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ECE 4363/5363Optical Engineering

Spring 2013

Lecture 1427 March 2013

Last class…• Semiconductor Photodetectors• Optical Receivers

Today…• Point-to-point link power budget analysis• Optical Amplifiers• Test and measurement

Point-to-Point Digital Transmission Links

• Link Requirements:– Transmission Distance– Data Rate or Bandwidth

A designer has the choice of the following:

1) Fiber -Multimode or single-modeCore size and refractive index profileAttenuationNumerical aperture

2) Source -Laser diode or LEDEmission wavelengthSpectral widthOutput powerSpeed (bandwidth)Effective emitting areaEmission pattern

3) Detector –Sensitivity (or responsivity)Speed (bandwidth)Operating λ

Model for a Point-to-Point Link

From: Optical Fiber Communications, 3rd Edition, by Gerd Keiser, McGraw-Hill, 2000.

Link Power Budget Analysis

PS - PR ≥ [ αf L + m(lc) + n(lsp) + system margin ]

where αf = fiber attenuation (dB/km)L = fiber length (km)m = number of connectorslc = loss per connector (dB)n = number of spliceslsp = loss per splice (dB)PS = source output power (dBm)PR = receiver sensitivity (dBm)

System Margin

• System margin is typically specified at 6 to 8 dB to allow for new components, component aging, and temperature fluctuations.

Link Rise Time Budget

• One accepted method for determining the dispersion limitiation of a fiber optic transmission system is to calculate the system rise time, tsys, and ensure that it does not exceed 70% of the NRZ bit period.

tsys = [ ( ttx)2 + ( tGVD)2 + ( tmod)2 + ( trx)2 ]1/2

where ttx = transmitter rise time (spec'd by manufacturer)

tmat = material dispersion rise time = DσλLor

tGVD = group-velocity dispersion ≈ |D|Lσλ

where D = material dispersionσλ = source spectral widthL = fiber length

tmod = modal rise time 0 for single-mode fibertrx = receiver rise time (spec'd by

manufacturer)

Purposes of Timing

• To allow the signal to be sampled by the receiver at the time when the SNR is at a maximum

• To maintain proper pulse spacing• To indicate the start and end of each timing

interval

Signal Coding

From: Optical Networks: A Practical Perspective, R. Ramaswami and K. N. Kumar, Morgan Kaufmann Publishers, Inc., 1998.

Signal Coding


Example

A 1550 nm single-mode digital fiber optic link needs to operate at 622 Mb/s over 80 km without amplifiers. A single-mode InGaAsP laser launches an average optical power of 0 dBm into the fiber. The fiber has a loss of 0.25 dB/km, and there is a splice with a loss of 0.1 dB every km. The coupling loss at the receiver is 0.5 dB, and the receiver uses an InGaAs APD with a sensitivity of –39 dBm.

a) Find the system margin.b) Find the system margin at 2.5 Gb/s with an APD

sensitivity of –31 dBm.

SolutionPS - PR ≥ [ αf L + m(lc) + n(lsp) + system margin ]

so we can calculate the system margin fromsystem margin ≤ PS - PR - αf L - m(lc) - n(lsp)

where PS = 0 dBmαf = 0.25 dB/kmL = 80 kmm = 1lc = 0.5 dBn = 79lsp = 0.1 dB

Solution (continued)a) PR = –39 dBm for a data rate of 622 Mb/ssystem margin ≤ 0 dBm – (-39 dBm) – (0.25 dB/km)(80 km)

– (1)(0.5 dB) – (79)(0.1 dB)system margin ≤ 10.6 dB, which is very respectable

b) PR = –31 dBm for a data rate of 2.5 Gb/ssystem margin ≤ 0 dBm – (-31 dBm) – (0.25 dB/km)(80 km)

– (1)(0.5 dB) – (79)(0.1 dB)system margin ≤ 2.6 dB, which is really not good enough to

ensure long-term, problem-free operation of the link

ExampleYou are assisting with the design of an OC-192 fiber

optic transmission link. Given a 1550 nm laser diode with a rise time of 25 ps and a spectral width of 0.1 nm, and a receiver with a rise time of 25 ps:

a) Determine the maximum dispersion-limited transmission distance through a fiber optimized for a 1310 nm source (assume a material dispersion of 15 ps/nm-km).

b) Determine the maximum dispersion-limited transmission distance through a dispersion-shifted fiber optimized for a 1550 nm source (assume a material dispersion of 2 ps/nm-km).

Solution

( ) ( ) ( ) ( )

( ) ( ) ( ) ( )

( ) ( ) ( ) ( )

12 2 2 2 2

mod

12 22 2 2

mod

12 22 2 2

mod

substituting for

and solving for

sys tx GVD rx

GVD

sys tx rx

sys tx rx

t t t t t

t D L

t t D L t t

L

t t t tL

D

λ

λ

λ

σ

σ

σ

⎡ ⎤= + + +⎣ ⎦=

⎡ ⎤= + + +⎣ ⎦

⎡ ⎤− − −⎢ ⎥⎣ ⎦=

Solution (continued)From the problem statement,

ttx = 25 pstmod ≈ 0trx = 25 psσλ = 0.1 nm

For an OC-192, the data rate is approximately 10 Gb/s, so the NRZ bit period isTb= 1x10-10 s = 100 ps. Thus, tsys should not exceed 70% of Tb, so set tsys=70 ps.

Solution (continued)

( ) ( ) ( )( )( )

12 2 2 2

max

max

a) transmission through a fiber optimized for a 1310 nm source with 15 / .

70 25 25

15 / 0.1

40.28

b) transmission through a dispersion-shifted fiberoptimized for

D ps nm km

ps ps psL

ps nm km nm

L km

= ⋅

⎡ ⎤− −⎣ ⎦=⋅

=

( ) ( ) ( )( )( )

12 2 2 2

max

max

a 1550 nm source with 2 / - .

70 25 25

2 / 0.1

302.08

D ps nm km

ps ps psL

ps nm km nm

L km

=

⎡ ⎤− −⎣ ⎦=⋅

=

Optical Amplifiers


Basic Operation


Gain vs. Input Signal Power


Energy-levels for EDFAs




Gain Behavior


Amplified-Spontaneous-Emission (ASE) Noise


ASE vs. Pump Power


EDFA Noise


Amplifier Cascades

From: Optical Networks: A Practical Perspective, R. Ramaswami andK. N. Kumar, Morgan Kaufmann Publishers, Inc., 1998.

SNR Degradation


Power vs. Amplifier Spacing



Gain Equalization


Automatic Gain Control (AGC)


Optical Supervisory Channel


Test & Measurement

Typical Problems

Low Levels

RXTXFiber Optic Cable

Dirty Connectors

Connectors not seated properly

Pinched Fibers

Tight bending radius’s

Bad Patchcords

Patchcord Patchcord

Low Transmit Levels

Ferrule Must be Clean

Key/Keyway must be engaged in mating

hardware

Avoid Tight Bending Radius’s

Avoid Stress Points Tie wrap Cinched Tight, Must

be Loose

Typical Problems

High Levels


Not enough Loss in Fiber Plant

Patchcord Patchcord

High Transmit Levels (LASER) Fixed Attenuator

Barrel Type Variable Attenuator

Screw adjustable Attenuator

5db increments

3 to 30db

3 to 30db

A

Attenuators

Typical Problems

No Receive Level


Dirty Connectors

Connectors not seated properly

Bad Patchcord (Open)

No Transmit Output

Wrong Fiber

Patchcord Patchcord

Ferrule Must be Clean

Key/Keyway must be engaged in mating

hardware

Avoid Tight Bending Radius’s

Avoid Stress Points Tie wrap Cinched Tight, Must

be Loose

Optical Loss Measurements

Patchcord PatchcordBulkhead

Connection

Power MeterLight Source

Reference Measurement

Relative Reference Measurement

-15.0dBmRef-0.00dBm

850nm

850nmReceived Level

Patchcord Patchcord

Bulkhead Connection

Power MeterLight Source

Attenuation Measurement ---Forward Direction

Loss

-5.00dBm

Bulkhead Connection

Fiber under test

850nm

850nm

Fiber Loss



Patchcord Patchcord

Bulkhead Connection

Power Meter Light Source

Attenuation Measurement ---Reverse Direction

Loss

-4.80dBm

Bulkhead Connection

Fiber under test

850nm

Fiber Loss850nm

Optical Time-Domain Reflectometry


OTDR Measurements

OTDR DEAD ZONES

Near End or

Attenuation DZ Splice DZ

Connector DZ

End of Cable

OTDR Measurements

Optical Return Loss

Large Reflection

Large Reflection

Small Reflection

Cleaved Fiber -14.5db

Flat Finish -14.5db

PC Finish - 45db

Dirty Connectors also cause Reflections

Increases Bit Error Rates

Increases Noise in Analog Systems

Optical Return Loss ---- Problems with Reflections

Pulse-Dispersion Measurements


Chromatic Dispersion Measurement(phase-shift method)


Optical Spectrum Analyzers

• Characterization of optical sources• EDFA gain and noise measurements




EDFA Gain Measurement


EDFA Noise Measurement


Eye-Pattern Measurement


Eye-Pattern Key Features

• Opening (height) and width of the eye• 20-80% rise and fall times for fiber systems

(10-90% times are often obscured by noise and jitter effects)

• Overshoot on logic ones and zeros• Undershoot on logic zero• Jitter in the eye pattern


3-bit Long NRZ Combinations


Eye Pattern Performance Parameters


• Width and height of the eye opening defines best sampling time

• Noise margin (%) = (V1/V2)x100%• Slope of the eye-pattern sides determines

sensitivity to timing errors• Timing jitter (%) = (ΔT/Tb)x100%• Nonlinearities of the channel transfer

characteristics will create asymmetry in the eye-pattern

Next class …

• Geometrical optics• Optical instruments

Documents

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