레이저응용광학의몇가지원리 - Holography - Optical memory - Short pulse generation - Nonlinear optics
홀로그래피 (Holography) Hologram : whole message - 1948, Dennis Gabor : 홀로그래피발명 (1972 년노벨상수상 ) - 1960, 미국 Michigan 대학, E. Leith, J. Upatnieks : 레이저이용, 투과형 off-axis 홀로그램. - 1960, 소련, Denisyuk : 반사형홀로그램 ( 슬릿이용 ), 밝은실내에서도가시화됨. - 1970, 미국 Benton : 무지개홀로그램 ( 백색광 ), 여러방향의모습을동시기록, 입체감이뛰어남. - 1977, 미국 Cross : 스테레오홀로그램 D. Gabor Denisyuk Benton
홀로그래피의원리 1. 사진 색과빛의세기를기록 깊이감이없음 2. 실물 film r 1 r 3 r 2 색과빛의세기및위상을감지 깊이감과시차를느낌 exp{ i( ω t r r k )}
홀로그램의제작및재생 ( 투과형 ) 1. 제작 2. 재생 mirror mirror laser beam object virtual image film 기준광과물체로부터의반사광을 간섭시켜그세기를기록하고현상 위상정보가기록됨. film 기준광만비추면홀로그램에회절된 광에의해물체의입체허상이보임.
홀로그램의종류 1. 투과형 2. 반사형
홀로그래피의응용 -Display (3차원영상, 입체 TV, Head up display, ) - Interferometry ( 변형, 진동, 유속, 결함측정, ) - Holographic optical memory - Pattern recognition - Diffractive optical elements(doe) - 신용카드, 지폐 ( 불법복제방지 ) - 광고, 장식, Holography 의예 This 10x8 inch two-step full color reflection holographic stereogram of an Acura NSX was made using a single laser wavelength (647 nm), and a single high-resolution photographic plate. Color was produced using the "In-Situ" color control technique developed by Spatial Imaging alum Julie Parker. Data courtesy of Alias Research and Honda R&D Company. Here is a large-scale one-step holographic stereogram. The film measures 1 x 1 meters, and the image volume occupies approximately 1 meter of depth. This hologram was produced in conjuction with research done for General Motors Corporation in 1991. It is composed of 1000 separate exposures, and displays a 45 degree viewing angle. 금속캔의하중에의한응력분포
광메모리 (optical memory) (Products) Capacity(Gbit/in 2 ) 10 2 10 1 10-1 CD ROM (650MB) PC(Phase change) MO(Magneto-Optic) Mixed 5.25 MO(1X) 5.25 MO(2X) DVD ROM (4.7GB*2) 1st DVD RAM 2.6GB*2 PD (650MB) MAMMOS ASMO CD ROM (650MB) HD DVD ~1.5GB*2 5.25 MO(8X) 2nd DVD RAM 4.7GB*2 IOPM 3rd DVD RAM ~15GB*2 Holography 3D multi-layer 1980 1990 2000 2010
1. CD(compact disc), DVD 종이, 마이크로필름, 오디오, 비디오테이프 자기테이프,
변형된부분인 pit 와원상태그대로인 land 에서의빛의반사도차이를감지 => 픽업
CD-RW
2. Holographic optical memory - CD/DVD보다수십배기록밀도높음 - 재료 : 광굴절무기단결정 (Photorefractive crystal) 광굴절고분자 (photorefractive polymer) 광변색성고분자 (photochromic polymer) 광이성화고분자 (Photoisomerization polymer) 홀버닝재료 (Spectral hole burning material) - 다중화 : 각도, 위상, 파장
Holographic data storage 514.5 nm Ar + ion laser PBS reference beam λ/2 WP shutter λ/2 WP image beam CCD camera BE SLM spatial filter polymer (PDR1) computer
Original images and reconstructed images in PDR1 film < Original Images > < Reconstructed Images >
HDDS (holographic digital data storage)
Input beam (signal arm) SLM Input beam (reference arm) Recording/Readout Head Signal Beam Lens of high NA θ s Reference Beam x z a Photorefractive Crystal 을이용한 VHDDS (Volume holographic digital data storage) -IBM Holographic Disk Axis of disk rotation Shift Mation Holographic data storage HM-100 -Holoplex z L CCD Output beam (reconstrnction arm)
Short pulse generation -Q switching - Mode locking 1. Q switching : sudden switching of the laser cavity Q from a low value to a high value Switching method - Rotating mirror - Electro-optic switching - Saturable absorber Pulse width ~ 10 ns full mirror active medium output coupler
Rate equations di dt dn dt ν Put, = cσ ( ν ) N2I ν 2 σ ( ν ) = N2Iν x = hν Iν chν N t cg t Γ I ν 21 y = N 2 N N 1 + K 2,, τ t 2 = cg t y y x x dx = ( y 1) x dτ xy τ
2. Mode locking : locking the phases of longitudinal modes by modulating the cavity Q Modulation method - Acousto-optic modulation - Electro-optic modulation - Saturable absorber modulator Pulse width ~ 10 fs full mirror active medium output coupler
Acousto-optic modulator : photoelastic effect 에의한공간적인굴절률변조 회절격자 입사광회절 acousto-optic cell 회절광 기준광 PZT(piezo-electric transducer) Electro-optic modulator : Electro-optic effect 에의한편광에따른굴절률변조 입사광의편광변화 편광기이용 AC polarizer
Mode locking pulse Electric filed of m-th longitudinal mode r E m ( z, t) = eˆ mem sin( kmz)sin( ω mt + φm) where, Assume, φ m = 0 k m π = m, ω m = k m c L Total electric field in the cavity ε (, ) 0 ( ) ( + ) E z t = { A ( z, t)cos k0( z ct) A ( z, t)cos k0( z )} 2 ct N N + where, N : number of modes
Mode-locked laser system and mode locking pulse Pulse width : 2L/cN fs Ti:sapphire laser system 2L/c Mode-locked pulse