Components of Optical Instruments

1
Components of Optical Instruments
Chapter 7
Chapter 7
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2
Components of optical instruments
1. Absorption
3. Emission and chemiluminescence
2. Fluorescence and phosphorescence
Source Wavelength Selector Sample
Detector Readout
Rgb back 146,184. 148
3
Construction Materials for Spectroscopic
Instruments
Wavelength 100 180 380 850
2000
18000 40000
Region VUV UV Visible
Near IR IR Far
IR
LiF
Fused silica or quartz
Materials for cells, windows, lenses and prisms
Corex glass
Silica glass
NaCl
KBr
TlBr or TlI
ZnSe
4
Sources for Spectroscopic Instruments
Wavelength 100 180 380 850 2000
18000 40000
Region VAC UV Visible Near IR
IR Far IR
Ar lamp
Sources Continous Line
Xe lamp
H2 or D2 lamp
Tungtstan lamp
Nernst glower ZnO2Y2O3
Nichrome wire
Glowbar SiC
Hollow cathode lamp
Lasers
5
Wavelength Selectors for Spectroscopic
Instruments
Wavelength 100 180 380 850
2000
18000 40000
Region VAC UV Visible
Near IR IR Far
IR
Fluorite prism
Wavelength selectors Continous Dis-continous
Fused silica or quartz prism
Glass prism
NaCl prism
KBr prism
3000 lines/mm Grating 50
lines/mm
Interference wedge
Interference filter
Glass filter
6
Detectors for Spectroscopic Instruments
Wavelength 100 180 380 850 2000
18000 40000
Region VAC UV Visible Near IR
IR Far IR
Photographic plates
Detectors Photo- electric Thermal
Photomultiplier tube
Photo tubes
Photo cells
Photo diodes
Charge coupled devices
Photo conductors
Thermocouples or bolometers
Golay pneumatic cell
Pyroelectric cell
7
Light Amplification by Stimulated Emission of
RadiationLASER

• Characteristics of a laser
• Spatially narrow and intense
• Highly monochromatic
• Coherence

8
Schematic of a Laser Source
Nonparallel radiation
Active lasing medium
Laser radiation
Mirror
Partially transmitting mirror
Radiation
Pumping source
Power supply
9
Processes in Laser Action
1- Pumping Excitation by electrical, radiant or
chemical energy
1- Pumping 2- Spontaneous emission
1- Pumping 2- Spontaneous emission 3- Stimulated
emission
1- Pumping 2- Spontaneous emission 3- Stimulated
emission 4- Absorption
Ey Ey Ey Ey Ex
Metastable Excited state
10
Processes in Laser Action
1- Pumping Excitation by electrical, radiant or
chemical energy
1- Pumping 2- Spontaneous emission
1- Pumping 2- Spontaneous emission 3- Stimulated
emission
1- Pumping 2- Spontaneous emission 3- Stimulated
emission 4- Absorption
Ey Ey Ey Ey Ex
Metastable Excited state
11
Population Inversion
Light attenuation by absorption

Noninverted population
Light amplification by stimulated emission
Inverted population
12
Laser Systems
E1
E1
Ey
Ey
Ex
E0
E0
Three level system
Four level system
13
Conduction and Valance Bands
Conduction band
Conduction band
Conduction band
Valence band
Valence band
Valence band
Conductor
Semiconductor
Insulator
14
Diode Laser
p-metal
Emitted radiation
3mm strip width
n- type GaAs substrate
n-metal
15
Frequency Doubling
974 nm laser
450 nm laser
Blue-green output
Nonlinear crystal
Laser diode
16
Wavelength Selectors

• Filters
• Interference
• Absorption
• Gratings
• Prisms

17
Interference Filters
Glass plate
Metal film
Dielectric layer
Metal film
Glass plate
18
Interference Filters
q
Metal film
t
Dielectric layer
Metal film

• Condition for reinforcement nl 2t/cos q
• If q lt 10o nl 2t
• lh
• l 2th/n

19
Wedge Type Interference Filters
Glass plate
Metal film
Dielectric layer
Metal film
Glass plate
l1
l2
20
Transmission Characteristic of Interference
Filters
100
Effective bandwidth 15 A
80
Effective bandwidth 15 A
60
Effective bandwidth 10 A
Percent Transmittance
Effective bandwidth
40
1/2 Peak height
20
0
5090 5110 6215 6225 6940
6960
Wavelength
21
Effective Bandwidth of Filters
100
Interference Filter
80
60
Effective bandwidth10nm
Percent Transmittance
40
Absorption Filter
Effective bandwidth 50nm
20
0
40 450
500 550

22
Coupling of Filters
100
Orange cut-off filter
Green filter
50
Combination of two filters
Percent Transmittance
0
400 500
600 700
Wavelength nm
23
Czerny Turner Grating Monochromator
Concave mirrors
Reflection grating
l1
l2
Entrance slit
Exit slit
Focal plane
24
Dispersion of Monochromators
200 300 400 500
600 700 800
nm
Grating
200 350
400 450 500 600 800
nm
Glass prism
200 250
300 350 400 500 800
nm
Quartz prism
cm
0 5 10
15 20
25
Distance along focal plane
25
Bunsen Prism Monochromator
Entrance slit
Focal plane
l1
Exit slit
Collimating lens
l2
Focusing lens
Prism
26
Dispersion by a Prism
a
r
i
Mirror
r
b
Quartz cornu
Littrow
27
The Echellette Grating
Detector
Source
Diffracted beams at refelected angle r
Monochromatic beams at incident angle i
d sin i d sin r l
r
n1
dsin i
i
dsin r
d
28
The Echellette Grating
Diffracted beams at refelected angle r
Source
dsin r
dsin i
d sin i d sin r 1.5 l
Monochromatic beams at incident angle i
r
dsin i
i
dsin r
d
29
Photolithography
UV lamp
Mask
Photoresist
Plate
Developing solution
Etching solution
30
Performance Characteristics of a Grating
Monochromator

• The purity of its radiant output
• Ability to resolve adjacent wavelengths
• Light-gathering power
• Spectral bandwidth.

31
Spectral Purity

• The exit beam of a monochromator is usually
  contaminated with small amounts of scattered or
  stray radiation with wavelengths far different
  from that of the instrument setting.
• Sources of stray radiation
• Reflections from optical parts due to mechanical
  imperfections.
• Scattering by dust particles in the atmosphere or
  on the surfaces of optical parts
• To minimize stray radiation
• Introduction of baffles
• Coating interior surfaces with flat black paint
• Sealing the monochromator

l l1
l
Exit slit
32
Dispersion of Grating Monochromators

•  

l1
l2
Focal plane
Entrance slit
Exit slit
33
Resolving Power

•  

34
Light-Gathering Power of Monochromators

•  

35
Echelle Monochromator
i
r i b 63o26
r
36
Echelle grating
30 Degree prism
240
260
220
108
260
280
300
98
340
320
300
280
Prism dispersion Diffraction order, n
88
400
380
360
340
320
78
500
420
480
440
460
68
500
600
620
580
540
560
520
58
48
720
680
760
800
740
700
640
660
480
Wavelength, nm Grating dispersion
37
Effect of Slit Width on Resolution

• Entrance and exit slits are identical in width
• Movement of the monochromator from a setting l1
  to l3 results in the image filling and being
  moved completely out of the slit.
• The bandwidth is defined as the span of
  monochromator settings (in units of wavelength)
  needed to move the image of the entrance slit
  across the exit slit.
• The effective bandwidth is one half the bandwidth

Exit slit
Detector
Effective Bandwidth
Power
Bandwidth

• l1 l2 l3
• Wavelength

38
Effect of Slit Width on Resolution
Slit width l2-l1
Slit width 0.75 (l2-l1)
Slit width 0.5 (l2-l1)
Detector
Effective Bandwidth
Power

• l1 l2 l3

39
Effect of Spectral Bandwidth
0.700
0.600
Absorbance
1.0 nm bandwidth
Absorbance
0.5 nm bandwidth
0.100
0.100
220
275
220
275
Wavelength, nm
Wavelength, nm
(a)
(b)
0.600
Absorbance
2.0 nm bandwidth
0.100
275
220
Wavelength, nm
(c)
40
Characteristics of Radiation Transducers
1. High sensitivity
2. High signal to noise ratio
3. Constant response over considerable range of
wavelength
4. Fast response time
5. Zero output signal in the absence of
illumination
6. The electrical signal would be directly
proportional to radiant power S kP S
kPkd
41
Types of Radiation Transducers

• Photon transducers
• UV, Vis, near IR
• Heat transducers
• IR, far IR

42
Response of Detectors
1015
Photomultiplier tube
1014
1013
CdS photoconductivity cell
Spectral response.
1012
GaAs photovoltaic cell
CdSe photoconductivity cell
1011
PbS photoconductivity cell
silicon photodiode
1010
Se/SeO photovoltaic cell
Thermocouple
Golay
109
200 600 1000
1400 1800 2200
Wavelength nm
43
Photon Transducers

• Photovoltaic
• Phototube
• Photomultiplier tubes
• Photoconductivity transducers
• Silicon photodiodes
• Charge-coupled device

44
Barrier Layer Cell
Thin layer of silver
Glass
Selenium
Plastic case
Iron
-

45
Characteristics of Photovoltaic Cells

• Cell current 10 100 mA
• No external electrical energy required.
• Usually used for low level signals.
• Low internal resistance amplification not
  convenient
• Fatigue
• Low cost

46
Photoelectric Effect
47
Phototube
Cathode
Wire anode
90 Vdc
48
Response of some Photoemissive Surfaces
80
K/Cs/Sb
60
Sensitivity, mA/w
40
Ga/As
Ag/O/Cs
20
400
600
800
1000
200
Wavelength, nm
49
Dynode Potential(V) Number of

electrons
1 90
1
5
2 180
10
7
3
3 270
100
4
4 360
103
6
2
1
5 450
104
8
6 540
105
9
7 630
106
Quartz envelope
Anode
Grill
8 720
107

9 810
108
Photoemissive Cathode
Anode 900V Gain 108
900V dc
Photoemissive Cathode
Dynodes 1-9
Anode
To readout
50
Features of Photomultiplier Tubes

• High sensitivity in UV, Vis, and NIR
• Limited by dark current
• Cooling to -30oC improves response
• Extremely fast time response
• Limited to measuring low-level signals

51
Silicon Diode
pn junction
Metal contact
Lead wire
n region
p region
Fig. 7.30
52
Silicon diode under Revese Bias
Reverse bias
Depletion layer
53
Silicon Diode under Forward Bias
54
Reset
Start

10-Bit shift register
Clock

-5V

Diode number
Spectrum

Diode number 1 2
1024
55
Photodiode Array
FET switch closes and the integrating capacitor
dischages
The integrating capacitor is charged proportionaly
Start Pulse
Clock Pulse
10-Bit shift register.
Integrating amplfier
Start
An output proportional to the charge is generated.
Clock
Capacitor dischages
Photodiodeconducts
FET switch
-5 V
FET switch closes
Diode number
Ohter capacitor charge up
The capacitor charges up
The capacitor is charged to the original value
Photodiode
10pF Capacitor
Read A start pulse initializes the shift
register and clock pulses sequentially close FET
switches. This charges up the capacitors and in
turn charges the integrating capacitor
proportionaly. The output data is a voltage
proportional to charge of the capacitor.
Irradiation Shutter is opened and the spectrum
is focused on the PAD. Note that some diodes
receive more photons and some less.
Charging After a start pulse the shift register
sequentially closes FET switches momentarily to
charge up the capacitors
At this stage the PDA is ready to start the cycle
again.
Diode No. 1 2
1024
56
Charge Coupled Devices
5V
10V
-
-
V2
V1
Electrodes
SiO2 Insulator
n- doped silicon
Substrate
57
Charge Coupled Devices
f1
Three phase clock output
f2
f3
Optical input
Metallic electrodes
Insulator SiO2
Potential well
p type silicon
58
Thermocouples
15V
Spectrophotometer slit
Thermocouple junction
To amplifier
Reference junction
-15V
59
Optical Fibers
Coating with refractive index n2
Light path
Fiber with refractive index n1
Refractive index of medium n3
Numerical aperturen3.sinqsqrt(n12n22) n1gtn2gtn3

60
Optical fibers of different lengths
3
4
3 3 star coupler
Fluorescence signals
Dye laser
5
2
1
6
Laser excitation
Monochromator and detection system
Sample 1
Sample 2
Sample 3
61
Iron Emission Spectrum
1.0
Fe(I)
0.8
Fe(I)
Ni(I)
0.6
Ni(I)
0.4
Cr(I)
Cr(I)
Fe(I)
0.2
Fe(I)
Cr(I)
Fe(II)
Cr(II)
Cr(II)
Cr(II)
0
33250 33275 33300
33325 33350
Wavenumber cm-1
62
Time Domain Spectroscopy
Frequency domain
Time domain
P(n)
P(n)
F2
P(I)
Frequency
F1
F2
P(n)
P(I)
63
Time Domain Spectroscopy
64
Michelson Interferometer
Movable mirror
Beam splitting mirror
Source
Fixed Mirror
Detector
l
-1
2
1
3
65
P(s)
P(d)
_

s cm-1
d cm
P(s)
P(d)
s cm-1
_

d cm
P(s)
P(d)
_
s cm-1

d cm