In an optical fibre, the spectrum of the Brillouin scattered light may have one or more peaks depending on the number of acoustic modes that interact with the lightwave in or near the core area.

Most fibres are characterized by a single acoustic mode and hence exhibit a typical single-peak Brillouin spectrum. Some fibres however, such as Dispersion Shifted Fibres (DSF), due to the presence to specific index profiles and/or multiple dopants, allow more acoustic modes and hence exhibit multiple peaks in the Brillouin spectrum.

Spectrum.JPG

When the interrogator processes the spectrum data for a simple “peak search”, all the peaks lower than the dominant one are ignored and a correct measurement is made. However, when the intensity difference is small (“dual-mode” fibres), the measurement noise may randomly alter the relative height of the peaks that is perceived by the equipment, making the dominant peak to appear lower than the stronger secondary one. In such cases the measured temperature or strain profile exhibits multiple large “jump” errors (that is frequently shifting up and down of a ~constant quantity) because at random positions it gets locally calculated according to the frequency shift of the wrong peak.

For further infromation you may refer to: 

Optical fiber cables for Brillouin distributed sensing applications