My TN Connection

Have you ever wondered why a stick appears to bend when you put it into standing water? Well this phenomena can best be summarized by the concept of refractive index or index of refraction. The index of refraction is basically the ratio between the speed of light in any given medium versus the speed of light in a vacuum. Since air and water have two different refractive indexes, the stick appears to bend as it is inserted into the water. Although this is a great parlor trick to amaze your children, how does this relate to optical fiber networks?

Index of refraction also comes into play within fiber optic cables. Even though both the core and cladding are made up of glass, the core has a higher refractive index than the cladding, so light that enters the core is reflected back whenever it hits the cladding. This bouncing back and forth effect is what determines a light mode’s wavelength. There are certain ranges of wavelengths at which fiber operates at it’s best. This is why multimode transmission wavelengths are characterized as 850nm or 1300nm, when the window may be 800nm to 900nm or 1250nm to 1350nm. Conversely, single-mode transmission wavelengths are usually characterized as 1310nm or 1550nm, while the windows are 1250nm to1350nm and 1500nm to 1600nm. These wavelengths best match the transmission properties of the light source used (LEDs and lasers) with the transmission qualities of the optical fiber.

Another important concept to consider when talking about the transmission of light through fiber is dispersion. Dispersion is the spreading of a light pulse as it travels down a fiber optic strand. If the pulse spreads too much, it becomes undistinguishable by the receiver as 0s and 1s and will result in bit errors and loss of information. The two main types of dispersion are Modal and Chromatic. Modal dispersion is a concern in multimode applications, where the various modes of light traveling down the fiber’s core arrive at the receiver at different times, causing a spreading effect . Chromatic dispersion occurs as a result of the range of wavelengths in the light source. Even within the same mode of light, different wavelengths travel at different speeds. Over distance, the varying wavelength speeds cause the light signal to spread. Chromatic dispersion is of more importance in single-mode applications.

Over the years much has been done to minimize the effects of dispersion in both multimode and single-mode optical networks. Modal dispersion was combated in multimode environments by the development of Graded-index fiber. Graded-index refers to the fact that the refractive index of the core gradually decreases farther from the center. The increased refraction in the center of the core slows the speed of some light waves, allowing all the light waves to reach the receiver at the same time.

Single mode fiber has also underwent numerous changes as the need to push information over longer distances has evolved. This evolution of single-mode has resulted in three classes of single-mode fiber being used today. The most widely deployed type is non dispersion-shifted fiber (NDSF). This fiber class was initially developed for use near the 1310nm window. As the addition of 1550nm optical systems arose, serious dispersion issues arose with using NDSF fiber at the 1550nm wavelength. From this dilemma, fiber manufacturers were prompted to develop dispersion-shifted fiber (DSF), which moved the wavelength of maximum bandwidth out to the 1550nm range. With the recent growth in popularity of WDM (Wave Division Multiplexing) technology, the need to revisit the properties of today’s single-mode fiber resurfaced. Even though dispersion-shifted fiber (DSF) worked well with a single 1550nm wavelength, it did not hold the integrity of the signal when transmitting many closely spaced wavelengths over a single fiber optic strand. This gave rise to non zero-dispersion-shifted fibers (NZ-DSF), where the wavelength of maximum bandwidth is moved near the 1550nm window, but outside of the window that is actually being used to transmit the signals. NZ-DSF appears to be meeting the demands of those optical networks that are deploying CWDM and DWDM, but it is only a matter of time before fiber manufacturers are again forced to re-evaluate their current fiber offering in order to meet the needs of future network applications.

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