There are several issues when trapping between bare optical fibres. It is hard to perfectly cleave the end of the fibre, and imperfect ends lead to a non-gaussian beam out of the fibre, which reduces the quality of the trap. The fibre ends are delicate, and also may bend, which leads to imperfect trap alignment.
By using ceramic ferrules to hold the fibre we overcome these problems. The end of the fibre can be polished to a high quality finish, and the alignment is rigid. An example of trapping is shown below in “Ferrule trap”. The bead is being moved by adjusting the power balance in the two fibres. These beads are 2.5 microns in diameter.
The size of the ferrules (3mm diameter as opposed to 125 microns for the bare fibres) leads to problems with the imaging. The separation between the ferrules is only approximately 100 microns, and this leads to diffraction. The image of the bead is elongated, and you get ghost images either side.