introduction to optical link design · 2015-03-19 · 17 the optical intensity (unit wm‐2) is...
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Introduction to Optical Link Design
• HMY 645• Lecture 04• Spring Semester 2015
Stavros IezekielDepartment of Electrical and
Computer EngineeringUniversity of Cyprus
BASIC CONSIDERATIONS IN COMMUNICATION LINKS
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• The aim of any communication link is the faithful (i.e. accurate) transmission of information from the transmitter (sender) to the receiver, over a channel.
• The channel is far from perfect, and can introduce noise and distortion.
• Ideally, the demodulated message at the receiver should match that from the source. Most systems today are digital, so for these we require error‐free transmission. However, there are still some analogue systems, and here we require a high signal‐to‐noise ratio.
• Many modern communication systems use complicated modulation schemes, especially since the advent of digital signal processing.
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• Detection in the presence of noise is one of the major challenges in a communications link, whether it is analogue or digital.
Analogue
Digital
The Impact of Noise
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NoiseNonlinearityDispersionAttenuation
ChannelReceiver
Transmittedsignal
Receivedsignal
t
Distortedand noisy
Noise
NoiseNonlinearityDispersionAttenuation
Transmittedsignal
Receivedsignal
t•Regeneratedpulse•Effects of channelare mitigated
RegenerativereceiverChannel
(Fibre)
Analogue Communication
Digital Communication
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Digital and Analogue Communications: Comparison
• Advantages of digital communications– 1. Signal regeneration– 2. Error detection and error correction is possible– 3. Greater dynamic range
• Disadvantages of digital communications:– 1. Generally requires more bandwidth than analogue – 2. Digital detection requires synchronisation(i.e. clock must be recovered at the receiver from the incoming data stream).
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Digital vs. Analogue Performance Criteria
• Analogue systems deal with continuous waveforms. Evaluate performance by fidelity criterion, e.g. signal‐to‐noise ratio (SNR).– i.e. SNR is a figure of merit largely used for analogue communications
• Digital communication systems transmit signals from a finite set (“alphabet”) that is known by the receiver. (e.g. Morse code).
• Hence a figure of merit for digital communications is the probability of incorrectly detecting a digit– also called the probability of error– often specify the bit error rate (BER)– a BER of 10‐9 or less is typically specified for many modern systems
• Note: Although SNR and BER are two different measures, for many optical systems they are related.
Probability of Error (Digital performance criterion)
• The bit error rate (BER) is obtained by dividing the number of errors (Ne) occurring over a time interval t by the number of pulses (ones and zeros) transmitted during this interval (Nt):
tBN
NNBER
T
e
t
e
• BT is the bit rate (bits/sec) and is equivalent to 1/TB where TB is the bit duration. We assume that a “1” is “high” and “0” is low for the duration of TB, but other line coding schemes are possible.
Common data (line coding) formats
Most optical systems use NRZ or RZ codes.
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So in a digital optical link, the decision points are made at the mid‐point of the bit:
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Recovered pulse train(output voltage)
Bit errors can be made here;the number depends on the SNRof the received signal
This is shown in more detail for the photoreceiver:
BASIC PRINCIPLES OF OPTICAL LINKS
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Informationsource
Informationrecipient
Opticaltransmitter
Opticalreceiver
Opticalfibre
Electrical‐to‐optical (E/O) conversion
Optical‐to‐electrical (O/E) conversion
Basic architecture of the simplest type of optical link
Alternative names for optical receiver: photoreceiver and photodetector
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In reality, an optical link will be more complex.
However, for the moment we will focus on the simpler version from the previous slide.
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There are two main reasons why we are interested in using photonics for digital (and sometimes analogue) communications:
1. Bandwidth2. Low loss optical fibres
THz200105.1103
6
8
cfTypical DFB laser
spectrum
Lasers offer a very high carrier frequency
2021
2112
ccfff
e.g. A wavelength span of 0.8 nm centred on 1550 nm gives 100 GHz bandwidth
Optical fibre offers large bandwidths
Typical single‐mode fibre attenuation plus optical amplifier gain, both versus wavelength.
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Lasers are of interest because they offer an (almost) monochromatic source of very high quality (coherent) light.
We can describe the light from a laser by an electric field:
ootjEtE exp)( 0
which implies that we can use amplitude modulation (E0), frequency modulation (ω0) or phase modulation (φ0). Note that the electric field as described above is complex, i.e. E0 is a complex number.
Note: in some cases, when we modulate the amplitude, we also end up modulating the frequency – when this happens, we say we have chirp.
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The optical intensity (unit Wm‐2) is directly proportional to the square of the electric field magnitude:
20 )(2
)( tEcntI
where c is the speed of light, n is the refractive index of the medium, and ε0 is the permittivity of free space. Hence:
20)( EtI
For a fixed cross‐sectional area, the optical power is directly proportional to the optical intensity, and so:
20)( EtPOptical
We mostly work with optical power, not intensity for two reasons. First, it can be measured directly. Second, as electrical engineers we prefer to use the symbol I for current, so if we used intensity instead of optical power it would lead to confusion in our equations!
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So we have two of the basic ingredients – an optical source (laser) and an optical transmission medium (optical fibre). The third basic ingredient is an optical receiver to convert the light coming from the output of a fibre into an electronic signal (usually current).
The simplest photo‐receiver is a photodiode connected to a load resistor:
LoadresistorRLPhotodiode
IP
RRRR tEE cos~
Incoming signal (from laser):
optical phase
optical frequency (of order 200 THz) = c / R
electric field amplitude; optical power is ER2
Electric field of incident optical signal
The frequency response of a photodiode is limited to about 100 GHz in state‐of‐the‐art devices. Consider an optical OOK (on‐off keying) waveform:
EnvelopeOptical carrier
The photodiode cannot detect the fast variations of the optical carrier, it can only respond to the modulation envelope, i.e. it acts as an envelope detector. (We also use the term direct detection.)
Photodiodes respond to the magnitude of the incoming electric field, and convert this to current.
In other words, photocurrent is proportional to the optical power incident on the photodiode.
Incoming electric field
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Photocurrent Slope is given by
responsivity
Input optical power
• In an ideal photodiode (no noise, no nonlinearity), there is a linear correspondence between input optical power and photocurrent.
• One consequence of this is that optical loss in dB is double the corresponding electrical loss in dB (1 dBo = 2 dBe). More about this later.
• Note that the photodiode is actually classified as a square law device, since optical power varies directly with the square of the electric field magnitude.
(A/W)tyResponsiviO
P
PI
This is with reference to the static characteristic
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LoadresistorRLPhotodiode
IP
RRRR tEE cos~
Incoming electric field (from an intensitymodulated laser):
The incident optical power is proportional to the square of the E‐field, i.e.:
RRRRincident tEEP 222 cos~
RRRincident tEP 22cos1221
the photodiode cannot detect the term 2R. Hence for intensity modulation/direct detection schemes (IM/DD),
2, RDDP EI
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• The majority of optical links are digital and are based on direct detection of intensity modulation
• This means that the optical power emitted by the source is modulated, and the modulated power is then detected by a simple photoreceiver (like the one shown on the previous slide) after passing through a length of optical fibre.
• The optical power can be modulated either directly (the current into a laser diode is modulated) or externally, as shown below:
TransimpedanceAmplifier
• The advantage of external modulators is that they can be modulated to many tens of GHz, and they can also be used to implement optical phase modulation.
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• In digital optical communications, an important figure of merit for photoreceivers is the receiver sensitivity. The receiver sensitivity PR sets the lower limit on the optical power needed to achieve binary transmission at BTbits per second with a specified BER.
• Sensitivity gets worse as we go up in bit rate:
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• One way of improving sensitivity is by using coherent detection of phase modulated light:
Note: PDs can only respond to intensity variations. Phase modulation is converted to intensity variation through mixing with LO laser.
but the receiver is more complex (and therefore more expensive also).
• However, we can improve the performance of IM/DD (intensity modulation/direct detection) by using optical amplifiers:
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1
2 2
3 3
N N
MUX DMUX
EDFA
Multiplexer
Demultiplexer
TX1
TX2
TX3
TXN RXN
RX1
RX2
RX3
1, 2,3 ..... N
1
Num
ber o
f wav
elen
gth
chan
nels
Data rate per channel (Gb/s)0.01 0.1 1 10 100 1000
0.1
1
10
100
1000
2003
1977 1995
1998
1998
1995
1986 1991
1993
1996
200120032001
Improving electronics
Impr
ovin
g ph
oton
ics 2006
2008
Total capacity
As a result of optical amplifiers, in long distance networks, WDM is pervasive:
which means we can relax bandwidth requirements by increasing the number of wavelength channels and the channel density. Even so, the trend is to higher bit rates for optoelectronics.
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DirectlyModulatedLaser Diode
Intensity modulatedopticalsignal(“AM”)
RF input
Photodiode
Optical input
RFout
Source and detector options
Optical input
Photo‐diode
CWLaser(LO)
+ Square‐lawDetection& LPF
RFout
Opticalcoupler
Externalmodulator
CWLaser RF input
ModulatedopticalSignal
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
ELECTRO‐OPTIC CONVERSION
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• In electrical‐to‐optical (E/O) conversion, our aim is to convert an electronic waveform (current or voltage, for example) to corresponding variations of optical power (or intensity).
In addition to having sufficient optical output power, adequate conversion efficiency and good impedance matching, we
require that the output is a faithful replica of the input – i.e. noise and nonlinearity are bad news.
• If we choose to use coherent detection, then we may wish to modulate the optical frequency (or phase) instead of the power.
• For the rest of the lecture, we will assume that we are not interested in coherent detection, but that we will use IM/DD instead.
PL (mW)
IL (mA)
sL (W/A)
LI
LPDrivecurrent
Optical power
Threshold current
The L‐I characteristic resembles the DC piecewise‐linear 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
Consider the light‐current characteristic of a laser diode. 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
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 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.
When characterising E/O devices at microwave frequencies, the intensity modulation response is an important parameter. Directly modulated laser diodes have a low‐pass second‐order response which places a limit on the bandwidth they can support:
So we can model our E/O component as a linear two‐port with a transfer function:
)()(
mL
mL
jijp
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
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 fact, most high‐speed and long distance networks use Mach‐Zehnder modulators for E/O conversion. 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.
VV
Bias point and modulation depth chosen to give incrementally linear slope
Opticalpower
This will depend on the
CW laser output power
as well as drive conditions
)(tvVB
)(0 tpP Externalmodulator
CWLaser RF input
+ Bias
External modulation
Modulatedlightoutput
OPTOELECTRONIC (O/E) CONVERSION
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• In optical‐to‐electrical (O/E) conversion, our aim is to convert an incoming time‐varying optical power into corresponding variations of electrical signal (current, perhaps followed by transimpedance stage).
In addition to having sufficient bandwidth andadequate conversion efficiency, we require that the output is a
faithful replica of the input – i.e. noise is bad news.
• If we elect to use coherent detection, then we may use a local oscillator laser in order to boost the sensitivity. Power handling will be important.
• Definitions used here: a photodiode is a one‐port electrical device with an optical input which is based on a semiconductor diode. A photodetector (also called a photoreceiver) is a circuit containing a photodiode as the front end followed by electronic amplification.
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Photocurrent Slope is given by responsivity
Input optical power
(A/W)tyResponsiviO
P
PI
qhcR
hfPqI
O
P
/
photons incidentof no.hpairseof numberefficiency Quantum
36
(Detector slope efficiency is another term for responsivity)
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When using external modulation, high optical power handling capability may be important, since increased CW laser power will improve the modulator slope efficiency.
POINT‐TO‐POINT DIGITAL LINK DESIGN
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Informationsource
Informationrecipient
Opticaltransmitter
Opticalreceiver
Opticalfibre
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• In digital communications, three key specifications are:
1. The length of the link (in km)2. The bit rate (in Mb/s or Gb/s)3.The bit error rate (BER).
• In addition, such considerations as component cost (persubscriber) and reliability also have to be taken care of. Insatisfying the link specifications, a designer has a numberof decisions to take, because ....
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DispersionModulationbandwidth
Sensitivity
Modulationbandwidth
Power
TransmissionMedium(Fibre)
Transmitter(e.g. laser)
Receiver(photodiode)
Attenuation
Bandwidth (bit rate) and repeater spacing aredetermined by:
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A. Choice of operating wavelength
• Short haul links (e.g. LANs) :‐ use short wavelengths (e.g.0.85 mm). Moderate fibre losses can be tolerated and thetechnology is cheap. By using multimode fibre, connectorsare more rugged than for single mode.
• Long haul links (e.g. transatlantic) :‐ use long wavelengthswhere attenuation and dispersion are low. (e.g. 1.3 mm ‐gives dispersion minimum, or 1.55 mm ‐ has attenuationminimum and is compatible with optical amplifiers).
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Loss of modern fibres
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0
10
20
-10
-20
Dispersion(ps/(nm.km))
Dispersion for a silica single‐mode fibre
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B. Choice of source
• Power :‐ laser couples more power into single mode fibre than LED, but high‐bit rate versions can be expensive and require temperature and optical power control. This makes them unsuitable for short links.
• Spectral width :‐ at short wavelengths (high materialdispersion) LEDs with large spectral widths might causeproblems with intersymbol interference. At 1.3 mm, we havevery low dispersion fibre, which combined with low spectralwidth lasers allows high bit rates (e.g. 10 Gb/s and above).
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50 to 100 nm
3 to 6 nm
< 1 pm
LED
Fabry-PerotLaser Diode
Single-ModeLaser Diode
Wavelength
Rel
ativ
e P
ower
Den
sity
Comparison of spectral widths
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Source Bandwidth:
LEDs: 3 dB bandwidth of a few hundreds of MHz can be achieved
Laser diodes: up to tens of GHz (approx. 30 GHz is max.)
External modulation to more than 100 GHz has been demonstrated.
Typical LD frequencyresponse (second‐order)
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C. Choice of fibre• Multimode :‐
– modal dispersion limited– can be used with LEDs and laser diodes– graded index multimode fibre can achieve reasonablereduction in modal dispersion.
• Single‐mode :– no modal dispersion problems– can only be used with laser diodes (high tolerancecoupling)
– can support > 1 Tb/s (using WDM)– small core diameter (8mm) leads to high tolerance (highprice) connectors.
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D. Choice of photodetctor
• PIN :‐– simpler construction than APD– relatively low sensitivity– available for short and long wavelengths– higher bandwidths achievable compared to APDs (up to100 GHz)
• APD :‐– better receiver sensitivity– temperature sensitive– high bias voltages
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Typical receiver sensitivities vs bit rate:
LINK POWER BUDGET
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Link Power Budget
• Put simply, the link power budget is an "accounting"procedure in which one calculates how much power canbe lost between the transmitter and the receiver for agiven receiver sensitivity (which depends on the bit rate)and transmitter power output. The resulting budget isallocated to connector losses, splice losses, fibre lossesand a safety margin (system margin).
• dB and dBm units are used in the link power budget.
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dB and dBm
• Advantage of this approach is that it replaces multiplication& division with addition/subtraction in calculation of linkgain/link loss.
G (dB)Pin (dBm) Pout (dBm) = Pin (dBm) + G (dB)
L (dB)Pin (dBm) Pout (dBm) = Pin (dBm) - L (dB)
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• power link budget
Lmax = PS - PR
PS (dBm) PR (dBm)L (dB)
LASER PHOTODIODEFIBRE
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LASER PHOTODIODE
FIBRE
Laser‐to‐fibre coupling loss;can be minimised using lenses
Fibre pigtail; v. short,negligible loss
Fibre splice (permanent connection)Introduces splice loss
Loss due to fibreconnector
Fibre loss dB/km
OPTICALAMPLIFIER
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LASER PHOTODIODEFIBRE
OPTICALAMPLIFIER
Distance along link (km)
Pow
er le
vel (
dBm
)
Total linkloss
Ma
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• In practical applications, we often use components that have connectors attached (i.e. they are connectorised). Fibre with one connector is known as a fibre pigtail. A length of fibre with connectors on both ends is called a patchcord.
• In many link budgets, the splice loss is often “lumped” together with the fibre loss.
• We also include a safety factor known as the system margin(Ma) to account for component degradation. A typical value for Ma is 6 dB.
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Example
• Calculate maximum link length for a system with: – a connectorised laser transmitter (PS = 3 dBm) – a connectorised receiver with sensitivity PR = ‐40 dBm– a fibre patchcord (F = 0.5 dB/km, including splice losses)– connector losses of C = 1 dB and system margin of 6 dB
Total link loss (dB) = PS - PR = F L + 2 C + Ma
Laser (PS)Receiver
(PR)Fibre
C C
F L
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F L = PS - PR - 2 C - Ma = 35 dB
Laser (PS)Receiver
(PR)C CF L
Hence Lmax = 35 / 0.5 = 70 km
PS = 3
Distance(km)
2Powerlevel (dBm)
N.B.Not toscale!
PR = -40
-34-33
700
C
C
F Lmax
Ma
LINK RISE‐TIME BUDGET
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Digital link design: Rise time budget
• In the previous section, we saw how the maximum linkdistance is affected by the fibre attenuation and also thesource power and the photoreceiver sensitivity for a givenbit rate; this gave us the link power budget.
Informationsource
Informationrecipient
Opticaltransmitter
Opticalreceiver
Opticalfibre
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However, recall that bit rate and repeater spacing are also determined by rise‐time considerations:
DispersionModulationbandwidth
Sensitivity
Modulationbandwidth
Power
TransmissionMedium(Fibre)
Transmitter(e.g. laser)
Receiver(photodiode)
Attenuation
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tTX
LASER PHOTODIODEFIBRE
• rise-time budget
tRX
tmat tmod
2222RXmodmatTXsys ttttt
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Concept of rise‐time• Any real‐life system with an input/output will have a finite bandwidth.
• For example, consider typical modulation response of a laser diode:
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• Previous diagram relates to sinusoidal (j) response. • The corresponding step‐response shows that it takes a
finite time to reach the steady‐state, and that in some cases there may even be relaxation oscillations:
iin(t)pLD (t)
t00
iin(t)
t0
0
pLD (t)
Typical step‐response of a laser diode, showing turn‐on delay andrelaxation oscillations (due to low damping factor)
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• Similarly, a photodiode will take a finite time to respond to step‐changes in the incidient optical power, as shown below for the case of a pulse input:
t00
pPD(t)
t0
0
iout (t)pPD(t)
iout(t)
Note: Output current pulse shape depends on the device capacitance and also the width of the depletion region. The above response is quite poor due to large junction capacitance.
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• Finally, the optical fibre itself will exhibit its own rise time due to the effects of dispersion.
• In the case of single‐mode fibres, this is entirely due to intramodaldispersion, with the main contribution to this being being material dispersion.
• In multimode fibres, the dominant effect is intermodal dispersion. (Although material dispersion also exists, it is negligible in comparison).
• Although attenuation is important, it does not have an impact on rise‐time. It affects the link power budget instead.
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• So, not surprisingly, the optical fibre link as a whole will have a rise‐time (and fall‐time) in response to a rectangular pulse input:
FIBREiin(t)
pLD (t) pPD(t)iout(t)
t00
iin(t)
t0
0
iout (t)
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Definition of rise‐time and fall‐time
N.B. y‐axis is voltage, current or optical power as appropriate
• Rise‐time: time taken to rise from 10% to 90% of the steady‐state value of the pulse.• Fall‐time: time taken to fall from 90% to 10% of the steady‐state value of the pulse.
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• Put simply, the rise‐time budget is an "accounting"procedure in which one calculates how much pulsespreading can be tolerated between the transmitterand the receiver for a given transmitter rise‐time,photoreceiver rise‐time and dispersion due to the fibre(both modal and chromatic, as appropriate).
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Rise‐time Budget• The total rise‐time of the fibre‐optic link is known as the
system rise time tsys.• It depends on the rise‐times of the individual systems
components, and assuming these are independent ofone another, they affect tsys as follows:
2222RXmodmatTXsys ttttt
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• tTX = optical transmitter rise‐time
• tRX = optical receiver rise‐time
• tmat = material dispersion rise‐time
• tmod = modal dispersion rise‐time (for MM fibre only)
• The usual requirement on tsys is:
tsys < 0.7
where is the pulse duration.
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• The pulse duration depends on the data format.
• Two main data formats are used: NRZ and RZ
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Consider an NRZ stream composed of alternating “0”s and “1”s, and an RZ stream composed entirely of “1”s:
For NRZ signalling, NRZ = 1/BT , hence:
NRZ
NRZ
RZ
RZ
T = 1/BT
101010.....
111111.....
For RZ signalling, RZ = 1/2BT , hence:
Tsys B
t 7.0
Tsys B
t 35.0
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tmat : material dispersion rise‐time
• due to material dispersion• significant in single‐mode fibres
tmat can usually be neglected if the spectral width is narrow (e.g. in DFB lasers) or if operation is at 1.3 mm.
tmat = Dmat L
• Dmat = material dispersion parameter (ps/nm.km)• = spectral width of optical source (in nm or m)
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tmod : modal dispersion rise‐time
• due to (inter)modal dispersion
• dominant in multimode fibres
• In theory tmod is proportional to fibre length. In a real system, pulse distortion increases less rapidly after a certain initial length because of mode coupling.
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• Empirically, it can be shown that tmod in ns is given by:
where q is between 0.5 and 1.0 (depends on amount of mode coupling), L is the fibre length (km) and BO is the 3dB electrical bandwidth (in GHz) of 1 km of fibre.
O
q
BLt 44.0
mod
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tRX : photoreceiver rise‐time
• Assuming a simple low‐pass RC characteristic for the frequency response of a photoreceiver, then we can relate tRX (in ns) to the 3 dB receiver bandwidth (BRX in units of GHz) as follows:
RXRX B
t 35.0
isignal inoise Cd RdA simple photodiode model;high frequency versions alsoinclude parasitics due to thepackaging
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tTX : optical transmitter rise‐time
• This is a function of both the intrinsic frequency response (of either the LED or the laser diode) along with any drive electronics.
• LED and laser diode data sheets usually specify the device rise time.
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Finally....
Although the above equations/analysis may appear to bestraightforward, be VERY careful in using units.
Bandwidths of laser diodes, for example, tend to be in theGHz range, so rise‐times tend to be quoted in ns.
However, it is possible to encounter a mix of units whenperforming rise‐time calculations.
If in any doubt, convert all quantities to SI units, andperform calculations in SI units, before converting to ns at theend.