ENE 623 Optical Networks
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Transcript of ENE 623 Optical Networks
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Lecture 7
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Tunable Semiconductor LasersWhat determines lasing frequency:
Gain spectrum A function of temperature.
Optical length of cavityMirror reflectance spectrumAny perturbation which affects refractive index
and/or lasing frequency.
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Single frequency laserDFB and DBG lasers
Tuning achieved by changing heat sink temperature.
Tuning by changing bias current which affects the number of carriers in tuning region.
M
442 ; integer
2
M
M
nLnLM M
c
cM
nL
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ModulatorsMach-Zehnder modulators (electro-optic
modulators)Electro-absorption modulators
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Phase Modulators
333e
ln r V
g
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Electrooptic Modulator
(A) Directional coupler geometry
(B) Mach-Zehnder configuration
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Mach-Zehnder modulatorSolve wave equation for mode field
distribution & propagation constant.
where k = constant
( , , ) ( , )
2
0
i z
eff
eff eff
u x y z u x y e
n
n n V kV
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Mach-Zehnder modulator
Thus, by applying V will cause a phase shift for propagating mode.
v
Pi Po
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Mach-Zehnder modulatorBy symmetry, equal amplitudes in 2 arms
after passing through the first branch.
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Mach-Zehnder modulatorFor the second branch, output depends on
relative phases of combining waves:2 waves in phase.
2 waves rad out of phase
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Mach-Zehnder modulatorWave amplitudes
1 2
222
4i iin
out out
AP A e e
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Mach-Zehnder modulator
1 22 2 2
1 2 1 2
2 21 2 1 2
2 21 2 1 2
1 2
cos cos sin sin
cos cos 2cos cos
sin sin 2sin sin
1 cos2
i i
inout
e e
PP
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Mach-Zehnder modulator
1 2
1 2
1 0
2 0
1 2 1 2
2 ; integer
0
22 eff eff
M M
cV
cV
LcV n n
Pout = Pin
Pout = 0
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Mach-Zehnder modulatorV is a swiching voltage which give Pout -rad
phase difference.V is determined by material and electrode
configuration.V is different for dissimilar polarizations.
1 cos2in
out
P VP
V
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Diffused optical waveguidesDiffused optical waveguides: Ti:LiNbO3 indiffused
waveguides.
Waveguide modes (linearly polarized or ‘LP’):TE mode – light polarized in plane of substrate
surfaceTM mode – light polarized normal to plane of
substrate surface.
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Diffused optical waveguides
Ti indiffused waveguides: Ti metal atoms cause refractive index increase for both TE and TM waves.
Proton exchanged waveguides: H atoms exchange with Li atoms in lattice. Refractive index increases for only one polarization; e.g, TE mode.
/ / . . /
/ / . . /
( , , ) ( , , ) ( , , )
( , , ) ( , , ) ( , , )
TE sub TE wg TE e o TE
TM sub TM wg TM e o TM
n x y z n n x y z n x y z
n x y z n n x y z n x y z
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Diffused optical waveguidesFor digital transmission, different V could
degrade ‘on-off radio’ or OOR. Ideally, we want OOR to be close to infinity.
Solutions for that are:Use polarized optical input.Use proton exchanged waveguides to eliminate TM modes
(get Pout only for TE mode).
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ExampleConsider a Mach-Zehnder modulator with an electrode
length of 2 cm and electrode gap width g of 12 mm, such that
with E the applied electric field, assumed to be constant between the electrodes, and KTE = 5.8 x 10-10 m/V and KTM = 2.0 x 10-10 m/V. What is VTE and VTM ?
Note: neff = n0 + Δn in one arm and neff = n0 - Δn in the other arm.
/
/
eff TE TE
eff TM TM
n K E
n K E