Double-clad Fibers

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). 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. However, if the overall fiber (not only the cladding) has a non-circular shape, this may cause problems when fusion splicing the fibers.

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. Such a structure can have a very high numerical aperture of at least 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 such 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.

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, 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. Note that photonic crystal fiber designs such as that shown in Figures 4 and 5 provide all-glass solutions with very high NA of the inner cladding.

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.

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.

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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