Why use the grub screw of the fiber receptacle type FC?
The bore hole of the fiber receptacle of the fiber coupler, into which the connector ferrule of the fiber is placed, has some free play with respect to the ferrule.
This is necessary, so that the ferrule can be inserted without force.
The free play is small, however, it is still in the order of a mode field of a single-mode fiber at short wavelengths.
Once the ferrule is inserted into the connector type FC, the connector cap nut is screwed on gently onto the receptacle until it is finger tight. Even though the connector is already fixed, the long lever of a fiber connector makes it likely that the ferrule in the bore hole moves laterally. This lateral movement of the ferrule and thus of the emitting point of the fiber, yields to a pointing error of the beam emitted from the coupler.
Tightening the small grub screw minimizes the free play in between ferrule and bore hole after the fiber connector is fixed and minimizes the possible pointing error.
Please note: In case of polarization-maintaining fibers it is important that this grub screw is fastened finger tight only. If it is fastened too tight, there is risk of affecting the polarization-maintenance of the fiber cable.
The fiber receptacle with this small grub screw is compatible with all standard fiber connectors of type FC. It is used by Schäfter+Kirchhoff for all Fiber Couplers/Collimators with a receptacle for connector type FC (as well as AVIO).