Optical Communication (Part 1)

In this technical blog post and the 2 parts later, we cover almost everything about the optical communication platform which is a reliable and robust means to send and receive most of data, information, and signal.

Step-index Multimode fiber

In step-index multimode fibers, the refractive index is the same at all points in the core region, and from the common border with the cladding region, the refractive index changes stepwise. In the first fibers, this step-index design was used. The main defect of these types of fibers is that the light rays that enter these types of fibers at the same time travel different optical Optical communication is used in fields such as transmission of telecommunication signals, transmission of radio programs, computer networks, space research, and satellite communications. In optical communication, two environments are often proposed for optical signal transmission:

1- Optical fiber

2- Free space

Optical fiber was proposed as a suitable transmission medium for communication in the late 70s, and with the passage of a short time, it was widely used and welcomed by telecommunication companies. Currently, most of the communication traffic in the world is established on optical fibers and the demand for optical fiber communication is increasing.

Optical communication through free space or FSO was proposed in the 80s and is used in high-capacity terrestrial and satellite communications. FSO systems are used in terrestrial communications due to the ability to transmit high bandwidth and establish fast links in ways such as the connection of news stations with radio and television broadcasting centers, military information applications, and LAN and MAN computer networks.

FSO systems can operate over distances of several kilometers as long as there is a clear line of sight between the source and destination and the optical receiver can reliably decode the transmitted message. FSO systems in satellite communications can provide high-speed links with long-range by using small, low-mass, and low-power subsystems, which makes them suitable for communications in space.

Various satellite arrays intended to provide global broadband coverage take advantage of these advantages and use laser communications for inter-satellite links of several hundred to thousands of satellites, effectively creating an optical mesh network in space. FSO links are much less useful in sensitive and real-time terrestrial communications due to their sensitivity to rain, snow, and fog weather conditions, as well as the passage of birds on the path of the laser, which causes interruptions or disruptions in communication.

Figure 1-(a) FSO links in satellite communication (b) Destructive effects of atmospheric factors on optical signal propagation in FSO ground communication link

With progress in the construction of active and passive optical equipment, optical fiber networks with different topologies in wide areas, urban, local, residential areas, and data centers provide high-capacity communications with good service quality and speeds from several megabits per second to several terabits per second. The variety of services are PDH, SDH, OTN telecommunication networks, Ethernet networks up to 800 Gb/s, and digital cable networks with interactive multimedia services under FTTX technologies.

In the optical fiber connection according to Figure 2, the electrical signal containing information such as audio, video, and data is converted into an optical signal and then directed into the optical fiber. The optical signal is converted into an electrical signal in the receiver and is used.

Figure 2- Content signal transmission by optical fiber communication link  

Advantages of Optical fiber

Optical fiber has significant advantages over other transmission media. Very high bandwidth and as a result high communication capacity, low energy loss, low attenuation and as a result longer distances, immunity to electromagnetic wave interference (can be installed next to high voltage power lines), lack of cross-talk, cheapness and The abundance of raw materials in nature, the simplicity of capacity development according to daily needs, longer life, message safety or security against eavesdropping, and ease of installation and repair of cables are among the advantages of optical fibers.

The structure of optical fibers

Optical fiber is a cylindrical transmission medium made of glass or plastic, which consists of two core regions and cladding with different breaking coefficients and two primary and secondary coating layers made of plastic (Figure 3). According to Snell’s law, for light to propagate along the optical fiber, the condition n1>n2 must be established, where n1 and n2 are the coefficients of the core and cladding, respectively.

Figure 3- Optical fiber structure

Light emission is weakened under the influence of inherent and acquired factors. These factors are mainly caused by ultraviolet absorption, infrared absorption, Rayleigh scattering, bending, and mechanical pressures on them. The curve of attenuation changes in terms of wavelength is shown in Figure 4.

Figure 4- Optical fiber attenuation curve by wavelength

Types of Optical Fibers

Optical fibers are divided into two major categories, multi-mode fibers, and single-mode fibers, in terms of the way they emit light.

Multimode fibers

These types of optical fibers are mainly used in the part of optical communication that is responsible for sending short-distance communication data, including communication between computers and data centers. In this type of fiber, when light is emitted into it, different modes are simultaneously emitted from inside the core. The core diameter of existing multimode fibers is usually 50 and 62.5 microns. Multimode fibers are divided into two groups, step and graded, in terms of refractive index that called step-index and graded-index multimode fibers.

paths due to the difference in the propagation angle, and as a result, the light rays exit at the end of the path at different times, which practically create a phenomenon known as “Dispersion” in optical fiber communication. Due to the dispersion phenomenon, these types of fibers are not useful at high frequencies and long distances. In Figure 5, how the propagation in multimode fibers with stepped refractive index is shown schematically, where n1 and n2 are the refractive index of core and cladding, respectively.

Figure 5- Light propagation in step-index multimode fiber

Graded-index Multimode fiber

In graded-index multimode fiber, the refractive index changes gradually in the entire core region. The highest refractive index is in the center of the core, and the closer the refractive index is to the common surface with the cladding, the lower its value is, and at the common surface, the refractive index of the core and the fiber cladding is equal. Due to the gradual change of the refractive index, the light wave is also gradually broken. For this reason, the path of light transmission is formed in an oscillatory way. The speed of light will be different in different areas of the core due to the difference in the refractive index. In the center of the core, because the refractive index of the fiber is high, the speed of light naturally decreases, and in the region far from the center, with the gradual decrease of the refractive index, the speed of light gradually increases. As a result, different rays reach the end of the fiber with gradual propagation speeds almost at the same time and the dispersion effect will reach its minimum value. Figure 6 shows how light propagates in a fiber with a gradual refractive index, where n1 and n2 are the refractive index of the core and cladding, respectively.

Figure 6- Light propagation in graded-index multimode fiber

Single-mode optical fiber

Single-mode optical fibers are mainly used in optical communication and have a smaller core compared to multi-mode fibers. There are different types of single-mode optical fibers and they are divided according to the working wavelength. The most common single mode fibers are normal single mode with a working wavelength of 1300 nm, single mode with dispersion shifted (DSF) with a working wavelength of 1550 nm, single mode with non-zero dispersion shifted (NZDSF) with a working wavelength of 1530 to 1565 nm and single mode with dispersion flat (DFF). The diameter of the core in this type of fiber is 9 to 10 microns and the cladding diameter is 125 microns, and with the coating, the final diameter reaches 250 microns. Due to the small diameter of the core in these types of fibers, more than one mode is not propagated in them. In single-mode fibers, dispersion is very low, and for this reason, these types of fibers are suitable for long-distance optical communication networks. Figure 7 shows how light propagates in a single-mode fiber.

Figure 7 – Light propagation in single-mode optical fiber