clearly
detailed schematics of the apparatus [H4].
A list of relevant websites and papers that will aid the student in understanding the laboratory
concepts or equipment should be included.
The experiment should incorporate a variety of skills and techniques [H4].
Table 5 lists the criteria pertaining to the training that a user of the Optical Tweezers System will
require.
Table 5 Training Criteria
Item Description of Criteria
ID
Tr1
The TA/Professor present during the laboratory session should be well acquainted with the
experimental procedure and apparatus and should be available for assistance.
Tr2
The student must require minimal initial training in order to execute the lab.
Tr3
An introduction to the lab should be given by the TA/Professor to acquaint the student with the
lab concepts, the equipment, expectations and clearly stated safety instructions.
Tr4
An instructor/TA manual should be provided, which includes additional details such as
information on the safety features and disassembling enclosures.
3.2 – Assessment of Revised Experiment
The revised experiment will encompass the requirements, constraints and criteria that have been
outlined, in order to create an improved laboratory experiment that will be usable by a modern
student. The focus will be placed on one of the methods to determine the stiffness of the optical
trap.
3.2.1 Experiment
3.2.2 Student Assessment
3.2.3 Experimental Protocol
3.2.4 Instructor/TA Training
3.3 – Deliverables
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ESC471 – Optical Tweezers Preliminary Design Report
3.3.1 Student Materials - Protocol
One of the main improvements that will be made to the experiment is the inclusion of a more
detailed procedure for executing the necessary steps. This will eliminate time spent on “figuring
out” the apparatus and leaves less room for dangerous practice through the inclusion of specific
instructions. As the optical tweezers operate with the use of a high-powered laser beam, such
provision is necessary to ensure that appropriate procedures are followed. The lab manual will
also include laser safety procedures/details to ensure that students are aware of proper practices.
3.3.2 Instructor/Teaching Assistant Materials
An instructor (TA) manual will be created, which will include additional details surrounding the
experiment. For instance, a description of the safety features that will be designed will be
outlined in the instructor/TA manual. These details will not be included in the student manual as
students should not have the necessary information to prompt tampering with the enclosures and
interlocks. However, the in the case that the instructor or TA requires access to the enclosed
components (such as to perform beam alignment), sufficient details will enable them to do so.
3.3.3 Safety Sheet
3.3.4 Safety Sign
While the optical tweezers are in operation, a temporary "Laser Work in Progress" sign must be
mounted on the outside of the door (MP248) which will be provided by Dr. Sandu Sonoc.
3.3.5 Lecture Presentation
“ An introduction to the lab should be given by the TA/Professor to acquaint the student with the lab
concepts, the equipment, expectations and clearly stated safety instructions.”
We also have to include a whole thing on laser safety.
4. BUDGET
4.1 – Laser Safety
4.1.1 Fibre Optic Enclosure
Item
Estimated Cost
<Transparent material for box>
Screws (#)
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ESC471 – Optical Tweezers Preliminary Design Report
4.1.2 Laser Beam Enclosure
Item
Estimated Cost
Neutral Density Filter
Circuitry
Screws (#)
4.1.3 Safety Interlock / Emergency Switch
Item
Estimated Cost
Touch sensor
Other circuit components for interlock w/ LM14S2
2.5mm mono phono jack
4.2 – Experiment
4.2.1 Experimental Materials
Item
Estimated Cost
Sample and bead
Laser Safety Goggles (2nd pair)
4.3 – Overall Budgetary Analysis
5. SCHEDULE
The laser safety design and development of an improved experiment will be executed in parallel.
-
Buying materials
Fabricating enclosures x2
Designing circuit (this should be done for the report)
Testing safety features
Going through lab x10
Adding details to lab
Laser safety
Deliverables (student, Ta manual, safety sheet on wall, etc.)
Week
October 18th, 2010

Laser Safety Tasks
Purchase circuit
components and enclosure
materials

Experiment Tasks
Order laser safety goggles
(2nd pair)
October 25th, 2010
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ESC471 – Optical Tweezers Preliminary Design Report
November 1st, 2010
November 8th, 2010
November 1st, 2010
November 15th, 2010
November 22nd, 2010
November 29th, 2010
*Identify critical tasks
6. REFERENCES
[H1] University of Toronto Physics Department. “Advanced Physics Laboratory Course Homepage.”
12 September 2010. http://www2.physics.utoronto.ca/~phy326/
[H2] Engineering Academic Calendar 2010 – 2011
http://www.undergrad.engineering.utoronto.ca/Assets/UndergradEng+Digital+Assets/calenda
r1011/Chapter+8.pdf
[H3] Panel on Undergraduate Engineering Education, Committee on the Education and
Utilization of the Engineer, Commission on Education and Technical Systems, National
Research Council. Engineering Undergraduate Education. Washington, DC: National
Academy
Press,
1986,
pp.82.
[Online].
Available:
http://www.nap.edu/openbook.php?record_id=589&page=82. Accessed Sept 26, 2010.
[H4] E. Bell, “Laboratory Exercises”, Biochemistry and Molecular Biology Education, vol. 29, no. 3,
2001. [Online]. Available: http://onlinelibrary.wiley.com/doi/10.1111/j.15393429.2001.tb00086.x/pdf. Accessed Sept 26, 2010.
APPENDIX A – Assessment of Alternative Solutions
Other solutions – critique
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ESC471 – Optical Tweezers Preliminary Design Report
APPENDIX B – Existing Optical Trap Experiments
The following information has been obtained from the “Optical Trap” laboratory manual, published
August 9, 2010, by the lab of Professor William Ryu, of the Department of Physics at the University of
Toronto.
Experiments
Optical traps operate best when the trapping laser wavelength and the diameter of the trapped bead
are comparable [4]. In this regime, neither of the two approximations are valid and theories regarding
trapping in this regime are very complex. Hence the stiffness of traps working in this regime cannot be
accurately predicted. Given the fact that different beads and laser powers will often be required during
experiments, empirical methods of determining the stiffness of an optical trap is crucial. The calibration
experiments outlined in this lab will take you through a variety of methods to determine the trap
stiffness. The first method will employ the equipartition theorem for a trapped bead undergoing thermal
fluctuations; the second method will seek to measure the detection of the trapped bead under external
forces produced by Stokes' drag; the third method will take advantage of the high data acquisition rates
offered by the quadrant detector to produce the power spectrum of a trapped bead and infer the
stiffness from theoretically predicted features.
Figure 3. Linear relationship of the displacement and velocity Appleyard 2007.
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ESC471 – Optical Tweezers Preliminary Design Report
1) Equipartition
For each degree of freedom in the motion of particle, there will be 1/2kBT of thermal energy, where kB
is the Boltzmann constant and T is the temperature in Kelvin. When the bead is not trapped, the thermal
energy is converted into kinetic energy and the bead will undergo random walk in the medium. If the
bead is in a harmonic trap, the thermal energies become potential energies manifested in a small
displacement. Over large sample sizes the fluctuations will give the result:
(1)
where {x2} is the variance of the x displacements (Instructions on recording .avi videos from CCD camera
are given in Appendix A and instructions on converting the videos to positional data are in Appendix D).
2) Stokes' Drag
A more direct way of measuring the stiffness is simply to apply an external force and measure the
deflection. At low Reynolds numbers, external force may be applied to the trapped bead by oscillating
the sample stage. The liquid inside the will move in unison with the sample stage and for liquid velocity v
the drag force applied to the bead will be:
(2)
Since the strength of the optical trap decreases as you move further into the medium, the optimal
working distance for the optical trap is a few microns above the coverslip. Because of the proximity of
the trapped bead to the boundary, Stokes' drag will not be suffcient in calculating the drag coefficient B.
For a better approximation of the drag coefficient you will have to correct for wall effects with the
formula:
(3)
where r is the radius of the bead and h is the height of the center of the bead with respect to the
coverslip surface. (Directions on finding the height, h, are given in Appendix C).
Once the bead is trapped, you can begin recording the positional data from the CCD camera while
oscillating the sample stage (Refer to Appendix A). If the bead is assumed to be in equilibrium, then the
drag force, Fd, will be balanced by the trapping force, Ft = kx, from which the stiffness, k, may be
calculated.
The maximal deflections of the bead will correspond to the maximal external force from the surrounding
liquid. Thus you will need to measure the maximal deflections at several different frequencies and find
the displacement to force relationship to calculate the stiffness. The maximal deflection, xmax, should be
half the width of the data in the x-direction and the maximal force, Fmax, should correspond to the
maximal velocity Fmax= Bvmax.
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ESC471 – Optical Tweezers Preliminary Design Report
3) Power Spectrum
In the low Reynolds number regime, the equation of motion for the bead will be that of a massless
damped oscillator:
(4)
Where x is the displacement of the bead and B is the drag coefficient.
The external Brownian force F(t) is random, and is essentially white noise with an amplitude of
(5)
The Fourier transform of equation (4) is:
(6)
Thus, the power spectrum of the position is given by:
(7)
Equation (7) is a Lorentzian with a cornering frequency of fc = k/ which will give the stiffness, k.
Since taking the power spectrum require high frequency components, the 30 fps acquisition rate of the
CCD camera will not be sufficient, thus a quadrant detector with a high data acquisition rate must be
employed (instruction for using and calibrating the quadrant detector are found in Appendix A). The
spectrums of many data sets (taken at a the same power and bead height) may be averaged to reduce
the noise level at high frequencies.
Figure 4. Ideal power spectrum with label of corner frequency
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ESC471 – Optical Tweezers Preliminary Design Report
Extra Experiments
Blur Correction for the CCD Camera
When using detection systems such as a CCD camera, it is important to note the precision of the
position measurements are limited by the exposure time of the camera [7]. The measured position X m, is
an average of the instantaneous positions X taken over a finite interval, W, which is the exposure time of
the CCD pixels. The simplest model is a step function exposure factor:
(8)
In the application of the equipartition theorem, this blur effect implies that {X2m} is less than or equal to
{X2}, which will lead to an over estimation of the trap stiffness. If we define the dimensionless
parameter,  in terms of the trap relaxation time,  and the exposure time W:
(9)
and the relaxation time, , is given by  = /k. The variance of X and Xm will be related by:
(10)
Where S() is the correction function:
(11)
Using the information above, correct for the blur effect in the equipartition experiment. Can you find a
way to calculate the correction term without knowing the drag coefficient ?
Using the Quadrant Detector
Perform the equipartition and Stokes' drag experiments using the quadrant detector. What are the
advantages?
Analysis Questions
1. Derive the expression for the gradient force for equation (3) in detail.
2. Show that the forces acting on the bead is in the low Reynold's number regime, and that the
assumptions of Stokes' drag are appropriate.
3. Derive equation (7), include an argument for the validity of equation (6).
4. Explain the disadvantages of the second method for determining trapping height.
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ESC471 – Optical Tweezers Preliminary Design Report