Since the first demonstration of distributed feedback quantum (DFB) quantum cascade (QC) lasers in 1997 [1] tremendous progress has been made in the development of single-mode emitting QC lasers [2][3][4]. Such kind of devices are of particular interest because a lot of different molecules, like CO 2 , N 2 O or CH 4 , have their fundamental modes in the mid-infrared spectral region (3-12 μm).This paper will cover the most recent developments in monolithic, single-mode emitting QC laser sources. A special focus will be put on the following topics: 1 st order DFB QC lasers with very low power dissipation. As can be seen in Fig. 1 (left panel) less than 1 W of power consumption at laser threshold in continuous-wave mode could be realized for temperatures up to 40°C [5]. The optical output power reaches values of up to 18.5 mW at -30°C. Such devices are the key to build compact and portable sensor systems. The concept of low power consumption can moreover be easily transferred to other wavelength ranges.To increase the optical output power of single-mode emitting QC lasers, two-sectional device geometries can be used. One (shorter) section comprises a DFB grating and acts as a single-mode seed. The second (longer) section consists of a simple Fabry-Pérot cavity and amplifies the radiation from the first section. Peak optical output powers higher than 1 W were realized (for preliminary results see Fig. 1 (middle panel); cp. also [6]). Such "master-oscillator power-amplifier" (MOPA) configurations have the advantage of being more robust in terms of single-mode emission. Long (one-sectional) DFB devices with shallow gratings are in contrast more susceptible to mode instabilities due to the weaker grating-coupling.
Fig. 1 (left panel)Light-electrical dissipation characteristics of a typical 3 μm wide and 1 mm long low dissipation device in continuous-wave operation. Less than 1 W is dissipated at laser threshold up to 40°C. (middle panel) Light-current characteristics of an uncoated MOPA-device with a peak power of higher than 1 W. (right panel)Twin-DFB spectrum lasing selectively at 1600 cm -1 or 1900 cm -1 or both wavelengths in parallel.The third topic will cover dual-wavelength single-mode emitting QC lasers. These so called twin-DFB devices consist of a dual color active region which is processed into a two-sectional device geometry. Both sections feature a 1 st order DFB grating dedicated to one of the two wavelengths. In contrast to previously demonstrated dual-color DFB QC lasers [7] both parts are electrically separated and can be driven individually. Both wavelengths can selectively be switched on and off while they are both sharing the same spatial origin of radiation. Since no mechanical parts are included, the device can switch very fast between both wavelengths. Typical spectra of such a twin-DFB device are shown in Fig. 1 (right panel).