Generally speaking, information transmission consists of three stages: information acquisition, information exchange, and information transmission.
Fiber optic communication is one of the information transmission links in this process.
Information transmission requires two necessary prerequisites:
a. A reliable signal source—in fiber optic communication, this is a laser.
b. A good transmission medium—in fiber optic communication, this means optical fiber.
With the successive emergence of these two necessary conditions, optical fiber communication began its rapid development, ushering in a new era for optical fiber communication, one of the most important transmission methods in the communications industry.
It's easy to imagine that atmospheric optical communication, utilizing the straight-line propagation of light in air, would require no wiring, making it simple and economical. In 1960, Thomas Maiman invented the ruby laser, producing monochromatic coherent light, making high-speed optical modulation possible.
Various atmospheric transmission system experiments followed, but it was later discovered that atmospheric transmission optical communication had many serious problems and was not practically usable.
In 1966, British-Chinese Kao Kuen and Hockham foresaw that glass could be used to make communication optical fibers (optical fibers) with an attenuation of 20 dB/km.
At that time, the best optical glass in the world had an attenuation of about 1000 dB/km.
In 1970, Corning Glass of the United States achieved a major breakthrough by first producing optical fiber with an attenuation of 20 dB/km, making optical fiber communication possible.
In 1974, fiber optic attenuation was reduced to 2 dB/km.
In 1980, optical fiber attenuation was as low as 0.2 dB/km (within the 1.55 μm long-wavelength low-attenuation window), close to the theoretical value. This made long-distance optical fiber communication possible.
Furthermore, due to continuous improvements in purification processes, the transmission window of optical fibers has shifted from a short wavelength window of 0.85μm to a long wavelength, low-attenuation window of 1.3μm and 1.55μm.
After 1976, various practical fiber optic communication systems emerged one after another. By 1980, many countries around the world had developed commercial fiber optic communication systems. From then on, fiber optic communication took a giant leap into the commercial era.
Since the commercialization of optical fiber, with the continuous development of technology, the varieties of optical fibers have gone through several important development stages.
Today, we will briefly review the process at each stage:
Phase 1: Multimode Fiber (First Window)
In July 1966, Chinese-American scientist Charles K. Kao published a landmark paper on the prospects of optical fiber transmission. This paper analyzed the main causes of transmission loss in optical fibers, theoretically explained the possibility of reducing loss to 20 dB/km, and proposed that such optical fibers could be used for communication.
In 2009, Kao Kuen was awarded the Nobel Prize in Physics for his outstanding contributions to the optical fiber industry.
Guided by the theory, four years later in 1970, Corning Incorporated in the United States actually produced optical fiber with a loss of 20 dB/km, proving the possibility of optical fiber as a communication medium.
Meanwhile, Bell Labs in the United States invented a semiconductor laser that uses gallium arsenide (GaAs) as a material. Due to its small size, it has been widely used in fiber optic communication systems.
In 1972, the transmission loss of optical fiber was reduced to 4 dB/km.
Thus began the era of fiber optic communication.
The period from 1972 to 1981 was the time for the research and application of multimode optical fibers.
The first optical fiber communication wavelength used was 850nm, which is called the first window.
Early development used step-index multimode fiber. Subsequently, A1a graded multimode fiber (50/125) was developed, with an attenuation of 3.0-3.5 dB/km, a bandwidth of 200-800 MHz·km, and a numerical aperture of 0.20±0.02 or 0.23±0.02.
Later, A1b type gradient multimode fiber (62.5/125) was developed and used, with an attenuation of 3.0-3.5dB/km, a bandwidth of 100-800MHz·km, and a numerical aperture of 0.275±0.015.
These two types of optical fibers, when combined with LEDs (light-emitting diodes) with wavelengths around 850nm, formed an early optical communication system.
At that time, the LED spectral width was 40nm, the injected light power was 5 or 20μW, and the maximum rate was 5 or 60Mb/s.
Phase Two: Multimode Fiber (Second Window)
In the late 1970s and early 1980s, fiber optic manufacturers developed a second window (1300nm).
Category A1a fiber has an attenuation of 0.8-1.5 dB/km and a bandwidth of 200-1200 MHz·km. Category A1b fiber has an attenuation of 0.8-1.5 dB/km and a bandwidth of 200-1000 MHz·km.
They are paired with high-emissivity LEDs with a spectral width of 120nm, an injected light power of 20μW, and a maximum rate of 100Mb/s.
Phase 3: G.652, G.653, and G.654 single-mode fiber (second and third windows)
The period from 1982 to 1992 was a period of large-scale application of G.652, G.653, and G.654 single-mode optical fibers, which opened up the second window (1310nm) and the third window (1550nm) for optical fibers.
Between 1973 and 1977, major optical fiber manufacturers around the world developed various advanced preform manufacturing processes—Corning developed OVD technology; NTT, Sumitomo, Furukawa, Fujikura, and others in Japan jointly developed VAD technology; Lucent improved MCVD technology; and Philips in the Netherlands developed PCVD technology.
In 1982, starting with the United States, and followed by countries such as Japan and Germany, a large-scale construction of G.652 single-mode fiber optic long-distance projects began globally. The surge in market demand for single-mode fiber optics stimulated large-scale production.
At this point, Corning's OVD further improved the deposition rate, and VAD, MCVD, and PCVD all used external sleeves as a measure to increase the size of the preform.
Subsequently, all companies followed the two-step mixing process to increase the size of the precast rods.
In the 1990s, Alcatel of France developed APVD technology (MCVD+ plasma spraying process).
Significant advancements in manufacturing technology by major fiber optic manufacturers have created better conditions for the widespread application of conventional single-mode fiber.
In 1984, the third window (1550nm) was put into use.
In the same year, the CCITT (International Telegraph and Telephone Consultative Committee) released the G.651 and G.652 standards.
By 1985, the loss of G.652 optical fiber at 1310nm had reached 0.35dB/km, and the loss at 1550nm had reached 0.21dB/km.
In 1985, the G.653 dispersion-shifted fiber, developed by Japan and the United States, was commercialized. Its feature is that the zero dispersion point is moved from the second window to the third window. The 1550nm wavelength not only has the lowest loss, but also the smallest dispersion.
In 1988, CCITT released the G.653 standard. This optical fiber was widely used in Japan's communication trunk lines.
In the early 1990s, the commercialization of erbium-doped fiber amplifiers (EDFAs) spurred the development of dense wavelength division multiplexing (DWDM).
However, the zero dispersion of G.653 fiber at a wavelength of 1550nm causes severe nonlinear interference between channels in DWDM systems, which is why it has not been widely adopted worldwide.
In 1995, my country constructed the Beijing-Kowloon optical cable project, using six G.653 optical fibers in a 24-core cable, but it was never put into operation. Subsequently, my country did not use G.653 optical fiber again.
During this period, a type of optical fiber with a shifted cutoff wavelength was also developed. It not only has low loss at 1550nm, but also low microbending loss, making it suitable for long-distance trunk systems and submarine optical cable systems that use optical amplifiers.
In 1988, CCITT released the G.654 standard.
Phase Four: Fiber Optic Window Fully Open, Comprehensive Development of Characteristics
From 1993 to 2006, the optical fiber communication window was expanded to the 4th and 5th windows and the S-band, and the optical fiber communication window was fully opened. Four new types of optical fibers were developed, and the characteristics of optical fibers became more perfect.
(1) Non-zero dispersion shifted single-mode fiber G.655 fiber (third and fourth windows)
To suppress four-wave mixing (FWM) and cross-phase modulation (XPM) in dense wavelength division multiplexing (DWDM) systems and reduce nonlinear interference between optical channels, non-zero dispersion shifted fiber (WZDSF) was introduced in 1993.
First, Lucent launched true wave fiber, and then Corning launched large effective area LEAF fiber.
These optical fibers initially operated in the third window, the C-band (1530-1565nm). After 1995, they were extended to the fourth window, the L-band (1565-1625nm).
In 1996, the ITU-T developed the G.655 standard. Since 1998, it has been widely used worldwide.
(2) Low water peak single-mode fiber G.652C (fifth window)
In 1998, Lucent launched full-wave fiber (i.e., low water peak fiber), which made the 1383nm water peak almost non-existent (attenuation <0.31dB/km), opening the fifth window of optical fiber, namely the E-band (1360-1460nm).
In 1999, China began using full-wave optical fiber to make optical cables for Jiujiang Telecom.
In 2000, ITU-T developed the G.652C standard.
In 2001, Corning developed low water peak optical fiber.
In 2002, G.652C optical fiber was promoted worldwide.
From then on, single-mode optical fiber has excellent attenuation performance in the wavelength range of 1260nm to 1625nm.
Optical communication uses light waves to carry information and achieve communication. Optical fiber is a transmission medium, similar to the transmission through copper wires or cables, but unlike copper wires or cables, it carries optical signals. Fiber optic communication is a communication method that uses optical fibers as the transmission medium.
Generally speaking, information transmission consists of three stages: information acquisition, information exchange, and information transmission.
Fiber optic communication is one of the information transmission links in this process.
Information transmission requires two necessary prerequisites:
a. A reliable signal source—in fiber optic communication, this is a laser.
b. A good transmission medium—in fiber optic communication, this means optical fiber.
With the successive emergence of these two necessary conditions, optical fiber communication began its rapid development, ushering in a new era for optical fiber communication, one of the most important transmission methods in the communications industry.
It's easy to imagine that atmospheric optical communication, utilizing the straight-line propagation of light in air, would require no wiring, making it simple and economical. In 1960, Thomas Maiman invented the ruby laser, producing monochromatic coherent light, making high-speed optical modulation possible.
Various atmospheric transmission system experiments followed, but it was later discovered that atmospheric transmission optical communication had many serious problems and was not practically usable.
In 1966, British-Chinese Kao Kuen and Hockham foresaw that glass could be used to make communication optical fibers (optical fibers) with an attenuation of 20 dB/km.
At that time, the best optical glass in the world had a degradation of about 1000 dB/km.
In 1970, Corning Glass of the United States achieved a major breakthrough by first producing optical fiber with an attenuation of 20 dB/km, making optical fiber communication possible.
In 1974, fiber optic attenuation was reduced to 2 dB/km.
In 1980, optical fiber attenuation was as low as 0.2 dB/km (within the 1.55 μm long-wavelength low-attenuation window), close to the theoretical value. This made long-distance optical fiber communication possible.
Furthermore, due to continuous improvements in purification processes, the transmission window of optical fibers has shifted from a short wavelength window of 0.85μm to a long wavelength, low-attenuation window of 1.3μm and 1.55μm.
After 1976, various practical fiber optic communication systems emerged one after another. By 1980, many countries around the world had developed commercial fiber optic communication systems. From then on, fiber optic communication took a giant leap into the commercial era.
Note: Currently, the lifespan of lasers exceeds 100,000 hours, and the typical transmission window and loss of optical fibers are as follows:
1550nm window loss: 0.18dB/KM
1310nm window loss: 0.35dB/KM
In optical fiber communication, we often mention the concept of transmission window. As we know, light of any wavelength can be transmitted in optical fiber, but the transmission loss of certain wavelengths of light in optical fiber is lower than that of other wavelengths. These specific wavelengths are what we call transmission windows. Currently, the most commonly used transmission windows are 850nm, 1310nm, and 1550nm, which were mentioned above.
The formula for calculating loss is as follows: Loss (dB) = 10lg(input/output).
The unit dB of loss is a relative unit, representing a ratio relationship rather than an absolute value concept.
