Double-clad fiber (DCF) is a class of
optical fiber
An optical fiber, or optical fibre in Commonwealth English, is a flexible, transparent fiber made by drawing glass (silica) or plastic to a diameter slightly thicker than that of a human hair. Optical fibers are used most often as a means to ...
with a structure consisting of three layers of optical material instead of the usual two. The inner-most layer is called the ''
core
Core or cores may refer to:
Science and technology
* Core (anatomy), everything except the appendages
* Core (manufacturing), used in casting and molding
* Core (optical fiber), the signal-carrying portion of an optical fiber
* Core, the centra ...
''. It is surrounded by the ''inner
cladding
Cladding is an outer layer of material covering another. It may refer to the following:
*Cladding (boiler), the layer of insulation and outer wrapping around a boiler shell
*Cladding (construction), materials applied to the exterior of buildings
...
'', which is surrounded by the ''outer cladding''. The three layers are made of materials with different
refractive indices
In optics, the refractive index (or refraction index) of an optical medium is a dimensionless number that gives the indication of the light bending ability of that medium.
The refractive index determines how much the path of light is bent, or ...
.
There are two different kinds of double-clad fibers. The first was developed early in optical fiber history with the purpose of engineering the
dispersion
Dispersion may refer to:
Economics and finance
* Dispersion (finance), a measure for the statistical distribution of portfolio returns
* Price dispersion, a variation in prices across sellers of the same item
*Wage dispersion, the amount of variat ...
of optical fibers. In these fibers, the core carries the majority of the light, and the inner and outer cladding alter the waveguide dispersion of the core-guided signal. The second kind of fiber was developed in the late 1980s for use with high power
fiber amplifiers and
fiber lasers. In these fibers, the core is doped with
active
Active may refer to:
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* ''Active'' (album), a 1992 album by Casiopea
* Active Records, a record label
Ships
* ''Active'' (ship), several commercial ships by that name
* HMS ''Active'', the name of various ships of the British Royal ...
dopant material; it both guides and amplifies the signal light. The inner cladding and core together guide the
pump
A pump is a device that moves fluids (liquids or gases), or sometimes slurries, by mechanical action, typically converted from electrical energy into hydraulic energy. Pumps can be classified into three major groups according to the method they u ...
light, which provides the energy needed to allow amplification in the core. In these fibers, the core has the highest refractive index and the outer cladding has the lowest. In most cases the outer cladding is made of a
polymer
A polymer (; Greek '' poly-'', "many" + ''-mer'', "part")
is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...
material rather than
glass
Glass is a non-crystalline, often transparent, amorphous solid that has widespread practical, technological, and decorative use in, for example, window panes, tableware, and optics. Glass is most often formed by rapid cooling (quenching) of ...
.
Dispersion-compensating fiber
In double-clad fiber for dispersion compensation, the inner cladding layer has lower refractive index than the outer layer. This type of fiber is also called ''depressed-inner-cladding fiber'' and ''W-profile fiber'' (from the fact that a symmetrical plot of its refractive index profile superficially resembles the letter W).
[
]
This type of double-clad fiber has the advantage of very low
microbending losses. It also has two zero-dispersion points, and low dispersion over a much wider
wavelength
In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats.
It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tro ...
range than standard singly clad fiber. Since the dispersion of such double-clad fibers can be engineered to a great extent, these fibers can be used for the compensation of
chromatic dispersion
In optics, and by analogy other branches of physics dealing with wave propagation, dispersion is the phenomenon in which the phase velocity of a wave depends on its frequency; sometimes the term chromatic dispersion is used for specificity to o ...
in
optical communications
Optical communication, also known as optical telecommunication, is communication at a distance using light to carry information. It can be performed visually or by using electronic devices. The earliest basic forms of optical communication date b ...
and other applications.
Fiber for amplifiers and fiber lasers
In modern double-clad fibers for high power fiber amplifiers and lasers, the inner cladding has a higher refractive index than the outer cladding. This enables the inner cladding to guide light by
total internal reflection
Total internal reflection (TIR) is the optical phenomenon in which waves arriving at the interface (boundary) from one medium to another (e.g., from water to air) are not refracted into the second ("external") medium, but completely reflected b ...
in the same way the core does, but for a different range of wavelengths. This allows
diode laser
The laser diode chip removed and placed on the eye of a needle for scale
A laser diode (LD, also injection laser diode or ILD, or diode laser) is a semiconductor device similar to a light-emitting diode in which a diode pumped directly with e ...
s, which have high power but low
radiance
In radiometry, radiance is the radiant flux emitted, reflected, transmitted or received by a given surface, per unit solid angle per unit projected area. Radiance is used to characterize diffuse emission and reflection of electromagnetic radiati ...
, to be used as the optical pump source. The pump light can be easily coupled into the large inner cladding, and propagates through the inner cladding while the signal propagates in the smaller core. The doped core gradually absorbs the cladding light as it propagates, driving the amplification process. This pumping scheme is often called ''cladding pumping'', which is an alternative to the conventional ''core pumping'', in which the pump light is coupled into the small core. The invention of cladding pumping by a Polaroid fiber research team (H. Po, ''et al.'') revolutionized the design of fiber amplifiers and lasers.
[
] Using this method, modern fiber lasers can produce continuous power up to several kilowatts, while the signal light in the core maintains near
diffraction-limited beam quality.
[
]
The shape of the cladding is very important, especially when the core diameter is small compared to the size of the inner cladding. Circular symmetry in a double-clad fiber seems to be the worst solution for a fiber laser; in this case, many
modes
Mode ( la, modus meaning "manner, tune, measure, due measure, rhythm, melody") may refer to:
Arts and entertainment
* '' MO''D''E (magazine)'', a defunct U.S. women's fashion magazine
* ''Mode'' magazine, a fictional fashion magazine which is ...
of the light in the cladding miss the core and hence cannot be used to pump it.
[
] In the language of
geometrical optics
Geometrical optics, or ray optics, is a model of optics that describes light propagation in terms of ''rays''. The ray in geometrical optics is an abstraction useful for approximating the paths along which light propagates under certain circumstan ...
, most of the
rays
Ray may refer to:
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* Ray (fish), any cartilaginous fish of the superorder Batoidea
* Ray (fish fin anatomy), a bony or horny spine on a fin
Science and mathematics
* Ray (geometry), half of a line proceeding from an initial point
* Ray (gra ...
of the pump light do not pass through the core, and hence cannot pump it.
Ray tracing,
[
] simulations of the paraxial propagation
[
] and mode analysis
[
] give similar results.
Chaotic fibers
In general, modes of a waveguide have "scars", which correspond to the classical trajectories. The scars may avoid the core, then
the mode is not coupled, and it is vain to excite such a mode in the double-clad fiber amplifier. The scars can be distributed more or less uniformly in
so-called
chaotic fibers
[
] have more complicated cross-sectional shape and provide more uniform distribution of
intensity in the inner cladding, allowing efficient use of the pump light. However, the scarring takes place even in chaotic fibers.
Spiral shape
An almost-circular shape with small spiral deformation seems to be the most efficient for
chaotic fibers. In such a fiber, the
angular momentum
In physics, angular momentum (rarely, moment of momentum or rotational momentum) is the rotational analog of linear momentum. It is an important physical quantity because it is a conserved quantity—the total angular momentum of a closed syst ...
of a ray increases at each reflection from the smooth wall, until the ray hits the "chunk", at which the spiral curve is broken (see figure at right). The core, placed in vicinity of this chunk, is intercepted more regularly by all the rays compared to other chaotic fibers. This behavior of rays has an analogy in wave optics. In the language of
modes
Mode ( la, modus meaning "manner, tune, measure, due measure, rhythm, melody") may refer to:
Arts and entertainment
* '' MO''D''E (magazine)'', a defunct U.S. women's fashion magazine
* ''Mode'' magazine, a fictional fashion magazine which is ...
, all the modes have non-zero derivative in vicinity of the chunk, and cannot avoid the core if it is placed there. One example of modes is shown in the figure below and to the right. Although some of modes show scarring and wide voids, none of these voids cover the core.
The property of DCFs with spiral-shaped cladding can be interpreted as conservation of angular momentum. The square of the derivative of a mode at the boundary can be interpreted as pressure. Modes (as well as rays) touching the spiral-shaped boundary transfer some angular momentum to it. This transfer of angular momentum should be compensated by pressure at the chunk. Therefore, no one mode can avoid the chunk. Modes can show strong scarring along the classical trajectories (rays) and wide voids, but at least one of scars should approach the chunk to compensate for the angular momentum transferred by the spiral part.
The interpretation in terms of angular momentum indicates the optimum size of the chunk. There is no reason to make the chunk larger than the core; a large chunk would not localize the scars sufficiently to provide coupling with the core. There is no reason to localize the scars within an angle smaller than the core: the small derivative to the radius makes the manufacturing less robust; the larger
is, the larger the fluctuations of shape that are allowed without breaking the condition
. Therefore, the size of the chunk should be of the same order as the size of the core.
More rigorously, the property of the spiral-shaped domain follows from the theorem about boundary behavior of modes of the
Dirichlet Laplacian.
[
] Although this theorem is formulated for the core-less domain, it prohibits the modes avoiding the core. A mode avoiding the core, then, should be similar to that of the core-less domain.
Stochastic optimization of the cladding shape confirms that an almost-circular spiral realizes the best coupling of pump into the core.
Tapered
Tapered double-clad fiber (T-DCF) is a fiber whose outer and inner claddings and core diameters vary smoothly with length. The core at narrow side of T-DCF supports propagation of the
fundamental mode
A normal mode of a dynamical system is a pattern of motion in which all parts of the system move sinusoidally with the same frequency and with a fixed phase relation. The free motion described by the normal modes takes place at fixed frequencies ...
only, whereas at the wide side the core is able to guide many modes. However, it was shown experimentally that light launched into the narrow end of a T-DCF propagates into the wide core without any changes of mode content.
As a result, at the wide (substantially multimode) end of T-DCF light propagates only in the lowest-order mode with excellent beam quality. Thus, tapered fiber is a unique and easy way to implement fundamental mode regime propagation (and amplification) in a multimode fiber.
Filling factor
The efficiency of absorption of pumping energy in the fiber is an important parameter of a double-clad fiber laser. In many cases this efficiency can be approximated with
:
where
:
is the cross-sectional area of the cladding
:
is the radius of the core (which is taken to be circular)
:
is the
absorption coefficient
The linear attenuation coefficient, attenuation coefficient, or narrow-beam attenuation coefficient characterizes how easily a volume of material can be penetrated by a beam of light, sound, particles, or other energy or matter. A coefficient valu ...
of pump light in the core
:
is the length of the double-clad fiber, and
:
is a
dimensionless
A dimensionless quantity (also known as a bare quantity, pure quantity, or scalar quantity as well as quantity of dimension one) is a quantity to which no physical dimension is assigned, with a corresponding SI unit of measurement of one (or 1) ...
adjusting parameter, which is sometimes called the "filling factor";