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.