T2b

Optical Fiber Characteristics

• When copper isn't sufficient.
• Lightweight.
• Robust to oxidation, water, electrical and optical interference.
• Long transmission distances.
• Less flexible (brittle).
• Difficult to join (requires melting).
• Easy to create thin cables:
  • Copper: Down to $0.03mm$ diameter.
  • Standard Fiber: Down to $8\mu m$ diameter ($0.008mm$).
  • Lab: Below $1\mu m$.
• Potentially more expensive than copper.

Language Transition

• Copper cables use frequency (f) measured in Hertz (Hz) or $s^{-1}$.
• Optical Fiber uses wavelength ($\lambda$) measured in meters.
• High frequency is equivalent to short wavelength.
• $v = f * \lambda$ (speed of light in the medium).
• $c_{vacuum} = 300,000 km/s$.
• $c<em>{copper, glass} \approx \frac{2}{3} c</em>{vacuum}$.

Performance Comparison

• Copper: Kilohertz ($10^3$) to Megahertz ($10^6$), Wireless to Gigahertz ($10^9$).
• Optical: Terahertz ($10^{12}$) and higher.
• Nyquist Theorem Consideration.
• VDSL at $12 MHz$ has a range of $25m$.
• WiFi at $2.4GHz$ has a range of $12.5cm$.
• Yellow light at $600nm$ corresponds to $500 THz$.

Physics of Optical Fiber

• Index of refraction: $n = \frac{c}{speed \ of \ light \ in \ material}$.
• Light changes direction when crossing between materials due to the change in speed.
• Fiber cable construction:
  • Glass fiber core.
  • Cladding (different type of glass).
  • Plastic jacket for protection.
• $n<em>{cladding} < n</em>{core}$.

Total Internal Reflection

• Achieved by selecting the correct angle and materials.
• Occurs within a 'critical angle'.
• Types of fiber:
  • Step-index fiber.
  • Graded-index fiber.
• Each ray represents a 'mode'.
• Modal distortion: Straight line is the slowest.
  *Reduces modal distortion

Multimode vs Singlemode Fibers

• Multimode fibers: Significant modal distortion.
• Singlemode fibers: Narrow core (few wavelengths).
• Singlemode performance is higher but more costly.

Fiber Standards

• Cable standards:
  • Multimode: OM1 ($62/125\mu m$), OM2-OM5 ($50/125 \mu m$).
  • Singlemode: ITU G.652-G.657 ($9/125 \mu m$).
• Performance expectations are more stringent than manufacturing requirements.
• Connector standards: Many standards vary by sector (30+ on Wikipedia).
• Incompatible cables: Mixing multimode and singlemode fibers mechanically works, but performance degrades.

Fiber Connectors

• Connectors must be perfect.
• Glass face to Glass face contact is important.
• Dust is a serious issue.
• Curved (or angled) faces are often used to reduce reflections.
• Terminating copper cables involves simple tools.
• Terminating fiber cables involves melting and polishing glass/plastic, which are thinner than human hair.
• Splicing fiber:
  • Melting (good).
  • Glueing (less good, but cheaper).

Losses in Fiber

• Attenuation: $0.4-3 dB/km$ due to:
  • Scattering (structures and materials).
  • Absorption (materials).
• Distances of many km are easily achievable.
• 8km yields 75% of the original light.
• Attenuation depends on wavelength.
• Fibers have multiple passbands due to materials and manufacturing techniques.
• Improvements are continuously being made to both absorption and range.
• First Window, Second Window, Third Window

Other Losses

• Chromatic dispersion:
  • Index of Refraction varies with wavelength.
  • A pure single wavelength is difficult to achieve (even with a laser).
  • Soliton pulses address this issue.
• Polarization mode dispersion:
  • Core shape helps mitigate this.

Loss Budgets

• Energy budget:
  • Amount of energy sent.
  • Amount of energy needed to be received for a clear signal.
• Loss factors:
  • Fiber loss: $0.4-1.0 dB/km$ for SMF.
  • Mechanical connector loss: $0.3dB$ each.
  • Physical splicing loss: $0.3dB$ each.

Bending Radius

• Copper: Tight bending radius (N times diameter).
• Fiber: Vulnerable to fractures, causing attenuation and interference.
• MMF is more resilient than SMF.
• Fiber testing: Send a pulse, analyze what gets through and what is reflected.

THz Channels

• A single wavelength at $500 THz$ can theoretically carry $1+Pb/s$ (very faint signal).
• Electronics limitations: Clarity over speed.
• Modulation techniques: ASK, PSK.
• Frequency-DM becomes Wavelength-DM (multiple wavelengths of light)
• Polarization Division Multiplexing.
• WDM types: Coarse (CWDM) and Dense (DWDM).
• Each wavelength is generally used as a separate channel.

Add-Drop Multiplexing

• Managing N wavelengths on a fiber.
• Adding or removing a wavelength (channel).
• Active electronic separation vs. Passive optical separation of channels.

Transmitting Over Light

• Digital data conversion to optical signaling (modulation).
  • OOK -> QPSK++
• Pulsing an LED:
  • Fast but cheap.
  • Limited brightness and pulse shaping.
  • Broad color range.
  • Used in MMF.
• Chopping a laser:
  • Semiconductor lasers are now $100\mu m$ in size.
  • Wavelength tunable on the fly.
  • Optical Mach-Zehnder modulator.
  • Used in SMF.

Multi-Core Cable Design

• Individual fibers are fragile.
• Cable bundles can contain up to 1024 fibers.
• Burying/hanging costs are substantial whether single or multiple fiber.
• Reasons for separate fiber paths:
  • Security.
  • Guaranteed performance.
  • Avoid interference.
  • New technologies.
• Fibers with sub-fibers or hollow channels.
• 'Dark' and 'grey' fiber concepts:
  • Empty glass to fill as needed.
  • Slot for adding a wavelength.

Data Transmission Speed

• Depends on the definition of "fast".
• Highest absolute speed (2012): $1Pb/s$ over $50km$, (2017) $10.2Pb/s$ over $11km$.
• High speed over long distances (2009): $100Gb/s$ over $7000km$ with $155\lambda$, (2016) $65Tb/s$ over $6600km$, (2021) $319Tb/s$ over $2900km$.
• Techniques: Multiple wavelengths, multiple cores, pre-distortion.
• Metric: bandwidth*distance.
  • 2021: $9x10^5 (Tb/s).km$
  • 2016: $4*10^5 (Tb/s).km$
  • 2012: $5*10^4 (Tb/s).km$
• 2012 used specialized fiber, 2009/2016 used commodity fiber.
• 2020 Internet traffic: $375,000PBytes/month = 1.4Pb/s$.

Transmission Distance

• Without effort: $1-2 km$ over MMF, $50-100 km$ over SMF.
• More distance:
  • Brighter lasers (difficult).
  • Regenerate/Repeat every $50-100 km$ (expensive).
  • Optical-electronic-optical (OEO) interfaces.
• Amplify every $50-100 km$:
  • Cheaper electronics, optical amplification (erbium doping).
  • Amplifies signal and noise.
• Record: 2015 - Melbourne to Melbourne, via Sydney&Perth = $10,358km$ @$100Gb/s$.

Single Fiber Use

• Fibre is not easily a shared medium - Point-to-point
• Can use a single fibre for RX and TX simultaneously via optical splitters.
• Risk of crosstalk in connectors and within fiber.
• Fibre pair is still more common.

Last Mile and Fiber

• Costs: Capital costs of installing fiber vs. copper, running/maintenance costs, performance comparison.
• NBN approach: Hybrid.
  • Push fiber as close as possible (FTTx).
  • Use existing copper for the last segment (DSL).
  • Monitor electronics.

Exchange Range

• Copper links = $4km$ circle
• Fibre links = $40+km$ circle

FTTx Models

• Leverage existing infrastructure (landline phones).
• Reduce average copper distance.
• Push fiber as deeply as affordable.
  • FTT Home
  • FTT Kerb
  • FTT Node
    *What needs power? Who is sharing? How many houses per ‘box’ What needs managing?

FTTx vs HFC

• Fiber To The … (FTTx):
  • FTTPremises/FTTHome
  • FTTBasement
  • FTTCurb/Kerb/dp
  • FTTNode
• Hybrid Fiber Coax (HFC):
  • FTTN for proximity.
  • Shared (coax) copper.
  • Serves 50-100s of houses in NBN.
• Cost Considerations: Prioritize run cost

(G)PON

• (Gigabit) “Passive” Optical Networking.
• Flexibility of optics.
• Cheap broadband.
• Minimal fibre usage.
• TDM to active ONU/ONT (Optical Network Units/Terminals).
• Actual electronics.
• TDM to passive splitters from OLT (Optical Line Terminals).
• Mirrors and lenses.
• WDM: RX and TX on a single fibre.
• SDM: More revenue from the cable.

Copper and Fiber

• Can link copper to fiber "easily" via optical/electronic convertors.
• Mix possible in campus/building/farm/home.
• Understand protocol and losses.