## 1 Introduction
Since complex coupled gratings show many advantages over index coupled gratings, a special manufacturing process was developed [25,26] to realize high performance DFB lasers with low threshold currents, high output efficiency and excellent side mode suppression ratios (SMSR). The lasers presented in this paper are realized as ridge waveguide structures with a width of 2-10 \(\mu\)m. The feedback grating consists of Cr and is defined laterally to the ridge (see figure 2). This metallic grating structure results in complex coupling without the need for any overgrowth steps. Since the standard fabrication for DFB lasers requires at least two growth steps, the presented process simplifies the realization of the lasers and reduces defect sources. The biggest advantage of complex coupled lasers over conventional index coupled devices however is the absence of a stopband in the optical spectrum, thus allowing to directly select the Bragg-mode. Furthermore, complex coupled devices benefit from higher mode stability [27-29], lower sensitivity of optical feedback reflections [30,31] and improved modulation properties [32,33]. Since only the evanescent part of the mode interacts with the grating, the spatial overlap is around \(10^{.4}\), two orders of magnitude lower than for an overgrown grating incorporated into the waveguide. Figure 2: SEM micrograph of a ridge waveguide structure with lateral Cr grating. This is compensated by the large imaginary part of the refractive index of the grating metal. The additional losses introduced by the Cr grating lead to only a slight penalty in threshold current and output efficiency [34]. The nodes of the DFB mode are in phase with the absorption grating, resulting in only very small damping of the DFB mode. Typical corresponding optical spectra are shown in Figure 3. The presented process can be used for various material systems, yielding monomode devices in the wavelength range from 760 nm to 3.0 \(\upmu\)m. Due to their excellent and customizable properties these devices are covering a wide field of applications from the telecommunications sector to gas sensing, medical and space applications (see Section 5). The presented DFB concept of lateral Cr gratings leading to complex coupling is also a very viable way to realize high quality widely tunable lasers for a large field of different applications. The most convenient way to incorporate the desired multitude of DFB wavelength channels into one device is the concept of binary superimposed gratings (BSG), which were first proposed by [PERSON] _et al._ for use in DBR lasers [35]. For a tunable grating with N optical emission peaks, N binary gratings are virtually superimposed on top of each other, leading to a staircase like grating with N+1 levels which are binarized subsequently [36, 37]. Two such gratings yielding comb-like mode spectra with slightly different mode spacing are implemented in two longitudinally coupled laser segments. While the mode spacing in segment 1 is \(\Delta\lambda_{t}\), the mode spacing in segment 2 is \(\Delta\lambda_{2}>\Delta\lambda_{t}\), thus avoiding the overlap of more than two peaks at one time. By changing the refractive index of a segment, the reflection peaks of the corresponding grating can be shifted. In principle, the refractive index can be adjusted by the temperature and the current. But in general a current-induced tuning of the refractive index is much more convenient and precise than temperature tuning. By shifting the two mode combs of the segments against each other, the Vernier effect can be utilized to select one desired wavelength channel (see Figure 4). If two peaks overlap, the corresponding wavelength is favored by suffering lower internal losses, resulting in monomode emission at this spectral position. Figure 3: Examples of monomode spectra of laterally coupled DFB lasers with Cr gratings emitting at different wavelengths. A simultaneous tuning of the segment currents results in continuous tuning of the selected wavelength.