Brillouin Frequency Shift
In Brillouin D-FOS the key measured parameter is the shift between the wavelength of the pump light and the wavelength at which Brillouin scattered light exhibits its peak intensity. The Brillouin shift is normally measured in terms of frequency rather than in terms of wavelength because in that way it allows a notation that is easier to handle with the required resolution.
For a step-index fibre, the Brillouin frequency shift depends on refractive index, pump wavelength and acoustic velocity and can be calculated from below equation.
In D-FOS technology “Brillouin shift” or “Brillouin frequency” typically identifies the “frequency shift of Brillouin peak at zero” (nSBS,0) that is the slope-intercept of Brillouin frequency shift at 0°C, no applied stress and atmospheric pressure (0.1 MPa). The actual value of nSBS,0 depends on the fibre doping and on its drawing conditions, and it may vary significantly even at different points of the same fibre.
In a typical Brillouin D-FOS sensing application, the absolute accuracy of vSBS,0 is not, in general, extremely important since it is usually taken a “baseline” or “measurement zero” just after the installation of the sensor and any further measurement is done differentially with respect to the baseline.
However, it has to be noted that vSBS depends on the pump wavelength, and this has to be taken into account when comparing the measurements taken with different test equipment or when replacing the interrogator equipment in an existing installation.
The waveguide properties of most optical fibres are obtained by doping the fibre core with Germanium. An increase of core Germanium (Ge) content lowers the acoustic velocity and consequently the Brillouin frequency shift. Common telecom fibres (ITU G.652) typically have a Ge doping of ~3%weight and a Brillouin shift around 10.845 GHz at room temperature. Ge doping is a little higher in bend-insensitive (ITU G.657) and dispersion-shifted fibres (ITU G.655 and G.653) that consequently have a Brillouin shift lower of few hundreds of MHz (typically ~10.65 GHz), but it can be larger (up to 20%weight and more) in some specialty fibres designed for extremely low bending losses or cladding modes suppression, that consequently may have very low (~9 GHz) Brillouin shift.
The Brillouin shift of some specialty fibres may be lower than or too close to the frequency scanning limit of some interrogators, preventing the possibility of using them for sensing unless a proper custom scanning frequency is asked when ordering the equipment.
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