Temperature characteristics of K-Rb hybrid optically pumped magnetometers with different density ratios

Optically pumped magnetometers (OPMs) that are equipped with hybrid cells of K and Rb have been studied for improving their sensitivity and biomagnetic field measurements. The densities of the two alkali metal atoms and their density ratio are especially important for hybrid OPMs. In this study, we fabricated five hybrid cells using different K and Rb atom densities and measured the output signal intensities by controlling their cell temperatures. The output signal intensity of OPMs has different temperature characteristics depending on the density ratios of K and Rb atoms. The densities of the two atoms at any temperature were estimated based on the Raoult's law, and we compared the experimental results with the calculated results based on the Bloch equations. Furthermore, the numerical calculations that were obtained based on the Bloch equation by incorporating a relaxation term due to the absorption of the probe beam exhibited good agreement with the experimental results. Finally, in case of n_K/n_Rb = 4.85, it is estimated that a sensitivity of 1.6 fT/Hz^1/2 can be achieved by increasing the temperature to 270 °C.