# A Widely Tunable Three-Section DBR Lasers for Multi-Species Gas Detection
## 3 Results and Discussion  
### Characterization of the DBR Laser  
First, the electro-optical characteristics of the device are measured at different temperature, as shown in Figure 3a, with a current of the DBR and phase section 0 mA. The optical power is measured from the gain section facet of the device by an integrating sphere. We can see that, with the temperature increasing from 15 to 35 \({}^{\circ}\)C, the threshold current of the device increases (20 to 30 mA) and the laser power decreases (9.9 to 6.9 mW). The data are analyzed in the empirical relation, \(Ith=I_{0}exp(T/T_{0})\), the characteristic temperature \(T_{0}\) is determined to be 53 K. The value of \(T_{0}\) obtained here is comparable to the conventional 1.65-\(\upmu\)m DFB lasers. The laser's emission wavelength redshifts with the temperature. The temperature-tuning rate is 0.11 nm/\({}^{\circ}\)C when the gain current is maintained at 60 mA, as shown in Figure 3b. This indicates that in addition to the tuning of DBR and phase currents, the tuning range of the DBR laser can also be expanded effectively by changing the temperature. Meanwhile the SMSR properties of the device are not deteriorated. Then, the wavelength tuning character is investigate by varying the phase and DBR current. The lasing wavelength dependence on the DBR current at 25 \({}^{\circ}\)C in a fixed gain current of 60 mA is shown in Figure 4a. It can be seen from the figure, the lasing wavelength can be switched digitally over 11 consecutive channels for about a tuning range of 9 nm, when increasing the DBR current from 0 to 100 mA. The channel wavelength blue shifts when the DBR current increasing. From the tuning characteristics of DBR section, it indicates that the tuning physical mechanism is determined by the free-carrier (FC) injection effect. However, a tuning saturation takes place when the DBR current is increased to 63 mA. Actually, the tuning saturation which is caused by the self-heating effect can be improved by designing and selecting the coupling coefficient \(\kappa\) and the length of Bragg grating carefully. The longer the PL wavelength of the grating material, the bigger effective refractive index change at the same density of the injected FC [21]. So, we can get a larger tuning range by selecting a narrower bandgap DBR material. Figure 3: (**a**) CW (continuous wave) L\(-\)I (Light-Current) characteristics of the DBR laser at the temperatures between 15 to 35 \({}^{\circ}\)C. Inset shows the threshold current dependence on mount temperature. (**b**) Lasing spectrum of the DBR laser at different temperature at 60 mA gain current. By changing the current in the DBR section, the lasing wavelength of the laser can be tuned continuously in the separate channel and it is not continuous between channels. This can be compensated by tuning the phase current. Through the phase current adjusting, the wavelength shift can cover the gaps between two channels where the mode hopping happens, which is shown in Figure 4b. The emission wavelength sweeps 0.88 nm continuously as the phase current increasing from 1 to 8 mA. The single channel range realized by the phase current is larger than the 0.8-nm mode-spacing. Therefore, a continuous tuning range can be realized by combining injected current of DBR and phase area. Combined with the temperature tuning (increasing the heat sink temperature from 15 to 35 \({}^{\circ}\)C), the tuning range of the device can be further increased from 9 to 11 nm, as shown in Figure 5a. For gas detection, the side mode suppression ratio (SMSR) is also a key parameter to DBR lasers. The SMSR of higher than 30 dB are obtained for all channels, as shown in Figure 5b. In practical applications, the beam quality of the laser is also an aspect that needs to be considered. Generally, the far-field characteristics can be used as a parameter to evaluate the quality of the laser beam. We measured the far-field characteristics of the DBR  
Figure 4: (**a**) Lasing wavelength dependence on DBR current.