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The second order mode cut-off wavelength (commonly shortened to cut-off) refers to the wavelength above which the fiber is single-mode; only at wavelengths above the cut-off will the fiber guide be single-mode. Cut-off wavelength is important because, in most cases, it determines your choice of fiber type. At wavelengths just below the cut-off, a few modes may be guided, whilst multi-mode fiber operates far below the second order cut-off point.

When referring to Fibercore products, the number in the fiber product code (i.e. HB450, SM600 etc.) typically represents the ‘nominal’ cut-off wavelength. If you need Single Mode operation, you should choose the fiber type with the closest cut-off wavelength range below the operating wavelength of your optical source. For example, if you are using a helium-neon laser, at 632.8nm, then you need HB600, while a 1550nm diode laser would call for HB1500.

The following key facts should help:

• The cut-off wavelength is the wavelength at which an optical fiber becomes single-mode.
• At wavelengths shorter than cut-off several optical modes may propagate - the fiber is multi-mode.

As the cut-off wavelength is approached, progressively fewer modes may propagate until, at cut-off, only the fundamental mode may propagate - the fiber is then single-mode.

At wavelengths longer than cut-off the guidance of the fundamental mode becomes progressively weaker, until eventually (usually at a wavelength several hundred nanometers above cut-off) the fiber ceases to guide - the fiber loses all optical function. This is often referred to as the fundamental mode cut-off, or bend edge.

Related Terms: Bend Edge, Core , Multimode, Refractive Index, Refractive Index Profile (RIP), Single-Mode, Wavelength