Efficient fluorescence coupling microlens integrated fiber magnetometer probe based on nitrogen vacancy centers

The nitrogen vacancy (NV) centers in diamonds have gathered increasing interest as an emerging quantum sensing platform with high sensitivity and spatial resolution. Integration of micro-sized diamond and fiber is an essential method to build an NV center endoscope probe and enable NV center sensors for practical application. However, the low fluorescence collection efficiency of fibers due to their small numerical aperture (NA) has limited the sensitivity of the sensors. In this paper, a cone-shape microlens was fabricated using the photopolymerization process at the end of a multimode fiber to boost the laser excitation and fluorescence collection efficiency of NV centers. Experiments demonstrated that over 21 times fluorescence intensity enhancement and 12 times sensitivity improvement were achieved. This fiber–microlens magnetometer probe exhibited a 2.1-nT/Hz¹/² sensitivity over a bandwidth of 100 Hz with ∼80-µm diameter diamond. This research presented a robust and large NA diamond integrated fiber–microlens magnetometer probe, which can also be expanded to magnetic field scan and real-time monitoring.