Cladding Mode Strippers
In some situations, it is necessary to remove light from the cladding of an optical fiber. Some examples:
- Sometimes, a single-mode fiber is used as a kind of mode cleaner. The wanted light is transmitted through the fiber core, and any other light, spoiling the beam quality, should be removed. In many cases, a polymer coating around the fiber cladding can serve as a cladding mode stripper. For that purpose, the refractive index of the coating should be slightly above that of the cladding, so that light can easily be transmitted from the cladding into the coating and then radiated into the ambient air by scattering at irregularities. (Alternatively, the coating may absorb the light.)
- In a high-power fiber amplifier made from a double-clad fiber, residual (unabsorbed) pump light (at the fiber end opposite to the pump end) may have to be removed from the pump cladding (inner cladding) in order to prevent it either from accompanying the amplified signal or from getting to the signal source. A special cladding mode stripper may be used for that purpose.
- When pump light is launched into a double-clad fiber (for example, from free space), some of the optical power may get into the outer cladding around the pump cladding. It may propagate in that outer cladding up to a location where the fiber has a polymer coating, and then destroy that coating via excessive heating. That problem may be avoided with a cladding stripper which attenuates light in the outer cladding, but not in the pump cladding.
Cladding mode strippers for use in high-power fiber lasers and amplifiers need to be able to handle substantial optical powers. It needs to be ensured that these powers are absorbed in a sufficiently widespread region, and that the generated heat can be removed safely, without damaging the mode stripper or any surrounding parts.
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