A robust laser cavity platform for NV-diamond singlet infrared absorption magnetometry

The negatively charged nitrogen-vacancy center NV$^-$ in diamond is a versatile platform for quantum magnetometry under ambient conditions. Recently, laser threshold magnetometry (LTM) has been proposed as a means to significantly enhance the sensitivity of NV-based magnetometers by incorporating a diamond hosting NV^- centers within a laser cavity and operating near threshold. While demonstrations have validated the concept, practical implementations remain technically demanding, requiring high pump powers and precise alignment of free-space cavities. It remains unclear whether the benefits of operating near threshold will outpace increased laser noise. In this work, we integrate an NV-diamond with a high NV^- content into a compact external cavity diode laser and demonstrate singlet infrared absorption optically detected magnetic resonance (ODMR). The system exhibits exceptional threshold current stability, enabling ODMR using the threshold current as the read-out parameter. We report a five-fold enhancement in the ODMR contrast by operating near threshold. The best magnetic field sensitivity of 7.6 mathrm{nT/sqrt{Hz}} (DC-500 Hz) is achieved well above threshold, while near threshold sensitivity is limited by increased probe laser noise. These results establish a compact and mechanically robust platform for singlet absorption-based NV^- magnetometry and highlight key trade-offs between contrast enhancement and laser noise near threshold.