What is a Fiber Optic Cable?
Optical fiber is a very thin strand of pure glass which acts as a waveguide for light over long distances. It uses a principle known as total internal reflection. Fiber optic cable is composed of two layers of glass, the core, which carries the actual light signal, and the cladding, which is a layer of a glass surrounding the core. The cladding has a lower refractive index than the core. This causes total internal reflection within the core. Total internal reflection in fiber optic cable is the optical phenomenon in which waves arriving at the interface from one medium to another are not refracted into the second medium but completely reflected into the first medium. Most fibers operate in duplex pairs i.e., one fiber is used to transmit and the other is used to receive. But it is possible to send both signals over a single strand. Optical fiber has advantages like high carrying capacity and very broad bandwidth, THz or Tbits/s. It has low transmission losses and does not dissipate heat. They are immune to cross-talk and electromagnetic interference, which are discussed below.
Working of Fiber Optic Cables
A fiber optic cable consists of one or more strands of glass, each only slightly thicker than a human hair as shown in the figure above. The center of each strand is called the core, which provides the pathway for light to travel. The core is surrounded by a layer of glass called cladding that reflects light inward to avoid loss of signal and allow the light to pass through bends in the cable. As the cladding has a lower refractive index than the core, therefore total internal reflection occurs within the core of the fiber optic cable. In fiber optic cable, total internal reflection occurs which is the optical phenomenon in which waves arriving at the interface from one medium to another are not refracted into the second medium but completely reflected onto the first medium. Thus, helping in the propagation of light through the optical fiber with minimum loss.
The two primary types of optical fiber cables are single-mode and multi-mode. Single-mode fiber uses extremely thin glass strands and a laser to generate light, while multi-mode optical fiber cables use LEDs. Single-mode optical fiber networks often use Wave Division Multiplexing techniques to increase the amount of data traffic that the strand can carry. WDM allows light at multiple different wavelengths to be combined and later separated, effectively transmitting multiple communication streams through a single light pulse.
Advantages of Fiber Optic Cables
Fiber cables offer several advantages over long-distance copper cabling. Fiber optics support a higher capacity, the amount of network bandwidth a fiber cable can easily exceed that of a copper cable with similar thickness. 10 Gbps, 40 Gbps, and 100 Gbps are standard Fiber cables.
Because light can travel for much longer distances over a fiber cable without losing its strength, the need for signal boosters is lessened.
A fiber optic cable is less susceptible to interference. A copper network cable requires shielding to protect it from electromagnetic interference. While this shielding helps, it is not sufficient to prevent interference when many cables are strung together in proximity to one another. The physical properties of fiber optic cables avoid most of these problems.
Different Types of Fiber Optic Cables
The optical fiber cable is classified based on three factors – the refractive index, the materials used, and the mode of propagation of light.
The basis of the refractive index fiber optic cables is of two types:
Step Index Fibers:
Step index fibers are optical fibers with a uniform refractive index at the core. The refractive index sharply decreases at the core-cladding interface so the refractive index of the cladding is lower than the core. This kind of index profile is mainly seen in single-mode fibers as well as in some multimode fibers. Step index fibers are generally manufactured by doping high-purity silica glass with different concentrations of materials like titanium, germanium, or boron.
Graded Index Fibers:
Graded index fibers are optical fibers whose core refractive index decreases as it moves away from the core center, and gradually approaches the cladding refractive index at the interface. The center of the core in such fibers has the maximum refractive index. The refractive index of the optical fiber decreases as the radial distance from the fiber axis increases. Since the part of the fiber core close to the fiber axis has a higher refractive index than the part near the cladding, the light rays follow a sinusoidal path through the fiber. This kind of index profile is seen in multimode fibers.
The main advantage of using graded index fibers over a step-index fiber is to minimize intermodal dispersion. A multimode fiber contains multiple modes and these modes do not necessarily travel with the same velocity due to dispersions. Thus each mode reaches at different times at the end of the fiber. This is known as intermodal dispersion.
In a step-index fiber, since the core has a uniform refractive index, there is no variation in the velocity within the core. So rays either travel through the center or in a zig-zag motion. Thus, the modes which enter the fiber at higher angles take a longer time to travel through the fiber which contributes to modal dispersion.
In graded-index fiber, the light follows a curved path. The modes that enter at large angles travel mostly through the low-index region in the core which allows them to travel faster than the high-index regions. This compensates for the longer paths of these modes and thus reduces the modal dispersion greatly.
Based on materials, fiber optic cables is of two types:
Plastic Optical Fibers:
Plastic fiber (POF) is made of highly transparent polymers such as polystyrene (PS), polymethyl methacrylate (PMMA), polycarbonate (PC) as the core material, PMMA, fluorine plastics as the cortex material of a kind of optical fiber (optical fiber). Excellent short distance data transmission medium. Polymethyl methacrylate is used as a core material for the transmission of light.
Glass Fibers:
Glass fiber or glass fiber is a material consisting of numerous extremely fine fibers of glass. Glass fibres are used as reinforcement of polymers in various fields such as aerospace, automobile, marine, sporting and leisure goods, and construction and civil engineering. A principal advantages of using glass fibers for reinforcement of polymers is their high performance per cost ratio.
Based on the mode of propagation of light, fiber optic cables are divided into:
Single-Mode Fibers:
Single-mode fibers are those fibers that allow only the propagation of a single value of incident angle through it. Any ray of light which comes within the acceptance angle can propagate inside an optical fiber. But all the rays which enter the fiber cannot sustain inside it. This happens because the rays which propagate via total internal reflection through the fiber can interfere among themselves. So only certain allowed angles of incidence which can constructively interfere inside the fiber can sustain inside it. These specific angles correspond to the Modes of the system. The allowed angles inside a fiber or the modes of a fiber depend on various factors, like, the wavelength of the incident light inside the fiber, the angle at which the ray is incident at the core-cladding interface, and also the diameter of the core of the fiber.
Fiber is categorized as a single-mode or multimode based on a factor known as the V number or normalized frequency. It is a dimensionless quantity that determines the number of modes in an optical fiber. It is defined as,
We know that the numerical aperture of fiber is given by,
where n1 and n2 correspond to the refractive indices of the core and cladding of the fiber respectively and a is the radius of the core of the fiber. Single-mode fibers are used for long-distance transmission of signals. Single-mode optical fibre allows one type of light mode to be propagated at a time. Single-mode fiber cables can be used for long-distance applications. Single-mode fiber optic cable‘s core diameter is up to 9 µm.
Multimode Fibers:
Used for short-distance transmission of signals. Multi-mode fiber cable can propagate multiple modes. The allowed angles inside a fiber or the modes of a fiber depend on various factors, like, the wavelength of the incident light inside the fiber, the angle at which the ray is incident at the core-cladding interface, and also the diameter of the core of the fiber. Multimode fiber core diameters are from 50 µm - 62.5 µm, which enables the multimode fiber to have a higher “light-gathering” ability and simplify connections. The cladding diameter of single mode and multimode fiber is 125 µm.
From the above graph, if the V number is less than 2.405, it is seen that optical fiber can support only single-mode and if the V number is greater, then the fiber can support multiple modes. So for an optical fiber to support multimode operation, the V number should be greater than or equal to 2.405. This is achieved by adjusting the core radius of the fiber. So, multimode fibers usually have a larger core radius. Many modes can be simultaneously propagated through a multimode fiber and the output is an overlap of all the propagating modes.
Fiber optic deployments
There are numerous types of fiber optic deployments:
Fiber to the Home (FTTH) or Fiber to the Premise (FTTP): The fiber cable ends where the living space begins. For example, a box on a home’s exterior wall. This is a full fibre link.
Fiber to the Building (FTTB): The optical fibre terminates at the building’s perimeter. For example, consider the box in an apartment building’s basement.
Fiber to the Node (FTTN): Here, fiber optic cable terminates at the street cabinet, which could be miles away from the customer’s location.
Fiber to the Curb or Cabinet (FTTC): This is similar to FTTN, except that the fiber cable is terminated closer to the premises, less than a mile away. Copper wire is used for the last-mile connection.
Fiber to the Antenna (FTTA): When an antenna is fed with optical fibre, this occurs also known as fibre backhaul. To meet the high 5G bandwidth requirements, 5G fiber optics like an FTTA architecture is required.
Fiber to the Desk (FTTD): This is a common commercial fiber optic cable deployment. Fiber, like copper cable, is distributed from a telecommunication room to the equipment outlets and terminated at a faceplate.
Optic fiber cable use in various fields:
Optical fiber cables are used across a wide range of fields due to their flexibility, high bandwidth, and immunity to electromagnetic interference. In medicine, optical fibers are manufactured as extremely thin and flexible strands that can be inserted into lungs, blood vessels, and other hollow parts of the body. They are widely used in diagnostic and imaging instruments such as endoscopes, allowing doctors to observe internal organs without invasive surgery. In telecommunications, fiber optic cables are the backbone of modern communication systems, transmitting information as light pulses over long distances. Compared to traditional coaxial cables, optical fibers support much higher data rates and can carry thousands of simultaneous conversations with minimal signal loss.
In networking, optical fibers connect servers, data centers, and end users, significantly improving data transmission speed, accuracy, and reliability. Industrial and commercial sectors use fiber optics for imaging in hard-to-reach or hazardous environments, as sensors for temperature and pressure measurement, and for signal transmission in areas where electromagnetic interference is a concern, such as factories and automobiles. Fiber optic cables are also extensively used in Broadcast and Community Antenna Television (CATV) systems to deliver HDTV, video-on-demand, internet services, and other multimedia content. In defense and government applications, optical fibers serve as hydrophones in SONAR and seismic systems and are used for secure, lightweight, and EMI-resistant wiring in submarines, aircraft, and military vehicles.
Additionally, optical fiber cables play an important role in data storage and sensing applications. They are used not only for high-speed data transmission but also for imaging, illumination, and monitoring a wide range of physical parameters. Fiber optics are integral to research, development, and testing across medical, industrial, defense, and communication technologies, making them a foundational component of modern technological infrastructure.
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