Lightwave_systems.ppt

Light Wave Systems
Dr Manoj Kumar
Professor & Head
Department of ECE
DAVIET,Jalandhar

Overview
• In this section we cover point-to-point digital
transmission link design issues (Ch8):
– Link power budget calculations
– Link rise time calculations
A link should satisfy both these budgets

Fig. 8-1: Simple point-to-point link
System Requirements
1. Transmission Distance
2. Data Rate for a given BER
This p-p link forms the basis for examining
more complex systems

Selecting the Fiber
Other factors to consider: attenuation (depends on?)
and distance-bandwidth product (depends on?) cost
of the connectors, splicing etc.
Then decide
• Multimode or single mode
• Step or graded index fiber
Bit rate and distance are the major factors

Selecting the
Optical Source
• Emission wavelength
• Spectral line width
(FWHM) and number
of modes
• Output power
• Stability
• Emission pattern
• Effective radiating area
LED
LASER

Selecting the detector
• Type of detector
– APD: High sensitivity but complex, high bias voltage
(40V or more) and expensive
– PIN: Simpler, thermally stable, low bias voltage (5V or
less) and less expensive
• Responsivity (that depends on the avalanche gain
& quantum efficiency)
• Operating wavelength and spectral selectivity
• Speed (capacitance) and photosensitive area
• Sensitivity (depends on noise and gain)

Typical bit rates at different
wavelengths
Wavelength LED Systems LASER Systems.
800-900 nm
(Typically
Multimode Fiber)
150 Mb/s.km 2500 Mb/s.km
1300 nm (Lowest
dispersion)
1500 Mb/s.km 25 Gb/s.km
(InGaAsP Laser)
1550 nm (Lowest
Attenuation)
1200 Mb/s.km Up to 500
Gb/s.km
(Best demo)

Design Considerations
• Link Power Budget
– There is enough power margin in the system to
meet the given BER
• Rise Time Budget
– Each element of the link is fast enough to meet
the given bit rate
These two budgets give necessary conditions
for satisfactory operation

Fig. 8-3: Receiver sensitivities Vs bit rate

Fig. 8-2: Optical power-loss model
ystem Margin
T s R c sp f
P P P ml nl L S

     
: Total loss; : Source power; : Rx sensitivity
connectors; splices
T s R
P P P
m n Try Ex: 8.1

Fig. 8-4: Example link-loss budget
Try Ex. 8.2

Rise Time Budget
• Total rise time depends on:
– Transmitter rise time (ttx)
– Group Velocity Dispersion (tGVD)
– Modal dispersion rise time (tmod)
– Receiver rise time (trx)
Total rise time of a digital link should not exceed
70% for a NRZ bit period, and 35% of a RZ bit period
1/ 2
2
1
n
sys i
i
t t

 
  
 


Rise Time…
MHz
in
bandwidth
receiver
is
where
ns;
350
rx
B
/B
t rx
rx 
tx
tx B
t /
350
 ns
Similarly
Assuming both transmitter and receiver as first order
low pass filters

Modal Dispersion Rise Time
Bandwidth BM(L) due to modal dispersion of a
link length L is empirically given by,
B0 is the BW per km (MHz-km product) and
q ~0.5-1 is the modal equilibrium factor
q
o
M L
B
L
B /
)
( 
(ns)
/
440
/
44
.
0 0
mod B
L
B
t q
M 


Group Velocity Dispersion
Where,
D is the dispersion parameter (ns/km/nm) given by eq. (3.57)
σλ is the half power spectral width of the source (nm)
L is the distance in km


L
D
tGVD |
|

2
/
1
2
0
2
2
2
2
2
2
2 440










B
L
L
D
t
t
t
q
rx
tx
sys 

Try examples 8.3 and 8.4

Fig. 8-6: 800 MHz-km Multimode Fiber at
800 nm, (BER=10-9)

Parameters for Fig 8-6
Power coupled
from LED : -13
dBm
Fiber loss 3.5
dB/km
System Margin 6
dB, couplers 1dB
(LED-PIN)
Dmat = 0.07
ns/(nm.km)
LED  50 nm
LASER 1 nm
Bo=800 MHz-km
q = 0.7 (modal)
Power coupled
from LASER = 0
dBm
Material
dispersion limit
with LASER is
off the graph
System Margin 8
dB (Laser-APD)

Fig. 8-7: Single Mode fiber, 1550 nm,
D = 2.5 ps/nm.km, 0.3 dB/km, two lasers

Analog Communication Links
Analog (RF) links are used in
Analog TV and audio services (Legacy)
Cable modem services
Satellite base stations
(Amplifier
Spontaneous
Emission)

Multi Channel Systems
Number of RF carriers can be summed and
directly modulate the laser

Multi Channel Systems
• These have the capability to multiplex
several RF channels
• Each RF channel is independent, it may
carry different type of data (analog video,
digital video, digital audio etc.)
• The data could be modulated onto the RF
carrier using different techniques (AM, FM,
QAM etc.)
• Nonlinearity is the major concern

Sub Carrier Multiplexing
• Each modulating RF carrier will look like a sub-
carrier
• Unmodulated optical signal is the main carrier
• Frequency division multiplexed (FDM) multi channel
systems also called as SCM
Frequency
Unmodulated (main) carrier
Sub-carriers
f1
f2
f1
f2
f0

Link Noise
Modal Noise: When a laser is coupled to a
multi mode fiber (MMF) modal noise
exists. To avoid this,
• Use LED with MMF
• Use a laser with large number of modes
• Use a MMF with large NA
• Use single mode fiber with laser

Modal noise at a connection of a SMF

Mode Partition Noise
• This is the dominant noise in single mode
fiber coupled with multimode laser
• Mode partition noise is associated with
intensity fluctuations in the longitudinal
modes of a laser diode
• Each longitudinal mode has different λ
• The SNR due to MPN can not be improved
by increasing the signal power

Fig. 8-12: Dynamic spectra of a laser
Laser output
spectrum vary
with time
giving
mode partition
noise

Fig. 8-16: Mode-Partition-Noise BER depends on
Receiver BER and System BER

Interferometric
Noise
due to multiple
reflections
• Increases RIN
• Laser instability
• Increases with
signal power
• Can be decreased by
having angled, low
back reflection
connectors
and isolators

Fig. 8-17: Chirping & extinction-ratio penalties

Lightwave_systems.ppt

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