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Lecture 7
Tunable Semiconductor Lasers
What determines lasing frequency:
Gain spectrum
A function of temperature.
Optical length of cavity
Mirror reflectance spectrum
Any perturbation which affects refractive index and/or
lasing frequency.
Single frequency laser
DFB and DBG lasers
Tuning achieved by changing heat sink temperature.
Tuning by changing bias current which affects the
number of carriers in tuning region.
4 nL
M
M
4 M nL
2M ; M integer
c
cM
2nL
Modulators
Mach-Zehnder modulators (electro-optic modulators)
Electro-absorption modulators
Phase Modulators
l
ne3r33 V
g
Electrooptic Modulator
(A) Directional coupler geometry
(B) Mach-Zehnder configuration
Mach-Zehnder modulator
Solve wave equation for mode field distribution &
propagation constant.
u ( x, y , z ) u ( x , y )e i z
2 neff
neff neff V 0 kV
where k = constant
Mach-Zehnder modulator
v
Pi
Po
Thus, by applying V will cause a phase shift for
propagating mode.
Mach-Zehnder modulator
By symmetry, equal amplitudes in 2 arms after passing
through the first branch.
Mach-Zehnder modulator
For the second branch, output depends on relative
phases of combining waves:
2 waves in phase.
2 waves rad out of phase
Mach-Zehnder modulator
Wave amplitudes
Pout A
2
out
Ain2 i1 i2
e e
4
2
Mach-Zehnder modulator
i1
e e
i2
2
cos 1 cos 2 sin 1 sin 2
2
cos 2 1 cos 2 2 2 cos 1 cos 2
sin 2 1 sin 2 2 2sin 1 sin 2
Pin
1 cos 1 2
Pout
2
2
Mach-Zehnder modulator
Pout = Pin
1 2 2M ; M
Pout = 0
1 2 0
integer
1 0 cV
2 0 cV
1 2 2cV
2 L
n
eff 1
neff 2
Mach-Zehnder modulator
V is a swiching voltage which give Pout -rad phase
difference.
V is determined by material and electrode
configuration.
V is different for dissimilar polarizations.
Pin
Pout
2
V
1 cos
V
Diffused optical waveguides
Diffused optical waveguides: Ti:LiNbO3 indiffused
waveguides.
Waveguide modes (linearly polarized or ‘LP’):
TE mode – light polarized in plane of substrate surface
TM mode – light polarized normal to plane of substrate
surface.
Diffused optical waveguides
nTE ( x, y, z ) nsub / TE nwg / TE ( x, y, z ) ne.o./ TE ( x, y, z )
nTM ( x, y, z ) nsub / TM nwg / TM ( x, y, z ) ne.o./ TM ( x, y, z )
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.
Diffused optical waveguides
For 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).
Example
Consider a Mach-Zehnder modulator with an electrode length of 2 cm
and electrode gap width g of 12 mm, such that
neff / TE KTE E
neff / TM KTM E
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.