Optical Fibers: Past And Present

The main message of his development was that fiber optic communication can be organized using optical fibers. In his research, Kao introduced the unique design features of optical fiber and its materials to the world. Researches of the scientist can rightfully be considered the basis of fiber optic telecommunications today.

The very first mention of the term “optical fiber” was first used in 1956 by NS Kapany from the USA. Today, fiber optic communication technologies have so firmly penetrated our lives that we no longer see anything surprising in them and perceive their presence as well as the presence of a water supply system in an apartment building. During the development of fiber optics, many interesting studies and experiments have been carried out.

Active talks about optical fiber LEDs began in the fifties of the last century. Then they began to make them from various kinds of transparent materials. However,  the transparency of those materials was not enough for good light conductivity. In 1966, a group of scientists led by Charles Kuen Kao concluded that quartz glass would be the most suitable material for fiber optic communications. 

Even then, Kao believed that with the help of fiber optics it would be possible to transmit information and soon this type of communication would replace the signal transmission via copper wires. Three years later, Kao received a fiber with an attenuation coefficient of 4 dB/km. This result was the first instance of ultra-transparent glass. 

A year later, Corning Incorporated produced optical fibers with a stepped refractive index profile and reached an attenuation coefficient of 20 dB/km at a wavelength of 633 nm. For the first time, a quartz optical fiber passed a light beam at a distance of up to 2 kilometers. Quantum data transfer is currently developing similarly as an experiment and commercial use over short distances. 

Today, optical fiber is used in many industries besides Telecom. It includes X-ray machines, where it provides galvanic isolation between a high voltage source and low-voltage control equipment. In this way, staff and patients receive isolation from the high-voltage part of the equipment. Optical fiber is used in distribution devices of electric substations as a sensor of the protection system.

Optical fibers are widely used in various types of measuring systems, where it is impossible to use traditional electrical devices. For example, their applications include temperature measurement systems in jet engines of an airplane, tomographic medical devices for the study of internal organs, including the brain, etc. Optical fiber sensors can measure vibration frequency, rotation, displacement, speed and acceleration, twisting, and other parameters.

Today, optical fiber-based gyroscopes are used, which operate based on the Sagnac effect. This gyroscope has no moving parts, which makes it very reliable. Even though modern navigation systems use a huge number of different sensors that determine the position of the object, the most independent system can be created only based on fiber optic gyroscopes.

Fiber optics are widely used in security alarm systems. Such a security system is arranged as follows: when an intruder enters the territory, the conditions for passing light through the light guide change, and an alarm is triggered. Optical fiber is actively used for decorative purposes, as a holiday decoration, in art and advertising.

New types of optical fibers are constantly being developed. For example, photonic-crystal light guides. The propagation of light in them is based on slightly different principles. These fiber optics can be used as a liquid, chemical, and gas sensor. Also, it can be used for transporting high-power radiation for industrial or medical purposes.

Fiber lasers with a continuous output power of several tens of kilowatts are no longer new. Weapons based on 5.5 kW lasers with 6 optical fiber were tested in the U.S. Navy in 2014. Fiber lasers cut metal and concrete. For example, the IPG Photonics metal cutting machine has a capacity of 100 kW.

The development of an optical fiber that could be used to transmit laser energy with the power of several kilowatts continues. In theory, the transmission of emission with a power of 10 kW over a 250 m long fiber with a core diameter of 150 microns is considered to be possible. It is also worth noting that multi-core optical fibers are being actively developed today. Their use will significantly increase the total bandwidth of the fiber optic network.

Fiber systems are in its fifties, but the fiber technology is not going to retire. Innovations in the field of optical fiber appear regularly and Telecom is not the only industry interested in the development of fiber optic technology. Optical fibers are used, for example, as sensors for measuring temperature, pressure, and mechanical stresses. 

Besides, fiber optic systems are often used as distributed spectroscopic and acoustic detectors for probing oil wells. In extreme conditions, cracks appear on the surface of the fiber. At high temperatures and pressures, hydrogen and moisture quickly penetrate the material, and its transparency and, as a result, other characteristics deteriorate.

Physicists have found out why this happens. Using Raman scattering, scientists have proved that allotropic carbon compounds (carbon nanotubes, fullerenes, graphenes) are present in the protective layer of optical fibers. Such nanostructures can play the role of additional channels for the penetration of hydrogen and moisture to the core of optical fibers, impairing transparency for light signals. The results of the study will help optimize the technological processes for creating optical fibers with a protective carbon layer so that they can be used in the exploration of oil wells.

Innovations in fiber optics

Standard optical fiber with a "step" change in the refractive index directs the light due to the effect of total internal reflection. This effect is easy to observe, if you look from below at a glass of water at a slight angle, the surface of the water will appear mirrored. Similarly, light beams in a fiber system are reflected from the walls and can propagate without loss over large distances. 

At the same time, the rather low values of the angle of internal reflection in the optical fiber and the wave nature of the light establish the conditions that the propagation of light is possible only at certain angles. In other words, the fiber supports the propagation of several discrete "modes". Optical fiber, which allows the propagation of only one mode, is called single-mode - these are the fibers that are most suitable for use in telecommunications.

What are the disadvantages of such a standard optical fiber with a stepwise change in the refractive index? In fact, none. Such a fiber optic system copes pretty well with all the applications for which it was originally developed. The problem is that modern industry needs something more.

It is not enough to perform only one job  - well-flexibility (literally and figuratively), the ability to integrate with other devices and devices is valued as highly as the classic reliability of conventional fiber optics. And such a fiber loses wherever some unusual properties are required, such as the ability to transmit high power, compatibility with various sensors and fiber optic lasers on rare-earth metals, possess high nonlinearity, birefringence, or dispersion. 

In other words, conventional optical fiber is only perfect for simple applications in telecommunications. A huge number of new applications appeared with the advent of such objects as microstructured fiber, an optical fiber with a photonic crystal, fiber lasers, mode synchronizers on carbon nanotubes, nanoplasma structures.

Microstructured fiber optics

Unlike optical fiber with a stepwise change in the refractive index, which is usually made of two or more types of glass (for example, germanium-doped glass has a higher refractive index and is used to produce the central part of the fiber), a microstructured fiber optics can be made entirely of one type of glass. The outer layer with a low refractive index is replaced here with a large number of cylindrical cavities filled with a specific gas or simply air.

The technique of producing such optical fibers was introduced in 1991 and has been constantly developing since then. The technology is based on a simple idea: glass tubes of relatively large size are stacked together in the desired structure, which is subsequently drawn under heating into an optical fiber with a specific arrangement of air cavities, the geometry of which is determined by the initial arrangement of the tubes. Depending on how the full internal reflection mechanism is implemented, these fibers can be divided into two types: cavity fibers and fibers on photonic crystals.

Cavity optical fibers

In the cavity fibers, the glass central part is surrounded by a set of cylindrical air cavities, which reduces the effective refractive index and greatly modifies the effect of total internal reflection. Since the size of the air cavities and the distance between them are comparable with the wavelength of light (hundreds of nanometers), the effective refractive index will also vary with the wavelength of transmitted light. The result of this is the ability of such an optical fiber to carry only one mode, regardless of the wavelength. Such fibers are commonly used to transmit high light powers and have low nonlinearity.

Optical fiber with a photonic crystal

Compared to all other fibers in the optical fiber with a photonic crystal does not use the total internal reflection. The collecting of light in the center of such a fiber system occurs due to the phenomenon of interference on a periodic structure with a size of the order of the wavelength created by the lattice of cylindrical cavities - a photonic crystal. 

Although the fact that the physics of photonic crystals, and especially their production, is still developing, we often face related phenomena in everyday life. These phenomena give a bright color to the wings of some butterflies and holograms on our credit cards. And in that and other cases, certain colors stand out from the white light due to interference. The advantage of such fiber optics is the low dispersion since light now propagates in an almost dispersion-free medium-air.

Frequency converters on cavity lasers

The ability to get rid of any restrictions on the environment in which light propagates inside the fiber opens up very interesting prospects and applications. Thus, light propagating inside the central part of the optical fiber filled with a certain gas will collect information about this gas (for example, due to Raman scattering).

Raman scattering can also be used for frequency converters. The passing light excites vibrational modes in the gas molecules that fill the fiber. The reemission of light at a lower frequency, as a rule, is a rather weak effect, however, in the case of an optical fiber, it is enhanced due to the enormous length at which the interaction takes place (along the entire length of the fiber), as well as due to local amplification of the electric field.

Fiber optic lasers

Fiber optic laser technology has developed rapidly over the past few decades. In such lasers, the active medium is located inside the optical fiber itself. The characteristics of fiber lasers are improving every year and have almost reached the characteristics of conventional lasers in terms of power, pulse duration, and generation bandwidth. At the same time, the combination of fiber lasers with microstructured fiber opens up new prospects for such devices. Such structures have low bending losses and increased selectivity between modes.

Mod synchronizers on carbon nanotubes

Currently, fiber lasers are used in a variety of fields, from telecommunications to laser surgery. One of the main advantages of such lasers is the ability to generate ultra-short pulses of light in the picosecond and sub-picosecond ranges. For such applications, fiber optic lasers use passive mod synchronizers, a device whose optical transparency varies with the intensity of the transmitted light. Recently, mode synchronizers on carbon nanotubes are more often used.

Chemical and biological sensors based on fiber optic technology

The development of nanotechnology and in particular nanoplasmonics, together with fiber optics, leads to the emergence of new devices and sensors. Nanoplasmonic structures allow efficiently converting plasmon resonances of various chemical compounds adsorbed on the surface of an optical fiber into optical signals propagating through a fiber. Multiple amplification of the local electric field makes these sensors sensitive to single molecules.

Recently, fiber optics is undergoing a rebirth through integration with various nanostructures. This symbiosis leads to completely new, sometimes unexpected, applications. There is no doubt that soon we will witness the widespread penetration of devices based on optical fibers in various areas of the industry from telecommunications to medicine.

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