Schematic diagram of the optical paths inside the OPM sensor and the electrical signal connections (heating coils not shown). PM: polarization-maintaining optical fiber; F: fiber port; L: lens; PBS: polarizing beam splitter; P: reflecting prism; Q: quarter-wave plate; C: potassium vapor cell; H: heat insulation; PD: four-quadrant photodiode; TIA: transimpedance amplifier; Gen Out: function generator; DAQ: data acquisition; Lock-in: virtual lock-in amplifier.