  We present a novel four-channel OPM sensor for magnetoencephalography that
utilizes a two-color pump/probe scheme on a single optical axis. We
characterize its performance across 18 built sensor modules. The new sensor
implements several improvements over our previously developed sensor including
lower vapor-cell operating temperature, improved probe-light detection optics,
and reduced optical power requirements. The sensor also has new electromagnetic
field coils on the sensor head which are designed using stream-function-based
current optimization. We detail the coil design methodology and present
experimental characterization of the coil performance. The magnetic sensitivity
of the sensor is on average 12.3 fT/rt-Hz across the 18 modules while the
average gradiometrically inferred sensitivity is about 6.0 fT/rt-Hz. The sensor
3-dB bandwidth is 100 Hz on average. The on-sensor coil performance is in good
agreement with the simulations.
