  We experimentally demonstrate a simple and robust optical fibers based method
to achieve simultaneously efficient excitation and fluorescence collection from
Nitrogen-Vacancy (NV) defects containing micro-crystalline diamond. We
fabricate a suitable micro-concave (MC) mirror that focuses scattered
excitation laser light into the diamond located at the focal point of the
mirror. At the same instance, the mirror also couples the fluorescence light
exiting out of the diamond crystal in the opposite direction of the optical
fiber back into the optical fiber within its light acceptance cone. This part
of fluorescence would have been otherwise lost from reaching the detector. Our
proof-of-principle demonstration achieves a 25 times improvement in
fluorescence collection compared to the case of not using any mirrors. The
increase in light collection favors getting high signal-to-noise ratio (SNR)
optically detected magnetic resonance (ODMR) signals hence offers a practical
advantage in fiber-based NV quantum sensors. Additionally, we compacted the NV
sensor system by replacing some bulky optical elements in the optical path with
a 1x2 fiber optical coupler in our optical system. This reduces the complexity
of the system and provides portability and robustness needed for applications
like magnetic endoscopy and remote-magnetic sensing.
