3 Bitrate Distance Graph for various point to point link technologies
4 System Design Determine wavelength, link distance, and bit-error rate Work out power budgetWork out risetime budgetWork out cost budget
5 Power Budget StepsStart with BER and bit rate, determine B based on coding methodB = 1/2RC gives the maximum load resistance R based on B and CBased on R and M, determine detector sensitivity (NEP), multiply by B1/2Add system margin, typically 6 dB, to determine necessary power at receiver
6 Power budget steps, continued Add power penalties, if necessary, for extinction ratio, intensity noise (includes S/N degradation by amplifiers), timing jitterAdd loss of fiber based on link distanceInclude loss contributions from connections and splicesEnd up with required power of transmitter, or maximum length of fiber for a given transmitter power
7 Power budget exampleImagine 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 We want to use a PIN photodiode, which at this wavelength should be InGaAs. The R0 for the diode is 0.9 A/W.
8 Bandwidth required for bit rate For NRZ format, B=0.5 times bit rateFor RZ format, B=bit rateFor this example, the bandwidth B is equal to the bit rate, 109 /s.
9 Bandwidth limit C=2 pF for this photodiode. B = 1/2RC, so the load resistance R must be (2BC)-1 = 79.6
10 Noise Equivalent Power (NEP) Signal power where S/N=1Units are W/Hz1/2In this case, M=1 and the dark current = 4 nA.The factor outside the radical is 1/R0. We can thus determine the NEP, which is 5.1x10-7 W, which equals dBm.
11 Q Factor and BERFor our BER of 10-9, Q=6 and S/N=12
12 Extinction ratio penalty Extinction ratio rex=P0/P1If our extinction ratio is 0.1, the penalty is 0.87 dB.
13 Intensity noise penalty rI=inverse of SNR of transmitted lightSince our S/N is 12, rI=0.83, which leads to a power penalty of 1.25 dB
14 Timing jitter penaltyParameter B=fraction of bit period over which apparent clock time variesIf our jitter represents 10% of the bit period, the power penalty is 0.34 dB
15 Fiber attenuationIf 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
16 Example results Minimum power required for receiver: -33.0 dBm Safety margin: 6.0 dBExtinction ratio power penalty: dBS/N power penalty: dBTiming jitter power penalty: dBFiber loss over 80 km: dBTotal= minimum transmitter power=-8.54 dBm=0.14 mW=140 W
17 Further stepsAlternatively, previous data could be used with a fixed transmitter power to determine maximum length of a fiber linkIf power budget does not add up, one canreplace PIN photodiode with APDadd an EDFA to the link
20 Rise time budget components bit rate and coding format determine upper limit of rise timerise time of transmitter (from manufacturer; laser faster than LED)pulse spread due to dispersionrise time of receiver (from manufacturer; PIN faster than APD)Rise time components are combined by taking the square root of sums of squares
21 Upper limit for rise time For NRZ format, Tr=0.70/BFor RZ format, Tr=0.35/BIn this case, choose RZ format. Tr must thus be less than or equal to 0.35/109 = 350 ps
22 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.
23 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:
24 Total rise timeFor 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 formatUse faster detector or transmitterUse graded-index fiber for less dispersion
25 Computer Based Link Simulation Computer Simulation is often used to model opticla links to account for the complex interaction between components and nonlinear effectsCommercial simulation tools are now available such as:Linksim from RSoftand the tools from VPI SystemsFiber-Optic Communication Systems-G. Agrawal
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