Engineering advances are often made by simplifying previous designs, as they turn out to be unnecessarily complicated. In recent years, the development of optical fibers for high-power amplifiers has been going towards designs consisting of a solid silica-glass core surrounded by a careful arrangement of microstructured air-holes along the fiber. With proper design of the air-hole structure, the light amplified in the doped core can propagate in a transverse single-mode while the core still has such a large area that the light intensity does not exceed the damage threshold of the glass.

The work presented here suggests a route using, probably, the simplest structure possible: the core consists of a 100 µm diameter pure silica rod, surrounded only by a low-index polymer. Conventional step-index fibers use two differently doped glasses for core and cladding, with a small index step (~0.2%) between them to achieve single-mode propagation, but the idea here is to deliberately support a high number of higher-order modes (HOMs). The low-index polymer allows a 4-times larger index step than can be achieved through the use of glass doping. The azimuthally symmetric fiber supports modes of large azimuthal orders with effective areas much higher than that of the fundamental mode, and are therefore potentially interesting for high-power amplifiers. Apart from the benefit of larger modal area, it is known that HOMs have increasing effective index separation with increasing modal order, making it more difficult for the modes to couple to each other during propagation along a fiber. If a HOM of sufficiently high order is excited cleanly at the input of the fiber, the HOM can therefore propagate very stably without coupling to other modes of the fiber. This is experimentally verified in the work, by showing that HOMs of azimuthal order less than about 10 are unstable over a 16 m long fiber, but that the modes become very stable for azimuthal orders above 10. It turns out that once the mode order is increased above a certain limit (~25 in the case studied here) it is no longer possible to observe pure modes at the fiber output. However, it is shown that this is not due to coupling between the modes along the fiber, but rather because modes of such high order cannot be excited cleanly at the fiber input by the spatial light modulator used in the experiments. Nevertheless, this shows that there is a range of HOMs that can both be individually excited cleanly at the input as well as propagate extremely stably along the fiber. It is also pointed out that devices for field profiling are getting increasingly better, and it is therefore plausible that modes of even higher orders than demonstrated here can be excited cleanly, and be exploited for stable propagation with large mode areas.

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