## 3 Monomode DFB Quantum Cascade lasers in the Mid Infrared Spectral Regime (MIR)  
The fundamental rotational-vibrational molecular absorption bands of most relevant smaller molecules are located in the MIR (wavelength range from 3 to 24 \(\upmu\)m) [42]. As a result, their line strengths in this region are several orders of magnitude higher than in the NIR, which offers great potential for increased detection sensitivity of trace gases. This makes laser sources in this spectral region very attractive for applications such as in-situ process control, environmental monitoring and biomedical diagnostics for which the development of non-invasive measurements are highly favorable [42,43]. The atmospheric windows at 3-5 \(\upmu\)m and 8-12 \(\upmu\)m enable other applications like free-space optical communications [44] and infrared countermeasures [45]. Before the first demonstration of QCL in 1994 [46], the only available semiconductor lasers operating in the MIR were lead-salt LD, which suffer from low operation temperatures and therefore need cooling with liquid nitrogen. Over the past 15 years the performance of QCL has greatly improved and operation above room temperature has been demonstrated in the wavelength range from 3.1 to 16 \(\upmu\)m [47,48]. Figure 6: Four segment BSG device. The grating segments allow for wide monomode tunability. The segments without grating allow for current controlled phases. The coverage of such a wide spectral region with a single laser type is possible because the stimulated emission in QCL relies on intersubband transitions in heterostructures within the conduction band. Therefore the energy separation of the upper and lower laser level can be tailored to achieve lasing at a designated wavelength [49]. Nowadays QCL operating in pulsed or continuous wave mode are commercially available by a number of suppliers. Mounted on thermo-electrically cooled submounts in standard housings, they are compact, rugged and maintenance-free light sources for research, industrial applications and environmental monitoring in the field or even outer space. QCL DFB gratings are most often implemented as either buried gratings [50] or as top gratings [51] usually defined by electron beam lithography and etched into the top of the laser ridge to enforce monomode emission. To provide efficient heat dissipation, laser ridges are generally processed as double channel ridge waveguides with thick electroplated gold around and on top of the laser ridge (see Figure 7) or as buried heterostructure ridges [52]. This is very important for operation at high duty-cycles or in continuous wave mode, because in comparison to diode lasers QCL exhibit considerably higher electrical power consumption and lower wallplug-efficiencies. Thermal management and optimal mounting is therefore indispensible. The temperature tuning rate of DFB QCL in the MIR depends on the emission wavelength and lies in the range of 0.05 to 0.18 cm-1/K. This yields about 4 to 13 cm-1 total tuning range on a standard Peltier cooler. Because the spectral gain region can be tailored to be a lot wider (up to 430 cm-1[14]) by incorporating bound to continuum active region designs [53] or multiple heterogeneous cascades [54], a lot of effort has been made to enable monomode emission over the entire available bandwidth. Concepts to extend the monomode tuning range include DFB laser arrays [55], multisegment lasers [56] and external cavity QCL. Especially the latter have shown extreme tuning ranges of up to 3.8 \(\upmu\)m [14]. Users of DFB QCL in gas spectroscopy typically operate pulsed QCL either in intra- or inter-pulse tuning mode to scan across the absorption line of interest. The former method relies on spectral tuning  
Figure 7: SEM micrograph of the facet of a double channel ridge waveguide with DFB top grating and 5 \(\upmu\)m thick electroplated gold. of the DFB mode within a long pulse (usually a few 100 ns) as a result of the current induced temperature rise in the waveguide during a single pulse (see Figure 8).