Abstract: Provided is a holding device for locking with another, similar holding device, comprising a locking member having a longitudinal axis, an external surface and a first end, an actuator for displacing said locking member along its longitudinal axis between a holding position and a released position, a guiding part, for engaging said external surface of said locking member and guiding its displacement functionally along its longitudinal axis, a light guiding part extending in at least part of said locking member, extending functionally parallel to said longitudinal axis, having a first light guiding part end at said first locking member end, a light source for optically coupling light into said light guiding part for providing light source light at said first light guiding part end, and a light detector, optically coupled to said light guiding part for detecting light entering said light guiding part at said first light guiding part end.