Design Considerations of Single Mode Fiber Network

The term ‘conventional single mode’ has been used to represent ITU-T recommendation G.652.B compliant single mode optical fiber. Recently the trend has got a shift towards ITU-T G.652.D low water peak fiber. Almost 90% of the optical fibers being deployed in today’s field is low water peak fiber. This implies that the role of ITU-T G.652.B fiber has taken over by ITU-T G.652.D fiber.

It is no surprise that low water peak fiber is preferred by fiber optic cable installers as it offers extended spectrum usage throughout the operational windows from 1260nm to 1625nm. This is particularly beneficial for optical fiber based communication systems that use coarse wavelength division multiplexing (CWDM). Low water peak fiber complying to ITU-T G.652.D recommendations has added advantage that it can support DWDM technology also. The trend in the back-bone fiber optic network is currently use of low water peak fiber.

There are reports from the fiber optic industry that the access network fiber preference is shifting towards bend insensitive and low water peak optical fibers. This is driven by the massive demand and deployment of fiber to the home cables in the recent years. The fiber choice in the access and premise is getting shifted to the latest available technologically advanced extra bend insensitive fiber that has additional advantage of low water peak also. Optical fiber manufacturers are eager to announce invention of optimized products from their test laboratories.

Single mode optical fibers having step index matched clad design are considered to be conventional optical fibers. The core of the optical fiber is silica doped with Germanium dioxide and the cladding is pure silica. The use of doping material has an effect on the thermal coefficient of the core portion of the fiber. Thus the difference in thermal coefficient of expansion of the core and cladding portion of single mode fiber creates differences in shrinking after fusion splicing. A fusion splicing machine is designed to take care of this difference in thermal coefficient of expansion of core and cladding.

Another important design parameter of a single mode fiber is the ‘delta’, which is the refractive index difference between the core and cladding. The actual diameter of the core also plays equally important role but not directly. The delta and the actual core diameter directly affect the mode field diameter (MFD) and cut-off wavelength. Therefore, actual diameter of core of a single mode fiber is not important to measure, but mode field diameter is important for a single mode fiber. Often, fiber optic cable installers make mistake in mentioning the core diameter for a single mode fiber, though it is not required to specify. This may be due to the fact that actual core diameter is important for a multimode fiber. Fiber optic cable installer who is familiar with multimode optical fiber may think of the same importance for core diameter of single mode fiber also.

Dispersion and bend performance of singlemode fiber are determined by the values of mode field diameter and cut-off wavelength. While designing a single mode optical fiber, the most important point to consider is that the optical fiber must meet the majority of international specifications available in the market. The most prominent international specifications are ITU-T recommendations, IEC standards, EIA/TIA standards etc.

Optical fiber manufacturers must possess a clear understanding on the inter-relationships of optical characteristics and their importance in the quality of the fiber. Dispersion and bend loss shall be specified in relation with mode field diameter and cut-off wavelength. The rhombus relation among these parameters is the vital design criteria of a single mode fiber.

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