Optical Fibres

The optical fibre is the medium for carrying the information from one point to another in the form of light. A basic fiber optic system consists of a transmitting device that converts an electric signal into in light signal and a receiver that accepts the light signal and converts it back to the electric signal. The Optic Fiber System may be from very simple to extremely sophisticated systems. Today, the long-distance telephone cable has now been replaced by Optical Fiber Cable. From the decades of 1980s onwards there has been phenomenal growth in the use & importance of Fiber Optic System.

Difference between Optical Fibres and Ordinary Cables

The difference is that signals are transmitted as light in optic communication. Conventional electronic communication relies on electrons passing through wires. Since fibers transmit signals as light rather than current, they are immune to electromagnetic interference which causes noisy data transmission in wires. Because of this, they are used where data security is critical.

• As no shielding is required against electromagnetic interference; they are smaller and more flexible, which makes installation much easier than metal cables.
• Fiber optic cable is more efficient than other cables as it has lower attenuation, mainly because light is not radiated out in the way that electricity is radiated from copper cables
• Non-conductive nature of fibers avoids spark hazards and damage to electronic equipment from power surges.
• They can transmit signals at higher speeds or over long distances.
• Copper wires are adequate for the vast bulk of computer data transmission over point to point links and local area networks. They react differently to tension as they are elastic and copper is inelastic.
• Optical fibers can be used to sense, illuminate, deliver laser power, display and image as well as to communicate. One disadvantage is that they are more expensive than other cables.

Optical Fibre Cable and Total Internal Reflection

In an optical fiber the light signal undergoes total internal reflection. The light hits the fibers at the glancing angle and is transmitted forward. They have different layers of glass protected in layers of buffers, namely, hard buffer, soft buffer, core glass, and cladding glass. The cladding glass has a low refractive index toward the core glass. When total internal reflection occurs the signal is transmitted. The soft and the hard buffer are protective coating which provide the necessary protection to the inner glass from external environments.

Type of Light Used in Optical Fibre Cable

Please note that Optical communication employs a beam of modulated monochromatic light to carry information from transmitter to receiver. The light spectrum spans a tremendous range in the electromagnetic spectrum, i.e.  extending from the region of  (10⁴ gigahertz to 10⁹ gigahertz) covering the far infrared to visible to near ultraviolet.

Construction material for Optical Fibres

The Optical Fibers are basically very thin strands of pure glass and many of them have diameter of a human hair. Most of them are made from silica, because of its higher refractive index. Other materials such as fluorozirconate, fluoroaluminate and chalcogenide, sapphire etc. are also used for making optical fibers for longer-wavelength infrared or other specialized applications.

Optical Fibres and attenuation coefficients

In the making of the Optical Fibers, the attenuation coefficients also matters. The attenuation coefficient is a quantity that characterizes how easily a material or medium can be penetrated by a beam of light, sound, particles or other energy or matter. A large attenuation coefficient means that the beam is quickly “attenuated” (weakened) as it passes through the medium, and a small attenuation coefficient means that the medium is relatively transparent to the beam.  The Plastic optical fibers (POF) have higher attenuation coefficients than glass fibers and that is why high attenuation limits the range of POF-based systems.

In comparison, Silica shows extremely low absorption and scattering losses of the order of 0.2 dB/km and has a fairly broad glass transformation range. Silica fiber also has high mechanical strength against both pulling and even bending, provided that the fiber is not too thick and that the surfaces have been well prepared during processing. Silica is also relatively chemically inert. In particular, it is not hygroscopic (does not absorb water), thus is comparatively better.

Doping of Silica

But as above mentioned, the silica has a refractive index of 1.5. For enhancing the quality of the Optical Fiber, the refractive index of the Silica is increased. The method is doping with other materials. These materials include the Germanium dioxide (GeO2) or Aluminum oxide (Al2O3). In certain situations, doping may also be used to lower the reflective index. The material typically used for lowering the refractive index is fluorine or Boron trioxide (B2O3)

Single-Mode and Multimode Fibers

There are two general categories of optical fiber: single-mode and multimode. Multimode fiber allows hundreds of modes of light to propagate through the fiber simultaneously. Additionally, the larger core diameter of multimode fiber facilitates the use of lower-cost optical transmitters.

On the other hand, the single mode fiber allows only one mode of light at a time to propagate and thus has smaller core.

Can we use Optical Fibers in Power transmission?

Yes. By using the photovoltaic cell. The optical fiber can be used to transmit power using a photovoltaic cell to convert the light into electricity, but it is NOT very efficient method. So, it is used in certain situations where it is desirable not to have a metallic conductor. Examples are devices used close to MRI machines, which produce strong magnetic fields.