In an approach to measuring a magnetic field of a sample, a system includes an optical fiber comprising a first end and a second end, the optical fiber having a core; and a nitrogen vacancy (NV) diamond sensor; where the first end of the optical fiber is configured to receive excitation light from an optical excitation source; and the NV diamond sensor is coupled to the second end of the optical fiber with an optical matching material, the optical matching material configured to optically match the NV diamond sensor to the core of the optical fiber.