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ME 677: Laser Material ProcessingInstructor: Ramesh Singh
Laser Optics-II
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ME 677: Laser Material ProcessingInstructor: Ramesh Singh
Outline
• Absorption• Modes• Irradiance
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ME 677: Laser Material ProcessingInstructor: Ramesh Singh
Reflectivity/Absorption
• Absorption coefficient will vary with the same effects as the reflectivity
• For opaque materials:– reflectivity = 1 - absorptivity
• For transparent materials:– reflectivity =1- (transmissivity + absorptivity)
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ME 677: Laser Material ProcessingInstructor: Ramesh Singh
Reflectivity• In metals the radiation is predominantly absorbed by free
electrons in an “electron gas”• Free electrons are free to oscillate and reradiate without
disturbing the solid atomic structure• The reflectivity increases from visible to high wavelength• As a wavefront arrives at a surface all the free electrons in the
surface vibrate in phase generating an electric field 180° out of phase with the incoming beam
• The sum of this field will be a beam whose angle of reflection equals the angle of incidence
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ME 677: Laser Material ProcessingInstructor: Ramesh Singh
Effect of Wavelength• Reflectivity is a function of the refractive index,
n, and the extinction coefficient, k
• At shorter wavelengths, the more energetic photons can be absorbed by a greater number of bound electrons– Reflectivity decreases and absorptivity increases
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ME 677: Laser Material ProcessingInstructor: Ramesh Singh
Reflectivity of Metals
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ME 677: Laser Material ProcessingInstructor: Ramesh Singh
Reflectivity of Non-metals
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ME 677: Laser Material ProcessingInstructor: Ramesh Singh
Effect of Temperature
• Temperature increase results in increase in phonon population and phonon-electron energy exchanges– Reflectivity decreases– Absorption increases
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ME 677: Laser Material ProcessingInstructor: Ramesh Singh
Surface Roughness
• Surface Roughness has a large effect on absorption due to:– The multiple reflections in the undulations– Also some "stimulated absorption" due to beam
interference with sideways reflected • If roughness is less than the beam wavelength, the
light will perceive the surface as flat
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ME 677: Laser Material ProcessingInstructor: Ramesh Singh
Case Study-1045 steel
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ME 677: Laser Material ProcessingInstructor: Ramesh Singh
Angle of Incidence• At certain angles the surface electrons may be constrained
from vibrating since to do so would involve leaving the surface. This they would be unable to do without absorbing the photon
• The electric vector is in the plane of incidence, the vibration of the electron is inclined to interfere with the surface and absorption is thus high
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ME 677: Laser Material ProcessingInstructor: Ramesh Singh
Refraction
• On transmission the ray undergoes refraction described by Snell’s law:
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ME 677: Laser Material ProcessingInstructor: Ramesh Singh
Refraction• Scattered intensity is a function of 1 /l4 Rayleigh
Scattering Law. • The normal form of a dispersion curve (refractive
index vs wavelength) is known as a Cauchy Equation:
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ME 677: Laser Material ProcessingInstructor: Ramesh Singh
Beam Mode• Two spatial modes describe the beam
– Longitudinal – Transverse
• Essentially independent of each other– Transverse dimension in a resonator is normally considerably smaller
than the longitudinal• The standing wave condition will be amplified, i.e., there can be
only integer number of half wavelengths in the cavity, – D= q. λ/2 or qλ=2D– q is a large integer referring to the number of nodes in the longitudinal
standing, D is the cavity length (mirror separation), and λ is the wavelength.
• The longitudinal mode number is large in industrial lasers and is normally ignored on beam characteristics and performance.
• The transverse electromagnetic mode (TEM) is more important.
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ME 677: Laser Material ProcessingInstructor: Ramesh Singh
Longitudinal Mode
• Longitudinal Mode (integral multiples of l/2)
l=2D, l=DD
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ME 677: Laser Material ProcessingInstructor: Ramesh Singh
Longitudnal Mode• The frequency of the axial mode of the cavity,
! = #$2&
• If λ is replaced by '(, where c is the speed of light and ν is the frequency, the frequency separation between two adjacent nodes ∆ν between adjacent modes (∆q =1) is given by,
Δ! = '*+
• The axial modes of the laser cavity consists of a large number of frequencies spaced apart by '*+ as illustrated. However, the given mode can oscillate if the gain exceeds the losses at that frequency
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ME 677: Laser Material ProcessingInstructor: Ramesh Singh
Spectral lines due to axial mode
(a)
(b)17
(a)
(b)
qq-3 q-2 q-1 q +1 q+2 q+3
q
Irrad
ianc
e
q
Irrad
ianc
e
Level representing losses
ME 677: Laser Material ProcessingInstructor: Ramesh Singh
Transverse Mode• TEM describes the variation in beam intensity with position in
a plane perpendicular to the direction of beam propagation– It characterizes the intensity maxima in the beam
• The TEM is determined by: – The geometry of the cavity– Alignment and spacing of internal cavity optics– Gain distribution and propagation properties of the active medium– Presence of apertures in the resonator
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ME 677: Laser Material ProcessingInstructor: Ramesh Singh
TEM• Rectangular Modes
– X axis– Y axis
• Circular Modes TEMpl– Radial, p– Angular, l
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Number of zero fields along X axis (m)
Num
ber o
f zer
o fie
lds a
long
Y a
xis (n
)
Number of radial zero fields (m)Nu
mbe
r of a
ngul
ar z
ero
field
s (n)
ME 677: Laser Material ProcessingInstructor: Ramesh Singh
Intensity Plots
a.TEM00; b. TEM 10; c. TEM01*
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ME 677: Laser Material ProcessingInstructor: Ramesh Singh
Propagation…• Generic equation for propagation:
! ", $, %
= !'()'2 "
+(%) )(2 $
+(%)+
+(%) ./ 01231
41 ./5 67/(82'2() 9:;<= 77> ./5
6?1@A(7)
Equations for Hermite Gaussian beam in (x-y coordinates) at beam waist (z=0)
2 2
2
0
0
2 2( , )
_ _min _
_ tan _ __ tan _ _min _ _
x yw
m nx yE x y E H H ew w
whereE Electric field amplitudeE No al amplitudex x dis ce from axisy y dis ce from axisw No al beam radius
æ ö+-ç ÷ç ÷è ø
æ ö æ ö= ç ÷ ç ÷ç ÷ ç ÷
è ø è ø
=====
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ME 677: Laser Material ProcessingInstructor: Ramesh Singh
Propagation…• Hn(x) is a Hermite polynomial
( ) ( ) 2 2
2 2
2
2
2
2
2
0
0
02
2
0
1
( ) 1
( , ) ._ ,
( )
( ) ( )
( )
nn x x
n n
x yw
rw
rw
dH x e edx
H x
E x y E eCircular coordinates
E r E e
I r E r
I r I e
-
æ ö+-ç ÷ç ÷è ø
æ ö-ç ÷ç ÷è ø
æ ö-ç ÷ç ÷è ø
= -
=
=
=
µ
=
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ME 677: Laser Material ProcessingInstructor: Ramesh Singh
Gaussian Beam
• Gaussian function goes out to infinity• Low powered lasers mimic the TEM00
• TEM00 beams can be focused to smallest spot as compared to any other distribution
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ME 677: Laser Material ProcessingInstructor: Ramesh Singh
Beam Properties• The point where irradiance drops to 1/e2 of
the peak• The radius containing 1- 1/e2 power• A two dimensional plot, the x value of which
95% of the plot area is contained between x and –x.
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ME 677: Laser Material ProcessingInstructor: Ramesh Singh
Gaussian Distribution
For different gaussian beams:
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2
2
2
2
2
0
0 2
9
0
0 2
( )2
Other beam,
( )9
rw
rw
I r I ePIw
I r I ePIw
p
p
æ ö-ç ÷ç ÷è ø
æ ö-ç ÷ç ÷è ø
=
=
=
=
ME 677: Laser Material ProcessingInstructor: Ramesh Singh
Equations for a generic intensity
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Intensity I(x, y, z) propagating along z axis. The second moments in x and y axes at a given location, z, can be defined as !"# $ = ∬ "'"̅ ) * ",,,- .".,
∬ * ",,,- .".,
!,# $ = ∬ ,'/, ) * ",,,- .".,∬ * ",,,- .".,
The centroids, 0̅ and /1 are defined by, 0̅ = ∬ " * ",,,- .".,
∬ * ",,,- .".,
/1 = ∬ , * ",,,- .".,∬ * ",,,- .".,
The beam dimensions (widths) in x and y are given by,23" = 4 !"($)23, = 4 !,($)
ME 677: Laser Material ProcessingInstructor: Ramesh Singh
Examples
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!(#′, &′) = )∗+, [1 − 012
32 ][1 −51262 ]
(1)!(#′, &′) = )∗+
, [1 − 01232 ][1 −
512
62 789:2;2] (2)
!(#′, &′) = 2∗+, [1 − &′2
02 ][1 −#′222 ] (1)
!(#′, &′) = 2∗+, [1 − &′2
02 ][1 −#′2
2231−&′ 202 4] (2)
ME 677: Laser Material ProcessingInstructor: Ramesh Singh
Propagation of laser beam
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2 210
0 2( , ) tan2z z R z
w r z krE r z E Exp Exp i kz
w w z R-
æ öé ùæ ö æ ö= - - - +ç ÷ê úç ÷ ç ÷ç ÷è ø è øë ûè ø
For a monochromatic beam propagating in z the complex electric field amplitude
where E0 is the peak amplitude; w is beam waste radius; k = 2π/l ; ZR is the Rayleigh length; Rz is the radius of curvature of the wave front
ME 677: Laser Material ProcessingInstructor: Ramesh Singh
The variation of beam radius in propagation
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2
0
20
1zR
R
zw wz
wz pl
æ ö= + ç ÷
è ø
=
ME 677: Laser Material ProcessingInstructor: Ramesh Singh
Propagation of Laser Beams• A laser beam propagating in space (lower case for TEM00 and upper case
for real beams)– Beam waist or minimum diameter, d0 /D0
– Beam waist diameter, dz/Dz at a location z from the waist– Beam waist or minimum radius, w0 /W0
– Beam waist radius, wz/Wz at a location z from the waist– q/Q = Full-angle beam divergence– l = Wavelength of light
QBEAM WAIST DIAMTER, DZ
Q
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ME 677: Laser Material ProcessingInstructor: Ramesh Singh
Propagation of Ideal Beam• For a TEM00 beam, the diameter dz for any distance z form the
waist is a hyperboloid2
0 20
2 2 2 20
0
00
1 2 1
2 1
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4
_ can be approximated by
2 tan2( )
z
z
zd dd
d d z
dDivergence TEM
Divergence
d dz z
lp
qlq
pq
q -
æ ö= + ç ÷
è ø
= +
=
=
æ ö-= ç ÷-è ø
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- 3 - 2 - 1 1 2 3
- 3
- 2
- 1
1
2
3
d2d1
z2z1
ME 677: Laser Material ProcessingInstructor: Ramesh Singh
Real Beam• Real beams can be defined in terms of TEM00
• It can be postulated a fictitious “embedded Gaussian beam” having a smaller dia d exists in the real beam; D=M.d , where M>1
qpl
pl
pl
pl
pl
2
0
2
2220
2
2
0
222
02
2
20
220
2
2
20
20
2
20
0
...4
...4
....41
....41
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MDMzDD
DMzDD
DMzDD
DMz
MD
MD
dzdd
z
z
z
z
z
==Q
Q+=
÷÷ø
öççè
æ+=
÷÷
ø
ö
çç
è
æ÷÷ø
öççè
æ+=
÷÷ø
öççè
æ+=
÷÷ø
öççè
æ+=
\
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ME 677: Laser Material ProcessingInstructor: Ramesh Singh
Focused Beam Calculations
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01
21
2 222
1
2 21 01
2 2 202 01
2
0
21
2 22 01
1 21
2
12 2 2 202 01 1
For ideal beam,
( )
( )4
1 1 11
4
4. .For real beam with ,
.
( )
( )
1 1 11
z f fz fd
z f
z dd d f f
MD
z f fz fDz f
zD D f f
pl
pl
lp
-= +
öæ- + ÷çç ÷
è ø
öæ öæ= - + ç ÷÷ç è øè ø
Q =
-= +
öæ- + ÷ç Qè ø
öæ= - +÷ç Qè ø
ME 677: Laser Material ProcessingInstructor: Ramesh Singh
Final Calculation• Once D02 is calculated, Q2 could be found
• Depth of focus where focal spot size changes by ±5%.• Approximate solution for focused beam diameter if lens is
placed at z from the beam waist
If unfocused beam diameter at z, is Dz.
2
02
4. ..MDlp
Q =
22202
22 Q+= zDD z
2
024. . .
. z
f MDDl
p=
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ME 677: Laser Material ProcessingInstructor: Ramesh Singh
Laser Optics Setup at IITB
Indian Patent Application No 442/MUM/2011 Filed on 17 February 2011
Method and device for generating laser beam of variable intensity distribution and variable spot size
ME 677: Laser Material ProcessingInstructor: Ramesh Singh
Compact Focusing
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The magnifying lens can be selected for a given magnification(m) and length (l=u+v). ! = #
$% + ' = ()# +
)$ =
)*
Solving the above set of equations, the focal length of magnifying lens (a biconvex lens) can be determined in terms of length(l) and magnification(m),
+ = ,-,.) /
ME 677: Laser Material ProcessingInstructor: Ramesh Singh
Aberrations
• Spherical Aberration• Thermal Distortion• Astigmatism• Damage
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ME 677: Laser Material ProcessingInstructor: Ramesh Singh
Spherical Aberration
• There are two reasons why a lens will not focus to a theoretical point– Diffraction limited problem – Spherical lens is not a perfect shape.
• Most lenses are made with a spherical shape since this can be accurately manufactured economically
• The alignment of the beam is not so critical as with a perfect aspheric shape
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ME 677: Laser Material ProcessingInstructor: Ramesh Singh
Thermal Distortion
• High power laser beams are absorbed by lenses/optics– Selection of right optics ZnSe with CO2– The power distribution in TEM00 causes more
severe gradients than Donut• Shape change of lens• Varies the refractive index, specially in ZnSe
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ME 677: Laser Material ProcessingInstructor: Ramesh Singh
Astigmatism and Damage
• Due to optical misalignment• Damage
– Due to dirt accumulation and burning on lens surface
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ME 677: Laser Material ProcessingInstructor: Ramesh Singh
Summary• Absorption• Beam Modes• Propagation• Focusing• Aberrations
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