Specialty Optical Fibers
The overwhelming majority of optical fibers for telecommunication acting on the world market are fibers that meet the international standards: ITUT Recommendation G.652 - G.656. The volume of the market remains large enough despite predictable temporary slowdown of its growth. On a background of these temporary fluctuations the pleasant exception is the confident growth (though not in such large volume) of the specialty optical fibers market. Manufacturers mark a growing interest in specialty fiber for use within optical components. Famous consultancy ElectroniCast predicts that the worldwide consumption of specialty optical fiber will rise. Many specialty fiber manufacturers are expanding their customer base by introducing them into aerospace, military and biomedical markets. Other manufacturers are currently seeing more opportunities for specialty fiber outside the telecom industry, such as in fiber-optic gyroscopes and sensors. However, in telecommunication the specialty fibers have made the most significant advances and there are still significant opportunities. It become clear, that the specialty fibers are being put to work in next-generation devices.
Now it is possible to name about twenty types of the specialty optical fibers with different constructive characteristics and basic properties.
The basic items of information about some widely distributed specialty optical fibers, conditionally classified on the most important areas of their application in optical communication, are shown below.
Fibers for Fiber Lasers and Amplifiers
Fibers Like an Active Medium
Erbium doped optical fibers are designed for optical amplifiers (EDFA) with a wide range of requirements to the technical characteristics, which is dedicated for DWDM, CATV and other telecom applications. EDFAs include: power amplifiers, pre-amplifiers and in-line amplifiers for C- and L-bands.
In a typical erbium-doped fiber amplifier design, erbium-doped fiber is pumped with a laser diode at 980 nm (or 1480nm) to provide gain in the 1550 nm region. The erbium-doped fiber must be designed to provide maximum pump absorption efficiency at 980 nm, as well as optimized signal gain at 1550 nm. This can be accomplished by designing the fiber with a high numerical aperture that is typically in the range from 0.23 to 0.25 in order to achieve a reasonable overlap of the fields of the pump and signal. The fiber cutoff wavelength is also of critical importance in the fiber design, as it determines the wavelengths at which single mode operation is distinguished from multimode operation. Typically, an erbium-doped fiber has such cutoff wavelength that ensures that the pump will propagate in the single mode regime that provides the maximum amount of overlap between the field and the erbium ions in the fiber core.
Rangeof values of some characteristics of typical erbium-doped fibers (C-Band):
Ytterbium-doped double-clad fiber and Co-doped Erbium-Ytterbium double-clad fibers are used in high power sources and amplifiers in the 1.5 µm region. These fibers were developed to match the requirements of high power optical amplifiers, industrial and military lasers, and IR sources. The fibers have been specifically designed to combine efficiently a single mode signal and high pumping power from a multimode diode into a passive double clad fiber. Associated with low-cost high power multimode diodes (915 or 976 nm) the double clad fiber can easily reach multi-watt output levels with efficient electrical to optical power ratio. Using step-index large-mode-area fibers in continuous-wave operation, output powers approach the kW regime with diffraction-limited beam quality. In the pulsed regime, an average power of about 100 W (even for femtosecond fiber laser systems) can be achieved. Ytterbium-doped dual-clad fiber amplifiers are an attractive technology for high power phased arrays. They offer many advantages including high gain and ease of thermal control.
Pump Delivery Fibers
These fibers have multimode core sizes selected to match the diameters of the inner claddings of Yb-doped double-clad fibers, which are used as active elements in fiber lasers and amplifiers. They deliver pump radiation in fiber lasers and output laser light in various fiber laser applications. They can be used differently as laser pump diode pigtails or as pump legs on couplers and combiners. Fiber combiners combine (add together) the output power of some pump lasers into one fiber and create a high-power pump.
These fibers have following features: multimode guide for pump radiation delivery, high NA (~0.45), attenuation at 915 nm about 3 dB/km, and high humidity stability. Some pump delivery fibers can redistribute the reverse-propagating light, which is the major cause of failure in multimode diodes used to pump high beam quality fiber lasers and fiber amplifiers.
Fibers for DWDM Add-Drop Multiplexers & Demultiplexers
The DWDM Add-Drop Multiplexers and Demultiplexers are usually created by using photosensitive fibers. The capability of an optical fiber under the action of light to change the core’s refractive index permanently has been named photosensitivity. When ultraviolet light illuminates the germanium-doped silica core of an optical fiber, the ultraviolet photons break atomic bonds and the refractive index of the core becomes changed permanently. Processing the fiber prior to irradiation using hydrogen-loading technology can enhance the refractive index change. Another feature of the photoinduced refractive index change is anisotropy, which is useful for fabricating rocking filters.
The photosensitive fibers are used to create Fiber Bragg Gratings (FBG), which are the main component of the OWDM Add-Drop Multiplexers and Demultiplexers. An FBG is a periodic variation of the refractive index of the fiber core along the length of the fiber. Illuminating the photosensitive fiber with a laser through a phase mask can create an FBG pattern. The principal feature of FBGs is that they reflect light in a narrow bandwidth that is centered about the Bragg wavelength. Fiber Bragg Grating shows high-reflectivity in a certain wavelength, low insertion loss, low transmission loss, high wavelength selectivity, and low crosstalk characteristics. Therefore it is a very promising device for composing an optical add/drop multiplexer. To separate the input signal from the counter propagating reflected signal, an optical non-reciprocal circulator is used. Each Add-Drop Multiplexer has two circulators: one for add-wavelength the second for drop-wavelength. A circulator typically has 0.5 to 1.0dB insertion loss. The insertion loss increases as more gratings and circulators are cascaded.
Range of values of some characteristics of typical photosensitive fibers:
Fibers for Optical Modulators
Two types of optical fiber modulators exist: in-line modulators and external modulators. Frequently both types are phase modulators.
External modulators can be built as a waveguide in a substrate. The waveguide makes the device to suite the fibers settled on the input and the output. These fibers are usually polarization maintaining (PM) fibers or ordinary single-mode (SM) fiber.
Another variant of external modulators is all-fiber acousto-optic modulators. Most commonly all-fiber acousto-optic modulators are frequency shifters based on surface acoustic wave (SAW). These modulators utilize polarization mode coupling in polarization maintaining fiber or spatial mode coupling in ordinary single mode fiber.
The in-linefiber acousto-optic modulator ordinarily contains an acoustic modulator, which modulates the optical phase of the light that propagates down an ordinary optical fiber, using stretching or squeezing optical fiber.
Thereby in optical fiber modulators are used polarization maintaining fiber or ordinary optical fiber.
Single mode fibers with birefringence guide optical radiation in two de-coupled modes, which are linearly polarized, mutually perpendicular, and have different phase travel speeds. PM fiber is designed to propagate only one polarization of the input light. The desirable direction of the polarization is obtained on the principle of a stress applying elliptical cladding surrounding a circular core, a circular cladding surrounding elliptical core, and others fiber structures. Depending on the applying structure, a PM fiber has a different name: "Bow-Tie”, "Oval Inner Clad", "PANDA", “3M Tiger”, et al.Range of values of some characteristics of typical polarization maintaining fibers:
* It is inversely proportional to the fiber's birefringence for a given wavelength and describes the length required for the linearly polarized mutually perpendicular modes to achieve different phase 2p .
Fibers for Optical Filters
Many types of optical filters exist now: Diffraction Grating Filters, Fabry-Perot Filters, Mach-Zehnder Interferometers Filters, All-fiber Filters, Gain Flattening Filters, and other types. In most of filters are used conventional fibers.
A coupled-waveguide Fabry-Perot (FP) filter is a resonator consisting of two parallel optical waveguides with partially reflecting mirrors at the two ends.
Mach-Zehnder filters are built by applying two directional couplers and two conventional fibers, one using as reference arm and another as the arm whose refractive index is varied with the control signal.
Fiber Bragg Grating filter consists a photosensitive fiber segment, which is formed as a Fiber Bragg Grating. The characteristics of such fibers are shown above in the item Fibers for WDM Add-Drop Multiplexers & Demultiplexers. If the grating period in the Fiber Bragg Grating is controlled, the filter becomes tunable. The grating period can be changed thermally or mechanically.
The Bragg principle is also used in All-fiber acousto-optic tunable filters, where the gratings are made by applying a high frequency acoustical signal on an optically transparent waveguide. All-fiber acousto-optic tunable filters include transducers, which are fiber surface, utilizing spatial mode coupling in ordinary single mode fiber, or fabricated on acousto-optic frequency shifters based on surface acoustic wave, which utilize polarization mode coupling in polarization maintaining fiber. The characteristics of these fibers are shown in the item Fiber for optical modulators.
In the gain flattening filters are used long-period grating with an index-varying period (~100 µm) written into bare fiber. Such grating provides coupling between the core modes and the cladding modes, which produce wavelength-depended loss. Gain flattening filters can be created using fused taper technology.
Fibers for Dispersion Compensators
Dispersion compensation can be executed by several methods. Depending of the method, for the compensation can be used specialty fibers or devices named dispersion compensators (Dispersion Compensating Modules). Some of the compensators include specialty fibers or optical waveguides.
These fibers have a high negative dispersion and also a negative dispersion slope.Range of values of some characteristics of typical dispersion compensating fibers:
One can see the attenuation of such fibers is higher than ordinary single mode fibers. The problem has been solved in some dispersion compensators.
These devices include Chirped Fiber Bragg Gratings (see Fibers for Optical Filters). The grating period of a Chirped Fiber Bragg Grating is made linearly variable along the grating. Because of this, different wavelengths are reflected back at different depths of the grating. Hence, their travel times are different and the chromatic dispersion becomes compensated. All linearly chirped compensators are not tunable. In tunable compensators the gratings must be nonlinearly chirped. The varying dispersion arises during mechanical or thermal stretching the fiber.Thereby for dispersion compensation are used optical fiber with negative dispersion and photosensitive fibers, which are used to fabricate Chirped Fiber Bragg Gratings.
Fibers for Supercontinuum Optical Sources
A particular example of specialty fiber is photonic-crystal fibers (PCF). Due to possessing a series of unique properties, they find now applications not only in fiber-optic telecommunications, but also in high-power transmission, sensitive sensors, nonlinear devices, and other areas. PCFs use an air-filled cladding region usually as a cladding surrounding a core where light is confined. Their internal periodic structure made of capillaries, filled with air, presents in cross-section a hexagonal or a square lattice. The holes in the lattice can be made circular or can be almost arbitrary shapes. Manipulating the type of lattice, lattice pitch, air hole shape and diameter, and refractive index of the glass allows obtaining features, which do not exist by ordinary fiber. Furthermore, PCFs can have two modes of operation, according to their light guiding mechanisms: index guiding mechanism and the photonic bandgap mechanism.
PCFs with a core, which have higher average refraction index than the microstructured cladding (for instance, a solid core), can operate on the same principle as ordinary optical fiber, i.e. according to index guiding mechanism. PCFs created by the microstructured cladding with a hollow (air) central channel guides the light predominantly through this channel due to the photonic bandgap mechanism, which can confine light in a lower-index core and even a hollow core.
If a PCF has a very small core and a low refraction index of the cladding (due to an extremely high filling fraction), it has an extremely small effective area and high nonlinear coefficients. These nonlinear properties make the PCF capable to supercontinuum generation that means to convert light with certain wavelength to both longer and shorter wavelengths in a wide range.Range of values of some characteristics of typical PCF for supercontinuum generation:
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