Quantum well lasers emitting between 3.0 and 3.4 μm for gas spectroscopy

The 3 to 4 μm range had long appeared inaccessible to quantum well lasers made on GaSb. Despite having excellent performance in the 2 to 3 μm range, GaInAsSb/AlGaAsSb quantum well lasers rapidly show their limits when crossing the 3 μm barrier (the highest wavelength reached with such a device was 3.04 μm under cw operation at 20°C). This situation was all the more regrettable because several gases have their strongest absorption lines in the 3 to 4 μm range: methane, for example, has a peak of absorption at 3.26 μm overhanging a weaker peak at 2.31μm by a factor 40. Works carried out in the University of Munich in 2005 gave new hopes to the world of laser diode spectroscopy. By replacing the quaternary AlGaAsSb barrier by a quinary AlGaInAsSb barrier, researchers were able to reach laser operation at 3.26 μm and room temperature in the pulsed mode. Since then, several teams have engaged in the objective of reaching cw operation at room temperature with such structures. We will give an insight into the phenomena responsible for the increase of threshold current with growing wavelength. Finally, we will present results obtained with a monomode DFB laser diode emitting at 3.37 μm having a threshold current of 140 mA at 18°C.