Download EXTERNAL MODULATION

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Direct modulation
Consider the static optical power-versus-current characteristic of a laser diode; if
we bias at point IB and then superimpose modulation, then the optical power will
track changes in this. We show it here for sinusoidal modulation:
Electrical-to-optical conversion: modulators
Stavros Iezekiel
PL (mW)
Department of Electrical and
Computer Engineering
University of Cyprus
IL
PL = P0 (1 + m cos ω mt ) = P0 + p (t )
PL (t ) = s L I L (t )
P0
PL
IL (mA)
I L = I B [1 + m cos ωmt ] = I B + i (t )
IB
• HMY 645
• Lecture 08
• Spring Semester 2015
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Although the method of direct modulation is a useful one, it suffers a number of
problems:
1. As well as the intensity of the light, the wavelength is modulated. (This
phenomenon is called chirp.) Along with fibre dispersion, this leads to a chirpinduced dispersion limit on transmission distance.
Ti-diffused optical waveguide
Electrodes
CW light
2. The maximum bandwidth we can modulate up to is only a few tens of GHz at the
very best.
3. The maximum quantum efficiency (η) in theory is 100%, and this places an upper
limit on the slope efficiency (and therefore the “gain”).
sL = η
hc
qλ
Lithium
niobate
substrate
Modulated light
EXTERNAL MODULATION
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In addition to direct modulation, we can also modulate the light from a laser with an external
component known as a modulator. Hence the terms external modulation and external modulator.
External modulation
CW
Laser
External
modulator Modulated
light
output
RF input
+ Bias
Optical
power
This will
depend on the
CW laser
output power
as well as drive
conditions
One advantage of external modulation is that it can be used to implement optical
phase modulation, which opens up the possibility of coherent optical communications
and therefore increased receiver sensitivity.
Laser emits constant optical power. This then
passes through an optical modulator (external
modulator) – this is a voltage driven device. As
we adjust the voltage, the amount of optical
power absorbed will vary. In this way, we
achieve modulation of the optical power
coming out of the modulator.
Bias point and
modulation depth
chosen to give
incrementally linear
slope
Vπ
P0 + p (t )
V
VB + v(t )
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Other advantages of external modulation compared to direct modulation:
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Material Considerations
• Laser diodes suffer from chirp which then introduces dispersion penalty. Not an issue
with external modulation.
Obviously the key requirement is that some optical property of the material must
change in response to a changing electrical parameter.
• It is possible to produce formats such as single sideband (SSB) or double-sideband
suppressed carrier (DSB-SC)
• Electro-optic effect
• An applied electric field changes the refractive index
• This leads to phase changes
• Can also produce intensity modulation
when combined with an interferometer
• Slope efficiency of laser diodes is limited by fundamental quantum efficiency (100%
max), whereas for external modulation the slope efficiency scales with CW laser power.
• Laser diodes limited to 30 GHz max (unless optical injection locking is used), whereas
up to at least 100 GHz has been reported with modulators.
• Many Mach-Zehnder modulators are based on lithium niobate and are difficult to
integrate with other components, but electro-absorption modulators lend themselves to
monolithic integration with driver electronics.
• Recent work by Intel, for example, on silicon modulators paves the way for integration
with CMOS electronics.
• Acousto-optic effect
• A sound wave (resulting from electric field applied to a piezoelectric) changes
the refractive index
• Electro-absorption effect
• Applied electric field changes the absorption
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To date, the dominant type of modulator is the lithium niobate Mach-Zehnder, which
is based on the electro-optic effect.
Comparison of electro-optic materials
Electro-optic effect:
A small change in refractive index n results from an electric field E:
Pockels effect
Only certain crystalline solids show the Pockels effect,
as it requires lack of inversion symmetry
It is linear with respect to electric field and hence
voltage
Moodie, CIP
1
1
=
+ rE + RE 2 + ...
n 2 n02
Kerr effect
Observed in all optical materials with varying
magnitudes, but generally weaker than Pockels
effect.
Apart from presence of electro-optic effect, other important material properties include
optical loss, maximum optical power handling capability and stability (thermal and
optical).
In general, the Pockels effect is used since it is stronger (Kerr effect is primarily
exploited for optical fibre solitons).
Modulators can be made from inorganic materials, semiconductors or polymers
The Pockels coefficients rij are elements of a 6 x 3 tensor
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However, for the moment lithium niobate (an inorganic material) dominates, not
because it excels with respect to loss, stability, maximum optical power or electro-optic
sensitivity, but because it offers the best compromise between all four key parameters.
Lithium niobate is also relatively cheap since it is also widely used in surface acoustic
wave filters. It can be grown using the Czochralski process in wafer sizes large enough to
accommodate the relatively long and narrow structures required for Mach-Zehnder
modulators.
Polymer modulator fabricated using SU-8 based
rubber stamp as a potential route to low cost
manufacturing.
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Light from a laser can be described by its electric field. To keep things simple we
consider a purely monochromatic laser (i.e. a “perfect” laser), for which the emitted
field at some fixed distance from the laser is given by:
Lucent
E (t ) = Eo (t )e j (ωo ( t ) t +φo ( t ))
Optical phase
Amplitude (complex quantity)
Optical frequency (i.e. 100’s of THz)
In analogy with electronic communications, we are able to modulate amplitude,
frequency or phase.
MACH-ZEHNDER
MODULATORS
Amplitude modulation in optical communications is known as intensity modulation,
and this is the most common approach. It can be achieved either through direct or
external modulation.
Frequency and phase modulation can only be achieved with an external modulator,
and can only be detected with a coherent photoreceiver. We will not consider these
techniques any further here.
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The optical intensity is directly proportional to the square of the electric field
magnitude. The optical power emitted by the laser is, in turn, directly proportional
to the intensity. So we can write:
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optical power ∝ E (t ) = Eo (t )
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External modulators that are based on the interferometer principle are known as
Mach-Zehnder modulators (MZM). To understand the basic principle, we need to
remember something about superposition (and constructive and destructive
interference).
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So the optical power varies only with variations in the amplitude of the electric field,
and this is achieved either through direct modulation or an external modulator.
We will now consider the operation of an external modulator based on the principle
of an interferometer:
Consider some examples:
Constructive
interference:
1.0
1.0
Electrical input (modulation)
=
Modulator
1.0
-0.2
1.2
+
-0.2i
=
=
0.8
time
"Quadrature phase" ±90°
interference:
+
+
0.2
Unmodulated
light from laser
Destructive
interference:
1-0.2i
time
time
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Now consider the optical waveguide structure of a MZM:
The two waveguide arms have equal
length, so the delay and hence phase
shift is equal for both paths.
Input light
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The waveguides are formed from titanium which is diffused onto a layer of lithium niobate,
which forms the substrate. Lithium niobate is a material that has a strong electro-optic effect –
if we apply a voltage to it, then its refractive index changes. We can show that this is equivalent
to introducing a phase shift.
Ti-diffused optical waveguide
Electrodes
CW light
Output light
Y-junction. The incoming light is
split equally into two paths at
this point. So the light on each
of these paths for an ideal
device will be 3 dB less in
optical power compared to the
input light.
Lithium
niobate
substrate
Second Y-junction. Here light from the two arms
is combined in phase. However, the optical power
of the output will be lower than that of the input
due to losses in the waveguides and at the Yjunctions. We refer to this as the insertion loss of
the MZM
Modulated light
In the MZM shown above, a voltage applied to the electrodes will introduce a phase shift into
the upper arm. For zero volts there is no phase shift and we have constructive interference, but
if we increase the voltage to some value (called Vπ) then there is a π radians relative phase shift
leading to total extinction. Values in between will lead to varying levels of absorption.
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Just as we have a light-current characteristic for a laser diode, we have a voltage-light
characteristic for a MZM:
Po Pi
 πVm 
Po T ff 

=
1 + cos
Pi
2 
 Vπ 
Po = T ff Pi
T ff < 1
1
The transfer characteristic is given by:
If we apply a bias voltage of nVπ/2 (where n is odd) and a small-signal
modulation component given by vm(t), then linearization of the above equation
around the bias point will yield:
Reduction due to
optical insertion loss
of modulator
T ff
 π (VB + vm (t ) )  T ff
Po T ff 
 ≈
=
1 + cos
Vπ
Pi
2 
 2

 π vm (t ) 

1 ±
Vπ 

from which the slope efficiency (in W/V) is obtained as:
0
0
1
Vm = Vπ
2
3
4
Vm Vπ
dPo T ff π
Pi
=
dvm 2Vπ
So by increasing the optical power from the CW laser, we can increase the
efficiency of the modulator.
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Choice of the bias point is an important consideration, because the sinusoidal shape of the
MZM transfer characteristic means there are no linear parts to the curve, so if we want to
have almost linear operation we must choose points on the curve that are good
approximations to a straight line. Also, we can show that the best bias points will be those for
which the slope of the characteristic is maximised (in order to prove efficiency).
So if we use an appropriate bias point (say 3Vπ/2), and then apply modulation, we
have the following:
Po Pi
If we assume constant CW input power, then:
Po =
T ff Pi 
 πVm 

1 + cos
2 
 Vπ 
dPo T ff Pi
=
dVm
2
Bias point and modulation
depth chosen to give
incrementally linear slope
1
Reduction due to
optical insertion loss
of modulator
T ff
Po + p (t )
Pi
 π
 πVm 

− sin 
V
V
π
π



Finding the maxima/minima for this derivative yields the following as suitable bias points:
0
0
Vm 1 3 5
= , , ,.....
Vπ 2 2 2
1
VB + v(t )
Vπ
2
3
4
Vm Vπ