The DTS systems based on the Raman scattering
detection use multimode optical fibers with core diameter
of 50 μm with large value of the numerical aperture
(NA) as a sensor. This requirement is implied by the need
of good guidance of the scattered light which has very small power. The spectral-attenuation characteristic of the
multimode optical fiber limits the range where those
systems could be used to 8 - 10 km in length.