Graphical illustration of a fiber optic communication system (ii)

“It can be seen from the following figure that the optical fiber communication system is mainly composed of three parts: an optical transmitter, an optical fiber and an optical receiver…”

Composition of optical fiber communication systems

  The optical fiber communication system, like all communication systems, consists of a transmitter, a channel, and a receiver. The following diagram shows a schematic diagram of an optical fiber communication system. An optical fiber communication system uses an optical fiber as a channel.
  It can be seen from the following figure that the optical fiber communication system is mainly composed of three parts: an optical transmitter, an optical fiber and an optical receiver, and the electric terminal is an electronic device that processes electrical signals. At the sending end, the electrical terminal processes the electrical signal to be transmitted and sends it to the transmitter. The optical transmitter turns the electrical signal into an optical signal and couples the optical signal to the optical fiber. The optical signal is transmitted to the receiving end through the optical fiber. The optical receiver restores the received optical signal to the original electrical signal, and then processes the electrical end to send the message to the user.

FOT Blog: Optical fiber communication system diagram

Optical fiber classification

  Optical fiber is an abbreviation of Optical Fiber (OF). In an optical communication system, an optical fiber is often simplified to a fiber, such as a fiber amplifier or a fiber backbone.
  Actually, an optical fiber refers to a core made of a transparent material, and a light-transmitting medium composed of a material having a refractive index slightly lower than the refractive index of the core. The light signal injected into the core is reflected by the cladding interface so that the optical signal is The core propagates forward.
  There are many types of optical fibers, and depending on the application, the required functions and performance vary. However, the principles for the design and manufacture of optical fibers for cable television and telecommunications are basically the same:

  • Low loss
  • Have a certain bandwidth and small dispersion;
  • Easy wiring;
  • High reliability
  • Manufacturing is relatively simple;
  • Cheap and so on.

  Fibers are classified according to operating wavelength, refractive index profile, transmission mode, raw materials, and manufacturing methods.

  1. Divided by working wavelength, there are ultraviolet optical fiber, considerable optical fiber, near-infrared optical fiber, infrared optical fiber (850nm, 1300nm, 1550nm) and so on.
  2. According to the distribution of refractive index, there are step (SI) type, near step type, gradient (GI) type, and others (such as triangular type, W type, convex type).
  3. According to the transmission mode, there are single-mode optical fibers (including polarization maintaining optical fibers, non-polarization maintaining optical fibers), and multi-mode optical fibers.
  4. According to raw materials, there are quartz glass, multi-component glass, plastics, composite materials (such as plastic cladding, liquid core, etc.), infrared materials and so on. According to the covered material can also be divided into inorganic materials (carbon, etc.), metal materials (copper, nickel, etc.) and plastics.
  5. According to the manufacturing method, there are vapor axial deposition (VAD), chemical vapor deposition (CAD), etc., drawing method includes the tube in method (Rod intube) and double crucible method.

Optical fiber line

  Fiber optic lines consist of fiber optics, fiber optic connectors, and fiber optic connectors. The optical fiber is the main body of the optical fiber line, and the optical fiber used in the optical fiber communication is mainly a fiber filament drawn from quartz glass. In a fiber optic communication system, an optical signal is transmitted as a channel from an optical transmitter to an optical receiver. Two important parameters that characterize the transmission characteristics of optical fibers are loss and dispersion, both of which affect the transmission distance and transmission capacity of optical fiber communication systems. The loss of the optical fiber directly determines the transmission distance of the optical fiber communication system, and the dispersion of the optical fiber causes the optical pulse to be broadened when transmitted in the optical fiber. Therefore, the dispersion of the optical fiber affects the transmission code rate or the communication capacity of the system.
  The development of optical fiber technology is centered on further reducing the loss and dispersion of the optical fiber. For example, from step fiber to graded fiber, from the short wavelength to the long wavelength, from the multimode fiber to the single mode fiber, the loss and dispersion of the fiber is continuously reduced, thereby greatly increasing the communication distance and communication capacity of the fiber communication system.

Optical transmitter

  The function of the optical transmitter is to convert the electrical signal into an optical signal and couple the optical signal into the optical fiber. The basic composition of the optical transmitter is shown in the figure below. It consists of a light source and a driver circuit. The light source is the “heart” of the optical transmitter. In optical fiber communications, semiconductor lasers (LDs) or semiconductor light emitting diodes (LEDs) are commonly used. The input electrical signal realizes modulation of the light source through the driving circuit, that is, directly modulates the injection current of the semiconductor light source, so that the intensity of the output optical signal changes with the input electrical signal, which is the commonly used direct light emphasis system (IM).

FOT Blog: Basic composition of optical transmitter

  The transmitted optical power is an important parameter of the optical transmitter, usually based on 1 mw and in dBm.
The emission power of the LED is low, generally less than -10dBm, and the transmission power of the semiconductor laser can reach 0-10dBm. Due to the limited ability of LEDs to operate, most high-performance fiber-optic communication systems use semiconductor lasers as light sources. The code rate is another important parameter of the optical transmitter. It is generally limited by the electronic circuit rather than the semiconductor laser itself. If designed well, the optical transmitter can have a bit rate of 10 to 15 Gb/s.

Optical receiver

  The role of the optical receiver is to restore the optical signal transmitted through the optical fiber to the original electrical signal. The basic composition of the optical receiver is as shown in the figure below. It consists of an optical detector and an amplifier circuit. Optical detectors are an important part of optical receivers. In optical fiber communications, semiconductor PIN photodiodes or semiconductor avalanche photodiodes (APDs) are usually used as optical detectors, which can convert the dimming signals transmitted from the optical fiber into corresponding ones. electric signal. Since the optical signal transmitted through the optical fiber is weak (down to the nanowatt level), the detected electrical signal is amplified by the amplifying circuit after the optical detection.

FOT Blog: Basic composition of optical receiver

  The receiver sensitivity is an important parameter of the optical receiver and it is a comprehensive indicator of the quality of the optical receiver. It reflects the ability of the receiver to receive weak optical signals when it is adjusted to the best state. The sensitivity of a digital optical receiver is generally defined as the minimum noise at a certain bit error rate, including the receiver internal noise (thermal and electrical noise), the optical transmitter noise (relative intensity noise), and the optical signal during the transmission of the optical fiber. Introduced noise and so on.

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