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"There's nothing like the eureka

moment of discovering something that

no-one knew before….I won't

compare it to sex, but it lasts

longer”, Professor Stephen W.

Hawking, Theoretical Physicist in

United Kingdom

CONTENT

1. LATEST HAPPENINGS IN OSA (NTU) STUDENT CHAPTER

----2

2. REFRACTION OF LIGHT PART ONE

a. OPTICAL TWEEZERS-- 3

3. MALAYSIA EATING SPREE —-- 6

4. REFRACTION OF LIGHT PART TWO

a. FIBER OPTICS SENSOR ---9

5. A LONELY STUDENT OVERSEAS CONFERENCE—11

BY STEVE LEE WOEI MING, BENJAMIN TAY CHIA MENG

O-SING Issue 3 Page 1 of 12 AUGUST 26, 2004

Disclaimer: O-SING is published by the Student Chapter OSA(NTU), School of Electrical and Electronic Engineering, Nanyang Technological University. Block S1, Nanyang

Avenue, Singapore 639798. Opinions expressed herein do not necessarily reflect the views of the University.

Copyright c 2003 Student Chapter OSA(NTU), All rights reserved. Material in this publication may not be reproduced without written permission.


The OSA student Chapter (NTU) has completed a number of activities since its

formation in early December 2003. The committee has managed to organize a

number of activities amidst their busy and working academic schedules. The

activities that we have accomplished are as follows: -

RESEARCH LIFESTYLE

A Joint Seminar by OSA /PhRC

Participants of the seminar taking a group photo

with Prof Kishan and Assoc.Prof Larry Yuan (Faculty Advisor)

The OSA (NTU) and PhRC (NTU) jointly organized the seminar with an aim to

promote research lifestyle to the local student community. The scope in the seminar will

be to promote and enhance the spirit of research to the local Photonics Research centre.

The seminar also hopes to interest and inspire both the local undergraduate the graduate

community to the area of research.

In this seminar, the OSA(NTU) and PhRC(NTU) invited a distinguish guest, a Tan

Chin Tuan Fellow, Professor Kishan Dholakia from St Andrews University, for a short but

inspirational seminar for the local Photonics research community in NTU.

During the seminar, Professor Kishan used a combination of scientific history and

interesting science questions to show us that the nature of science is full of creativity

moments, fun but on the other hand requires perseverance. But if effort is put into one's

work, he will be able to make a difference in the world of science. Most importantly is the

O-SING Issue 3 Page 2 of 12 AUGUST 26, 2004

Disclaimer: O-SING is published by the Student Chapter OSA(NTU), School of Electrical and Electronic Engineering, Nanyang Technological University. Block S1, Nanyang

Avenue, Singapore 639798. Opinions expressed herein do not necessarily reflect the views of the University.

Copyright c 2003 Student Chapter OSA(NTU), All rights reserved. Material in this publication may not be reproduced without written permission.


focus that one must have in order to succeed in any scientific field. After the seminar,

refreshments were offered to the attendees of the seminar.

Prof Kishan giving his talk in lightning speed Participant helping themselves to the

Refreshments.

Many students were so inspired with the talk by Professor Kishan that they stayed

on after even after the seminar has ended.

The presentation slides can be found at

http://www.ntu.edu.sg/home/WMLee/NTU_presentation.pdf

More photos of the seminar can be found at

http://www.ntu.edu.sg/studorgn/OSA/studentchapter/favorite.htm

BY: Steve Lee Woei Ming

O-SING Issue 3 Page 3 of 12 AUGUST 26, 2004

Disclaimer: O-SING is published by the Student Chapter OSA(NTU), School of Electrical and Electronic Engineering, Nanyang Technological University. Block S1, Nanyang

Avenue, Singapore 639798. Opinions expressed herein do not necessarily reflect the views of the University.

Copyright c 2003 Student Chapter OSA(NTU), All rights reserved. Material in this publication may not be reproduced without written permission.


Conventional optical tweezers are often known as 3-dimensional optical traps, whereby a

tightly focus laser beam is used to trap the microscopic neutral particles such as cells, membranes

and small dielectric spheres. This type of optical tweezers are introduced by Arthur Ashkin in 1980s.

In order to understand the working mechanism of the optical tweezers, the properties of

light must be considered. It is widely known that visible light is a part of the family of

electromagnetic radiation. Hence,

c= λ.

v ----------------------(1)

where c is the speed of light, λ is the wavelength, v is the frequency of light.

Furthermore, each photon possess energy and momentum which is

E = hv . ----------------------(2)

where E is the energy of light, h is the Planck’s constant, v is the frequency of light.

hv .

p = ---------------------------------(3)

λ

where p is the momentum of light, h is the Planck’s constant, v is the frequency of light and λ is the

wavelength.

The linear momentum of light is absorbed by the object, which is being trapped. This

trapping is known as radiation pressure. The interacting forces between the particle and laser beam

are better known as scattering and gradient force. One important parameter to know is that the

optical trap operates in two regimes according to the shape of the particles being trapped. The Mie

regime where the overall diameter of the particle is much larger compared with the wavelength of

the laser. While in the Rayleigh regime is where the overall diameter of the particle is much smaller

compared with the wavelength of the laser. As the result, most of the particles in the Mie regime are

spheres and ellipsoids. And in the Rayleigh regime, the particles are not as restricted in particle

shapes. In general, the smaller the particle the less the trapping power (per watt) is required. In most

cases, the dipole model and the conservation of momentum model are being used for explaining the

working mechanisms of the optical tweezers with respect to the shape of the particles.

For the particle in the Rayleigh regime, the dipole model is used. The electromagnetic

radiation of the light will induce dipole to form with the particles. The scattering force, (Eq.4), comes

from the reflected and absorbed light. This vector force has a magnitude proportional to the intensity

of light and a direction point toward the propagation of laser light. Furthermore, when light interacts

with the particles, polarization occurs. This polarization will cause the particle to experiences a

gradient force. This vector force has a magnitude proportional to the gradient intensity and a

direction pointing towards the direction of intensity gradient.

< s > σ

Fscattering = nm

-----------------(4)

c

2

2

8 4 2 ⎛m−1⎞ where σ = Π( kr) r ⎜ 2 ⎟ σ is cross-sectional area of a sphere, r is radius, < s >is the time

3 ⎝m+ 1⎠

averaged Poynting vector, n is the refractive index of the particle, nm is the refractive index of the

medium, m is the relative index and k= 2 m n Π

is wave number of the light.

λ

α 2

Fgradient = ∇< E >-----------------(5)

2

O-SING Issue 3 Page 4 of 12 AUGUST 26, 2004

Disclaimer: O-SING is published by the Student Chapter OSA(NTU), School of Electrical and Electronic Engineering, Nanyang Technological University. Block S1, Nanyang

Avenue, Singapore 639798. Opinions expressed herein do not necessarily reflect the views of the University.

Copyright c 2003 Student Chapter OSA(NTU), All rights reserved. Material in this publication may not be reproduced without written permission.


2

2

2 2⎛m−1⎞

m 2

where α = nr ⎜

⎝m 2

⎟ , r is radius, ∇ < E >is the divergence of the intensity of the light, nm

+ 1⎠

is the refractive index of the medium and m is the relative index.

For particles in the Mie regime, the conservation of momentum is used. The forces exerted

onto the dielectric sphere are due to the refraction of the light rays. The total force acting onto the

sphere will be the summation of all the force acting onto it. These same forces explained in terms of

scattering force, gradient force and the total proposed by Ashkin [1] .

2

np⎛

T 1

[ cos(2θ − 2 r) + Rcos2θ]


Fscattering = ⎜1+ R cos(2 θ ) −

2

⎟ ------(6)

c ⎜ 1+ R + 2Rcos2r ⎟

⎝ ⎠

2

np⎛

T [ cos(2θ− 2 r) + Rcos2θ]


1


Fgradient =


⎜R

sin (2 θ)


2

c

⎜ − ⎟

⎜ ⎝ 1+ R + 2Rcos2r ⎠⎟ -------------(7)

Qnm1 p

Ftotal = ------(8)

c

where P is the power of the incident light and rat hitting a dielectric sphere, r are the angles of

incidence and refraction, R and T are the Fresnel reflection and transmission coefficients, Q is the

dimensionless efficiency, nm is the index of refraction of the suspending medium and c is the speed of

light.

The trapping could be better explained through a schematic diagram as shown in Figure 3. Where a

particle encounters the laser beam will be pushed towards the center of the beam, if the particle's

index of refraction is higher than that of the surrounding medium. Light is deflected in the particle,

resulting in a gradient force that pushes the particle vertically to the propagation of the laser beam,

towards the largest intensity of light (the middle of the laser beam). By focussing the light, the

gradient force pushes the particle backwards as well. If this force overcomes the propagation force of

the laser beam, the particle is trapped.

In conclusion, refraction of light when it is being transmitted through the micron-size glass spheres

causes the gradient forces that allow trapping to occur.

BY Steve Lee Woei Ming

O-SING Issue 3 Page 5 of 12 AUGUST 26, 2004

Disclaimer: O-SING is published by the Student Chapter OSA(NTU), School of Electrical and Electronic Engineering, Nanyang Technological University. Block S1, Nanyang

Avenue, Singapore 639798. Opinions expressed herein do not necessarily reflect the views of the University.

Copyright c 2003 Student Chapter OSA(NTU), All rights reserved. Material in this publication may not be reproduced without written permission.


FOOD!!! FUN!!! SATISFACTION!!!

The most recent OSA (NTU) student Chapter trip took place across the borders of Singapore and

Malaysia. The OSA Student Chapter set foot in State of Johor in Malaysia.

Our trip started off from Kranji Mrt Station, where we boarded a chartered private van from

Singapore to Malaysia. The ride across the causeway was filled with excitement and morning daze as

many of us stayed up late to catch the Olympic telecast on TV from the previous night. Upon

reaching the customs office, we had a smooth ride to our breakfast destination. We reached a famous

shop in Johor Jaya where they serve delicious Indian ROTI PRATA and malay-style NASI LEMAK.

To many of us, breakfast was the time where we warmed up our stomach for the brand new day

before we engaged in more culinary delights for lunch, dinner and dessert.

After breakfast, we headed towards Kota Tinggi for a glimpse of the wilderness of Malaysia. We

embarked on our own little trek up the famous Kota Tingi Waterfall in Johor.

We Attempted to scale the waterfall Crossed the streams Scaling a new waterfall peak.

The waterfall was great fun as we attempt to scale the side of the waterfall to search for the water

source. A JOB FOR US….SCIENTISTS IN THE MAKING!!!!

The climb was full of sweat but fun. Most of us completed the climb with great satisfaction before

heading for LUNCH TIME….CHINESE “DIM SUM”!!!!

O-SING Issue 3 Page 6 of 12 AUGUST 26, 2004

Disclaimer: O-SING is published by the Student Chapter OSA(NTU), School of Electrical and Electronic Engineering, Nanyang Technological University. Block S1, Nanyang

Avenue, Singapore 639798. Opinions expressed herein do not necessarily reflect the views of the University.

Copyright c 2003 Student Chapter OSA(NTU), All rights reserved. Material in this publication may not be reproduced without written permission.


DIM SUM TIME!!!!

Dim Sum provided our much needed energy intake

after our arduous climb at Kota Tinggi waterfall. We

were extremely delighted to have such a fantastic

lunch-break before we progressed to our next dining

area which is around 40 kilometers away from our

lunching location. As our dining place is just beside

the Johor Singapore Channel, we needed much

energy to help overcome our long ride.

After around two hours of traveling, we reached our

dining location, which is just beside the channel and

built on water. The people staying in this place are

ORANG LAUT (Sea people in Malay). We entered

the restaurant and went straight to the KELONG

(Fish Breeding on sea). At the Kelong, we can select

the types of seafood that we would like to consume as

for dinner. The seafood was extremely fresh as they

were still alive when we were choosing them.

KELONG AREA!!!! SELECTING OUR OUR CHEF AT

DINNER!!! WORK !!!

GREAT FOOD!!!! FINAL SCOOP OF FRESH FISH!!!

After our sumptuous dinner, we moved towards our final destination in Johor Bahru before

proceeding back home. We went to the TREE-TOP, it is a great place to hang out, the restaurant is

built around two large trees. There, we had our dessert and enjoyed some contemporary Indonesia

music.

O-SING Issue 3 Page 7 of 12 AUGUST 26, 2004

Disclaimer: O-SING is published by the Student Chapter OSA(NTU), School of Electrical and Electronic Engineering, Nanyang Technological University. Block S1, Nanyang

Avenue, Singapore 639798. Opinions expressed herein do not necessarily reflect the views of the University.

Copyright c 2003 Student Chapter OSA(NTU), All rights reserved. Material in this publication may not be reproduced without written permission.


GREAT MUSIC !!!! POSING AT THE TREETOP!!!

The “treetop” was a great place to relax and chill out after a week of hard work in lab. This is the

prefect place for us to fine tune our sanity back in before heading for a new week of work.

AN AMAZING SUNSET AT THE KELONG FOR US TO COME TO TERMS WITH MOTHER

NATURE !!!!

BY Steve Lee Woei Ming

O-SING Issue 3 Page 8 of 12 AUGUST 26, 2004

Disclaimer: O-SING is published by the Student Chapter OSA(NTU), School of Electrical and Electronic Engineering, Nanyang Technological University. Block S1, Nanyang

Avenue, Singapore 639798. Opinions expressed herein do not necessarily reflect the views of the University.

Copyright c 2003 Student Chapter OSA(NTU), All rights reserved. Material in this publication may not be reproduced without written permission.


Excerpt from: Kuang-An Chang, Ho-Joon Lim and Chin B Su, “A fibre optic Fresnel ratio meter for

measurements of solute concentration and refractive index change in fluids”, Meas. Sci. Technol. 13

(2002) 1962–1965.

Here, we introduce a new normalization technique that greatly enhances resolution when applied to

conventional fibre optic reflectometry that is based on Fresnel reflection from the fibre tip. The

overall technique provides accuracy, simplicity, near non-invasiveness because of the small

dimension of the optical fibre, high spatial resolution of less than 10 µm and almost unlimited

temporal resolution.

Figure 1. The experimental setup for concentrationmeasurement. The light source is a 1.3 µm wavelength

multi-longitudinal diode laser.

Figure 2. Two typical pairs of reflectance pulses from the fluid–fibre and air–fibre interfaces for 0 and 3%

salt concentrations.

O-SING Issue 3 Page 9 of 12 AUGUST 26, 2004

Disclaimer: O-SING is published by the Student Chapter OSA(NTU), School of Electrical and Electronic Engineering, Nanyang Technological University. Block S1, Nanyang

Avenue, Singapore 639798. Opinions expressed herein do not necessarily reflect the views of the University.

Copyright c 2003 Student Chapter OSA(NTU), All rights reserved. Material in this publication may not be reproduced without written permission.


The idea of this technique is to measure the reflected optical power due to Fresnel reflection from the

fibre–fluid interface. A change in the reflected optical power measures the change in the refractive

index of the fluid. However, to increase measurement resolution, the reflected power is normalized

by a similar reflected power from a reference fibre–air interface, with both reflected signals derived

from the same optical source, and detected by a common detector. The normalization procedure

corrects for fluctuations that are common to the signal and reference arms. The idea mentioned

above was implemented by the experimental setup shown in figure 1. A train of optical pulses,

derived from a diode laser driven by a pulse generator, is launched into a standard

telecommunications graded singlemode optical fibre (SMF-28 fibre). The optical pulse is divided into

two paths by a 2×2 single-mode fused fibre couplerwith a nominal 50%:50% splitting ratio. The end

of the signal fibre is immersed in the test fluid while the end of the reference fibre rests free in air.

Returned pulses due to Fresnel reflections from both fibre ends are detected by a dc-coupled detector.

The pulses are subsequently amplified and processed by a computer. The experiment is conducted in

the pulse mode so that the reflected power from the signal and reference fibres can be temporally

separated. Thus, the signal path consists of only a few metres of fibre while the reference path is a

spool of long fibre. The length of the long fibre used is arbitrary as long as the returned signal and

reference pulses can be temporally separated. The long fibre length is not a problem because the

optical wavguide effect of the fibre remains intact independent of the fibre length, unless the fibre is

excessively bent during the course of the experiment causing radiation loss. A pair of time-separated

optical pulses reflected from the fibre–fluid and fibre–air interface is shown in figure 2. It is

important to note that both pulses are derived from a common optical pulse but delayed in time by

2L/c, where L is the length difference between the two optical paths and c is the speed of light in the

fibre. This assures that any pulse amplitude fluctuation will not limit the measurement precision. The

first pulsewhich represents the overlay of two reflected pulses from the fibre–fluid interface with two

different NaCl concentrations of 0 and 0.03 g cm−3 (3% salt) shows the amplitude dependence on

solute concentration. The second pulse is the reference pulse. It is noted that, unlike the second pulse,

the first pulse does not have a flat top. This is due to the additional contribution originating from

backscattered Rayleigh light from the reference pulse travelling in the long fibre. However, the

Rayleigh backscattered light does not affect the differential index measurement because its

magnitude is proportional to the laser power which is corrected by this normalization technique. The

amplitude of each pulse is made almost equal by adjusting the splice loss while splicing the reference

arm to the fibre coupler when the tip of the signal arm is immersed in pure water. This gives the

reference value for the amplitude ratio. Any deviation from this ratio is indicative of an index change.

However, any NaCl concentration can be taken as the reference ratio.

The technique gives a refractive index measurement resolution of about 2 × 10−5 and salt

concentration resolution of about 2 × 10−4 g cm−3 limited by temperature and polarization effects.

This ratio meter should be appropriate for various measurements, in particular for measurements of

the spatially and temporally dependent concentration field in an environment such as a water tank.

From: Benjamin Tay Chia Meng

O-SING Issue 3 Page 10 of 12 AUGUST 26, 2004

Disclaimer: O-SING is published by the Student Chapter OSA(NTU), School of Electrical and Electronic Engineering, Nanyang Technological University. Block S1, Nanyang

Avenue, Singapore 639798. Opinions expressed herein do not necessarily reflect the views of the University.

Copyright c 2003 Student Chapter OSA(NTU), All rights reserved. Material in this publication may not be reproduced without written permission.


The first overseas conference for any PhD student is always a scary prospect, presenting your work

in front of all the top researchers in your field while being a greenhorn is not that appealing for a

student’s holiday trip. But I suppose it is a part of any graduate student’s life. This time, my time has

come, I was summoned to the far ends of the world….USA. The air travel was a great drag as I took

around 20 hours from Singapore to Denver on two transits and three airports at Japan, Los Angeles

and finally Denver.

Once I reached Denver, it was a direct trip to my hotel room to relax myself. I checked into the

WARWICK Hotel situated on Grant Street in Denver Downtown taking the SKYShuttle from the

airport at a cost of USD$18 per trip.

My BEDS ARE WAITING FOR ME A VERY LARGE BATHROOM

After night’s sleep, it was TREASURE HUNT time where I have to search for the convention

centre in the whole of Denver Downtown. Luckily I got there in the nick of time.

Capital Hill, Denver 16th Street, California St. A old USA building before reaching

the convention centre

THE DENVER’s

Convention Centre

O-SING Issue 3 Page 11 of 12 AUGUST 26, 2004

Disclaimer: O-SING is published by the Student Chapter OSA(NTU), School of Electrical and Electronic Engineering, Nanyang Technological University. Block S1, Nanyang

Avenue, Singapore 639798. Opinions expressed herein do not necessarily reflect the views of the University.

Copyright c 2003 Student Chapter OSA(NTU), All rights reserved. Material in this publication may not be reproduced without written permission.


Me presenting my work!!!! ME again presenting my work!!!

Then came my time to present my work in the place where some of the top researchers were

present, at first I am kind of nervous of saying the wrong time but as I talked more about my

work….my nervousness was dispelled eventually. No longer was I scare…infact I am became

confident about my work. After my talk, Professor Masuhara from Japan complimented my talk

by saying that , “It was interesting”. While Professor Kishan from UK came and complimented

on my work and discussed some ideas that he had in mind for my work, it was rewarding and

fulfilling for me to have the chance to hear some feedback from these top researchers in the field

of optical trapping. What more can I say?! Concentrate on the physics and all the trouble will be

dispelled.

Attending a conference is also about meeting old friends and making new ones. I had the pleasure

of meeting some of the most eminent scientists in the conference. One particular guy was Graham

Wright from UK. He was a really enjoyable guy to hang out with when we went for a late drink or

for dinner.

The trip started to be a bit daunting for me but ended with great satisfaction.

A composite photo of the friends I meet in Denver…I’M LOVING IT!!

BY Steve Lee Woei Ming

O-SING Issue 3 Page 12 of 12 AUGUST 26, 2004

Disclaimer: O-SING is published by the Student Chapter OSA(NTU), School of Electrical and Electronic Engineering, Nanyang Technological University. Block S1, Nanyang

Avenue, Singapore 639798. Opinions expressed herein do not necessarily reflect the views of the University.

Copyright c 2003 Student Chapter OSA(NTU), All rights reserved. Material in this publication may not be reproduced without written permission.

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