Opticalampliﬁers - Login ERC WEBopti500.cian-erc.org/opti500/pdf/25...

/ 40 Cédric Ware <[email protected]>

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Optical amplifiers


2011

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Introduction

High-speed wired communications: optical fibers

Primary limiting factor: attenuation

λ (nm)

(dB/km)

1550 nm

C L

Attenuation

0.2 dB/km

E SO U1

1000 1100 1200 1300 1500

0.20.3

1600

0.5

1400


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Introduction

Avoid signal regenerators (O-E-O bulky; all-optical not mature)

⇒ Optical amplifierssince 1993: long-distance transmissions2000s: metropolitan networksnow: extended-range access networksenvisioned: all-optical signal processing

⇒ Transmission bandwidth = amplifiers’ gain bandwidth


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Optical amplification

Optical amplification based on stimulated emission:

hν

Spontaneous emission

hνhν

Absorption

hνhν

Stimulated emission

Need more electrons in excited state than in fundamental state⇒ population inversion


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Parameters of an amplifier

Fundamental parameters:λ, bandwidthGain, saturation / output power

System / technological parameters:Noise, signal distortionSpeed, transient managementPackaging, bulkiness, consumptionCost

Extra functionalities:Dispersion compensationChannel add/dropMonitoring


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Gain and saturation

Gain:

Pout = GPin

Pin (G − 1) < Ppump Pin G Pout

Ppump

Saturation / max. output power

Gain (dB)

Pin (dBm)

3 dB

P satin

20

20

10

0

30

-40 -20 0


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Amplification noise

Amplifiers add noise (else violate uncertainty principle)Amplified spontaneous emission (ASE)Noise transfer from pumpVacuum fluctuations ...

Noise Figure:

NF =SNRinSNRout

assuming quantum-noise-limited in-put signal

NF > 3 dB (for a high-gain optical amplifier)


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Noise from a chain of amplifiers

Amplifier chain: the first amplifier’s noise dominatesG1NF1

G2NF2

GnNFn

NF = NF1 +NF2 − 1

G1+

NF3 − 1G1G2

+ . . . (Friis formula)

(Not to confuse with transmission chain, which has strongattenuation between amplifiers)

Attenuation: quantum noise not affected⇒ NF (attenuator) = attenuation

Insertion loss: attenuation at amplifier input⇒ Strong influence on NF


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Signal distortions

Dispersion (chromatic and polarization) in long amplifiers

Polarization-dependent gain (PDG)

High power ⇒ non-linearityWDM ⇒four-wave mixing, crosstalkSoliton-like pulse compression

Gain saturation rapidityFast gain ⇒ non-linearity, distorted bitsSlow gain ⇒ modulation-transparent, problems with transients


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Packaging

Pumping typeselectrical ⇒ easy integrationoptical ⇒ must insert pump, separate signal at output

Packagingall-integrated / discrete componentsrackable unitsbulkiness, electrical consumptionsubmarine cables: fit in cable, remote power supply...

Integrationphotoreceiver + preamplifierloss-less splitteractive switching matrix


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Functionalities of amplifiers

WDM amplificationSimultaneous amplification of λ combGain equalization

Gain controlGain variation rapidityInput power fluctuation handling

Inter-stage accessDispersion compensationROADM: channel add-drop

MonitoringCheck operationOptical power of individual λ channelsChannel estimation


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Typical usage configurations

Transmission line

Emitter

Booster PreamplifierIn-line

Receiver

... in a mesh networkDifferent channels → different pathsVariable traffic, packet network ⇒ power fluctuationsReconfigurable channel add-drop (ROADM)

“Loss-less” splitter: 1× N + integrated amplifiers


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Needs for different usages

Transmission NetworkBooster In-line Preamp Metro Access

High gain important critical criticalHigh Pout critical importantLow NF andinsertion loss

important critical

Polarizationindependence important critical critical critical critical

Bandwidth wide narrow Coarse WDMDispersion mgmt DCF multi-spanAdd/drop ROADMLow consumpt important importantLow cost important critical


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Erbium-doped fiber amplifiers

Currently most-used amplifiers: EDFAs

Er3+ ions in silica (glass) fiber (codoped Al2O3, GeO2, P2O5...possibly TeO2 ou ZBLAN/fluoride → stronger doping)Optical pumping: 980 nm, used to be 1480 nm, more efficient beforegood 980-nm lasersAmplification in C-band (1530-1565 nm) or L-band (1565-1600 nm)Setup:

Pump980 nm Counterpropagating

pumpCopropagating

pump

Signal

IsolatorEr3+-doped fiber


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EDFA: gain spectrumPopulation inversionNet gain

λ (nm)

40%60%

1500 1520 1540 1560 1580 1600

100%

Adjustement: pump power and λ, fiber length...Special-glass fibers (TeO2, ZBLAN)Gain-flattening filters (GFFs)


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Gain-flattening filters

Gain-flattening filters → gain equalization

www.bookham.com

Interference filters or Fiber Bragg gratings (FBGs)Complex designSensitive → temperature variations

Active temperature controlAthermic packaging that compensates dilatation

Insertion loss⇒ Between stages (before input: NF ↗, after output: Pout ↘)


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EDFA: pumping

Single-mode fiber required for the signal⇒ low numerical aperture ⇒coupling losses when injecting

Single-mode not needed for pump⇒ double-cladding fiber, V-groove injection

(high-power amplifiers)

Core (doped)Cladding

V-groove


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EDFA characteristics

C- or L-bandAll-fiber ⇒ low insertion lossGain up to 40 dB, Pout > 23 dBm, polarization-independentNF down to ∼ 3 dB (lab) ; 4–6 dB in practiceLong-lifetime excited states (few ms)⇒ gain = constant over each bit⇒ good linearity

Drawbacks:Optical pumping ⇒ complexSensitive to traffic fluctuations (on packet networks)


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Modern EDFAs

Usage: all applications on C + L bandsDynamic gain equalizationPower monitoring (not on individual WDM channels: too costly)2+ stages, mid-point access → DCF, add-drop

ICTON 2006 115 Tu.C1.2

1-4244-0236-0/06/$20.00 ©2006 IEEE

Optical Amplifiers for Modern Networks David Menashe, Alex Shlifer and Uri Ghera

RED-C Optical Networks, Atidim Tech. Park, Bldg 3. P.O.B 58101, Tel-Aviv 61580, Israel Tel: (9723) 6476789, Fax: (9723) 6476990, e-mail: [email protected]

ABSTRACT Recent trends in optical networks, such as Reconfigurable Optical Add Drop Multiplexing (ROADM) and optical cross connects, require advanced optical amplifiers based on both Erbium Doped Fibre Amplifiers (EDFAs) and Raman technology. To address the dynamic nature of modern networks, EDFAs should provide broadband variable gain operation, flexible mid-stage access, fast transient response to dynamic events, and advanced spectral monitoring and control to adjust to changing spectral conditions in the network. An important supplement to EDFA technology is the use of Distributed Raman Amplification (DRA) to achieve transmission over multi-span Ultra Long Haul (ULH) links in all optical networks, as well as very high loss repeaterless links. Keywords: optical amplifiers, EDFAs, distributed Raman amplifiers, optical channel monitoring, ROADM.

1. INTRODUCTION The rapid growth of IP traffic, which now dominates most service provider networks, has placed new demands on the optical layer of the network. The unpredictable nature of the traffic, coupled with the need to provide multiple broadband services at ever decreasing cost, has led service providers to demand flexible reconfigurable optical networks which are self-managed and can seamlessly adjust to dynamic traffic conditions. In the short to intermediate term, this demand has led to the widespread interest in and deployment of Reconfigurable Optical Add Drop Multiplexing (ROADM), which allows individual wavelength to be dynamically and remotely dropped or added at sites along an optical link. In the longer term, Optical Cross Connect (OXCs), also known as high degree nodes, will be required to create all-optical mesh networks where different wavelengths traverse different and diverse paths within a multi-point mesh network. Additionally, enhanced protection and restoration capabilities are required at the optical layer to support carrier class Ethernet and other advanced services.

Being key components in any optical network, optical amplifiers need to support these new requirements. In particular, they are required to support: Broadband operation over a large dynamic range with respect to input/output power and gain: Flexible mid-stage access for advanced optical modules such as ROADM devices, Wavelength Blockers (WBs) and Dynamic Gain Equalizers (DGEs); Fast transient response to sudden dynamic changes in input power; and Detection of and adjustment to changes in spectral composition of the input signal. In addition, amplifiers should have minimum Noise Figure (NF) in order to enhance system OSNR and enable transmission over longer distances without electronic regeneration. In this respect, the use of DRA in conjunction with conventional EDFAs is a key enabler for ULH transmission and other demanding applications.

2. ADVANCED EDFA MODULES EDFAs have been widely deployed since the early 90’s, with the basic technology and components now being both mature and well understood [1]. The basic EDFA design consists of a length of Erbium Doped Fibre (EDF), pumped by a 980nm pump laser diode. The addition of input and output isolators and detectors, together with an electronic control unit, represents a single stage amplifier module, as shown in Fig. 1a. Such a module is typically operated in Automatic Gain Control (AGC) or Automatic Output Power Control (APC), where the input and output detectors supply the required feedback to the control unit, which in turn controls the pump power.

Figure 1. (a) Basic Single Stage Amplifier Module, (b) Broadband Variable Gain EDFA.

Isolator

EDF Pump Combiner

Isolator

980nm Pump

Input Detector

Output Detector

Control Unit

(a)

Gain Stage I

VOA GFF Gain

Stage II

Control Unit

(b)

D. Menashe, “Optical Amplifiers for Modern Networks”, ICTON 2006.


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EDFA packaging

FDDatasheet

Media ConvertersRepeaters and Optimizers

[email protected]

Optical Amplifi ers

Overview

Fiber Driver® EM316EDFA modules provide a multi-function, low-noise, Erbium-Doped Fiber Amplifi er (EDFA) solution ideal for metro Dense Wavelength Division Multiplexing (DWDM) as well as single wavelength distance extension applications.

The EM316EDFA-BR and EM316EDFA-LPR are part of the Fiber Driver optical multi-service platform solution family. The EM316EDFA-BR is an optical booster extending transmission range. The EM316EDFA-LPR may be either an in-line repeater or a pre-amplifi er that can strengthen weak signals, optionally extending the link range as well. By performing two functions, the EM316EDFA-LPR also reduces some inventory requirements.

EM316DMR3G-3R

EM316GEMX2R

EM316-2XFP

DWDMMUX

BOOSTER IN-LINE PRE-AMP DWDMMUX

EM316DMR3G-3R

EM316GEMX2R

EM316-2XFP

OPTICAL OUTPUT

PWR/NMS

OUTPUT

INPUT

SD

OK

OPTICAL OUTPUT

PWR/NMS

OUTPUT

INPUT

SD

OK OPTICAL OUTPUT

PWR/NMS

OUTPUT

INPUT

SD

OK

90 km 80 km+9dBm

+4dBm

Long Haul Application

EM316EDFA-BR

A B B

NC316BU-16/AC

EM316EDFA-LPR EM316EDFA-LPR

NC316BU-16/ACNC316BU-16/AC

Features

Two models- EM316EDFA-BR (booster amplifi er) - EM316EDFA-LPR (in-line and pre-amplifi er)

Applications - Metro DWDM distance extension - Single wavelength distance extension

Automatic Gain Control (AGC)

Managed and non-managed operation

Advanced performance monitoring - Input and output power levels - Signal gain- Temperature- Supply voltage

Gain fl attering fi lters (GFF)

Manual (SNMP management) and automatic power shutdown

Status LEDs- Input power OK- Output power OK

Hot-swap support

Fiber Driver® two-slot and sixteen-slot chassis compatibility

MRV [email protected]


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Other doped-glass amplifiers

Same principle as EDFAs:EDWA: doped waveguide instead of fiber

Short length (few cm), low bulkObsoleted by mini-EDFAs (fiber spool fits < 10 cm)

EYDFA : codoping erbium-ytterbiumHigh output power (30–45 dBm)Only part of C band (1540–1560 nm)

Thulium amplifier (lab)Tm3+ ions in fluoride glassS-band amplification(Depending on pump: 700 nm, 800 nm, 1µm, 1.4µm, and/or 1.56µm)

Short-λ amplifiers (lab)Praseodymium or neodymium → O-bandYtterbium → λ ∼ 1µm


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Semiconductor optical amplifiers

SOA = semiconductor laser without cavity→ Fabry-Perot laser + antireflection-coated facets

Substrate

Electrode

Antireflectioncoatings

dotsQuantum

wellsQuantum

Bulk

Active layer


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SOA packaging

SOA module:chip mounted on basebias current 200mA – 2 A (according to active layer volume)Peltier thermoelectric module → cooling, temperature controllensed fibers or microlenses

L. Spiekman, “Semiconductor optical amplifiers for reconfigurable optical networks”, J. OpticalNetworking 6 (11), Nov 2007.


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SOA characteristics

Gain determined by energy band structure

Valence band

Conduction band

hν

EF c

EF v

Gr (dB)

λ (nm)-5

-4

-3

-2

-1

0

1500 1520 1540 1560


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Bulk SOAs

Bulk / quantum-well (QW) SOAs:Mature technology, same wavelengths as lasersG ∼ 20 dB, BW > 50 nm, NF ∼ 6 dBLow polarization dependency, low ripplePsat < 20 dBm, τ ∼ 100 ps–1 ns; nonlinearities

L. Spiekman, “Semiconductor optical amplifiers for reconfigurable optical networks”, J. OpticalNetworking 6 (11), Nov 2007.


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Quantum-dot SOAsQuantum-dot (QD) SOAs:

G ∼ 10–25 dB, BW ∼ 100 nm, τ ∼ few psExcellent linearityDevelopment underway; almost mature → C-band

T. Akiyama et al., “Quantum-Dot Semiconductor Op-tical Amplifiers”, Proc. IEEE 95 (9), Sept 2007.


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SOAs: poor amplifiers

Historically, SOA problems:

SOA fast (∼ 1 ns) ⇒ bit-timescale signal distortions

t t

through SOANRZ signal

NRZ signal

Nonlinearities, four-wave mixing ⇒ problem with WDM

⇒ EDFA preferred, except:Niche: transmissions outside C-bandNiche: integrated amplifiers (e. g. with photodiode)Active MZI gatesSignal processing: λ conversion, regeneration...


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SOAs: comeback

Currently, SOAs making a comeback:Long-distance transmissions changing techniques

Constant-envelope modulations (NRZ-xPSK)Packet networks ⇒ transients on packet timescales

Development of novel metro+access networksLow cost preferredCoarse WDM ⇒ less FWM, need wide bandwidthShorter distances/lower powers ⇒ small signals ⇒ SOAs ∼ linear“Extender-boxes” → long-range access networks (> 20 km)


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SOA improvements

Quantum-dot SOAs:Very wide bandwidthUltrafast electron transitions + wetting layer ⇒ gain is clamped

LOA: SOA + VCSELActive layer sandwiched between Bragg reflectors

⇒ Laser perpendicular to signal propagation⇒ Clamps carrier density ⇒ better linearity


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New SOA usages

“Novel functionalities” of the 1990s (nonlinear effects)All-optical signal processingWavelength conversionModulation format conversionRegenerationLogic gates

→ Still not widespread outside labs

Integration / use as on-off switchLoss-less splittersSwitching matricesRSOAs: replaces laser + modulator for wavelength-independent opticalnetwork units in access networks


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Nonlinear effects (for amplification)

Several-photon interactionsInteractions with non-electronic energy levels: phonons

Vibrationlevel

Pump

Signal

Virtual level

amplificationParametric

amplificationRaman/Brillouin

Nonradiative

(phonon)transition

Pump(2 photons) (1 photon)

Idler

Conservation ofenergy &momentum:ωp = ωs + ωphonon~kp = ~ks + ~kphonon


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Phonon types

Acoustic phonons: lattice vibrations, low frequencies→ Brillouin effectOptical phonons: molecular vibrations, high frequencies→ Raman effect

k

Acousticphonons

Opticalphonons

E


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Brillouin scattering

Phase matching: kphonon ∝ ωphonon thus, for significant frequencydifference (hence gain), need large kphonon.

⇒ counterpropagating pump (−kp = ks − k ′phonon).

Very narrow bandwidth: few 10 MHz.

⇒ Application: possibly low-bitrate WDM demultiplexing.

→ Mostly, parasitic effect that limits optical power.


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Raman amplification

Raman-effect fiber amplifier (RFA): same setup as EDFA, but the “active”fiber is a standard, long fiber, and λpump chosen as a function of λsignal.

Detuning

Raman gain

(THz)0 10 20 30 40

Gain peak: ∆ν ∼ 12THz (∆λ ∼ 100 nm).


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Pum configuration

Phase matching: kphonon may be large or small compared to kopt forsimilar frequencies, so pumping can be copropagating(kp = ks + kphonon) or counterpropagating (−kp = ks − k ′

phonon)

But: very fast effect ⇒ transfers pump noise to signal

If counterpropagating pump, noise ends up averaged over each bit

⇒ Counterpropagating pump


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Raman amplifier = distributed amplification

Localized amplifiers: between fiber spans

Distributed amplifiers: gain along tail of transmission

⇒ less attenuation noise, better overall noise figure.

hνBo/2−60

−40

−20

0 100 200 300km

SignalNoise

S (dist.)N (dist.)

dBm


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Pros/cons of Raman amplification

Pros:Works at any λDistributed amplification ⇒ better NFDual pumping ⇒ gain over whole transmission span

Cons:Non-uniform gain⇒ Multiple pumps

Need long fiber for significant gain⇒ directly over transmission fiber

⇒ Usage: in-line amplification.


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Hybrid Raman–EDFA amplification

Pump ∼ 1460–1480 nm, standard + doped fibers:C-band EDFA + Raman

⇒ 2.5 Gbps over 370 km with single amplifier stage

Pump @ 1480 nm

Signal

Er3+dopedfiber

285 km 85 km


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Amplifiers vs applications

Current usages:EDFAs mature → all telecom applications

installed in ∼ all amplified networksonly C and L bands; require control → transients

Raman → long-range transmissionsdeployed in recent systems

SOA → low costbeginning to be used

Under development:SOA → special functions (RSOAs; all-optical processing)QD-SOA: very promising

catch up with EDFA when available in C band?

Research or non-telecom usages:EYDFA (high power); Tm, Pr, Yb (λ < 1500 nm)


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References, further reading

E. Desurvire, “Erbium-Doped Fiber Amplifiers, Device and System Developments”,Wiley-Interscience, 2002.

D. Menashe, “Optical Amplifiers for Modern Networks”, ICTON 2006.

L. Spiekman, “Semiconductor optical amplifiers for reconfigurable optical networks”, J.Optical Networking 6 (11), Nov 2007.

T. Akiyama et al., “Quantum-Dot Semiconductor Optical Amplifiers”, Proc. IEEE 95 (9),Sept 2007.

C. Headley, G. P. Agrawal, “Raman amplification in fiber optical communicationsystems”, Elsevier Academic Press, 2005.

S. Jiang et al., “Full characterization of modern transmission fibers for Ramanamplified-based communication systems”, Optics Express 15 (8), Apr 2007.


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