The rapid and accurate determination of physiological concentrations of biologicaly important gases like CO2 and O2 is an important factor in many clinical and bioprocess situations. The optical sensors based on phase fluorometry are developed as an alternative to electrode methodologies. A directly modulated infrared diode laser was frequency doubled in an intracavity KTP quasi-phase-matched wavequide, producing modulated laser light at 425 nm, which was subsequently employed for phase fluorometric detection of CO2 and O2 gases.