## Single Mode Fiber (Single Mode Fiber Optic Cable)

When the fiber core is so small that only light ray at 0° incident angle can stably pass through the length of fiber without much loss, this kind of fiber is called single mode fiber. The basic requirement for single mode fiber is that the core be small enough to restrict transmission to a singe mode. This lowest-order mode can propagate in all fibers with smaller cores (as long as light can physically enter the fiber).

The most common type of single mode fiber has a core diameter of 8 to 10 μm and is designed for use in the near infrared (the most common are 1310nm and 1550nm). Please note that the mode structure depends on the wavelength of the light used, so that this fiber actually supports a small number of additional modes at visible wavelengths. Multi mode fiber, by comparison, is manufactured with core diameters as small as 50um and as large as hundreds of microns.

The following picture shows the fiber structure of a single mode fiber.

What Are the Conditions for Single Mode Transmission?

To calculate the number of modes Nm in a step-index fiber, Nm can be simplified as:

Where
D is core diameter of the fiber
λ is the operating wavelength
nf is refractive index of the fiber core
nc is refractive index of the fiber cladding

Reducing the core diameter sufficiently can limit transmission to a single mode. The following formula defines the maximum core diameter, D, which limits transmission to a single mode at a particular wavelength, λ :

If the core is any larger, the fiber can carry two modes.

Mode Field Diameter (MFD)

The typical core diameter of communication single mode fibers is from 8~10um for operating wavelength 1.31um to 1.5um. Fiber with a core diameter less than about ten times the wavelength of the propagating light cannot be modeled using geometric optics as we did in the explanation of step-index multimode fiber. Instead, it must be analyzed as an electromagnetic structure, by solution of Maxwell's equations as reduced to the electromagnetic wave equation.

So even though the fiber cladding confines the light within the fiber core, some light does penetrate into the cladding, despite the fact that it nominally undergoes total internal reflection. This occurs both in single mode and multimode fibers, but this phenomenon is more significant in single mode fibers.

For a Gaussian power distribution (lasers used in communications are Gaussian power distribution) in a single mode optical fiber, the mode field diameter (MFD) is defined as the point at which the electric and magnetic field strengths are reduced to 1/e of their maximum values, i.e., the diameter at which power is reduced to 1/e2 (0.135) of the peak power (because the power is proportional to the square of the field strength). For single mode fibers, the peak power is at the center of the core.

Mode field diameter is slightly larger than the core diameter, as shown in the following illustration.

References on how to measure mode field diameter for a single mode fiber

• EIA/TIA-455-191 (FOTP-191), Measurement of Mode-Field Diameter of Single-Mode Optical Fiber.
• http://www.corning.com/docs/opticalfiber/mm16_08-01.pdf Corning’s paper on how to measure mode field diameter for a single mode fiber
• Measurement of Mode-Field Diameter of Single-Mode Optical Fiber, Fiberoptic Test Procedure FOTP-191, Telecommunications Industry Association, Standards and Technology Department, 2500 Wilson Blvd., Suite 300, Arlington, VA, 22201 (1998).
• Measurement of the Effective Area of Single-Mode Optical Fiber, Fiberoptic Test Procedure FOTP-132, Telecommunications Industry Association, Standards and Technology Department 2500 Wilson Blvd., Suite 300, Arlington, VA, 22201 (1998).