This article describes the advances made in the development of a specific optical spectrometer based on the Optical Feedback-Cavity Enhanced Absorption Spectroscopy technique for exploring past climate by probing the original composition of the atmosphere stored in the ice sheet of a glacier. Based on significant technological progresses and unconventional approaches, SUBGLACIOR will be a revolutionary tool for ice-core research: the optical spectrometer, directly embedded in the drilling probe, will provide in situ real-time measurements of deuterium isotopic variations (δ(2)H ) and CH4 concentrations down to 3500 m of ice depth within a single Antarctic season. The instrument will provide simultaneous and real-time vertical profiles of these two key climate signatures in order to evaluate if a target site can offer ice cores as old as 1.5 million years by providing direct insight into past temperatures and climate cycles. The spectrometer has a noise equivalent absorption coefficient of 2.8 × 10(-10) cm(-1) Hz(-1/2), corresponding to a detection limit of 0.2 ppbv for CH4 and a precision of 0.2‰ on the δ(2)H of H2O within 1 min acquisition time.