Photonic Crystal Fiber Dictionary
On our website you will find a phletora of acronyms and photonic phenomena which can be somewhat obscure is you are not directly involved in the fiber manufacturing business. To guide you, we have compiled a small photonic crystal fiber dictionary, which list definitions and explanations the most used acronyms.
More definitions can be found in the Photonics Dictionary.
Air-clad fibers
The PCF equivalent to the double-clad fiber
is the air-clad LMA fiber. The fiber consists of a LMA
structure with an active, doped core, which is placed inside an air-clad pump
guide. Due to the greatly enhanced index contrast, the air-clad can provide
very large numerical
apertures determined by the bridge-width in the air-clad. Consequently,
the NA is only limited by the practical handling of the fibers where cleaving
of the fibers becomes increasingly challenging at NAs above 0.6 (the exact
limit depends on fiber design and cleaving equipment and can be as high as
0.7). Moreover, as the fiber is air-glass, the thermal conductivity is greatly
improved compared to polymer clad fibers, and there is no material degradation
[8]. The power density is only limited by the damage threshold of silica.
The combination of very large MFD and high NA makes it possible to create
lasers and amplifiers with very short fiber lengths, drastically reducing
the nonlinear effects.
See also our section on air-clad fiber
products .
DC - Double Cladding Fibers
High power fibers are commonly designed with a double-cladding structure, where a second low index region acts as cladding for a large pump core. In the center of the pump core is located a much smaller doped signal core . The major advantage of the double cladding design, over the more traditional core pumped variety, is the large pump area and high numerical aperture, enabling pumping with relatively low-cost multimode diode stacks.
Standard double-cladding fibers utilize a low-index polymer coating to create the cladding for the pump core. The obtainable refractive index of the polymer limits the numerical aperture of the fibers (in praxis to below 0.48) which limits the pump power absorption pr. fiber length and thereby how short a fiber laser can be made. The polymer potentially degrades at high pump powers and elevated temperatures. Limited heat conductivity of the polymer may isolate the pump core thermally and cause undesired heating of the fiber. Both problems may cause reduced reliability and catastrophic breakdown at high powers. In addition, the step-index core technology limits the mode-field diameter (MFD) to around 25 µm for single-mode operation.
The disadvantages of the traditional polymer clad fibers can be overcome by using air-clad fibers instead.
See also our section on air-clad fiber products.
Holey Fibers
LMA - Large Mode Area
The LMA fiber is a sub group of the photonic crystal fibers.
NA - Numerical Aperture
The sine of the vertex angle of the largest cone of meridional rays that can enter or leave an optical system or element, multiplied by the refractive index of the medium in which the vertex of the cone is located. Generally measured with respect to an object or image point, and will vary as that point is moved.
See also our section on passive high NA fibers and double-clad active fibers.
Microstructured fibers
PCF - Photonic Crystal Fiber
The term photonic crystal fiber is inspired by the unique cladding structure of this fiber class. Standard fibers guide light by total internal reflection between a core with a high refractive index (typically germanium doped silica), embedded in a cladding with a lower in-dex (typical pure or fluorine-doped silica). The index differences in PCFs are obtained by forming a matrix of different material with high and low refractive index. In this way, a hybrid material is created with properties not obtainable in solid materials (e.g. very low index or novel dispersion). The hybrid material cladding can be constructed with a structure similar to that found in cer-tain crystal, which is where the term photonic crystal fiber originates. The fibers are not fabricated in crystal-line materials as the name might indicate.
There are two fundamental classes of PCFs: In-dex-guiding PCFs and fibers that confine light through a photonic bandgap (PBG).
An index guiding PCF comprises a solid glass high-index core embedded in an air-filled cladding structure where a number of air holes are arranged in a pattern that runs along the length of the fiber, creating a hybrid air-silica material with a refractive index lower than the core. This air-silica matrix structure has given rise to several other names like microstructured and holey fi-bers, but despite the difference in terminology, they all refer to the same fiber type.
Compared to the well-known standard fiber, one of the most novel features of the triangular PCFs is the possibility to design fibers, which exhibit no second-order mode cut-off, rendering them single-mode at any wavelength. This fiber type is known as endlessly single-mode fibers, and can be realized by choosing suffi-ciently small holes in the cladding.
See also our technology tutorial.
PM - Polarization Maintaning
Single-mode fiber that preserves the plane of polarization of the light launched into it as the beam propagates through its length. Also known as a polarization-preserving fiber. The polarization is maintained by introducing asymmetry in the fiber structure, either in its shape (geometrical birefringence) or in its internal stresses (stress-induced birefringence). Because of this asymmetry, the two perpendicularly polarized modes transmitted by the fiber have different propagation constants, reducing cross-coupling between them as compared with conventional single-mode fiber.
See also our section on polarization-maintaining fiber products.
PZ - Polarizing
A polarizer is an optical device capable of transforming unpolarized or natural light into polarized light, usually by selective transmission of polarized rays. A polarizing fiber does exactly the same, by only transmitting one polarization state in a certain wavelength range.
See also our section on polarizing fiber products.
SCG - Supercontinuum Generation
Supercontinuum generation is the formation of broad continuous spectra by
propagation of high power pulses through nonlinear media, and was first observed
in 1970 by Alfano and Shapiro. The term supercontinuum does not cover a specific
phenomenon but rather a plethora of nonlinear effects, which, in combination,
lead to extreme pulse broadening.
Provided enough power is available, supercontinuum generation can be observed
in a drop of water, but the nonlinear effects involved in the spectral broad-ening
are highly dependent on the dispersion of the me-dia and clever dispersion
design can significantly reduce power requirements. The widest spectra are
obtained when the pump pulses are launched close to the zero-dispersion wavelength
of the nonlinear media and the introduction of the nonlinear PCFs with zero-dispersion
wavelengths in range of the Ti:Sapphire fem-tosecond laser systems, therefore,
led to a boom in su-percontinuum experiments.
Supercontinua combine high brightness with broad spectral coverage –
a combination offered by no other technology.
See also our section on supercontinuum generation and nonlinear fiber products.
Supercontinuum
application note 
Step-index fibers
A step-index fiber is the traditional way of making a fiber waveguide. A
high index core (typically germanium-doped silica) is embedded in a cladding
region with a lower index (typically pure silica). Due to total internal reflection
on the core/cladding boundary, light launched below a certain critical angle
(defining the Numerical
Aperture of the fiber), is confined in the core. The silica cladding is
protected by on ore more layers of polymer coating.
The standard step-index fiber forms the basis of all optical telecommunication
networks.
