Light of Communication

Today， Internet has become an indispensable part of daily life， then how information on the Internet is transmitted？

It is transmitted via communication technologies. Fiber optic communication， cable communication and wireless communication constitute a "communication world". The ability of communication is called bandwidth. Fiber optic communication is a communication technology which can provide maximum bandwidth， and more than 80% of network flow is transmitted on optical network. How does fiber optical communication technology， one of the internet cornerstones develop？

Optical communication truly becoming a reality is when the light has been fully understood.

The ancients have learned some basic optical phenomena and laws， such as descriptions of pinhole camera model and concave mirror imaging in China's Monist Canon in the 5th Century B.C.， and Euclid's Law of Reflection in ancient Greece， etc. While modern optics started roughly in the 17th century with Snell， a Dutch proposing law of refraction as the starting point， since then， a wealth of knowledge of optics has been accumulated in centuries. But until the mid-nineteenth century， Maxwell summarized people's understanding of electricity and magnetism in a very beautiful system of equations and found that the speed of electromagnetic waves was exactly equal to the known speed of light， thereby he identified that light was electromagnetic waves. Maxwell's work may be called the most outstanding achievement in the 19th century， and is a revolutionary change for human beings to understand the nature of light.

Radio waves， microwaves， infrared light， visible light， ultraviolet light， X-rays and Gamma rays are electromagnetic waves， propagate at the speed of light， constitute a world of electromagnetic， and are also the carriers various communication technologies use today. Definition of vibration frequency can be too compared to drumbeat； the vibration frequency is 1 Hz when the drum is beat once per second. The higher the frequency of electromagnetic waves is， the bandwidth which can be used to carry information is greater. Available bandwidth of fiber optic communication is much larger than that of mobile communication.

Maxwell's Electromagnetic Wave Theory made wireless telegraphy， wire telephone and other communication technologies come to our life， but these technologies were insufficient to constitute the optical communication technologies today. In 1905， Einstein introduced the concept of light quantum （photon） in a paper explaining photoelectric effect， namely the energy of light is not continuous， but one by one， and equals to the result a constant timing the frequency of light， which is to say that a beam of light is composed of many discrete photons. The concept of photon and quantum mechanics established by Heisenberg， Schrodinger， Dirac and others about 20 years later make people aware that some properties of light are like those of waves， and some others are like those of particles， the phenomenon is called wave-particle duality. It is another revolutionary understanding of the nature of light.

Semiconductor Laser

Light Sources in Fiber

Optic Communication

Gradual deepening of people's awareness of the nature of light makes core technologies today's optical communication relies on， such as lasers， fiber optics， optical amplifiers， and photo detectors possible.

Invention of lasers benefits from the idea of stimulated radiation proposed by Einstein in 1916. Generally， common luminescence in nature is spontaneous radiation， so the direction， frequency， etc. of emitted photons are random. If the photons enter an electron having absorbed photons and being excited to a higher energy level， the excited electron can no longer absorb photons. Einstein believed that at this moment， the electron could be excited to emit a photon which was the same as the incident photon. The process was called stimulated radiation.

Principle and realization of lasers were accomplished by Townes et al. until the 1950s. Unlike ordinary light sources， laser has good coherence， and is suitable to be used as a carrier for optical communications because it has light waves of the same frequency， troughs and peaks of the same location， and good directionality. The laser that plays the most important role in information industry is double heterostructure semiconductor laser emerging in 1961， the success of which is attributed to Alfaro from Russia and Cromer from the United States who won Nobel Prize in Physics in 2000 because of which.

Optical Fiber -

Best Transmission

Medium

Optical fiber is a kind of fiber made of glass or plastics， and can be used as a tool for optical conduction. Principle of optical fiber transmission is total reflection of light. Semiconductor laser solves the problem of light source for optical communication， while settlement of transmission media owes to Charles Kuhn Kao， "father of fiber optic communication". Charles Kuhn Kao has not invented optical fiber. In fact， glass optical fiber was used in gastro scope and other situations very early. But until the 1950s， few people believed that fiber optic communication was workable in that optical glass has great loss， about 1000 dB/km， so optical fiber cannot transmit so far.

Charles Kuhn Kao's contribution was fully reflected in a classical article in 1966. In that article， he first pointed out the source of loss of quartz fiber， and predicted that there must be low-loss "valley" in quartz fiber， and stated that loss of quartz fiber could be reduced to 20 dB/km or less by improving purity of quartz and reducing the absorption of impurities. Secondly， he designed single-mode round optical fiber structure with the fiber core in the middle and the cladding outside. Such structure is perfect， easy to implement， and suitable for long distance transmission. Therefore， Charles Kuhn Kao won Nobel Prize in physics in 2009

WDM

Makes "Rainbow"

Packed into Optical Fiber

With breakthroughs in two key technologies， semiconductor laser and low-loss optical fiber， fiber optic communication enters booming period， five generations have been developed by now， and currently it is the fifth generation， namely WDM transmission system that is most widely used in main lines of communication. New generation optical fiber communication system using coherent detection and digital signal processing is being developed and begins to be tried out. The overall trend is higher transmission capacity， longer distances， and reduced transmission cost.

The so-called WDM is to modulate signals on light waves of different colors （wavelengths）， transmit these light waves over a single optical fiber， then separate these light waves of different colors （wavelengths） on receiving end， just like packing a rainbow into an optical fiber to transmit for thousands of kilometers and then releasing the rainbow， thus， the capacity of such optical fiber can be increased dozens of times， even one hundred times.

Although quartz single-mode optical fiber has smaller loss， but the signal of which will weaken to 1% of the original signal after 100 km transmission， so a EDFA is required to amplify the signal. With EDFA， high-speed optical signal can be transmitted for thousands of kilometers and even across oceans requiring no repeater. With EDFA， WDM system is today's major optical communication technology， and is the key to low cost of Internet communications. For example， cost of Atlantic cables is reduced to 1/3500 of the original in a dozen year.

Multi-dimensional

Ultra-large-capacity

Communication

Light is a kind of electromagnetic waves， has 6 physical parameters， i.e.， frequency， phase， polarization， amplitude， time， and spatial distribution which are six dimensions that can be utilized to increase communication capacity. For example， WDM is to increase the number of signals in the dimension of frequency （wavelength） to make the transmission capacity increase a hundredfold. Increasing the modulation rates for signals of different wavelengths is to increase the transmission capacity in the dimension of time. Phase and amplitude of light can be used to make advanced complex signals available.

However， frequently-used optical signal reception mode is to measure the power of optical signal directly， which is called direct detection， in this way， information on phase and polarization of optical signal will be lost. In recent years， employment of coherent detection， namely comparing a local optical signal with a transmitted optical signal， not only makes the amplitude of optical signal known， but also restores polarization and phase of optical signal. Thus， all six dimensions of light waves can be used to greatly increase the transmission capacity. Since its birth， the transmission capacity of optical fiber is approximately 10-fold increased every four years. In 2012， Japan's NTT Basic Research Laboratories reported a coherent light communication system which has the highest single-mode optical fiber transmission capacity， 102 terabit/s （1 terabit = 1012）. China's Fiber Home reported similar results in 2014. However， there will encounter a bottleneck in further increasing the capacity of quartz single-mode optical fiber， and a more feasible approach might use SDM techniques， such as making multi-core fiber by increasing the number of cores in optical fiber， making few-mode fiber by enlarging the diameter of fiber core so that the optical fiber can support several modes， and each core or mode transmits different signals. In 2014， NTT Basic Research Laboratories reported an amazing fact that the transmission capacity is up to 1 Petabyte/s （1 Petabyte=1015） with multi-core fiber optical communication system， in other words， the world's seven billion people telephone with the signal transmitting over a single optical fiber at one time， while less than a quarter of transmission capacity is used.