EE 230: Optical Fiber Communication Lecture 17

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EE 230 Optical Fiber Communication Lecture 17
System Considerations
From the movie Warriors of the Net
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Basic Network Topologies
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Bitrate Distance Graph for various point to point
link technologies
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System Design

• Determine wavelength, link distance, and
  bit-error rate
• Work out power budget
• Work out risetime budget
• Work out cost budget

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Power Budget Steps

• Start with BER and bit rate, determine bandwidth
  B based on coding method
• Given BER, determine Q factor and S/N ratio
• B 1/2?RLC gives the maximum load resistance RL
  based on B and C
• Based on RL and M, determine detector sensitivity
  (minimum power for S/N)
• Add system margin, typically 6 dB, to determine
  necessary power at receiver

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Power budget steps, continued

• Add power penalties, if necessary, for extinction
  ratio, intensity noise (includes S/N degradation
  by amplifiers), timing jitter
• Add loss of fiber based on link distance
• Include loss contributions from connections and
  splices
• End up with required power of transmitter, or
  maximum length of fiber for a given transmitter
  power

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Power budget example

• Imagine we want to set up a link operating at
  1550 nm with a bit rate of 1 Gb/s using the RZ
  format and a BER of 10-9. We want to use a PIN
  photodiode, which at this wavelength should be
  InGaAs. The R0 for the diode is 0.9 A/W and its
  dark current is 4 nA.

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Bandwidth required for bit rate

• For NRZ format, B0.5 times bit rate
• For RZ format, Bbit rate
• For this example, the bandwidth B is equal to the
  bit rate, 109 /s.

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Bandwidth limit

• C2 pF for this photodiode.
• B 1/2?RLC, so the load resistance RL must be
  (2?BC)-1 79.6 ?

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Q Factor, S/N Ratio, and BER

• For our BER of 10-9, Q6 and S/N144

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Minimum signal power required

• Method 1
• Signal to noise ratio is equal to
• where R0 is responsivity, M is multiplier, id
• is dark current, and PS is signal power. If we
  set this expression equal to the SNR of 144 we
  calculated, the required signal power PS turns
  out to be 6.09x10-6 W -22.2 dBm.

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Noise Equivalent Power (NEP)

• Method 2
• NEP is the signal power at which S/N1
• Units are W/Hz1/2
• In this case, M1 and the dark current 4 nA.
• The factor outside the radical is 1/R0. We can
  thus determine the NEP by multiplying the above
  result by the square root of the bandwidth. The
  result is 5.1x10-7 W, which equals -33.0 dBm.

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NEP Method, continued

• Since Q is proportional to the square root of
  SNR, multiplying the NEP by that square root will
  give us the minimum acceptable power.
• In this case, SNR144, so the minimum power is 12
  times NEP 6.12x10-6 W -22.1 dB, the same
  result as the other method.

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Extinction ratio penalty

• Corrects for emission from transmitter during 0
  bits
• Extinction ratio rexP0/P1
• If our extinction ratio is 0.1, the penalty is
  0.87 dB.

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Intensity noise penalty

• Corrects for power fluctuations during 1 bits
• rI inverse of SNR of transmitter and amplifier
  output
• If the SNR of the amplified source is 13 dB 20,
  then rI 0.05 and we will have to raise the
  power at the receiver by dI to keep the desired
  SNR.
• Since Q 6, dI 0.41 dB.

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Timing jitter penalty

• Parameter B?fraction of bit period over which
  apparent clock time varies
• If our jitter represents 10 of the bit period,
  the power penalty is 0.34 dB

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Fiber attenuation

• If the attenuation in the fiber is 0.2 dB/km and
  the link is 80 km long, the total loss in the
  fiber will be 16.0 dB

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Example results

• Minimum power required for receiver
• -22.1 dBm
• Safety margin 6.0 dB
• Extinction ratio power penalty 0.87 dB
• S/N power penalty 0.41 dB
• Timing jitter power penalty 0.34 dB
• Fiber loss over 80 km 16.0 dB
• Total minimum transmitter power
• 1.52 dBm 1.42 mW

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Further steps

• Alternatively, previous data could be used with a
  fixed transmitter power to determine maximum
  length of a fiber link
• If power budget does not add up, one can
• replace PIN photodiode with APD
• add an EDFA to the link

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Power Budget Example
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Risetime Budget
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Rise time budget components

• bit rate and coding format determine upper limit
  of rise time
• rise time of transmitter (from manufacturer
  laser faster than LED)
• pulse spread due to dispersion
• rise time of receiver (from manufacturer PIN
  faster than APD)
• Rise time components are combined by taking the
  square root of sums of squares

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Upper limit for rise time

• For NRZ format, Tr0.70/B
• For RZ format, Tr0.35/B
• In this case, choose RZ format. Tr must thus be
  less than or equal to 0.35/109 350 ps
• To add a safety margin of x, multiply B by
  (1x/100) before calculating rise time budget
  limit

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Group Velocity Dispersion-based rise time

• Calculate from laser optical bandwidth if known,
  or from modulation rate
• In this case, D17 ps/nm-km, L80 km, and
  ??0.016 nm, so tf21.8 ps.

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Modal dispersion rise time

• For multimode fiber, time spread due to modal
  dispersion is based on core index and fiber
  length L.
• For step-index fiber
• For graded-index fiber

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Total rise time

• For this example, tMD0, tTR100 ps, tRC0.5 ns,
  and tGVD 21.8 ps as before. tr is therefore 510
  ps, and the rise time budget does not meet the
  limit.
• Can use NRZ format
• Use faster detector or transmitter
• Use graded-index fiber for less dispersion

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Computer Based Link Simulation
Computer Simulation is often used to model
opticla links to account for the complex
interaction between components and nonlinear
effects Commercial simulation tools are now
available such as Linksim from RSoft and the
tools from VPI Systems
Fiber-Optic Communication Systems-G. Agrawal