The technology of double-clad fibers is important in the area of active fiber optics, particularly for high-power fiber lasers and amplifiers. Only with double-clad fibers, very high output powers of fiber-based amplifiers and lasers are possible.
For the invention of double-clad fiber designs, one only had a choice between the following two approaches:
- One could realize fiber lasers or amplifiers based on an active single-mode fiber. That way, one 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.
- When using a highly multimode fiber, one can easily launch much higher pump power and consequently achieve much higher output powers. However, with a multimode core one usually obtains a quite poor beam quality. A high beam quality, however, is highly desirable for many applications.
That 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 (called pump 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 pump 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 launching of pump light from high-power laser diodes (e.g. beam-shaped high-power diode bars), despite their poor beam quality. Fig. 1 shows the example of a simple high-power fiber amplifier.
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. Industrial devices typically contain fiber-optic pump combiners for better robustness and stability.
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. For example, coated V grooves cut into the inner cladding can be used to reflect pump light into the inner cladding. In other cases, one exploits coupling of pump light from additional pump fibers wound around the active fiber.
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.
The simplest kind of design has a circular pump cladding and a centered core (first design in Figure 2). The actual glass fiber may not differ at all from a normal core-pumped fiber, except that one uses a suitable coating with sufficiently low refractive index (e.g. fluorinated acrylate) to obtain a sufficiently large numerical aperture of the pump cladding. This kind of double-clad fiber is thus easy to make and use. However, but such fibers with a radially symmetric design 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 (see Figure 3). As a result, the gain and power efficiency are compromised. Only to a limited extent, this problem can be solved by strongly coiling the fiber.
This numerical simulation, done with the software RP Fiber Power, shows that the pump intensity distribution develops a “hole” in the core region. The remaining pump light exhibits quite incomplete absorption. With a D-shaped core, for example, this effect can be mitigated.
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. However, if the overall fiber (not only the cladding) has a non-circular shape, this may cause problems when fusion splicing the fibers. Splicing is also more difficult if the fibers have off-centered cores, since those then need to be properly aligned.
Fiber designs with a non-circular shape of the pump cladding often have another glass layer and not only the polymeric coating around the pump cladding. They are then sometimes called triple-clad fibers (see below), still counting the coating as another cladding.
Double-clad fibers can also be made as photonic crystal fibers as shown in Figure 4. Here, the multimode pump core is suspended by very thin struts in the air cladding, through which the pump light cannot escape. (Note that pump leakage resulting from an imperfect air cladding may damage the fiber coating.) Such a structure can have a very high numerical aperture of at least 0.6 for the pump light, which further reduces the requirements concerning the brightness of the pump source. The thickness of the struts can be chosen such that at the same time one achieves good mechanical stability, high thermal conductivity (resulting in a not dramatically increased temperature of the fiber core during high power operation), 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 light in the core is achieved as in other photonic crystal fibers.
There are triple-clad fibers, having one more undoped cladding. While such fibers overall behave similar to double-clad fibers, that refinement can bring some advantages.
One possibility is to have an all-glass double-clad waveguide structure, not relying at all on a polymeric coating for the optical function. It is then also more easily possible to use a pump cladding of modified shape, for example a rectangular or octagonal shape, while keeping the overall cross-section circular.
In other cases, there is another undoped cladding of relatively small diameter around the fiber core  which is used to optimize the guided mode properties.
For more details, see the article on triple-clad 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. That area ratio should not be too large – for too reasons:
- The pump absorption becomes weak, so that one requires a longer length of fiber. That is particularly problematic 11 runs into problems with fibers nonlinearities, e.g. when amplifying intense signal pulses.
- The pump intensity in the core gets smaller, 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 (possibly made a triple-clad fibers), and 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. Therefore, all-glass designs are often preferred for high output powers. One may thus use triple-clad designs, but photonic crystal fiber designs such as that shown in Figures 4 and 5 also provide all-glass solutions, and that with very high NA of the inner cladding.
Erbium-doped double-clad fibers would normally be difficult to use, since the low absorption cross-section combined with the limited realistic doping concentration of erbium in silicate glasses would lead to quite poor pump absorption. A good solution is ytterbium co-doping – see the article on erbium-ytterbium-doped laser gain media.
Coupling Light into Double-clad Fibers
In research setups, light is often coupled from free space into a double-clad fiber, as shown in Figure 1. For industrial lasers, however, this approach is not sufficient stable and robust. They should be based on an all-fiber setup e.g. as shown in Figure 6, where fiber-coupled pump laser diodes are directly connected to the active fiber via some passive transport fibers, avoiding any air spaces in the beam path. One then requires fiber-optic pump combiners (or pump couplers), i.e., special types of fiber couplers used for interfacing to the active fiber.
Various designs of high-power pump combiners have been developed. They are partly offered by the manufacturers of double-clad fibers, and partly by other more specialized manufacturers. The advances in the development of fiber pump combiners with high power handling capability, preservation of radiance, compatibility to certain double-clad fibers etc. have been an important contribution to the progress on high-power fiber devices.
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 (sometimes above 80%) combined with a high 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.
Typical Problems with Double-clad Fibers
It has already been mentioned above that rather incomplete pump absorption can result from cladding modes with weak core overlap. Even if strong mode mixing is ensured with a suitable design, the pump absorption is reduced according to the limited overlap of pump light with the doped fiber core. Therefore, one typically requires an accordingly longer length of active fiber. That can be detrimental e.g. in terms of fiber nonlinearities. Also, the larger total amount of dopant ions can make it more difficult to achieve laser or amplifier operation with short signal wavelengths, and the increased amount of fluorescent light can decrease the power conversion efficiency.
Some of the signal light may be coupled out of the core into the pump cladding, e.g. as a result of bending or by a fiber Bragg grating. That light will then remain in the pump cladding and will not (as for other fibers) get lost via the coating. One may need some type of cladding light stripper (cladding mode stripper) to remove such light, if it would be disturbing in the device output. That may also be the case for residual pump light.
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See also: triple-clad fibers, fibers, rare-earth-doped fibers, photonic crystal fibers, fiber-optic pump combiners, high-power fiber lasers and amplifiers, power scaling of lasers, brightness, side pumping, cladding mode strippers
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