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Volume Holograms
Kogelnik’s Coupled Wave Theory
Physics 545
Volume HologramGeometry, Grating Vector K ,Reading Wave R and Signal Wave
S
Wave EquationFrom
Maxwell’s
Equations
for
a
non‐magnetic
dispersive
material
of
permeability μ
= 1 and no volume charges:
(Del)2E –
μ0
σ(Partial E/ w.r.t t) –
μ0
ε0
ε(Partial E/ w.r.t t)2
= 0 (1)
Which
has
a
plane
wave
solution
propagating
in
the
z
direction
and
linear
polarization in the y direction of the form:
E(x, z, t) = Re{ a(x, z) exp(iωt)}. (2)Where a(x, z) is the complex amplitude
Inserting (2) into (1) and employing complex notation gives:
(Del)2
a(x, z) + k2a
= 0 (3)
k2
= μ0
ε0
εω2
‐
iωμ0
σ
= (ω/c)2ε
– iωμ0
σ
(4)
Relative Constants: ε
and σ• ε
can
be
considered
to
be
composed
of
an
average
component ε0
and
a
sinusodially vaying component ε1
• Values vary as a cosine function in the direction of the grating vector K
• σ
will have the same properties
• Define r(x, y, z) to be the position vector of any point within the medium
ε
= ε0
+ ε1
cos(K . r) (5)σ
= σ0
+ σ1
cos(K . r) (6)
Inserting
(5) and (6) into (4)
k2
= β2
– 2iαβ
+ 2κβ[exp(i
K . r) + exp(‐i
K . r)] (7)
α
= μ0
cσ0
/2(ε0
)1/2
; β
= 2π(ε0
)1/2/λ
Coupling Constant κ
κ
= ¼[ 2πε1
/(ε0
)1/2/λ ‐ iμcσ1
/(ε0
)1/2]
• Central parameter in coupled wave theory
•Incident
and
diffracted
waves
are
coupled
via
spatial
modulations
of
the
index of refraction n and/or spatial modulations of the absorption constant
• Expressing β, α
and α1
= μcσ1
/(ε0
)1/2
in terms of the optical media
gives
κ
= πn1
/λ
– iα1
/2 (8)
• Where n1
is the amplitude of index of refraction modulation
• (8) was determined using the following approximations
α
« 2πn/λ
; α1
« 2πn/λ
; n1
« n
Bragg Condition
Θ
= Θ0
+ ∆
Θ
= Θ0
ρ
= propagation vector of R, σ
= propagation vector of Sσ,
ρ
and
K
are co‐planar σ
= ρ
–
KΘ
= Θ0
(Bragg Condition) K/2 = ρsin(Θ0
)ρ
= β
= σ
Coupled Wave Equations
A
solution
to
(3)
is
a
superposition
of
the
reading
and
signal
waves
given by:
a
= R(z)exp[‐ρ.r] + S(z)exp[‐σ.r] (9)
Inserting (9) into (3) gives two coupled equations for R(z) and S(z):
R’’ – 2iρz
R’
–
ρ2R + β2R – 2iαβR + 2κβS = 0 (10)
S’’ – 2iσz
S’
–
σ2S + β2S – 2iαβS + 2κβR = 0 (11)
Third & fourth terms in (10) sum to 0(Bragg Condition)
;
Sum of
third & fourth terms in (11) can be expressed in terms of small
deviations ∆
from the Bragg Condition and Γ
= β∆
sin(2Θ0
)
β2‐ σ2
≈
2β2∆sin(2Θ0
) ≈
2βΓ
Nicer Version
Ignoring the R’’
and S’’, since R and S change slowly (10) and (11) become:
CR
R’
+ αR = ‐iκS CR
= ρz
/β
Cs
S’
+ (α
+ iΓ)S = ‐iκR Cs
= σz
/β
R’’
+ (α/CR
+α/ Cs + iΓ/ Cs
)R’
+ (α2
+ iΓα
+ κ2)R/ CR
Cs
= 0
Which has a solution of the form R(z) = exp[Υz]
Solving for Υ
Υ1,2
= ‐1/2(α/CR
+α/ Cs
+ iΓ/ Cs
) ± ½[( α/CR
‐ α/ Cs ‐
iΓ/ Cs
)2
‐
4κ2/ CR
Cs
]1/2
R(z) = R1
exp[Υ1
z] + R2
exp[Υ2
z] and S(z) = S1
exp[Υ1
z] + S2
exp[Υ2
z]
Boundary Conditions & Analysis
Reading wave R has unit amplitude (normalized)
Signal wave S(0) = 0 as does S’(0) =0
R(0) = R1
+ R2
= 1; S(0) = S1
+ S2
= 0; S’(0) = Υ1
S1
+ Υ2
S2
S(d) = iκ/[Cs
(Υ2
–
Υ1
)][ exp[Υ2
d] ‐
exp[Υ1
d]
Phase Grating: α1
= α
= 0 Diffraction from spatial modulation of n
Define two parameters for the analysis
ν
=κd/cos(Θ0
) = πn1
d/λ
cos(Θ0
) and ζ
= ∆βdsin(Θ0
) = Γd/(2cos(Θ0
))
S(d) = ‐i[exp[‐iζ]sin((ζ2
–
ν2)1/2)]/(1 + ζ2/ ν2)
K
σρ
Θ0Θ0
z = 0 z = d
Θ
= Θ0
and ζ
= 0
S(d) = ‐isin(ν)
|S(d)|2
= 1
ν
=
π/2
πn1
d/λ
cos(Θ0
) = π/2
n1
d/ cos(Θ0
) = λ/2
When the optical path length of the
incident wave changes by λ/2 100% efficiency is obtainable
100% efficiency is not obtainable
when the angle of incidence differs
from the Bragg angle
General Results: η
= |S(d)|2/ |R(0)|2
= |S(d)|2
ζ ≠ 0
References
1.
Coupled
Wave
Theory
for
Thick
Hologram
Gratings,
Kogelnik,
H. The Bell System Technical Journal, May 23, 19692.
Optical Holography, Collier, R., Burckhardt, C., Lin, L. Academic
Press, 1971 pgs 228 –
264
3.Fundamentals of Photonics 2nd
Edition, Saleh,B., Teich, M. Wiley
Interscience
2007
4.Electromagnetic
Fields
2nd
Edition,
Wangness,
R.,
John
Wiley
and Sons 1986 5. Conversations with Professor S. Mendez, University of Louisville