Easier Launching into Fibers with Large Mode Area?

Posted on 2007-04-28 as a part of the Photonics Spotlight.

Permanent link: http://www.rp-photonics.com/spotlight_2007_04_28.html

Author: Dr. Rüdiger Paschotta, RP Photonics Consulting GmbH

Ref.: encyclopedia articles on large mode area fibers, single-mode fibers, multimode fibers, effective mode area

It seems to be obvious that launching into a single-mode fiber is difficult because it normally has a small effective mode area. And that the job should become easier with large mode area fibers. However, this expectation is not quite right, as long as one is dealing with single-mode fibers.

If a large mode area (LMA) fiber is supporting only a single propagation mode (per polarization direction), it will be relatively easy to hit the fiber core with an incident beam, but for efficient launching the incident beam also needs to come from quite precisely the right direction, and is therefore not necessarily easier. This essentially results from the small numerical aperture (NA), which is related to the angular acceptance.

An interesting detail is that, precisely speaking, the angular acceptance is determined by the mode area and not really by the NA. At least for a given shape of the intensity distribution of the fiber mode, the mode field distribution depends only on the effective mode area. Furthermore, the efficiency with which any incident beam can be coupled into the fiber can be calculated from an overlap integral of the two field distributions. The NA does not enter there! So a somewhat higher NA might still be compatible with single-mode guidance, but it would not make it easier to launch light into the fiber. Some common intuitive pictures can be quite misleading in this context.