## 1 Introduction  
In recent years the importance of lasers in optical sensing has been continuously increasing [1]. Many apparatus use long-known techniques like fluorescence measurements after excitation by photons, e.g., to detect petroleum contamination in soils [2]. Other systems use cavity ring down or cavity leak-out spectroscopy, e.g., for medical applications with CO [3], or use the excitation of rotational-vibrational spectra for in-situ gas analysis [4] or detection of chemical compounds even in the liquid phase [5]. Excellent scientific books on laser spectroscopy [6] and laser in environmental and life science [7] show the growing interest in the scientific and industrial world. International conferences on laser spectroscopy [8], laser field-applications [9] and tunable diode laser spectroscopy (TDLS) [10] have been generating a steady output of excellent publications as well as scientific exchange for years. Wherever rapid measurement, cost effective monitoring or miniaturization of devices is crucial, the outstanding characteristics of tunable semiconductor lasers show their advantages. In modern environmental analysis these aspects play an important role, hence tunable semiconductor lasers are the workhorse for such applications. Since simple Fabry-Perot (FP) laserdiodes (LD) show poor tuning behavior and no monomode spectrum, they are not suited for high resolution applications. Such FP LD in an external cavity [11, 12] yield good tuning ranges as well as monomode spectra. Such systems are available for high resolution spectroscopy at all wavelengths between 400 nm [13] and 11 \(\upmu\)m [14], however, they incorporate mechanical parts which make them hardly usable for industrial environments, for field measurements or in production sites. This problem is overcome by distributed feedback (DFB) lasers. The key element for monomode emission and the tuning characteristics is the DFB-grating. Through a special technology, based on lateral metal Bragg gratings incorporated in the laser structure, it is nowadays possible to fabricate monomode emitting semiconductor lasers on any III/V-based material, leading to a broad accessible wavelength range from 760 nm up to 3 \(\upmu\)m. In this part of the spectrum many industrial and environmental gas species exhibit strong absorption features [15]. Figure 1 shows absorption features of some exemplary gases in this wavelength range, which is covered by tunable diode lasers made from base-materials like Gallium-Arsenide (760 nm to 1 \(\upmu\)m), Indium-Phosphide (1 \(\upmu\)m to 1.8 \(\upmu\)m) and Antimonides (1.6 \(\upmu\)m to 3.0 \(\upmu\)m). Such devices have been used for many years in products for industrial applications, e.g., in such diverse and complicated cases as the detection of alkali atoms in combustion plants [16] or photoacoustic gas sensing with a quartz tuning fork [17], for sensitive and spatially resolved H\({}_{2}\)O vapor detection [18] and for temperature analysis in shock waves [19]. At wavelengths between 3 \(\upmu\)m and 16 \(\upmu\)m however, even stronger absorption features occur. In this region lead salt LD were used in the past, but they have been largely replaced by quantum cascade lasers (QCL) during the last few years. Figure 1: Absorption features of different gases in the 760–3,000 nm wavelength range. As there have been many good review papers about spectroscopy in the past [20-23], we give a brief review about highly reliable semiconductor lasers with DFB structures enabling tunable monomode emission and high optical power in the wavelength range from 760 nm to 16 \(\mu\)m. Typical applications are shown exemplary in the last section. **2. DFB Lasers in the 760 nm to 3,000 nm Wavelength Range**  
There are various ways to realize monomode semiconductor devices, e.g., distributed Bragg reflectors (DBR) [23], external cavity devices [11,12] or DFB LD. The latter exhibit either pure index coupling or--as presented in this paper--complex coupling.