The mandrel diameter and number of turns are chosen to eliminate certain modes in a reproducible way. It is empirically observed that more than 5 full 360-degree wraps creates little additional loss, so 3 to 5 turns are commonly specified. The mandrel diameter affects how far into the mode volume the modal unbinding occurs. Experimentally, one plots the transmitted power from a wrapped fibre into which a uniform modal power distribution has been excited, as a function of mandrel diameter, maintaining a constant number of turns. This reveals step-like reductions in transmitted power as the diameter decreases, where each step is the point at which the mandrel is beginning to affect the next-lower mode group. For best measurement reproducibility, one would select a diameter that is not near such a transition, although this may not be possible if measurements must be performed over a range of wavelengths. Total mode volume in a fiber is a function of wavelength, so the mandrel diameter at which the mode group transitions occur will change with wavelength.
The mandrel diameter and number of turns are chosen to eliminate certain modes in a reproducible way. It is empirically observed that more than 5 full 360-degree wraps creates little additional loss, so 3 to 5 turns are commonly specified. The mandrel diameter affects how far into the mode volume the modal unbinding occurs. Experimentally, one plots the transmitted power from a wrapped fibre into which a uniform modal power distribution has been excited, as a function of mandrel diame.ter, maintaining a constant number of turns. This reveals step-like reductions in transmitted power as the diameter decreases, where each step is the point at which the mandrel is beginning to affect the next-lower mode group. For best measurement reproducibility, one would select a diameter that is not near such a transition, although this may not be possible if measurements must be performed over a range of wavelengths. Total mode volume in a fiber is a function of wavelength, so the mandrel diameter at which the mode group transitions occur will change with wavelength