Optical Fiber Transmission Media

Optical Fiber

Introduction

• Optical fiber is a type of transmission media.
• In copper media, electrical pulses travel, while in optical fiber, light pulses carry data.
• Understanding the properties of optical fiber materials is crucial.
• Topics to be covered include:
  • Overview of optical fiber communication.
  • Theory and principles of operation.
  • Characteristics and specifications.
  • Transmission properties.

Light as a Communication Medium

• Light can be used to transmit data through optical fiber cables.
• The speed of light is approximately 300 \times 10^6 meters per second, making it an excellent medium for data transmission.
• Optical fiber supports large bandwidths, making it attractive for long-distance communication.
• Undersea cables utilize optical fiber for high-bandwidth communication between continents, supporting terabytes of data transmission.

John Tyndall's Experiment (1880)

• John Tyndall demonstrated that light could be propagated within a jet stream of water using total internal reflection.
• He used a bucket of water with a hole and a mirror to reflect light into the water stream.
• The light was trapped within the water due to the refractive index, illuminating his hand when placed at the end of the jet stream.
• This experiment demonstrated the principle of total internal reflection, which is used in modern optical fiber communication.

Optical Fiber Basics

• Optical fibers use total internal reflection to transmit light signals over long distances.
• The glass used in optical fiber is 99% pure to minimize impurities.
• The remaining 1% impurity is used by Optical Time Domain Reflectometers (OTDR) for fault detection.
• OTDR sends a laser signal and detects reflected light caused by irregularities or breaks in the fiber, allowing location of faults.

Optical Fiber Specifications

• Typical diameter is 250 micrometers.
• Common carrier network cables come in configurations such as 144F, 288F, and 312F (number of fibers).
• Carriers like Telstra use these cables to connect central offices.
• Optical fiber connects transmitter and receiver equipment.

Myths vs. Reality

• Myth: Optical fibers are fragile.
  • Reality: They are made of glass or silica, but are protected during manufacturing.
• Myth: Optical fibers are difficult to work with.
  • Reality: Technical expertise is needed, especially for fault location (using OTDR) and splicing.
  • Splicing involves fusing broken glass fibers together using specialized equipment (fusion splicing), which is expensive.

Advantages of Optical Fiber

• Scale bandwidth: Fiber provides larger bandwidth as compared to copper cables.
• Speed: Faster data transmission.
• Attenuation: Longer transmission distances (e.g., 200 km links) without repeater stations.
• Crosstalk: No crosstalk due to absence of radio frequency interference (RFI) and electromagnetic interference (EMI).
• Weight: Lightweight compared to copper cables.
• Lightning: Not prone to lightning strikes.
• Safety: No electromagnetic emissions, fire, shock, or spark hazards.
• Health and Safety: Safer working environment but laser safety is a consideration due to lasers used as light sources.
• Operating temperature: Wide range (-40 to 80 degrees).
• Cost: With the cost of copper increasing, the cable cost is not the biggest problem, it's the transmitter receiver cost.

Disadvantages of Optical Fiber

*Cost of transmitter and receivers is higher. There are two types available:
*LED source
*Laser source
*Only LED sources can be used with multimode.
*High maintenance cost compared to copper or wireless.

Comparison with Wireless and Electrical Cables

• Fiber vs. Wireless/Satellite: Bandwidth is the biggest advantage of fiber.
• Fiber vs. Electrical Cables: No EMI, RFI, or electromagnetic induction issues.
• Optical fiber cables can be run alongside electrical cables.
• In Australia, electrical cables must have a three-meter separation from copper-based data cables.
• Optical fiber can be tied with electrical cables.
• Wi-Fi: Biggest advantage is mobility and reduced cost of reconfiguring devices.

Carrier Network Components

Core Network

• Permanent optical fiber links.
• Trunk networks connect telephone exchanges within a city.
• Intercapital networks connect cities to cities (long-distance communication).

Access Network (Customer Access Networks - CAN)

• Utilizes optical fiber cable.
• National Broadband Network Co (NBN Co) is responsible for laying down this infrastructure.
• Flexible and accessible network.
• Consists of:
  • Optical Line Terminal (OLT): Covers a small area or suburb.
  • Ethernet fan-out switch.
  • Fiber access node.
  • Distribution fiber.
  • Fiber distribution hub.
  • Drop optical fiber: Connects to Optical Network Terminal (ONT) at homes.
  • Internal or external ONT.
  • Wireless router for in-home connectivity.

Multi Dwelling Units

• Optical fiber to each floor.
• Copper-based technology (VDSL modems) for older buildings.

VDSL

• Very High Speed Digital Subscriber Line
• VDSL1: Up to 52 Mbps download, 16 Mbps upload.
• VDSL2: Up to 100 Mbps in both directions (up to 1.5 km distance).

Deployment

• Local optical fiber with drop fiber cables.
• Existing electrical poles can be used.

Basic Construction of Optical Fiber Cable

• Two glass layers:
  • Core glass.
  • Cladding glass.
• Protective coatings (e.g., UV acrylate).
• Core and cladding are made of optical transparent material (e.g., silica glass).
• Color coding for identification.
• Layers surrounding the glass core are designed for protection and to reflect UV and other harmful rays.
• UV cured acrylate has a 250 micrometer coating.
• Two types: 900 micrometer tight buffer fibers and 250 micrometer tight fibers.

Total Internal Reflection

Refractive Index

• Ratio of the velocity of light in a vacuum to the velocity of light in a medium.
• n = \frac{c}{v}, where:
  • n is the refractive index.
  • c is the speed of light in a vacuum (300 \times 10^6 m/s).
  • v is the speed of light in the medium.
• Example: Glass has a refractive index of 1.5.
• Refractive index determines the light's bending power through a medium.

Refraction and Reflection

• Refraction: Occurs when a light ray strikes the boundary of two dissimilar transmission mediums at an angle greater than the critical angle of acceptance. Light bends and goes out of core.
• Reflection: Occurs when the light strikes the boundary at an angle equal to or below the critical angle of acceptance. The light is trapped within the core.
• Acceptance Cone: Light is sent within the acceptance cone angle to ensure it enters the core.

Optical Fiber Types

• Multimode Optical Fiber: Supports multiple modes and rays.
• Single Mode Optical Fiber: Supports only one mode or ray of light through the center of the core.
• Multimode fibers are not advisable or cost-effective for long distance because of dispersion within the cable.

Acceptance Cone

Light rays are accepted through either laser or light emitting mode
*Capability of fiber to receive light is determined by numerical aperture.
*Those that enter within the cone will remain in the fiber.

Multimode Optical Fiber

• Core glass is 62.5 micrometer.
• Cladding is one twenty five micrometers.
• Esrylate coating is 250 micrometers.
• OM1, multimode optical fiber.
• Core of 50 micrometer is OM2.

Refractive Index Profiles

• Step Index: Abrupt change in refractive index at the core-cladding boundary.
• Graded Index: Gradual change in refractive index to reduce modal dispersion. Reduces steep angle of reflections.
  • Example: OM3 offers 1 Gbps to 10 Gbps transmission up to 300 meters.
  • OM4 offers 10 Gbps transmission up to 550 meters.

Single Mode Optical Fiber

• Used for long-haul applications (core and access networks).

• High bit rates (up to 40 Gbps).

• Core refractive index is approximately 1.47.

• Advantage: Reduced core diameter (approximately 9 micrometers), limiting light ray path to one mode, which removes modal dispersion (intersymbol interference).

• Construction:

  • Core: 9 micrometers.
  • Cladding: 125 micrometers.
  • Only one mode of propagation is possible.

• The smaller difference in the refractive index, it allows allows the total internal reflection to happen.

Types of Single Mode Optical Fiber Cable

• g.652d: core of 9 micrometers supporting wavelengths of 1310 and 1550 nanometers has attenuation of .35 to .22 dBs per kilometers.
• g.657a: is bond sensitive and is very popular and the same attenuation, same everything. It is used best for patch chords and pigtails.

Single Mode vs. Multimode

• Multimode is suitable for in-building connections.
• Single mode is suitable for customer access networks, intra-exchange networks, and trunk networks.
• Bandwidth available in single mode is greater than terabytes per second.
• Bandwidth for multimode is up to 10 Gbps but distance is less than 300 meters.

Frequencies

Using Frequencies for transceivers for transmission/reception is preferred to using wavelength because frequencies produce really big single.
C = f \times \lambda where and the spectrum of light are visible in the infrared region.
Frequency spectrum for single mode, LED for Multimode: LED Sources used LED for 850 NM and 3800 NM. for laser is 13 10 NM and 15 15 NM.

Fiber attenuation

Fiber attenuation has 3 major causes:

• relay train scattering: its scattering of light energy and it creates a dispersion of light.
• Light absorption light photons are absorbed by impurities: The energy is converted into heat due to molecular resonance.
• The best areas are one there's least attenuation so three/four windows or bandwidth of transmitter receivers.

External Physical conditions for attenuation

• Splice joints. Performance of insertion loss. Mechanical splicing causes loss in signals
• localized pressure points act actually create attenuation: Occurring due to poor installation or environmental conditions. they can be observed with a naked eye. they can be macro or microbanded
• MBR: minimum bend radius of 30/60 mm or 15mm in G652 or G657. expensive cables can use 90 degree bends.