# A Widely Tunable Three-Section DBR Lasers for Multi-Species Gas Detection
## 1 Introduction  
In recent years, multi-gas detection has important applications in many fields such as production process optimization, medical health and environmental protection [1; 2; 3]. Due to its reliable, rapid, non-contact features, and low spectral interference, tunable diode laser absorption spectroscopy (TDLAS) is very promising in many different techniques [4; 5; 6]. A highly precise TDLAS gas sensing system requires its laser source possessing several special properties such as single-longitudinal-mode (SLM) and continuous wave (CW) operation, continuous tuning, mode-hop-free, and narrow spectral linewidth. TDLAS system commonly use distributed feedback (DFB) lasers as the light sources because of its simple fabrication process and narrow linewidth. Since the tuning range of DFB lasers is relatively small (one or two nanometers), it need multiple lasers in the multiple gases sensing system [7], which make the system very complex. In application, cost-effective monitoring, rapid measurement and equipment miniaturization is of great importance for an excellent gas sensing system, thus the widely tunable semiconductor lasers show their advantages when they are applied in such system [8; 9]. At present, because of the rapid development of communications, the DBR lasers emitting at 1.5 and 1.3 \(\mu\)m have been reported [10; 11; 12; 13; 14]. Furthermore, an array waveguide based tunable quantum-dot (QD) laser with a ring structure, operating at around 1.6-1.8 \(\mu\)m has been developed by [PERSON] et al. fe tomography systems [15; 16; 17; 18]. However, the 1.6-1.9 \(\mu\)m monolithic quantum-well DBR lasers have yet to be explored. The spectral range (1.6 to 1.9 \(\mu\)m) is consistent with the absorption spectrum of many gas species in environmental and industrial interest [19], such as water (H\({}_{2}\)O), methane (CH\({}_{4}\)), nitric oxide (NO), carbonic oxide (CO), hydrogen chloride (HCL), carbon dioxide (CO\({}_{2}\)), et al. So, this near-infrared wavelength range is important for spectroscopic and gas sensing applications. Although the 1.6-1.9 \(\mu\)m wavelength band is in the overtone and combination absorption region whose absorption features is much weakerthan that of the fundamental absorption region (2-20 \(\upmu\)m), however, it is completely enough to achieve a sub ppm detection accuracy based on the TDLAS detection schemes [20]. It can be seen from the above, a widely tunable DBR laser is an ideal light source in the multiple gas detection applications.  
By simulation, the photoluminescence (PL) wavelength of the grating material is proportional to the wavelength tuning range of the laser [21]. In order to get a larger tuning range, the InGaAsP bulk materials with a PL wavelength of 1.45 \(\upmu\)m which has the big refractive index difference under a fixed injection current are adopted as the butt-joint material in DBR section. Through the design, we get a total tuning range of 11 nm. The lasing spectra can cover the absorption lines of multiple gases, (HCL, H\({}_{2}\)O, N\({}_{2}\)O, NO, CH\({}_{4}\)) which are extracted from the HITRAN (high-resolution transmission molecular absorption) database, as shown in Figure 1. Sensing of those gases is attractive in environmental and industrial applications. Furthermore, we also study its application for measuring two spectral features of molecular gases (water and methane). It demonstrates that the DBR lasers we fabricate are capable of accurate wavelength scanning and are applicable for gas sensor. To our knowledge, this is the first demonstration of 1.8 \(\upmu\)m wavelength monolithic tunable DBR quantum-well laser diode for multispecies gas detection.