by Acad. Yevgeny DIANOV, Director of the Fiber Optics Scientific Center, RAS (Moscow)

History of human civilization is also history of development of communication means. Advances in science and technology were accompanied by emergence of new information transmission methods. Suffice it to mention the invention of wire telegraph and telephone in the 19th century. But the 20th century was characterized by distribution of radio communications including satellite one. Wavelength reduction of carrier radio radiation (frequency increase) allowed transmission of more and more data volumes. But even these opportunities could not keep up with the growing requirements of society. Specialists became aware of the necessity to find a new method of communication, i.e. using optical radiation, which promised growth of information transmission rate more than 10,000 times, if compared with radio communication. The current scales of using optical communication via fiber lightguide confirmed the expectations. However this material information carrier also has its limitations. What is to be done? Let us clear it up.

Spectrum of optical losses of fiber lightguides. O, E, S, C, L, U are designations of spectral bands used in literature. EDFA is an amplification band of erbium fiber amplifier. ALLWave and SSMF are types of fiber lightguides.

All continents of our planet are connected at present by under-water fiber-optic lines. By 2010 the total extent of laid fiber lightguides including transatlantic and transpacific under-water systems reached 1 bln km. In developed countries such light-guide is now connected to each house thus providing access to wide-band information through Internet. According to the estimates the number of the global network users will reach 5 bln in 2015. By the way, this very network emerged due to the development of the fiber-optic communication.

The outstanding results in creation of the said systems were achieved owing to extensive fundamental and applied research, engineering developments of laser radiation sources, fiber lightguides and other elementary base. This research was carried out by a number of scientific centers and companies in many countries of the world, and it was described in detail in a special issue of the J. of Lightware Technology (USA) in 2008. Regular international conferences on fiberoptic communication played a prominent information coordination role in conducting these studies. Of special note are two of them: the annual Optical Fiber Communication Conference (USA) and the annual European Conference on Optical Communication.

Referring to the most recent history we shall cite only several results which actually determined the development trends of the information transmission high-speed systems. First of all, there are two outstanding breakthroughs in optics: creation of lasers (1960) and development of glass fiber lightguides with extremely low optical losses (1970). The continuous generation of a semiconductor laser based on GaAIAs double heterostructure at room temperature realized both in the USA and the USSR (in the laboratory of Zhores Alferov, Nobel Prize Winner of 2000) signified the emergence of a compact and efficient radiation source for fiber-optic communication systems. Lasers first used for such purposes (1980) operated at a wavelength of 0.85 mem, and an information transmission rate in the first commercial systems made up a very modest value of 45 Mb/s. The transmission medium was represented by a multimodal (transmitting radiation in the form of many types of vibrations) fiber lightguide, whose intermo-dal dispersion* restricted the information transmission rate.

The fundamental research in fiber lightguides** was a next stage in the development of broadband fiber optic communication systems. It appeared that van-

* In a multimodal lightguide, different types of oscillations spread at various rates which results in extension of a light pulse in time or in its dispersion.---Ed.

** See: A. Prokhorov, Ye. Dianov, "Fiber Optics: Problems and Prospects", Science in the USSR, No. 3, 1987.--Ed.

Diagram of dense wavelength division multiplexing (DWDM)

ishing dispersion of quartz glass and consequently single-mode fiber lightguides is located near a wavelength (A.) equal to 1.3 mem and minimal optical losses equal to ~0.2 dB/km were close to λ = 1.55 mem. Proceeding from the obtained data the researchers focused considerable efforts on development of the element base (primarily single-mode fiber lightguides, semiconductor lasers and photodetectors) for these spectral fields. It allowed creation of communication systems with considerably higher rate of information transmission which operated on the said wavelengths. But the further growth of rate appeared impossible due to relative tardiness of such important element of these systems as opto-electro-optic signal regenerators, in which it transformed first from optical to electric signal then amplified and transformed again to optical one. Therefore a problem arose of developing a broadband fiber optical signal amplifier.

The intensive studies in this field resulted in the development of two perspective devices. The advantage of the first device, namely, erbium fiber amplifier (EDFA), consists in high efficiency and compliance of a spectral band of amplification with a field of low optical losses of fiber lightguides. EDFA promoted emergence of a new generation of fiber optic communication systems operating on a waveband close to 1.5 mem. But as appeared the erbium amplifier had also disadvantages, in particular, a narrow amplification band from 1,530 to 1,610 Nm, i.e. maximum 80 Nm, which restricts information transmission rate. But the other device using light stimulated Raman scattering called the Raman fiber amplifier (RFA) allows to get a broader band of amplification on any wavelength. In contradistiction to EDFA, the efficiency of RFA is not high but nevertheless it is used in commercial fiber optic communication systems.

Application of spectral multiplexing was another striking result. The gist of it is that a single-mode fiber lightguide serves as a material carrier of information transmission for about 100 independent channels with different wavelengths of carrier radiation but within limitations of optical amplifier. The overall rate of information transmission in such systems is equal to nb, where n is a number of channels and b is an information transmission rate in one spectral channel. If it is granted that 10 Gbps (1 Gb = 10⁹ bit) are transmitted through one channel and there are 100 channels, the overall rate is 1 Tbps (1 Tb = 10¹² bit).

Cross-section of a fiber lightguide with an air core (a) and spectra of optical losses of this lightguide (b).

Today the standard rate of information transmission through one spectral channel is 40 Gbps. Now work is under way on bringing this value to 100 Gbps, while the number of channels exceeds 100, therefore the "carrying capacity" of one fiber lightguide in commercial systems reaches 10Tbps and in experimental systems up to 100 Tbps. These are extremely high parameters! However, it must be recognized that information needs of modern society is steadily increasing (in developed countries by 30-40 percent annually). Most probably such or even higher dynamics will preserve in years to come. This fact is confirmed by forecasts of the American company ElectroniCast Consultants, published in the Optics and Photonics News journal in November of 2013. It is assumed that at least until 2017 the world market of optic fiber related to telecommunication needs will increase by more than 50 percent annually.

Why such sharp rise in information needs in developed countries? First of all, it is an increase of the number of Internet users with predominating video information (especially in social networks), which is a very large part of transmitted signals. Considerable inflows of various data is required for economy, education and science, in particular, for handling such world problems as the global climate change, ecology and other things. But perhaps the largest body of information is necessary for creation of a modern national infrastructure. The terabit fiber optic networks turned into a peculiar kind of the nervous system of the developed society, which by an analogy with the human nervous system provides efficient performance of all state authorities.

If such growth of information need preserves, in 10 years there will arise a necessity of increasing the rate of its fiber lightguide transmission to the level of ~1 petabyte/s (1 petabyte = 10¹⁵ bytes) and in 20 years--100 petabyte/s. But using standard glass single-mode lightguides it is not possible to cope with such task as they are an ideal transmitting medium only as regards certain, though sufficiently high, rates, i.e. around 100 Tbps. Their further increase results in limitations: nonlinearity, dispersion and optical losses of glass fiber lightguides. One more limitation is connected with a rather narrow amplification band of the existing erbium fiber amplifiers.

So what are new approaches to creation of light-guides with a high information capacity? At present specialists in the world are studying ways of breaking the achieved limit of information transmission rate through a fiber lightguide which is equal to 100 Tbps. The emphasis is on three ways of solving this problem. First, creation of an air-core lightguide, which has low optical losses. Secondly, expansion of a spectral field for information transmission, which is reduced to creation of efficient fiber amplifiers. And finally, channel spatial multiplexing.

Now we shall briefly discuss the results of this research. The experiments of the 1960s on information transmission through free atmosphere by laser radiation revealed that it was not a suitable transmission medium due to meteorological conditions, spatial and

Amplification spectrum of a bismuth fiber amplifier.

time instability of optical losses and its density. Besides, such parameters as nonlinearity, dispersion and optical losses of air atmosphere as such are at least by an order of magnitude less than in glass fiber lightguides. However, to maintain its stable parameters the atmosphere should not be free. Therefore, an idea to produce fiber lightguides with air core, which will be isolated from external factors, has come to the forefront. In this case a glass coating in the form of a photon crystal provides a mechanism of light dissemination along the core. Such lightguides were created and their characteristics were studied in detail (article by P. Roberts and coauthors (Great Britain) published in 2005 in the American Optics Express journal, Vol. 13, No. 1). It is true that their minimum optical losses make up at present 1.2 dB/km, which is substantially higher than in ordinary glass fiber lightguides (<0.2 dB/km on the wavelength of 1.5 mem). Solution of this problem requires further fundamental studies, and now specialists and scientists search for other ways of increasing the rate of information transmission.

As one of the possible approaches we can name expansion of a spectral field where optical losses are minimal. Its parameters are known: over the range from 1,300 to 1,700 Nm such losses make up less than 0.35 dB/km. Unfortunately, there are no fiber optic amplifiers available as yet for this spectral field. Their creation is a priority task.

In 2005 the Scientific Center for Fiber Optics, RAS, in cooperation with the Institute of Chemistry of High-Purity Substances (Nizhni Novgorod) developed for the first time in the world a technology of creation of fiber bismuth-doped lightguides, whose luminescence spectrum overlaps the said spectral area. That same year these research teams demonstrated for the first time a bismuth fiber laser with continuous generation over a range of 1,150-1,300 Nm and later created a family of bismuth fiber lasers which generated in a spectral area of 1,150-1,550 Nm. Application of a new promising active medium allowed to make the first move towards creation of efficient fiber amplifiers for the spectral area of 1,300-1,500 Nm. A bismuth fiber amplifier of 65 mW with the maximum amplification factor of 25 dB at laser diode pumping on the 1,310 Nm wavelength was developed for the first time for the spectral area of 1,425-1,465 Nm.

One more method of increasing information transmission rate which has drawn much attention of specialists recently is spatial channel multiplexing by creation of multi-core and also low-mode fiber light-guides in which every mode is a carrier of independent channels. At present specialists developed and studied a number of multi-core fiber lightguides including seven-, twelve- and nineteen-core ones. Their specifications include low optical losses of all cores, low cross noise between neighboring cores and not too large

Typical structure of a seven-core fiber lightguide (a) and a profile of the refractive index of multi-core lightguide cores (b).

diameter of such lightguides (coating diameter ≤200 mem). The value of cross noise depends on a distance between neighboring cores. Besides, overlapping of optical fields in neighboring cores, which certainly reduces cross noise can be achieved by selection of a special refractive index profile.

In 2012, the 38th European Optical Communication Conference discussed development data on multi-core fiber lightguides, appropriate optical-fiber amplifiers and attempts of information transmission with channel spatial multiplexing. For example, the Japanese researcher Katsunori Imamura and his coauthors suggested a seven-core fiber lightguide with the 186 mem cladding diameter and a distance of 55 mem between neighboring cores. The cross noise at a distance of 100 km from the lightguide makes up 40 dB.

Development of a transmission medium in the form of a multi-core fiber lightguide for information transmission for long distances (≥ 100 km) requires creation of respective optical amplifiers. At the conference Yokihiro Tsuchida with coauthors (Japan) submitted specifications of a seven-core erbium fiber amplifier. The lightguide length is 16 m, diameter 180 mem, distance between erbium-alloyed cores 45 mem. Were received: amplification >15dB at 40 mW pumping, noise factor <7 dB, cross noise <--40 dB.

Hidenori Takakhashi and coauthors (Japan) submitted results of the first demonstration of signal transmission with channel spatial multiplexing (seven-core fiber lightguide) and a seven-core erbium amplifier for a distance of 6,160 km and a rate of 35.8 Tbps. Besides, 40 spectral channels with an information transmission rate of 128 Gbps were put into each core of lightguides.

But the most notable research result was submitted at the Conference by Hidehiko Takara and coauthors of information transmission through a twelve-core light-guide at a rate of 1.01 petabyte/s for a distance of 52 km. In this experiment 222 spectral channels each with an information transmission rate of 456 Gbps were put into every core. The carrier radiation of channels occupied areas of 1,526.44-1,565.09 Nm and 1,567.95-1,620.06 Nm, and carrier radiation frequencies of neighboring channels were spaced at 50 GHz.

The submitted results show application perspectives of channel spatial multiplexing for increasing information capacity of fiber lightguides to the petabyte level. Development of petabyte systems of information transmission and creation of petaflop supercomputers signify that Mankind is on the road to the peta era in the field of information processing, transmission and use.