  On-chip magnetic field sensing with Nitrogen-Vacancy (NV) centers in diamond
requires scalable integration of 3D waveguides into diamond substrates. Here,
we develop a sensing array device with an ensemble of shallow implanted NV
centers integrated with arrays of laser-written waveguides for excitation and
readout of NV signals. Our approach enables an easy-to-operate on-chip
magnetometer with a pixel size proportional to the Gaussian mode area of each
waveguide. The performed continuous wave optically detected magnetic resonance
on each waveguide gives an average dc-sensitivity value of $195 \pm 3
{nT}/\sqrt{Hz}$, which can be improved with lock-in-detection or
pulsed-microwave sequences. We apply a magnetic field to separate the four NV
crystallographic orientations of the magnetic resonance and then utilize a DC
current through a straight wire antenna close to the waveguide to prove the
sensor capabilities of our device. We reconstruct the complete vector magnetic
field in the NV crystal frame using three different NV crystallographic
orientations. By knowing the polarization axis of the waveguide mode, we
project the magnetic field vector into the lab frame.
