Optical Fibre Communication Systems Lecture 8 - Systems
Professor Z Ghassemlooy Electronics & It Division School of Engineering Sheffield Hallam University U.K. www.shu.ac.uk/ocr Prof. Z Ghassemlooy
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Contents System Design Digital Systems
Link Power Budget Link Rise Time (Bandwidth) Budget Transmission Distance
Analogue Systems
Prof. Z Ghassemlooy
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Fiber Optic System Design i There are many factors that must be considered to ensure that enough light reaches the receiver. Without the right amount of light, the entire system will not operate properly.
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Fiber Optic System Design- Step-byStep i Select the most appropriate optical transmitter and receiver combination based upon the signal to be transmitted (Analog, Digital, Audio, Video, RS-232, RS-422, RS-485, etc.). i Determine the operating power available (AC, DC, etc.). i Determine the special modifications (if any) necessary (Impedances, bandwidths, connectors, fiber size, etc.). i Carry out system link power budget. i Carry out system rise time budget (I.e. bandwidth budget). i If it is discovered that the fiber bandwidth is inadequate for transmitting the required signal over the necessary distance, then either select a different transmitter/receiver (wavelength) combination, or consider the use of a lower loss premium fiber Prof. Z Ghassemlooy
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Digital Systems iCompared with analogue systems: – It Gives superior performance – It reduces problems associated with the optical source nonlinearities and temperature dependency (in baseband transmission)
iProvide ideal channel for data transmission iInformation is carried in the baseband using Intensity Modulation (IM).
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Link Power Budget
Pt Po
Total loss LT = αf L + lc + lsp
Pt − Po = LT + SM Po = Receiver sensitivity (i.e. minimum power requirement) SM = System margin (to ensure that small variation the system operating parameters do not result in an unacceptable decrease in system performance) Prof. Z Ghassemlooy
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Link Power Budget - Example 1 Parameters
Value
dB
3 mW
4.8 dBm -3.7 dB
Transmitter Average transmitted power Fibre coupling losses
Channel
-15.7 dB -10 dB -0.79 dB 0 dB
Fibre loss Splitting losses Splice & Connector losses Fibre dispersion & nonlinearity
Receiver Signal power at the receiver Receiver sensitivity
All lossess
System Margin (-20 dBm -(-30 dBm))
Prof. Z Ghassemlooy
-26.79 dBm -31 dBm +4.1 dB
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Link Power Budget - Example 2 i Transmitter – Date rate = 500 Mb/s – Source Laser @ 1300 nm – Coupling power = 2 mW (3 dBm) into a 10 um fibre.
i Channel – Mono mode fibre of length 60 km and a loss of 0.3 dB/km – Connector loss = 1 dB/connector – Splicing every 5 km with a loss = 0.5 dB /splice
i Receiver: – PIN @ 1300 nm – BER = 10-9
i System margin = ? Prof. Z Ghassemlooy
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Link Power Budget - Example 2 contd. Receiver sensitivity -29 dBm
Pt − Po = LT + SM LT = 2(1 dB) + 0.3(60) + 0.5 (11) = 25.5 dB thus 3 +29 = 25.5 dB+SM therefore SM = 5.5 dB G Keiser Prof. Z Ghassemlooy
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Link-Power Budget - Example 3 Launch power into fibre
1 dB
Link power budget can be shown graphically in terms of receiver sensitivity Vs. the data rate L Launch power into fibre LED/PIN, @ 20 Mbps G Keiser Prof. Z Ghassemlooy
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Link-Power Budget - contd. iDispersion -equalisation penalty is given as:
(
DL = 2 2σBT 2
)
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(dB)
Where BT is the bit rate, σ is the rms pulse width. Therefore, the total channel loss is given as:
Total loss LT = αf L + lc + lsp + DL
(dB)
DL is only significant in wideband multi-mode fibre systems Prof. Z Ghassemlooy
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Rise Time Budget iThe system design must also take into account the temporal response of the system components. iThe total loss LT (given in the power budget section) is determined in the absence of the any pulse broadening due to dispersion. iFinite bandwidth of the system (transmitter, channel, receiver) may results in pulse spreading (i.e. intersymbol interference), giving a reduction in the receiver sencitivity. I.e. worsening of BER or SNR iThe additional loss penalty is known as dispersionequalisation or ISI penalty. Prof. Z Ghassemlooy
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Rise Time Budget - contd. ⎛ 2⎞ = ⎜⎜ ∑ ti ⎟⎟ ⎝ i −1 ⎠ N
t sys
The total system rise time
(
t sys = t
2
s
+t
2
inter
Fibre Source intermodal
+t
2
intra
+t
2
0 .5
)
0. 5
d
Fibre intramodal Detector
Note - 3 dB bandwidth of a simple low pass RC filter is given as: B=
1 2πRC
With a step input voltage into the RC filter, the rise time of the output voltage is: 0.35 t r = 2.2 B =
B
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Rise Time Budget - contd. t sys = t r =
For a fibre optic link: 1
0
0.35 B
1
Bit rate R = B = 1 / τ
For RZ data format τ
BRZ =
0.35 t sys
Bit rate R = B = 1/ 2τ
For NRZ data format 2τ
BNRZ Prof. Z Ghassemlooy
0.75 = t sys 14
Transmission Distance -1st window Multi-mode, Input power Pt = -13 dB LED (0 dBm laser), fibre loss = 3.5 dB/km, SM = 6 dB, BER = 10-9
(0.07ns/(nm-km) @ λ=800 nm)
G Keiser
Po: -51dBm Si PIN -64 dBm Si APD
for fibre with bandwidth of 800 MHz/km
Po: -38dBm -57dBm
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Transmission Distance -3rd window D = 2.5 ps/(nm.km), fibre loss = 0.3 dB/km@ 1550nm, Pt = 0 dBm laser, Po = 11.5 log B -71dBm forAPD, and = 11.5 log B- 60.5 dBm for pin
G Keiser
B
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Analogue System iThe system must have sufficient bandwidth to pass the HIGEST FREQUENCIES. iLink Power budget is the same as in digital systems iRise Time budget is also the same, except for the system bandwidth which is defined as:
Bsys
0.35 = t sys
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