Download Fiber Optics: Communication and Other Applications by Henry Zanger - A Non-Mathematical Approach
Fiber optics is a technology that uses thin strands of glass or plastic called optical fibers to transmit light signals over long distances. Fiber optics has revolutionized the field of communication and has many other applications in various domains such as sensing, imaging, lighting, and medicine. In this article, we will explore the basics of fiber optics, how it works, what are its advantages and disadvantages, and how it is used for different purposes.
Physics of Light
Before we dive into the details of fiber optics, we need to understand some basic concepts about light. Light is a form of electromagnetic radiation that travels in waves. The wavelength of light is the distance between two consecutive peaks or troughs of a wave, and the frequency of light is the number of waves that pass a point in one second. The wavelength and frequency of light are inversely related, meaning that the shorter the wavelength, the higher the frequency, and vice versa. The speed of light in a vacuum is constant and equal to about 300,000 kilometers per second, but it changes when it enters a different medium such as air, water, or glass. The refractive index of a medium is the ratio of the speed of light in a vacuum to the speed of light in that medium. The higher the refractive index, the slower the light travels in that medium.
When light travels from one medium to another with a different refractive index, it changes its direction. This phenomenon is called refraction. The angle of incidence is the angle between the incoming light ray and the normal (a perpendicular line) to the surface of the medium. The angle of refraction is the angle between the outgoing light ray and the normal. The relationship between these angles is given by Snell's law, which states that the product of the sine of the angle of incidence and the refractive index of the first medium is equal to the product of the sine of the angle of refraction and the refractive index of the second medium.
When light hits a boundary between two media at an angle greater than a certain value called the critical angle, it does not enter the second medium but reflects back into the first medium. This phenomenon is called total internal reflection. Total internal reflection is the principle behind fiber optics, as we will see later.
Principles of Fiber Optics
Fiber optics is based on the idea of guiding light through a thin and flexible fiber using total internal reflection. A fiber optic cable consists of two main parts: the core and the cladding. The core is the central part of the fiber where the light travels, and it has a higher refractive index than the cladding. The cladding is the outer layer that surrounds and protects the core, and it has a lower refractive index than the core. The difference in refractive index between the core and the cladding creates a boundary that reflects light back into the core when it reaches a certain angle.
When a light source such as a laser or an LED emits light into one end of a fiber optic cable, some of the light rays enter at an angle smaller than the critical angle and escape through the cladding. These rays are called leaky rays and they contribute to signal loss. However, some of the light rays enter at an angle greater than or equal to the critical angle and bounce back and forth inside the core without escaping through the cladding. These rays are called guided rays and they carry information along the fiber.
The performance and quality of fiber optic transmission depend on several factors such as attenuation, dispersion, bandwidth, and noise. Attenuation is the reduction in signal strength as it travels along the fiber due to absorption, scattering, or bending of light. Attenuation is measured in decibels (dB) per kilometer (km) and it varies with the wavelength and type of fiber. Dispersion is the spreading or distortion of signal pulses as they travel along the fiber due to different speeds or paths of light rays. Dispersion limits the data rate and distance of transmission and it can be classified into two types: modal dispersion and chromatic dispersion. Modal dispersion occurs when different modes (patterns) of propagation have different speeds or distances in multimode fibers. Chromatic dispersion occurs when different wavelengths (colors) of light have different speeds or distances in single-mode or multimode fibers. Bandwidth is the maximum amount of data that can be transmitted per unit time over a fiber optic cable. Bandwidth is measured in hertz (Hz) or bits per second (bps) and it depends on the attenuation, dispersion, length, and type of fiber. Noise is the unwanted or random variation in signal amplitude or quality due to external or internal sources such as electromagnetic interference, thermal fluctuations, or optical amplifiers. Noise affects the signal-to-noise ratio (SNR), which is the ratio of signal power to noise power, and it determines the bit error rate (BER), which is the probability of transmitting or receiving an incorrect bit.
Optical fibers are thin strands of glass or plastic that can transmit light signals over long distances with low loss and high bandwidth. Optical fibers are made by drawing molten glass or plastic through a tiny hole called a preform. The preform has a core surrounded by a cladding with different refractive indices to create total internal reflection. Optical fibers can be classified into two main types: 71b2f0854b