  Efficiently excite nitrogen-vacancy (NV) centers in diamond and collect their
fluorescence significantly benefit the fiber-optic-based NV sensors. Here,
using a tapered optical fiber (TOF) tip, we significantly improve the
efficiency of the laser excitation and fluorescence collection of the NV, thus
enhance the sensitivity of the fiber-optic based micron-sized diamond magnetic
sensor. Numerical calculation shows that the TOF tip delivers a high numerical
aperture (NA) and has a high fluorescence excitation and collection efficiency.
Experiments demonstrate that using such TOF tip can obtain up to over 7-fold
the fluorescence excitation efficiency and over15-fold the fluorescence
collection efficiency of a flat-ended (non-TOF) fiber. Such fluorescence
collection enhances the sensitivity of the optical fiber-based diamond NV
magnetometer, thus extending its potential application region.
