1. Chapter 8
Basic System Design

2. System factors for designing from scratch: Design Verification
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

3. System factors (continued)
Available choices
Detector type
PIN diode, APD, IDP
Modulation scheme
OOK, multilevel, coherent
End-end bit error rate
<10-9 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

4. 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

5. 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)
SNR requirements
Minimum power at detector
BER
Safety margin (Ma)

6. 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.

7. Risetime budget includes the following:
Risetime of the source, TS
Risetime of the fiber (dispersion), TF
Risetime of the amplifier, TA
Risetime of the detector, TD

8. 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

9. 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 =
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; dB/km loss; dispersion 1ns/km; 4 connectors with dB 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 5.3kΩ

10. 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

11. 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

12. 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

13. 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.

14. 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.

15. 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

16. 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

17. 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.

18. Solution
a)35Mbps
Pi – Po = [(Fiber cable loss + Splice losses ) x L + Connector loss + Ma ]dB
[-3dBm – (-55 dBm)] = ( )L
0.5L = 52 –2-7
L = 86km
b) 400 Mbps
[-3dBm – (-44 dBm)] = ( )L
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 + Ma]dB
[-3dBm – (-44 dBm)] = ( )L
0.5L = 41 – – 7
L = 61km
Note: a reduction of 3 km in the maximum length without repeaters when DL is taken to account.

19. 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?.

20. 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 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.

21. Solution The power launched into the fiber 100µW = -10 dBm
Minimum power required at the receiver dBm
Total system margin dB
Fiber loss 7 x dB
Splice loss 6 x dB
Connector loss dB
Safety margin dB
Excess power margin = 61 dB 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..

22. 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 (M+Mg) = 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

23. 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)
= = 1.73ns

24. 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 detector’s 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,

25. 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.

26. 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.