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Ask RP Photonics for advice concerning double-clad fibers, or concerning the design of cladding-pumped fiber lasers and amplifiers. The powerful modeling software RP Fiber Power is available for such purposes.
Definition: optical fibers with different waveguide structures for pump and signal light
A fiber laser or amplifier based on an ordinary doped single-mode fiber can generate a diffraction-limited output, but it restricts the pump sources to those with diffraction-limited beam quality and thus normally to those with low power. On the other hand, the use of multimode fibers usually (although not always) leads to poor beam quality.
This dilemma has been resolved with the invention of double-clad fiber designs, which allow cladding pumping of fiber devices. Here, the laser light propagates in a single-mode (or multimode) core, which is surrounded by an inner cladding in which the pump light propagates. Only the core (or sometimes a ring around the core) is rare-earth-doped. The pump light is restricted to the inner cladding by an outer cladding with lower refractive index, and also partly propagates in the single-mode core, where it can be absorbed by the laser-active ions. The inner cladding has a significantly larger area (compared with that of the core) and typically a much higher numerical aperture, so that it can support a large number of propagation modes, allowing the efficient launch of the output, e.g. of high-power laser diodes (e.g. beam-shaped high-power diode bars), despite their poor beam quality.
Figure 1: Cladding-pumped fiber amplifier based on a double-clad fiber. The signal light is launched into the doped core, while the pump light is launched into the inner cladding. The core is D-shaped for more efficient pump absorption.
The pump light does not necessarily need to be injected into the fiber ends, as shown in Figure 1. It is also possible to use side pumping techniques, where access to the fiber ends is not required for pumping.
Double-clad Fiber Designs
There are a variety of different designs of double-clad fibers. Figure 2 shows the fiber cross-sections for the most important design types.
Figure 2: Various designs of double-clad fibers. The fiber core is shown in blue, the inner cladding in light gray, and the outer cladding in dark gray. An additional polymer coating, as often used, is not shown.
The simplest kind of design has a circular pump cladding and a centered core (first design in Figure 2). This is relatively easy to make and use, but in this kind of fibers there are propagation modes of the inner cladding (related to helical rays) which have hardly any overlap with the core, so that some significant part of the pump light exhibits incomplete absorption. As a result, the gain and power efficiency are compromised. To a limited extent, this problem can be solved by strongly coiling the fiber.
Modes with poor core overlap can be avoided by using a modified design with a lower symmetry. Examples are designs with an off-centered core or a non-circular (e.g. elliptical, D-shaped or rectangular) inner cladding. Such pump claddings are also often better matching the properties of pump sources such as beam-shaped diode bars.
Figure 3: Structure of a photonic crystal fiber with an air cladding.
Double-clad fibers can also be made as photonic crystal fibers as shown in Figure 3. Here, the multimode pump core is suspended by very thin struts in the air cladding, through which the pump light cannot escape. Such a structure can have a very high numerical aperture of e.g. 0.6 for the pump light; this further reduces the requirements concerning the brightness of the pump source. The thickness of the struts can be chosen so that at the same time one achieves good mechanical stability, high thermal conductivity, and minimal pump losses. Another advantage of this type of fiber is that pump light is kept away from the protective polymer coating, avoiding any damage by absorbed pump light. The guidance of the core is achieved as in other photonic crystal fibers.
Parameters and Fabrication Methods of Double-clad Fibers
Besides the properties of the fiber core, the ratio of the areas of inner cladding and core is an important parameter. This area ratio should not be too large, because otherwise the effective pump absorption length becomes large, and the pump intensity in the core is small, resulting in low excitation levels which can also compromise the power efficiency. Area ratios of the order of 100-1000 are common. Pump sources with improved brightness allow the use of fibers with a smaller area ratio, thus also with a smaller length, which also reduces the impact of various types of nonlinearities.
In many cases, the core and inner cladding of a double-clad fiber are similar to those of a normal core-pumped fiber, except that in addition there is the lower-index outer cladding. If the inner cladding is made of silica, the outer cladding may consist of fluorine-doped silica. The numerical aperture for the inner cladding can then be e.g. ∼0.28. Larger values are possible with polymer outer claddings, but these cannot tolerate very high temperatures and may introduce higher propagation losses for the pump light.
Applications
Double-clad fibers are extensively used for cladding-pumped high-power fiber lasers and amplifiers. Such devices can have a fairly high power conversion efficiency combined with good beam quality. As the beam quality of the output can be diffraction-limited whereas that of the pump can be poor, the brightness of the laser or amplifier output can be much higher than that of the pump source. Particularly if this increase in brightness is essential for an application, the cladding-pumped fiber laser may be called a brightness converter.
Bibliography
| [1] | E. Snitzer et al., "Double-clad, offset-core Nd fiber laser" (first report of cladding pumping), Proc. Conf. Optical Fiber Sensors, Postdeadline paper PD5 (1988) |
| [2] | D. J. Ripin et al., "High efficiency side-coupling of light into optical fibres using imbedded v-grooves", Electron. Lett. 31, 2204 (1995) |
| [3] | V. Dominic et al., "110 W fibre laser", Electron. Lett. 35, 1158 (1999) |
| [4] | G. C. Valley, "Modeling cladding-pumped Er/Yb fiber amplifiers", Opt. Fiber Technol. 7, 21 (2001) |
| [5] | D. Kouznetsov and J. V. Moloney, "Efficiency of pump absorption in double-clad fiber amplifiers. II. Broken circular symmetry", J. Opt. Soc. Am. B 19 (6), 1259 (2002) |
| [6] | D. Kouznetsov and J. V. Moloney, "Efficiency of pump absorption in double-clad fiber amplifiers. III: Calculation of modes", J. Opt. Soc. Am. B 19 (6), 1304 (2003) |
| [7] | Y. Jeong et al., "Ytterbium-doped large-core fiber laser with 1.36 kW continuous-wave output power", Opt. Express 12 (25), 6088 (2004) |
| [8] | L. J. Cooper et al., "High-power Yb-doped multicore ribbon fiber laser", Opt. Lett. 30 (21), 2906 (2005) |
| [9] | V. Filippov et al., "Double clad tapered fiber for high power applications", Opt. Express 16 (3), 1929 (2008) |
See also: fibers, rare-earth-doped fibers, photonic crystal fibers, high-power fiber lasers and amplifiers, power scaling of lasers, brightness, side pumping
Category: fibers and other waveguides
