Fiber Optic Fundamentals
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Cable Types & Construction
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Connectors & Termination
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Splicing
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Testing & Troubleshooting
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Installation & Network Design
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Standards & Safety
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Quick Facts
- Exam
- CFOT
- Credential
- Certified Fiber Optic Technician
- Body
- FOA
- Questions
- 100
- Time
- ~2 hours
- Pass
- 70/100 (70%)
- Format
- MC, matching, T/F
- Level
- Foundational
- Validity
- 3 years
Fiber Jacket Color Code
Yellow single, orange legacy, aqua laser, lime OM5
Yellow: singlemodeOrange: OM1/OM2Aqua: OM3/OM4Lime: OM5
Singlemode vs Multimode Fiber
Singlemode
- 9/125 µm core
- No modal dispersion
- Long-haul, low loss
Multimode
- 50 or 62.5 µm core
- Has modal dispersion
- Short LAN runs
Distance and cost tradeoff
Fiber Type Selection
- Long-haul, lowest loss→Singlemode fiber(9/125 µm)
- Legacy short LAN run→OM1 or OM2(62.5 or 50 µm)
- 10G up to 300m→OM3 fiber(Aqua jacket)
- 10G up to 550m→OM4 fiber(Aqua jacket)
- Multi-wavelength WDM LAN→OM5 fiber(Lime jacket)
Fiber Types & Sizes
- Singlemode (OS1/OS2)
- 9 µm core
- OM1
- 62.5/125 µm, legacy
- OM2
- 50/125 µm, legacy
- OM3
- 50/125 µm, 10G/300m
- OM4
- 50/125 µm, 10G/550m
- OM5
- 50/125 µm, wideband WDM
- Cladding diameter
- 125 µm, all types
Light Propagation Principles
- Total internal reflection
- Light bounces at boundary
- Numerical aperture (NA)
- Max light acceptance angle
- NA formula
- sqrt(n1² − n2²)
- Core index
- Higher than cladding index
- Step-index fiber
- Abrupt core-cladding boundary
- Graded-index fiber
- Gradual refractive index change
- Rayleigh scattering
- Intrinsic glass density loss
- Fresnel reflection
- Glass-air interface reflection
Wavelengths & Attenuation
- Multimode windows
- 850 nm and 1300 nm
- Singlemode windows
- 1310 nm and 1550 nm
- SM loss @1310nm
- ~0.35 dB per km
- SM loss @1550nm
- ~0.20 dB per km
- MM loss @850nm
- ~3.5 dB per km
- MM loss @1300nm
- ~1.5 dB per km
- 1310 nm
- Zero chromatic dispersion point
- VCSEL
- 850 nm laser source
Dispersion & Jacket Colors
- Modal dispersion
- Multimode only, mode spreading
- Chromatic dispersion
- All fiber, wavelength spreading
- Yellow jacket
- Singlemode fiber
- Orange jacket
- Legacy OM1/OM2 multimode
- Aqua jacket
- Laser-optimized OM3/OM4
- Lime jacket
- OM5 wideband multimode
Loose Tube vs Tight Buffer
Loose tube
- Gel-filled tube
- Outdoor, OSP use
- Needs fan-out kit
Tight buffer
- 900 µm coating
- Indoor premises use
- Direct termination ready
Outdoor vs indoor cable
Cable Construction Picker
- Outdoor buried or aerial→Loose tube cable(Gel-filled tubes)
- Indoor direct termination→Tight buffer cable(900 µm coating)
- High-density mass splicing→Ribbon cable(12-fiber flat array)
- Rodent or crush risk→Armored cable(Steel/aluminum layer)
- Aerial, no messenger wire→ADSS cable(All-dielectric self-supporting)
Cable Construction Types
- Loose tube
- Gel-filled, outdoor/OSP use
- Tight buffer
- 900 µm, indoor use
- Ribbon cable
- 12-fiber flat array
- Armored cable
- Steel/aluminum, rodent protection
- Distribution cable
- Common jacket, needs fan-out
- Breakout cable
- Individual jacketed subunits
- Simplex cable
- Single fiber
- Duplex cable
- Two fibers, zipcord
- ADSS
- All-dielectric self-supporting, aerial
Cable Components & Handling
- Strength members
- Aramid yarn or fiberglass
- Buffer coating
- 250 µm primary coating
- Fan-out kit
- Loose tube to connector
- Cable lubricant
- Reduces pulling friction
- Velcro straps
- Support without crushing fiber
Polish Color Memory Aid
Blue is flat UPC; green is angled APC
Blue: UPC flatGreen: APC angledNever mix themAPC for PON
UPC vs APC Polish
UPC
- Blue color code
- Flat/curved polish
- -50 dB return loss
APC
- Green color code
- 8° angled polish
- -65 dB return loss
Never mate the two
Connector Selection Guide
- Need high port density→LC connector(1.25 mm ferrule)
- Legacy multimode link→ST connector(Bayonet twist mount)
- PON, FTTH, RF video→APC connector(Green, low reflectance)
- General SM/MM link→SC connector(Blue, push-pull)
- 40/100G parallel optics→MPO/MTP connector(12 or 24 fiber)
- Fast field termination→PPS connector(No epoxy cure)
Connector Types
- SC connector
- 2.5 mm, push-pull
- ST connector
- 2.5 mm, bayonet twist
- LC connector
- 1.25 mm, high density
- MPO/MTP
- 12 or 24 fibers
- Ferrule
- Aligns and holds fiber
- Adapter
- Mates two connectors
- Pigtail
- Connector one end, bare other
- Patch cord
- Connectors on both ends
Connector Polish Types
- UPC
- Blue, flat polish
- UPC return loss
- Better than -50 dB
- APC
- Green, 8° angle
- APC return loss
- Better than -65 dB
- APC required for
- PON, RF video overlay
- Never mate
- APC with UPC
Termination Methods
- Epoxy/polish
- Lowest loss, slowest method
- Polishing film grits
- 12, 3, 1, 0.3 µm
- Pre-polished splice (PPS)
- Fast, slightly higher loss
- Fiber cleaver
- Scores and breaks fiber
- Max cleave angle
- ~1-2 degrees accepted
Splice & Connector Loss Order
Fusion lowest, mechanical higher, connector highest loss
Fusion: 0.02-0.05 dBMechanical: 0.1-0.5 dBConnector: up to 0.75 dB
Fusion vs Mechanical Splice
Fusion splice
- Melts glass together
- 0.02-0.05 dB loss
- Needs fusion splicer
Mechanical splice
- Gel plus alignment fixture
- 0.1-0.5 dB loss
- No power required
Lowest loss vs speed
Splicing Method Picker
- Lowest loss required→Fusion splice(0.02-0.05 dB)
- No power available→Mechanical splice(0.1-0.5 dB)
- High fiber count cable→Mass fusion splice(12 fibers at once)
- Fast field splice-on→PPS connector(1-2 minutes each)
- Splicer rejects cleave→Re-cleave the fiber(Angle under 1-2°)
Splicing Methods
- Fusion splicing
- Melts fiber ends together
- Fusion splice loss (SM)
- 0.02-0.05 dB typical
- Mechanical splicing
- V-groove plus index gel
- Mechanical splice loss
- 0.1-0.5 dB typical
- Index matching gel
- Reduces Fresnel reflection loss
- Mass fusion splicing
- 12 fibers, one arc
- Max TIA splice loss
- 0.3 dB allowed
- Splice protector
- Heat-shrink sleeve plus rod
- Splice tray/closure
- Organizes and protects splices
dB vs dBm Power Units
dB compares two points; dBm is absolute
dB: relative lossdBm: power vs 1mW3dB: half power10dB: tenth power
OTDR vs OLTS Testing
OTDR
- Tests from one end
- Locates each event
- Shows distance to fault
OLTS
- Tests both fiber ends
- Measures total link loss
- TIA-568 reference method
Where vs how much
Test Tool Picker
- Need total link loss→OLTS(Source and meter)
- Need event location→OTDR(Distance trace)
- Quick break check→VFL(Visible red light)
- Check live traffic→Fiber identifier(No disconnect needed)
- Certify installed link→OLTS both wavelengths(TIA-568 method)
- Troubleshoot bad segment→OTDR(Locates fault position)
Test Equipment
- OTDR
- Traces events, one end
- OLTS
- Source plus meter, total loss
- VFL
- Visible red fault locator
- Fiber identifier
- Detects traffic, no disconnect
Insertion Loss vs Return Loss
Insertion loss
- Power lost forward
- Lower value is better
- Measured in dB
Return loss
- Power reflected backward
- Higher value is better
- Measured in dB
Forward loss vs reflection
OTDR Trace Reading
- Reflective event
- Connector or mechanical splice
- Non-reflective event
- Fusion splice or macrobend
- Dead zone
- Blind spot after reflection
- Launch fiber
- Hides first connector loss
- Break signature
- Spike then no backscatter
Loss Math & Units
- dB
- Relative loss ratio
- dBm
- Absolute power vs 1mW
- 3 dB loss
- 50% power lost
- 10 dB loss
- 90% power lost
- Power budget
- Source dBm minus sensitivity
- Link loss formula
- Source dBm minus received dBm
- Safety margin
- ~3 dB typical buffer
- Max connector loss (TIA)
- 0.75 dB per pair
Insertion vs Return Loss
- Insertion loss
- Power lost, forward direction
- Return loss
- Power reflected, higher better
- Two-wavelength MM test
- 850 nm and 1300 nm
- Reference method
- Zero test set first
FTTH vs FTTN Architecture
FTTH
- Fiber to the home
- Highest bandwidth
- No copper segment
FTTN
- Fiber to the node
- Copper last leg
- Lower deployment cost
All-fiber vs hybrid copper
Installation Practices
- Bend radius (under tension)
- 10x cable diameter minimum
- OSP pulling tension max
- ~600 lbf dielectric cable
- Conduit fill ratio
- 30-40% recommended maximum
- Cable lubricant use
- Reduces friction, prevents overstress
- As-built documentation
- Routes, splices, OTDR traces
Network Architectures
- FTTH
- Fiber to the home
- FTTN
- Fiber to node, copper last
- OLT
- Central office PON equipment
- ONT/ONU
- Subscriber premises equipment
- ODN
- Passive fiber plus splitters
- 1:32 splitter loss
- ~15-17 dB
- WDM
- Multiple wavelengths, one fiber
- CWDM spacing
- 20 nm channels
- DWDM spacing
- 0.8 nm or less
- GPON downstream
- 1490 nm wavelength
- GPON upstream
- 1310 nm wavelength
Standards & Codes
- TIA-568
- Structured cabling standard
- NEC Article 770
- Fiber installation code
- OFNP
- Plenum-rated fiber cable
- OFNR
- Riser-rated fiber cable
- FOTP-171
- OLTS attenuation test method
Safety & Cleaning
- Laser check first
- Verify source off, inspecting
- Never look
- Into live fiber end
- Fiber shard disposal
- Dark tape, sealed container
- Common telecom laser class
- 1M or 3R
- Video inspection probe
- Never direct-view live scope
- Clean and inspect
- Every connector, every mating
Common Traps
dB vs dBm Confusion
dB is relative loss ≠ dBm is absolute power
UPC vs APC Mating
Match same polish type ≠ Never mix angled and flat
OTDR vs OLTS Purpose
OTDR finds fault location ≠ OLTS gives total loss
Insertion vs Return Loss
Insertion is forward loss ≠ Return is reflected loss
Cladding vs Core Diameter
Cladding always 125 µm ≠ Core size varies by type
Loose Tube vs Tight Buffer
Loose tube needs fan-out ≠ Tight buffer direct terminates
Modal vs Chromatic Dispersion
Modal hits multimode only ≠ Chromatic affects all fiber
Last Minute
- 1.100 questions, 70% to pass
- 2.Cladding is always 125 µm
- 3.Yellow jacket means singlemode fiber
- 4.Aqua jacket means OM3/OM4 fiber
- 5.APC is green; UPC is blue
- 6.Never mate APC with UPC
- 7.Fusion splicing has lowest loss
- 8.OTDR locates faults; OLTS totals loss
- 9.3 dB loss equals half power
- 10.Bend radius: 10x cable diameter
- 11.Verify laser off before inspecting
- 12.Clean and inspect before every mating
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