1 Nonlinear Optics Lab Hanyang Univ 1 LASER

1. 레이저 기초 이론 Nonlinear Optics Lab. Hanyang Univ.

1) 역사 LASER : Light Amplification by the Stimulated Emission of Radiation 1916, A. Einstein : predicted stimulated emission(유도 방출) 1954, C. H. Townes et al. : developed a MASER(메이저) 1958, A. Schawlow, C. H. Townes : adapted the principle of MASER to light 1960, T. H. Maiman : Ruby laser @ 694. 3 nm 1961, A. Javan : He-Ne laser @ 1. 15 mm, 632. 8 nm … Nonlinear Optics Lab. Hanyang Univ.

2) 아인슈타인의 복사 양자론 E 2 E 1 [light-matter interaction] (stimulated) absorption spontaneous emission stimulated emission B 12 N 1 r A 21 N 2 B 21 N 2 r [radiative processes] * N 1, N 2 : No. of atoms at E 1, E 2 * r : photon density * A 21=1/t 21 : spontaneous emission rate * B 12, B 21 : stimulated absorption/emission coefficients Nonlinear Optics Lab. Hanyang Univ.

자발 방출과 유도 방출 Spontaneous emission Stimulated emission Phase and propagation direction of created photon is random. Created photon has the same phase, frequency, polarization, and propagation direction as the input photon. Nonlinear Optics Lab. Hanyang Univ.

아인슈타인의 A, B 상수 Rate equation : (thermal equilibrium) (Boltzman distribution of atoms) (Planck’s blackbody radiation law) (population inversion) Light amplification ! (Lasing) Nonlinear Optics Lab. Hanyang Univ.

3) 레이저의 기본 구조 및 3 요소 resonator mirrors active medium laser pumping source relaxation laser pumping relaxation Nonlinear Optics Lab. Hanyang Univ.

1) Pumping source (펌핑 소스) - Optical : Nd-YAG, Ruby, Dye, Ti: sapphire, … - Electrical : He-Ne, Ar+, CO 2, N 2, LD, … - Chemical : HF, I 2, … 2) Active medium (이득 매질) - Gas : He-Ne, Ar+, CO 2, N 2, … - Liquid : Dye - Solid : Nd-YAG, Ruby, Ti: sapphire, LD, … 3) Resonator (공진기) - Total reflector / Output coupler - Maximizing the light amplification / Extracting laser light - Resonance condition : ml/2=L (m: integer) Nonlinear Optics Lab. Hanyang Univ.

4) 레이저 광의 특성 Monochromaticity (단색성) - Linewidth(FWHM) : 7. 5 k. Hz (He-Ne laser) << 940 MHz (low pressure Cd lamp) Coherence (결맞음) - Definite phase correlation in the radiation field at different locations(spatial) and different times(temporal) Directionality (지향성) - Divergence angle : f=1. 27 l/D < q=2. 44 l/D (diffraction limit angle) Brightness (높은 휘도) - Radiance : 106 W/cm 2 -sr (4 m. W, He-Ne laser) << 250 W/cm 2 -sr (super-high-pressure Hg lamp) Focusability (집속도) - Focusing diameter : d ~ f f Nonlinear Optics Lab. Hanyang Univ.

5) 레이저 주파수 안정화 Resonance condition for laser oscillation ; => ex) n=6 x 1014 Hz (Ar+ laser), d=1 m, 1 MHz stabilization => => Frequency instabilities - Long-term drift : temperature, gas pressure - Short-term fluctuation : acoustic vibration, fluctuation of medium Nonlinear Optics Lab. Hanyang Univ.

주파수 안정화 장치 (active nd control) 1) Reference frequency (기준 주파수) - FP interferometer’s transmission curve - Atomic/molecular transition line profile - Reference(Master) laser 2) Frequency discrimination (주파수 분별) : nd l 3) Electronic control system (전자조절 장치) : PID servo Nonlinear Optics Lab. Hanyang Univ.

예) FP 공진기를 이용한 주파수 안정화 Nonlinear Optics Lab. Hanyang Univ.

6) 초단 펄스광 생성 (모드 록킹) Mode-locking : controlling the many oscillating modes to have definite phase relation (practically, Df=0) each other by optical modulation. Mode-locking methods : - Active type : Pockel’s cell, AO modulator, EO modulator - Passive type : Saturable absorbers <Active mode-locking by an AO modulator> Nonlinear Optics Lab. Hanyang Univ.

Mode-locked pulse Modulator’s transmission : where, : round-trip angular frequency Field amplitude of k-th mode : (k+1)-th mode (k-1)-th mode Number of modes : Nonlinear Optics Lab. Hanyang Univ.

Total amplitude : where, Total intensity of laser : Nonlinear Optics Lab. Hanyang Univ.

Mode-locking laser system and mode locking pulse Pulse width : 2 d/c. N 2 d/c fs Ti: sapphire laser system Mode-locked pulse Nonlinear Optics Lab. Hanyang Univ.

2. 레이저 광속의 전파 Nonlinear Optics Lab. Hanyang Univ.

1) 레이저 광속의 공간적 분포 (가우스 광속) Maxwell’s curl equations => Put, : Scalar wave equation (monochromatic wave) => Helmholtz equation : Assume, => Put, => Nonlinear Optics Lab. Hanyang Univ.

=> v is must be a complex ! => Assume, => put, ( is pure imaginary. : real) At z = z 0, v Beam radius at z=0, : Beam Waist at arbitrary z, : Complex beam radius => Nonlinear Optics Lab. Hanyang Univ.

=> => Nonlinear Optics Lab. Hanyang Univ.

Wave field where, : Beam radius : Radius of curvature of the wave front : Confocal parameter(2 z 0) or Rayleigh range Nonlinear Optics Lab. Hanyang Univ.

Gaussian beam spread angle : I Gaussian profile Near field (~ plane wave) Far field (~ spherical wave) Nonlinear Optics Lab. Hanyang Univ.

2) 가우스 광속의 전파 - ABCD law Matrix method (Ray optics) yi ai optical elements ao yo : ray-transfer matrix Nonlinear Optics Lab. Hanyang Univ.

Ray-transfer matrices 1) Free space q r 1 z 1 d 2) Refracting surface R n 1 q 1 r 2 z 2 r 2 = r 1 + qd q : constant (paraxial ray approximation) s r n 2 q 2 s’ n 1/s + n 2/s’ = (n 2 -n 1)/R r : constant q 2 = q 1 n 1/n 2 – (1 - n 1/n 2) (r 1/R) … Nonlinear Optics Lab. Hanyang Univ.

Nonlinear Optics Lab. Hanyang Univ.

Nonlinear Optics Lab. Hanyang Univ.

가우스 광속에 대한 ABCD 법칙 optical system ABCD law for Gaussian beam : Nonlinear Optics Lab. Hanyang Univ.

예) 가우스 광속의 집속 ? q 1 ? Nonlinear Optics Lab. Hanyang Univ.

- If a strong positive lens is used ; => : f-number => ; The smaller the f# fo the lens, the smaller the beam waist at the focused spot. - If => Note) To satisfy this condition, the beam is expanded before being focused. Nonlinear Optics Lab. Hanyang Univ.

Selecting beam-shaping optics for optimum propagation Nonlinear Optics Lab. Hanyang Univ.

If the optics has a diameter of , nearly 99% of the beam gets through. But, diffraction effects caused by sharp-edged circular aperture produce ripple effects on intensity patterns in near field and a reduction of on-axis intensity in the far field of about 17%. => Nonlinear Optics Lab. Hanyang Univ.

3. 비선형 광학 Nonlinear Optics Lab. Hanyang Univ.

1) 비선형 분극과 파동 방정식 Polarization(분극) of medium : linear 2 nd order 3 rd order - Mostly, the 1 st linear term is dominant, and the higher order terms are negligible : linear medium - In some medium, the 2 nd or 3 rd order term is comparable to the 1 st term : nonlinear medium - Invention of Laser high intensity : easy to observe the nonlinear effects Wave equation : new wave source term Nonlinear Optics Lab. Hanyang Univ.

2) 제 2차 비선형 효과 SH(second harmonics) SF(sum frequency) DF(difference frequency) OR(optical rectification) - SHG(second harmonic generation) - SFG(sum frequency generation) - DFG(difference frequency generation) - Optical rectification - Parametric oscillation - Electro-optic effect Nonlinear Optics Lab. Hanyang Univ.

2차 비선형 효과의 조건 1 : noncentrosymmetric medium 1. Centro symmetric media - Potential energy for electric dipole : Lorentz picture of an atom - Equation of motion : - Perturbation expansion method : Nonlinear Optics Lab. Hanyang Univ.

에 대한 계수는 0 : Centrosymmetric medium 에서 2차 비선형 효과는 발생하지 않음 2. Noncentrosymmetric media - Potential energy for electric dipole : - Equation of motion : Nonlinear Optics Lab. Hanyang Univ.

Nonlinear Optics Lab. Hanyang Univ.

2 nd order susceptibility : : SHG : SFG : DFG : OR Nonlinear Optics Lab. Hanyang Univ.

2차 비선형 효과의 조건 2 : Phase matching condition ex) 2 nd harmonic generation optic axis w w 2 w Wave equation : Total electric field : where, Nonlinear Optics Lab. Hanyang Univ.

2 nd harmonic intensity : optic axis qm Phase matching condition : 결정과 광의 편광이 특정 각도 조건을 만족해야 함. Nonlinear Optics Lab. Hanyang Univ.

2차 비선형 효과의 응용 - New wave generation : SHG, SFG, DFG - Parametric down conversion - Modulator, switch : electro-optic effect <Parametric down conversion> V output input polarizer electrooptic crystal l/4 wave plate polarizer <Electrooptic modulator> Nonlinear Optics Lab. Hanyang Univ.

3) 제 3차 비선형 효과 - THG(third harmonic generation) - Phase conjugate wave(위상공액파) generation - Self focusing - Saturated absorption - Optical bistability Nonlinear Optics Lab. Hanyang Univ.

Phase conjugate wave generation by four wave mixing(4광파 혼합) pump conjugate signal Phase matching condition : Nonlinear crystal pump Signal wave : where, Conjugate wave : where, wave front reversal beam direction reversal Nonlinear Optics Lab. Hanyang Univ.

위상공액파의 응용 Aberration correction Optical resonator mirror 2 6 1 5 9 3 4 7 8 Phase change : wave front … ordinary mirror PCM 2회 왕복 마다 위상 일치(공진기 길이에 무관) Nonlinear Optics Lab. Hanyang Univ.

Self focusing Optical bistability Self focusing Self defocusing bistability Nonlinear Optics Lab. Hanyang Univ.

4. 레이저 응용 첨단 연구 및 기술 Nonlinear Optics Lab. Hanyang Univ.

1) 레이저 응용 분야 Laser light-matter interaction - Material processing (레이저 가공 및 처리) - Laser induced fusion (레이저 핵융합) - Biomedical optics (의광학) - Advanced Laser spectroscopy (첨단 레이저 분광학) - Laser weapons (레이저 무기) - Environment (환경) / pollution (오염) … Laser information - Optical communication (광통신) - Holography (홀로그래피) - Optical memory / Display (광메모리 / 광디스플레이) - Optical computing (광컴퓨터) - Metrology (광계측) - Entertainments (문화/오락) - Printers/copiers (프린터/복사기) … Nonlinear Optics Lab. Hanyang Univ.

2) 레이저 가공 및 처리 레이저 가공의 장점 - Almost no limit in material selection Metal, wood, acryl, paper, rubber, textile, glass, … - Locality of processing - Noncontact processing High precision / Small sample / Minimizing the material change - Ease of access Remote control / Inaccessible / Harmful area -Cutting (절단) / Welding (용접) / Engraving (조판) / Making (마킹) / Scribing (줄긋기) / Surface hardening (담금질) Nonlinear Optics Lab. Hanyang Univ.

주요 가공용 레이저 - CO 2 laser - Nd: YAG laser - Diode laser Power -10 ~ 500 W engraver textile 용도와 재료에 따름 ex) - 10 W (비금속 마킹) - 60 W(금속 용접) - 50~ 500 W(절단) cutter marker Nonlinear Optics Lab. Hanyang Univ.

레이저 처리 - Crystallization(결정화) of amorphous Si : Short processing time / good crystal quality - Surface hardening (담금질) : Fast heating & cooling / only near the surface - Surface alloying (with auxiliary material) : Enhanced hardness / improved corrosion resistance / protection of material - Laser deposition (레이저 증착) : Applicable to compounds that are difficult or impossible to produce in thin-film form by other techniques. ex) complex ceramic materials (High Tc superconductors) magnetic materials (YIG, Ni-Mn-Ga, …) Nonlinear Optics Lab. Hanyang Univ.

핵융합용 레이저시스템 개발 Rawrence Rivermore National Laboratory : Nova - 10 beam lines, 45 k. J, 30 TW Rochester university : Omega - 60 beam lines, 45 k. J Osaka university : Gekko - 12 beam lines, 12 k. J target Nova Gekko (Osaka Univ. ) Target (Rochester Univ. ) Nonlinear Optics Lab. Hanyang Univ.

의광학 영상법 광결맞음 단층영상법 (optical coherence tomography, OCT) - 광원 : fs-Ti: sapphire laser(800 nm), SLD(superluminance diode, 800 nm/1300 nm) - 마이켈슨 간섭계의 원리 Nonlinear Optics Lab. Hanyang Univ.

레이저 임상치료 안과 망막 박리: Ruby laser(694 nm) 혈관 응고 : Ar+ laser(488, 514 nm) 안구 유리체 치료 : Ar+ laser(488, 514 nm), Kr+ laser(413 nm) 백내장, 녹내장(안압과 밀접) : Nd-YAG pulse laser(1. 064 mm) 라식 : Excimer laser(Ar. F : 193 nm) 피부과 사마귀, 티눈, 주름, 흉터, 수두자국, 모발 등 제거 : CO 2 laser(10. 6 mm) 모반(붉은색) : Copper vapor laser(577 nm), Pulsed Dye laser(585 nm) 모반(검은색) : Ar laser(514 nm), Nd: YAG laser(532 nm), Nd: YAG laser(1. 064 mm), CO 2 laser(10. 6 mm) 부인과 종양 : CO 2 laser(10. 6 mm) 자궁 질환 및 불임 : Nd: YAG laser(1. 064 mm) 신경외과 뇌종양 : Nd: YAG(1. 064 mm), Nd: YLF(1. 053 mm) => fs 레이저 치과 경조직(법랑질, 상아질) : Er: YAG(2. 94 mm) –열효과 최소화 미백 : LD(830 nm) – 활성산소 약품과 병행 Nonlinear Optics Lab. Hanyang Univ.

5) 첨단 레이저 분광학 EIT(electromagnetically induced transparency) - Optical pulse manipulation - Quantum information processing - Nonlinear optics - Single photon switching … Nonlinear Optics Lab. Hanyang Univ.

Laser cooling BEC - High resolution spectroscopy - BEC (Bose-Einstein Condensation) - Atomic optics / interferometry - Atomic layer deposition … well-collimated atom laser beam Nonlinear Optics Lab. Hanyang Univ.

6) 광통신 Compact : 전선에 비해 가볍고 작음 Low Cost : 같은 전송능력에서 전선보다 매우 저렴. No electrical connection (ground loop) No EMI (electromagnetic interference) Transmission capacity : 10 Gbps 상용, 100 Gbps 연구중 Long repeater spacing : 40 km 상용, 120 km 가능 (전선 : 12 km) Better security : tapping 불가 광통신의 원리 : TIR(total internal reflection, 내부전반사) q 2 q 1 (n 1 > n 2) cladding (n 2) q>qc 1 cladding (n 2) core (n 1) <광섬유(optical fiber) 기본 구조> critical angle (q 2=p/2) : qc 1= sin-1(n 2/n 1) Nonlinear Optics Lab. Hanyang Univ.

광통신의 제한점 Optoelectric conversion efficiency energy loss Difficulty in joining fiber cable : fusion splice, connector transmission loss Cable bending loss Relatively expensive components Difficulty in controlling optical signal 광자밴드갭(photonic bandgap)을 이용한 신개념 광섬유/광도파로 광결정 (photonic crystals) 광결정섬유와 도파로 Nonlinear Optics Lab. Hanyang Univ.

광통신용 레이저 광원 LD(laser diode) - Small size, high efficiency, integrability with electronic components - Ease of pumping and modulation by current injection - Spectral linewidth is larger than that of other lasers - Blue LD(Ga. N-series) - Ease of operating a single mode : DBR/DFB LD - Elliptical beam 신개념 PC VCSEL(surface emitting laser) H. K. Park, S. H. Kim, S. H. Kwon, Y. G. Ju, J. K. Yang, J. H. Baek, S. B. Kim, Y. H. Lee, Electrically driven single-cell photonic crystal laser, Science 305, 1444(2004). Nonlinear Optics Lab. Hanyang Univ.

7) 홀로그래피 (Holography) Hologram : whole message D. Gabor 1. 제작 2. 재생 mirror laser beam object virtual image film Nonlinear Optics Lab. Hanyang Univ.

홀로그래피의 응용 Display (3차원 영상, 입체 TV, Head up display, …) Interferometry (변형, 진동, 유속, 결함 측정, …) Holographic optical memory Pattern recognition Diffractive optical elements(DOE) 신용카드, 지폐(불법복제 방지) 광고, 장식, … 금속캔의 하중에 의한 응력분포 Nonlinear Optics Lab. Hanyang Univ.

Holographic optical memory Nonlinear Optics Lab. Hanyang Univ.

HDDS (holographic digital data storage) Photorefractive Crystal을 이용한 VHDDS(volume HDDS) - IBM Holographic data storage HM-100 -Holoplex Microoptics Lab. (한양대) Nonlinear Optics Lab. Hanyang Univ.

References 1) F. L. Pedrotti, S. J. , L. S. Pedrotti, “Introduction to optics”, Prentice Hall, New Jersey(1993). 2) W. Demtroder, “Laser spectroscopy”, Springer-Verlag, New York(1998). 3) R. W. Boyd, “Nonlinear optics”, Academic Press, Inc. , New York(1992). 4) B. E. A. Saleh, M. C. Teich, “Fundamentals of Photonics”, John Wiley & Sons, Inc. , New York(1991). 5) E. Kapon, “Semiconductor laser II”, Academic Press, Inc. , San Diego(1999). 6) A. Yariv, “Optical electronics in modern communications”, Oxford University Press, Oxford(1997). 7) H. K. Park, Y. H. Lee, et al. , Electrically driven single-cell photonic crystal laser, Science 305, 1444(2004). 8) 김법민, Ultrashort pulse laser를 이용한 생체조직의 제거와 미세수술, 광학과 기술, 7(2), 27(2003). 9) 이병하, 극초단 레이저를 이용한 고분해능 OCT 시스템, 광학과 기술, 7(2), 34(2003). 10) 김법민, 임현수, 생체조직광학과 레이저치료, 광학과 기술, 7(3), 6(2003). 11) 이병하, 오정택, 광학 이미징의 원리와 응용, 광학과 기술, 7(3), 14(2003). 12) 윤길원, 박승한, 강신원, 광기술을 이용한 의료/바이오 계측, 광학과 기술, 7(3), 27(2003). 13) 고동섭, 권혁제, 유용성, 김찬윤, 안광학 진단기기, 광학과 기술, 7(3), 36(2003). 14) 정세채, 생체 조직의 레이저 손상과 기작 연구 및 레이저 안전성 평가기술, 광학과 기술, 7(3), 45(2003). 15) http: //wed. edunet 4 u. net/~laser/미래의레이저/세계적개발현황. htm 16) http: //www. ehwaemc. com/ : 이화메카트로닉스 사 홈페이지 17) http: //www. k 2 laser. co. kr/scanhawk. htm : K 2 레이저시스템 사 홈페이지 18) http: //microoptics. hanyang. ac. kr/home/holography/ : 한양대 마이크로광학연구실 홈페이지 19) http: //www. holoplex. com : Holoplex 사 홈페이지 20) http: //www. ibm. com : IBM 사 홈페이지 Nonlinear Optics Lab. Hanyang Univ.