## 3 Results and Discussion  
The power-current (PI) characteristics with CW operation at 20 \({}^{\circ}\)C are shown in Figure 3. The threshold current of the DFB laser was measured to be 20 mA. Figure 3 shows that when the I\({}_{\text{DFB}}\) was fixed, an increase in the I\({}_{\text{Tap}}\) resulted in increased output power. As the current was increased to 3.0 A, the output power reached 401.5 mW. During this period, the curve maintained a good linear trend and did not reach the non-linear region of saturation, so we hypothesize that the output power will be significantly improved significantly at higher currents. Figure 4 shows the spectra of different I\({}_{\text{DFB}}\) values (from 50 mA to 700 mA, measured every 50 mA) at a fixed I\({}_{\text{Tap}}\) value of 0.5 A at 20 \({}^{\circ}\)C (measured with AQ6370C spectrum analyzer with a spectral resolution of 0.02 nm). As the current increased, a significant number of carriers were injected into the device, causing the refractive index of each layer to change. In addition, the injection of carriers caused a build-up of heat in the device, which eventually led to the narrowing of the bandgap of the gain material in the active region. This means that the gain peak and the corresponding lasing wavelength to the long-wavelength cause a red shift. The wavelength redshift of Figure 4 was from 1548.44 nm to  
Figure 3: Measured output power with I\({}_{\text{DFB}}\) = 700 mA as a function of I\({}_{\text{Tap}}\). Figure 2: Delayed self-heterodyne linewidth test system. 1550.52 nm (2.08 nm). The spectrum shows that several side peaks appeared in addition to the main laser peak as the current increased. Although side peaks also appeared, the SMSRs at all lasing wavelengths were still over 37 dB, indicating that the device always displayed acceptable single-mode operating characteristics. Figure 5 shows the variation curve of the central lasing wavelength obtained by changing the I\({}_{\text{DFB}}\) at different temperatures. When the temperature was constant, the lasing wavelength increased as the values of I\({}_{\text{DFB}}\) increased. Similarly, as the temperature increased from 10 \({}^{\circ}\)C to 50 \({}^{\circ}\)C, the lasing wavelength also increased. It can be seen from Figure 5 that the change in lasing wavelength for different current wavelengths at different temperatures is almost linear. This is due to the fact that the surface Bragg grating in the device is minimally affected when the temperature increases. However, the bandgap and refractive index do change in a linear relationship. This also limits the rate of change of the lasing wavelength for the DFB laser with temperature. However, this also makes the single-mode stability of the device excellent, and no mode-hopping phenomenon occurs. The achieved quasi-continuous tuning range is 6.156 nm (from 1547.468 nm to 1553.624 nm), and the rate of change of lasing wavelength with temperature was 0.154 nm/\({}^{\circ}\)C.  
Figure 4: Measured spectrum under different I\({}_{\text{DFB}}\) values where I\({}_{\text{Tap}}\) = 0.5 A.  
Figure 5: Measured lasing wavelength under different values of I\({}_{\text{DFB}}\) at different temperatures. As shown in Figure 6a, the maximum SMSR of the device exceeded 55 dB. At the same time, the SMSR at different temperatures and different values of I\({}_{\text{DFB}}\) were also measured, which are displayed in Figure 6b. It is clear that when the temperature and the current were lower, and just after the laser had begun lasing, the SMSR increased proportionally with the increase in current. As the I\({}_{\text{DFB}}\) increased slightly, a side mode appeared, causing the side mode and the main mode to compete. This caused the SMSR to become smaller as the I\({}_{\text{DFB}}\) increased. This situation improved as the current continued to increase. This is because, at the high current level, the main mode dominates, which suppresses the increase in the side mode, increasing the size of the SMSR.