# A Widely Tunable Three-Section DBR Lasers for Multi-Species Gas Detection
## 3 Results and Discussion
### Experimental Setup of TDLAS  
To demonstrate the DBR laser's possibilities in multi-gas detection, the experimental setup of the gas detection system based on TDLAS technology is schematically illustrated in Figure 7. For the convenience of gas sensing, an industry standard 14-pin butterfly packaging scheme of the DBR laser chip is adopted and the whole device is integrated with thermistor, thermo-electric cooler (TEC), optical isolator, and the photo-detector (PD).  
Temperature control circuit ensures the DBR laser working at a proper temperature, stabilizing the output power, and locking laser emission peak to the absorption line of the target gas. The laser is modulated by two waveform generators. One is to generate a low frequency triangular wave providing a slow wavelength scan by a function generator. The other is to superimpose a high frequency sinusoidal modulation to the laser by a lock-in amplifier. The modulated light is guided to the gas cell which is filled with target gas and is detected by a photo-detector (PD) which transforms the optical signals to the electrical ones. Finally, the electrical signals are sent to another lock-in amplifier where such signals are demodulated, outputting the signals in a second harmonic type.  
Figure 6: Lasing wavelength varies with work time.  
According to the spectra extracted from Hitran database, we choose absorption lines around 1781 nm which corresponds to that of methane and water for detection experiment, as shown in Figure 8. The peak emission wavelength of the DBR laser is 1781 nm with a SMSR of 40 dB when the gain current is 75 mA at 22 \({}^{\circ}\)C. For H\({}_{2}\)O detection, a 1 kHz triangular ramp (gain current varied from 62 to 86 mA) and a 10 kHz sinusoidal modulation (with the amplitude of 6.48 mA) are applied to the driving current in the gain section while the current in the DBR section is set to 0 mA. For methane detection, the applied signal in the gain section is the same with H\({}_{2}\)O applied except an additional 4 mA current in the DBR section.  
The second harmonic curve (the \"2f\" curve) of H\({}_{2}\)O and CH\({}_{4}\) is shown in Figure 9. The current in the valley bottom of the \"2f\" curve is consistent with the driving current of the DBR laser in which the emitting wavelength corresponding to the sample gas absorption line. In other words, the \"2f\" curve contains a dip where the sample gas absorbed at the wavelength corresponding to that laser current. The current at the dip represents the driving current of the laser at the wavelength of interest.  
Figure 8: The spectrum of DBR laser near the absorption lines of the measured gas. Inset is the selected absorption lines of methane and water in the gas detection experiment.  
Figure 7: TDLAS (tunable diode laser absorption spectroscopy) experimental setup for the gas detection.  
The theoretical formula of TDLAS is based on the Beer-Lambert law. For a single frequency laser, the light intensity changes can be described as follows [24]:  
\[I(\
u)=sl_{0}(\
u)exp[-\alpha(\
u)CL] \tag{1}\]  
where \(s\) is the light collection efficiency, which is the ratio of laser power received by the photo-detector to the original optical power without gas absorption; \(I(\
u)\) and \(I_{0}(\
u)\) are the transmitted and incident laser intensities, respectively; \(\alpha(\
u)\) is the spectral absorption coefficient in unit length and unit concentration at the frequency of \(\
u\); \(C\) is the concentration of sample gas; \(L\) is the length of optical path. The working principle is that, when a light beam at a specific wavelength which is modulated passes through the target gas at a path length \(L\) and the transmitted laser power is measured by a PD. The amount of light intensity lost (absorbed by the sample gas) is proportional to the gas concentration in the path of light. Analyzing this second harmonic data extracted from the lock-in amplifier, the gas concentration value can be calculated.