We show the operation of an optically pumped magnetometer (OPM) array in a 50 microT magnetic field. The various components for a fully optical and non magnetic detector unit were constructed and evaluated, from which a prototype unit was assembled with fiber coupled electronics. In this unit the magnetometers were operated using the Intensity Modulated (IM) method and heated with an off-resonant laser. Calculations on the temperature distribution were used to design the magnetometer array. Different magnetometers in such a detector unit were characterized and showed identical performance. Without applying noise reduction schemes, the obtained magnetic field resolution is a factor 2.5 above the shot noise level down to frequencies of about 7 Hz.