It’s known that multi-mode (MMF) fiber is available in OM1, OM2, OM3 (e.g. OM3 patch cable) and OM4 versions. How about single-mode fiber (SMF)? There are a variety of SMFs with carefully optimized characteristics available commercially: ITU-T G.652, 653, 654, 655, 656 or 657 compliant. Since several types exist, do you know how to choose a right SMF type to light-up your multi-terabit per second system? The past decades has witnessed the evolution of SMF designs, and most deployed SMFs nowadays are G.652D, G.655 or G.656 compliant fibers. (G.657A is essentially a more expressive version of G.652D, with a superior bending loss performance.) So, in this text, discussion is made on choosing G.652D, G.655 and G.656 compliant fibers against three parameters: attenuation, PMD and CD.

Attenuation is the reduction or loss of optical power as light travels through an optical fiber and is measured in decibels per kilometer (dB/km). G.652D offers respectable attenuation coefficients, when compared with G.655 and G.656. However, It should be remembered that even a meager 0.01dB/km attenuation improvement would reduce a 100km loss budget by a full dB - but let's not quibble. No attenuation coefficients for G.655 and G.656 at 1310? It was not, as you may immediately assume, an oversight. Both G.655 and G.656 are optimized to support long-haul systems and therefore could not care less about running at 1310nm. A cut-off wavelength is the minimum wavelength at which a particular fiber will support SM transmission. At ≤1260nm, G.652D has the lowest cut-off wavelength - with the cut-off wavelengths for G.655 and G.656 sitting at ≤1480nm and≤1450 respectively - which explains why we have no attenuation coefficient for them at 1310nm.

PMD, polarization-mode dispersion, is an unwanted effect caused by asymmetrical properties in an optical fiber that spreads the optical pulse of a signal. Slight asymmetry in an optical fiber causes the polarized modes of the light pulse to travel at marginally different speeds, distorting the signal and is reported in ps / √km, or "ps per root km". Oddly enough, G.652 and co all possess decent-looking PMD coefficients. Now then, popping a 40-Gbps laser onto fiber up against an ultra-low 0.04 ps / √km, the calculator reveals that the PMD coefficient admissible fiber length is 3,900 km and even at 0.1 ps / √km, a distance of 625km is achievable.

PMD is particularly troublesome for both high data-rate-per-channel and high wavelength channel count systems, largely because of its random nature. Fiber manufacturer’s PMD specifications are accurate for the fiber itself, but don’t incorporate PMD incurred as a result of installation, which in many cases can be many orders of magnitude larger. It is hardly surprising that questionable installation practices are likely to cause imperfect fiber symmetry - the obvious implications are incomprehensible data streams and mental anguish. Moreover, PMD unlike chromatic dispersion is also affected by environmental conditions, making it unpredictable and extremely difficult to find ways to undo or offset its effect.

CD is called chromatic dispersion to emphasize its wavelength-dependent nature, and it has nothing to do with the loss of light. This phenomenon occurs because different wavelengths of light travel at different speeds. Thus, when the allowable CD is exceeded - light pulses representing a bit-stream will be rendered illegible. It is expressed in ps/ (nm·km). At 2.5-Gbps CD is not an issue - however, lower data rates are seldom desirable. But at 10-Gbps, it is a big issue and the issue gets even bigger at 40-Gbps.

G.652D’s high CD coefficient is very poor next to the competition. G.655 and G.656, variants of non-zero dispersion-shifted fiber (NZ-DSF), comprehensively address G.652D’s shortcomings. It should be noted that nowadays some optical fiber manufacturers don’t bother with distinguishing between G.655 and G.656 - referring to their offerings as G.655/6 compliant.

It seems that this approach creates more problems than it is likely to solve - by unacceptably amplifying non-linear four-wave mixing and limiting the fiber to single-wavelength operation - in other words, no DWDM. That, in fact, is why CD should not be completely lampooned. Research revealed that the fiber-friendly CD value lies in the range of 6-11 ps/nm·km. Therefore, and particularly for high-capacity transport, the best-suited fiber is one in which dispersion is kept within a tight range, being neither too high nor too low.

After discussion, it’s clear to make the conclusion that only thing that genuinely separates fiber types for high-bit-rate systems is the third parameter: CD. Actually, one of the most important considerations in the fiber selection process is the fact that optical signals may need to be amplified along a route, which will be discussed in other articles.

Different kinds of SMFs have different characteristics. Hope this passage help you obtain a clear understanding of three parameters considered in single-mode fiber selection. As a professional fiber patch cord manufacturer, Fiberstore supplies various kinds of SMFs for long-haul applications, like LC to LC fiber patch cable single mode. Certainly, MMFs can also be found here. You can visit Fiberstore for more information about fiber optic cables.

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