2.1 Spectroscopy 2.4 Angle measurement Optical Engineering

Universität Karlsruhe (TH)

Optical Engineering

Martina Gerken29.11.2007

5.2

Course outline1. Imaging optics2. Optical sensors

2.1 Spectroscopy2.2 Material characterization2.3 Distance measurement2.4 Angle measurement2.5 Optical mouse

3. Optics in data storage4. Introduction to displays5. Fourier optics6. Diffractive optics and holograms7. Integrated optics8. Computerized imaging

5.3

Absorption spectroscopy• Measure concentration using Lambert-Beer law

– Presumption: Concentration N0 is distributed homogeneously in test cell– σ is absorption cross-section

Source: http://www.tu-darmstadt.de/fb/mb/ekt/laser/Absorptionsspe_1.pdf

Lambert-Beer law:

5.4

Absorption laser spectroscopy• Wavelength changed with tunable laser• High wavelength resolution possible


5.5

Example: Gas analysis• Continuous in-situ monitoring of CO und O2 under pulsed fuel injection


5.6

Interference spectroscopy• Used for characterization of thin optical layers

• Measurement of spectrally resolved intensity of transmitted or reflected beam

• Thickness and complex refractive index calculated from evaluation of interferences– Refractive index n and absorption coefficient t k depend on wavelength

d = ?

n(λ)sub

n(λ) = ?, k(λ) = ?

5.7

Antireflective coatings• Interference effects if coherence length of light longer than layer thickness • Antireflective coating uses destructive interference in reflection

– For T=100%: R1=R2 and λ/2-pathlength difference

Source: http://de.wikipedia.org

5.8

Antireflective coating - calculation• Calculation

– π-phase difference (corresponds to λ/2) upon reflection from optically thinner to thicker medium

– Amplitude reflection coefficient

• Thinnest antireflective coating for λ und 0°:

• T=100 % only for specific wavelengths• Multilayer antireflective coatings for larger wavelength interval

1

11,,0

+

++°= +

−=

ii

iiii nn

nnrθ

sAR nnn 0=

ARAR n

d4

λ=

⇒+−

=+−

22

0

0

sAR

sAR

AR

AR

nnnn

nnnn

⇒⎟⎠⎞

⎜⎝⎛ +=

ARAR n

md λ212 m = 0; 1; 2; ... (order)

5.9

Ideal antireflective coating for glass• Transition air (n=1,0) to glass (n=1,5) at 0°• Ideal antireflective coating for λ=633 nm

– nAr=1,22 and dAr=130 nm

400 450 500 550 600 650 70095

95.5

96

96.5

97

Wellenlänge / nm

Tran

smis

sion

sgra

d / %

400 450 500 550 600 650 70097

97.5

98

98.5

99

99.5

100

Wellenlänge / nm

Tran

smis

sion

sgra

d / %

Without AR-layer With ideal AR-layer

5.10

MgF2 - antireflective coating for glass• Transition air (n=1,0) to glass (n=1,5) at 0°• MgF2 - antireflective coating for λ=633 nm

– nMgF2=1,38 and dMgF2=115 nm• For wavelengths with constructive interference reflection identical to

transmission coefficient of glass

400 450 500 550 600 650 70096.5

97

97.5

98

98.5

99

Wellenlänge / nm

Tran

smis

sion

sgra

d / %

200 400 600 800 1000 120096

96.5

97

97.5

98

98.5

99

Wellenlänge / nm

Tran

smis

sion

sgra

d / %

(No exactly correct as constant nMgF2 used.)

5.11

Layers with higher index on glass• Layers:

– Air (n=1,0) – 500 nm layer with n=1,55– Glass (n=1,5)

• Calculation at 0°

• Layers: – Air (n=1,0) – 500 nm layer with n=2,5– Glass (n=1,5)

• Calculation at 0°

400 450 500 550 600 650 70060

65

70

75

80

85

90

95

100

Wellenlänge / nm

Tran

smis

sion

sgra

d / %

400 450 500 550 600 650 70094

94.5

95

95.5

96

Wellenlänge / nm

Tran

smis

sion

sgra

d / %

5.12

• Works well for transmission spectra of transparent wavelength region exhibiting several interference minima

200 400 600 800 1000 12000

0.2

0.4

0.6

0.8

1

Wellenlänge in nm

Tran

smis

sion

Interference spectroscopy: Calculation of order m

• First order of minimum far away from absorption is calculated using:

λ2 λ1

⎥⎦

⎤⎢⎣

⎡−

=21

21 λλ

λm( ) ⇒+=2

12

21

11

λλ mm

m2 m1

5.13

• Thickness d for constant n calculated from λ and m– For layers with higher refractive index on substrate:

Interference spectroscopy: Calculation of d

200 400 600 800 1000 12000

0.2

0.4

0.6

0.8

1

Wellenlänge in nm

Tran

smis

sion

56784

and

Maximum

Minimum2λmnd =

221 λ

⎟⎠⎞

⎜⎝⎛ += mnd

• Refractive index is calculated from transmission amplitude in mimimum

5.14

Exercise: Determine layer thickness• Determine the layer thickness of the two samples from the spectra!

• In which wavelength region do the samples absorb?

5.15

Application areas of interference spectroscopy• Layers need to be sufficiently (optically) thick (e.g., nd > 300 nm), to exhibit

several extrema• Method fails if narrow absorption features present in region of measurement • Method not suited for layers with strong absorption if extrema not well visible

• Alternative method: ellipsometry– Measurement of polarization state of light reflected from surface– Change in wavelength and/or change in incidence angle delivers more

information

5.16

Dielectric mirror• Based on interference effects in several thin layers

– Particularly interesting for high intensity applications, since absorption is smaller than in metals

• Bragg-mirror: Periodic layer sequence

01 1 2 24

d n d nλ= =

Constructive interference of 1+2 (as in single layer)

Constructive interference of 2+3:

Phase change λ/2 upon reflection on optically thicker medium

(“Face-Change “)

n1d1

n2d2

1 2 3

0 0 1 01 1 0

1

222 4 2

nL d nn

λ λ λ λ∆ = + = + =

n2d2

5.17

Bragg-mirror• GaAs-AlAs-Bragg mirror on GaAs-substrate

– dGaAs= 61 nm and nGaAs=3.5– dAlAs=73 nm and nAlAs=2.9

• Reflection at normal incidence from air for 4, 10 and 30 periods

5.18

Substrate to be coated

Material in liner is heated with electron beam

Material melts, evaporates, and forms thin film on substrate surface

Local material temperature up to ca. 2000°C. Liner cooled with water to prevent melting.

RecipientPressure ca. 1 x 10-6mbar(room pressure: 1 bar)

e-

Manufacture of thin films using e-beam evaporation

5.19

Typical layers for e-beam evaporation are:170 nm Tantalumpentoxid (Ta2O5, n = 2,2) and 260 nm Siliciondioxid (SiO2, n = 1,45).Bragg mirror obtained by repeated coating with these two layer types:

Glass substrate

SiO2-Ta2O5 dielectric mirror

Accurate control of layer thicknesses necessary for mirror at desired wavelength!

λB = 500 nm

Control of layer thickness5.20

Halogen lamp= White light source

Spectrometer

Thin-film filteron substrate

Opt. fiber

If transmission spectrum is monitored during deposition, layer thickness may be controlled accurately!

Interference spectroscopy for layer analysis

5.21

Insitu-monitoring of layer thickness

Substrate

e-


SpectrometerPC

Setup:White light coupled into optical fiber using collimator lens.

Fiber guides light into evacuated chamber and onto substrate.

Transmitted light collected into fiber on other side of substrate.

Transmitted light analyzed with spectrometer.

5.22

Univex e-beam system at LTI

5.23

Evaluation:Transmission spectrum taken every second.

Spectrum compared with calculated nominal values of desired layer.

If nominal spectrum reached, deposition is stopped.

Substrate

e-

SpectrometerPC

Insitu-monitoring of layer thickness


5.24

Note: If no polarization filter is used combined transmission spectrum for TM- and TE-polarized light obtained.

Minimum Maximum

Wavelength in nm

Tran

smis

sion

Example of monitoring signal

5.25

Other optical methods for material characterization• Here only two methods of optical material characterization discussed

– Absorption spectroscopy– Interference spectroscopy (transmission, reflection)– (Imaging discussed in section on microscopes)

• Many other methods exist– Raman spectroscopy– Fluorescence spectroscopy– Photoluminescence spectroscopy– Pump-probe spectroscopy– Photo-modulated reflection measurements– Electro-modulated reflection measurements– Four-wave mixing– …

5.26

Course outline1. Imaging optics2. Optical sensors

2.1 Spectroscopy2.2 Material characterization2.3 Distance measurement2.4 Angle measurement2.5 Optical mouse

3. Optics in data storage4. Introduction to displays5. Fourier optics6. Diffractive optics and holograms7. Integrated optics8. Computerized imaging

5.27

Methods of distance measurement

• Optical methods– Optical triangulation– Optical time of flight measurements– Optical interference measurements

Source: http:// de.wikipedia.org/wiki/Entfernungsmessung

5.28

Distance measurement using triangulation• Measurement with fixed basis

– Basis fixed– Angle β changed until adjacent

leg and hypotenuse meet at object

• Measurement with fixed angle– Angle β fixed– Length of leg changed until

adjacent leg and hypotenuse meet at object

Source: http://www.epsonrd1.de

5.29

Visualization

• Mixed view– Focused when image and

ghost congruent

• Cut view– Focused when image

sections without offset

• Inverse view– Focused when inverse view

without offset

Source: Naumann/Schröder, Bauelemente der Optik, 1992

5.30

Examples for indicator images

• Mixed view indicator

Source: http://www.epsonrd1.de

• Cut view indicator

5.31

Geometric beam splitter• Splits beam in two or more subsections (“Aperture splitting”)

– Aperture should not be too small. Otherwise, diffraction effects reduce resolution.

Source: Schröder, Technische Optik, 1990

Groove mirror Rhomboid prism Lamellar/Point-shapedmetallization

5.32

Physical beam splitter• Beam splitting using partially reflective surfaces

– Utilization of Fresnel reflection– Dielectric thin-film coatings (Interference filters) for low loss splitting

• Both beams have initial aperture– Resolution is maintained

• Realization as– Beam splitting plate– Beam splitting cube– Beam splitting foil (Very thin strained plastic foil to avoid effect of plane

plate thickness. Mechanically instable)


5.33

Example: Beam splitting cube• Cemented prisms with interference filter on hypotenuse

– Wavelength dependent due to filter

Source: www.linos.com/ , www.edmundoptics.com

5.34

Example: Variable beam splitting plate• Wedge shaped evaporated dielectric layers• Reflection / transmission depends on location on beam splitting plate

(adjustment travel)

Source: www.linos.com/

5.35

Example: Polarizing cube• Splitting ratio depends on polarization state of incident light• Works only in limited wavelength interval, e.g.,

– 450-550 nm: λ0 =500 nm – 550-700 nm: λ0 =633 nm – 700-900 nm: λ0 =800 nm

Quellen: www.linos.com

5.36

Periodic beam splitting• Beam switched periodically by swinging or rotating screen

• Example of beam splitting plate


5.37

Example: Mixed view distance measurement• Change in angle obtained by rotation of prism

– Leverage for accurate angle measurement


5.38

Variable beam deflection• “Optical leverage” with prism wedges generates small deflection for large

turning or rotation• Plane plate micrometer• Turning/displacement of lens pair

• Rotating wedge pair

Source: Schröder, Technische Optik, 1990;Naumann/Schröder, Bauelemente der Optik, 1992

5.39

Example: Inverse view distance measurement• Change in angle by lens displacement• Image separation with prism system


5.40

Example: Distance sensor

Control of rotation axis of body

Control of unbalance of body

5.41

Example: Distance sensor

Measurement of layer thickness

Essential for automation

5.42

Light stripe sensor• Laser line or LED-line projected onto object at defined angle • Reflected light imaged onto camera • Contour correspondes to height profile

Source: http:// www.machinevisiononline.org; http://www.sick.de

5.43

Color coded triangulation• Press report 01/2007: With color coded triangulation three dimensional

images may be formed of moving objects. – 3D-Machine vision extended by Siemens– Projector illuminates object with parallel stripes– Camera records pattern corresponding to height profile– Computer program calculated 3D-image in fraction of second– Light stripes coded color and time redundant– Depending on coding 3D-data determined from video image and moving

objects recorded.

Source: http:// www.pro-physik.de; http://www.siemens.de

5.44

Compilation of questions• Which factors limit the resolution of a spectrometer?• What is absorption spectroscopy?• What is interference spectroscopy?• How should the refractive index and layer thickness be chosen for an

antireflective layer?• Sketch the transmission spectrum of a MgF2–layer (n=1,38) on glass!• For two given spectra of layers on glass: Which layer has the higher refractive

index? Which layer is thicker?• Why is a Bragg-mirror reflective?• What is triangulation?• Sketch an optical system for visual triangulation!• Which optical elements may be employed as beam splitters?• How does a light stripe sensor work?• What is color coded triangulation?

Documents

2.1 Spectroscopy 2.4 Angle measurement Optical Engineering