## 5 Applications
### Medical Applications  
Measuring the glucose level is crucial for patients suffering from diabetes. Nowadays the standard measurement procedures require a small blood drop usually drawn by pricking a finger tip each time the glucose level has to be checked, usually up to six times a day. A non-invasive alternative has been developed making use of multiple laser diodes emitting at different wavelengths [68]. The wavelengths are chosen to represent various characteristic positions in the very broad absorption band of glucose. A multiple channels scanning system allows to quantify the glucose level by triggering the different laser diodes sequentially. The light is guided onto the nail bed and conclusions about the glucose level can be drawn from the intensity of the refracted light. Due to the very stable wavelength and power characteristics of complex coupled DFB LD, this approach yields very reliable results providing a practical way to measure the blood glucose levels. Another major medical application of DFB laser diodes is their use for non-invasive breath tests. The best-known diagnostic test based on this measurement technique is the \({}^{13}\)C-Urea Breath Test (UBT) used to detect _Helicobacter pylori_ infections in humans. For the UBT, the patient orally receives a small dose of urea isotope-marked with \({}^{13}\)C. If _Helicobacter pylori_ bacteria are present in the patient's stomach, the isotope-marked substance will be split into \({}^{13}\)CO\({}_{2}\) and ammonia. The \({}^{13}\)CO\({}_{2}\) diffuses via the bloodstream into the patient's lungs. Consequently it can be detected in the patient's breath by laser based spectroscopy. A technique which is ideally suited for this purpose is photoacoustic spectroscopy (PAS) [69; 70]. In contrast to transmission spectroscopy, PAS enables offset-free measurements. Furthermore, the relatively long absorption cells usually associated with transmission spectroscopy can be replaced by much smaller ones facilitating the development of small and handy analytical equipment. The breath gas is guided into a gas cell and exposed to the light of a tunable DFB laser source wavelength-matched to an absorption wavelength of CO\({}_{2}\). The laser light therefore is absorbed by the CO\({}_{2}\) molecules and transferred into kinetic energy of the surrounding molecules via inelastic collisions. A modulation of the exciting laser source results in local pressure fluctuations _viz._ a sound wave that can be monitored using a microphone and, in some cases, phase sensitive lock in detection schemes. For modulation frequencies below 1 MHz, the photoacoustic signal is directly proportional to the concentration of the absorbing molecules, the absorption cross section of the molecular transition and the intensity of the laser source [70]. DFB lasers offer several advantages over gas lasers often used in PAS measurements. First of all, diode lasers can be modulated directly via the injection current without the use of mechanical choppers, therefore avoiding coherent noise caused by the rotating blade. Furthermore, the frequency shift caused by a current modulation of \(\pm\)1% is around \(\pm\)1 GHz, relatively small in comparison with the absorption bandwidth of solids therefore eliminating window signals often deteriorating sensitivity when using a gas laser [71, 72]. As mentioned above, the narrow spectral linewidth of complex coupled DFB lasers enables to resolve different gas isotopes. In the case of UBT measurements, this is taken advantage of in order to discriminate between \({}^{13}\)CO\({}_{2}\) and \({}^{12}\)CO\({}_{2}\). Figure 11 shows the emission spectrum of a DFB LD emitting right within a rotational-vibrational absorption band of CO\({}_{2}\) (see Figure 1) at a wavelength of 2,043 nm. The high SMSR of typically \(>\)35 dB and the narrow spectral linewidth of \(<\)3 MHz guarantee high spectral selectivity and enable cross interference free measurements.