CDSM (Coupled Dark State Magnetometer) is an optically-pumped scalar magnetometer based on two-photon spectroscopy of free alkali atoms. In a special laser-based excitation mode, three different magnetic field dependent resonances arise in the presence of an external magnetic field. They reach their maximal strengths at different angles between the magnetic field direction and the optical path of the laser excitation field through of the sensor unit. According to this angle, the strongest resonance is selected for the actual measurement. This enables an omni-directional all-optical scalar magnetic field measurement without the need for moving parts, feedback coils and active electronics at the sensor. The extended measurement range, which is a lot larger than the magnitude of the Earth’s magnetic field (60,000 nanotesla), is a further advantage of this measurement principle.

In general, scalar magnetometers measure the magnitude of the magnetic field with high accuracy, while vector magnetometers (mostly fluxgate magnetometers) measure the magnitude and the direction of the magnetic field. Fluxgate magnetometers must be recalibrated in flight regularly. Therefore, for a certain mission scenario, full science return can only be achieved with a combination of vector and scalar magnetometers.

CDSM comprises a mixed signal electronics unit, a Vertical Cavity Surface Emitting Laser (VCSEL; with housing, temperature stabilisation and fibre coupler), a sensor unit, and a sensor harness with optical fibres. The electronics as well as the laser are located in an electronics box inside the spacecraft, whereas the sensor unit is designed for boom mounting.

The sensor consists of two fibre couplers for the connection of the outbound and inbound fibres, a polariser and a quarter wave plate to secure the required laser light characteristics, a cylindrical glass cell, which contains a few micrograms of 87Rb metal for the actual measurement process and Neon as buffer-gas, and a metal housing for a precise alignment of the optical components.