IEG4030-Tutorial 10

IEG4030-Tutorial 10 1 Power Penalty:

Under some non-ideal conditions, the averaged received power needs to be increased in order to maintain the same system

performance. The increase of signal power (in dB) required to achieve the same SNR or BER after a particular degradation

is introduced. This is called power penalty.

Non-transmission-related power penalty mechanisms: extinction ration, intensity noise and timing jitter

Transmission-related power penalty mechanisms: modal noise, dispersive pulse broadening, mode-partition noise,

frequency chirping, fiber nonlinearity, and reflection feedback and noise.

1.1 . Extinction Ratio:

Some power is emitted by most transmitters even in the off-state.

Definition of extinction ratio: 10 / PPrex =

Extinction ratio induced power penalty: )11

(log10 10ex

exex r

r−+

=δ

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IEG4030-Tutorial 10

1.2 Intensity Noise:

Light emitted by any transmitter exhibits power fluctuations. Such fluctuations are intensity noise. The optical receiver

converts power fluctuations into current fluctuations which add to those resulting from shot noise and thermal noise. So,

SNR degraded. Intensity noise parameter is simply inversed of the SNR of light emitted by the transmitter. Ir

)1(log10 2210 QrII −−=δ

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The power penalty will become infinite when , which implies that the receiver can not operate at specific BER even

if the received optical power is increased indefinitely.

1−= QrI

1.3 Dispersive Pulse Broadening:

Dispersion→pulse broadening→peak power reduces.

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Note that pulse broadening factor =peak power reduction factor. bf

])4(1[log5log10 21010 λδδ LDBf rbD −−==

The power penalty will become infinite when λδLDBr =0.25. This is because of ISI induced by dispersion.

1.4 Fiber Nonlinearity:

Due to the fiber nonlinearity, system performance may be degraded under high optical power.

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SBS, SRS and FWM will reduce or increase particular wavelength channel’s power.

SPM or XPM affect the phase of signal, thus inducing chirping. Such effects plus chromatic dispersion will cause phase to

intensity conversion→waveform distortion→power penalty

2 Chromatic dispersion:

2.1 Impairments:

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2.2 Solutions:

a) Dispersion Shifted Fiber (DSF)

Not suitable for WDM transmission since it is more likely to introduce nonlinearity impairments (such as FWM)

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b) Dispersion Compensation

In WDM transmission, dispersion slope compensation also needs to be considered.

c) Soliton Transmission

The balance of nonlinearity and dispersion during transmission in SMF→ pulse will not change its shape in transmission

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3 CATV (Community Antenna TeleVision):

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The denominator contains three terms: shot noise, thermal noise and RIN noise

CSO (composite-second-order distortion) and CTB (composite-triple-beat noise): caused due to the nonlinearity of the

modulator’s switching curve. Usually, CATV requires very high linearity of the modulators.

Reflection → RIN increases → signal degrades

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4 OTDR (Optical Time Domain Reflectometry):

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