Over the past seven years, the coherent digital optical transmission enables a new generation of high speed optical communication. Coherent transmission technologies have delivered improvement in the capacity-reach product for long-haul fiber optic transmission systems. The coherent digital optical transmission is able to reach bit rates up to 100 Gbit/s. One of the keys of the ideal coherent optical communication is by narrowing laser linewidth. Several approaches to narrow the laser linewidth have been proposed. For instances by adding the length of external cavity resonator as shown by Schawlow-Townes equation and the other one is by controlling laser current injection. This paper is proposed to analyze the structure of coherent laser system to obtain a coherent laser linewidth as narrow as possible by adding external cavity and controlling the laser current injection. The analysis and simulations are performed using Optiwave software. Based on our proposed laser coherent system, we are able to obtain linewidth of 100 kHz. This linewidth is sufficient enough to support for high speed 100 Gbit/s optical communication system.