Laser based sensors

• References:
1) “Lasers and optical engineering,” P. Das, Springer-
Verlag
2) “Optics, light and lasers,” Dieter Meschede, Wiley-VCH
3) “Optics,” E. Hecht, Addison-Weseley
4) “Optical fiber devices,” J. P. Goure, I. Verrier, Institute
of Physics
5) “Laser-aided diagnostics of plasmas and gases,” K.
Muraoka, M. Maeda, Institure of Physics
6) “Laser fundamentals,” W. Silfvast, Cambridge
 1) Laser based sensors

•Optical sensor: an introduction

•Application of optical sensors
•Considerations in design of sensing system
•Optical fiber based sensors
•Measurement issues
•New areas of optical sensors
 Optical sensor: an introduction
What’s an optical sensor
A sensor that is based on the use of light
to create the “sensing element”
What can optical sensors sense?
Temperature
Pressure, stress
Displacement, strain
Vibrations, acoustics
Velocity, flow
Acceleration
Absolute and relative rotation
Liquid level
Magnetic field, current
Electric field, voltage
Absorption / emission spectra
Species identification / concentration, etc. etc.
 Optical sensors
Why the interest in optical sensors:
High sensitivity
Small size
Immunity to EMI (electro magnetic interference)
High insulation
Large bandwidth
Distributed / multiplexed sensing
Remote sensing
Compatibility with fiber-optic telemetry
Low power, weight
Etc. etc.

Limitation
Cost (?)
Applications of optical sensors
Navigation
Flight vehicles / automobiles
“smart” structures
Robotics
Undersea monitoring
Chemical plants
Mines
Pollution monitoring
Electric / gas / water utilities
Building management
Medical diagnostics
Security systems
Accurate clocks
Accurate voltage and current refrences
Etc. etc.
 How do optical sensors “sense”?
By detecting changes in the properties of light due to a
disturbance

Properties of light include: Disturbance

Intensity / amplitude
Propagation direction
Phase
Polarization
Frequency / wavelength Light Transducer Detector
Etc. etc. source

ur uur i⎛⎜ 2λπ ⎞

z − 2π f t ⎟
E = E0 e ⎝ ⎠
Various techniques
 Direction-based

Application: beam deflection measurements

 Phase-based

Application: electric field, magnetic field, temperature measurements

 Polarization based

Application: stress, strain, temperature,

Wavelength-based (absorption)

Application: specie identification

 Wavelength-based (emission)

Application: specie identification

 Light and its property

Electro-magnetic wave
In vacuum, speed of light:
c =υ λ
 Electromagnetic spectrum : c=νλ
λ 1Ao1nm 1μm 1mm 1m 1km
Wavelength
(m) 10-13 10-1010-9 10-6 10-3 100 103 105

ν (1THz) (1GHz) (1MHz) (1kHz)

Frequency
(Hz) 1022 1015 1012 109 106 103
Light

Radiofrequency
Microwaves
Ultraviolet

Infrared
Visible
X-Rays
γ-Rays

Far infrared: 10 to 1000 μm

middle infrared: 1 to 10 μm
near infrared: 0.75 to 1 μm
visible: 0.4 to 0.75 μm
ultraviolet: 0.2 to 0.4 μm
0.4μm 0.75μm vacuum uv: 0.1 to 0.2 μm
extreme uv: 10 nm to 100 nm
soft X-rays: 1 nm to 20-30 nm
 Wave-equation
E ( z , t ) = E0 cos(kz − ωt )
= Re E0 e (
i kz −ωt )

E0 ⎡ i( kz −ωt ) − i( kz −ωt ) ⎤
= ⎣ e +e ⎦
2
Where k=2π/λ, ω=2πf, T=1/f= λ/c

H = n ( ε 0 / μ0 )
1/ 2
E , in a medium having refractive index n

ε 0 = 8.85 × 10−12 F/m, permittivity of vacuum

μ0 = 4π × 10−7 H/m, permeability of vacuum

Speed of light in vacuum= 3×108 m/s

 Photon energy = h υ
Where h=6.63×10-34 J.s, υ = frequency

1
Photon intensity = n ( ε 0 / μ0 ) E
1/ 2 2

2
Polarization
 Quarter-wave plate
Isotropic crystals: n1=n2=n3

Uniaxial crystals: n1=n2=no

n3=ne

Biaxial crystals: n1≠ n2≠ n3

 Two-beam interference
Michelson interferometer

Application: displacement, strain, motion, pressure, temperature, rotation, magnetic

field, electric field
Two-beam interference
 Two-beam interference
Etotal = E1 + E2 = E01 cos(kz1 − ωt ) + E02 cos(kz2 − ωt )
1 1
I total = Etotal 2 = ( E1 + E2 )
2

2 2
1
= ⎡⎣ I1 + I 2 + 2( I1I 2 )1/ 2 cos(kz1 − kz2 ) ⎤⎦
2
1
I max = ⎡⎣ I1 + I 2 + 2( I1I 2 )1/ 2 ⎤⎦
2
1
I min = ⎡⎣ I1 + I 2 − 2( I1I 2 )1/ 2 ⎤⎦
2
I −I
Fringe visibility = V = max min
I max + I min
When I1 = I 2 = I , and perfect polarization alignment
1
I total = [ 2 I + 2 I cos(kz1 − kz2 )]
2
I max = 2 I , I min = 0 ⇒ V = 1
 Different configurations
Michelson Application: Plasma diagnostics,
specie identification, optical coherent
tomography

Mach-Zehnder
Specie identification, plasma diagnostics

Application: Gyroscopic measurements

Sagnac
Temporal coherence
High temporal coherence:

τ is very long

Δf = 1/τ is very small

Can predict amplitude

and phase at any time, at
a given position
 Spatial coherence

High spatial coherence

• Wave is well behaved in space

• Can predict amplitude and phase at any position, at a given

time
 Multiple beam interference

Applications: displacement, strain, motion, pressure, temperature, frequency,

wavelength
Multiple interference
Transmission property
Interaction of light with matter

F = qE

F = ma ⇒ Electrons with high acceleration

 Lorentz oscillator

d 2x dx k e
2
+σ + x= E (t )
dt dt M e Me
 Atom model
Nucleus
Electron E5 n=6

Energy
n=5
n=4
n=3
E1 n=2
E0 n=1

Energy level diagram of atom

E2 - E1 = h ν21
 Absorption and emission

Stimulated Absorption (Absorption) :

Spontaneous emission :

N2 E2 N2 E2

hν21 hν21

N1 E1 N1 E1

Absorption between Spontaneous emission

two levels between two levels
 Stimulated emission
Stimulated emission :

N2 E2 N2 E2
hν21
hν21
hν21

N1 E1 N1 E1

Photon-particle collision Stimulated emission

Stimulated emission responsible for optical frequency

amplification requires:
Population inversion
 Laser induced fluorescence

Figure: Two level system

Application: time-of-flight studies, nano-scale analysis

Figure: Multi-level system

 Photo-ionization

Figure: Various schemes for photo-ionization: (a)

single-photon ionization, (b) two-photon ionization, (c)
two-step ionization, (d) two-photon excited ionization,
(e) three-step ionization.
Application: specie identification, isotope identification
Laser propagating through a gas or plasma

Thomson scattering by charged particles

Mie scattering
Rayleigh scattering
Raman scattering
Brillouin scattering
 Propagation of light

Figure: Propagation of light along dielectrics

Behavior of light at interfaces

Figure: Reflection and transmission at interfaces

Figure: (a) A wave reflecting at an
interface. (b) Electron oscillators and
Brewster’s law, (c) Polarization of light
that occurs on reflection from a dielectric,
such as glass, water, or plastic
Brewster’s angle

tan(θ p ) = nt / ni
 Propagation of light in fibers and
waveguides
Rays reflected in a clad
optical fiber

Partially reflected at each core-

cladding interface and quickly
leak out of the fiber

Fiber optics is used for efficiently conducting light from one

point in space to another via transparent, dielectric fibers
Advantages: 1) large information carrying capacity,
2) immunity from EMI, 3) small size and weight
 Optical fiber

SiO2 + GeO2 : increases n2, and n2 > n1

Evanescent field
 Numerical aperture

The maximum acceptance angle ⇒ Numerical aperture

sin(θ 0 ) = ( n f − nc )
1 2 2 1/ 2

no
NA = no sin(θ 0 ) = ( n − n
2
f c)
2 1/ 2
 Optical loss in fiber

dB = −10 log( Po / Pi )
−α L /10 = log( Po / Pi ) ⇒ Po / Pi = 10−α L /10
Fiber optic configurations

Multi-mode fiber

Dispersion shifted fiber

Single-mode fiber
Spatial characteristics
Condition for single-mode operation
 Waveguides

Single-channel
Multi-channel
 Different configurations

Optical fibers are used for transmission over long distances.

Mechanically flexible.

Waveguides on the surface of a suitable substrate (e.g.

LiNbO3) play an important role in integrated optics.
Coupling can be performed via an edge or by frustrated total
internal reflection with a prism on top.