Components of Components of Optical InstrumentsOptical Instruments

2004. 42004. 4Yongsik LeeYongsik Lee

7A General Design7A General Design

► Optical spectroscopic methodsOptical spectroscopic methods Absorption (figure 7-1 a)Absorption (figure 7-1 a) Fluorescence (figure 7-1b)Fluorescence (figure 7-1b) PhosphorescencePhosphorescence ScatteringScattering Emission (figure 7-1c)Emission (figure 7-1c) Chemiluminescence (figure 7-1c)Chemiluminescence (figure 7-1c)

► 5 Components5 Components Stable source of radiant energy (figure 7-3a)Stable source of radiant energy (figure 7-3a) Transparent container (figure 7-2a)Transparent container (figure 7-2a) Isolater for a restricted region of the spectrum (figure 7-2b)Isolater for a restricted region of the spectrum (figure 7-2b) Radiation detector (figure 7-3b)Radiation detector (figure 7-3b) Signal processor and readoutSignal processor and readout

7B Sources of Radiation7B Sources of Radiation

► Two Types of sourcesTwo Types of sources Continuum soureceContinuum sourece Line sourceLine source

► LasersLasers

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

Continuum sourceContinuum source

► Continuum sourcesContinuum sources Deuterium lamp (D2)Deuterium lamp (D2)

►Common for UVCommon for UV High pressure, gas-filled arc lamp (Ar, Xe, Hg)High pressure, gas-filled arc lamp (Ar, Xe, Hg)

► Intense source of UV, VISIntense source of UV, VIS W filament lampW filament lamp

►Universally used for VISUniversally used for VIS Inert solids heated at 1500-2000 KInert solids heated at 1500-2000 K

►Maximum blackbody radiation around 1.5-1.9 micrometerMaximum blackbody radiation around 1.5-1.9 micrometer► For IRFor IR

Line SourecesLine Soureces

► Hg, Na vapor lampHg, Na vapor lamp► Hollow cathode lampHollow cathode lamp► Electrodeless dischargElectrodeless discharg

e lampe lamp► LasersLasers

7B-3 Laser Sources7B-3 Laser Sources

► Light amplification by stimulated emission of radiationLight amplification by stimulated emission of radiation► CharacteristicsCharacteristics

High intensitiesHigh intensities Narrow bandwidthNarrow bandwidth CoherentCoherent Spatially narrowSpatially narrow Highly monochromaticHighly monochromatic Short pulses (pico- femto- sec) or CW (continuous wave)Short pulses (pico- femto- sec) or CW (continuous wave)

► HistoryHistory 1960’s1960’s Used in spectroscopy and in kineticsUsed in spectroscopy and in kinetics

Dye LaserDye Laser

► Converting laser frequenConverting laser frequencycy From high power, short wFrom high power, short w

avelength intense laseravelength intense laser Dye laserDye laser Frequency multiplingFrequency multipling(Non Linear Optical effect)(Non Linear Optical effect) Frequency mixingFrequency mixing

Components of LasersComponents of Lasers

► Lasing medium – solid, gas, liquidLasing medium – solid, gas, liquid►Mirrors - resonatorMirrors - resonator► Pumping sourcePumping source

Mechanism of Laser ActionMechanism of Laser Action

► PumpingPumping Metastable stateMetastable state

► Spontaneous Spontaneous emissionemission

Mechanism of Laser Action(2)Mechanism of Laser Action(2)

► Stimulated emissionStimulated emission Same frequencySame frequency Same phaseSame phase coherentcoherent

Population inversionPopulation inversion

3 level and 4 level laser3 level and 4 level laser

Semiconductor Diode LaserSemiconductor Diode Laser

► Electron-hole pairElectron-hole pair Lasing at junctionLasing at junction Band gap = hBand gap = h

► LED vs. LDLED vs. LD Light emitting diodeLight emitting diode Laser diodeLaser diode

► DBR (Distributed Bragg RefDBR (Distributed Bragg Reflector) laser diodelector) laser diode SpectroscopySpectroscopy Optical drives (CD, DVD)Optical drives (CD, DVD)

► cavity surface emitting semcavity surface emitting semiconductor laser iconductor laser

< N2O 의 IR 흡수 그래프 >4.5 μ m 에서 최대흡수를 보여 이 영역대의 분석선을

선택하였으며 , 가장 높은 감도를 획득 !

p-n 접합 레이저발진

N2O 발생

~ ~ ~ ~ ~

검출기

N2O 발생농도에 따라 흡수가

일어나며 이는 Beer-Lambert

법칙에 따름

오실로스코프로 데이터를

전송하고 , 다시 컴퓨터로

전송하여 분석 / 해석

다이오드 레이저의 기본원리 및 개념

수수신모듈 ( 검출기 ) 과 송신모듈 ( 레이저 ) 의 분리구성 ⇒ 원거리 on site 분석 가능 !

• N2O 의 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 MgF2 재질을 사용 .

• Lens : 빔을 모아주는 역할을 하며 사용 파장을 고려하여 CaF2 재질을 선택 .

• GM : 빔을 반사시키는 역할 , Iris : 빔의 크기 조절 , Chopper : Io 측정

• N2O 의 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 MgF2 재질을 사용 .

• Lens : 빔을 모아주는 역할을 하며 사용 파장을 고려하여 CaF2 재질을 선택 .

• GM : 빔을 반사시키는 역할 , Iris : 빔의 크기 조절 , Chopper : Io 측정

수신 모듈 송신 모듈

Dual beam 다이오드 레이저 분광 시스템

• 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/N 비를 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/S 를

이용하면 멀티빔 배열로 여러 site 를 동시에 측정할 수 있다 .

Multi-beam 배열

출동출동 , , 현장을 뛰는 사람들현장을 뛰는 사람들 !!

►안양 하수처리장에서 안양 하수처리장에서 발생하는 온실기체 발생하는 온실기체 N2N2O O 측정측정

►적외선 다이오드 적외선 다이오드 레이저 분광법레이저 분광법

Nonlinear Optical effectsNonlinear Optical effects

►비선형광학 효과비선형광학 효과 (NLO)(NLO)► Linear PolarizationLinear Polarization

P = P = EE 분극은 외부 전기장에 선형 비례분극은 외부 전기장에 선형 비례 = polarizability= polarizability

► At high radiation intensityAt high radiation intensity P=P=E + E + EE + EE + EEE + …EEE + … - Hyperpolarizability- Hyperpolarizability Doubling of YAG 1064 nm to 266 nmDoubling of YAG 1064 nm to 266 nm Ammonium dihydrogen phosphate(ADP) crystalAmmonium dihydrogen phosphate(ADP) crystal

7C Wavelength selectors7C Wavelength selectors

►FilterFilter Absorption filterAbsorption filter Interference filter (Fabry-Perot filter)Interference filter (Fabry-Perot filter) Interference wedgeInterference wedge

►MonochromatorMonochromator Grating typeGrating type Prism typePrism type

Wavelength selectorsWavelength selectors

Typical wavelength selector Typical wavelength selector outputoutput

Interference FilterInterference Filter

►Optical interferenceOptical interference►Dielectric material(CaFDielectric material(CaF22 or MgF or MgF22) between ) between

metal filmsmetal films

Order of interferenceOrder of interference

► Cos(90-Cos(90-) = d/(x/2)) = d/(x/2)► X = nX = n► nn(air)(air)sin(sin() ) 2d2d► Available UV, VIS, IR ( up to 14 Available UV, VIS, IR ( up to 14 m)m)► Effective bandwidth = 1.5% of peak usuallyEffective bandwidth = 1.5% of peak usually

Specially 0.15% possibleSpecially 0.15% possible► Maximum transmittance = 10%Maximum transmittance = 10%

Interference WedgeInterference Wedge

► Thickness of wedgeThickness of wedge Transmitted Transmitted

wavelength changewavelength change

► For UV, VIS, IRFor UV, VIS, IR Can be used instead of Can be used instead of

prism or gratingprism or grating

Absorption FilterAbsorption Filter

Characteristics of absorption Characteristics of absorption filterfilter

► Cheaper than interference filterCheaper than interference filter Worse than interference filterWorse than interference filter But widely usedBut widely used

► Absorption some spectral rangeAbsorption some spectral range Effective bandwidth = 30 ~ 250 nmEffective bandwidth = 30 ~ 250 nm %T = less than 10%%T = less than 10% Cut-off filterCut-off filter

► typestypes Dye suspended in gelatinDye suspended in gelatin Colored glass (stable to heat)Colored glass (stable to heat)

Combination of two filtersCombination of two filters

7C-2 Monochromator7C-2 Monochromator

► PurposePurpose Scanning spectrum in Scanning spectrum in

frequency domainfrequency domain► ComponentsComponents

Entrance slit (+ Entrance slit (+ window)window)

collimating lens or collimating lens or mirrormirror

Dispersion element - Dispersion element - Prism or gratingPrism or grating

Focusing Focusing Exit slit (+ window)Exit slit (+ window)

PrismPrism

GratingGrating

►GratingGrating Transmission typeTransmission type Reflection type – widely used in modern Reflection type – widely used in modern

instrumentsinstruments

►Replica and masterReplica and master UV/VIS = 300-2000 Grooves/mmUV/VIS = 300-2000 Grooves/mm IR = 10-200 grooves/mmIR = 10-200 grooves/mm Groove size, spacing, parallelGroove size, spacing, parallel

Types of gratingsTypes of gratings

► Echellette-type gratingEchellette-type grating► Concace type gratingConcace type grating

Collimating/focusing is not necessaryCollimating/focusing is not necessary Cost effective and more energy throughputCost effective and more energy throughput

► Holographic gratingHolographic grating Master with hologram method (perfect lines)Master with hologram method (perfect lines) Two lasers with different angles - fringesTwo lasers with different angles - fringes 6000/mm for 50 cm possible6000/mm for 50 cm possible Replica grating are possible with same qualityReplica grating are possible with same quality

Echellette-type GratingEchellette-type Grating

►Optical path Optical path differencedifference In bold lineIn bold line Multiples of Multiples of

wavelengthwavelength N = orderN = order

►High order lines are High order lines are removed by filtersremoved by filters

► Example 7-1Example 7-1

Orders of echellet type gratingOrders of echellet type grating

Performance Characteristics of MonoPerformance Characteristics of Monochromatorschromators

► Spectral puritySpectral purity Free of unwanted, scattered or stray radiationFree of unwanted, scattered or stray radiation Reflections of optical componentsReflections of optical components mechanical imperfections particularly in gratingsmechanical imperfections particularly in gratings scattering by dust particlesscattering by dust particles

►DispersionDispersion ability to separate small wavelength differences

► Light gatheringLight gathering► Spectral bandwidthSpectral bandwidth

► Spectral purity increaseSpectral purity increase Use entrance and exit windows Use Dust and light-tight housing Coat interior with light absorbing paint

► DispersionDispersion ability to separate small wavelength differences Linear dispersion (D) or reciprocal linear dispersion (D-1) variation in across the focal plane

► Light gatheringLight gathering light collection efficiency f/number

Dispersion of gratingDispersion of grating

► dispersion Ability to separate different wavelengths range of wavelengths exiting the monochromator Related to dispersion and entrance/exit slit widths

► Angular dispersionAngular dispersion dr/ddr/d (change in the angle)/(change in wavelength)(change in the angle)/(change in wavelength) nn = d(sin i + sin r) holding i constant = d(sin i + sin r) holding i constant ndnd = d(0 + cos r)dr = d(0 + cos r)dr Dr/dDr/d = n/(d cos r) = n/(d cos r)

Linear dispersionLinear dispersion

► Definition of linear dispersion (D)Definition of linear dispersion (D) Variation in wavelength (Variation in wavelength ( as a function of y as a function of y Y = the distance along the exit slit focal plane (AB in figure 7-16)Y = the distance along the exit slit focal plane (AB in figure 7-16) D = dy/dD = dy/d = Fdr/d = Fdr/d F = focal length of the monochromatorF = focal length of the monochromator

► Reciprocal linear dispersion (DReciprocal linear dispersion (D-1)) DD-1 = d = d/dy= d/dy= dFdrFdr DD-1 = d = d/dy= (d cos r)/dy= (d cos r)nFnF

► Practical applicationPractical application Angular dispersion increases as d decreasesAngular dispersion increases as d decreases If angle r is small, we assume cos r ~ 1If angle r is small, we assume cos r ~ 1 Linear dispersion of a grating is almost constantLinear dispersion of a grating is almost constant Dimensions = nm/mm or angstrom/mmDimensions = nm/mm or angstrom/mm

Resolving power (R)Resolving power (R)

► DefinitionDefinition Limit of its ability to separate adjacent images that have a slight diffLimit of its ability to separate adjacent images that have a slight diff

erence in wavelengtherence in wavelength R =R =dd Where Where is average wavelength of the two is average wavelength of the two

► Typical values of RTypical values of R 1000 – 10000 for UV/VIS monochromator1000 – 10000 for UV/VIS monochromator It can be shown that R = nNIt can be shown that R = nN Where n = diffraction order, N = number of grating blazesWhere n = diffraction order, N = number of grating blazes

► Better RBetter R Longer gratingsLonger gratings Smaller blaze spacingsSmaller blaze spacings Higher diffraction ordersHigher diffraction orders

Light gathering powerLight gathering power

► DefinitionDefinition A measure of the ability of a monochromator to collect thA measure of the ability of a monochromator to collect th

e radiatione radiation f/number = F/df/number = F/d F = the focal length of the collimating mirror or lensF = the focal length of the collimating mirror or lens d = its diameterd = its diameter

► PracticePractice f/2 lens gathers 4 times light than f/4f/2 lens gathers 4 times light than f/4 Usually f/1 to f/10 for most monochromatorsUsually f/1 to f/10 for most monochromators

Echelle monochromatorEchelle monochromator

► Echelle monochromatorEchelle monochromator Two dispersing elementTwo dispersing element Echelle gratingEchelle grating Low-dispersion element (prLow-dispersion element (pr

ism or another grating)ism or another grating)

Echelle gratingEchelle grating► Described byDescribed by

G. R. Harrison In 1949G. R. Harrison In 1949► differs from a conventional gradiffers from a conventional gra

ting (echelette)ting (echelette) High angle of incidenceHigh angle of incidence Short side of blaze is usedShort side of blaze is used An echelle is coarse (300 or fAn echelle is coarse (300 or f

ewer grooves/mm)ewer grooves/mm) i.e., it has fewer grooves per i.e., it has fewer grooves per

millimeter than an echelettemillimeter than an echelette is used at high angles in high is used at high angles in high

diffraction orders. diffraction orders.

Echelle grating AdvantagesEchelle grating Advantages► The advantage of an echelle The advantage of an echelle

high efficiency and low polarization effects over large spectral intervhigh efficiency and low polarization effects over large spectral intervalsals

Together with high dispersion, this leads to compact, high-resolution Together with high dispersion, this leads to compact, high-resolution instruments.instruments.

► An important limitation of echelleAn important limitation of echelle the orders overlap unless separated optically, for instance by a crosthe orders overlap unless separated optically, for instance by a cros

s-dispersing element. s-dispersing element. A prism or echelette grating is often used for this purpose. A prism or echelette grating is often used for this purpose. For broad spectral range, to use many sucessive ordersFor broad spectral range, to use many sucessive orders

► http://www.gratinglab.com/library/technotes/technote6.ashttp://www.gratinglab.com/library/technotes/technote6.aspp

Monochromator SlitsMonochromator Slits

►Good slitsGood slits Two pieces of metal to give sharp edgesTwo pieces of metal to give sharp edges Parallel to one anotherParallel to one another Spacing can be adjusted in some modelsSpacing can be adjusted in some models

►Entrance slitEntrance slit Serves as a radiation sourceServes as a radiation source Focusing on the slit planeFocusing on the slit plane

Effect of slit width on Effect of slit width on resolutionresolution

► BandwidthBandwidth Defined as a span of monocDefined as a span of monoc

hromator setting hromator setting needed to move the image of needed to move the image of

the entrance slit across the the entrance slit across the exit slitexit slit

► Effective bandwidthEffective bandwidth effeff

½ of the bandwidth½ of the bandwidth When two slits are identicalWhen two slits are identical

Calculating slit widthCalculating slit width

► Effective bandwidth(Effective bandwidth(effeff) and D) and D-1

DD-1 = = yy When When y = w = (slit width)y = w = (slit width) DD-1 = = eff eff /w/w

► ExampleExample Recpiprocal linear dispersion = 1.2nm/mmRecpiprocal linear dispersion = 1.2nm/mm Sodium lines at 589.0 nm and 589.6 nmSodium lines at 589.0 nm and 589.6 nm Required slit width?Required slit width? eff eff = ½ (589.6-589.0) = 0.3 nm= ½ (589.6-589.0) = 0.3 nm W = 0.3 nm/(1.2 nm/mm) = 0.25 mmW = 0.3 nm/(1.2 nm/mm) = 0.25 mm Practically, narrower than the theoretical values is necesPractically, narrower than the theoretical values is neces

sary to achieve a desired resolutionsary to achieve a desired resolution

Choice of slit widthsChoice of slit widths

► Variable slits for Variable slits for effective bandwidtheffective bandwidth

►Narrow spectrumNarrow spectrum Minimal slit widthMinimal slit width Bet decrease in the Bet decrease in the

radiant powerradiant power

►Quantitative analysisQuantitative analysis Wider slit width Wider slit width for “more” radiant for “more” radiant

powerpower

7D Sample Containers7D Sample Containers

►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)

7E Radiation Transducers7E Radiation Transducers

► Photon transducer - quantum eventPhoton transducer - quantum event PhotovoltaicPhotovoltaic Photo tubePhoto tube Photomultiplier (PMT)Photomultiplier (PMT) Si photodiodeSi photodiode Charge transfer device (CTD)Charge transfer device (CTD)

► Thermal detector – average power of heatingThermal detector – average power of heating ThermocoupleThermocouple BolometerBolometer Pyroelectric detectorPyroelectric detector

Properties of Ideal Properties of Ideal TransducersTransducers

►high sensitivity►low noise►wide wavelength response►linear output (S=k·)►low dark current (small current when =0) (S

=k·+kd)

Relative response curveRelative response curve

Figure 7-25

Photovolatic cellPhotovolatic cell► Structure

metal-semiconductor-metal sandwiches

► produce voltage when irradiated 350-750 nm 550 nm maximum response 10-100 microA

► Barrier-layer cell Low-price Amplification difficulty Low sensensitivity for weak radi

ation Fatigue effect

Vacuum PhototubeVacuum Phototube

► Structure Wire anode and semi

cylinder cathode in a vacuum tube

Photosensitive material► electrons produced by

irradiation of cathode travel to anode.

► l response depends on cathode material (200-1000 nm) High sensitivity Red response UV response Flat response

Photomultiplier tubePhotomultiplier tube

► irradiation of cathode produces electrons

► series of anodes (dynodes) increases gain 105-107 electrons per

photon.

► Low incident fluxes only!

PM Tube in usePM Tube in use

►Good response to Good response to UV/VISUV/VIS

► Fast responseFast response►Dark current - Dark current -

decrease when decrease when coolingcooling

►Housing/dark room Housing/dark room conditioncondition

► Photon countingPhoton counting

HamamatsuHamamatsu

► http://www.hpk.co.jphttp://www.hpk.co.jp

Silicon diodeSilicon diode

► P-n junction diodeP-n junction diode► 190-1100 nm190-1100 nm► SensitivitySensitivity

Vacuum phototube < Vacuum phototube < silicon diode < PMTsilicon diode < PMT

Multi-channel transducerMulti-channel transducer► Conventional multi-channel detector

Photographic plate or Film strip Relatively sensitive (10-100 photons) Limitation - Takes long time (DP & E)

► Modern multi channel photon detector► Types

1D Photodiode arrays, PDA 2D Charge-injection devices, CID 2D Charge-coupled devices, CCD

► Photodiode arrays (Fig 7-31) multichannel transducer photon striking n-type Si creates free electrons which travel to p-type Si. Many junctions in a row - spatially sensitive Charge collection = charge transfer device (CTD)

Thermal transducers

► sensitive to IR (> 750 nm) IR lack the energy to cause photoemission Photoconduction type should be used Absorbing element’s heat capacity must be small to gen

erate temperature changes Background thermal noise

►Vacuum housing►Beam chopping

► Three types thermocouples - junction thermometer bolometers - resistance thermometer pyroelectric devices - piezoelectric effect

ThermocoupleThermocouple► Junction thermometerJunction thermometer

two thin wires of two thin wires of differentdifferent metals, soldered or welded metals, soldered or welded together at two ends to form two junctions. together at two ends to form two junctions.

One junction is held at a reference temperature (usually One junction is held at a reference temperature (usually ice water) and the other at the temperature to be ice water) and the other at the temperature to be measured. measured.

The temperature difference generates a voltage that can The temperature difference generates a voltage that can be measured with a volt-meter. Thermocouples be measured with a volt-meter. Thermocouples

ThermocoupleThermocouple

► Two metal piece junctionTwo metal piece junction Common wire materials are copper and constantan (a coCommon wire materials are copper and constantan (a co

pper – nickel alloy). pper – nickel alloy). Bi or Sb wire for IRBi or Sb wire for IR Colored in black – absorptionColored in black – absorption In a vacuum boxIn a vacuum box

► 1/1000000 K detection1/1000000 K detection Corresponds to 6-8 Corresponds to 6-8 V/WV/W work in the temperature ranges –270C to 2300C.work in the temperature ranges –270C to 2300C.

► ThermopileThermopile A series of thermocouplesA series of thermocouples

BolometersBolometers

►Resistance thermometerResistance thermometer Metals - Pt or NiMetals - Pt or Ni Semiconductor (thermistor)Semiconductor (thermistor) Change of R as T changesChange of R as T changes

►Ge bolometer is best for 5-400 cmGe bolometer is best for 5-400 cm -1-1

Imaging bolometer at LANLImaging bolometer at LANL

Pyroelectric detectorPyroelectric detector► Insulator waferInsulator wafer

Triglycine sulface-DTriglycine sulface-D► Temperature dependent elTemperature dependent el

ectric polarizationectric polarization Generally disappears when ElGenerally disappears when El

ectric field is removedectric field is removed► For pyroelectric materialsFor pyroelectric materials

T dependent capacitor is creT dependent capacitor is createdated

Fast response timeFast response time► usageusage

FT-IR detectorFT-IR detector Night vision systemNight vision system

DetectorDetector

7H Types of optical 7H Types of optical instrumentsinstruments

► SpectroscopeSpectroscope Atomic emission linesAtomic emission lines

► ColorimeterColorimeter Absorption where human eye as detectorAbsorption where human eye as detector

► PhotometerPhotometer Source, filter, photoelectric tranducer, signal processor, readoutSource, filter, photoelectric tranducer, signal processor, readout

► SpectrographSpectrograph Entire spectrum measuring deviceEntire spectrum measuring device

► SpectrometerSpectrometer Spectrum intensitySpectrum intensity

► SpectrophotometerSpectrophotometer Spectrometer + photoelectric transducer + double beamSpectrometer + photoelectric transducer + double beam

7I Fourier Transformation7I Fourier Transformation

►HistoryHistory In 1950s, astronomyIn 1950s, astronomy

►Separate weak signals from noiseSeparate weak signals from noise

Late 1960s, FT-NIR & FT-IRLate 1960s, FT-NIR & FT-IR

►Fourier transformFourier transform

Advantages of FTAdvantages of FT

► Throughput / Jaquinot advantageThroughput / Jaquinot advantage Few optics and slitsFew optics and slits Less dispersion, high intensityLess dispersion, high intensity Usually to improve resolution decrease slit width Usually to improve resolution decrease slit width but less light makes spectrum "noisier" (S/N)but less light makes spectrum "noisier" (S/N)

► High ResolutionHigh Resolution = 6 ppm= 6 ppm

► Short time scaleShort time scale Simultaneously measure all spectrum at once saves timeSimultaneously measure all spectrum at once saves time frequency scanning vs. time domain scanningfrequency scanning vs. time domain scanning Fellgett or multiplex advantageFellgett or multiplex advantage

Freq-domain / time-domainFreq-domain / time-domain

Time domain spectroscopyTime domain spectroscopy

►Unfortunately, no detector can respond on 10-14 s time scale

►Use Michelson interferometer to measure signal proportional to time varying signal

Michelson interferometerMichelson interferometer

InterrerogramInterrerogram

►retardation Difference in pathlength

►interferogram Plot signal vs. cosine wave with frequency proportional to light f

requency but signal varies at much lower frequency

►One full cycle when mirror moves distance /2 (round-trip = )

modulationmodulation

► Velocity of moving mirrVelocity of moving mirror(MM)or(MM)

► Time to move Time to move /2 cm/2 cm►Bolometer, pyroelectri

c, photoconducting IR detectors can "see“ changes on 10-4 s time scale!

Analysis of interferogramAnalysis of interferogram

► Computer needed to turn complex interferogram into spectrum

► Figure 7-43 (b) resolved lines (c) unresolved lines

► FT Time -> Frequency

► inverse FT Frequency -> Time

Resolution of FT Resolution of FT spectrometerspectrometer

►Two closely spaced lines only separated if one complete "beat" is recorded.

►As lines get closer together, must increase.

HomeworkHomework

►7-7, 9, 13, 19, 227-7, 9, 13, 19, 22