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Link power and rise time budget analysis
- 1. 1 Link Power andRise Time Budget Analysis MEC
- 2. 2 Contents • Point toPoint Links. • Fiber Losses. • Allowable Loss. • Link Power Budget Analysis. • Rise Time Budget Analysis. • 3 - dB Bandwidth. • Transmission Distance Limits.
- 3. 3 Point to PointLink • Simplest transmission link has transmitter and receiver – places least demand on technology. • Link design involves several variables, source, fiber & detector characteristics & several iterations. User
- 4. 4 Point to PointLink • Performance and Cost Constraints. • Careful choice of components to ensure desired performance over expected life time. • Key System Requirements: - Desired/Possible Transmission Distance. - Data Rate/Channel Bandwidth. - Bit Error Rate.
- 5. 5 Point to PointLink – Designer Choice • Multimode or Single Mode Fiber. - Core Size. - Core Refractive Index Profile. - Bandwidth or Dispersion. - Attenuation. - Numerical Aperture / Mode Field Diameter.
- 6. 6 Point to PointLink – Designer Choice • LED or Laser Diode Source - Emission Wavelength. - Spectral Line Width. - Output Power. - Effective Radiating Area. - Emission Pattern. - Number of Emitting Modes.
- 7. 7 Point to PointLink – Designer Choice • pin or Avalanche Photodiode. - Responsivity - Operating Wavelength. - Speed. - Sensitivity. • Analysis for desired system performance: - Link Power Budget Analysis. - Rise Time Budget Analysis.
- 8. 8 Why Link PowerBudget Analysis? • To determine: - Power margin between optical transmitter output and minimum receiver sensitivity to establish specified BER. - Margin allotted to connector, splice, fiber loss, additional margins due to component degradation, temperature effects. - Component change/repeater insertion requirements for desired transmission distance.
- 9. 9 Link Power BudgetAnalysis • Received optical power depends on the amount of light coupled into the fiber, fiber losses, losses at connectors and splice. • Link loss budget derived from loss contributions of each element (dB). • If Pin and Pout are optical powers into and out of the loss element, loss (dB) = 10log • Link power margin for component ageing, temperature fluctuations, components added in future, 6 - 8dB if no future additions. out in P P
- 10. 10 Link Power BudgetAnalysis Two connectors, Loss = 2Ic Sensitivity = PR PS PT = PS - PR
- 11. 11 Link Loss Budget •Considers total optical power loss PT allowed between source and detector, loss due to cable attenuation, connector and splice losses and system margin. • PT = PS – PR = 2Ic + αfL + System Margin, PS - optical power emerging from the end of the fiber flylead attached to light source, PR – receiver sensitivity, Ic – connector loss, αf – fiber attenuation (dB/km), L- fiber length, system margin taken as 6 dB.
- 12. 12 Receiver Sensitivity BER =10-11 for InGaAs APD BER = 10-9 for pin and Si APD
- 13. 13 Link Loss Budget 800nm LED/pin @ 20 Mb/s
- 14. 14 Why Rise TimeAnalysis? • Ensure overall desired system performance. • Determine distance/dispersion limitations. • Considers transmitter rise time, material dispersion rise time, modal dispersion rise time, receiver rise time. • Total transition time degradation to be within limits.
- 15. 15 Rise Time Budget •Total transition time degradation not to exceed 70% of an NRZ bit period, 35% bit period of an RZ data (Bit Period = 1/Data Rate). • Transmitter and Receiver rise times known to the designer. • Transmitter rise time attributed to source and the driving circuitry.
- 16. 16 Rise Time Budget •Receiver rise time (10% – 90%) attributed to photodetector response, 3 dB electrical bandwidth of receiver front end (Brx). • Receiver front end response modelled as a first order low pass filter u(t) – unit step function. • Receiver front end rise time
- 17. 17 Rise Time Budget •Receiver rise time defined between g(t) = 0.1 to g(t) = 0.9. • For multimode fibers, rise time depends on modal and material dispersions. • In 800 – 900 nm range, material dispersion adds about 0.07 ns/nm.km to rise time. • Material dispersion effects to be neglected for lasers & for LEDs at longer wavelengths.
- 18. 18 Rise Time Budget •Total rise time of the link is the root sum square of the rise times of each contributor tj to the pulse rise time degradation. • ttx – transmitter rise time, tmat – material dispersion rise time, tmod – fiber modal dispersion rise time, trx – receiver rise time.
- 19. 19 Fiber Bandwidth • FiberBandwidth resulting from modal dispersion inversely proportional to cable length. • For long continuous fiber, no joints, fiber bandwidth decreases linearly with increasing distance for lengths L < modal equilibrium length Lc. • For L > Lc, steady state equilibrium established, bandwidth decreases as L0.5.
- 20. 20 Fiber Bandwidth • PracticalCase : several fibers joined to form link. • Modal redistribution occurs at fiber to fiber joints – misaligned joints, different core indices & different degrees of mode mixing in individual fibers. • Value of index grading parameter α that minimizes pulse dispersion depends on wavelength, fibers optimized for different wavelengths have different indices.
- 21. 21 Fiber Bandwidth • Variationsin α at same wavelength leads to overcompensated & undercompensated refractive index profiles. • Total Route Bandwidth a function of order in which fibers are joined. • Alternate over & undercompensated profiles to attain a more modal delay equalization – time consuming & unwieldy. • Initial fiber control final link characteristics.
- 22. 22 Fiber Bandwidth • BandwidthBM in a fiber of length L, (0.5<q<1, B0 – bandwidth of 1 km length) q = 0.5 if steady state equilibrium, q = 1 if little mode mixing, typically q = 0.7. Also, Bn – Bandwidth of the nth fiber section
- 23. 23 Pulse Broadening • Iftn – pulse broadening of the nth section, pulse broadening occuring over N cable sections: • Empirical expression for pulse broadening (0<rpk <1 – correlation coefficient between pth and kth fiber):
- 24. 24 Fiber Rise Timeand 3 dB Bandwidth • Optical power emerging from a fiber has a gaussian temporal response (σ – rms pulse width) • Time t½ required for the pulse to reach its maximum value ie;
- 25. 25 Fiber Rise Timeand 3 dB Bandwidth • Full width of the pulse at its half maximum value: • 3-dB optical bandwidth – modulation frequency f3 dB at which received optical power has fallen to 0.5 of zero frequency value.
- 26. 26 Fiber Rise Timeand 3 dB Bandwidth • Letting tFWHM be the rise time resulting from modal dispersion, • tmod is given by: tmod in ns and Bm in MHz.
- 27. 27 Fiber Rise Timeand 3 dB Bandwidth all times in ns, σλ – source spectral width, Dmat – fiber material dispersion factor ( = 0.07 ns/nm.km @ 800 – 900 nm).
- 28. 28 Transmission Distance Limits 800MHz-km fiber 800 nm source BER = 10-9 3.5dB/km
- 29. 29 Transmission Distance Limits SingleMode Links 1550 nm DFB LD, D - 2.5 ps/nm.km Attenuation - 0.3 dB/km
- 30. 30 References • https://www.youtube.com/watch?v=NctDb6p WpoA • https://www.youtube.com/watch?v=mqKaVbo _dsE •https://www.youtube.com/watch?v=mmvyDS 7jsN0 • https://www.youtube.com/watch?v=SWvWpl8 Txx0 • https://www.youtube.com/watch?v=QRABE0 qMBJE
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