Optical Fiber: Principles, Types, and Applications
1. Understanding Optical Fiber and its Operation
An optical fiber is essentially a thin, flexible rod of transparent material, such as glass or plastic, designed to guide light along its length. It consists of a central **core** surrounded by a **cladding** layer.
Light pulses, representing data, are transmitted through this very fine strand of material.
2. Refractive Index and Light Speed
When light travels through a transparent medium denser than a vacuum, it logically moves at a slower speed than the speed of light in a vacuum.
The speed of light in a vacuum can be approximated as:
a. c = 300,000 km/s (or 300 mm/µs), where c represents the speed of light in a vacuum.
b. To determine the new speed of light within the medium, we use the following equation:
c. New speed of light = Speed of light in a vacuum
Refractive index
Each fiber strand comprises a core made of plastic or glass (silicon oxide and germanium) with a high refractive index. This core is enveloped by a layer of similar material with a slightly lower refractive index. When light encounters a boundary with a lower refractive index, a significant portion of it is reflected. The greater the difference in refractive indices and the larger the angle of incidence, the more light is reflected. This phenomenon is known as **total internal reflection**.
The fiber’s operation relies on transmitting a light beam through the core in such a way that it doesn’t pass through the cladding but instead reflects and continues to propagate. This is achieved when the core’s refractive index is greater than the cladding’s, and the angle of incidence exceeds the critical angle.
3. Critical Angle and Total Internal Reflection
A special case of refraction occurs when the refracted ray travels along the boundary between two media, perpendicular to the normal line.
This is known as the **critical angle** of refraction, and it can be calculated as follows:
- The critical angle of refraction helps classify light beams based on their angles of incidence relative to the critical angle.
- For angles smaller than the critical angle, the light beam is refracted and escapes from the denser medium into the less dense medium.
- For angles larger than the critical angle, the light is reflected within the denser medium.
- This phenomenon is called **Total Internal Reflection** and forms the fundamental principle behind the operation of all optical fibers.
4. Attenuation in Optical Fiber
Attenuation refers to the loss of power in a light signal as it propagates through the transmission medium. Its coefficient is related to the distance.
- Attenuation:
- Rayleigh Scattering: Caused by the scattering of light due to irregularities, discontinuities, and inhomogeneities in the material, especially when imperfections are ≤ λ. It’s inherent in all transparent materials and proportional to 1/λ4.
- Material Absorption: Results from the interaction between light and matter.
- Intrinsic absorption is caused by the mechanical resonance of glass molecules (absorbing infrared energy) and transitions from stimulated electronic bands (absorbing ultraviolet energy).
- Extrinsic absorption is caused by impurities like transition metal ions.
- Linear Dispersive Attenuation: Arises from the coupling of different distribution modes, as well as tensions and extreme curvatures.
- Nonlinear Dispersive Attenuation: A complex phenomenon involving the emergence of higher modes (generation of different, higher frequencies). Known as stimulated Raman and Brillouin emission, it’s used in optical amplifiers and requires relatively high power levels.
5. Dispersion in Optical Fiber
Depending on the core’s refractive index profile, there are two main types of multimode fiber:
Step-index: In this type, the core has a uniform refractive index throughout its cylindrical section. It exhibits high modal dispersion.
Graded-index: Here, the refractive index varies gradually across the core, resulting in lower modal dispersion. The core is typically composed of different materials.
Power loss through the medium, known as **attenuation**, is expressed in decibels (dB) with a positive value. It’s caused by factors such as reduced bandwidth, speed, and efficiency. Multimode fibers generally experience greater loss due to light scattering caused by impurities. The primary causes of loss in the medium include:
- Absorption losses
- Rayleigh scattering loss
- Chromatic dispersion
- Radiation losses
- Modal dispersion
- Coupling losses
6. Types of Optical Fibers
The different paths a light beam can take inside a fiber are called **propagation modes**. Based on these modes, there are two primary types of optical fiber: **multimode** and **single-mode**.
Multimode fiber allows multiple light beams to travel along different paths or modes. Consequently, not all beams arrive simultaneously.
Single-mode fiber is designed to propagate only one mode of light. This is achieved by reducing the core diameter to a size (8.3 to 10 microns) that permits only a single mode of propagation.
7. Methods for Joining Optical Fibers
Fusion Splicing: Involves permanently joining fibers by fusing them together.
Mechanical Splices: Fibers are inserted into an alignment mechanism and then secured with epoxy adhesive.
Alignment mechanisms include:
- V-groove: Carved into a metal, ceramic, or plastic substrate.
- Three-cylinder base: Fibers are aligned using three tubes.
- Tube set base: Fibers are inserted into a glass tube or sleeve with precisely circular holes (3 mm larger than the fiber diameter).
- Square-type base: Fibers are inserted into a square-section tube at an angle, directing them towards a corner.
The epoxy adhesive also serves as a refractive index matching element. Optimization can be achieved by rotating one fiber. Insertion losses of 0.1 to 0.5 dB are typically achieved.
Mechanical Splices: A horizontally divided tube with a V-type base at the bottom and a flat top. The space between is filled with an index-matching gel. Fibers are inserted (at a fixed length) and then sealed with a pressure staple to push them together. There are versions for planar multi-fiber connections.
Electric Arc Fusion Splicing:
- Cleaning: All protective coatings must be removed from the fiber ends to be spliced. Side-cutting pliers are used. The fiber is then carefully cleaned with a solvent (usually acetone).
- Alignment: Splicing is performed on the faces of two fibers, preparing them for fusion. Several methods can be used depending on the type of joint.
Flame Fusion Splices: Follow the same process as arc fusion until the pre-fusion stage.
Mechanical Splices: Require cleaning and cutting processes.