Fibers with end caps
The maximum power that can be guided within a fiber is mainly restricted by the power density at the fiber end-faces, when not considering bulk, nonlinear optical effects within the fiber, such as Brillouin scattering. Extreme power densities can cause scorching of the end-face or photo-contamination by the generation of a dipole trap.
Regular fiber without end cap
For a regular fiber the divergence of the beam starts at the fiber exit.
For 100 mW laser power coupled into typical fibers, the power density at the end-face without an end cap reaches multiple kW/mm2. This leads to the stated problems especially for small wavelengths.
Fiber with end cap
The described detrimental effects can be obviated using a fiber end cap, in which a short piece of fiber (<300 µm) without a core is spliced onto the polarization-maintaining fiber .
Without a fiber core to confine the beam, the mode field diameter of the beam already starts to diverge within the fiber end cap and the resulting beam area at the end-face is significantly larger. The numerical aperture of a fiber is not affected by an end cap.
For 100 mW laser power coupled into typical fibers, the power density at the end-face without an end cap reaches multiple kW/mm2, where as it is only hundreds of W/mm2 with an end cap.
Working with end cap fibers
The end cap fibers are terminated with standard FC-connectors and profit from all benefits known for these
standard connectors. Also, this means, that the fibers can then be used with 60SMS laser beam couplers or with 60FC fiber collimators for example, all of which have an easily and precisely adjustable focus position. This is of great
importance when swapping a fiber without end cap for one with end caps, since in this case the focus position of the lens within the collimator or coupler has to be adjusted typically <200 µm to correct for the divergence within the end cap.
Polarization extinction ratio of end cap fibers
The fiber cables with end caps made by Schäfter+Kirchhoff typically have a polarization extinction ≥ 200:1 (21 dB) or ≥ 400:1 (23 dB) for λ ≥ 780 nm. For more details on PM fibers please see here.