Quantentechnologie

In our fiber cavity QED project we use optical fibers as cavity mirrors. We fabricate the cavity mirrors directly onto the plane end facets of cleaved single-mode fibers based on a procedure developed by J. Reichel [1]: A pulse of a focused CO₂ laser beam a few milliseconds long ablates a few micrometer of glass from the center of the fiber end facet, leaving a shallow depression of approximately Gaussian shape. In this way, small radii of curvature (~100 µm) can be achieved, yielding small mode waists and mode volumes. Furthermore, a slight reflow of the molten glass under the influence of its surface tension produces a very smooth surface which can be coated with a reflective coating of low scatter loss. Our fiber endfacet machine also incorporates a Mirau interferometric microscope, which allows high-resolution surface characterization by phase-shift interferometry. Mode-matching of cavity and fiber modes together with a low-loss optical coating can achieve the desired high optical in- and outcoupling efficiency. The influence of the non-spherical mirror shape on the finesse and ways to reduce polarization mode splitting (birefringence) may need further investigation.

Fig.  1: Left: Cross-sectional image of a cleaved fiber endfacet after being processed with a CO2 laser beam in our interference microscope. Middle: True phase information obtained independently for each pixel using the phase shift interferometry technique. Right: Reconstructed surface tpopgraphy (false colour) obtained via a phase-unwrapping algorithm. Radius of curvature and ellipticity can now be determined precisely.

Fig. 2: Schematics for the setup for fiber end facet mirror fabrication

References
1. D Hunger et al, New J. Phys. 12, 065038 (2010) http://iopscience.iop.org/1367-2630/12/6/065038