In 1841, two scientists, Daniel Colladon and Jacques Babinet, performed a simple experiment:
Drill a hole in a wooden bucket filled with water, and then use a lamp to illuminate the water from above the bucket. The results surprised the audience. People saw that the shining water flowed out of the small holes in the bucket, the water flow bent, and the light also followed.
Light was captured by the twisting water.
Why is this? Doesn't light travel straight?
This phenomenon is called the total internal reflection of light. That is, the light is emitted from the water to the air. When the incident angle is greater than a certain angle, the refracted light disappears, and all the light is reflected back into the water. On the surface, light seems to bend in the current. In fact, in a curved stream of water, light still travels in a straight line, but multiple total reflections occur on the inner surface, and the light propagates forward after multiple total reflections.
In 1880, Alexander Graham Bell invented the "optical telephone". Bell focused the sunlight into a very narrow beam and irradiated it on a very thin mirror. When the "sound wave" of people's voice made this thin mirror vibrate, the change in the intensity of "reflected light" made the inductive detector Changes occur, changing the "resistance" value. The receiving end uses the changed "resistance" value to generate current and restore it to the original "sound wave".
When Bell tested the "optical phone" successfully, he wrote:
"I heard the laughter, cough, and singing of light."
However, his invention can only propagate for about 200 meters, because the light intensity transmitted by the air beam will rapidly decrease with distance.
Bell had predicted at the time that this invention would be far more interesting in the scientific world than telephones, gramophones, and microphones.
Because light decays quickly in the air, people have thought of using matter to conduct light, as shown by Daniel Colladon and Jacques Babinet, to let "light waves" propagate in the water column.
However, in the nearly 60 years after the bucket demonstration in 1841, the principle of total internal reflection of light was only used in the field of short-distance propagation. For example, in medicine, dentists use curved glass rods to direct light into a patient's mouth for surgical lighting.
Although glass fiber has been widely used since the Renaissance, glass workers can produce exquisite vases and crafts. However, in order to solve the long-distance transmission of the light guide, the glass rod must be drawn into a very strong and flexible glass fiber.
In 1887, a British scientist named Charles Vernon Boys put a bow near a heated glass rod. When the glass rod was hot enough, he fired an arrow, which drove the hot glass out of the laboratory. A long thin glass fiber.
This "optical fiber" is 9 feet long (about 2.74 meters)
This undoubtedly makes the development of optical fiber communication a big step forward. However, like what happened after the bucket demonstration in 1841, the experiment was ultimately an experiment, and we waited another 50 years for the next step.
It was not until 1938 that Owens Illinois Glass of the United States and Nitto Textile of Japan began to produce long glass fibers.
However, the fiber produced at this time is bare fiber and has no cladding.
We know that the propagation of optical fibers uses the principle of total internal reflection. The angle of total internal reflection is determined by the refractive index of the medium. Bare fibers can cause light leakage, and light can even leak out from oil stains attached to the fiber.
The problem of cladding was not resolved until after 1950.
In 1951, the photophysicist Brian O'Brian proposed the concept of cladding. Then someone tried to use margarine as a cladding, but it was not practical.
In 1956, a student at the University of Michigan made a glass-clad fiber. He used a low-refractive-index glass tube to fuse to a high-refractive-index glass rod.
Glass cladding quickly became the norm, and later plastic cladding also appeared.
As we all know, optical fiber uses light in fibers made of glass or plastic to form light transmission based on the principle of total internal reflection. Generally, a transmitting device at one end of an optical fiber uses a light emitting diode or a laser beam to transmit light pulses to the optical fiber, and a receiving device at the other end of the optical fiber uses a photosensitive element to detect the pulse.
In the 1960s, telecommunications engineers were looking for more ways to transmit bandwidth. Radio and microwave frequencies can no longer meet the growing bandwidth requirements of televisions and telephones, so they want to find a higher frequency to carry signals. The telephone company believes that the upcoming video phone will increase the demand for bandwidth.
In 1960, Theodore Maiman showed people a laser. This has ignited people's interest in optical communication. Lasers seem to be a promising communication method that can solve the problem of transmission bandwidth. Many laboratories have started experiments.
However, they soon discovered that air is not a good medium for laser communication, and it is too severely affected by the weather. Naturally, engineers turned their attention to optical fibers.
With the luminous source, the problem of the cladding is also solved, and it seems that the day when the optical fiber communication will come is not far away. However, the following questions made many people retreat.
With the cladding fiber, it can only be made into a flexible endoscope to enter the throat, stomach, and colon of the human body. When it is used in an endoscope, half of the energy lost by light transmission is 3 meters. It is okay for human internal organs examination, but it is used for long-distance optical communication.
Optical fiber propagation loss is too large to be suitable for communication, and many engineers have abandoned the attempt of optical fiber communication.
But there are always people who refuse to give up lightly. They decided that they must find out what factors affect fiber loss.
Finally in 1966, the young engineer, British borrowed Chinese, Gao Ke (KCKao) reached a breakthrough conclusion in the history of fiber optic communication:
The loss is mainly due to impurities contained in the material, not the glass itself.
"Father of Fiber"
He predicts that when the beam travels at least 500 meters in high-purity fiber, there is still 10% of energy remaining.
Sao Nian, are you kidding me? For many people, this prophecy is like a myth.
In July 1966, the prospect of high-fiber optical transmission published a historic paper. This article analyzes the main causes of optical fiber transmission loss, theoretically expounds the idea that the loss can be reduced to 20dB / km, and proposes that such optical fiber will be used for communication.
Now everyone knows that after 43 years, Gao Yan won the 2009 Nobel Prize in Physics for this paper.
But at the time, many people thought it was a nightmare.
Like a preacher, Gao Yan promoted his faith everywhere. He went to Japan, Germany, and even the famous Bell Labs in the United States. Gao Yan is very stubborn about what he believes. Perhaps because of this "stubbornness", Gao's dissertation eliminated doubts in the academic and industrial circles, and proved the feasibility of optical fiber transmission of information. Everyone immediately followed.
Later, the optical fiber became a hot spot, the industry invested manpower and financial resources, and scientists and engineers went all out.
Four years later, Corning, USA, actually pulled out 20dB / km of fiber.
Corning has achieved a result consistent with the theory and broke through the 20 dB / km attenuation level proposed by Gao Yan, proving the possibility of optical fiber as a communication medium.
At the same time, semiconductor lasers using gallium arsenide (GaAs) as materials have also been invented by Bell Labs, and they have been widely used in fiber optic communication systems due to their small size.
At this point, optical fiber has really begun to apply to optical fiber communications. Therefore, we call 1966 the birth year of fiber optic communication.
Since then, fiber optic communication has officially begun ...
In 1972, the transmission loss was reduced to 4dB / km.
In 1973, the Wuhan University of Posts and Telecommunications of the Ministry of Posts and Telecommunications began to study optical fiber communications.
In 1974, the Bell Institute in the United States invented a low-loss optical fiber manufacturing method-CVD method (vapor deposition method), which reduced the optical fiber transmission loss to 1.1dB / km.
In 1976, Bell Labs completed an optical fiber communication experimental system in Atlanta, using a fiber optic cable containing 144 optical fibers manufactured by Western Electric Company. An optical fiber communication system with a rate of 44.7 Mbit / s was born in an underground channel.
At this time, Nippon Telegraph and Telephone Corporation started a laboratory test of a 64km, 32Mbit / s abrupt refractive index fiber system, and successfully developed a semiconductor laser with a wavelength of 1.3 microns.
In 1978, China developed its own communication optical cable , which used multimode fiber and the core structure of the cable was layer twisted.
In 1979, the Japan Telegraph and Telephone Corporation developed a 0.2db / km ultra-low loss quartz fiber (1.5 microns).
A commercial optical fiber communication system is introduced. This optical fiber communication system in the history of humankind uses a gallium arsenide laser with a wavelength of 800 nm as a light source, and the transmission rate reaches 45 Mb / s. A repeater is required to enhance the signal every 10 kilometers.
Then, the second generation of commercial fiber-optic communication systems also came out. It uses an InGaAsP laser with a wavelength of 1300nm.
Although early optical fiber communication systems affected signal quality due to dispersion issues, the invention of single-mode fiber in 1981 overcomes this problem.
By 1987, the transmission rate of a commercial optical fiber communication system was as high as 1.7 Gb / s, which was nearly forty times faster than the speed of an optical fiber communication system. At the same time, the problem of transmitted power and signal attenuation has also been significantly improved. A repeater is required to enhance the signal at 50 km intervals.
In the late 1980s, the birth of EDFA was a milestone in the history of fiber optic communications. It enables optical fiber communication to directly perform optical relay, enables long-distance high-speed transmission, and promotes the birth of DWDM.
In the third-generation optical fiber communication system, a laser with a wavelength of 1550 nm was used as the light source, and the attenuation of the signal has been as low as 0.2 decibels per kilometer (0.2 dB / km). Previous fiber-optic communication systems using GaAs lasers often encountered pulse spreading problems, and scientists designed excellent dispersion-shifted fibers to address these issues. When this kind of fiber transmits 1550nm light waves, the dispersion is almost zero, because it can limit the laser's spectrum to a single longitudinal mode (longitudinal mode).
These technological breakthroughs made the transmission rate of the third-generation optical fiber communication system reach 2.5Gb / s, and the interval between repeaters can reach 100 kilometers.
The fourth-generation optical fiber communication system introduces optical amplifiers, further reducing the need for repeaters. In addition, wavelength-division multiplexing (WDM) technology has significantly increased the transmission rate.
The development of these two technologies has greatly increased the capacity of optical fiber communication systems by doubling every six months. By 2001, it had reached an astonishing rate of 10Tb / s, which is 200 times the optical fiber communication system of the 1980s As much as possible.
Later, the transmission rate was further increased to 14Tb / s, and a repeater was needed every 160 kilometers.
The focus of the development of the fifth generation optical fiber communication system is to extend the wavelength operating range of the wavelength division multiplexer. The traditional wavelength range, also commonly known as "C band", is between 1530nm and 1570nm, and the low-loss band of the dry fiber in the new zone extends between 1300nm and 1650nm.
Another developing technology is the introduction of the concept of optical soliton, which uses the non-linear effects of optical fibers to allow pulse waves to resist dispersion and maintain the original waveform.
As one of the great technological achievements made by human society in the 20th century, optical fiber communication technology is an irreplaceable and important cornerstone for humankind to enter the information age. Without the invention of optical fiber communication, there would be no comfortable and convenient Internet life.