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Nonlinear Optics Lab. Hanyang Univ.
Chapter 6. Processes Resulting from
the Intensity-Dependent Refractive Index
- Optical phase conjugation
- Self-focusing
- Optical bistability
- Two-beam coupling
- Optical solitons
Reference :
R.W. Boyd, “Nonlinear Optics”, Academic Press, INC.
- Photorefractive effect (Chapter 10)
: cannot be described by a nonlinear susceptibility c(n) for any value of n
Nonlinear Optics Lab. Hanyang Univ.
6.1 Optical Phase Conjugation
: Generation of a time-reversed wavefront
Signal wave :
rk*
s
**
s
rk
sss
s
s
AˆE
AˆE
i
s
i
ee
ee
c.c.E),r(E~
ss tiet
Phase conjugate wave : c.c.E),r(E~ *
sc tiert where, r : amplitude reflection coefficient
Nonlinear Optics Lab. Hanyang Univ.
rk*
s
**
ssAˆE
i
s eeProperties of phase conjugate wave :
1) *ˆse : The polarization state of circular polarized light does not change in reflection from PCM
Ex) ji/2i
00s ee ee
i) In reflection from metallic mirror [-phase shift]
ii) In reflection from PCM [-phase shift &
y component : i/2 -i/2 (- : delayed)]
2) *A s : The wavefront is reversed
)r(*)r( tt AA i
ss
i
ss eaea
3) : The incident wave is reflected back into its direction of incidencerk s i
e
ss kk
Nonlinear Optics Lab. Hanyang Univ.
Aberration correction by optical phase conjugation
Wave equation :
0
~)r(~
2
2
2
2
t
E
cE
Solution : ..)r(),r(~ )( cceAtE tkzi
With slow varying approximation, 02)r( 2
2
22
z
AikAk
cAT
Since the equation is generally valid, so is its complex conjugate, which is given explicitly by
02)r( *
*2
2
2*2
z
AikAk
cAT
Solution : ..)r(),r(~ )(* cceAtE tkzi
c
: A wave propagating in the –z direction whose complex amplitude
is everywhere the complex of the forward-going wave
Nonlinear Optics Lab. Hanyang Univ.
Nonlinear Optics Lab. Hanyang Univ.
Phase Conjugation by Degenerated Four-Wave Mixing
1) Qualitative understanding
Four interacting waves : ..)r(..)r(),r(~ )r(
cceAcceEtEtki
i
ti
iii )4,3,2,1( i
Nonlinear polarization :r)(*
321
)3(*
321
)3( 32166
kkkiNL eAAAEEEP cc
(021 kk Counter-propagating waves)
r*
321
)3( 36
ik
eAAAc
New wave (k4) source term !
So, A4 is proportional to A3* (complex conjugate of A3)
and its propagation direction is –k3(in the case of perfect phase matching)
Nonlinear Optics Lab. Hanyang Univ.
2) Rigorous treatment
Total field amplitude :4321 EEEEE
Nonlinear polarization : *2)3( )(3 EEPNL c
*
243
*
441
*
331
*
221
*
1
2
1
)3(
1 22223 EEEEEEEEEEEEEEP c
*
143
*
442
*
332
*
112
*
2
2
2
)3(
2 22223 EEEEEEEEEEEEEEP c
*
421
*
443
*
223
*
113
*
3
2
3
)3(
3 22223 EEEEEEEEEEEEEEP c
*
321
*
334
*
224
*
114
*
4
2
4
)3(
4 22223 EEEEEEEEEEEEEEP c
4321 ,, EEEE Neglect the 2nd order terms of E3 and E4
*
221
*
1
2
1
)3(
1 23 EEEEEP c
*
112
*
2
2
2
)3(
2 23 EEEEEP c
*
421
*
223
*
113
)3(
3 2223 EEEEEEEEEP c
*
321
*
224
*
114
)3(
4 2223 EEEEEEEEEP c
Nonlinear Optics Lab. Hanyang Univ.
Wave equation :
ii
i Ptct
E
cE
~4~
~2
2
22
2
2
2
where, ..)r(..)r(),r(
~ )r(cceAcceEtE
tki
i
ti
iii
(1) Pump waves, A1 and A2 (slow varying approximation)
111
2
2
2
1
)3(1 ||2||6
AiAAAnc
i
dz
dAc
222
2
1
2
2
)3(2 ||2||6
AiAAAnc
i
dz
dAc
# Each wave shifts the phase of the other wave
by twice as much as it shift its own phase
# Since only the phase of the pump waves are
affected by nonlinear coupling, the quantities
|A1|2 and |A2|
2 are spatially invariant, and hence
the k1 and k2 are in fact constantSolution :zi
eAzA 1)0()( 11
zieAzA 2)0()( 22
*
3
)(
21
*
321421)0()0( AeAAAAAP
zi
(6.1.15)
: Nonlinear polarization responsible for producing the phase
conjugate wave varies spatially.
)0()0()()()0()0( 21212121 AAzAzAAA
Therefore, two pump beams should have equal intensities :
Nonlinear Optics Lab. Hanyang Univ.
(2) Signal (A3) and conjugate waves (A4)
*
433
*
4213
2
2
2
1
)3(3 )||2|(|12
AiAiAAAAAAnc
i
dz
dAc
*
343
*
3214
2
2
2
1
)3(4 )||2|(|12
AiAiAAAAAAnc
i
dz
dAc
where, )||2|(|12 2
2
2
1
)3(
3 AAnc
i c
21
)3(12AA
nc
ic
put, zieAA 3'
33
zieAA 4'
44
'
3
'
4
'
4
'
3
Aidz
dA
Aidz
dA
)4,3(0|| '2
2
'
iii A
dz
dA
Nonlinear Optics Lab. Hanyang Univ.
Solution :
)0(||cos
)](|sin[|
|)(
||cos
||cos)(
)0(||cos
)](|cos[|)(
||cos
||sin||)(
'*
3
'
4
'
4
'*
3
'
4
'*
3
AL
LziLA
L
zzA
AL
LzLA
L
zizA
0)('
4 LA ( conjugate wave at z=L is zero)
)0()||(tan||
)0(
||)/cos0()(
'*
3
'
4
'*
3
'*
3
ALi
A
LALA
i) )0()( '*
3
'*
3 ALA
~0)0('
4Aii)
: amplification
: depends on L|| (can exceed 100% pump wave energy)
Nonlinear Optics Lab. Hanyang Univ.
Processes of degenerated four-wave mixing: One photon from each of the pump waves is annihilated
and one photon is added to each of the signal and conjugate waves
one photon transition two photon transition wave-vectors
Amplification of A3 and over 100% conversion of A4/A3 are possible
Nonlinear Optics Lab. Hanyang Univ.
Experimental set-ups
0)( 21 kk 0)( 43 kk
1A2A
3A
4A
43 AA
Nonlinear Optics Lab. Hanyang Univ.
17.5 Optical Resonator with Phase Conjugate Reflectors (A. Yariv)
189
178
167
156
145
134
123
12
),(
),(
),(),(
),(
)),((
),(),(
nm
nm
nmnm
nm
nm
nmnm
l
l
ll
R
Rl
lR
lRl
R
R ),( nml
# The self-consistence condition is satisfied
automatically every two round trips.
The phase conjugate resonator is stable
regardless of the radius of curvature R
of the mirror and the spacing l.
Nonlinear Optics Lab. Hanyang Univ.
17.6 The ABCD Formalism of Phase Conjugate Optical Resonator
The wave incident upon the PCM :
)
2(exp)()
2(exp)(E
2
2
22
i
iiiq
krkztiε
w
rkrkztiε
rr
)
2(exp)()
2(exp)(E
2*
2
22*
r
iirq
krkztiε
w
rkrkztiε
rr
2
11
w
i-
qi
where,
Reflected conjugate wave :
*2
111
ir qw
i
q
1-0
01
DC
BA,
DC
BA*
*
Mi
ir
q
Ray transfer matrix for the PCM mirror
By comparing the ABCD law for ordinary optical elements,
Nonlinear Optics Lab. Hanyang Univ.
ABCD law at any plane following the PCM :
T
*
T
T
*
T
DC
BA
i
iout
q
Example)
10
01
1R
2-
01
DC
BA
10
01
DC
BA
1R
2-
01
DC
BA
11
11
1M
Matrix after one round trip :
Matrix after two round trip :
IMM
10
01
10
01
1R
2-
01
10
01
1R
2-
012
12
: Self-consistence condition is satisfied
automatically every two round trips
Nonlinear Optics Lab. Hanyang Univ.
17.7 Dynamic Distortion Correction within a Laser Resonator
Phase conjugate resonator
Distortion corrected beam
Nonlinear Optics Lab. Hanyang Univ.
17.8 Holographic Analogs of Phase Conjugate Optics
1) Holography
recording reading
Nonlinear Optics Lab. Hanyang Univ.
2) Phase conjugate optics
43 AA
Holography by phase conjugation
- Real time processing (no developing process)
- Distortion free image transmission
Nonlinear Optics Lab. Hanyang Univ.
17.9 Imaging through a Distorted Medium
Distortion free
transmission (A2)
Nonlinear Optics Lab. Hanyang Univ.
6.2 Self-Focusing of Light
I20 nnn 22
e)I( wrr Gaussian beam :
defocusing-self
focusing-self
: 0
: 0
2
2
n
n
Nonlinear Optics Lab. Hanyang Univ.
Self-Trapping
: Beam spread due to diffraction is precisely compensated by the contraction due to self-focusing
Simple model for self-trapping
Critical angle for total internal reflection :nn
n
0
00cos
00
2
1
00
0
2
0
2
12
11
nn
n
n
Nonlinear Optics Lab. Hanyang Univ.
A laser beam of diameter d will contain rays within a cone whose maximum angular extent
is of the order of magnitude of diffraction angle ;
dndnd
00
6.02
So, the condition for self-trapping :
0 d
2
0
0
0
2
1
0
6.0
2
16.02
dn
nndnn
n
21
20 I26.0
nnd I2nn
Critical laser power :
20
2
2
8
6.0I
4P
nndcr
# Independent of the beam diameter
Ex) CS2, n2=2.6x10-14 cm2/W, n0=1.7, =1m
Pcr = 33 kW
Nonlinear Optics Lab. Hanyang Univ.
Simple model of self-focusing
2w0
zf
nn 2
10
nn 0
)2
1()()( 0
21
2
0
2
0 nnwznnz ff
2
0000
2
1
2
1)(
f
ffffz
wnznznznnz
2
1
00
0
0
2
1
00
22
nnw
nn
wz f
where, : critical angle
I2nn
21
2
02
0
21
2
00
2I
Pn
nw
n
nwz f
where, I2
1 2
0wP : total power
21
2
00 1
6.0
2
cr
f
PP
wnz
Nonlinear Optics Lab. Hanyang Univ.
6.3 Optical Bistability
: Two different output intensities for a given input intensity
Switch in optical computing and in optical computing
Bistability in a nonlinear medium inside of a Fabry-Perot resonator
TR 22
,
Intensity reflectance and transmittance :
,where, : amplitude reflectance and transmittance
likle 2
22 AA
212 AAA
likle
22
12
1
AA (6.3.3)
Nonlinear Optics Lab. Hanyang Univ.
1) Absorptive Bistability
In the case when only the absorption coefficient depends nonlinearly on the field intensity,
at the resonance condition, Re ikl22
Assume, 1l
)1(1
AA 1
2lR
2
A2I ii nc
12 2
II
1 (1 )
T
R l
Introducing the dimensionless parameter C,R
lRC
1
2
12
)1(
I1I
CT (6.3.7)
Nonlinear Optics Lab. Hanyang Univ.
Assume the absorption coefficient obeys the relation valid for a two-level saturable absorber ;
SII1
0
Intracavity intensity : 2
'
22 I2II )1(
,I2I1
00
2
0
RlR
CC
CS
where,
2
2
021
I2I11II
S
CT
(6.3.7)
23 II T
Nonlinear Optics Lab. Hanyang Univ.
2) Dispersive Bistability
In the case when only the refractive index depends nonlinearly on the field intensity,
)I(,0 fn
(6.3.3) iδikl eRe
1
A
1
AA 1
22
12
ieR2where,
20
lc
n
00 2 : linear phase shift
: nonlinear phase shiftlc
n
I2 22
2sin41
I
)cos21(
I
)1)(1(
II
22
1
2
112
TR
T
RR
T
eReR
Tiδiδ
Similarly as before,
Nonlinear Optics Lab. Hanyang Univ.
2sin41
1
I
I22
1
2
TR
T
220 I4
l
Cn
23 II T