Special Optical Fibers: The Overview

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Special optical fibers: the overview
At present, optical fibers are widely used not only in fiber-optic data transmission lines but also in various fiber optic cables and sensors of physical quantities and other fiber-based devices. The specifics of this application require the creation of optical fibers with special properties.
The main purpose of special optical fibers is to perform various operations with light signals (amplification, modulation, filtration, etc.), as well as the operation of fibers in special modes and conditions (for example, under high mechanical loads – shock or
static, high temperature, radiation, humidity, UV, average IR, and far-IR ranges), so the requirements for optical losses in such fibers fade into the background.
The typical length of special optical fibers is not kilometers, as, in the case of long-distance fiber cables, it achieves from one to several tens of meters. Today, manufacturers of fiber optic solutions note a growing interest in specialized fibers for use in optical components.
For example, global consumption of special optical fibers in 2007 amounted ...
... to more than $ 1.2 billion. Many manufacturers of special optical fibers are expanding their customers in the field of biomedicine, aviation, input/output, and military industries. Other manufacturers see more opportunities for using special fiber optic cables in sensors and fiber optic gyroscopes.
Nevertheless, the use of special optical fibers in communication systems has made more significant progress and promises many new opportunities. It is already clear that in any case of further development, special fiber cables will be used in the equipment of next-generation communication networks.
Currently, there are about twenty types of special optical fibers that differ in their design characteristics and basic properties. The following basic information about some of the widely used special optical fibers is provided based on the most important areas of their application in communications.

Optical fiber for lasers and amplifiers
Ytterbium fiber with a double-clad is used in high-power radiation sources and amplifiers. These fiber optic cables are designed to meet the requirements for high-power amplifiers, industrial and military lasers, and infrared sources.
The optical fibers are specifically designed to effectively combine a single-mode signal and high pumping power from a multimode diode into a passive double-clad fiber. The combination of low-cost, high-output multimode diodes with these fibers allows for easily achieving multi-watt power levels with an effective ratio of electrical power to optical power.
These fiber cables have a multimode core that corresponds in size to the diameter of the inner clad of the ytterbium fiber used as an active element for fiber lasers and amplifiers. They are used to transfer radiation energy from the optical pump source of a fiber laser (or amplifier) to its active element and deliver laser output radiation for various applications.

Optical fibers for optical multiplexers and demultiplexers
Optical multiplexers and demultiplexers of an input/output are typically created with the use of photosensitive fibers. The ability of an optical fiber to change the refractive index of the core under the influence of light is called the fiber's photosensitivity.
Photosensitive fibers are used to create fiber Bragg gratings, which are the main component of radiation input-output multiplexers and demultiplexers. A fiber Bragg grating is an optical fiber with a periodic change in the refractive index along with its core.
By irradiating a photosensitive fiber with a laser beam through a phase mask, a fiber Bragg grating can be created. The main property of this grating is the reflection of light propagating through the fiber in a narrow band that is centered around the Bragg wavelength.

Optical fibers for modulators
There are two types of optical waveguide modulators: planar and fiber. Both types are most often phase modulators. Thus, both polarizing fibers and conventional optical fibers are used in these modulators.

Optical fibers for filters
Currently, there are numerous types of optical fiber filters: filters on diffraction or Bragg gratings, Fabry–Perot and Mach–Zander filters, etc.
For example, a Bragg filter is a photosensitive optical fiber with a Bragg grating formed on part of it. If you change (control) the period of the FBG filter, it becomes a tunable filter. The grating period can be changed by heating or mechanical stresses.

Optical fibers for dispersion compensation
Dispersion compensation can be performed using several methods. For example, special fiber cables or dispersion-compensating modules can be used.
These fiber optic cables have a large negative dispersion, as well as a negative slope of the dispersion curve. A wide range of operations can be performed using fibers that compensate for dispersion. The second example of dispersion compensation is fiber Bragg gratings with a variable period.

Optical fibers for supercontinuum sources
Photonic crystal fibers are a special example of special optical fibers. Thanks to the appearance of a series of unique properties, they are used not only in optical communication, but also in high-power transmission, sensitive sensors, non-linear devices, and other areas.
Manipulating the type of grating, its step, the shape of the air channels, and the refractive index of the glass allows for obtaining properties that do not exist in conventional fiber optic cables. For example, nonlinear properties make photon-crystal fibers capable of generating a supercontinuum, i.e. converting light of a certain wavelength into light with longer and shorter waves. Thus, it is possible to create broadband light sources based on new principles.

Fiber optic amplifiers
It is known that the optical signal is attenuated by 10-20 dB at every 50-100 km of fiber optic cables. This fact requires compensation. Previously, the only way to compensate for losses in the line was the use of regenerators in the existing communication lines.
Currently, three types of optical amplifiers have been developed for fiber optic systems: semiconductor optical amplifiers, fiber amplifiers based on rare-earth ions (for example, erbium), and Raman fiber amplifiers.
The most widespread use is currently found in optical fiber amplifiers. The current level of technology development allows for employing various impurities into quartz fiber, in particular, rare earth elements. Erbium optical fiber amplifiers are the most common at present.
Advantages of erbium fiber amplifiers include:
– high energy transfer from the pump to signal > 50 %;
- simultaneous amplification over a wide range of wavelengths, i.e. they are suitable for WDM systems;
- output limit greater than 10-25 dB / m;
– the gain time constant is large enough for overcoming modulation interference;
- low noise factor;
– polarization independence (which reduces loss);
- the opportunity to use these optical fibers in remote systems;
- the erbium amplifier can also operate in the S and L ranges.
The disadvantages of erbium fiber amplifiers include:
- large dimensions of the erbium amplifier module;
- the inability to integrate with semiconductor devices;
- amplified spontaneous emission (ASE);
– crosstalk;
- gain limit.

Raman fiber amplifier
Raman amplifiers are promising for use in fiber optic systems due to their following fundamental advantages: they can amplify at any wavelength; the fiber light guide itself can be used as the active medium of Raman amplifiers; the gain spectrum of these amplifiers depends on the pump spectrum (wavelength), so the selection of pump sources can form a very wide (more than 100 nm) gain band; Raman amplifiers have a low noise level.
The main disadvantage of Raman amplifiers is their low conversion efficiency, which requires the use of a fairly powerful continuous pump radiation to obtain the typical signal gain of 30 dB for fiber optic systems.

Double-clad activated optical fibers
An appropriate pump is required for any laser to work. In particular, fiber lasers use optical fiber pumping. It is proposed to use double-clad optical fibers to increase the output power of fiber lasers and simplify the input of radiation from semiconductor laser diodes into the fiber light guide.

Photonic crystal activated fibers
Recently, photonic crystal fiber-based lasers have been rapidly developed. Photonic crystal waveguides and optical fibers are a new type of waveguides. Their appearance is associated with the creation and research of new fiber optic systems – photonic crystals. They have the following distinctive features in comparison to conventional fibers:
- high numerical aperture;
- large core diameter, which can support the single-mode operation. As a result, high pumping powers and generation without noticeable heating can be realized in photonic crystal fibers;
- the absence of non-linear effects;
- high anisotropy of the optical fiber structure, allowing transmission of radiation with a high degree of polarization.

Anisotropic single-mode fiber cables
Along with the long-distance lines, fiber optic cables are widely used in a wide variety of measurement, diagnostic, and highly sensitive monitoring and control systems. Anisotropic single-mode optical fibers promote the development of sensors for measuring various physical quantities and such unique devices as fiber optic gyroscopes.
Many manufacturers of special optical fibers are expanding their customers in the field of biomedicine, aviation, and military industries. Other manufacturers see more opportunities for using special fiber optic cables in sensors and fiber optic gyroscopes. Nevertheless, the use of special optical fibers in communication systems has made more significant progress and promises many new opportunities. It is already clear that in any case of further development, special fiber cables will be used in the equipment of next-generation communication networks.
If you would like to obtain an optical fiber product, you should choose the Optromix company. Optromix is a provider of top quality special fibers and broad spectra optical fiber solutions. The company delivers the best quality special fibers and fiber optic cables, fiber optic bundles, spectroscopy fiber optic probes, probe couplers, and accessories for process spectroscopy to clients. If you have any questions or would like to buy an optical fiber, please contact us at info@optromix.com