2010 REU Lecture Optics and Instrument Design An overview

Erik Richard [email protected]

303.735.6629

Optics – REU Lecture 2010 Richard 1

Outline

•Brief Review: Nature of Light (Electromagnetic Radiation) –Propagation of E&M waves –Interaction with matter –Wave-particle duality –Atomic, Molecular absorptions and emssions –Blackbody Radiation – Planck –Sensors • Brief Review: Optics Concepts - Refraction, Reflection, Diffraction, Polarization - Prisms vs. gratings –Imaging characteristics of lenses and mirrors –Aberrations –”You can’t always get what you want” –Detectors •Instrument Design, Function and Calibration-Dave Crotzer Optics – REU Lecture 2010 Richard 2

Nature of Light (Electromagnetic Radiation)

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. Optics – REU Lecture 2010 Richard 3

Electromagnetic Spectrum

• Velocity, frequency and wavelength are related: c= l*n where: • c=3x10 8 m/sec is the velocity in vacuum  l and n are the wavelength and frequency respectively • Electromagnetic radiation is typically classified by wavelength: Optics – REU Lecture 2010 Richard 4

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 n =hc/ l • Where h=Planck’s constant h=6.63e

-34 Joule hz -1 • A visible light photon ( l =400 nm) has n =7.5 x 10 14 hz and E=4.97 x 10 -19 J Optics – REU Lecture 2010 Richard 5

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 surface. 1ev=1.6x10

-19 J n -W. W is the work function (depth of the ‘potential well’) for electrons in the Optics – REU Lecture 2010 Richard 6

A closer look at the Sun’s spectrum

Note log-scale for irradiance The hotter and higher layers produce complex EUV (10-120 nm) emissions dominated by multiply ionized atoms with irradiances in excess of the photospheric Planck distribution.

Optics – REU Lecture 2010 Richard 7

Atmospheric absorption of solar radiation

N 2 , O, O 2 Solar FUV and MUV radiation is the primary source of energy for earth’s upper atmosphere.

stratosphere troposphere

O 3

~99% solar radiation penetrates to the troposphere Altitude “contour” for attenuation by a factor of 1/e I(km) = 37% x I o

Optics – REU Lecture 2010 Richard 8

Optics – REU Lecture 2010

Atmospheric Absorption in the Wavelength Range from 1 to 15



m

Richard 9

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 2010 Richard 10

Optics – REU Lecture 2010

Blackbody Radiation Curves

u

(  ,

T

)  2

hc

2  5  

e hc



kT

1  1    Richard 11

Black body radiation

•

Planck distributions Hot objects emit A LOT more radiation than cool objects I (W/m 2 ) =



x T 4 The hotter the object, the shorter the peak wavelength T x



max = constant

Richard 12 Optics – REU Lecture 2010

Solar Spectral Irradiance

SORCE Instruments measure total solar irradiance and solar spectral irradiance in the 1 -2000 nm wavelength range.

Optics – REU Lecture 2010 Richard 13

Solar Cycle Irradiance Variations

Optics – REU Lecture 2010 The FUV irradiance varies by ~ 10-100% but the MUV irradiance varies by ~ 1-10% during an 11 year solar cycle. Richard 14

Solar variability across the spectrum

• 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.

Optics – REU Lecture 2010 Richard 15

Optics – REU Lecture 2010

Atmospheric Observation Modes

Direct Solar Radiation

(Solar or Stellar)

Richard 16

Optics – REU Lecture 2010

Functional Classes of Sensors

Richard 17

Elements of optical sensors characteristics

Sensor

What is there?

Spectral Characteristics Spectral bandwidth (  ) Resolution (  ) Out of band rejection Polarization sensitivity Scattered light

How much is there?

Radiometric Characteristics Detection accuracy Signal to noise Dynamic range Quantization level Flat fielding Linearity of sensitivity Noise equivalent power

Where is it located?

(or Pattern Recognition)

Geometric Characteristics Field of view Instan. Field of view Spectral band registration Alignments MTF’s Optical distortion Optics – REU Lecture 2010 Richard 18

The challenge

So… there is light (sometimes lots, sometimes not so much) How do we collect it?

How do we separate it?

How do we detect it?

How do we record it?

Finally, How do we know we’re correct?

Optics – REU Lecture 2010 Richard 19

Optics – REU Lecture 2010

Snell’s Law of Refraction - Derivation

Richard 20

Reflection and refraction

refractive index



speed of speed of light in vacuum light in medium Glass

:

n

 1.52

Water

:

n Air

:

n

 1.33

 1.000292

As measured with respect to the surface normal

:

angle of incidence



angle of reflection

Snell

'

s law

:

n

sin  

n

'sin  ' Optics – REU Lecture 2010 Richard 21

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

'sin  ' 

n

sin  sin 

c



n n

' sin 90 o 

n n

' Examples:

Water



to



air



c

 sin  1   1 1.33

   48.6

 Optics – REU Lecture 2010

Glass



to



air



c

 sin  1   1 1.52

   41.1

 Richard 22

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 2010 Richard 23

Polarization

p-polarized waves are linearly polarized waves parallel to the plane of incidence s-polarized waves are linearly polarized waves perpendicular to the plane of incidence

From German “parallel” & “senkrecht” Optics – REU Lecture 2010 Richard 24

Brewster’s Angle



n

sin   90 o 

n

'sin   

n

'sin(90 o   ) 

n

'cos   

B

 arctan  

n

'

n

  Examples:

Water



to



air



B

 tan  1   1.33

1    53.1



Glass



to



air



B

 tan  1   1.52

1    56.6

 Optics – REU Lecture 2010 Richard 25

Fresnel Reflection Equations

Polarization dependent Reflection fraction vs. incident angle

R s

(  )    sin(  sin(       )  )   2   

n

cos 

n

cos  cos    2 cos   

R p

(  )    tan(   tan(      )  )   2   

n

cos  

n

cos   Normal incidence

R

  

n n

  2 cos  cos   2  Augustin-Jean Fresnel 1788-1827 Examples: Air-to-water : R=2.0% Air-to-glass : R=4.2% Optics – REU Lecture 2010 Richard 26

Fresnel Reflection

Air-to-salt salt-to-air

Optics – REU Lecture 2010

Salt: AgCl (near-IR)

Richard 27

Familiar Examples of Brewster and TIR

Brewster’s: HeNe laser cell Round trip gain must exceed round trip reflection losses to achieve laser output Want to MINIMIZE reflection here Optics – REU Lecture 2010 TIR: Diamond cutting Want to MAXIMIZE reflection here Brilliant diamond cut must maximize light return through the top.

Richard 28

Rainbow

Optics – REU Lecture 2010 Richard 29

Optics – REU Lecture 2010

Prism refraction

n

 1

n

     1    2   sin sin  1  1  

n

 

n

sin sin  2  2     1   2    2 Richard 30

Optics – REU Lecture 2010 Richard 31

Optical dispersion issues

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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 ll < 150 ppm) In-flight re-calibration – Prism transmission calibration – Duty cycling 2 independent spectrometers Optics – REU Lecture 2010 Richard 33

Optics – REU Lecture 2010 n’

SIM Prism in Littrow

Al coated Back surface 2   sin  1   sin 

n

'    sin  1   sin( 

n

'   )   Richard 34

Optics – REU Lecture 2010

SIM Optical Image Quality

Richard 35

Optics – REU Lecture 2010 Richard 36

Optics – REU Lecture 2010

SIM Measures the Full Solar Spectrum

Richard 37

Optical displacements “Careful!”

Optics – REU Lecture 2010 For small angles:

d



t



n

 1

n

Richard 38

Focal length (thin lens)

Optics – REU Lecture 2010 Richard 39

C

h r o m a t i

c

Aberration

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C

h r o m a t i

c

Aberration

Optics – REU Lecture 2010 Richard 41

C

h r o m a t i

c

Aberration

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Focal ratio (f/#)

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Focal ratio con’t

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Optical Transmission

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Reflection or Refraction?

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Reflection

Optics – REU Lecture 2010 Richard 48

Auxiliary Optical Elements for Gratings

Lenses are often used as elements to collimate and reimage light in a diffraction grating spectrometer.

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 2010 Richard 49

Diffraction grating fundamentals

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 = dsin  & AB = -dsin  m  = d(sin  + sin  ) Note: sign convention is “minus” when diffracted beam is on opposite side of grating normal than incidence beam; “plus” when on same side Optics – REU Lecture 2010 Richard 50

Diffraction grating fundamentals

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  m that obey the grating equation: m

.

 = d .

(sin(  )+sin(  m )). In the special case that m=0, a grating acts like a plane mirror and  =  Optics – REU Lecture 2010 Blue (400 nm) and red (650 nm) light are dispersed into orders m=0, ± 1, and ± 2 Richard 51

Grating example

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

 1

mm

1450

x

10 6

nm mm

 689.7

nm

  689.7

nm

 sin 48  

n

sin 20    Wavelength (nm) 748.4

nm n

 n=1 n=2 n=3 20 10 0 -10 748 632 513 393 374 316 256 196 249 211 171 131 Optics – REU Lecture 2010 Richard 52

Reflection Grating Geometry

Gratings work best in collimated light and auxiliary optical elements are required to make a complete instrument Plane waves, incident on the grating, are diffracted into zero and first order Rotating the grating causes the diffraction angle to change 650 nm 400 nm Zero order Optics – REU Lecture 2010  

d

 (sin(  )  sin(  ))   Richard 53 

Typical Plane Grating Monochromator Design

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.

Grating spectrometer using two concave mirrors to collimate and focus the spectrum Entrance Slit Exit Slit Detector Optics – REU Lecture 2010 Richard 54

Optics – REU Lecture 2010

Diffraction

Fraunhofer single slit diffraction involves the spreading out of waves past openings which are on the same order of the wavelength of the wave

Richard 55

Na spectral lines

Resolving Power

Instrument & Detector Na D-lines D 1 =589.6 nm D 2 =589.0 nm Optics – REU Lecture 2010 Richard 56

Free spectral range

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    

m

 1  1

m

Optics – REU Lecture 2010 Richard 57

Resolving Power

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).

Optics – REU Lecture 2010 Richard 58

SOLSTICE: Channel Assembly

Optics – REU Lecture 2010 ‘A’ Channel During Preliminary Alignment Test Richard 59

SOLSTICE: Channel Assembly

Optics – REU Lecture 2010 Richard 60

Solstice Instrument The

SOL

ar-

ST

ellar

I

rradiance

C

omparison

E

xperiment 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 Channels on the IM SOLSTICE B Optics – REU Lecture 2010 SOLSTICE A Single SOLSTICE Channel - Dimensions: 88 x 40 x 19 cm - Mass: 18 kg - Electrical Interface: GCI Box Richard 61

SOLSTICE Grating Spectrometer

• SOLSTICE cleanly resolves the Mg II h & k lines Optics – REU Lecture 2010 Richard 62

Optical Aberrations

Optics – REU Lecture 2010 Richard 63

Optical Aberrations

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Optical Aberrations

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Optical Aberrations

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Spherical Aberration

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Coma

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Astigmatism

Optics – REU Lecture 2010 Richard 70

Optical Aberrations

Optics – REU Lecture 2010 Richard 71

Optics – REU Lecture 2010 Richard 72

Optical Aberrations

Optics – REU Lecture 2010 Richard 73

Focus!!!

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Optics – REU Lecture 2010

Unwanted & Scattered Light

Richard 76

Optics – REU Lecture 2010

Cassegrain Baffling Example

Richard 77

The End Game

Optics – REU Lecture 2010 Richard 78

Optical Detection

Optics – REU Lecture 2010 Richard 79

Optics – REU Lecture 2010

“What’s the Frequency--Albert?”

Richard 80

Photomultiplier Tube Detectors Single photon detection (pulse counting) with an PMT Output pulse Ground -1200 V •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 10 6 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 Optics – REU Lecture 2010 Richard 81

Optics – REU Lecture 2010 Richard 82

More Nomenclature

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Optics – REU Lecture 2010 Richard 85