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(c) Sergiusz Patela Optical waveguide amplifiers

Optical amplifier functions

1. In-line amplifier

2. Power booster

3. Detector preamplifier

Transmitter

Receiver Preamplifier

In-line Amplifier

Power Booster

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Optical amplifiers - classification

1. Semiconductor optical amplifiers (SOA, 400 - 2000 nm band)

2. Raman optical amplifiers

3. Brillouin optical amplifiers

4. Erbium doped fiber amplifier (EDFA, 1500-1600 nm band), alsoPDFFA(1300 nm band)

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SOA principle of operation

An SOA is based on the same technology as a Fabry-Perot diode laser. Theamplification function is achieved by externally pumping the energy levels ofthe material. In order to get only the amplification function, it is necessary to

protect the device against self-oscillations generating the laser effect. This isaccomplished by blocking cavity reflections using both an antireflection (AR)coating and the technique of angle cleaving the chip facets. SOAs areelectrically pumped by injected current

P in R~ 0 R~ 0P out = P in e g L

L

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SOA advantages

1. Compactness

2. Integration potential

3. High power output4. Broad choice of operating wavelength (400-2000 nm)

5. Low price with high volume production

Disadvantages

1. High coupling loss

2. Polarization dependence3. High noise figure (as compared with EDFA)

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SOA parameters (State-of-the-art)

Wavelength ~1300 nm or ~1550 nm residual reflectivity of less than 10 -4 to ensure a gain ripple below 0.5

dB

low optical loss to achieve a net gain as high as 30 dB high material gain to allow low-drive current operation (20 to 30 dB

fiber-to-fiber gain for a 100-mA drive current) high output saturation power, defined as the output power for which

the gain is reduced by 3 dB chip-to-fiber coupling loss of less than 3 dB per facet, which is

achieved using integrated mode-expanding tapered waveguides at theoutput facets.

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Polarisation sensitivity of SOA

Polarisation sensitivity of SOA is less than 0.5 dB The polarization state of the optical signal coming from a link fibre is

usually random.

Material gain is isotropic in bulk material The strip waveguide in the SOA active area is polarisation sensitive

(differential gain between transverse-electric (TE) and transverse-magnetic (TM) modes)

Polarization sensitivity as low as 0.3 dB can be achieved with a nearsquare (0.4 m * 0.6 m) active waveguide having almost the sameconfinement factor for both polarization states.

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Example of SOA design

Semiconductor Laser Chip

Anti-reflectionCoating

Tapered Fiberno 1

Tapered Fiberno 2

Waveguide

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Optical amplification - EDFA

Erbium Doped Fiber Amplifier PumpLaser

InputSignal

Erbium Doped Fiber

AmplifiedOutputSignal

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Erbium Doped Fiber Amplifier

EDFA optical amplifiers are made of short lengths (a few meters) ofoptical fiber doped with the element erbium. A pumping laser exciteserbium ions in the fiber, which can then give their energy to theoptical signals passing through.

Amplified wavelength: about 1550nm

Pump wavelengths: 980 and/or 1480.

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Erbium Excitation/Emission

Pump signals - 1480 nm and/or 980 nm. Erbium ions stays in excitedstate for about 10 ms (time is longer for 980 nm).

Excited State 1Erbium ion

1480 mnpump

Ground state

Signal Amplified1550nm

signal

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EDFA amplifier basic structure

Opticalinput Coupler

Optical

output

LDpump

Er 3+ doped fiber

Wavelength [ m]

25 dB

Ampl.

40 nm

1.55

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ASE

Amplified Spontaneous Emission (ASE): A backgroundnoise mechanism common to all types of erbium-doped fiber

amplifiers (EDFAs). It contributes to the noise figure of theEDFA which causes loss of signal-to-noise-ratio (SNR).

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EDFA parameters

wide bandwidth - 40 nm (5000 GHz)

high amplification - 30 do 40 dB

high output power - do +20dBm (100 mW)

low noise - 4 dB (Noise factor F)

pump wavelength - 980 or 1480 nm

no dispersion compensation

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Block diagram of a full-featured EDFA amplifier

Coupler

1

OpticalInput

1550 nm

99%

1%

Isolator 1 Isolator 3

Isolator 2

Coupler

2

99%

1% 1%

Erbium DopedFiber

WDM 1 WDM 2 OpticalOutput

1550 nm

Detector 1Input

Monitor

Detector 2Reflection

Monitor

Detector 3OutputMonitor

980 nmPump

Laser 1

980 nmPump

Laser 2

Microcontroller-based Control and

Monitoring Circuitry

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Two-stage EDFA with Mid-stage Access

OpticalInput1550 nm

OpticalOutput1550 nm

Isolator 1 Isolator 4Isolator 2 Isolator 3WDM 1 WDM 2

Erbium DopedFiber 1

Erbium DopedFiber 2

980 nmPump

Laser 1

980 nmPump

Laser 2

Microcontroller-based Control and

Monitoring Circuitry

Mid-stage access

(e.g. dispersioncompensation module)

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Raman Scattering

The principle of Raman scattering is that a lower wavelength pump-laser light traveling down an optical fiber along with thesignal, scatters off atoms in the fiber, loses some energy to theatoms, and then continues its journey with the same wavelengthas the signal.

SignalwavelengthPump

wavelength

Ramanscattering

Signal

wavelength

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Comparison of EDFA, SOA and EDWA

Technology Description Advantage Disadvantage SuppliersErbium DopedFiber Amplifier (EDFA)

Standard beyond 2005

Knowntechnology;

provenmanufacturable

Higher cost Corning, JDS Uniphase,Agere, Alcatel Optronics,

Nortel HPOCS, Avanex,Altamar, Onetta, Keopsys

Semiconductor OpticalAmplifier (SOA)

Discrete InP- basedcomponent

Potential lowcost; ease of integration;small size

Low output power; highnoise; immaturetechnology

Alcatel Optronics, Genoa,JDS Uniphase, NortelHPOCS, Optical Crossing,Kamelian, OptoSpeed

Erbium DopedWaveguideAmplifier (EDWA)

Planar waveguideamplifier

Potential lowcost; small size

Immaturetechnology;difficult tomanufacture

Teem Photonics, Cisilias, Northstar Photonics,Symmorphix

Christine Mulrooney, Optical Amplifiers--the Metro Story, CIR Whitepaper

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Books

1. P. C. Becker, N.A. Olsson, J.R. Simpson, Erbium-Doped FiberAmplifiers: Fundamentals and Technology Academic Pr 1999

2. Emmanuel Desurvire Erbium-Doped Fiber Amplifiers : Principles andApplications, John Wiley & Sons 1994

3. S. Sudo (Editor), Optical Fiber Amplifiers : Materials, Devices, andApplications (Artech House Optoelectronics Library), Artech House

19974. Govind P. Agrawal, Nonlinear Fiber Optics, Second Edition,

Academic Pr 1995

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