Optical Transmission Basics

Optical Transmission Basics

Sang-Kook Han [email protected]

YONSEI UNIVERSITY BTN LAB

Optical fiber transmission link

Drive circuit

Light source

O/E/O regenerator

Optical Amp. (EDFA)

Photo- detector

Signal restore

Electrical Input signal

Electrical Signal out

Amplifier

Receiver

Transmitter Fiber link

Fiber link

Optical coupler or Star coupler

To other node


Optical Source Light Sources : LED, LASER (Laser Diode), etc. General Requirements

Pure Spectrum (narrow ∆λ) High freq. modulation response High output power & stability Low power consumption

Light Emitting Diodes (LED)

Forward biased PN junction semiconductor Spontaneous emission process ⇒ Broad spectrum & incoherent Output wavelength depends on Eg of semiconductor Surface emitting & edge emitting Usually, for short distance communication


Laser Diodes (LD) Differences from LED

Two cleaved mirrors for optical feedback Need higher current injection (threshold current) Narrower linewidth (∆λ) Waveguide structure to confine photons in active region Optical gain by stimulated emission ⇒ Spontaneous Emission vs Stimulated Emission

LD Structure Double Hetero(DH) Junction (p-i-n) Broad strip LD, Ridge LD, Buried LD

Lasing Condition Total Gain ≥ Total Loss (mirror, absorption, scattering etc) In phase condition (round trip) ⇒ need Ith to start lasing


LD P-I curve

Temperature Sensitivity of LD output

Digital modulation : high SE Analog modulation : high SE, linear P-I curve

Ith(T) = I0 exp(T/T0) T0 : characteristic temperature typical 50~70K for 1.55µm LD

CAdTd oo /2~1≅

λ

LD characteristics (1)

dIdP = Slope Efficiency (W/A)

P

I

High Tap

P

I Ith

LD


LD characteristics (2) Spectral Characteristics (longitudinal mode)

Many longitudinal modes in FP-LD Single longitudinal mode LD needed = DFB-LD Linewidth (∆λ3dB)

FP-LD = 1~2Å (8Å ↔ 100GHz at 1.5µm) DFB = less than 1GHz

Far Field Pattern

Spot size converter to make θ⊥≈ θ||

λ

vertical

horizontal θ⊥ θll

Ridge LD 40~50˚ 10~20 ˚

Buriel LD 40~50˚ 30~40 ˚

Multi-mode spectrum of LD

p

n


Distributed-Feedback LD (DFB-LD) Structure

Grating period

m=1 , neff≈3.4 ⇒ Λ ≅ 0.2µm for 1.55µm

Characteristics Narrow linewidth Single longitudinal mode Less temperature sensitivity (≤ 0.1nm/˚C) Linear P-I curve Fabrication difficulty Low yield

oλ

eff

o

nm2

λ=Λ

Λ

Loss profile


p

i

n

cavity

n-doped Bragg

Reflector Substrate

Light

Vertical Cavity Surface Emitting Laser (VCSEL) Characteristics

Vertical resonance with wavelength selective mirrors Very low Ith possible Array structure preferred Circular beam pattern 0.85 µm (1.3 & 1.55µm under development)

Applications

Optical network interconnect Ethernet transceiver module CD / DVD Visible wavelength (plastic fiber, display etc.)

p-doped Bragg

Reflector

fiber Metal Contact


Optical Modulation Optical transmitters

External Modulation : Optical source + modulator + driving circuit + coupler +etc Direct Modulation : Optical source + driving circuit + coupler +etc

Direct vs. External modulation Direct Modulation - Data rate limited by frequency chirping - Inexpensive

External Modulation - Up to 40Gb/s possible - Expensive - High power data needed

LD

IDC

data

Modulated Light LD Modulator

IDC data

Modulated Light CW Light


Direct Modulation of LD LD direct modulation

Modulated currents is injected into LD Nonlinear P-I curve cause distortion of

modulated optical signal

Frequency chirping Caused by refractive index change

due to carrier variation Can increase chromatic dispersion

β(linewidth enhancement factor)

Parameter of frequency chirping

IIDC

P

λ

dNddNdn

//

αβ =

modulated

unmodulated

Modulated optical signal

Electrical signal


External Modulation Mach-Zehnder Modulator

Use phase variation of passing LiNbO3 with controlled electrode Single arm type vs. dual arm type

Modulation Transfer function Mach-Zehnder Modulator

DC bias

AC signal


Modulated Signals

Power spectrum of 193THz (1554.4nm) optical carrier being intensity modulated at 1GHz

Received Spectrum (after PD)


Optical Amplification – EDFA (1)

Coupler Filter

Input light (1.55µm)

Pump light (0.98µm or 1.48 µm)

EDF

Amplified output light (1.55µm)

Pump light (0.98µm or 1.48 µm)

Fiber Amplifier : EDFA (erbium-doped fiber amplifier) Wideband amplification : C-band & L-band Stimulated emission by optical pumping High gain(20~30dB), large gain bandwidth Low noise, small ASE, Polarization insensitive


Optical Amplification – EDFA (2) Characteristics of EDFA

Gain saturation : by increasing input power, gain is decreased wavelength dependent gain @ low input power : flattening tech. required

signal-controlled automatic gain control (AGC) long-period fiber Bragg grating as gain equalizer

Example : Mitsubishi EDFA FA155D-1875


Optical Amplification – Applications (1) In-line optical amplifier

Optical repeater to compensate for transmission loss and increase the distance between regenerative repeaters

Preamplifier Front-end preamplifier for optical receiver

Optical Tx G Optical

Rx

Fiber lines


Rx

In-line amplifier

Preamplifier


Optical Amplification – Applications (2) Power (booster) amplifier

Placing the device immediately after optical transmitter to booster the transmitted power

LAN booster amplifier boosting optical signal in front of star coupler


Rx

Optical Tx G

Long fiber link

LAN booster amplifier

Power (booster) amplifier

Receiver stations

Star coupler


Optical Amplification – Applications (3) Optical Amplifier in WDM system

Simultaneous amplification of several channels in optical domain Need no O/E/O conversion : Simple !!

WDM Mux

λ1 λ2

λ1

λ2

λ n

λ n . . . WDM

Demux

λ1

λ2

Rx

Rx

Rx

λ1 λ2 λ n . . .

λ n

Tx

Tx

Tx

. . .

λ1 λ2 λ n . . . λ1 λ2 λ n

. . . Optical Amp (EDFA, etc.)


Optical Fiber – SI fiber & GRIN fiber Step-index (SI) fiber vs. Graded-index (GRIN) fiber

Refractive index profile – step & graded index

Comparison between SI & GAIN fiber Size Function Difficulty in usage

[SI Fiber] • Small ∆n for single-mode operation • Small NA, difficult coupling

[GRIN Fiber] • Mostly multi-mode operation • Small mode dispersion

n 2

n 1 n1

n2 n 2

n 1 n1

n2


Optical Fiber – other types Dispersion-Shifted (DS) & Dispersion-Flattened (DF) fibers

Standard fiber : low dispersion @1.3um, low loss @1.5um DSF : low dispersion & loss @1.5um long haul transmission DFF : low dispersion @1.3 & 1.5um 1.3/1.5um CWDM

Polarization-Maintaining (PM) & Single-Polarization (SP) fibers Single-mode fiber support 2 modes with orthogonal state of polarization PMF & SPF control the state of polarization (SOP) using fiber core structure

Disp

ersio

n [p

s/nm

·km

]

Wavelength [μm]

Conventional Single-Mode Dispersion Flattened

Dispersion Shifted

Representative designs of polarization-sensitive fibers


Optical Fiber - Materials Glass fibers

GeO2-SiO2 core, SiO2 cladding P2O5-SiO2 core, SiO2 cladding SiO2 core, B2O3-SiO2 cladding GeO2-B2O3-SiO2 core, B2O3-SiO2 cladding

Active glass fibers Rare-earth element (atomic # 57~71) doped fiber Amplification, attenuation, phase retardation

Plastic optical fibers For optical LAN, optical interconnection of servers High-bandwidth graded-index polymer(plastic) optical fiber(POF) Core : polymethylmethacrylate or perfluorinted polymer

core

cladding


Optical Fiber - Attenuation Attenuation of a silica glass fiber

Minimum loss for silica fiber : 1.55µm @ third window 1.3µm and 1.55µm region are most practical


Optical Fiber - Dispersion Dispersion

Material dispersion : pulse at difference wavelength have different velocities

Waveguide dispersion : pulse at difference wavelength(but propagating in the same mode) travel at slightly different angles

Modal distortion : different axial velocities because of the path difference

PMD (polarization-mode dispersion) : the two orthogonally polarized waves do not travel at the same speed because of the birefringence (It will be explained again in WDM section)


Photo-Detector – PIN PD PIN PD

Use photo-absorption in semiconductor Advantage : high speed operation Disadvantage : low sensitivity

i

n+

hf

Anti-reflection coating

p+ Metal contact

SiO2

Depletion layer

Light

Band gap Eg

Photo-generated electron

Conduction band

Valence band Depletion region

Photon hν≫Eg

p

i

n


Photo-Detector - APD APD (avalanche PD)

Use avalanche effect (multiplication of photocurrent) Advantage : high sensitivity Disadvantage : low speed operation

Electric field

Avalanche region

Minimum field required for impact ionization

Depletion region

WM

n+

p

i(π)

p+


Optical Receiver (1) Structure of Optical Receiver

Photodetection + Signal (E) shaping + Data & Clock recovery

Photodiode

Voltage supply Automatic

gain control Clock

recovery

Decision circuit Filter

Preamplifier Optical signal

Amplifier

Linear channel Data recovery

Data

Front end

Photodectection Signal shaping Data & Clock recovery


Optical Receiver (2) Front end

Optical coupling – butt coupling into AR PD window Preamplifier – Large amplification & small impedance

High impedance front end Low impedance front end Trans impedance front end : high amp & small impedance

Linear Channel

Main Amplifier : Automatic gain control (AGC) Low pass filter : voltage pulse shaping by reduce the noise

Reduce Rx noise ∝ Rx BW Since filter ∆f < B ⇒ ISI reduced LPF design needed


Optical Receiver (3) Data recovery

Clock recovery f(=B) component separation out of the received signal Synchronization of decision circuit by TB= 1/B (bit slot) different circuit for RZ & NRZ codes

Decision circuit Decide ‘0’ or ‘1’ data using a proper threshold level Sampling time determined by clock recovery Extinction ratio (ER) of the signal becomes important

Bit Error Rate (BER) : normally set < 10-9


Optical Receiver Performance (1) BER (Bit Error Rate)

Depend on the decision threshold ID

BER minimum @

( )[ ]

−+

−=+=

2241)0/1(1/0

21

0

1

1

1

σσIIerfcIIerfcPPBER DD

0011 /)(/)( σσ IIIIQ DD −=−≡


Optical Receiver Performance (2) Eye pattern (for NRZ)

Eye closing (vs. Eye opening) Vertical axis : intensity noise Horizontal axis : time jitter

Ideal eye pattern

Bit slot Degraded eye pattern


Optical Receiver Performance (3) Receiver sensitivity

The minimum average received power Prec required by the receiver to operate at a BER of 10-9

Receiver sensitivity degradation : ER (extinction ratio), RIN (relative intensity noise), timing jitter


Receiver Noise (1) Receiver noise

Current fluctuations add the noise over the input optical power intensity noise

Shot noise (white noise) current fluctuation by random electron generation and flow I(t) = IP + is(t) where, is(t)=stationary random process

Thermal noise (Nyquist noise)

Temperature dependent random electron motion in conductor Random thermal motion of electron in RL of front end I(t) = IP + is(t) + iT(t)


fFR

TkfIIq nL

BTdPS ∆=∆+=

2,)(2 22 σσ

fFR

TkfIRPq

PRIpowernoise

powersignalAveSNRn

L

Bdin

inP

∆+∆+===

4)(2

. 22

2

2

σ

inPfTFk

RRSNRnB

L 22

4 ∆=

Receiver Noise (2) σ (rms of noise currents) → variance

where, Id = dark current, Fn = Amplifier Noise Figure (NF)

Pin Receiver SNR

In practice, σT ≫σS


WDM definition WDM(Wavelength Division Multiplexing)

Fiber-optic transmission techniques that employs different light wavelength to transmit data parallel-by-bit

Modulation format independent Increasing capacity using existing fiber Enhance network flexibility

Classification of WDM CWDM(Coarse WDM) : 20nm(or 10nm) channel spacing @1.5um DWDM(Dense WDM) : 0.2, 0.4, 0.8nm channel spacing @1.5um

WDM Mux . . .

λ1 λ2

λ1

λ2

λ n

λ n . . .

Client

Client

Client


WDM transmission System WDM(wavelength division multiplexing)

Carry several optical signal of different wavelength in single fiber

Tx

Tx

Tx

Tx

Rx

Rx

Rx

Rx

Source1

Source2

Source3

Source4

fiber1

fiber2

fiber3

fiber4

Tx

Tx

Tx

Rx

Rx

Rx

Rx

Source2

Source3

Source4

fiber Mux Demux

Different wavelength

<Traditional optical transmission system>

<WDM transmission system>

Source1

Optical amp, regenerator etc.

Tx

Can use Same wavelength


Necessity of WDM (1) TDM(Time Division Multiplexing)

Combines traffics from multiple inputs onto one common high capacity output Allows high flexibility in managing traffic : fixed bandwidth Data format dependent : only for Digital Requires electrical Mux/Demux function

Problems of TDM system High speed electric devices needed 10Gbps system available, 40Gbps thought to be Max

Scheme of TDM Mux/Demux


Necessity of WDM (2) Advantage of WDM

Overcome the constrains of TDM Independence of input data bit rate and format Merges optical flexibility in expanding bandwidth Reduce costly Mux/Demux function, reuse existing optical signal

Scheme of WDM Mux/Demux


WDM wavelength Expending the optical spectrum

wavelength allocation for more bandwidth and enhanced flexibility

196.0 195.0 194.0 193.0 192.0 191.0 190.0 186.0 199.0

f(THz)

l (nm) 1510 1530 1535 1540 1545 1550 1555 1560 1565 1570

C-Band OA Flat Gain Region

1610

L-Band OA Flat Gain Region


WDM transmission link

Multichannel point-to-point fiber link

Tx

Tx

Tx

Rx

Rx

Rx

• • • • • •

MUX DEMUX

λ1

λ2

λN

λ1

λ2

λN •••

Switching Matrix

High bandwidth

λ1, λ2, •••, λN


Evolution of WDM network

λ1

λ2

λN

λ1

λ2

λN

• • • •

λ4 λ4 λk λk

oxc • • • • • •

WDM/Pt-to-Pt Optical Transport

Fixed WDM/ Multipoint Network

Optical XC Reconfigurable WDM

Multipoint Network

Capacity Expansion

Bandwidth Management

∆f(100GHz) 0.8nm

∆f(50, 25GHz) DWDM


WDM devices - LD Laser diode for WDM

DFB-LD : single-mode Tunable LD : tuning output

wavelength MFL(multi-frequency laser) Accurate frequency locking required

Frequency locking technology

Need electrical and optical loop-back circuits

Feedback control scheme: ±0.5GHz accuracy (J.of lightwave technology, Vol.18, No.8 August, 2000)


1 2 3 4 1 2 3 4

FSR

WDM devices - AWG Arrayed Waveguide Grating (AWG)

Provides a fixed routing of an optical signal from a given input port to a given output port based on the wavelength of the signal

Wavelength Grating Router (WGR)


WDM devices – Thin film filter Thin film filter

Thin film select wanted wavelength, others reflected

Thin film WDM Mux/Demux Array of thin film filter 1x16 Mux/Demux commercially

available Cheaper than AWG

Reflecting Wavelength

Transmitting Wavelength

Thin film filter

WDM Mux/Demux using Thin film filter array


WDM transmitter Multi-wavelength source : N×LD, Tunable LD or MFL Wavelength multiplexer, modulator and driving circuit

••• λ1, λ2, •••, λN

WL MUX

λ1

λ2

λN

• • • • • •

LD

MOD

MOD

MOD

MOD

DATA


WDM receiver Wavelength demultiplexer N×PD, decision circuit

••• λ1, λ2, •••, λN

WL D E M U X

λ1

λ2

λN

• • • • • •

Decision Circuit

Decision Circuit

Decision Circuit

Decision Circuit

• • •

PD

DATA

DATA

DATA

DATA


Polarization Mode Dispersion Polarization Mode Dispersion (PMD)

Caused by slight fiber asymmetry Different polarization propagates at different speeds Critical limitation at high speed rate (~OC-192) Current fiber : PMD < 0.5 ps/km1/2

Many installed spans : PMD > 10 ps/km1/2


PMD effect on OC-192 Data PMD changes randomly with environment effects (e.g. temperature) Requires adaptive compensation

Analog Optical Modulation (Radio over fiber system)


Radio-over-Fiber System

cloud

Switching Node

Building System

Fiber-optic micro/pico-cell radio system

ITS System

Satellite Comm. System

BWLL System

YONSEI UNIVERSITY BTN LAB 49

Center Station

Department store or Hall

Tunnel or Underground street

Buildings

Optical Fibers

・ Cellular/PCS ・ W-CDMA ・ ITS ・ Satellite Comm.

Optical Network for Mobile Systems Optical network has

great potential for mobile systems


LD : Laser Diode PD : Photo-Diode

E/O LD

PD

RF signal f1GHz

O/E

Antenna Unit

RF modulator

Base Station

f1 Frequency [GHz]

Mag.

Optical Fiber

Mobile Communications Network

Optical Fiber RF- mod LD

Base Station

Antenna Unit

PD

PD

PD

Advantages of RF optical transmission It is very easy to add to or drop services for various RF modulations No frequency difference is caused in each antenna unit. Configuration in antenna unit is radically simplified


Analog SCM Optical link

Wide bandwidth Simple and Cost effective

Power Combiner

modulator

modulator

modulator

channel 1

channel 2

channel n

f s1

f s2

f sn

Optical Source

SMF Photo Diode

Channel Filter

demod.

demod

demod.

local osc. 1

local osc. 2

local osc. n

f s1

f s2

f sn

Optical Link


Modulated Optical Signal

DSB (Double Side Band) Optical Signal

Optical Intensity Modulation

Modulation

Modulation

fs1

fs2

Channel 1

Channel 2

fs1 fs2

0

0

Power Combiner

Optical Source

SMF

f 0 -fs2 f 0 -fs1 f 0 f 0 +fs1 f 0 +fs2


I

P

Digital vs. Analog Modulation Digital Modulation

High Extinction Ratio is required

Analog Modulation High Linearity is required

I

P


SSMF

Electric Signal Electric Signal

Rx (Photodiode)

Optical Modulator

Tx (Laser)

Electric Signal

Rx (Photodiode)

Tx (Laser) SSMF

Electric Signal

Direct & External Modulation Direct Modulation

External Modulation


Optical Modulation Technique Size Power Consumption Frequency Bandwidth

Direct Modulation (DFB-LD)

Small Little ≤ 10GHz

External Modulation

Electro absorption Small Intermediate > 60 GHz

Mach-Zehnder (LiNbO3)

Large Much > 60GHz

Comparison of modulation Methods


Electric Modulation Index

Ppeak

Bias Current of LD [mA]

Optic

al Po

wer P

o [m

W]

Ith Ib

Optical Signal

Ps

A

B

I vs. L characteristic in LD Optical modulation index is very important parameter to design an optical link.

thb

ace II

Im

−=

Optical Modulation Index

A

B

p

ppm

s

speako =

−=

Iac

Modulation Index


I

P P

f

P

f I

P

Carrier Signal

Carrier Signal

Nonlinear Signal

Two tone RF signal

Nonlinear effects Linear Case (Ideal LD)

Nonlinear Case (Practical LD)


Nonlinear Distortion

f(RF)

p

RF Carrier[dBm]

p IMD3[dBm] CNR[dB]

Noise floor

p Optical Carrier

Input RF Signal

SSMF Rx (PD)

Tx (LD)

Modulated Optical Signal

Detected RF Signal

f(opt)

f(RF)


Nonlinear component

HMD (Harmonic Distortion) IMD (Intermodulation Distortion)

Frequency [Hz]

RF Carrier ω1, ω2

2nd HMD 2ω1, 2ω2

2nd IMD ω1± ω2

3rd HMD 3ω1, 3ω2

3rd IMD 2ω1± ω2, 2ω2± ω1

5th HMD 5ω1, 5ω2

5th IMD1 4ω1± ω2, 4ω2± ω1

5th IMD2 3ω1± 2ω2, 3ω2± 2ω1


CSO & CTB Re

ceive

d RF

Pow

er [d

Bm]

Frequency [Hz]

f1 f2

2f2-f1 2f1-f2

3f2-2f1

2f2

f1+f2

3f1-2f2

2f1

3f2

2f1+f2 2f2+f1

3f1

frequency CSO at the rmsproduct te todue distortion Total

PowerCarrier RF=CSO

frequency CTB at the rmsproduct te todue distortion Total

PowerCarrier RF=CTB


Outp

ut R

F Po

wer [

dBm

]

Input RF Power [dBm]

Noise floor

IMD3 (2f1-f2 or 2f2-f1 ) RF Carrier (f1 or f2 )

SFDR [dB]

LDR [dB]

SFDR

SFDR (Spurious Free Dynamic Range) LDR (Linear Dynamic Range) Noise – shot noise, thermal noise, relative intensity noise (RIN) etc.


Narrow- & Broad- Band

In narrow band : IMD3 acts as a dominant noise In broad band : IMD2 acts as a dominant noise So, reduction of these components increases CNR

p

f(RF)

1 octave

IMD5

IMD3 IMD2

Carrier dominant noise

Broad Band Narrow Band


Fiber-induced Distortion

Chromatic Dispersion due to group velocity distortion (GVD) Dispersion Induced Distortion (DID)

LO

LO

RF OUT

LD Modulator PD

RF Signal


Chromatic Dispersion Effects Chromatic dispersion

Including both Material and Waveguide dispersion Double sidebands progress with different velocity in dispersive media

Following effects (in analog optical link)

Dispersion induced carrier suppression : Refractive index change induced phase difference : Periodic decrease of the received RF carrier power Dispersion induced distortion : Phase distortion : Nonlinear components power increase


Dispersion induced Carrier Suppression

Dispersion induced carrier suppression (DICS) Intensity modulation generates two optical sidebands (DSB) A phase difference between the two sidebands

Periodic reduction of the received RF carrier power

Network Analyzer Optical Modulator (f0)

f m

SSMF transmission PD

f o f o +f m f o -f m


Dispersion induced Distortion

Dispersion induced Distortion (DID) Periodic decrease of RF carrier power by dispersion Increase of IMDs component power due to nonlinear phase shift by dispersion

Deterioration of CTB for transmission length

Transmission

Fundamentals

Intrinsic distortions

Fundamentals DIDs

Passive Optical Network (introduction)


Motivation Optical access network for multimedia information era.

Requires high speed, wide-band, high capacity, convenient multimedia service


Optical Access Network

PON : OLT, ONU, passive optical splitter Improved transmission reliability, easy of maintenance Easy to upgrade without changing the infrastructure Transparent to bit rates and modulation format

FTTH: all optical fiber to the home FTTB: optical fiber to the building FTTC: optical fiber to the ONU & coaxial cable to the curb/cab


TPON (Passive Optical Network) TPON (PON for telephony)

Downstream : broadcast (TDM) From CO to ONU Upstream : combine ONU’s fixed multiplexed time slot Number of ONU : limits splitting loss by star coupler Low cost optical component System performance were up to sum of bit rate of total ONU

Laser

Receiver

CO

Splitter/ combiner

RN

Laser

Receiver ONU

Laser

Receiver ONU

Laser

Receiver ONU

••• ••• ••• •••

CO: central office RN: remote mode


WDM PON – Basic concepts Broadcast & Select Networks based on :

Wavelength routing Networks based on :


PSPON vs. WDM-PON PSPON vs. WDM-PON

PSPON WDM-PON Bandwidth Share-dynamic BW allocation Higher BW per ONU

Power budget Large splitting loss High Sensitivity

Low insertion loss

Upgrade Flexibility Easy Hard

Privacy Need encryption Reduce crosstalk

Diagnosis Difficult OTDR-good

Optical Transmission Basics

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