L5 Optical Fiber Link and LAN Design

Table of content Transmission Type Elements in Network Design Factors for Evaluating Fiber Optic System Design Link Budget Considerations Power Budget Power Budget Requirement Example : Long-haul Transmission System Example : LAN

Table of content (cont.) Bandwidth Budget System Rise Time Example on STM-4, STM-16 and STM-64 Budget Summary Sensitivity Analysis Eye Diagrams Signal to Noise Ratio (SNR) Cost/ Performance Considerations Summary

Transmission Types Two types of transmissions: 1. Link (point to point) 2. Network a. point to multipoint b. Mesh c. Ring

Elements of Link/ Network Design Transmitter : Operating wavelength (), Linewidth (), Rise time, Bit-rate, Line format, Power level Fiber : SMF/MMF, Fiber type – SMF28, DSF, etc, Cable loss, Spool length Rx : PSEN, PSAT, Rise time

Elements of Link/ Network Design (cont.) Connection: No. of splice, Splice loss No. of connectors, Connector Loss In Line Devices: Splitter, Filter, Attenuator, Amplifier Insertion loss, Gain

The Main Question In Digital System - Data Rate - Bit Error Rate In Analog System - Bandwidth - Signal to Noise Ratios The Main Problems Attenuation and Loss Dispersion

Factors for Evaluating Fiber Optic System Design

Factors for Evaluating Fiber Optic System Design (cont.)

Optical Transmitter/ Sources LEDs Output Power Modulation Bandwidth Center Wavelength Spectral Width Source Size Far-Field Pattern Laser Diodes Output Power Modulation Bandwidth Center Wavelength, Number of Modes Chirp, Linewidth Mode Field of the Gaussian beam

Optical Fiber Multimode Fiber Attenuation Multimode Dispersion Chromatic Dispertion Numerical Aperture Core Diameter Single-Mode Fiber Attenuation Chromatic Dispersion Cutoff Wavelength Spot Size

Optical Receiver/ Photodiode Risetime/Bandwidth Response Wavelength Range Saturation Level Minimum Detection Level

Simple Link TX RX Medium and Devices OA

Link Budget Considerations Three types of budgets: (1) Power Budget Bandwidth or Rise Time Budget ?

POWER BUDGET

Power Budget Requirements: PB : PRX > PMIN PRX = Received Power PMIN = Minimum Power at a certain BER PRX = PTX – Total Losses + Total Gain - PMARGIN PTX = Transmitted Power PMARGIN ≈ 6 dB

Requirements Cont’d: Loss,L = LIL + Lfiber + Lconn. + Lnon-linear LIL = Insertion Loss Lfiber = Fiber Loss Lconn.= Connector Loss Lnon-linear= Non-linear Loss Gain,G = Gainamp + Gnon-linear Gainamp = Amplifier Gain Gnon-linear = Non-linear Gain

dB, dBm, mW dB = 10 log (P1/P2)

Decibel to Power Conversion

Decibel to Power Conversion

Power Budget Measurement for Long Haul Transmission Example: Power Budget Measurement for Long Haul Transmission 185 km Connector Splice PSEN = -28 dBm PTx = 0 dBm IS THIS SYSTEM GOOD? Attenuation Coefficient,  = 0.25 dB/km Dispersion Coefficient, D = 18 ps/nm-km Number of Splice = 46 Splice Loss = 0.1 dB Connector Loss = 0.2 dB PMargin = 6 dB

CONCLUSION: BAD SYSTEM!! Simple Calculation…. Fiber Loss = 0.25 dB/km X 185 km = 46.3 dB Splice Loss = 0.1 dB X 46 = 4.6 dB CONCLUSION: BAD SYSTEM!! Connector Loss = 0.2 dB X 2 = 0.4 dB Total Losses = 46.3 + 4.6 + 0.4 = 51.3 dB PMargin = 6 dB PRX = PTX – Total Losses – PMargin = 0 – 51.3 – 6 PRX = -57.3 dB Power Budget, PRX < PSEN !!

How To Solve? Answer… But… And… Place an amplifier What is the gain value? ? And… Where is the location?

Now…Where to put the amplifier? First we calculate the amplifier’s gain.. Gain  PSEN - PRX Gain  -28 – (-57.3) Gain  29.3 dB Gain  30 dB To make it easy, Now…Where to put the amplifier?

Three choices available for the location Power Amplifier – At the transmitter Preamplifier – At the receiver In Line – Any point along fiber

Let us check one by one… Power Amplifier: PTX + Gain = POUT 0 + 30 = 30 dBm But is there any power amplifier with 30 dBm POUT? NO, THERE ISN’T Hence …

What about Preamplifier? Remember… POUT received = -57 dBm Preamplifier with 30 dB available? Yes But, can it take –57 dBm? Typically, NO Hence …

Let us check In Line Amplifiers 30 dB gain amplifier available here… But, What value can it take? Typically –30 dBm So… Now, we can find the location…

Where is the –30 dBm point? PTX – Loss At That Point = 0 dBm – 30 dB Assume Other Loss = 0, Loss At That Point = Fiber Loss, 30 =  x Length of That Point Remember  = 0.25, Point Length = 30/0.25 = 120 km But 120 km from Tx, No. of splice = 120/4 = 30 Splice Loss = 0.1 dB x 30 = 3 dB

Also remember connector loss at amplifier and Tx… + 1 connector at Tx 2 connectors Connector Loss = 0.2 dB x 3 = 0.6 dB Actually, at 120 km, Total Losses = Fiber Loss + Splice Loss + Connector Loss = 30 + 3 + 0.6 = 33.6 dB 33.6 dB > 30 dB!! NOT GOOD! Now, We have excess of 3.6 dB…Find the distance, Fiber Loss Length = 3.6/0.25 = 14.4 km Good Location = 120 km – 14.4 km = 105.6 km

CONFIRM…105.6 KM IS A GOOD LOCATION!! Let us confirm the answer… At 105.6 km from Tx, Fiber Loss = 0.25 x 105.6 = 26.4 dB No. of Splice at 105.6 km = 105.6/4 =26.4 = 27 Splice Loss = 0.1 x 27 = 2.7 dB Total Losses = 26.4 + 2.7 = 29.1 dB 29.1 dB < 30 dB !! CONFIRM…105.6 KM IS A GOOD LOCATION!! PTx = 0 dBm 185 km PSEN = -28 dBm Splice Connector 105.6 KM

Power Budget Measurement for LAN Example: Power Budget Measurement for LAN Server A Server B 500 m Using 850nm PSEN = -25 dBm PTx = -15 dBm IS THIS SYSTEM GOOD? Attenuation Coefficient,  = 4.5 dB/km Dispersion Coefficient, D = 18 ps/nm-km Number of Splice = 0 Splice Loss = 0 dB Connector Loss = 0.5 dB PMargin = 2 dB

BANDWIDTH BUDGET

System Rise Time Calculate the total rise times Tx, Fiber, Rx Calculate Fiber rise time, TFiber Tfiber = D x  x L D = Dispersion Coefficient  = Linewidth L = Fiber Length Tx Rise Time, TTX = normally given by manufacturer Rx Rise Time, TRX = normally given by manufacturer

Tsys=1.1(TTX2+TRX2+Tfiber2)1/2 Total Rise time, Tsys: Tsys=1.1(TTX2+TRX2+Tfiber2)1/2

Bandwidth Budget T T’ RX TX OA OA Δτ = T’ - T Medium and Devices

What is a good Rise time? PW = 1/BitRate for NRZ 1/2BitRate for RZ For a good reception of signal Tsys < 0.7 x Pulse Width (PW) PW = 1/BitRate for NRZ 1/2BitRate for RZ

Rise Time Budget Measurement for Long Haul Application Example: Rise Time Budget Measurement for Long Haul Application Tx rise time, TTX = 0.1 ns Rx rise time, TRX= 0.5 ns Linewidth() = 0.15 nm Dispersion Coefficient, D = 18 ps/nm-km Fiber length = 150km Bit Rate = 622Mbps Format = RZ

Simple Calculation…. TSYS = 0.77 ns Fiber rise time, TF =Length x D x Linewidth() = 150 km x 18 x 0.15 nm = 0.4 ns Total Rise time, TSYS = 1.1 TLS2 + TPD2 + TF2 = 1.1 0.01 + 0.25 + 0.16 TSYS = 0.77 ns

Good Rise Time Budget!! Let say, Bit Rate = STM 4 = 622 Mbps Format = RZ Tsys < 0.7 x Pulse Width (PW) Pulse Width (PW) = 1/(622x106) = 1.6 ns 0.77 ns < 0.7 x 1.6 ns 0.77 ns < 1.1 ns !! Good Rise Time Budget!!

Bad Rise Time Budget!! Let say, Bit Rate = STM 16 = 2.5 Gbps Format = RZ Tsys < 0.7 x Pulse Width (PW) Pulse Width (PW) = 1/(2.5x109) = 0.4 ns 0.77 ns < 0.7 x 0.4 ns 0.77 ns ≥ 0.28 ns !! Bad Rise Time Budget!!

Budget Summary Option Δλ Power Budget Financial A Source (LED vs. LD)   Option Power Budget Bandwidth Budget Financial A Source (LED vs. LD) Δλ 850nm Mediocre Bad Cheap 1310nm Good Less expensive 1550nm Very good Expensive Modulation Bandwidth LED NA LD Output Power Radiation pattern LED (far-field pattern) LD (Gaussian beam)

Budget Summary B Fiber Option Power Budget Attenuation MM Cheap SM Bandwidth Budget Financial   Attenuation MM Mediocre Cheap SM Good Expensive Dispersion Numerical Aperture (NA) Core Diameter

Budget Summary C Receiver (PIN vs. APD) Rise time/ Bandwidth PIN Cheap Option Power Budget Bandwidth Budget Financial   Rise time/ Bandwidth PIN Mediocre Cheap APD Good Expensive Response wavelength range Saturation Level Minimum detection level

Sensitivity Analysis Minimum optical power that must be present at the receiver in order to achieve the performance level required for a given system.

Factors will affect this analysis 1. Source Intensity Noise - Refers to noise generated by the LED or Laser Phase Noise - the difference in the phases of two optical wavetrains separated by time, cut out of the optical wave Amplitude Noise - caused by the laser emission process. 2. Fiber Noise Relates to modal partition noise 3. Receiver Noise Photodiode, conversion resistor

4. Time Jitter and Intersymbol Interference Time Jitter - short term variation or instability in the duration of a specified interval Intersymbol Interference result of other bits interfering with the bit of interest inversely proportional to the bandwidth Eye diagrams - to see the effects of time jitter and intersymbol interference

5. Bit error rate - main quality criterion for a digital transmission system BER = Q [IMIN2/ (4 . N0 . B) ] where : N0 = Noise power spectral density (A2/Hz) IMIN = Minimum effective signal amplitude (Amps) B = Bandwidth Q(x) = Cumulative distribution function (Gaussian distribution)

Eye Diagrams

Signal to Noise Ratio SNR = S/N SNR (dB) = 10 log10 (S/N) S - represents the information to be transmitted N - integration of all noise factors over the full system bandwidth SNR (dB) = 10 log10 (S/N)

Cost/Performance Considerations Components considerations such as : Light Emitter Type Emitter Wavelength Connector Type Fiber Type Detector Type

Summary The key factors that determine how far one can transmit over fiber are transmitter optical output power, operating wavelength, fiber attenuation, fiber bandwidth and receiver optical sensitivity. The decibel (dB) is a convenient means of comparing two power levels. The optical link loss budget analyzes a link to ensure that sufficient power is available to meet the demands of a given application.

Summary Rise and fall times determine the overall response time and the resulting bandwidth. A sensitivity analysis determines the amount of optical power that must be received for a system to perform properly. Bit errors may be caused by source intensity noise, fiber noise, receiver noise, time jitter and intersymbol interference. The five characteristics of a pulse are rise time, period, fall time, width and amplitude.

TUTORIAL

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