by Academician Alexander PROKHOROV, Director of the Institute of General Physics of the USSR Academy of Sciences, Academician Secretary of the Department of General Physics and Astronomy of the USSR Academy of Sciences,
Yevgeni DIANOV, Dr. Sc. (Phys. & Math.), now-Academician, Head of the Department of Fiber Optics at the same Institute
The introduction of lasers* has triggered off numerous ideas on their application, one of the first being data transmission. Certain attempts had already been made to transmit data via the atmosphere with the aid of lasers; these experiments had been carried out during the 1960s all over the world, including the Soviet Union. The first optical communication link was established between Moscow State University on Lenin Hills and a building on Zubovskaya Square in Moscow which at that time was heralded as a miracle.
Such links proved to be very inefficient and specialists discovered that the atmosphere was a highly unstable medium, far too unreliable for practical communications.
Optical communications appeared to be faced by an insurmountable obstacle - the absence of a suitable transmission medium. Then the fiber light guides were recalled: thin glass fibers of special structure would, indeed, be suitable for this purpose, but those available at that time were very low-grade, with optical losses of about 1,000 dB/km. In other words, light was attenuated by a factor of two over a distance of but 1 m, while a system can operate efficiently only if such attenuation occurs over a distance of at least one kilometer. An impasse appeared to have been reached.
The situation, however, quickly changed due to two circumstances. First of all, in 1966 the high optical losses in glass were shown to be determined by the production technology and not by fundamental effects. Improved technology reduces the losses to less than 20 dB/km, which is acceptable for practical use in data transmission systems.
Secondly, in 1970 Academician Zhores Alferov ** developed semiconductor lasers operating in the CW mode at room temperature, which proved most promising for optical communication systems due to their small size (about that of a match head), low cost, low power consumption, and tuning capabilities.
Now, work could be carried out on a qualitatively new level. In 1973 we began developing fiber light guides with low losses on the basis of quartz glass; this research was carried out jointly with a group of colleagues from the Institute of Chemistry of the USSR Academy of Sciences (the city of Gorky) headed by G. Devyatikh, Corresponding Member of the USSR Academy of Sciences (from 1974 Academician). This resulted in the creation by 1975 of the first Soviet samples of optical fibers with losses below 10 dB/km. Working jointly with chemists, we developed a method of producing low-loss light guides which compared favorably with the world's best.
The light guides are long thin fibers of high-purity glass and consist of a core with a diameter ranging from single to dozens of microns and a cladding of glass with a lower refractive index and an outer diameter of about 100 mum. Light passing from a more dense medium into a less dense one (for instance, from water into air or, in the present case, from glass with a high refractive index into glass with a lower refractive index) experiences total reflection from the boundary between these media at certain incidence angles. This total internal reflection effect provides light propagation to considerable distances in the absence of excessive attenuation, the latter being dependent on the quality of the light.
Radiation can be transmitted as a unimode or multimode oscillation depending on such parameters as the core diameter, the difference between refractive indices of core and cladding, and the wavelength of the light signal. These modes differ in the propagation velocity, distribution of intensity
* A. Prokhorov, N. Basov and American researcher Ch. Gowns were awarded a Nobel Prize for their introduction. - Ed.
** Nobel prize laureate in physics for 2000. - Ed.
across the light-guide cross section and direction of the electric field vector. Correspondingly, light guides are divided into unimode and multimode types.
Fiber light-guide quality is evaluated by several main characteristics, the main one of which is the above-mentioned optical loss which has now been reduced to its practical minimum (light is attenuated by a factor of two on a guide length of approximately 20 km).
Another major characteristic is the information passband defined as the difference between the lowest and highest frequencies. The wider this passband, the greater the amount of information that can be transmitted during a time unit.
Creating wideband light guides has proved to be a rather complicated problem. In ordinary optical communication systems information is transmitted as a certain sequence of optical pulses. In multimode light guides various oscillation modes propagate with different velocities, thus leading to light pulses being prolonged, i.e., to dispersion effects. It can sometimes lead to pulse overlapping at the light-guide's output, and this results in interference and loss of information.
There are two ways out of this situation: one is either to use unimode light guides, or use multimode light guides but only those that transmit various oscillation modes at one and the same velocity To achieve the latter, the fiber light guide has to feature a density that gradually varies along its cross section so that its refractive index varies correspondingly If the latter varies according to a parabolic law, the propagation velocities of various modes will be approximately equal, so that all oscillation modes will arrive at the output almost simultaneously
Working in cooperation with colleagues from the Institute of Chemistry of the USSR Academy of Sciences, we succeeded in solving a complex technological problem-the production of multimode light guides with extremely wide passbands, of the order of one gigahertz at 1-km guide lengths. In ordinary light guides, in which the refractive index is constant across the core cross section and experiences an abrupt change at the core cladding boundary, the passband is more than an order of magnitude narrower.
Another criterion of fiber light-guide quality is their mechanical strength. Its resistance to rupture is very high, exceeding that of steel wire; however, the presence of any surface defects, such as cracks or scratches, greatly reduces this resistance and leads to rupture under large enough loads.
The technology of production of fiber light guides capable of withstanding stretching loads up to 7 kg at short sections and up to 1.5 kg at sections above 1 km in length was developed jointly with chemical engineers from Gorky.
The thermophysical properties of the polymer coating are very important in determining the temperature range in which the light guide can be successfully used. Under certain conditions the most commonly used polymers crystallize, become rigid and cause microbands in the light guide, thus increasing optical losses at low temperatures. We succeeded in developing polymers, in which crystallization takes place at temperatures below - 100 0 C, i.e., the crystallization point is shifted outside the actual operating range (-60 0 to + 100 0 C). New materials, which do not have any analogs worldwide, are based on organosilicon polymers.
Throughout this research fundamental physical problems were treated alongside engineering and technological problems. One of these fundamental problems was that of nonlinear propagation of optical radiation along fiber light guides.
In contrast to linear effects, nonlinear effects depend on light intensity A light field of sufficiently high intensity changes the optical properties of the medium (say its refractive index) and this affects the processes taking place within it. Nonlinear optical effects have been known for a long time, but it was only with the appearance of lasers that their detailed study begun.
Fiber light guides are especially suited for observation of nonlinear effects because their very low optical losses make it
possible for light to interact with the medium over large distances (of the order of kilometers) and at the same time this light is confined within extremely small (down to several microns) cross sections of the light guide.
Shortly after low-loss light guides had been developed, our Institute launched a wide range of investigations into the field of nonlinear fiber optics. At this juncture we shall consider one such effect which has had far-reaching practical application, as well as great scientific importance.
This effect is the phase self-modulation of light, which is produced by variations of the refractive index of the medium (in the present case - glass) under the action of a light pulse of sufficient power. The passage of such a pulse through a medium, in which the low- frequency ("red") components travel slower than the high- frequency ("blue") components results in the latter catching up with "red" components and thus the pulse compression. A suitable medium for this purpose is a fiber light guide designed to transmit only certain wavelengths.
Light guides with properly selected parameters provide pulse compression ratios in excess of 100:1 - pulse durations of the order of 200 femtoseconds (1 femtosecond=10 -15 s) were obtained. Pulses of similar short durations had been generated previously as well, but they were always located on very wide pedestals which contained a major share of the total pulse energy and were thus unsuitable for practical use. We managed to concentrate almost all the energy in the pulse itself, so that it could be used as a tool to study high-speed processes in various materials and in biological objects.
With properly selected pulse power and radiation wavelength pulse propagation along a light guide without pulse waveform distortion can be obtained. This propagation mode is called the soliton mode.
As mentioned earlier, the stretching of optical pulses results in crosstalk interference and restricts data-transmission rates. This makes solitons especially attractive for optical communication systems. Theoretical studies, carried out in our Institute and at other scientific centers, disclosed that in the soliton mode the data- transmission rate is restricted only by the optical losses in the light guide and nonlinear interaction between individual solitons. Various means of amplifying solitons with the object of increasing the transmission rate have been proposed.
The development of high-quality single-mode and multimode light guides and the insight gained into processes taking place during optical radiation propagation along light guides, served as the basis for solving various problems of application.
First of all, light guides restored their role as a transmission medium, for which they were initially designed. At present, their main fields of application are telephone communications and cable television.
The principle of operation of optical communications and data- transmission systems is very simple. Any kind of information - voice, television, data, etc.-can be recorded in a sequence of electric signals, which are then used to modulate the emission of a laser; a semiconductor laser converts these electrical signals into optical signals which are then fed into a light guide and distributed along it at the desired distances. A receiver at the far end of the transmission line converts optical signals back to electric signals which are subsequently displayed on a screen as television images or reproduced as voices in telephone communications.
As a rule, signals are transmitted over considerable distances - tens and hundreds of kilometers. Transmission along a path of a certain length results in signal attenuation and distortion. Optical signals are therefore converted back to electric signals, which are then amplified and reshaped in a repeater and, after conversion to optical signals, again fed into the transmission system. When used for communications within a city, when link lengths are of the order of tens of kilometers, fiber light guides eliminate the need to use complex and expensive retransmitters. With traditional copper wire cable, transmission of television signals require that retransmitters are installed every 500 m. Apart from this fiber-optics communication systems are more cost-efficient for yet another reason - they do not require copper and other expensive materials for their manufacture, the main raw material used being common sand, from which quartz glass is produced.
During the latter 1970s the efforts of scientists from our Institute and their colleagues from Gorky resulted in the production of batches of low-loss fiber light guides for the first city telephone networks in Moscow, Leningrad and Gorky.
Another highly promising development-fiber light guides based on oxygen-free glasses - merits a mention. These fiber light guides are designed to transmit infrared radiation, and we estimate optical losses in them to be lower than that of light guides based on quartz glasses by a factor of 10 to 20. This means that repeaters in telecommunication lines can be spaced at about 1,000-km intervals.
The scope of fiber light-guide application is not limited to communications and data-transmission systems alone. A high- sensitivity rotation sensor was developed at our Institute to take low rotation rate measurements required for the precise orientation of ships, satellites, balloons, etc. The electronic circuitry developed for this purpose provides measurement of very small phase shifts between the light rays; the sensor is able to detect rotation rates of the order of 0.1 and even 0.01 of a degree per hour. As compared to other rotation sensors (mechanical laser), the fiber-optics sensor is potentially much cheaper and simpler in design and manufacture.
Another highly attractive field of potential application is the use of fiber light guides in medicine, where a wide range of laser radiations with wavelengths ranging from ultraviolet to infrared are employed. Flexible thin fibers can be used to conduct the laser radiation to any part of the patient's body or to any internal organ.
New polycrystal light guides based on heavy metal halogenides (e.g., AgCl) have been developed at the Institute and are designed to transmit infrared radiation with wavelengths ranging from 5 to 10 mum. At present, however, their optical losses are rather high, though this is no obstacle to their use in medicine where radiation of relatively low power (10 to 50 mW) need to be transmitted over comparatively short distances (of the order of several meters).
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