Optical Networks

Poompat Saengudomlert

Session 6

Transmission Link Budgets

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3.3 Transmission Power Budget

A power budget accounts for various losses.

link lossconnector losssplice lossdevice insertion loss, e.g., filter and switch

Main constraint: minimum receive optical power called the receiversensitivity

In practice, 6-8 dB of margin is common.

Typical condition:

Ptx − total loss−margin ≥ Prx

Ptx: transmit powerPrx: receiver sensitivity

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Example:

Maximum transmission distance due to loss.

Parameter Value

Wavelength 850 nmBit rate 20 MbpsBER requirement ≤ 10−9

GaAlAs LED transmit power (Ptx) −13 dBmSi pin photodiode receiver sensitivity (Prx) −42 dBmConnector loss (lcon) 1 dBNumber of connectors (Ncon) 2 (1 at transmitter/receiver)System margin (lmargin) 6 dBFiber loss (αatt) 3.5 dB/km

Maximum distance calculation

L =Ptx − Nconlcon − lmargin − Prx

αatt

=−13− 2× 1− 6− (−42)

3.5= 6 km

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Example (continued):

Graphical illustration of the power budget

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Example:

Power budget verification for SONET/SDH OC-48

Parameter Value

Wavelength 1550 nmBit rate 2.5 GbpsLaser diode transmit power (Ptx) 3 dBmInGaAs APD receiver sensitivity (Prx) −32 dBmJumper cable loss (for connecting fibers 3 dB

to equipment rack)Number of jumper cables 2 (1 at transmitter/receiver)Connector loss (lcon) 1 dBNumber of connectors (Ncon) 4 (2 at transmitter/receiver)System margin (lmargin) 6 dBFiber loss (αatt) 0.3 dB/kmFiber length (L) 60 km

Receiver power computation

3− 4× 1− 60× 0.3− 3× 2− 6 = −31 ≥ −32 dBm

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Power Budget with EDFA

Challenge: EDFA gain depending on total input power⇒ Find the total input power before identify the gain.

Example:

WDM transmission link without/with EDFA

Parameter Value

Transmit power (Ptx) 0 dBmReceiver sensitivity (Prx) −30 dBmConnector loss (lcon) 1 dBMUX/DMUX loss 2 dBFiber loss (αatt) 0.1 dB/kmLoss margin (lmargin) 6 dB

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Example (continued):

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Example (continued):

Receive power: not enough without EDFA

0����Ptx

− 5× 1� �� �connector

− 2����MUX

− 0.1× 200� �� �fiber

− 2����DMUX

− 6����margin

= −35 dBm

Input power to EDFA on each WDM channel

0����Ptx

− 3× 1� �� �connector

− 2����MUX

− 0.1× 100� �� �fiber

= −15 dBm

⇒ total input power −15 + 6 = −9 dBm ⇒ gain = 9 dB

Receive power: enough with EDFA

−6����EDFA output

− 3× 1� �� �connector

− 2����DMUX

− 0.1× 100� �� �fiber

− 6����margin

= −27 dBm

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3.4 Rise-Time Budget

A rise-time budget accounts for various types of dispersion.

ttx: transmitter rise timetchro: chromatic dispersiontmod: intermodal dispersion (for multimode fibers only)trx: receiver rise time

Total dispersion

ttotal =√

t2tx + t2chro + t2mod + t2rx

Practical rules for transmission with bit period Tbit

Non-return-to-zero (NRZ) modulation: ttotal ≤ 0.70× Tbit

Return-to-zero (RZ) modulation: ttotal ≤ 0.35× Tbit

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Parameter Values

ttx: typically given as a transmitter specification

trx: detector response modeled as lowpass filter(1− e−2πBrxt

)u(t)

Brx is the 3-dB receiver bandwidth.The 10-to-90-percent rise time is

trx = 0.35/Brx

tchro: In terms of dispersion parameter D, fiber length L, and 3-dBtransmitter bandwidth σλ

tchro = |D|Lσλtmod: In terms of fiber length L, bandwidth-distance product of fiberB0, and empirical parameter q

tmod = 0.44Lq/B0

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Example:

Rise-time budget verification

Parameter Value

Bit rate 20 MbpsLED transmitter rise-time (ttx) 15 nsLED transmitter spectral width (σλ) 40 nmTotal chromatic dispersion (|D|Lσλ) 21 nsReceiver bandwidth (Brx) 25 MHzFiber bandwidth-distance product (B0) 400 MHz·kmFiber length (L) 6 kmThe q parameter (q) 0.7Fiber type multimodeModulation format NRZ

Total rise-time√

(15× 10−9)2 + (21× 10−9)2 +

(0.44× 60.7

400× 106

)2

+

(0.35

25× 106

)2

≈ 29.6 ns ⇒ OK since 0.7× Tb = 0.720 Mbps = 35 ns

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Example:

Rise-time budget verification

Parameter Value

Bit rate 2.5 Gbps1550-nm laser transmitter rise-time (ttx) 25 ps1550-nm laser spectral width (σλ) 0.1 nmChromatic dispersion (|D|) 2 ps/nm/kmInGaAs APD receiver bandwidth (Brx) 2.5 GHzFiber bandwidth-distance product (B0) 400 MHz·kmFiber length (L) 60 kmThe q parameter (q) 0.7Fiber type single-modeModulation format NRZ

Total rise-time√

(25× 10−12)2 + (2× 60× 0.1× 10−12)2 + 0 +

(0.35

2.5× 109

)2

≈ 0.143 ns ⇒ OK since 0.7× Tb = 0.725 Gbps = 0.28 ns

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Loss vs. Dispersion Limited Systems

The optical reach is the minimum between two limits.

Typically, loss limited at low bit rates, dispersion limited at high bit ratesP. Saengudomlert (2018) Optical Networks Session 6 13 / 15

3.5 Line Coding

Line coding is performed on transmitted data bits so that timingrecovery can be performed at the receiver.

Each codeword has at least one level transition, which is useful fortiming recovery.

Example (4B5B line code):

symbol 4B 5B symbol 4B 5B0 0000 11110 8 1000 100101 0001 01001 9 1001 100112 0010 10100 A 1010 101103 0011 10101 B 1011 101114 0100 01010 C 1100 110105 0101 01011 D 1101 110116 0110 01110 E 1110 111007 0111 01111 F 1111 11101

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Known mBnB Line Codes

Overhead in transmitted bits for line coding: n−mm × 100%

Key parameters of known mBnB line codes

line code n/m Nmax Ndisp

3B4B 1.33 4 ±36B8B 1.33 6 ±35B6B 1.20 6 ±47B8B 1.14 9 ±79B10B 1.11 11 ±8

Nmax: longest number of consecutive identical bits

Ndisp: upper bound on accumulated disparity between bits 0 and 1

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