Figure 7.14.: Interference pattern between the crystal face and the fiber end ob-
served with the camera. From left to right the fiber alignment is better and better until the two faces are parallel (right).
7.3 Alignment of the Cavity
7.3.2.1. Fiber Holders:
To align all the system one needs to start with the fibers. With standard single mode fibers, it is possible to buy a long connected fiber, cut it in the middle and cleave the bare parts. The connected parts of the fiber can then be fitted into standard coupling systems, which are relatively easy to align. With a photonic fiber, one still needs to cleave one side, but these fibers usually come without connectors, so it will be necessary to create a connector to hold the fiber at the other end and couple light in it; however, this will impair our ability to disconnect and reconnect a fiber, and still maintain a relatively good coupling.
Even with standard connectors, for fibers at the level of the homodyne, disconnect- ing and reconnecting the LO fiber or the signal fiber means having to realign the homodyne detections, so it is something to avoid. It is important to notice that pulling too much on these fibers close to the connectors can misalign them slightly, so it is a good idea to always tape the fibers carefully to the table after the connectors to avoid movements.
I usually start by aligning the signal fiber coupler to have some light in the cleaved fiber end with only the fiber holder placed on the base (Figure 7.5.c). In the schematic Figure 7.13 it corresponds to align the alignment beam (a) into the cou- pler of the fiber signal with the flip mirror up. For the standard single mode fiber, I used standard FC/APC connectors and couplers (F220APC-1064 ([67])) for the photonic fiber I used an old version of the cavity fiber holder with a 1 inch cylinder to go in a standard mount and a fine tube holding the bare fiber in a sheath with a screw. I put this holder in a translational stage (SM1Z ([67])) with a lens with a short focal to make the coupler. Then I aligned the other fiber corresponding to the LO (beam (b)) with two similar couplers.
When the signal fiber is in position in the fiber holder (Figure 7.6), it is a good idea to clean the end of it, because it could have become dirty going in the mount. I usually check the surface (Figure 7.8) with a camera attached to a microscope lens (X20) and clean the fiber with a lens tissue.
7.3.2.2. Crystal Holder and White Light Interferometry:
The crystal needs to be placed in the crystal holder with the HR face in the direction of the fiber, then the holder is placed on the foot with the peltier in the middle. The foot is then screwed to the base in front of the fiber holder along the L shape designed for it. It is possible to look on the side with a camera (in this experiment we used the DNT DigiMicro camera ([22]) which is cheap and can see the light at 1064nm, with a good resolution and which is quite reliable for this kind of camera). It is possible to get the fiber under a few millimeters to the crystal without touching it, but to get closer and to align the fiber to the crystal, a more precise system is required. I used a camera (DCC1545M ([67])) connected to a beam splitter (CM1- BP145B1([67])) and a X20 microscope lens placed behind the crystal to create a
Chapter 7 Experimental Method
BS
LED
Camera Microscope Fiber mount Crystal MountFigure 7.15.: White light interference system. The light from a red LED is col-
limated and sent to the crystal and the fiber end through a beam splitter. The light interferes between these two surfaces and returns to the camera. If the two surfaces are close enough, we observe interferences.
white light interferometer (Figure 7.15) to see interference patterns between the crystal and the face of the fiber. With this system, I can look at the fiber end through the crystal and measure the separation and the misalignment very precisely. The beam splitter of the interferometer is in a cage that allows other components like tubes, cameras and microscopes to be screwed to it. I used a simple red LED with a short focal lens to make the illumination, and the whole is placed on two or three translational stages to be able to move the system in all directions.
Finding the fiber with the camera is not always easy. A good trick is to use some light going in the fiber by the other end from the laser. With enough power the light is everywhere and it is easy to find the center and to focus it with the camera reducing each time the power to avoid burning the camera. When the light is perfectly in one dot, we can usually see the image of the fiber very well. The distance between the fiber and the crystal can then be reduced in moving the translational stage of the fiber holder until some interference patterns can be seen (Figure 7.14). This step is a bit dangerous at first, because it is hard to know if the interference patterns are not visible because the distance is still too far, or if the fiber and the crystal are already touching, but there is too much of an angle to see anything or the light illumination is not focused correctly leading to the damaging of the crystal. I used to use the piezo on the fiber mount moving at low frequency. The movement is visible with the camera because of the very long arm, so the contact becomes obvious, but when the lighting in the white light interferometer is set correctly, the interferences are very
7.3 Alignment of the Cavity
visible and this step becomes useless. When the interference pattern can be seen, it is possible to know what is the angle between the surface of the end of the fiber and the crystal, and in which direction is the misalignment. Then I usually back the fiber off a little bit for safety and I adjust the mirror mount of the fiber holder to move the fiber in an orthogonal direction from the interference pattern. This movement also needs care, because moving the angle of the fiber mount generally also moves the distance between the two interfaces, and one doesn’t want to scratch the crystal with the fiber. If the camera is not moved it is possible to continue to monitor the fiber and the appearance of the interference pattern. The fiber may have arrived at the end of the crystal during this procedure. If the misalignment is horizontally, it can be adjusted by the other translational stage of the fiber holder. If the misalignment is vertically, it will be necessary to back off the fiber a bit more, and move the mirror mount’s height . A fiber holder less long and better care during the initial manufacturing of the holders may make these steps useless, making the need for an adjustable mount unnecessary and therefore increase the general stability of the system. When the interference pattern is flat, the fiber and the crystal are aligned.
At this point it is possible to back off completely the fiber with the translation stage and come back without changing the angle. So it is possible to back off, touch very slightly the fiber with a drop of index liquid (G608N3 ([67])) and go back to contact. It is not possible to see the pattern anymore with the index liquid, but the contact is very obvious, as the liquid spreads at the contact with the crystal. The liquid adds a little bit of stability to the system, because it is a bit elastic and a bit sticky, but it doesn’t change substantially the reflection coefficient.