Fiber Optics Transmission Systems

As the core of the fiber is so small that one ray of light at an angle of 0 ° incident can stably pass through the fiber length, without much loss, this type of fiber called single-mode optical fiber. The basic requirement for single-mode fiber is that the basic limit small enough to transfer to a mode of singing. This lowest order mode can propagate in all fibers with small cores (as long as light can be physically fibers).

The most common type of single mode fiber has a base diameter of 8-10 mu m and is designed for use in the near infrared (the most common are designed 1550nm and 1310nm). Please note that the structure of method depends on the wavelength of light used, so that these fibers actually a few extra modes to wavelengths visible support. multi-mode fiber, compared with core diameters as small as 50um multimode and as large as a hundred microns.

The image below shows the structure of a single mode optical fiber.

What Are the Conditions for Single Mode Transmission?

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

Where
D is core diameter of the fiber
λ is the operating wavelength
n_f is refractive index of the fiber core
n_c 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 core diameter typical communication single-mode fibers from 8 ~ 10um for the operation of 1.31um 1.5um wavelength. Fibre with a base diameter of less than about ten times the wavelength of light propagation can be modeled using geometric optics, as in explaining the multimode fiber with step index have. Instead, they must be analyzed, reduced by solving Maxwell’s equations of the electromagnetic wave equation as an electromagnetic structure.

Thus, although the fiber cladding, the light confined in the heart of the fiber, the light penetrates into the mantle, despite the fact that it undergoes total internal reflection nominally. This is done both in mono mode and multimode fibers, but the phenomenon is important in single mode optical fiber.

be used for a Gaussian distribution (laser in the Gaussian distribution of communication) in a single mode fiber, the mode field diameter (MFD) as the point where the intensities of electric and magnetic fields up to 1 / e is defined reduce their peak values, the diameter of the power 1/e2 (0135) of the peak power (because the force is proportional to the square of the field strength) is reduced. For singlemode fibers, the peak in the middle of the nucleus.

Mode field diameter is slightly larger than the base diameter, as shown in the following figure.

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).

Advantages of single-mode fiber

dispersion single mode fiber modal, modal noise and other effects that come with multiple gears, single-mode fiber capable of carrying signals at much higher rates than multimode fiber. You are span standard option for high data rates and long distance (over a few kilometers), the laser diode based telecommunications transmission technology used fiber.

Disadvantages of singlemode fiber

Since the heart of the single-mode fiber is much smaller than the multimode fiber optic core, the light coupling single mode optical fiber requires much tighter tolerances than the light coupling into the veins over multimode fiber. However, these tolerances were more available.

singlemode fiber optic components and devices are also more expensive than their counterparts multimode fiber to multi-mode is widely used in systems where low cost connections and distances and transmission speeds must be taken are used modestly.

Single Mode Optical Fibers from Corning