NA of The Single Mode Fiber

Wang Jing
SUNY at Stony Brook
Optics Rotation Project 1, Fall 2001

Advisors: Prof. Hal Metcalf / Dr. John Noé


The purpose of my project is to study the properties of Numerical Aperture (NA) of the single-mode step-index fiber. The basic requirement for single mode fiber is that the core be small enough to restrict transmission to a single mode. Single mode fiber is optical fiber that is designed for the transmission of a single mode of light as a carrier and is used for long-distance signal transmission.

The number of the modes allowed in a given fiber is determined by a relationship between the wavelength of the light passing through the fiber, the core diameter of the fiber, and the material of the fiber. This relationship is known as the Normalized Frequency Parameter, or V number. The mathematical description of the V number is V= 2 * (pi) * NA * a / lambda

		NA =     Numerical Aperture(see below)
		a  =     fiber radius (microns)
                lambda = wavelength (microns)

A single-mode fiber has a V number that is less than 2.405, for most optical wavelengths. It will propagate light in a single guided mode.

A multi-mode fiber has a V number that is greater than 2.405, for most optical wavelength and therefore will propagate in many paths through the fiber.

The numerical aperture (NA) is a measurement of the ability of an optical fiber to capture light. All fibers have acceptance angle. The sine of the half of the acceptance angle of a fiber is known as the NA. The NA of the fiber, and also the acceptance angle, is determined by the ratio of refractive indices of the optical fiber core and its cladding. Rays entering the a fiber at a angle greater than the NA will not be reflected internally.

Experimental Set-up



Apparatus description:

Measurement and Result

Another Interesting Thing (Discovered by Doug Broege)

If we put a " rainbow glasses" very near at the end of fiber. We can see an interesting phenomenon as shown above. And there is no such a phenomenon if the laser beam dose not pass through the fiber. This maybe because???


I am very appreciated for Dr. John Noé's instruction and generous help in the experiment. I would like to thank Prof. Harold Metcalf for his much help. Many thanks to Doug Broege, Xueqing Liu and Xiyue Miao for helping me make the experimental set-ups.

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