Fiber Optic Theory and Testing

2

Objective

• Understand Fiber Optic Basics• Understand Issues that Impact Fiber Optic

Link and Channel Performance• Understand How to Determine Installed Link

and Channel Performance. • Demonstrate Fiber Testing and

Troubleshooting

3

Optical Fiber• Uses light pulses instead of electrical signals• Core & Cladding are composed of glass• n1 of the core > n2 of the cladding • Core diameter defines fiber type• Cladding diameter = 125 µm• Coating is UV curable urethane acrylate (2-Layers)• Coating diameter = 250 µm

4

Optical Fiber Types

• Single Mode

• Multimode

The radius, r, and index of refraction, n1, of the core determines the number of modes allowed to propagate:Number of Modes ≈ ∆(2πncorercore/λ)

5

STEP-INDEX MULTIMODE FIBER

GRADED-INDEX MULTIMODE FIBER

SINGLE-MODE FIBER

6

• LowerCost

• Very small core• Lower Attenuation• Higher Bandwidth

125 µ

Inexpensive Cable

Expensive Splicing

Longer Distance High

Capacity

8 - 9 µ 62.5 µ

Single Mode• Higher

Cost• Very Large Core• Higher Attenuation• Lower Bandwidth

Expensive Cable Inexpensive Splicing Shorter Distance Lower

Capacity

Multi-mode

7

Transmission Sources • Fabry-Perot (FP) and Distributed Feedback (DFB) Lasers

– Used for singlemode: 1310 nm or 1550 nm– Narrow spectrum (can be less than 1 nm)– Narrow beam width (does not fill multimode fibers)– Highest power and fastest switching– Most expensive (especially DFB)

• Light Emitting Diodes (LED)– Used for multimode: 850 nm or 1300 nm – Wide beam width fills multimode fibers– Wider spectrum (typically 50 nm)– Inexpensive– Cannot modulate as fast as lasers

• VCSEL’s– Vertical Cavity Surface Emitting Laser– Used for multimode at 850 and 1300 nm– Quite narrow spectrum– Narrow beam width (does not fill multimode fibers)– Much less expensive than FP or DFB lasers

Wavelength

Wavelength

8

Wavelength Windows Operation

800 900 1000 1100 1200 1300 1400 1500 1600

Att

enu

atio

n (

dB

/K

m)

L Ban

d

1st Window

2ndW

indo

w

5thW

indo

w

4thW

indo

w

3rdW

indo

w

C - Band 1530 - 1560L - Band 1565 - 1610

Wavelength (nm)

0.1

0.7

1.3

1.9

2.5

Reference Point: Visible Light is between 450 and 650 nm

Theoretical Minimum Attenuation of Single Mode

Fiber

C B

and

O B

and

E Band

S B

and

6thW

indo

w

9

Factors Affecting Optical Fiber Performance• Factors Affecting Light Losses or Attenuation

– Intrinsic– Bending Losses– Splice Losses

• Factors Affecting Light Pulse Broadening (Bandwidth)– Chromatic Dispersion– Modal Dispersion– Polarization Mode Dispersion

10

Attenuation

• Typical Attenuation for various types of optical fiber

Fiber Type 850 nm 1310 nm 1550 nm

Single Mode N/A 0.35 dB/km 0.25 dB/km

Multimode 3.5 dB/km 1.0 dB/km N/A

11

Sources of Attenuation• Intrinsic

– Raleigh Scattering– Water Peak Absorption (except of zero water peak fiber)

• Splice Loss– Fusion: core alignment– Mechanical: core alignment, dirt on end face, reflection– Mode Field Diameter in Single Mode Fibers– Numerical Aperture Mismatch in Multimode Fibers

12

Sources of Attenuation

• Macrobending (Single Mode Fiber)– Bending radius ~ 2 – 15 mm– Affects long wavelengths first– Affected mostly by fiber design

13

Sources of Attenuation

• Microbending (All Fiber)– Bending radius ~ radius of core– Can occur during optical fiber manufacturing process– Can be induced during installation due to point

pressures– Affects all wavelengths, but increases slightly with

wavelength– Order of Sensitivity (least to highest): SM, 62.5 µ, 50 µ– Affected by Coating and Cable Design

14

Dispersion or Pulse Broadening

• Chromatic Dispersion (Single Mode Fibers)

– Laser output is distribution of wavelengths– Different wavelengths travel different speeds– Dispersion compensating fiber

15

Dispersion or Pulse Broadening

• Polarization Mode Dispersion (Single Mode Fibers)

– Radially imperfect core– Causes delay in 1 of 2 Orthogonal Modes

16

Dispersion or Pulse Broadening

• Modal Dispersion (Multi-mode Fibers)

– Mode is quantum level in light pulse– Each mode occupies different area of core– Imperfect core structure causes modes to have

different speeds

17

18

Measuring Modal Dispersion

• Over-Filled Launch (OFL)– Uses LED– Completely fills all modes of multimode fiber

• Differential Modal Dispersion– Uses Laser– Injects pulses of light from one side of the core to the other at

micron intervals– Measures Pulse Intensity and Time of Arrival– Effective Modal Bandwidth (EFL) is determined from this test

19

Network might not work with “wrong” source

Over-filled launch = over estimates loss

Under-filled launch = under estimates loss

Pessimistic result

Optimistic result

Effects of Modal Dispersion

20

1. Reduces link loss variation2. Was developed to keep up with components used in high

speed networks (850 nm VCSEL, OM3/4 fiber)3. Was intended for >1GbE4. Targets 850 nm and 50 um cabling5. Can be used for all sources and links6. Improves supplier to supplier consistency

EF tightly controls the number of mode groups

EF is a new multimode launch condition metric that:

The “Encircled Flux” standard

21

How is EF measured?

Measured at output of test cord

Source

Reference grade test cord

mandrel

Near field measurement

EF output Test cord

output

22

Encircled Flux solves the problem by:

1. Controlling the number of mode groups launched from the test cord

2. Requiring better test cords from suppliers3. Formulating a tight standards-based template4. Advising all test equipment suppliers to use the

same template.

23

Launch Controller in use

24

Differential Modal Dispersion

Standard 62.5 µ vs. Laser Optimized 50 µ Fiber:Received pulse at 10 Gb/s over 300 meters

62.5 µ fiber

10 Gb/sBit Period

Laser Optimized 50µ Fiber

10 Gb/sBit Period

FiberCore

Center

25

10 GB Ethernet

– Approved by TIA in June 2002– A trend finds it’s continuation

10004 10 16 100 10 52 266 100

10000

02000400060008000

1000012000

Toke

nRi

ng 4

Mb

10BA

SE-

FOIL

Toke

nRi

ng 1

6FD

DI/T

PPM

D10

BASE

-FL AT

M

Fibr

eCh

anne

l10

0BAS

E-FX 10

00BA

SE-S

X10

GBA

SE-

S

1986 1987 1989 1992 1993 1993 1994 1995 1998 2002

Mbp

s Higher Speeds

0500

100015002000250030003500

Toke

nRi

ng 4

Mb

10BA

SE-

FOIL

Toke

nRi

ng 1

6

FDDI

/TP

PMD

10BA

SE-

FL ATM

Fibr

eCh

anne

l

100B

ASE-

FX 1000

BASE

-SX

10G

BASE

-S

1986 1987 1989 1992 1993 1993 1994 1995 1998 2002

Met

ers

Shorter Distances

13 12.5 13

1112.5

10

6

11

3.562.60

2468

101214

Toke

nR

ing

4 M

b

10B

AS

E-

FOIL

Toke

nR

ing

16M

bFD

DI/T

PP

MD

10B

AS

E-

FL ATM

Fibr

eC

hann

el

100B

AS

E-

FX 1000

BA

SE

-SX

10G

BA

SE

-S

1986 1987 1989 1992 1993 1993 1994 1995 1998 2002

dB

Smaller loss budgets

26

Smart Testing & Troubleshooting

• Eliminate common problems with good practices during installation and maintenance– Verify continuity, polarity, adequate end-face condition with

basic tools to ensure best termination and installation practices

• Perform complete cable certification per TIA TSB140– Basic certification (Tier 1):– Extended certification (Tier 2):

27

Tier 1 Test with LSPM (Light Source/Power Meter)

RemoteMain

28

Tier 1 Fiber Certification• Basic (Tier 1) certification of

fiber links

– Required for standards compliance

– Uses absolute power/loss measurement

– Best for measuring TOTAL (end-to-end) loss of a fiber channel

– Test against loss limits based on industry standards for current application

29

Tier 1 FAQ- What is the correct way to set a reference?

1. Set the reference using the test reference cord (sets P0 to 0dB).2. Attach tail cord to cable under test and measure P1 Loss = - (P1 - P0)3. Measures loss of two connectors and cable (fiber).

Most accurate and repeatable reference method:1Jumper Reference Method (also called “method B”)

30

Tier 1 FAQ- Why am I required to use a mandrel?

31

What does the Mandrel do?

Mandrel wrap with LED allows testing 50um and 62.5um

32

Tier 1 FAQ- When to use LED (MFM) and when to use VCSEL (GFM) source?

CladdingCore

Light

Under-filled launch provides under-estimated loss testing.

33

Tier 2 Test with OTDR• Single-ended testing of

fibers• Increase the quality of

fiber link installation• Troubleshoot faulty fiber

links

34

Tier 2 Fiber Certification• Extended (Tier 2)

certification of a fiber link

– Complements Tier 1 fiber certification

– Ensure that the fiber link meets expectations for current and future applications

35

Test Example: -Extended (Tier 2) certification

100 m 7 m 110 m

Pass/Fail loss budget is 3.2 dB noted for Gigabit in 568-B.1, Annex E

Result of Tier 1 Certification is 2.67 dB

36

Test Example: -Extended (Tier 2) certification100 m 7 m 110 m

Location(m)

850nm(dB)

Event Pass/Fail

0 .18 Reflect Pass100 .14 Reflect Pass107 1.4 Reflect Fail217 .19 Reflect Pass

1.91db loss at connections plus .76db loss for cable = 2.67db total link loss

37

Questions?