In the last decade, coherent narrow linewidth optical system technology has contributed to reaching a new generation called high-speed optical communication. This technology has improved the transmission capacity of long-haul fiber optic transmission systems. State of the art in the market is a bit rate up to 100 Gbps per channel, where in one fiber optic can be traversed by 80 channels or 8 TB per fiber. The main key of the high-speed transmission is narrowing the laser linewidth which will increase the laser coherency. Some techniques have been proposed before such as the use of quantum well structure, distributed feedback braggers and application of external cavity resonators that have been able to narrow semiconductor linewidth lasers. In this paper we make a theoretical analysis to attempt for narrowing the linewidth semiconductor laser with laser current injection control. We intend the application of narrowing linewidth technique for the free-space optical communications in the case of visible light communication where the demand of laser source with narrower linewidth of signal is one of the key to improve the performance. The analysis is performed based on Schawlow-Townes equation. We prove that increasing the current injection twice causes linewidth constriction twice. The analysis and simulations are performed using Optiwave software.