Ⅰ. Introduction
With the development of optical materials, low-loss optical fiber materials were produced. The materials hugely alleviated the loss issue of transmitting signals, making long-distance information transmission possible. The birth of fiber optic cable helps large-scale data circulation and information processing. It become one of the most crucial infrastructures in the information age.
Ⅱ. Structural composition
The basic structure of fiber optic cable is very simple and consists of a few key components:
Core. Made of high refractive index glass. It’s essential to total internal reflection of the light signals traveling along the fiber.
Cladding. Made of low refractive index glass to form a refractive index difference with the core. It is important as it is needed for total internal reflection.
Coating. A layer that protects the fiber from being exposed to external elements.
Ⅲ. Working Principle
Digital storage of the signal. Information like text, images, and video are usually stored as digital signals in a computer as long sequences of 0s and 1s in the form of binary data.
From Digital Signals to Optical Signals. Digital signals are translated into varying light wave characteristics (for example, amplitude, frequency, phase, etc.) through a process called “modulation“. Then lasers or LEDs convert the modulated signals the modulated signals, and the optical signals then enter into the optical fiber.
Transmission of Optical Signal in Optical Fibre. The refractive index in the core (where the optical signal will travel) is high and in the cladding (where the optical signal can be reflected) is low. The difference value between these two refractive indexes leads to total internal reflection, which allows the optical signal to be “trapped” in the fiber and guides the optical signal running along the fiber.
Optical Fiber Amplifiers. Because of the physical limitations of transmission materials, optical fiber signals can never be perfectly reflected during transmission, there is always some attenuation, hence optical fiber amplifiers are used to amplify the signal strength, to transmit over longer distances.
Ⅳ. Types: Single-mode and Multi-mode
Fiber optic cable can be classified into single-mode fibers (SMF) and multi-mode fibers (MMF). “Mode” refers to the different paths in which the light signal can propagate through the fiber.
Structure
The SMF has a very small core diameter of about 8-10 microns and only allows the light signal to propagate in the center path of the core, whereas the MMF has a much larger core diameter, typically around 50 microns (common OM2, OM3, OM4 fibers) or 62.5 microns, and therefore allows the light signal propagate in multiple paths.
Evolution
During the initial stages of fiber optic transmission, MMF with larger cores was designed to transmit many light modes at the same time. However, with the development of fiber optic technology, SMF emerged. The introduction of SMF overcame many of the speed and distance transmission problems.
Single-Mode vs. Multi-Mode Fiber Comparison
The advantages of SMF are lower signal attenuation, allowing long-distance high-speed signal transmission with high signal quality and high bandwidth, but they are more costly and difficult to install.
MMF has many modes of propagation, hence it is susceptible to dispersion, thus speeding rate, distance, and bandwidth are not very high; however, it is easy to produce, low in cost, and easy to install and maintain, thus widely used for short-distance, low to medium speed signal transmission.
Ⅴ. Wavelength Division Multiplexing (WDM): Improving Fiber Utilization
Principle
WDM can transmit multiple signals simultaneously through a single fiber optic cable at varying wavelengths (colors); for instance, the same fiber using WDM can transmit simultaneously a signal at a wavelength of 1550nm and another at 1530nm.
Advantages and Disadvantages of WDM
WDM is a great benefit to both the efficiency and bandwidth of transmission signals, but the varying losses experienced by different wavelengths can lead to some signals suffering from greater attenuation than others.
Note
WDM means transmitting many signals each with a different wavelength over the same fiber without changing the signals in any way whether by amplitude, frequency and phase. This is unlike multi-mode fiber which transmits many signals at fixed wavelength but with differing characteristics, over the same fiber. A helpful analogy to use is to imagine WDM as dividing a fiber into many wavelength channels, whereas traditional fiber optic communication uses only one of those channels to transmit signals.
Classification of WDM: CWDM vs DWDM
WDM is further divided into Coarse Wavelength Division Multiplexing (CWDM) and Dense Wavelength Division Multiplexing (DWDM).
CWDM has a wider wavelength spacing, typically 20nm. DWDM has a narrower wavelength spacing, typically 0.8nm or 0.4nm.
Accordingly, CWDM supports 8 to 18 wavelengths, whereas DWDM can support 40, 80 and even 160 wavelengths.
This is why DWDM provides a much higher bandwidth than CWDM and is more suitable for long-distance transmission, but is more reliant on fiber amplifiers, whereas CWDM is more appropriate for medium to short-distance transmission.
Ⅵ. Advantages of Optical Fiber Transmission
High Speed and Low Latency. In the process of optical fiber travel, light signals can reach closely to the speed of light, enabling high-speed, low-latency signal transmission.
Low Loss. With the purity of fiber continuously improved, the absorption losses caused by fiber impurities are correspondingly reduced, making the signal transmission more efficient.
Long Distance. The low-loss character of fiber transmission promotes long-distance signal transmission, particularly SMF, can even transmit over millions of miles.
High Bandwidth. Compared with electrical signal, optical signal support much higher bandwidth. MMF admits simultaneous transmission of light signals with different characteristics, WDM enables parallel transmission of multiple wavelength signals. These ways help fiber optic cable face the bandwidth demands better.
Resistance to Interference. Unlike electrical signals, which are vulnerable to electromagnetic interference (EMI) and radio frequency interference (RFI), optical fibers are immune to these disruptions. This makes fiber optics ideal for environments with high levels of interference, such as industrial and military applications.
Ⅶ. Conclusion
Fiber optic cable has a widespread use in data centers, internet networks, and more, driving the development of the Internet of Things (IoT) applications such as smart cities and smart healthcare, industrial automation, and high-tech innovations like autonomous driving and aerospace.
In the future, optical fiber will increasingly integrate with the digital age, continuously advancing societal progress.
