Fiber Optic Cable

Fiber optic cable is a modern communication medium consisting of a core made of glass or plastic, designed to transmit light signals at high speeds with minimal loss. These cables transmit data via photons rather than traditional electrical signals, thereby playing a crucial role in communication needs requiring long distances and high bandwidth.

Fiber optic cable image (AAnatolia Agency)

History

The foundations of fiber optic communication were laid in the 19th century with John Tyndall’s demonstration of the principle of total internal reflection. In the 1960s, with the advancement of laser technology, it was demonstrated that glass fibers could transmit light over short distances.

• 1970s: The first commercial systems were established in the United Kingdom and the United States.
• 1988: The first transatlantic fiber optic cable was laid by Tyco Telecommunication.
• Türkiye: The first manual exchange was installed in 1909; fiber infrastructure became widespread in the 2000s. 4.5G services began to be delivered over fiber networks as of 2016.

Fiber Optic Structure

Fiber optic cables consist of three fundamental layers:

• Core: The central region made of pure glass through which light is transmitted.
• Cladding: An outer layer with a lower refractive index that ensures total internal reflection of light within the core.
• Coating/Buffer: A protective layer, typically made of plastic or polymer, that shields the fiber from external influences.

Fiber optic cable structure (medium)

Dimensions

• Singlemode fiber: 9 µm core, 125 µm cladding
• Multimode fiber: 50/62.5 µm core, 125 µm cladding

Operating Principle

Fiber optic cables operate based on fundamental principles of optical physics—particularly total internal reflection, critical angle, and refractive index difference. When light is directed into the core such that it cannot pass into the cladding, it undergoes total internal reflection within the medium. This allows light waves (photons) to be transmitted as signals over very long distances. This principle is applied at the optical boundary between the core, which has a higher refractive index, and the cladding, which has a lower refractive index.

1. By Material

• Glass Fiber: Offers the highest performance. Made from ultra-pure silicon dioxide.
• Plastic-Coated Silica Fiber: Features a glass core with a plastic cladding. Economical but less efficient.
• Plastic Optical Fiber: The lowest cost option and suitable for shortest distances.

2. By Refractive Index Profile

• Step-Index: Features a sharp transition in refractive index. Commonly used in short-distance applications such as automotive systems.
• Graded-Index: The refractive index gradually decreases from the core toward the outer edge. This reduces signal dispersion and enables more stable data transmission.

3. By Mode Type

• Singlemode: Transmits light in only one mode. Provides long-distance transmission (up to 50 km) and high bandwidth. Attenuation: ~0.25 dB/km.
• Multimode: Carries multiple light modes. Preferred for short-distance communication (300 m to 2 km). Attenuation: ~2.5 dB/km.

Standards

Fiber optic cables are classified according to standards established by international telecommunications and industry organizations. These standards vary based on the cable’s physical structure, bandwidth capacity, and application areas.

Multimode fibers: Widely used for short-distance, high-speed network connections. These cables are classified as OM1, OM2, OM3, and OM4. OM1 fibers have a 62.5 micrometer core diameter and offer low bandwidth. OM2, OM3, and OM4 have a 50 micrometer core diameter and represent variants capable of operating at progressively higher frequencies. OM3 and OM4 are particularly preferred for next-generation network applications such as 10 Gigabit Ethernet due to their ability to support longer distances.

Singlemode fibers: Used for longer-distance and higher-capacity data transmission. The most widely adopted international standards for this type include the ITU-T G.65x series. The G.652 standard is commonly used in local area networks within buildings and FTTx (Fiber to the x) applications. The G.655 standard covers non-zero dispersion-shifted (NZD) fibers that enable high-speed transmission with lower dispersion and are used in terrestrial long-haul networks. Finally, the G.657 standard defines bend-insensitive fiber structures and is preferred in access networks requiring tight installation conditions. These standards determine the physical durability and signal transmission performance of fiber optic cables, enabling appropriate selection for specific applications.

Singlemode Fibers

ITU-T standards:

• G.652.x: For local area and FTTx applications
• G.655.x: For long-distance, high-bandwidth applications
• G.657.x: Specifically designed for bend-insensitive structures

Advantages

• High bandwidth
• Immunity to electromagnetic interference
• Lightweight and small diameter (thinner than a human hair)
• Low signal loss over long distances
• Secure data transmission (difficult to tap)
• No risk of electrical shock as they do not carry electric current

Disadvantages

• High equipment and installation cost
• Requires precise installation and testing
• Cost-to-benefit ratio may be low in localized applications

Applications

• Telecommunications and internet infrastructure
• Backbone communication in campuses, residential complexes, and shopping malls
• Building automation systems
• Security systems, CCTV, fire alarms
• Medical devices (endoscopy, laser surgery)
• Aerospace and military applications
• Nuclear energy and magnetically sensitive facilities
• Geophysical observations: seismic activity, glaciers, permafrost, volcano monitoring

Scientific Applications (Sensor Use)

Fiber optic cables are not only data transmission media but also serve as high-sensitivity sensors:

• Seismology: Detection of earthquake epicenters, shock intensity, and fault lines
• Glaciers and Avalanches: Monitoring ice movement, glacial earthquakes, and snow mass dynamics
• Vulcanology: Measuring magma pressure and pre-eruption vibrations
• Permafrost: Environmental analysis of thawing frozen soil and greenhouse gas emissions

In these applications, microscopic imperfections in the cable affect the reflection time of light, enabling precise monitoring of environmental changes.

Fiber optic cables play a critical role in both traditional communication and scientific measurement fields. By enabling high-performance data transmission based on light, and by being usable even in complex environments, they have become the backbone of modern technologies.