EE 230: Optical Fiber Communication Lecture 17

From the movieWarriors of the Net

System Considerations

Basic Network Topologies

Bitrate Distance Graph for various point to point link technologies

System Design

• Determine wavelength, link distance, and bit-error rate

• Work out power budget

• Work out risetime budget

• Work out cost budget

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 = 1/2RLC 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

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

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.

Bandwidth required for bit rate

• For NRZ format, B=0.5 times bit rate

• For RZ format, B=bit rate

For this example, the bandwidth B is equal to the bit rate, 109 /s.

Bandwidth limit

C=2 pF for this photodiode.

B = 1/2RLC, so the load resistance RL must be (2BC)-1 = 79.6

Q Factor, S/N Ratio, and BER

For our BER of 10-9, Q=6 and S/N=144

Noise

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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 2NEP is the signal power at which S/N=1Units are W/Hz1/2

In this case, M=1 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, SNR=144, so the minimum power is 12 times NEP = 6.12x10-6 W = -22.1 dB, the same result as the other method.

Extinction ratio penalty

Corrects for emission from transmitter during “0” bits

Extinction ratio rex=P0/P1

If our extinction ratio is 0.1, the penalty is 0.87 dB.

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Intensity noise penaltyCorrects 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 δI to keep the desired SNR.

Since Q = 6, δI = 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

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

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

Power Budget Example

Risetime Budget

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

Upper limit for rise time

• For NRZ format, Tr=0.70/B

• For RZ format, Tr=0.35/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 (1+x/100) before calculating rise time budget limit

Group Velocity Dispersion-based rise time

Calculate from laser optical bandwidth if known, or from modulation rate:

In this case, D=17 ps/nm-km, L=80 km, and =0.016 nm, so tf=21.8 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, tMD=0, tTR=100 ps, tRC=0.5 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