Semiconductor Laser Coherency in Relationship to the Linewidth: An Analysis

In the last decade, the coherent technology of narrow linewidth optical system has contributed to achieving a new generation called high speed optical communication. This technology has improved the capacity of transmitting long-haul fiber optic transmission systems. State of the art in the market is the bit rate up to 100 Gbps per channel, where in one fiber optic can be traversed by 80 channels or 8 Tbps per fiber. The main key to high speed transmission is narrowing the linewidth laser that will increase laser coherency. Narrowing Linewidth laser is also widely used for power laser cutting applications, because narrowing linewidth results in reducing chromatic aberration that narrowing of the focal diameter, such that laser cutting becomes more effective. Several techniques have been known previously such as the use of a quantum well structure, distributed Bragg feedback and the use of an external cavity resonator that can obviously narrow the semiconductor laser linewidth. In this paper, we make a new theoretical analysis to narrow the semiconductor laser linewidth by controlling laser current injection. The analysis is performed based on the Schawlow-Townes equation and its new revision. We prove that by increasing current injection causes narrowing of the linewidth. The analysis and simulations are performed using Optiwave software.