A humidity sensing device for measuring a humidity of a reactant stream in a fuel cell system is provided. The battery cell arrangement further has a frame arrangement which includes a first frame element and a second frame element which frames the cell body on all sides on the rim. The amplifier includes a wavelength controller configured to adjust the wavelength range of at least one of the optical sources to vary the frequency difference in a manner sufficient to vary optical intensity noise produced when the optical energy from the multiple optical sources is combined. Systems for controlling this apparatus are also disclosed.