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This Detail Specification applies to a Class IVa dispersion-unshifted single-mode optical fiber that has low attenuation at wavelengths near the water peak (typically near 1383 nm). Class IVa single-mode fiber has a nominal zero-dispersion wavelength in the 1310 nm transmission window. The fiber is commonly referred to as a dispersion-unshifted (or non-shifted) fiber, and its dispersion characteristic is approximated by a differentiated Sellmeier equation (see 4.4.4 of the Sectional Specification). This Detail Specification applies to a sub-category of a Class IVa fiber that is termed "Low Water Peak" fiber. These fibers are characterized by having a low, environmentally stable attenuation coefficient in the vicinity of 1383 nm - traditionally referred to as the "water peak."