ECE 455 Lecture 11
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Optical Sources and Modulation of Light
• HMY 455 • Lecture 10 • Fall Semester 2016
Stavros Iezekiel Department of Electrical and
Computer Engineering
University of Cyprus
ECE 455 Lecture 11
ELECTRO-OPTIC CONVERSION
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ECE 455 Lecture 11
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• In electrical-to-optical (E/O) conversion, our aim is to convert an electronic waveform (current or voltage) to corresponding variations of optical power.
In addition to having sufficient optical output power,
adequate conversion efficiency and good impedance matching, we require that the output is an accurate copy of the input
– i.e. nonlinearity and noise can be bad news. • If we choose to use coherent detection, then we might want to modulate the optical frequency (or phase) instead of the power.
• In thise case we must use external modulators.
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Electrical-to- Optical
Modulation
Optical -to-Electrical
Demodulation
Optical fibre
ωOPT ωOPT
ωOPT
ωOPT ωOPT
E/O “upconversion” O/E “downconversion”
Simplistic model of E/O & O/E conversion
ωRF
ωRF
ωRF
ωRF
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Directly Modulated Laser Diode
Intensity modulated
optical signal (“AM”)
RF input
Photodiode
Optical input
RF out
Source and detector options
Optical input
Photo- diode
CW Laser (LO)
+ Square-law Detection & LPF
RF out
Optical coupler
External modulator
CW Laser RF input
Modulated optical Signal
Intensity,
Phase, or Frequency
External modulation
Direct modulation
Direct detection Coherent detection
Direct intensity modulation / Direct detection (IM/DD) • Simple technique, cheap • Problems can include:
• Chirp • Nonlinearity
Coherent detection of: • Amplitude • Phase • or Frequency
Offers better sensitivity, but increased receiver complexity compared to direct detection
External Intensity modulation / Direct detection (IM/DD) • No chirp problems • Larger bandwidth compared to direct modulation • Relatively expensive
ECE 455 Lecture 11
)cos(1 mmBL tmII
oomm tjtmEtE expcos1)( 0
m = modulation index
For small-signal modulation m <<1 so we can use Bessel functions to expand the electric field expression. We can show that this contains multiple frequency components of the form mo n
Drive current:
Forward biased
laser diode
IB = bias current
However, the optical intensity will be given by the square of the electric field magnitude:
mmtmEtE cos1)(2
0
2
and so the optical output power is of the form: mmtmP cos10
where P0 is the average power, m is the microwave modulation frequency and m the phase.
Emitted electric field at a fixed point in space:
Chirp is neglected ( is fixed) 0
I
If we apply a sinusoidal microwave signal of frequency m to an E/O component, the resulting optical field will contain a central optical frequency 0 and multiple sidebands. The exact form of the spectrum depends on the E/O device and bias conditions. For a directly modulated laser diode, we have:
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Impact of microwave modulation on the optical spectrum of a laser diode as the modulation frequency is varied
ECE 455 Lecture 11
PL (mW)
IL (mA)
sL (W/A)
LI
LPDrive current
Optical power
Threshold current
The L-I characteristic is similar to the I-V characteristic of a diode. Above threshold and below saturation, the L-I characteristic can be approximated very well by a straight line segment with a slope given by:
L
LL
I
Ps
Slope efficiency in W/A
L-I characteristic
If we consider the light-current characteristic of a laser diode, the above result makes sense. The L-I characteristic is a plot of optical output power versus drive current:
Ideally we the slope efficiency to be as high as possible but it is fundamentally limited by the quantum efficiency of the laser.
saturation
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ECE 455 Lecture 11
PL (mW)
IL (mA)
)(cos1 tiItmII BmBL
BI
)(cos1 00 tpPtmPP mL
0P
)()( tIstP LLL
Although it is not obvious from the L-I curve, the slope efficiency is actually frequency-dependent. At a given frequency, the sinusoidal components of the current and optical power can be described using phasors, and they are related via:
)(
)()(
mL
mLmL
ji
jpjs
This is referred to as the intensity modulation response
Hence if we ensure that the drive current does not go below threshold or into saturation, the optical power will follow the drive current. The “DC” components are related via: BLIsP 0
This is also the average optical power
where iL(jm) is the modulation current phasor and pL(jm) is the corresponding output optical power phasor.
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ECE 455 Lecture 11
The intensity modulation response of a directly modulated laser diode is a low-pass second-order response which places a limit on the bandwidth they can support:
We can model our E/O component as a linear two-port with a transfer function:
)(
)(
mL
mL
ji
jp
m
Resonance peak: The laser is modulated at frequencies below this point
)( mjp
)( mji
)( mL js E/O
t
)(ti
t
)(tpModulation current
Optical power has same frequency but with an
amplitude and phase change
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ECE 455 Lecture 11
Although we have used a directly modulated laser diode to look at E/O conversion, a similar approach can be used with a modulator. In this case, the device is driven with voltage instead of current, and the light-voltage characteristic has a sinusoidal shape as opposed to a diode-like curve.
V V
Bias point and modulation depth chosen to give incrementally linear slope
Optical power
This will depend on the
CW laser output power
as well as drive conditions
)(tvVB
)(0 tpP External modulator
CW Laser
RF input + Bias
External modulation
Modulated light output
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ECE 455 Lecture 11
OPTICAL SOURCES FOR COMMUNICATIONS
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• Optoelectronic sources convert electrical energy to light energy (i.e. they “convert” electrons to photons).
• There are two main sources for optical communications:
– monochromatic incoherent sources (LEDs: light emitting diodes)
– monochromatic coherent sources (laser diodes)
– Both LEDs and laser diodes are semiconductor optoelectronic devices that can be modulated at high-speeds (laser diodes much more so than LEDs).
– For high-speed long distance links, laser diodes are used. These can be modulated directly or externally.
– Direct modulation is achieved by varying the drive current, external through varying the optical power with an external device (a modulator).
ECE 455 Lecture 11
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• Optical fibre sources should have the following properties:
(a) compatibility for launching light into fibre
(b) linearity
(c) emit light at wavelengths where fibre is low loss and has low dispersion
(d) wide modulation bandwidth (i.e. small rise time)
(e) deliver sufficient optical power to overcome losses
(f) narrow spectral linewidth to minimise chromatic dispersion
(g) maintain stable optical output against environmental changes and ageing
(h) be reliable, low cost and compatible with drive electronics
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Surface-emitting LED
Laser Diode (Fabry-Perot) resonator cavity
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(a) Compatibility for launching light into fibre
• LED gives very poor coupling into single- mode fibre, but is OK for multimode • Laser diode power is more efficiently coupled into single-mode fibre (directional beam)
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(b) Linearity
LED Laser diode
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(c) Operating wavelengths:
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(d) Bandwidth: • LEDs: 3 dB bandwidth up to a few hundreds of MHz can be achieved
• Laser diodes: up to tens of GHz (approx. 30 GHz is max.)
• Laser diodes can also be externally modulated, up to at least 100 GHz
(depending on the external modulator material)
• However, in addition to the frequency response as described in slide 14, for digital applications the time-domain characteristics are important, and in particular the rise time:
Optical power
Drive current
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Typical for a long wavelength LED:
Gaussian profile
(f) Spectral width:
Spectral width, at FWHM (full width-half maximum)
Typical for a Fabry-Perot laser diode (gives multimode output). Mode spacing is determined by cavity length (mirror-mirror spacing)
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In a distributed feedback (DFB) laser diode, feedback is provided by a grating, which selects a single mode whilst suppressing all the others
Relative optical power
DFB lasers give a single-mode spectrum. (Only one “line”)
Spectral linewidth (In the case of a DFB, this is the same as the spectral width)
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(g) Temperature dependence: in laser diodes, the threshold current has a distinct temperature dependence; this means that temperature control circuits are required, which adds to the cost of laser transmitters.
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LEDs: • Good points: cheap, easy to drive (no thermal or optical power stabilisation needed) • Bad points: low bandwidth, large spectral width, high source-to-fibre coupling loss for single mode fibres • Conclusions: best used with multimode fibres in LAN-type applications for low bit rates
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Laser diodes: • Good points: large bandwidths, narrow spectral linewidth, can couple several mW into single mode fibre • Bad points: relatively expensive, most need power and temperature stabilisation circuits, source-to-fibre coupling can be difficult.
• However, VCSELs (vertical cavity surface emitting lasers are cheap, and are used in many multimode fibre links, in some cases up to several Gb/s)
• Conclusions: best used with single-mode fibres in high-speed (often 10 Gb/s plus) long distance applications, and VCSELs have now become popular for multimode fibres (e.g. active optical cables for data centres).
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OPTICAL SPECTRUM OPTICAL SPECTRUM
FABRY-PEROT DIODE LASER DISTRIBUTED-FEEDBACK DIODE LASER (DFB)
•Simple structure •High power available •Major application: CD players •Cost: $10 - 500
•Low laser noise •High linearity •Major application: CATV distribution •Cost: $500 – 5,000
VERTICAL-CAVITY SURFACE-EMITTING LASER (VCSEL)
•Testable at wafer level •Circular, low-divergence beam •Major application: LAN links •Cost: $10 -500
OPTICAL SPECTRUM
Basic laser structures: Summary
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ECE 455 Lecture 11
EXTERNAL MODULATION
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External
modulator
CW
Laser Bias
+ modulation
Modulated
light
output
In addition to direct modulation of a
laser we can also modulate the optical
power with the following arrangement,
known as external modulation:
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:
V V
Optical
power
This will
depend on
the CW laser
output power
as well as
drive
conditions
)(tvVB
)(0 tpP
External modulation
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Modulated
optical source
[E/O modulation]
Photoreceiver
[O/E demodulation] Single-mode
optical fibre
Directly
Modulated
Laser Diode
Intensity
modulated
optical
signal
(“AM”)
RF input
Direct modulation (intensity modulation)
RF input RF output
External
modulator
CW
Laser RF input
Modulated
optical
Signal
Intensity,
Phase,
or Frequency
Modulation
External modulation
Photo
-diode
Modulated
optical
input
RF
output
Direct detection
Modulated
optical
input
Photo-
diode
CW
Laser
(LO)
+ Square-law
Detection
& LPF
RF
output
Optical
coupler
Coherent detection
OR
Source options:
OR
Receiver options: m
m
OMicrowave signals
reside as sidebands
on an optical carrier
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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:
))()(()()(
tttj
oooetEtE
Amplitude (complex quantity) Optical frequency (i.e. 100’s of THz)
Optical phase
In analogy with electronic communications, we are able to modulate amplitude, frequency or phase. 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.
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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:
22)()( tEtE o power optical
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:
Modulator
Electrical input (modulation)
Unmodulated light from laser
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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). Consider some examples:
+
=
time
1.0
-0.2
0.8
Destructive interference:
1.0
0.2
1.2
+
=
time
Constructive interference:
+
=
time
"Quadrature phase" ±90° interference:
1.0
-0.2i
1-0.2i
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Now consider the optical waveguide structure of a MZM:
Input 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.
The two waveguide arms have equal length, so the delay and hence phase shift is equal for both paths.
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 Y-junctions. We refer to this as the insertion loss of the MZM
Output light
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CW light
Modulated light
Ti-diffused optical waveguide
Lithium niobate substrate
Electrodes
The waveguides are formed from titanium on 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.
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 V) then there is a radians relative phase shift leading to total extinction. Values in between will lead to varying levels of absorption.
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0 1 2 3 4 0
1
VVm
io PP
ffT
Reduction due to optical insertion loss of modulator
The light-voltage (L-V) characteristic for a MZM:
VVm
1
ff
iffo
T
PTP
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The transfer characteristic is given by:
V
VT
P
P mff
i
o cos12
We can use this to find bias points at which the slope of the L-V characteristic is maximized:
0sin2
V
V
V
PT
dV
dP miff
m
o
This gives:
,.....2
5,
2
3,
2
1
V
Vm
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0 1 2 3 4 0
1
VVm
io PP
ffT
Reduction due to optical insertion loss of modulator
Bias point and modulation depth chosen to give incrementally linear slope. We can show that at this point,
V
tvV mB )(
i
o
P
tpP )(
So if we use an appropriate bias point (say 3V/2), and then apply modulation, we have the following:
i
ff
m
o PV
T
dv
dP
2
So by increasing the optical power from the CW laser, we can increase the efficiency of the modulator.