RP Photonics logo
VL logo part of the
Virtual
Library

The Photonics Spotlight

Easier Launching into Fibers with Large Mode Area?

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 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.

This article is a posting of the Photonics Spotlight. You may send comments and suggestions to RP Photonics Consulting GmbH. You may also link to this page, because its location is permanent. See also the Encyclopedia of Laser Physics and Technology.

arrow

This encyclopedia is provided by
RP Photonics Consulting GmbH.

Utilize the expertise of the author, Dr. Rüdiger Paschotta, also in the form of technical consulting services!

RP Q-switch

A powerful software tool for designing
Q-switched lasers. See the details.

Onefive logo

Onefive

Low-noise
femtosecond,
picosecond,
and tunable single-frequency lasers for OEM and R&D applications.

TRUMPF logo

TRUMPF-Laser

a leading supplier of industrial diode pumped solid state lasers,
CO2 lasers, and laser systems for material processing.

A.L.S. logo

A.L.S. GmbH

Picosecond laser diodes
<30 ps, 375 – 1600 nm, >1 Wp, single shot – 120 MHz

RP Fiber Power

This software helps to design and analyze fiber amplifiers and lasers.

Field Guide to Lasers

This new book by Dr. Paschotta explains principles and types of lasers.

Your Advertisement at This Place

will be seen by many thousands of visitors per month. These banners receive far over 100'000 page hits per month. Check the details.