Double-clad fiber

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Fig.1. Cross-section of circular DCF with offset core.

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Fig.2. Cross-section of DCF with rectangular inner cladding

In fiber optics, a double-clad fiber (or doubly clad fiber, or DCF) is an optical fiber that has a relatively small-diameter core and two layers of large-diameter cladding. Usually, both cladding layers have lower refractive index than the core, and the inner cladding layer has lower refractive index than the outer layer. This allows the inner cladding to carry multimode light of a different wavelength from that carried in the core of the fiber. 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). Mostly, DCF fibers are used as fiber laser.

Fiber lasers and optical amplifiers

Double-clad fibers often are used for fiber lasers and optical amplifiers, because the core can be doped to act as the gain medium while the inner cladding layer carries a pump beam used to maintain the population inversion in the core. For such applications, the core may be single-mode or may be multimode with a low numerical aperture. The use of double-clad fibers allows fiber lasers to be scaled to higher powers than can otherwise be achieved.

The shape of the cladding is very important at narrow core and wide cladding. Circular symmetry in a double-clad fiber seems to be the worst solution for a fiber laser; in this case, many modes of the light in the cladding miss the core and hence cannot be used to pump it. In the language of geometrical optics, most of the rays 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

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Fig.3. Spiral-shaped cladding (blue), its chunk (red), and 3 segments of a ray (green).

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Fig.4. Modes of spiral-shaped DCF by .

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 scaring 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 of a ray increases at each reflection from the smooth wall, until the ray hits the chunk [fig.3] of the spiral curve. The core placed in vicinity of this chunk is visited by all the rays more regularly, than in other chaotic fibers. This behavior of rays has analogy in the wave optics. On the language of modes, all the modes have non-zero derivative in vicinity of the chunk, and cannot avoid the core placed there. One example of modes is shown in fig.4. Although some of modes are scared and show wide voids, none of these voids cover the core.

The property of the DCF 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 it some angular momentum. This transfer of angular momentum should be compensated with 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 the angular momentum transferredby the spiral part.

The interpretation with angular momentum indicates the reasonable size of the chunk. There is no reason to make this 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 locaize the scars within an angle smaller than the core: the small derivative to the radius makes the manufacturing less robust; the larger is <math>R'(\phi)</math>, the larger fluctuations of shape are allowed without breaking the condition <math>R'(\phi)>0</math>. Therefore, the size of the chunk should be of order of 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, shold be similar to that of the core-less domain.

The stochastic optimization of the shape of cladding confirm that almost circular spiral realizes the best coupling of pump into the core .

Filling factor

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Fig.5. Estimates of the pump efficiency in a double-clad fiber with <math>F=0.8 </math> (blue) and <math> F=0.9</math> (red) , discussed in compared to the results of the ray tracing simulations(black curves).

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

<math>1- \exp\left( - F \frac{\pi r^2}{S}\alpha L \right) ,</math>

where

<math>~S~</math> is the cross-sectional area of the cladding

<math>~r~</math> is the radius of the core (which is taken to be circular)

<math>~\alpha~</math> is the absorption coefficient of pump light in the core

<math>~L~</math> is the length of the double-clad fiber, and

<math>~F~</math> is a dimensionless adjusting parameter, which is sometimes called the "filling factor"; <math>~0<F<1~</math>.

The filling factor may depend on the initial distribution of the pump light, the shape of the cladding, and the position of the core within it.

The exponential behavior of the efficiency of absorption of pump in the core is not obvious. We could expect, that some modes of the cladding (or some rays) are better coupled to the core than others; therefore, the "true" dependence could be combination of several exponentials; and the only comparison with simulations (Fig.5) justifies this approximation. In particular, this approximation does not work for circular fibers, see the initial work by Bedo et all, cited below. For chaotic fibers, <math>~F~</math> approaches unity. The value of <math>~F~</math> can be estimated by numerical analysis with propagation of waves, expansion by modes or by geometrical optics ray tracing, and values 0.8 and 0.9 are only empiric adjusitng parameters, which provide good agreement of the simple estimate with numerical simulations for two specific classes of double-clad fibers, circular ofset and rectangular. Obviously, the simple estimate above fails, as the ofset parameter becomes small compared to the size of cladding.

The filling factor <math>~F~</math> approaches unity especially quickly in the spiral-shaped cladding, due to the special boundary behavior of the modes of the Dirichlet Laplacian. Designers of double-clad fiber look for a reasonable compromise between the optimized shape (for the efficient couplung of pump into the core) and the simplicity of the manufacturing of the preform used to draw the fibers.

The power scaling of a fiber laser is limited by the unwanted nonlinear effects, such as Brillouin scattering and the Raman conversion. Therefore the powerful fiber laser should be short. However, for the efficient operation, the pump shold be absorbed in the core along the short length; and the estimate (1) provides the optimistic estimate for this case. In particular, the higher step of the index of refraction from the inner cladding to the outher one allows to focus (and confine) the pump in a smaller inner cladding. As a limiting case, the index step can be of order of two, from glass to air . The estimate with filling factor gives an estimate, how shorter can be made an efficient DCF due to reduction of size of the inner cladding.

Alternative structures

For the good choice of the shape of the cladding, the filing factor <math>~F~</math> above approaches unity; the following enhancement is possible at various kinds of tapering of the cladding ; non-convential shapes of such cladding are suggested . The planar waveguides with gain medium take an intermediate position between conventional solid-state lasers and the conventional double-clad lasers. The planar waveduide may confine multi-mode pump and high-quality signal, allowing the efficient coupling of pump and the diffraction-limited output

Other applications

A 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 range than a singly-clad fiber. Double-clad fibers can also be used for the compensation of chromatic dispersion in optical communications and other applications.

Notes and references

• Federal Standard 1037C