COMPONENTS OF OPTICAL
INSTRUMENTS
Chapter 7_II
UV, Visible and IR Instruments
Topics
A.
B.
C.
D.
E.
F.
G.
H.
I.
GENERAL DESIGNS
SOURCES
WAVELENGTH SELECTORS
SAMPLE CONTAINERS
RADIATION TRANSDUCERS
SIGNAL PROCESSORS AND READOUT
FIBER OPTICS
TYPES OF OPTICAL INSTRUMENTS
PRINCIPLES OF FOURIER TRANSFORM
OPTICAL MEASUREMENTS
1
Semiconductor Diodes Lasers
Semiconductor diode
• A device that has greater conductivity in one direction than
in the other
Chap.2 p. 45
Forward biased
current
Reverse biased
Resistance to current
2
Semiconductor diodes
• Made by forming n-type and p-type region within a single silicon (Si)
or germanium (Ge) crystal
• Interface between the regions: pn junction
• Si : group IV element, semiconductor
• n-type: doped with group V element
– Arsenic (As) or Antimony (Sb) introduces one unpaired (non bonding)
electron in the lattice
– negative charges are the majority charge carriers
• p-type: doped with group III element
– Gallium (Ga) or Indium (In) introduces a “hole” in the Si lattice
– Movement of holes from silicon atom to silicon atom constitute a
current.
– positive charges are the majority charge carriers
• pn junction: add p-type imputrity in minute Si chip doped with an ntype impurity
Semiconductor diode lasers
• Electrons are excited into the conduction band by
application of a forward bias voltage across the semiconductor
• Some electrons relax back to the valence band by
emitting light of energy equal to the band gap, Eg = hν
• LEDs are fabricated so as to enhance their light emitting
capacity
• Example:
– Gallium Arsenic Phosphide: 650 nm
– Gallium Aluminum Arsenide: 900 nm
• Applications
– Indicator lights
– Readout devices
• Limitation: red and IR ER only.
3
Nonlinear Optical Effects with Lasers
• Linear optical phenomena
– Relationship between
polarization and field strength
is linear
– P= αE
– α:
• Non linear phenomena
P = αE m sin ωt + β E m2 sin 2 ωt
– at high radiation intensities,
especially when E
1
sin 2 ωt = (1 − cos 2ωt )
approaches the binding
2
energy of electrons.
P = αE + βE 2 + γE 3 + ...
α f β fγ
P = αE m sin ωt +
βE m2
2
(1 − cos 2ωt )
C. Wavelength Selectors
• Function: to isolate a
narrow band of
wavelengths
• Why:
– To improves
selectivity
– To insure linear
response
• Effective bandwidth
4
C-1 Filters
• Interference filters:
– Based on destructive
and constructive
interference of
radiation
– Consists of a dielectric
(transparent nonconducting
substance:CaF2 or
MgF2) sandwiched
between two
semitransparent
metallic films.
– Efffective bandwidth:
2- 5 nm
λ=
2tη
n
wavelength of radiation transmitted
after internal reflection
t: thickness of dielectric
n: order of interference
η : retfractive index of the material
• Absorption filters
– Selectively absorb
portions of the spectrum
of the source.
– Colored glass
– Dyes suspended in
gelatin sandwiched
between glass plates
– Effective bandwidth: 30 250 nm
– Transmittance can be
only 10% at the band
peak!
5
C-2 Monochromators
•
Components of Monochromators
1. Dispersing element: Grating or Prism:
2. Slits: narrow rectangular opening
3. Lenses: for collimating and focusing
4. Mirrors: for reflection
5. Windows: for transmission
Components of Monochromators
Dispersion by Prism and Grating
Linear Dispersion is constant for a grating monochromator
Linear Dispersion is wavelength dependent for a prism monochromator
6
Prism Monochromators
Prism material must have a large dη/dλ
Hartman equation for the refractive index of glass
c
λ − λ0
dη
c
=−
(λ − λ )
dλ
η = η0 +
2
0
dη
dλ is large for shorter wavelengths
• Angular dispersion
Angular dispersion = a.d. =
dθ dθ dη
=
×
dλ dη dλ
dθ
: dispersion, depends on geometry of prism
dη
dn
: material dependent
dλ
2 sin α 2
−c
dθ
=
×
dλ (λ − λ ) (1 − η sin α 2)
2
2
2
θ
12
o
The angular dispersion is a function of the prism apex angle and
the refractive index
7
•Resolving Power
R=
λ
dη
=b×
∆λ
dλ
b: width at base of prism
R changes as
dη
varies
dλ
with
wavelength
8