Abstract: A laser absorption spectroscopy exhaust gas sensor includes an optical cell with porous walls having pores with a mean diameter in the range of 0.1 nm to 1 mm; gold mirrors within the optical cell positioned to support a multi-pass optical path within the optical cell; an active heating element adapted to heat the optical cell to prevent condensation; a laser adapted to generate a laser beam; an optical detector adapted to detect a returning laser beam; and a processor for controlling the laser and the active heating element and for analyzing signals from the optical detector to identify a gas in the optical cell.