Advantages of Fiber Optic Communication
● Large communication capacity
● Long relay distance
● Immune to electromagnetic interference
● Abundant resources
● Lightweight and small size of fiber optics
A Brief History of Optical Communication Development
● Over 2000 years ago: Beacon towers – lights, flag signals
● 1880: Optical telephone – wireless optical communication
● 1970: Fiber optic communication
● 1966: Dr. Kao Kuen, the "father of fiber optics," first proposed the idea of fiber optic communication.
● 1970: Kapron of Corning Institution produced fiber optics with a loss of 20 dB/km.
● 1977: Chicago's first commercial 45 Mb/s line.
Electromagnetic Spectrum
Refraction/Reflection and Total Internal Reflection of Light
Because light travels at different speeds in different substances, when light travels from one substance to another, refraction and reflection occur at the interface between the two substances. Furthermore, the angle of refracted light changes with the angle of incident light. When the angle of incident light reaches or exceeds a certain angle, the refracted light disappears, and all the incident light is reflected back; this is total internal reflection. Different materials refract light of the same wavelength at different angles (i.e., different materials have different refractive indices), and the same material refracts light of different wavelengths at different angles. Fiber optic communication is based on these principles. Reflectivity distribution: An important parameter characterizing optical materials is the refractive index, denoted by N. The ratio of the speed of light C in a vacuum to the speed of light V in a material is the refractive index of the material.
N＝C/V
The refractive index of the quartz glass used in fiber optic communication is approximately 1.5.
Fiber Optic Structure
A bare fiber optic cable generally consists of three layers:
First layer: A high-refractive-index glass core (core diameter is generally 9-10μm, (single-mode) 50 or 62.5 (multimode).
Second layer: A low-refractive-index silicon glass cladding in the middle (diameter is generally 125μm).
Third layer: An outermost reinforcing resin coating.
Basic Knowledge of Light
1) Core: High refractive index, used to transmit light;
2) Coating: Low refractive index, together with the core, forms conditions for total internal reflection;
3) Jacket: High strength, can withstand greater impact, and protects the optical fiber.
3mm Fiber Optic Cable Orange MM Multimode
Yellow SM Singlemode
Fiber optic cable dimensions:
Outer diameter is typically 125µm (an average human hair is 100µm)
Inner diameter: Single-mode 9µm, Multimode 50/62.5µm
Numerical Aperture
Not all light incident on the end face of an optical fiber is transmitted through the fiber; only light incident within a certain angular range is transmitted. This angle is called the numerical aperture of the optical fiber. A larger numerical aperture is advantageous for fiber optic splicing. The numerical aperture varies between optical fibers manufactured by different companies.
Types of Optical Fibers
Based on the transmission mode of light within the fiber, optical fibers can be classified as:
Multi-Mode (MM)
Single-Mode (SM)
Multi-Mode Fiber: Has a thicker central glass core (50 or 62.5 μm), allowing the transmission of multiple modes of light. However, its intermodal dispersion is significant, limiting the frequency of transmitted digital signals, and this limitation worsens with increasing distance. For example, a 600 MB/km fiber will only have a 300 MB/km bandwidth at 2 km. Therefore, multi-mode fiber has a relatively short transmission distance, typically only a few kilometers.
Single-Mode Fiber: Has a thinner central glass core (typically 9 or 10 μm in diameter), allowing only one mode of light to be transmitted. It is essentially a type of step-index fiber, but with a very small core diameter. Theoretically, it only allows a single path of straight-line light to enter the fiber and propagate in a straight line within the core. Fiber pulse broadening is minimal. Therefore, its intermodal dispersion is very small, making it suitable for long-distance communication. However, its chromatic dispersion plays a major role, which means that single-mode fiber has high requirements for the spectral width and stability of the light source, that is, the spectral width must be narrow and the stability must be good.
Fiber Optic Classification
By Material:
● Glass Fiber: Both the core and cladding are glass. Low loss, long transmission distance, high cost.
● Silicone Fiber with Sheath: The core is glass, the cladding is plastic. Similar characteristics to glass fiber, lower cost.
● Plastic Fiber: Both the core and cladding are plastic. High loss, very short transmission distance, very low price. Widely used in home appliances, audio equipment, and short-distance image transmission.
● By Optimal Transmission Frequency Window:Conventional Single-Mode Fiber and Dispersion-Shifted Single-Mode Fiber.
● Conventional: Fiber manufacturers optimize the transmission frequency of the fiber for a single wavelength, such as 1300nm.
● Dispersion-Shifted: Fiber manufacturers optimize the transmission frequency of the fiber for two wavelengths, such as 1300nm and 1550nm.
● Abruptly Shifted: The refractive index changes abruptly from the core to the glass cladding. Low cost, high intermodal dispersion. Suitable for short-distance, low-speed communication, such as industrial control. However, single-mode fibers have very low intermodal dispersion, so they are all graded-index fibers.
●Graded-index fibers: The refractive index gradually decreases from the core to the cladding, allowing high-mode light to propagate sinusoidally. This reduces intermodal dispersion, increases fiber bandwidth, and extends transmission distance, but it is more expensive. Most multimode fibers today are graded-index fibers.