This work presents an original method based on the variance properties of the microwave absorption spectrum of an ensemble of nitrogen vacancy centers in diamond. The spectrum is measured optically. A compact and simple device is designed to optimize the photon collection. We conduct a quantitative comparison of the ensemble's optical signal in both the visible and near infrared range. Using the enhanced signal-to-noise ratio achieved through the device geometry we perform real-time DC magnetometry at moderate light and microwave powers. Under these conditions, the amplitude of a DC magnetic field can be extracted from the variance of the microwave absorption spectrum in a fast and reproducible manner, without the burden of complex fitting techniques.
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