Double-clad fiber

A double-clad fiber has three concentric regions of decreasing refractive index. The core, a few to a few tens of micrometers across, guides the signal in one or a few modes. The inner cladding, typically 125–600 µm across, is itself a large multimode waveguide, because it is surrounded by an outer cladding of still lower index: either a low-index polymer, which gives a numerical aperture of about 0.46 for the inner cladding, or fluorine-doped silica, which gives about 0.22 in all-glass designs. Light launched into the inner cladding stays confined to it while crossing the core again and again. The structure is the basis of cladding pumping in fiber lasers and amplifiers, and it is also used to collect light in fiber-based imaging.

Cladding pumping

In a rare-earth-doped double-clad fiber the core contains the active ions, for example ytterbium, and the pump from multimode diode lasers is coupled into the inner cladding. Each time a pump ray passes through the doped core, part of it is absorbed. To a first approximation the pump absorption per unit length scales with the ratio of the core area to the inner-cladding area:

$$\alpha_{clad} \approx \alpha_{core}\, \left(\frac{d_{core}}{d_{clad}}\right)^{2}$$

For a 20/400 µm fiber the ratio is 0.0025. A core whose small-signal absorption at 976 nm is 500 dB/m therefore gives a cladding absorption of about 1.25 dB/m, and 10 m of fiber absorbs 12.5 dB, or 94% of the launched pump. Designers choose fiber length from this number: longer fiber absorbs more pump but adds nonlinearity and background loss.

The advantage is in étendue. Étendue scales as $(d \cdot \mathrm{NA})^2$, so an inner cladding of 400 µm at NA 0.46 accepts about 23,500 times the étendue of a 20 µm core at NA 0.06. Low-brightness diode bars, which could never be coupled into the core directly, fit easily into the cladding, and the optical pumping converts their power into a near-diffraction-limited beam in the core.

Cladding shape and skew rays

A perfectly circular inner cladding with a centered core is a poor pump absorber. Many pump rays travel helical (skew) paths that circle the fiber axis and never pass through the core, so the absorption saturates well below the value the area ratio predicts. Practical fibers break the circular symmetry: D-shaped, octagonal, hexagonal or offset-core cross sections scramble the ray paths so that all the pump eventually crosses the core. Coiling the fiber in a kidney or figure-eight shape adds further mode mixing. The inner cladding of a 400 µm, NA 0.46 fiber at 976 nm has a V-number near 590 and supports on the order of 10⁵ modes, so geometric ray arguments describe it well.

Other applications

Double-clad fibers serve as passive components as well. In endoscopic optical coherence tomography and fluorescence imaging, the single-mode core delivers the illumination and receives the coherent signal, while the multimode inner cladding collects diffusely scattered or fluorescent light with a much larger acceptance. Double-clad fiber couplers separate the two channels at the proximal end. Passive double-clad fibers matched to active ones are used for pump combiners, delivery fibers and the leads of fiber-laser components.

Measurement and handling

The inner-cladding NA is measured from the far-field cone of light launched to overfill the cladding. Pump absorption is measured by the cutback method on a coiled fiber with a pump diode at the absorption peak (976 nm for ytterbium); the result depends on coiling and launch conditions, so it is quoted with the measurement configuration. Core parameters, such as the mode field diameter, are measured as in any single-mode fiber.

Several pitfalls are specific to the structure:

• The polymer outer cladding is the guiding boundary for the pump. Stripping it, touching it with a higher-index material, or letting it overheat releases pump light at that point, a common cause of burned splices and failed recoats. Recoating after fusion splicing must use low-index polymer.
• Unabsorbed pump and signal light leaking into the inner cladding propagate to the output and degrade the beam. Cladding light strippers, high-index material applied over a stripped section, remove it deliberately and must be heat-sunk.
• A quoted cladding absorption is a bulk figure. With poor mode mixing, absorption is not exponential in length: the first meters absorb faster and later meters slower, so a long fiber can absorb less total pump than the quoted dB/m predicts.

Common questions

What is the difference between single-clad and double-clad fiber?

A single-clad fiber, such as standard telecommunications fiber, guides light only in the core; the silica cladding is bounded by a coating of higher index that strips any light reaching it. A double-clad fiber has a low-index outer layer that turns the cladding into a second waveguide, so light can be guided in the core and in the cladding at the same time.

Why is the core of a high-power fiber so large?

A larger core lowers the intensity for a given power, which raises the threshold for stimulated Raman and Brillouin scattering, and it raises the core-to-cladding area ratio, so the pump is absorbed in a shorter fiber. Large-mode-area cores of 20–30 µm are slightly multimode and are kept effectively single-mode by a low core NA and by coiling, which strips the higher-order modes through bend loss.

References: D. J. Richardson, J. Nilsson, W. A. Clarkson, J. Opt. Soc. Am. B 27, B63 (2010); M. N. Zervas, C. A. Codemard, IEEE J. Sel. Top. Quantum Electron. 20, 0904123 (2014); B. E. A. Saleh, M. C. Teich, Fundamentals of Photonics, 3rd ed. (Wiley, 2019).