Fiber Optics in Communication: Advantages, Applications, and Technology

Posted on Nov 29, 2024 in Technology

Fiber Optics

To surf the internet, you need more than just a computer, a modem, and software; patience is also key. Cyberspace can be slow. Users may wait minutes for a page to load or hours to download a program.

This slowness is because phone lines—how most of the 50 million users connect to the internet—were not designed to carry the volume of video, graphics, and text that travels across the network.

However, phone lines are not the only way to access cyberspace. Fiber optic internet service offers a faster alternative.

Origin and Evolution

The history of fiber optic communication is relatively recent. A test system was installed in England in 1977, and two years later, substantial orders for the material were being produced.

Earlier, in 1959, physics research in optics led to the discovery of the laser, a new application of light. Applied to telecommunications, lasers enabled message transmission at unprecedented speeds and distances.

However, this laser usage was limited by the lack of suitable transmission lines for the electromagnetic waves generated. This led scientists to develop optical fiber.

In 1966, the use of optical fiber for communication was proposed. Using light to carry information involves electromagnetic waves similar to radio waves, but with wavelengths measured in microns rather than meters or centimeters.

While the concept of light wave communication was long-standing, theoretical work demonstrating the feasibility of transmitting light through flexible, transparent fiber emerged in the mid-1970s.

The main technical hurdle was the light absorption within the fibers. For practical communication, light signals must travel many miles. Ordinary glass only transmits light a few meters. New, ultra-pure glasses developed in the early 1970s overcame this limitation, propelling the fiber optics industry. Lasers or light-emitting diodes send light through these fiber optic cables. Miniaturizing these components for fiber optic systems required extensive research and development. Lasers produce intense, “coherent” light that stays in a narrow beam, while light-emitting diodes produce “incoherent” light that is less focused. The choice depends on the specific fiber optic circuit design.

What is Fiber Optics?

Before explaining fiber optics, let’s review some optics basics. Light travels at its maximum speed in a vacuum. This speed decreases when light travels through other media. When light passes from one medium to another, its speed changes, and it undergoes reflection (bouncing off the interface, like a mirror) and refraction (bending as it changes speed and direction, like a spoon in water). See the diagram to the right.

A medium’s refractive index “n” is calculated by dividing the speed of light in a vacuum by the speed of light in that medium. Reflection and refraction at the boundary between two media depend on their refractive indices. A key law of refraction is…

Concept of Fiber Optics

Fiber optic circuits are hair-thin filaments of glass or plastic (10 to 300 microns) that transmit messages as light beams. These beams travel from end to end, even around curves, without interruption.

Fiber optics can replace copper wires in various settings, from small, self-contained environments (like aircraft data processing systems) to large geographical networks (like long-distance telephone lines).

Light transmission in fiber relies on total internal reflection. Light traveling through the fiber’s core hits the outer surface at an angle greater than the critical angle, causing complete reflection back into the core. This allows light to travel long distances, reflecting thousands of times. To minimize light scattering due to surface impurities, the core is coated with a lower-refractive-index glass layer. Reflections occur at the interface between the core and this cladding.

In essence, fiber optics is a superior light guide. Fiber optic signals experience less attenuation than copper signals, as information isn’t lost through refraction or scattering. Copper signals weaken due to the material’s resistance to electromagnetic wave propagation. Fiber optic cables can also carry multiple signals at different frequencies (multiplexing), similar to carrying multiple phone conversations on a single electrical wire. Fiber optics can also transmit light directly, among other advantages.

Manufacture of Fiber Optics

The images below illustrate single-mode fiber manufacturing. Each stage is shown in a short film sequence.

The first stage involves assembling a glass tube and rod concentrically. The assembly is heated to ensure the glass rod’s homogeneity.

A 1-meter-long, 10-cm-diameter glass rod can yield about 150 km of single-mode fiber.

What are fiber optics made of?

Most optical fibers are made from silica (sand), a more abundant raw material than copper. A few kilograms of glass can produce approximately 43 kilometers of fiber. The two main components are the core (innermost, light-guiding part) and the cladding (surrounding protective layer). The core consists of one or more thin glass or plastic strands (50-125 microns in diameter). A plastic or other material jacket protects the core and cladding from moisture, crushing, rodents, and other environmental risks.

How does Fiber Optics work?

A fiber optic transmission system includes a transmitter (active component) that converts electromagnetic waves into optical power (light). This light signal travels through the fibers to a receiver (optical detector) at the other end, which converts the light back into electromagnetic energy, recreating the original signal. The basic transmission system consists of: input signal, amplifier, light source, optical correction, optical fiber line (first section), splicing, fiber optic line (second section), optical correction, receiver, amplifier, output signal.

Simply put, fiber optics acts as a conduit for light signals generated by LED or laser transmitters.

LEDs and laser diodes are ideal for fiber optic transmission due to their quick control via current, small size, brightness, wavelength, and low operating voltage.

What devices are involved?

The main components of a fiber optic link are the transmitter, receiver, and fiber guide. The transmitter includes an analog/digital interface, voltage-to-current converter, light source, and source-to-fiber adapter. The fiber guide is an ultra-pure glass or plastic wire. The receiver includes a fiber-to-detector coupler, photodetector, current-to-voltage converter, amplifier, and analog/digital interface. The transmitter’s light source is modulated by an analog or digital signal, with impedance matching and amplitude limiting. The voltage-to-current converter acts as an electrical interface between input circuits and the light source (LED or laser diode). Light emission is proportional to the excitation current. The source-to-fiber coupler mechanically connects the source to the cable. The fiber has a core, cladding, and protective layer. The detector coupler is also a mechanical interface. The light detector (PIN diode or APD) converts light into direct current. A current-to-voltage converter transforms detector current changes into output voltage changes.

Components and Types of Optical Fiber

Fiber Optic Components

• Core: Made of silica, fused quartz, or plastic, this is where light propagates. Diameter: 50/62.5 μm (multimode) or 9 μm (singlemode).
• Cladding: Usually the same material as the core, but with additives to confine light within the core.
• Protective Coating: Typically plastic, this provides mechanical protection.

Optical Fiber Types

Singlemode Fiber

This fiber offers the highest potential information capacity (100 GHz/km bandwidth). While achieving high data rates, it’s complex to implement. Only light rays traveling along the fiber’s axis are transmitted, hence “single mode.” The core diameter is similar to the signal wavelength (5-8 μm). If the core and cladding have significantly different refractive indices, it’s a step-index singlemode fiber. While offering high bandwidth, its small size makes connection challenging.

Graded-Index Multimode Fiber

This fiber offers bandwidth up to 500 MHz/km. The core’s refractive index decreases gradually from center to cladding, focusing light rays towards the axis. This reduces dispersion between different propagation modes. The standard size is 62.5/125 μm (core/cladding), but other sizes exist (e.g., 100/140 μm step-index, 50/125 μm graded-index).

Step-Index Multimode Fiber

Made of glass (30 dB/km attenuation) or plastic (100 dB/km attenuation), this fiber offers bandwidth up to 40 MHz/km. The core has a uniform refractive index significantly higher than the cladding, creating a sharp index step. Download the graph for a visual representation.

What type of connectors are used?

Fiber optics use couplers and connectors:

• Couplers: These provide the mechanical connection between two connectorized fiber optic cables, allowing light to continue its path. “Hybrid” couplers can connect different connector designs.

• Connectors:
  1. The 568SC connector is recommended for maintaining polarity. The two connector positions on the 568SC adapter (A and B) ensure correct polarity and allow reverse polarity pairs.
  2. Existing BFOC/2.5 (ST-type) systems can still be used.
  3. Multimode connectors/adapters are ivory, while singlemode ones are blue.

Fiber optic termination requires connectors or spliced pigtails (connectorized armored cable) using fusion splicing. Various connector types exist, depending on application and standards.

• ST: Commonly used for singlemode/multimode fiber in data networking and multimode premises cabling.
• FC: Used for singlemode/multimode fiber in telephony and CATV (singlemode, angled singlemode).
• SC: Used for singlemode/multimode fiber, typically in singlemode telephone applications.

Characteristics of Optical Fiber

General Characteristics

• Stronger Covering: The special covering is extruded at high pressure directly onto the core, creating a helical inner edge that secures subcables. It contains 25% more material than conventional covers.
• Dual Use (Indoor/Outdoor): Water, fungus, and UV resistance, the tough covering, 900-micron buffer, 100-kpsi tested fibers, and broad environmental performance enhance reliability.
• Greater Moisture Protection: In gel-filled loose tube cables, gel channels can allow water migration to terminations, potentially shortening fiber lifespan. Multiple protective layers around the fiber combat moisture intrusion, increasing reliability in humid environments.
• Flame-Retardant Protection: New flame-retardant materials reduce the risks associated with older flammable gel-filled cables, improving safety and compliance with installation standards.
• High-Density Packaging: Maximizing fiber count in a smaller diameter simplifies installation in tight spaces with sharp bends. 72-fiber cables with 50% smaller diameters than conventional cables have been achieved.

Technical Features

Fiber transmits analog/digital information. Electromagnetic waves travel at light speed. Fiber consists of a core (transmission region) and a cladding (essential for propagation). Transmission capacity depends on: a) fiber geometry, b) material properties (optical design), and c) light source spectral width (wider width reduces capacity).

Fiber has a smaller footprint than other media. A 10-fiber cable (8-10 mm diameter) carries as much or more information than 10 coaxial tubes. Fiber optic cable is much lighter than metal wires, simplifying installation. Silica has a wide operating temperature range (melting point: 600°C). Fiber operates smoothly from -55°C to +125°C without degradation.

Mechanical Features

Individual optical fibers within a cable lack sufficient tensile strength for direct use. Protective coverings safeguard the fiber, especially in harsh environments. Key considerations include sensitivity to bending and microbending, mechanical strength, and aging characteristics.

Microbending and tension are assessed through tests for:

• Tension: Stretching/contracting forces exceeding the fiber’s elasticity can cause breakage or microbends.
• Compression: Lateral force.
• Impact: Primarily affects cable protection.
• Bending: Curvature angle limits exist, but the jacket prevents exceeding them.
• Torsion: Combined lateral and tensile stress.
• Thermal Stress: Varies depending on whether the fiber is glass or synthetic.

Minimizing additional losses and attenuation changes with temperature is crucial. Design choices sometimes prioritize other properties like strength, joint quality, fiber density, or production cost.

Advantages and Disadvantages of Fiber Optics

Advantages

• High-speed internet access (up to 2 Mbps).
• Continuous, unlimited 24/7 access without congestion.
• Real-time video and audio.
• Easy installation.
• Immunity to noise and interference.
• Secure transmission due to minimal light loss.
• No electrical signals, eliminating shock hazards and making it suitable for explosive environments.
• Smaller footprint and lighter weight than traditional media.
• High signal capacity.
• Abundant raw material (silica).
• Support for digital technology.

Disadvantages

• Limited availability (only in areas with fiber optic infrastructure).
• High connection and installation costs.
• Fiber fragility.
• Limited connector availability.
• Difficult field repair.

Applications of Optical Fiber

Internet

Fiber optic internet service overcomes cyberspace’s biggest limitation: slow speeds. While traditional connections are limited to 28,000 or 33,600 bps, fiber offers speeds up to 2 Mbps.

Networks

Fiber optics are increasingly used in networks due to light’s high frequency and information-carrying capacity. Long-distance communication networks utilize laser-based fiber optic systems for transcontinental and transoceanic connections. Fiber optic signals can travel much farther than electrical signals before requiring repeaters (100 km vs. 1.5 km). New optical amplifiers further extend this range.

Fiber optics are also prevalent in local area networks (LANs), connecting local subscribers (e.g., PCs, printers). This improves performance and simplifies adding new users. New optical and electro-optical components are further enhancing fiber system capacity.

LANs enable data, application, and resource sharing (e.g., printers). LAN computers are typically within a few kilometers of each other, commonly used in offices or campuses. LANs facilitate efficient information transfer within user groups and reduce costs.

Wide area networks (WANs) and private branch exchanges (PBXs) connect computing resources over larger distances. WANs are similar to LANs but connect computers across countries, requiring specialized hardware and leased communication services. PBXs provide continuous connections for data and telephone transmissions but are less suited for short-duration data bursts.

Telephony

Fiber optic transmission systems are widely used in public telecommunications networks due to standardized interfaces. However, their use in subscriber lines requires further consideration.

Existing copper wires suffice for basic telephone service. Fiber optics become essential for broadband services like video conferencing and video telephony. The BIGFON project (broadband integrated fiber optic telecommunications urban network) has provided valuable experience in this area. The current strategy involves expanding broadband services to include radio and television distribution within an integrated broadband telecommunications network (IBFN).

Comparison with Other Media

Compared to Coaxial Cables

Fiber Optics vs. Satellite Communications

Fiber optics are more cost-effective than satellite for short distances and high traffic volumes. For a 2000 km route, fiber is cheaper than satellite. Fiber optic signal quality is superior due to lower latency (under 100 ms vs. ~500 ms for satellite). Satellite’s high latency requires expensive echo suppressors, impacting reliability and quality. While satellite adapts to digital technology, its advantages are less pronounced in analog systems due to bandwidth requirements.