optical amplifier

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Chapter 8

• Basic System Design

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System factors for designing from scratch Design
Verification
Factor Available choices
Type of fiber Single mode, multimode, plastic
Dispersion Repeaters, compensation
Fiber nonlinearities Fiber characteristics, wavelengths used, transmitter power
Operating wavelength (band) 780, 850, 1310, 1550, 1625 nm typical
Transmitter power 0.1 to 20 mw typical usually expressed in dBm
Light source LED, laser
Receiver characteristics Sensitivity, overload
Multiplexing scheme None, CWDM, DWDM
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System factors (continued)
Factor Available choices
Detector type PIN diode, APD, IDP
Modulation scheme OOK, multilevel, coherent
End-end bit error rate lt10-9 typical may be much lower
Signal-to-noise ratio Specified in dB for major stages
Max number of connectors Loss increases with number of connectors
Max number of splices Loss increases with number of splices
Environmental Humidity, temperature, sunlight exposure
Mechanical Flammability, strength, indoor/outdoor/submarine
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Optical link loss budget

• Key calculations in designing a simple fiber
  optic link
• Objective is to determine launch power and
  receiver sensitivity
• Variables
• Environmental and aging
• Connector losses
• Cable losses
• Splices
• Amplifier
• Other components

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• The basic system design verification can be done
  through
• 1- Power budget The Ratio of PT/PR expressed in
  dB is the amount of acceptable loss that can be
  incurred.
• 2- Rise time budget A rise-time budget analysis
  is a convenient method to determine the
  dispersion limitation of an optical link.
• The power budget involves the power level
  calculations from the transmitter to the
  receiver.

• 1. Attenuation
• 2. Coupled power
• Other losses
• Equalization penalty (DL)

1. SNR requirements
2. Minimum power at detector
3. BER
4. Safety margin (Ma)

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The system margin can be expressed as
Ma PT(dBm)-PR(dBm)- system loss. A
()positive system margin ensures proper
operation of the circuit. A (-) negative value
indicates that insufficient power will be reach
the detector to achieve the required BER.

• The optical power budget is then assembled taking
  into account ALL these parameters.
• Pi (Po CL Ma DL) dBm
• where Pi mean input power launched in the
  fiber
• Po mean optical power required at the receiver
• CL total channel loss
• DL dispersion-equalization or ISI penalty,

The sensitivity of the detector is the minimum
detectable power.
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• Risetime budget includes the following

1. Risetime of the source, TS
2. Risetime of the fiber (dispersion), TF
3. Risetime of the amplifier, TA
4. Risetime of the detector, TD

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The risetime budget is assembled as Tsyst
1.1(TS2 TF2 TD2 TA2)1/2 For
non-return-to-zero (NRZ) data For return-to
zero (RZ) data
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Example 8.1
We need to design a digital link to connect two
points 10-km apart. The bit rate needed is 30Mb/s
with BER 10-12. Determine whether the
components listed are suitable for the
link. Source LED 820nm GaAsAl couples 12µW
into 50µm fiber risetime 11ns Fiber Step
Index fiber 50µm core NA 0.24 5.0 dB/km
loss dispersion 1ns/km 4 connectors with
1.0dB loss per connector Detector PIN
photodiode R 0.38A/W Cj 1.5pF, Id 10pA
risetime 3.5ns minimum mean optical power
- 86dBm Calculate also the SNR of the link if
RL given is 5.3kO
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Solution
For this example, 3 factors need to be
considered a) Bandwidth b) Power
levels c) Error rate (SNR)
Risetime Budget We start with the risetime
budget. Assume using NRZ coding, the system
risetime is given by Also Tsyst
1.1(TS2 TF2 TD2)1/2
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Now we can assemble the total system
risetime Total system risetime 23.3
ns Risetime of the source, TS 11.0ns Risetime
of the fiber (dispersion), TF 10 x 1.0ns
10.0ns Allowance for the detector risetime, TD
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Power Budget Total power launched into fiber
-19dBm Losses Fiber attenuation 5dB/km x 10
50dB 4 connectors 1dB x 4 4dB Power
available at detector ( -19dBm 50dB- 4dB)
-73 dBm Since power available at the detector is
73 dBm, the sensitivity of the detector must be
less than this. The safety margin, Ma
-73-(-86) dB 13dB
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The choice of components are suitable
because a) TD calculated is greater than TD
given b) Total power available at the detector
is greater than the minimum power required by
the detector i.e Ma is positive.
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Example 8.2 An optical link is to be designed to
operate over an 8-km length without repeater. The
risetime of the chosen components
are Source 8 ns Fiber Intermodal 5
ns/km Intramodal 1 ns/km Detector 6ns From
the system risetime considerations estimate the
maximum bit rate that may be achieved on the link
using NRZ code.
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Solution Tsyst 1.1(TS2 TF2 TD2) 1.1
82 (8 x 5)2 (8 x 1)2 62)1/2 46.2
ns Max bit rate Maximum bit rate
15.2Mbps Or 3 dB optical BW 7.6MHz
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Exercise 1 The following parameters were chosen
for a long haul single mode optical fiber system
operating at 1.3µm. Mean power launched from
laser 0 dBm Cabled fiber loss 0.4
dB/km Splice loss 0.1 dB/km Connector loss at
transmitter and receiver 1 dB each Mean power
required at the APD When operating at
35Mbps(BER 10-9) -65 dBm When operating at
400Mbps(BER 10-9) -54 dBm Required safety
margin 7 dB
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Estimate a) maximum possible link length without
repeaters when operating at 35Mbps. It may be
assumed that there is no dispersion-equalization
penalty at this rate. b) maximum possible link
length without repeaters when operating at
400Mbps. c) the reduction in the maximum
possible link length without repeaters of (b)
when there is dispersion- equalization penalty
of 1.5dB.
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• Solution
• a)35Mbps
• Pi Po (Fiber cable loss Splice losses ) x
  L Connector loss Ma dB
• -3dBm (-55 dBm) (0.4 0.1)L
  2 7
• 0.5L 52 2-7
• L 86km
• b) 400 Mbps
• Pi Po (Fiber cable loss Splice losses ) x
  L Connector loss Ma dB
• -3dBm (-44 dBm) (0.4 0.1)L
  2 7
• 0.5L 41 2-7
• L 64km
• Including dispersion-equalization penalty of
  1.5dB
• Pi Po (Fiber cable loss Splice losses ) x
  L Connector loss DL MadB
• -3dBm (-44 dBm) (0.4 0.1)L 2 1.5
  7
• 0.5L 41 2 -1.5 7
• L 61km
• Note a reduction of 3 km in the maximum length
  without repeaters when DL is taken to account.

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Exercise 2
Calculate the flux density to construct an
optical link of 15 km and bandwidth of 100 Mb/s.
Components are chosen with the following
characteristics Receiver sensitivity -50 dBm (at
100 Mb/s), fiber loss 2 dB/km and transmitter
launch power into the fiber is 0 dBm, detector
coupling loss is 1 dB. It is anticipated that in
addition, 10 splices each of loss 0.4 db are
required. Determine where the system operate with
sufficient power margin or not?.
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Example 8.4 An optical link was designed to
transmit data at a rate of 20 Mbps using RZ
coding. The length of the link is 7 km and uses
an LED at 0.85µm. The channel used is a GRIN
fiber with 50µm core and attenuation of 2.6dB/km.
The cable requires splicing every kilometer
with a loss of 0.5dB per splice. The connector
used at the receiver has a loss of 1.5dB. The
power launched into the fiber is 100µW. The
minimum power required at the receiver is 71dBm
to give a BER of 10-10. It is also predicted that
a safety margin of 6dB will be required. Show
by suitable method that the choice of components
is suitable for the link.
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Solution The power launched into the fiber 100µW
-10 dBm Minimum power required at the receiver
- 71dBm Total system margin 61
dB Fiber loss 7 x 2.6 18.2dB Splice loss 6 x
0.5 3.0 dB Connector loss 6.0 dB Safety
margin 28.7dB Excess power margin 61 dB -
28.7 dB 32.3 dB Based on the figure given, the
system is stable and provides an excess of 2.3 dB
power margin. The system is suitable for the link
and has safety margin to support future splices
if needed..
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Example 8.5 An optical communication system is
given with the following specifications Laser
? 1.55µm, ?? 0.15nm, power 5dBm, tr
1.0ns Detector tD 0.5ns, sensitivity
-40dBm Pre-amp t A 1.3ns Fiber total
dispersion (MMg) 15.5 psnm-1km-1, length
100km, ? 0.25dB/km Source coupling loss
3dB Connector (2) loss 2dB Splice (50) loss
5dB System 400 Mbps, NRZ, 100km
Show by suitable method that the choice of
components is suitable for the link
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Solution For risetime budget system budget,
Tsyst 1.75ns source ts
1.0ns (1) fiber tF
0.25ns (2) detector tD 0.5ns pre-amp tA
1.3ns for receiver, total
1.39ns (3) ?System risetime from (1),(2) and
(3) 11111111 1.73ns
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Since the calculated Tsyst is less than the
available Tsyst the components is suitable to
support the 400 Mbps signal. For the power
budget Laser power output 5 dBm Source coupling
loss 3 dB Connector loss 2 dB Splice loss 5
dB Attenuation in the fiber 25 dB Total loss 35
dB Power available at the receiver (5 dBm -35
dB) -30 dBm The detectors sensitivity is -40
dBm which is 10 dB less. Therefore the chosen
components will allow sufficient power to arrive
at the detector. Safety margin is 10 dB,
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• Exercise An analog optical link of length 2km
  employs an LED which launches mean optical power
  of 10 dBm into a multimode optical fiber. The
  fiber cable exhibits loss of 3.5 dB/km with
  splice losses calculated at 0.7 dB/km . In
  addition there is a connector loss at the
  receiver of 1.6 dB. The PIN photodiode receiver
  has a sensitivity of -25dBm for an SNR of 50 dB
  and with a modulation index of 0.5. it is
  estimated that a safety margin of 4 dB is
  required. Assume threre is no dispersion
  equalization penalty
• Perform an optical power budget for the system
  operating under the above conditions and
  ascertain its viability.
• Estimate any possible increase in link length
  which may be achieved using an injection laser
  source which launches mean optical power of 0 dBm
  into the fiber cable. In this case the fafety
  margin must be increased to 7 dB.

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Exercise A single TV channel is transmitted
over an analog optical link using direct
intensity modulation. The video which has a
bandwidth of 5 MHz and a ratio of luminance to
composite video of 0.7 is transmitted with a
modulation index of 0.8. The receiver contains
p-i-n photodetector with a responsitivity of 0.5
A/W and a preamplifier with an effective input
impedance of 1 M ohms together with a noise
figure of 1.5dB. Assuming the receiver is
operating at a temperature of 20 C and
neglecting the dark current in the photodiode,
determine the average incident optical power
required at the receiver (i.e, receiver
sensitivity) in order to maintain a peak to peak
signal power to rms noise power ratio of 55 dB.
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