Edge and vertical surface emitting lasers around 2.0–2.5 μm and their applications

Abstract

One of the main concerns in environmental issues is the accumulation of pollutant gases in the atmosphere. This gives rise to a need for convenient ways of detecting these gases. In this paper we review some fundamentals of a technique called ‘tunable diode laser absorption spectroscopy’, which is suitable for making a portable gas analyser. We focus on the 2.2–2.4 μm range which corresponds both to a transparency window in the atmosphere and to high absorption bands of the main pollutant gases. We present two GaInAsSb/AlGaAsSb laser diodes which meet well the requirements for a portable gas analyser: a laser diode emitting at 2.38 μm exhibiting a monomode emission (side–mode suppression ratio 30 dB) and a laser diode emitting at 2.30 μm able to operate continuous wave up to 140°C. Thanks to these diodes we designed a set–up for gas detection using the tunable diode laser absorption spectroscopy technique. We have reached a sensitivity as good as 1 ppm for CH4 at atmospheric pressure. Finally, we present a promising technique called ‘intra cavity laser absorption spectroscopy’ making use of a new device, the ‘vertical external cavity surface emitting laser’, which may allow us to increase sensitivity by a factor of 105.