Components of Optical Instruments

1. Components of Optical Instruments 2004. 4 Yongsik Lee

2. 7A General Design • Optical spectroscopic methods • Absorption (figure 7-1 a) • Fluorescence (figure 7-1b) • Phosphorescence • Scattering • Emission (figure 7-1c) • Chemiluminescence (figure 7-1c) • 5 Components • Stable source of radiant energy (figure 7-3a) • Transparent container (figure 7-2a) • Isolater for a restricted region of the spectrum (figure 7-2b) • Radiation detector (figure 7-3b) • Signal processor and readout

3. 7B Sources of Radiation • Two Types of sources • Continuum sourece • Line source • Lasers

4. Sources for spectroscopic instruments (Figure 7-3a)

5. Continuum source • Continuum sources • Deuterium lamp (D2) • Common for UV • High pressure, gas-filled arc lamp (Ar, Xe, Hg) • Intense source of UV, VIS • W filament lamp • Universally used for VIS • Inert solids heated at 1500-2000 K • Maximum blackbody radiation around 1.5-1.9 micrometer • For IR

6. Line Soureces • Hg, Na vapor lamp • Hollow cathode lamp • Electrodeless discharge lamp • Lasers

7. 7B-3 Laser Sources • Light amplification by stimulated emission of radiation • Characteristics • High intensities • Narrow bandwidth • Coherent • Spatially narrow • Highly monochromatic • Short pulses (pico- femto- sec) or CW (continuous wave) • History • 1960’s • Used in spectroscopy and in kinetics

8. Dye Laser • Converting laser frequency • From high power, short wavelength intense laser • Dye laser • Frequency multipling (Non Linear Optical effect) • Frequency mixing

9. Components of Lasers • Lasing medium – solid, gas, liquid • Mirrors - resonator • Pumping source

10. Mechanism of Laser Action • Pumping • Metastable state • Spontaneous emission

11. Mechanism of Laser Action(2) • Stimulated emission • Same frequency • Same phase • coherent

12. Population inversion

13. 3 level and 4 level laser

14. Semiconductor Diode Laser • Electron-hole pair • Lasing at junction • Band gap = hn • LED vs. LD • Light emitting diode • Laser diode • DBR (Distributed Bragg Reflector) laser diode • Spectroscopy • Optical drives (CD, DVD) • cavity surface emitting semiconductor laser

15. 다이오드 레이저의 기본원리 및 개념 p-n 접합 레이저발진 ~ ~ ~ ~ ~ N2O 발생농도에 따라 흡수가 일어나며 이는 Beer-Lambert 법칙에 따름 N2O 발생 오실로스코프로 데이터를 전송하고, 다시 컴퓨터로 전송하여 분석/해석 검출기 < N2O의 IR 흡수 그래프 > 4.5μm에서 최대흡수를 보여 이 영역대의 분석선을 선택하였으며, 가장 높은 감도를 획득 !

16. Dual beam 다이오드 레이저 분광 시스템 수신모듈 (검출기) 과 송신모듈 (레이저) 의 분리구성 ⇒원거리 on site분석 가능 ! 수신 모듈 송신 모듈 • N2O의 파장에 따른 흡수도를 고려, 2230GMP(2230±15 cm-1)를 설치하였음. • 검출기의 파장과 온도에 따른 감도를 고려, InSb 검출기(IS-010-E-LN4) 사용. • B/S : 빔을 나누어 주어 두 반응조의 동시 분석이 가능하도록 함. • 사용하는 빔의 파장과 반사/투과율을 고려하여 MgF2재질을 사용. • Lens : 빔을 모아주는 역할을 하며 사용 파장을 고려하여 CaF2재질을 선택. • GM : 빔을 반사시키는 역할 , Iris : 빔의 크기 조절, Chopper : Io 측정

17. Multi-beam 배열 • 레이저 조정기를 이용하여 온도와 • 전류를 조절해서 레이저 발진 주파 • 수의 변조/제어. • (Laser Components, L5830) • 록-인 증폭기를 이용하여 레이저의 • 구동 진동수와 위상의 동조가 가능. • 이를 이용하여 신호를 안정시키고 • S/N비를 증가시켰음. • (Stanford Research System, SRS830) • 오실로스코프 : 디지털 신호를 아날 • 로그 신호로 변환하여, 필요에 따라 • 컴퓨터로 전송하여 해석. 멀티 채널 • 로 되어있으므로 동시에 여러 신호 • 를 받아서 해석할 수 있다. • (LeCroy, 9310A) • Scope Explorer : 오실로스코프와 • 컴퓨터를 GPIB 인터페이스로 연결 • 하여 컴퓨터로 원격 조정할 수 있게 • 하는 프로그램 • (LeCroy, version 2.19.0beta) Dual beam alignment 그림에서는 dual beam 세팅을 예로 보였으며, 적당한 B/S를 이용하면 멀티빔 배열로 여러 site를 동시에 측정할 수 있다.

18. 출동, 현장을 뛰는 사람들! • 안양 하수처리장에서 발생하는 온실기체 N2O 측정 • 적외선 다이오드 레이저 분광법

19. Nonlinear Optical effects • 비선형광학 효과(NLO) • Linear Polarization • P = aE • 분극은 외부 전기장에 선형 비례 • a = polarizability • At high radiation intensity • P=aE + bEE + gEEE + … • b, g - Hyperpolarizability • Doubling of YAG 1064 nm to 266 nm • Ammonium dihydrogen phosphate(ADP) crystal

20. 7C Wavelength selectors • Filter • Absorption filter • Interference filter (Fabry-Perot filter) • Interference wedge • Monochromator • Grating type • Prism type

21. Wavelength selectors

22. Typical wavelength selector output

23. Interference Filter • Optical interference • Dielectric material(CaF2 or MgF2) between metal films

24. Order of interference • Cos(90-q) = d/(x/2) • X = nl • nl(air)sin(q) = 2d • Available UV, VIS, IR ( up to 14 mm) • Effective bandwidth = 1.5% of peak usually • Specially 0.15% possible • Maximum transmittance = 10%

25. Interference Wedge • Thickness of wedge • Transmitted wavelength change • For UV, VIS, IR • Can be used instead of prism or grating

26. Absorption Filter

27. Characteristics of absorption filter • Cheaper than interference filter • Worse than interference filter • But widely used • Absorption some spectral range • Effective bandwidth = 30 ~ 250 nm • %T = less than 10% • Cut-off filter • types • Dye suspended in gelatin • Colored glass (stable to heat)

28. Combination of two filters

29. 7C-2 Monochromator • Purpose • Scanning spectrum in frequency domain • Components • Entrance slit (+ window) • collimating lens or mirror • Dispersion element - Prism or grating • Focusing • Exit slit (+ window)

30. Prism

31. Grating • Grating • Transmission type • Reflection type – widely used in modern instruments • Replica and master • UV/VIS = 300-2000 Grooves/mm • IR = 10-200 grooves/mm • Groove size, spacing, parallel

32. Types of gratings • Echellette-type grating • Concace type grating • Collimating/focusing is not necessary • Cost effective and more energy throughput • Holographic grating • Master with hologram method (perfect lines) • Two lasers with different angles - fringes • 6000/mm for 50 cm possible • Replica grating are possible with same quality

33. Echellette-type Grating • Optical path difference • In bold line • Multiples of wavelength • N = order • High order lines are removed by filters • Example 7-1

34. Orders of echellet type grating

35. Performance Characteristics of Monochromators • Spectral purity • Free of unwanted, scattered or stray radiation • Reflections of optical components • mechanical imperfections particularly in gratings • scattering by dust particles • Dispersion • ability to separate small wavelength differences • Light gathering • Spectral bandwidth

36. Spectral purity increase • Use entrance and exit windows • Use Dust and light-tight housing • Coat interior with light absorbing paint • Dispersion • ability to separate small wavelength differences • Linear dispersion (D) or reciprocal linear dispersion (D-1) • variation in l across the focal plane • Light gathering • light collection efficiency • f/number

37. Dispersion of grating • dispersion • Ability to separate different wavelengths • range of wavelengths exiting the monochromator • Related to dispersion and entrance/exit slit widths • Angular dispersion • dr/dl • (change in the angle)/(change in wavelength) • nl = d(sin i + sin r) holding i constant • ndl = d(0 + cos r)dr • Dr/dl = n/(d cos r)

38. Linear dispersion • Definition of linear dispersion (D) • Variation in wavelength (l) as a function of y • Y = the distance along the exit slit focal plane (AB in figure 7-16) • D = dy/dl = Fdr/dl • F = focal length of the monochromator • Reciprocal linear dispersion (D-1) • D-1 = dl/dy= dl/Fdr • D-1 = dl/dy= (d cos r) /nF • Practical application • Angular dispersion increases as d decreases • If angle r is small, we assume cos r ~ 1 • Linear dispersion of a grating is almost constant • Dimensions = nm/mm or angstrom/mm

39. Resolving power (R) • Definition • Limit of its ability to separate adjacent images that have a slight difference in wavelength • R = l/dl • Where l is average wavelength of the two • Typical values of R • 1000 – 10000 for UV/VIS monochromator • It can be shown that R = nN • Where n = diffraction order, N = number of grating blazes • Better R • Longer gratings • Smaller blaze spacings • Higher diffraction orders

40. Light gathering power • Definition • A measure of the ability of a monochromator to collect the radiation • f/number = F/d • F = the focal length of the collimating mirror or lens • d = its diameter • Practice • f/2 lens gathers 4 times light than f/4 • Usually f/1 to f/10 for most monochromators

41. Echelle monochromator • Echelle monochromator • Two dispersing element • Echelle grating • Low-dispersion element (prism or another grating)

42. Echelle grating • Described by • G. R. Harrison In 1949 • differs from a conventional grating (echelette) • High angle of incidence • Short side of blaze is used • An echelle is coarse (300 or fewer grooves/mm) • i.e., it has fewer grooves per millimeter than an echelette • is used at high angles in high diffraction orders.

43. Echelle grating Advantages • The advantage of an echelle • high efficiency and low polarization effects over large spectral intervals • Together with high dispersion, this leads to compact, high-resolution instruments. • An important limitation of echelle • the orders overlap unless separated optically, for instance by a cross-dispersing element. • A prism or echelette grating is often used for this purpose. • For broad spectral range, to use many sucessive orders • http://www.gratinglab.com/library/technotes/technote6.asp

44. Monochromator Slits • Good slits • Two pieces of metal to give sharp edges • Parallel to one another • Spacing can be adjusted in some models • Entrance slit • Serves as a radiation source • Focusing on the slit plane

45. Effect of slit width on resolution • Bandwidth • Defined as a span of monochromator setting • needed to move the image of the entrance slit across the exit slit • Effective bandwidth • Dleff • ½ of the bandwidth • When two slits are identical

46. Calculating slit width • Effective bandwidth(Dleff) and D-1 • D-1 = Dl/Dy • When Dy = w = (slit width) • D-1 = Dleff /w • Example • Recpiprocal linear dispersion = 1.2nm/mm • Sodium lines at 589.0 nm and 589.6 nm • Required slit width? • Dleff = ½ (589.6-589.0) = 0.3 nm • W = 0.3 nm/(1.2 nm/mm) = 0.25 mm • Practically, narrower than the theoretical values is necessary to achieve a desired resolution

47. Choice of slit widths • Variable slits for effective bandwidth • Narrow spectrum • Minimal slit width • Bet decrease in the radiant power • Quantitative analysis • Wider slit width • for “more” radiant power

48. 7D Sample Containers

49. Cuvettes • Cells • Made of suitable material (see table 7-2): • Glass 400-3000 nm (vis-near IR) • Silica/quartz 200-3000 nm (UV-near IR) • NaCl 200-15,000 nm (UV-far IR) • Plastic containers (VIS)

50. 7E Radiation Transducers • Photon transducer - quantum event • Photovoltaic • Photo tube • Photomultiplier (PMT) • Si photodiode • Charge transfer device (CTD) • Thermal detector – average power of heating • Thermocouple • Bolometer • Pyroelectric detector