# A single-mode optical fiber with a large core

I know that the core of a single-mode optical fiber is small (approximately 8 μm), because the core diameter is small, the light is propagated in one mode. And I accidentally read this article which mentions a single-mode optical fiber with a large core: https://www.rp-photonics.com/large_core_fibers.html
I do not understand why a single-mode optical fiber has a large core ? What is the purpose of this fiber, transfer communication or energy ? With a large core, is light propagated in one mode ?

One can make a large core fiber single moded by adjusting the core-cladding index difference so that the waveguide parameter:

$$V = \frac{2\,\pi\,\rho}{\lambda}\sqrt{n_{co}^2-n_{cl}^2}$$

is less than the first zero $\omega_{01}\approx 2.405$ of the zeroth order first kind Bessel function.

This is often done when high optical powers must be borne by the single mode to:

1. Reduce nonlinear effects;
2. Reduce solarization and other damage wrought by high intensity fields

because the intensity is inversely proportional to the mode field diameter squared.

• Thanks, I have another question for you to answer: Between one large core optical fiber and multiple small core optical fibers, which transfer better energy? – viet nguyenduc Oct 9 '17 at 22:20

there is modal dispersion during the signal spreading. And transmission distance is greatly influenced by the dispersion. Luckily, single mode fiber have the ability to transmitting data for miles without losing too much data. Thus it can carry information for a longer distance than the multimode fiber.

• Welcome to Physics Stack Exchange! Could you make this answer a bit more on-target to the question of large core vs small core? That'll make it a more valuable answer. – Bob Jacobsen May 16 '18 at 6:16