# Purely Gain-Coupled Distributed-Feedback Bragg Semiconductor Laser Diode Emitting at 770 nm
## 3 Results and Discussion  
The power-current-voltage characteristics with continuous-wave (CW) operation at 20 \({}^{\circ}\)C are shown in Figure 3. The DFB lasers have a cavity length of 1 mm, with facet coating (high reflection \(>\)99%, antireflection \(<\)0.5%). The maximum CW power of the devices in single-longitudinal-mode operation is as high as 116.3 mW at 350 mA, which is better than that of the laterally coupled ridge-waveguide (LC-RWG) surface gratings of the DFB lasers (which is 28.9 mW), reported in [37], and better than commercial DBR lasers (which is 80 mW) [31]. An ECDL can get higher output power [37], even more than 700 mW. However, an ECDL uses external optical element to feedback and select the frequency of the laser, which increases the effective length of the resonator. The stability of the coupling optical path and working environment of external optical element is higher. In addition, the external optical frequency selective component coupling optical path and the working environment have high requirements for stability. To realize single longitudinal mode, single chip solutions are always compact and highly integrated as compared with external cavities [32]. Compared with commercially available DBR lasers [31], our device shows the advantage of easy fabrication process with only periodic injection windows, instead of using nanoscale grating fabrication or secondary epitaxy. Different from DBR lasers, our periodically injected current assured our device is working at single longitudinal mode under gain coupled mechanism and with a higher output power of 116.3 mW. Below the threshold current, the output power of the laser is negligibly small; above the threshold current, when the gain is able to cover the intrinsic and mirror losses, the output power increases almost linearly until saturation, as shown in Figure 3. The power-current curve becomes nonlinear when saturation occurs. There are two main reasons for nonlinearity at saturation as follows: (i) As the current increases,  
Figure 3: Power-current-voltage curve of the devices at 20 \({}^{\circ}\)C.  
and the gap between the peak and the valley of the gain in the laser decreases, weakening the gain-coupling effect. This results in multimode lasing, and the current-power curve no longer conforms to linearity. (ii) When the current increases further, gain saturation generally also appears in the quantum well, and there is no gain contrast. The increase in heat reduces the gain of the quantum well, leading to nonlinearity. As shown in Figure 4, the wavelength gradually red shifts as the current increases. The single-longitudinal-mode lasing spectrum of our devices in the injection current range of 290-370 mA is pretty well, and the side-mode-suppression ratios (SMSRs) are greater than 30 dB. At 20 \({}^{\circ}\)C, the maximum SMSR of our devices is 31.15 dB at 310 mA, and the tuning range of the wavelength vs. current is from 770.91 nm to 771.24 nm in the injection current range 290-370 mA. The mode-hop-free tuning range for the FP mode is 0.052 nm. As shown in Figure 5a, the SMSR of coated DFB lasers in single-longitudinal-mode operation is as high as 33.25 dB for emission at 770.09 nm (measured with a YOKOGAWA AQ6370C optical spectrum analyzer linked by a single mode fiber with core diameter of 5 \(\upmu\)m). Figure 5a shows a typical device spectrum. There are many small peaks along with the main one; the typical interval between these adjacent small peaks is 0.0665 nm. This value closely matches the interval of the FP modes. Figure 5b shows that the wavelength gradually red shifts as the temperature increases. In addition to the gain-coupled effect, the device also has a mode-selection mechanism from the FP cavity. This mechanism has the effect of narrowing the linewidth while causing tuning-wavelength discontinuities, as shown in Figure 5b. Since the gain-coupled effect is relatively weak, it only plays a secondary role in mode selection as compared with the FP modes.