Single-Mode Fibers with Large Mode Areas

Posted on 2006-08-10. Permanent link: http://www.rp-photonics.com/spotlight_2006_08_10.html

Author: Dr. Rüdiger Paschotta, RP Photonics Consulting GmbH

Ref.: J. M. Fini, "Bend-resistant design of conventional and microstructure fibers with very large mode area", Opt. Express 14 (1), 69 (2006); encyclopedia articles on large mode area fibers and high-power fiber lasers and amplifiers

In recent years, the output power available from fiber lasers and fiber amplifiers has been enormously increased. However, this has led to strongly increased optical intensities in the fiber core. Significant further increases of laser intensity are not possible, particularly for fiber devices generating short or ultrashort pulses, since the core glass may be damaged, and optical nonlinearities cause detrimental effects. A simple way of reducing the optical intensities is to use multimode fibers, but this degrades the obtainable beam quality, thus killing one of the essential advantages of fiber devices. Therefore, there is a race for developing single-mode fiber designs with larger and larger effective mode areas.

It is important to realize that the comparison of the performance of different fiber designs is not as simple as one might think. One aspect is that such fibers are often operated while they are strongly bent (coiled) – not only in order to reduce the space requirements, but sometimes also to attenuate higher-order modes in fibers which support not a single transverse mode, but several ones. It has been realized (see the reference above), that for some fiber designs the mode area can be strongly decreased under such conditions, while other designs (e.g. those with a parabolic refractive index profile) may start off with a somewhat smaller mode area but largely preserve this area when being bent. Therefore, one should compare mode areas under the conditions in which such fibers are actually used.

A fair performance comparison, however, involves some other aspects as well. A truly single-mode fiber is clearly preferably when stable and reliable generation of a beam with high beam quality is important. For fibers supporting a few modes, it can be essential to consider the propagation constants of the modes – similar values for different modes can increase mode coupling – as well as the dependence of the bend losses of different modes on the bending radius. This shows that a fair comparison is not a trivial task, and the results may well depend on the actual circumstances under which a fiber is going to be used.