The present disclosure includes an optical head for a spectroscopic device configured to produce a beam of light, a reference signal detector, and a signal detector. A mirror reflects the beam of light from the light source toward a multi-sectioned window having a top side and a bottom side. The bottom side includes an AR coating and the top side has a first section including an AR coating and a second section including a beam splitter with transmittance and reflectance. A first beam path for the beam of light is defined by the mirror, the second section of the multi-sectioned window, and the reference signal detector. A second beam path for the beam of light is defined by the mirror, the second section of the multi-sectioned window, a medium contained in the spectroscopic device, an additional mirror, the first section of the multi-sectioned window, and the signal detector.