Fiber and Fiber Mount

Ferrule M3 M3 M3 M3 second sheath fiber uncoated fiber first sheath Piezo a) b)

Figure 7.6.: a) A schematic of the fiber holder for the standard fiber. A groove

with M3 screws can hold a ferrule in front of a groove holding the fiber sheath, allowing us to put the fiber easily in the ferrule. A piezo glued to this piece and another piece allows us to control the translation of the fiber. The small middle hole is too small for the piece to be made in one time; a very long and thin drill bit would be required. A simple solution is to make two pieces and glue them together. b) An image of the fiber holder on a mirror mount.

The cavity is directly coupled to a fiber placed against the high reflective coated surface of the crystal. The end surface of the fiber needs to be as flat as possible and perfectly parallel to the crystal surface otherwise the transmitted beam will not

Chapter 7 Experimental Method be optimally coupled to the fiber and some losses will appear. (Figure 7.9) shows the effect of misalignment on the coupling.

Two types of fibers have been used in this thesis: a standard single mode fiber for 1064nm (P3-980A-FC-5 [67]) and a large mode area polarization maintaining photonic crystal fiber (LMA-PM-15 [67]). They do not have the same diameters, so different fiber mounts had to be used. For the photonic fiber, a commercial fiber holder (BFTU [67]) is fixed in a 1 inch custom ring that can be fixed into a standard mirror mount (Figure 7.5.a). The alignment is hard because the fiber is held far away from the crystal and only on one point. It usually creates a large angle with the mount which needs to be compensated for during alignment, moreover the system gets a lot of acoustic noise and may move when moving the fiber. The single mode fiber is held by a more carefully designed system (Figure 7.6) which gives a much better stability and makes the alignment easier.

The end of the fiber is made flat by using a cleaver from Nyfors ([29]). But it necessitates the removal of a few centimeters of coating sheath from the fiber, which makes it quite fragile, especially the single mode fiber which has a smaller diameter (125µmcompared to 230µmfor the photonic fiber).

A ferrule is used to hold the uncoated fiber. It is difficult to put the fiber in , but the ferrule holds it very well and in a very convenient axis, simplifying the alignment. Moreover, the holder makes the placement of the fiber in the ferrule quite easy. A ferrule is slid into the front of the holder and falls where the diameter becomes thinner, see Figure 7.6. A screw hole allows us to fix the ferrule to make sure that it doesn’t slide during the process. The fiber is placed in a plastic sheath ([31]) and slid at the back of the holder in the small diameter part of the holder. The fiber usually emerges exactly in front of the hole of the ferrule and slides into it. When the coated part of the fiber meets the ferrule, it can’t go through and it blocks the fiber. The ferrule can be unscrewed and will slide to the end of the mount. Another screw hole allows us to fix the ferrule at the end of the holder giving a very good stability to the system. Two screws at the back of the holder gently squeeze the fiber to prevent it from moving at all. By using several sheaths of different diameters that can slide in each other (PTFE AWG 30 T and PTFE 0,9 x 2,4 mm [31]) I made sure that all the fiber is covered to protect it against bending and to also thermalize it. The holder also contains a piezo sandwiched between two pieces of plastic. Moving this piezo can be useful during an initial alignment. But with a good illumination used in the white light interferometry system (Figure 7.14) it is very easy to see fringes without it. In a future design, the piezo will be removed and the holder shortened which will add a lot to the mechanical stability of the system.

The holder is designed in a 1 inch cylindrical shape to fit into a mirror mount. It gives two degrees of rotation allowing us to align the fiber perpendicular to the crystal. Unfortunately, the holder is quite long so moving the rotation degrees can create a big translation laterally and vertically. It is necessary to be able to compensate in those two directions. The mirror mount is attached to a regular sliding foot to

7.2 Other experimental consideration

be able to adjust the height. This sliding foot is itself fixed on two micro-metric translational stages (M-UMR5.16 [65]): one to adjust the lateral mismatch, the other one to control the distance between the fiber and the crystal to get contact between them when the alignment is well done. The micro metric translation and especially the sliding fit are contributing to the instability of the system and how much acoustic noise gets coupled to the cavity. In a future design, the holder will be shorter and the mechanical alignment of every piece simpler. Hopefully, it will not be necessary anymore to compensate in those two directions allowing the mirror mount to be mounted on a rigid piece and increasing a lot the general stability.