Longitudinal mode competition in chirped grating distributed feedback lasers

Gain competition often inhibits the simultaneous lasing of multiple longitudinal modes in homogeneously broadened laser systems. A stability analysis is developed to demonstrate that the two lowest order degenerate longitudinal modes in an index-coupled distributed feedback (DFB) laser will lase simultaneously when the index grating is chirped asymmetrically along the axis of the device. This chirped grating structure is shown to decrease the gain competition by reducing the spatial overlap between the degenerate modes. Stable mode beating between the two lowest order lasing modes results, and this beating produces high-frequency output self-pulsations which can be used for millimeter-wave and soliton pulse train generation. An exact closed-form expression for the output intensity of an antisymmetrically chirped index-coupled DFB laser, as a function of the unsaturated gain, is also derived. The expression is valid for arbitrary levels of gain saturation.