ee 230: optical fiber communication lecture 7 from the movie warriors of the net optical...

EE 230: Optical Fiber Communication Lecture 7

From the movieWarriors of the Net

Optical Amplifiers-the Basics

Amplifier Types and Applications

Fiber Optics Communication Technology-Mynbaev & Scheiner

Amplifiers are used to overcome fiber loss They are used in 4 basic applications:

In-line amplifiers for periodic power boosting

Power Amplifier to increase the power to greater levels than possible from the source

Pre-amplifier to increase the received power sensitivity

Distribution loss compensation in local area or cable networks

Characteristics of all amplifiers

• They operate by creating a population inversion, where there are more individuals in a high energy state than in a lower one

• The incoming pulses of signal on the fiber induce stimulated emission

• They saturate above a certain signal power

• They add noise to the signal

Comparison of Real and Ideal Amplifier

Inhomogeneous Gain Broadening

Lasers-Siegman

Inhomogeneous broadening

The individual atomic responses within and inhomogeneously broadened transition all add up to yield the measured lineshape

A Gaussian inhomogeneously broadened atomic lineshape such as produced by doppler broadening in atoms

Interaction of Atoms with Light

Rate Equations and Populations

Unstimulated Population densities in 2 ‘ level atom

Energy levels 1 and 2 and their decay times. By means of pumping, the population density of level 2 is increased at the rate R2 while that of level 1 is decreased at the rate R1

For large N or No (also called inversion density)We want 2 long, but 21 not too small, 1 and R1 large

Idealy 21~sp<<20 so 2~sp

Population densities with a strong resonant signal

Ideal Amplifier System

Pump process with large crosssection

Third excited state with very short lifetime,no fluorescence

Second excited state with very longlifetime and high cross section for stimulated emission

Energy gap between first andsecond excited states matches telecommunication frequencies

First excited state with very shortlifetime

Amplified Spontaneous Emission

Noise Figure Measurement


Noise Figure

in

out

321system

1 1 2 1 2 3 1

SNRNoise Figure

SNR

A perfect amplifier would have a Noise figure of 1 or 0 dB

Noise figure of an amplifier cascade

F = ........

For lowest overall noise figur

nn nkn

k

FF FF

G GG GG G G -

º

+ + + +

e you should put the lowest noise amplifier first

3 main types and 3 Big Ideas

The main types of optical amplifiers are:

•Semiconductor amplifiers (lasers that aren’t lasing)•Doped fiber amplifiers•Raman and Brillouin Amplifiers

The three big ideas

•Gain and gain bandwidth•Gain saturation•Noise and noise figure

Laser Amplifiers

Semiconductor Optical Amplifiers


Types of SOA

Fabry-Perot AmplifierHigh gain but non-uniform gain spectrum

Traveling wave amplifierBroadband but very low facet reflectivities are needed

Gain as a function of frequencyRipples are caused by the cavity modesThe overall gain curve is due to the width of the atomic transition in the semi-conductor

Fundamentals fo Multiaccess Optical Fiber Networks Dennis J. G. Mestgagh

Amplifier Bandwidths


Comparison of the bandwidths of Fabry Perot and Traveling wave amplifiers

Traveling Wave SOA


To make a traveling wave Semiconductor Optical Amplifier the Fabry-Perot cavity resonances must be supressed. To accomplish this the reflectivity must be reduced.

Three approaches are commonly used:

Anti-reflection coating

Tilted Active Region

Use of transparent window regions

Saturation Power


Gain saturation and saturation power

Semiconductor Optical amplifiers saturate silmilarly to a 2 level atom

The typical saturation output power for

SOAs is around 5-10 mW

Crosstalk in Semiconductor Amplifiers

Rate equation for pump current

If Φ suddenly goes to zero, as in 1-0 sequence,

Time constant is (ns)

If Φ suddenly turns on,which is smaller

)()()(

0 tNtNn

actN

qLWD

I

dt

dN

/1)( teqLWD

ItN

downT

11

n

acTup

Parameters on previous slide

• N=carrier density (cm-3)

• I=pump current (amp=coul/s)

• q=charge on electron (coul)

• L,w,d=cavity dimensions (cm3)=recombination lifetime (s)=confinement factor (unitless)=photon density (cm-3)

• a=gain coefficient (cm-1)

Crosstalk in semiconductor amplifiers

If time constant for spontaneous decay of excited state is shorter than the bit duration, the population of the excited state will vary sharply with the optical power in the fiber, and gain will depend on the fraction of 1s and 0s in the data stream.

If time constant is long, then the population in the excited state will be constant, dependent upon the pump power but not the signal power.

Reduction of Polarization Dependence


Three main approaches

Connect the amplifiers in series

Residual facet reflectivitycan cause undesired coupling between amplifiers resulting in poor noise and dynamic performance

Connect them in parallelGood solution but complex

Double pass with polarizaion rotation

Automatic 6 db loss due to coupler

Undesired effects in an SOA


Cross saturation can cause undesired coupling between channels

•This can be used for wave length conversion and “controlling light with light”

If used for multiple channels in a switched network gain must be adjusted as channels are added and dropped

Four wave mixing is also quite pronounced in SOAs

•Causes undesired coupling of light between channels•Can however also be used to advantage in wavelength converters.

High coupling loss

Polarization sensitive gain

Short Pulse Amplification in SOAs

Semiconductor amplifier advantages

• Are the right size to be integrated with waveguide photonic devices (short path length requirement)

• Can easily be integrated as preamplifiers at the receiver end

• Use same technology as diode lasers• Gain relatively independent of wavelength• Are pumped with current, not another laser

Semiconductor amplifier disadvantages

• Polarization dependence

• Self-phase modulation leading to chirp

• Cross-phase modulation

• Four-wave mixing and crosstalk

• Extremely short (ns) excited state lifetimes

ee 230: optical fiber communication lecture 7 from the movie warriors of the net optical...

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