What kind of propagation is used in optical fiber communication? Analysis of optical fiber communication principle

Fiber-optical communication (Fiber-opTIc communicaTIon), also used for fiber-optic communication. Optical fiber communication is a communication method in which light is used as an information carrier and optical fiber is used as a transmission medium. First, an electrical signal is converted into an optical signal, and then an optical signal is transmitted through the optical fiber, which is a type of wired communication. The light can be carried after being modulated. Since the 1980s, fiber-optic communication systems have revolutionized the telecommunications industry and played a very important role in the digital age. The optical fiber communication has the advantages of large transmission capacity and good confidentiality. Fiber-optic communication has now become the most important form of wired communication today.

Optical fiber communication is a communication method in which light waves are used as information carriers and optical fibers are used as transmission media. In principle, the basic material elements that make up fiber optic communication are fiber optics, light sources, and photodetectors. In addition to classification according to manufacturing process, material composition and optical characteristics, optical fibers are often classified according to their applications, and can be classified into communication optical fibers and sensing optical fibers. The transmission medium fiber is divided into general and special, and the functional device fiber refers to an optical fiber for performing functions of amplification, shaping, frequency division, frequency multiplication, modulation, and optical oscillation of light waves, and is often used for a certain functional device. The form appears.

At the transmitting end, the transmitted information (such as voice) is first converted into an electrical signal, and then modulated onto the laser beam emitted by the laser, so that the intensity of the light changes with the amplitude (frequency) of the electrical signal, and passes through the optical fiber. The principle of total reflection is transmitted; at the receiving end, the detector converts it into an electrical signal after receiving the optical signal, and recovers the original information after demodulation.

Optical communication utilizes the principle of total reflection. When the injection angle of light satisfies certain conditions, light can form total reflection in the optical fiber, thereby achieving the purpose of long-distance transmission. The light guiding properties of an optical fiber are based on total reflection of the light ray at the core and cladding interfaces, limiting light transmission in the core. There are two kinds of light in the fiber, namely meridian light and oblique light. The meridional light is the light light on the meridional plane, and the oblique light is the light that is not transmitted through the fiber axis.

The principle of optical communication is explained below by taking light transmission in a step fiber as an example.

As shown in the figure, a step-type fiber has a core refractive index of n1, a cladding refractive index of n2, and n1"n2, and an air refractive index of n0. The meridional light transmitted in the optical fiber, referred to as the inner light, is such that when the incident angle of the interface between the core and the cladding is greater than θc, it is ensured that the light generates multiple reflections in the core, so that the light is transmitted along the optical fiber. However, the incident angle of the internal light depends on the angle of refraction θ2 generated by the incident light from the air entering the core. Therefore, the incident angle θi of the incident light at the air and core interface defines whether the light can be in the optical fiber. It is transmitted in total reflection, corresponding to the critical angle θc of the incident angle of the internal light, and the incident angle θi of the incident light of the optical fiber has a maximum value θmax.

When light is incident on the end face of the core at θi θmax, the internal ray will be projected onto the core and cladding interface at an angle of incidence less than θc. Such light refracts in the cladding at an angle of less than 90 degrees, and the light will be incident on the cladding, exposing the fiber very quickly.

When light is incident on the end face of the core at θi "θmax, the incident ray will be projected into the core and cladding interface at an angle of incidence greater than θc within the fiber. Such light refracts more than 90 degrees in the cladding, which will produce multiple reflections at the core and cladding interfaces, allowing light to travel along the fiber.

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