Double-clad fiber

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.

In these fibers, the core is doped with active dopant material; it both guides and amplifies the signal light.

It also has two zero-dispersion points, and low dispersion over a much wider wavelength range than standard singly clad fiber.

This enables the inner cladding to guide light by total internal reflection in the same way the core does, but for a different range of wavelengths.

This allows diode lasers, which have high power but low radiance, to be used as the optical pump source.

The doped core gradually absorbs the cladding light as it propagates, driving the amplification process.

[3] 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.

Ray tracing,[6] simulations of the paraxial propagation[7] and mode analysis[8] give similar results.

The core, placed in vicinity of this chunk, is intercepted more regularly by all the rays compared to other chaotic fibers.

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

More rigorously, the property of the spiral-shaped domain follows from the theorem about boundary behavior of modes of the Dirichlet Laplacian.

Stochastic optimization of the cladding shape confirms that an almost-circular spiral realizes the best coupling of pump into the core.

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.

[12] As a result, at the wide (substantially multimode) end of T-DCF light propagates only in the lowest-order mode with excellent beam quality.

In many cases this efficiency can be approximated with[2] where 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.

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 empirical adjusting parameters, which provide good agreement of the simple estimate with numerical simulations for two specific classes of double-clad fibers: circular offset and rectangular.

Obviously, the simple estimate above fails when the offset parameter becomes small compared to the size of cladding.

approaches unity especially quickly in the spiral-shaped cladding, due to the special boundary behavior of the modes of the Dirichlet Laplacian.

For efficient operation, however, the pump should be absorbed in the core along the short length; the estimate above applies in this optimistic case.

Refractive index profile of dispersion-compensating double-clad fiber. c:core, i:inner cladding, o:outer cladding.
Refractive index profile of double-clad fiber for high power fiber lasers and amplifiers. c:core, i:inner cladding, o:outer cladding.
Schematic diagram of cladding-pumped double-clad fiber laser
Cross-section of circular DCF with offset core
Cross-section of DCF with rectangular inner cladding [ 2 ]
Spiral-shaped cladding (blue), its chunk (red), and 3 segments of a ray (green).
Modes of spiral-shaped double-clad fiber. [ 8 ]
Estimates of the pump efficiency in a double-clad fiber with (blue) and (red), discussed in [ 2 ] compared to the results of the ray tracing simulations [ 6 ] (black curves).