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Photonics Engineering Prof. Hosoeng Kim Optical Signal & Laser Application Lab. Chung-Ang University

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Photonics Engineering

Prof. Hosoeng Kim

Optical Signal & Laser Application Lab.

Chung-Ang University

광응용공학 (PHOTONICS ENGINEERING)

담당교수 (Instructor) 김호성 (Kim Hoseong) 5292 http://prof.cau.ac.kr/~laser 010-2514-5292

강의시간 / 강의실 (Room/Time) 207 관 ( 봅스트홀 ) 604 강의실 월 5,6 / 수 6

상담가능시간 (Office Hour for Advice) 월 10-11 E-mail [email protected]

과목 개요 (Course Description)

This course introduces light applications such as optical communication and Flat Panel Display to

understand these applications, this course covers properties of light, lenses , optical systems, light

sources including lasers, Detectors and Fiber optics.

수업 목표 (Course Goals)

Understanding the optical engineering and improve design skill.

강의 진행 방식

Power Point, Demonstration, Design Project (optical receiver design and simulation)

학습 평가 방법 Homework 10%, Midterm 25%, Project 30%, Final 35%

기타 안내 사항

It is not necessary to buy two text books but it is recommended strongly to buy one of two.

Slides of PowerPoint can be dowloaded from my homepage.

교재 : Optics Hecht, Optoelectronics and Photonics Kasap (2001)

주별강의 내용 / 과제 (Weekly Course Schedule/Assignments)

1 주차 강의내용 (1 week Topic(s)) 1.Propagation of light.

2 주차 강의내용 (2 week Topic(s)) 1.Propagation of light.

3 주차 강의내용 (3 week Topic(s)) 2.Lenses and aberrations

4 주차 강의내용 (4 week Topic(s)) 2.Lenses and aberrations

5 주차 강의내용 (5 week Topic(s)) 3.Simple optical instruments.

6 주차 강의내용 (6 week Topic(s)) 4.Detectors.

7 주차 강의내용 (7 week Topic(s)) 5.Light modulators.

8 주차 강의내용 (8 week Topic(s)) 6.Illuminators and condensers.

9 주차 강의내용 (9 week Topic(s)) 7.Lasers.

10 주차 강의내용 (10 week Topic(s)) 7.Lasers.

11 주차 강의내용 (11 week Topic(s)) 8.Diffraction theory.

12 주차 강의내용 (12 week Topic(s)) 9.Interference.

13 주차 강의내용 (13 week Topic(s)) 10. Optical communication

14 주차 강의내용 (14 week Topic(s)) 11. Displays

15 주차 강의내용 (15 week Topic(s)) 12. Holography, and 3D Display

16 주차 강의내용 (16 week Topic(s)) Exam

Prof. Hosoeng Kim(1)• 1976 년 2 월 경복고등학교 졸업 ( 마지막 고교입시 ) • 1976 년 3 월 서울대학교 전기공학과 입학 English Discussion 동아리• 1977 년 겨울방학 AFKN 받아 쓰기• 1978 년 9 월 English Discussion 동아리 회장• 1979 년 3 월 서울시 성수동에서 노동 야학 10 월 박정희 서거 운동권 ? 취업 ? 진학 ? 군입대 ?• 1980 년 2 월 동 대학 전기공학과 학사 3 월 동 대학 전기공학과 대학원 입학 5 월 광주사태 , 휴교령 9 월 개학 , 공부에 몰두함 박사 및 교수를 목표로 함• 1982 년 2 월 동 대학 전기공학과 석사

Prof. Hosoeng Kim(2)• 1982 년 3 월 -8 월 석사장교 소위 임관 및 전역• 1983 년 3 월 -1983 년 8 월 서울대 박사과정• 1983 년 9 월 - 1986 년 8 월 금성전기기술연구소 주임연구원 ( 연구소장 권유 ) 3 년 후 유학보장 Low Noise Amplifier and downconvertor for satellite communication

• 1984 년 4 월 결혼 , 86 년 장녀 출생• 1986 년 9 월 - 1988 년 8 월 ( 새로운 학문을 , 내 돈으

로… ) State University of NY at Buffalo 공학석사 • 1988 년 9 월 - 1992 년 8 월 동 대학 공학박사 “Laser Applications to Superconducting Thin Film Deposition and

Laser Drilling”• 1993 년 3 월 - 현재 중앙대학교 전자전기공학부 교수 • Laser Applications : Laser Metrology, MOEMS,

Light sensors, Optical system design, circuits…..

What One’s Future Will Be• 성공은 능력이 아니라 의지에 달려 있다 .• 4 학년의 고민 - 진로 : 취업 ( 중앙대생이라면 언제라도 가능 ) 진학 : 본교 , 유학 , 석사 , 박사 ? ( 돈 , 머리 , 체력 . 의지는 기본 )• 꿈은 이루어진다 !!!• 배우자 고르기 (controllable, observable)• CEO 되기 : 지성인 ? 지식인 인가 ?

너 자신을 알라 !!!

Laser

Light Amplification by Stimulated Emission of Radiation

(lase (v))

FeaturesMonochromatism

CoherenceSmall divergenceUltra short pulse

Monochromatism (single frequency or color)

• Very narrow bandwidth (linewidth) ex) stabilized HeNe laser: λ=632.8 nm f=4.74x1014

Hz ∆λ= 10-6(nm), ∆f=1 MHz• Very high quality factor: ~108

• Immune to chromatic aberration

• Very small focus: a few micrometer diameter• Application to Interferometer.

RedBlue

Coherence( 可干涉性 , 결맞음 ) • The spatial and temporal phase variation of the

electric field of the two waves are the same. • Characteristics of stimulated emission.• Spatial coherence, temporal coherence

• Holography, Interference, Speckles.

Small Divergence• Divergence: The bending of rays away from each

other or the spreading of a laser beam with increased distance from the exit aperture.

θ

• Divergence angle of typical HeNe laser 1 mr = 0.0573o • Alignment, distance measurements (earth to

moon)• Weapon guiding : Gulf War• Tight focusing• 800 m in diameter on the moon (400,000 km from

earth) Apollo 11

Ultra Short Pulse• ∆t = 10 fs ~ us

- λ=3 x 108 x 10 x 10-15 = 3 x 10-6 m = 3 μm

- Diameter of human hair : 100 μm• Ultra high intensity(=power density W/cm2)

-1 mJ, 10 ps, on 10-4 cm2 I > 1012 W/cm2

• Laser drilling, cutting, welding• Laser Weapon

- SDI (Strategic Defense Initiative), MD

Chapter 1Basic Properties of Light

Light is described using 3 pictures. Waves PhotonsRays

seemingly contradictory!Waves

[Reading Assignment: Hecht, Chapter 2 (most of this should be review), 3.2, 3.3, 3.4.4, 3.5, 3.6]

A propagating “disturbance” in electric and magnetic field (simultaneously!)

Example:

E(V/m)

Z(m)

A

-A

O

∙T0 ∙

T1 ∙T2 ∙

T3 ∙T4 ∙

T5

• At a fixed point in space, the electric field oscillates in time. At a fixed point in time, we see a wave train frozen.

• This is called a plane-wave because the field is constant everywhere in the x-y plane at a given z.

2ˆ cos 2xE Ea z ft

2ˆ cos 2yH Ha z ft

Another way to draw the plane wave is

“wave-fronts” surface of constant “phase” or“phase-fronts”

• The wavefront advances by a distance λ, in a time 1/f.

One of the many remarkable properties of light is it’s universal, constant speed:

So the velocity is υ = distance/time = λf.

• The physics of electromagnetic (EM) wave propagation is valid for arbitrary λ, f. On Earth, we can generate, manipulate and/or detect EM waves with wavelength from ~100 km all the way down to ~ 10-6Ǻ. Usually we describe light by wavelength rather then frequency, except in the microwave and radio regions.

• The electromagnetic spectrum encompasses the complete range of frequency/wavelength. Different regions have different names. Radio, microwave, infrared, visible, ultraviolet, x-ray, γ -ray.

Index of Refraction

• When light travels in materials, the speed is modified:

Usually n ≥ 1. (It can be < 1)

• The reason is that the electric field shakes the electrons, which tends to drag the field.

• Plane wave still has the same form:2

ˆ cos 2xE Ea z ft

But the effective wavelength becomes modified by n.

If we define the vacuum wavelength, λ vac=c/f , then in the material, .vacc

nf n

The wavelength becomes shorter, if n ≥ 1.

RedBlue

Dispersion

The index of refraction in most materials depends on wavelength:

In air – the index depends also on air pressure, humidity, and temperature which leads to many beautiful atmospheric effects.

cf

n

- Waves of the component frequencies travel with different velocities, causing a distortion in the signal wave shape.

- This phenomenon is called dispersion.

- All information-bearing signals consist of a band of frequencies.

Wavelength units (length)

Visible light: 4000~7000 Ǻ, 400 ~700 nm, 0.4 ~0.7 μm

Old New Frequency Ranges (GHz)

Ka K 26.5-40

K K 20-26.5

K J 18-20

Ku J 12.4-18

X J 10-12.4

X I 8-10

C H 6-8

C G 4-6

S F 3-4

S E 2-3

L D 1-2

UHF C 0.5-1

Spherical Waves

Another type of ideal light wave. Constant phase fronts are circular, emanating from a point source. Far away from the source, the radius of the circle becomes so large that we can approximate the wave as a plane wave.

For spherical waves, we have

2ˆ. plane wave cos 2xcf E Ea z ft

0

2cos 2r ft

E Er

Huygens’ PrincipleVery useful model for wave propagation.• Every point on a wavefront is regarded as a secondary point source generating a spherical wavelet.

• The advance of the wave front is found at the envelope of all these wavelets.

• Generally, this seems to give parallel wavefronts. But things get interesting at edges. This leads to diffraction (more later).

Rays• Follow a point on the wavefront. As the wavefront advances the point traces a straight line. This is a ray of light.

• For many cases, we can forget the waves and just trace rays in optical systems. This allows a vast simplification of our analysis and design processes. Virtually all optical design is done with rays. Highly sophisticated optical design CAD programs are available for ray tracing.

Photons (light “particles”)

• This picture has light represented by tiny bundles of energy (or quanta), following straight line paths along the rays.

• The coexistence of electromagnetic wave physics and photon physics is the central paradox of quantum mechanics.

• One photon has an energy given by

E = hν, ν : frequency in Hz

h = 6.62 × 10-34 J-s : plank’s constant

1 W = 1 J per second

For 2 eV visible photons,

1 W = 6.3 × 1018 eV/s = 3.15 × 1018 photons/sec

Photoelectric Effect Novel Prize to Einstein

20

1

2m eV

0 0( )eV h

0h 21

2h m

Interaction of Radiation and Matters

Stimulated absorption

Spontaneous emission• random phase, polarization

Stimulated emission• Same Phase• Same polarization• Laser!!

∆E = hv ∆E = hv

∆E = hv

1

2

• Atoms have energy states corresponding to electron orbits.• One atom “jumps” from a higher energy state to a lower energy state and emits one photon.

• Photons are not point particles. They have a wave-like property. A useful picture is the wave-packet.

Many photon packets can be thought of as superimposing to make up a plane wave, spherical wave or any other

The typical photon energy unit is the electron-Volt. This is defined as the energy required to push oneelectron across a one-Volt potential,

1eV= 1.6 ×10-19 J

Typical visible photon energy : 1.2 ~ 2.3 eV

Reflection and Refraction, Snell’s Law

[Reading assignment: Hecht 4.3, 4.4, 4.7]

An important element of optics is the interface between two materials with different index of refraction.

Total ReflectionIf n1 > n2, then we can have

The refracted ray disappears! The light is totally reflected. This usually occurs inside a prism , and iscalled total internal reflection. θC = “critical angle”. For a typical glass with n = 1.5, the critical angle is:

θC = 20.9o

Light Impinging at a Surface [Cheng p411, 8-10.2,8-10.3]The plane containing the light ray propagation vector and the surface normal is called the “plane of incidence”

• For a general polarization state incident on a surface, we choose s and p directions to decompose the polarization effects.

Fresnel Reflection Coefficients

Near θ1 = 0,2

2 1

2 1s p

n nR R

n n

: Fresnel Loss

1 2

2

2 1

4s p

n nT T

n n

1 2If / 2, tan , and 0pR No reflection for p-polarization wave!

1 21

1

tanB

n

n

Brewster angle or Polarizing angle. [Hecht 8.6]

Polarization [Cheng p364, Hecht 8.1, 8.2]

• Light waves have transverse polarization.

• The electric field vector points in a direction perpendicular to the propagation direction (ray direction).

• The magnetic field vector is orthogonal to propagation direction. Generally, we can ignore the magnetic field.

• The E-field vector can lie anywhere in transverse plane

Polarization State• The e-field oscillates in time at a given point in space• For light wave propagating in z-direction, let’s look in the x-y plane.

AM: linear polarization, with E field perpendicular to the ground TV: linear polarization, with E field parallel to the ground FM: circular polarization with the surface perpendicular to the ground

- Set z=0, 1 2

10 20

(0, ) (0, ) (0, )

cos( ) sin( ).

x y

x y

E t a E t a E t

a E t a E t

- Analytically, 1 2

10 20

(0, ) (0, )cos( ) , sin( ) ,

E t E tt t

E E

- Which leads to the following equation for an ellipse:

2 2

1 2

10 20

(0, ) (0, )1.

E t E t

E E

- Elliptically polarized if

- Circularly polarized if

10 20E E10 20E E

0

y

xE1

ωE(0,t)

<Circular polarization>

α

10 20( , ) cos( ) cos( ).2x yE z t a E t kz a E t kz

E2

1 2 10 20( ) ( ) ( ) ,jkz jkzx y x yE z a E z a E z a E e a jE e

Polarizers are devices which select one polarization.• Polarizing sheet has an allowed direction– transmits polarization component of incident light along the allowed direction– transmitted light is linearly polarized

• Polarizing beamsplitter (can be used to analyze inputpolarization state)

• Polaroid sunglasses are filter out the light of which E-field is parallel to the ground since .R R

1

2

B

i

RsR R

pR Rs pR R