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Optics – REU Lecture 2009 Richard 1
REU Lecture
Optics and Optical Design
Erik Richarderik.richard@lasp.colorado.edu
303.735.6629
Optics – REU Lecture 2009 Richard 2
Outline
•Brief Review: Nature of Light (Electromagnetic Radiation)–Propagation of E&M waves–Interaction with matter–Wave-particle duality
• Brief Review: Optics Concepts- Refraction - Reflection- Diffraction grating characteristics–Imaging characteristics of lenses and mirrors–Detectors
•Instrument Design and Function–Drawings–Block Diagram–Mechanisms
Optics – REU Lecture 2009 Richard 3
Classical Definition: Energy Propagating in the form of waves– Many physical processes give rise to E&M radiation including
accelerating charged particles and emission by atoms and molecules.
Nature of Light (Electromagnetic Radiation)
Optics – REU Lecture 2009 Richard 4
Electromagnetic Spectrum
• Velocity, frequency and wavelength are related: c=where: • c=3x108 m/sec is the velocity in vacuum and are the wavelength and frequency respectively
• Electromagnetic radiation is typically classified by wavelength:
Optics – REU Lecture 2009 Richard 5
Nature of Light: Wave-Particle Duality
• Light behaves like a wave– While propagating in free space (e.g. radio waves)
– On a macroscopic scale (e.g. while heating a thermometer)
– Demonstrates interference and diffraction effects
• Light behaves as a stream of particles (called photons)– When it interacts with matter on a microscopic scale
– Is emitted or absorbed by atoms and molecules
• Photons:– Travel at speed of light
– Possess energy: E=h=hc/• Where h=Planck’s constant h=6.63e-34 Joule hz-1
• A visible light photon ( =400 nm) has=7.5 x 1014 hz and E=4.97 x 10-19 J
Optics – REU Lecture 2009 Richard 6
Nature of Light: Photon Examples
Atoms and Molecules Photoelectric Effect
The nature of the interaction depends on photon wavelength (energy).
Electron kinetic energy: K.E.=h-W. W is the work function (depth of the ‘potential well’) for electrons in the
surface. 1ev=1.6x10-19J
Optics – REU Lecture 2009 Richard 7
The hotter and higher layers produce complex EUV (10-120 nm) emissionsdominated by multiply ionized atoms with irradiances in excess of the photospheric Planck distribution.
A closer look at the Sun’s spectrum
Note log-scale for irradiance
Optics – REU Lecture 2009 Richard 8
Alt
itu
de (
km)
Atmospheric absorption of solar radiation
Altitude “contour” for attenuation bya factor of 1/e
~99% solar radiationpenetrates to the
troposphere
I(km) = 37% x Io
troposphere
stratosphere
Solar FUV and MUV radiation is the primary source of energy for earth’s upper atmosphere.
O3
N2, O, O2
Optics – REU Lecture 2009 Richard 9
Atmospheric Absorption in the Wavelength
Range from 1 to 15 m
Optics – REU Lecture 2009 Richard 10
Black Body Radiation
• An object radiates unique spectral radiant flux depending on the temperature and emissivity of the object. This radiation is called thermal radiation because it mainly depends on temperature. Thermal radiation can be expressed in terms of black body theory.
Black body radiation is defined as thermal radiation of a black body, and can be given by Planck's law as a function of temperature T and wavelength
Optics – REU Lecture 2009 Richard 11
Blackbody Radiation Curves
€
u(λ ,T) =2hc 2
λ 5
1
ehc
λkT −1
⎛
⎝ ⎜ ⎜
⎞
⎠ ⎟ ⎟
Optics – REU Lecture 2009 Richard 12
Black body radiation
• Planck distributions
Hot objects emit A LOT moreradiation than cool objects
The hotter the object, theshorter the peak wavelength
I (W/m2) = x T4
T x max = constant
Optics – REU Lecture 2009 Richard 13
Solar Spectral Irradiance
SORCE Instruments measure total solar irradiance and solar spectral irradiance in the 1 -2000 nm wavelength range.
Optics – REU Lecture 2009 Richard 14
Solar Cycle Irradiance Variations
The FUV irradiance varies by ~ 10-100% but the MUV irradiance varies by ~ 1-10% during an 11 year solar cycle.
Optics – REU Lecture 2009 Richard 15
• Solar irradiance modulated by presence of magnetic structures on the surface of the Sun……Solar Rotation (short) Solar Cycle (longer)
• The character of the variability is a strong function of wavelength.
Greatest absolute variability occurs in mid visible
Greatest relative variability occurs in the ultraviolet.
Solar variability across the spectrum
Optics – REU Lecture 2009 Richard 16
Atmospheric Observation Modes
Direct Solar Radiation
Optics – REU Lecture 2009 Richard 17
Functional Classes of Sensors
Optics – REU Lecture 2009 Richard 18
Element of optical sensors characteristics
Sensor
Spectral bandwidth () Resolution ()
Out of band rejectionPolarization sensitivity
Scattered light
Detection accuracySignal to noiseDynamic range
Quantization levelFlat fielding
Linearity of sensitivityNoise equivalent power
Field of viewInstan. Field of view
Spectral band registrationAlignments
MTF’sOptical distortion
Spectral Characteristics Radiometric Characteristics Geometric Characteristics
Optics – REU Lecture 2009 Richard 19
refractive index = speed of light in vacuumspeed of light in medium
Glass : n =1.52Water : n=1.33Air : n=1.000292
As measured with respect to the surface normal :
angle of incidence = angle of reflection
Snell 's law :
nsinθ = n'sinθ '
Reflection and refraction
Optics – REU Lecture 2009 Richard 20
Critical angle for refraction
An interesting thing happens when light is going from a material with higher index to lower index, e.g. water-to-air or glass-to-air…there is an angle at which the light will not pass into the other material and will be reflected at the surface.
Using Snell’s law:
n 'sinθ ' = nsinθ
sinθc = nn'
sin90o = nn'
Examples:
Water−to−air
θc =sin−1 11.33
⎛⎝⎜
⎞⎠⎟=48.6°
Glass−to−air
θc =sin−1 11.52
⎛⎝⎜
⎞⎠⎟=41.1°
Optics – REU Lecture 2009 Richard 21
Total internal reflection
At angles > critical angle, light undergoes total internal reflection
It is common in laser experiments to use “roof-top” prisms at 90° reflectors.(Note:surfaces are typically antireflection coated)
Optics – REU Lecture 2009 Richard 22
Water−to−air
θB =tan−1 1.331
⎛⎝⎜
⎞⎠⎟=53.1°
Glass−to−air
θB =tan−1 1.521
⎛⎝⎜
⎞⎠⎟=56.6°
Examples:
θ + ′θ =90o
nsinθ = n 'sin ′θ = n 'sin(90o −θ ) = n 'cosθ
∴ θB = arctann '
n⎛⎝⎜
⎞⎠⎟
Brewster’s Angle
Optics – REU Lecture 2009 Richard 23
Fresnel Reflection Equations
Rs (θ) =sin( ′θ −θ)sin( ′θ +θ)
⎡
⎣⎢
⎤
⎦⎥
2
=ncosθ − ′n cos ′θncosθ + ′n cos ′θ
⎡⎣⎢
⎤⎦⎥
2
Rp(θ) =tan( ′θ −θ)tan( ′θ +θ)
⎡
⎣⎢
⎤
⎦⎥
2
=ncos ′θ − ′n cosθncos ′θ + ′n cosθ
⎡⎣⎢
⎤⎦⎥
2
R =n− ′nn+ ′n
⎛⎝⎜
⎞⎠⎟
2Examples: Air-to-water : R=2.0%Air-to-glass : R=4.2%
Polarization dependent Reflection fraction vs. incident angle
Normal incidence
Augustin-Jean Fresnel1788-1827
Optics – REU Lecture 2009 Richard 24
Fresnel Reflection
Air-to-salt salt-to-air
Salt: AgCl (near-IR)
Optics – REU Lecture 2009 Richard 25
Brewster’s: HeNe laser cell
TIR: Diamond cutting
Familiar Examples of Brewster and TIR
Want to MINIMIZE reflection here
Round trip gain must exceed round trip reflection losses to achieve laser output
Want to MAXIMIZE reflection here
Brilliant diamond cut must maximize light return through the top.
Optics – REU Lecture 2009 Richard 26
α
θ1 ′θ1
α
β γ′θ2
θ2
δ
′nn
sinθ1
sin ′θ1
=′n
n=
sinθ2
sin ′θ2
δ =θ1 +θ2 −α
Prism refraction
Optics – REU Lecture 2009 Richard 27
Optics – REU Lecture 2009 Richard 28
Second issue: Optical dispersion
Optics – REU Lecture 2009 Richard 29
Spectral Irradiance Monitor SIM
• Measure 2 absolute solar irradiance spectra per day
• Wide spectral coverage– 200-2400 nm
• High measurement accuracy– Goal of 0.1% (1)
• High measurement precision– SNR 500 @ 300 nm – SNR 20000 @ 800 nm
• High wavelength precision– 1.3 m knowledge in the focal
plane– (or < 150 ppm)
• In-flight re-calibration– Prism transmission calibration– Duty cycling 2 independent
spectrometers
Optics – REU Lecture 2009 Richard 30
SIM Prism in Littrow
2θ = sin−1 sinγn'
⎛⎝⎜
⎞⎠⎟+ sin−1 sin(γ −φ)
n'⎛⎝⎜
⎞⎠⎟
n’
Al coatedBack surface
Optics – REU Lecture 2009 Richard 31
SIM Optical Image Quality
Optics – REU Lecture 2009 Richard 32
Optics – REU Lecture 2009 Richard 33
SIM Measures the Full Solar Spectrum
Optics – REU Lecture 2009 Richard 34
d ≈t⋅n−1
nFor small angles:
Optical displacements “Careful!”
Optics – REU Lecture 2009 Richard 35
Focal length (thin lens)
Optics – REU Lecture 2009 Richard 36
Chromatic Aberration
Optics – REU Lecture 2009 Richard 37
Chromatic Aberration
Optics – REU Lecture 2009 Richard 38
Chromatic Aberration
Optics – REU Lecture 2009 Richard 39
Focal ratio (f/#)
Optics – REU Lecture 2009 Richard 40
Focal ratio con’t
Optics – REU Lecture 2009 Richard 41
Optics – REU Lecture 2009 Richard 42
Optical Transmission
Optics – REU Lecture 2009 Richard 43
Reflection or Refraction?
Optics – REU Lecture 2009 Richard 44
Reflection
Optics – REU Lecture 2009 Richard 45
Beam 2 travels a greater distance than beam 1 by
(CD - AB)
For constructive interference
m = (CD-AB)
m is an integer called the diffraction order
CD = dsinα & AB = -dsinβ
m = d(sinα + sinβ)
Note: sign convention is “minus” when diffracted beam is on opposite side of gratingnormal than incidence beam; “plus” when on same side
Diffraction grating fundamentals
Optics – REU Lecture 2009 Richard 46
Diffraction gratings use the interference pattern from a large number of equally spaced parallel grooves to disperse light by wavelength.
Light with wavelength that is incident on a grating with angle a is diffracted into a discrete number of angles βm that obey the grating equation: m. = d.(sin(α)+sin(βm)). In the special case that m=0, a grating acts like a plane mirror and β=-α
Blue (400 nm) and red (650 nm) light are dispersed into orders m=0,±1, and ±2
Diffraction grating fundamentals
Optics – REU Lecture 2009 Richard 47
Illuminate a grating with a blaze density of 1450 /mm With collimated white light and a incidence angle of 48°, What are the ’s appearing at diffraction angles of +20°, +10°, 0° and -10°?
d =1mm1450
x 106 nmmm
= 689.7 nm
=689.7nm
nsin 48° + sin 20°( ) =
748.4
nnm
β n=1 n=2 n=3
20 748 374 249
10 632 316 211
0 513 256 171
-10 393 196 131
Wavelength (nm)
Grating example
Optics – REU Lecture 2009 Richard 48
Plane waves, incident on the grating, are diffracted into zero and first order
650 nm
400 nm
Zero order
Reflection Grating Geometry
Rotating the grating causes the diffraction angle to change
α
€
λ=d•(sin(α)+sin(β))
Gratings work best in collimated light and auxiliary optical elements are required to make a complete instrument
Optics – REU Lecture 2009 Richard 49
Lenses are often used as elements to collimate and reimage light in a diffraction grating spectrometer.
Auxiliary Optical Elements for Gratings
Imaging geometry for a concave mirror. Tilted mirrors:1. Produce collimated light when p=f (q=infinity).2. Focus collimated light to a spot with q=f (p=infinity).
Optics – REU Lecture 2009 Richard 50
Entrance Slit
Exit Slit
Detector
Grating spectrometer using two concave mirrors to collimate and focus the spectrum
Only light that leaves the grating at the correct angle will pass through the exit slit. Tuning the grating through a small angle counter clockwise will block the red light and allow the blue light to reach the detector.
Typical Plane Grating Monochromator Design
Optics – REU Lecture 2009 Richard 51
Resolving Power
Na spectral lines
Na D-linesD1=589.6 nmD2=589.0 nmInstrument & Detector
Optics – REU Lecture 2009 Richard 52
For a given set of incidence and diffraction angles, the grating equation is satisfied for a different wavelength for each integral diffraction order m. Thus light of several wavelengths (each in a different order) will be diffracted along the same direction: light of wavelength λ in order m is diffracted along the same direction as light of wavelength λ/2 in order 2m, etc.
The range of wavelengths in a given spectral order for which superposition of light from adjacent orders does not occur is called the free spectral range Fλ.
1 + Δλ =m +1
m λ1
Free spectral range
Optics – REU Lecture 2009 Richard 53
The resolving power R of a grating is a measure of its ability to separate adjacent spectral lines of average wavelength λ. It is usually expressed as the dimensionless quantity
R =
=mN
Here ∆λ is the limit of resolution, the difference in wavelength between two lines of equal intensity that can be distinguished (that is, the peaks of two wavelengths λ1 and λ2 for which the separation |λ1 - λ2| < ∆λ will be ambiguous).
Resolving Power
Optics – REU Lecture 2009 Richard 54
SOLSTICE: Channel Assembly
‘A’ Channel During Preliminary Alignment Test
Optics – REU Lecture 2009 Richard 55
SOLSTICE: Channel Assembly
Optics – REU Lecture 2009 Richard 56
Solstice Instrument
The SOLar-STellar Irradiance Comparison Experiment consists of two identical channels mounted to the SORCE Instrument Module on orthogonal axes. They each measure solar and stellar spectral irradiances in the 115 - 320 nm wavelength range.
SOLSTICE B
SOLSTICE A
SOLSTICE Channels on the IM
Single SOLSTICE Channel - Dimensions: 88 x 40 x 19 cm - Mass: 18 kg - Electrical Interface: GCI Box
Optics – REU Lecture 2009 Richard 57
SOLSTICE Grating Spectrometer
• SOLSTICE cleanly resolves the Mg II h & k lines
Optics – REU Lecture 2009 Richard 58
Optics – REU Lecture 2009 Richard 59
Optical Aberrations
Optics – REU Lecture 2009 Richard 60
Optical Aberrations
Optics – REU Lecture 2009 Richard 61
Optical Aberrations
Optics – REU Lecture 2009 Richard 62
Optics – REU Lecture 2009 Richard 63
Optical Aberrations
Optics – REU Lecture 2009 Richard 64
Spherical Aberration
Optics – REU Lecture 2009 Richard 65
Coma
Optics – REU Lecture 2009 Richard 66
Astigmatism
Optics – REU Lecture 2009 Richard 67
Astigmatism
Optics – REU Lecture 2009 Richard 68
Optical Aberrations
Optics – REU Lecture 2009 Richard 69
Optics – REU Lecture 2009 Richard 70
Optical Aberrations
Optics – REU Lecture 2009 Richard 71
Optics – REU Lecture 2009 Richard 72
Unwanted & Scattered Light
Optics – REU Lecture 2009 Richard 73
Cassegrain Baffling Example
Optics – REU Lecture 2009 Richard 74
The End Game
Optics – REU Lecture 2009 Richard 75
Optical Detection
Optics – REU Lecture 2009 Richard 76
“What’s the Frequency--Albert?”
Optics – REU Lecture 2009 Richard 77
Photomultiplier Tube Detectors
-1200 VGround
Output pulse
•A photon enters the window and ejects an electron from the photocathode (photoelectric effect)•The single photoelectron is accelerated through a 1200 volt potential down series of 10 dynodes (120 volts/dynode) producing a 106 electron pulse.•The electron pulse is amplified and detected in a pulse-amplifier-discriminator circuit.•Solstice uses two PMT’s in each channel that are optimized for a specified wavelength range
–CsTe (‘F’) Detector Photocathode) 170-320 nm
–CsI (‘G’) Detector Photocathode) 115-180 nm
Single photon detection (pulse counting) with an PMT
Optics – REU Lecture 2009 Richard 78
Optics – REU Lecture 2009 Richard 79
More Nomenclature
Optics – REU Lecture 2009 Richard 80
Optics – REU Lecture 2009 Richard 81
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