## 4 Conclusions  
The output characteristics of the gain-switched semiconductor laser are affected by the modulation signal and the intrinsic parameters of the laser. However, it is impossible to improve the performance of  
Figure 4: Simulation results of output pulses at different \(T_{2}\), when \(I_{1}=34\) mA, \(T_{1}=0.971\) ns, \(I_{2}=64\) mA: (**a**) Time domain graph; (**b**) the peak power and full width at half maximum.  
Figure 5: (**a**) Simulation results of the relation between \(I_{2}\) and \(T_{2}\) based on conclusion one (green trace) and conclusion two (blue trace), when \(I_{1}=34\) mA, \(T_{1}=0.971\) ns; (**b**) The peak power and full width at half maximum of the output pulse when \(I_{2}\) and \(T_{2}\) and satisfy the fitting curve in (**a**).  
the output pulse by controlling the internal factors under the condition that the laser has been fabricated. According to the fact that the accumulation speed mainly determines the peak power and width of the output pulse after the carrier density reaches the threshold, we describe a method for generating ultra-short pulses with high extinction ratio based on step rectangular wave modulation. Then, through numerical simulation, the influence of amplitude \(I_{2}\) and width \(T_{2}\) of the second modulation current on the output pulse is discussed. It is concluded that when \(I_{2}\) is constant, \(T_{2}\) should be adjusted until the maximum time of output pulse power is equal to the injection current termination time. The moment is precisely the same as the injection current termination time; when \(T_{2}\) is fixed, \(I_{2}\) should be adjusted so that the output peak power just reaches saturation. At last, when the laser output pulse is optimal, the inverse proportional relationship between \(I_{2}\) and \(T_{2}\) is obtained by curve fitting. Compared with the rectangular wave modulation, it is shown that the step-pulse can increase the output pulse peak power by increasing the second modulation current and reduce the pulse width to saturation at the best working point. It should be noted that we did not consider the effect of temperature during modulation because of the mature of temperature control, which the precision could reach 0.01 degrees centigrade.  
Although our simulation is based on the parameters in Table 1, the results are applicable to other semiconductors with different parameters because the gain-switched process is always the same. The gain-switched pulse lasers with rectangular wave modulation have been studied for a long time and can be found in commercial products. Our method is an improvement to them, and it is not difficult to be achieved. In the real experiment, we can use the differential circuit in the falling edge of a square wave to generate spike as a scheme of generating step-pulse. Our method has excellent guidance for further improving the performance of the gain-switched semiconductor lasers and solving the application requirements for a high extinction ratio of short pulse signals, such as the optical time domain reflectometry.  
[PERSON] proposed the idea. [PERSON] and [PERSON] conceived and designed the simulations, analyzed the data, and wrote the paper. [PERSON], [PERSON] and [PERSON] reviewed the paper and simulation results.  
This research was supported by the funds described in the Acknowledgements Section.  
This work is currently supported by the National Science Foundation of China (NSFC) (No. 61705033), and the Fundamental Research Funds for the Central Universities (No. ZYGX2016J014).  
The authors declare no conflicts of interest.