Optical fiber, short for optical fiber, is a fiber made of glass or plastic that acts as a light-transmitting tool. How many types of optical fiber can it be divided into? What specifications does it have? What are the explanations of terms related to transmission technology?
First, the classification of optical fibers
Optical fibers are divided into single-mode optical fibers (Single-mode) and multi-mode optical fibers (Multi-mode) according to the transmission mode.
Fiber Basics
Single-mode fiber: a single optical path that transmits only one mode of light (that is, only transmits a beam of light that enters the fiber from a certain angle). Since modal dispersion is completely avoided, the transmission frequency band of single-mode fiber is very wide, so It is suitable for large-capacity, long-distance transmission systems, using light-emitting diodes or lasers as light sources, and using two bands of 1310nm and 1550nm.
Multi-mode fiber: multiple optical paths, which can transmit light of multiple modes in one fiber at the same time, due to dispersion and phase difference, its transmission performance is poor, the frequency band is narrow, the capacity is small, and the distance is short, the laser is used as the light source , using two bands of 850nm and 1300nm.
1. Specifications of optical fibers (commonly used)
Single mode: 8/125μm, 9/125μm, 10/125μm
Multimode: 50/125μm European Standard; 62.5/125μm American Standard Industrial, Medical and Low Speed Network: 100/140μm, 200/230μm
Plastic: 98/1000μm for car control.
(Note: fiber inner diameter/fiber outer diameter)
2. Optical fiber loss
2. Explanation of terms related to transmission technology
1. Digital differential multiplexing
Refers to a kind of transmission of multiple digitized data, voice and video signals on the same channel at the same time by interleaving bit pulses in different channels or time slots.
2. Wavelength Division Multiplexing Technology
Wavelength division multiplexing (WDM) is to combine two or more optical carrier signals of different wavelengths (carrying various information) at the transmitting end through a multiplexer (also known as a multiplexer, Multiplexer), and coupled to the optical carrier. The technology of transmitting in the same fiber of the line; at the receiving end, the optical carriers of various wavelengths are separated by a demultiplexer (also known as a demultiplexer or a demultiplexer), and then the optical receiver is used for Further processing to restore the original signal. This technique of simultaneously transmitting two or more optical signals of different wavelengths in the same fiber is called wavelength division multiplexing.
3. CWDM (Coarse Wavelength Division Multiplexing) and DWDM (Dense Wavelength Division Multiplexing)
The design of the communication system is different, and the spacing width between each wavelength is also different. According to the different channel spacing, WDM can be subdivided into CWDM (Coarse Wavelength Division Multiplexing) and DWDM (Dense Wavelength Division Multiplexing).
CWDM: The wavelength interval is ≥20nm, usually eight bands from 1470 to 1610nm (one for every 20nm).
DWDM: The wavelength interval is less than 10nm, and the window is also around 1550nm. Optical devices are more expensive, and optical transceivers are not commonly used.
There are two main differences between CWDM and DWDM: First, the CWDM carrier channel spacing is wide, so the same optical fiber can be multiplexed to about 10 wavelengths of light waves, while DWDM can multiplex more than 10 wavelengths of light waves; Second, CWDM modulated lasers use uncooled lasers, while DWDMs use cooled lasers. Cooled lasers are temperature tuned and uncooled lasers are electronically tuned. Temperature tuning is difficult and expensive to implement because the temperature distribution is very non-uniform over a wide range of wavelengths. CWDM avoids this difficulty and thus greatly reduces the cost. The cost of the entire CWDM system is only 30% of that of DWDM.
4. Relay
Optical fiber transmission is also lossy, so the transmitted signal power will gradually attenuate, and when the attenuation reaches a certain level, the signal will be distorted, which will lead to receiving errors. The maximum transmission distance of the optical transceiver is about 120km. When this distance is exceeded, we complete it through relay. At present, there are three types of relays: 1, analog relay 2, digital relay 3, optical relay:
1) Analog relay: Convert the attenuated optical signal transmitted from a distance into an analog carrier signal, and then convert it into an optical signal after processing and send it away with a new light wave. Problem: The process of analog repeating itself will cause signal attenuation and distortion, and it is difficult to ensure signal quality and cannot be repeated many times.
2) Digital relay: Convert the attenuated optical signal from a distance into a digital electrical signal, and then convert it into an optical signal and use a new light wave to send it away. Since the digital signal can be error corrected and reproduced, signal attenuation and distortion will not occur due to multiple conversions, which can ensure the quality of signal transmission and can be repeated infinitely.
3)Optical relay: directly amplify the attenuated optical signal transmitted from a distance and continue forward transmission. This is the simplest way, the signal does not need to be converted in any way, but optical amplifiers are very expensive and difficult to use.
